Photochemistry of Phenyl-Substituted 1-Methylpyrazoles

Article abstract of DOI:10.1021/jo970897r

Direct irradiation of 1-methyl-4-phenylpyrazole (2) in methanol results in regiospecific phototransposition to 1-methyl-4-phenylimidazole (4) and in photocleavage to (E)/(Z)-3-(N-methylamino)-2-phenylpropenenitrile (5) and (E)/(Z)-2-(N-methylaimno)-1-phenylethenyl isocyanide (6). Deuterium labeling confirms that the phototransposition occurs via the P₄ permutation pathway. Separate experiments show that 5 and 6 undergo (Z) → (E) isomerization and photocyclization to imidazole 4. Quantum yields for these reactions show that the sequence 2 → 6 → 4 is a major pathway for the P₄ phototransposition of 2 → 4. Isocyanides were also detected as intermediates in the P₄ phototransposition of a variety of other pyrazoles confirming the generality of this pathway in pyrazole photochemistry. Direct irradiation of 1-methyl-5-phenylpyrazole (3) resulted in the formation of 1-methyl-5-phenylimidazole (7), 1-methyl-2-phenylimidazole (8), and 1-methyl-4-phenylimidazole (4). Deuterium labeling revealed that these products were formed by P₄, P₆, and P₇ permutation pathways, respectively. (E)/(Z)-3-(N-methylamino)-3-phenylpropenenitrile (9) and (E)/(Z)-2-(N-methylamino)-2-phenylethenyl isocyanide (10) photocleavage products were also formed in this reaction. Irradiation of 3 in furan solvent did not result in phototransposition but led to the formation of endo and exo adducts formed by Diels-Alder reaction of furan with 4-phenyl-5-methyl-1,5-diazabicyclo[2.1.0]pent-2-ene. This constitutes the first direct evidence for the formation of a 1,5-diazabicyclo[2.1.0]hex-2-ene from photolysis of a pyrazole and is consistent with the electrocyclic ring closure - heteroatom migration mechanism suggested for the P₆ and P₇ phototranspositions.

Full text of DOI:10.1021/jo970897r

J . Org. Chem. 1997, 62, 8325-8334
8325
P h otoch em istr y of P h en yl-Su bstitu ted 1-Meth ylp yr a zoles
J ames W. Pavlik* and Naod Kebede
Department of Chemistry and Biochemistry, Worcester Polytechnic Institute,
Worcester, Massachusetts 01609
Received May 20, 1997X
Direct irradiation of 1-methyl-4-phenylpyrazole (2) in methanol results in regiospecific phototrans-
position to 1-methyl-4-phenylimidazole (4) and in photocleavage to (E)/(Z)-3-(N-methylamino)-2-
phenylpropenenitrile (5) and (E)/(Z)-2-(N-methylamino)-1-phenylethenyl isocyanide (6). Deuterium
labeling confirms that the phototransposition occurs via the P4 permutation pathway. Separate
experiments show that 5 and 6 undergo (Z) f (E) isomerization and photocyclization to imidazole
4. Quantum yields for these reactions show that the sequence 2 f 6 f 4 is a major pathway for
the P4 phototransposition of 2 f 4. Isocyanides were also detected as intermediates in the P4
phototransposition of a variety of other pyrazoles confirming the generality of this pathway in
pyrazole photochemistry. Direct irradiation of 1-methyl-5-phenylpyrazole (3) resulted in the
formation of 1-methyl-5-phenylimidazole (7), 1-methyl-2-phenylimidazole (8), and 1-methyl-4-
phenylimidazole (4). Deuterium labeling revealed that these products were formed by P4, P6, and
P7 permutation pathways, respectively. (E)/(Z)-3-(N-methylamino)-3-phenylpropenenitrile (9) and
(E)/(Z)-2-(N-methylamino)-2-phenylethenyl isocyanide (10) photocleavage products were also formed
in this reaction. Irradiation of 3 in furan solvent did not result in phototransposition but led to
the formation of endo and exo adducts formed by Diels-Alder reaction of furan with 4-phenyl-5-
methyl-1,5-diazabicyclo[2.1.0]pent-2-ene. This constitutes the first direct evidence for the formation
of a 1,5-diazabicyclo[2.1.0]hex-2-ene from photolysis of a pyrazole and is consistent with the
electrocyclic ring closure-heteroatom migration mechanism suggested for the P6 and P7 pho-
totranspositions.
In tr od u ction
solvent. Thus, after irradiation of a solution of 1 in
methanol (3.0 mL, 2.0 × 10-2 M) for 42 h, 42% of 1 was
consumed, but no volatile products could be detected by
gas-liquid chromatographic (GLC) analysis.
1-Methylpyrazoles undergo phototransposition to 1-
methylimidazoles and photocleavage to N-methylena-
mino nitriles.1-5 Permutation pattern analysis6,7 re-
vealed that the phototransposition occurs by the P6, P7,
and/or the P4 permutation pathways and thus by up to
three different mechanistic routes. Replacement of the
1-methyl group with phenyl substantially alters the
regiochemistry of the phototransposition. Thus, 1-phe-
nylpyrazoles transpose regiospecifically to 1-phenylimi-
dazoles via the P4 pathway.8 In this report we present
the results of our study of the photochemistry of 1-me-
thylpyrazoles substituted with phenyl at the carbon
atoms of the pyrazole ring rather than at nitrogen. These
studies provide information regarding the nature of
intermediates on the phototransposition pathways.
In contrast, we have recently shown that 1-methyl-4-
phenylpyrazole (2) undergoes phototransposition to
1-methyl-4-phenylimidazole (4) and photocleavage result-
ing in the formation of (E)/(Z)-3-(N-methylamino)-2-
phenylpropenenitrile (5) and (E)/(Z)-2-(N-methylamino)-
1-phenylethenyl isocyanide (6).9 The permutation pattern
Resu lts a n d Discu ssion
for the phototransposition of 2 to 4 was determined by
studying the phototransposition of 5-deuterio-1-methyl-
4-phenylpyrazole (2-5-d1), which was synthesized by
exchange of the C-5 proton by treatment of 2 with
butyllithium followed by quenching of the lithium reagent
with D2O. The mass spectrum of the resulting product
exhibited a molecular ion at m/e 159, indicating complete
1-Methyl-3-phenylpyrazole (1) does not undergo trans-
position even after prolonged photolysis in methanol
X Abstract published in Advance ACS Abstracts, November 1, 1997.
(1) Tiefenthaler, H.; Do¨rscheln, W.; Go¨eth, H.; Schmid, H. Helv.
Chim. Acta 1967, 50, 2244.
(2) (a) Beak, P.; Miesel, J . L.; Messer, W. R. Tetrahedron Lett. 1967,
5315. (b) Beak, P.; Messer, W. R. Tetrahedron 1969, 25, 3287.
(3) Barltrop, J . A.; Day, A. C.; Mack, A. G.; Shahrisa, A.; Waka-
matsu, S. J . Chem. Soc., Chem. Commun. 1981, 604.
(4) Wakamatsu, S.; Barltrop, J . A.; Day, A. C. Chem. Lett. 1982,
667.
(5) Pavlik, J . W.; Kurzweil, E. M. J . Org. Chem., 1991, 56, 6313.
(6) For a discussion of permutation pattern analysis in aromatic
phototransposition chemistry and its application to the analysis of the
phototransposition reactions of five-membered heteroaromatics, see:
Barltrop, J . A.; Day, A. C. J . Chem. Soc., Chem. Commun. 1975, 177.
Barltrop, J . A.; Day, A. C.; Moxon, P. D.; Ward, R. W. J . Chem. Soc.,
Chem. Commun. 1975, 786.
(7) For five-membered heterocycles containing two heteroatoms
there are 12 different ways of transposing the five ring atoms resulting
in 12 permutation patterns identified P1-P12. For a table showing these
permutation patterns see ref 5.
1
deuteration of 2. In addition, the H NMR spectrum of
the deuterated product exhibited a one-proton singlet at
δ 7.80 for the C-3 proton of 2 but showed no signal at δ
7.60 where the C-5 proton of 2 is known to absorb. This
confirms that the product is 5-deuterio-1-methyl-4-phe-
nylpyrazole (2-5-d1).
Irradiation of a solution of 2-5-d1 in methanol (20 mL,
2.0 × 10-2 M) for 3 h followed by preparative layer
chromatography led to the isolation of the phototrans-
position product. The mass spectrum of this material
(8) Pavlik, J . W.; Connors, R. E.; Burns, D. S.; Kurzweil, E. M. J .
Am. Chem. Soc. 1993, 115, 7645.
(9) Pavlik, J . W.; Kebede, N.; Bird, N. P.; Day, A. C.; Barltrop, J . A.
J . Org. Chem. 1995, 60, 8138
S0022-3263(97)00897-9 CCC: $14.00 © 1997 American Chemical Society

8326 J . Org. Chem., Vol. 62, No. 24, 1997
Pavlik and Kebede
exhibited a base peak at m/e 159 showing that the
quantity of imidazole 4.10 Irradiation of the solution for
3.0 h under the same conditions resulted in the consump-
tion of 42% of (Z)-5 and the formation of (E)-5 and
imidazole 4 in yields of 36% and 33%, respectively.
deuterium label had not been lost during the reaction
1
and isolation. The H NMR spectrum exhibited a one-
proton singlet at δ 7.71 for the C-2 proton of 4 but no
signal at δ 7.15 where the C-5 proton of 1-methyl-4-
phenylimidazole is known to absorb. This shows that
5-deuterio-1-methyl-4-phenylpyrazole (2-5-d1) has un-
dergone phototransposition to 5-deuterio-1-methyl-4-
phenylimidazole (4-5-d1) and confirms that the transpo-
sition has occurred via the P4 permutation pathway.
Unlike 1-methyl-4-phenylpyrazole (2), 1-methyl-5-phe-
nylpyrazole (3) did not undergo regiospecific phototrans-
position. When a solution of 3 in methanol (3.0 mL, 2.0
× 10-2 M) was irradiated for 3.0 h, GLC analysis showed
the consumption of 76% of 3 and the formation of three
volatile products that were identified as 1-methyl-5-
phenylimidazole (7), 1-methyl-2-phenylimidazole (8), and
1-methyl-4-phenylimidazole (4) in yields of 30, 29, and
31%, respectively. These photoproducts were identified
by direct comparison of their chromatographic and
spectroscopic properties with those of authentic samples
of these compounds synthesized in this laboratory.
Monitoring the photoreaction of 1-methyl-4-phenylpyra-
zole (2) as a function of irradiation time revealed the
continuous consumption of 2 and the formation of imi-
dazole 4 and nitrile 5. During the 3 h of irradiation,
however, 1H NMR analysis showed that the concentration
of isocyanide 6 reached a maximum after approximately
1 h and then decreased upon further irradiation. Indeed,
when the reaction was discontinued after 1 h of irradia-
1
tion, UV-absorption and H NMR analysis showed that
21% of 2 had been consumed, while 4, 5, and 6 had been
formed in yields of 17%, 12%, and 68%, respectively.
The formation and subsequent consumption of 6 sug-
gested that the isocyanide might be an intermediate in
the conversion of 2 to 4 and/or 5. This possibility was
explored by monitoring the direct irradiation of purified
(Z)-6. A solution of (Z)-6 in methanol (3.0 mL, 1.44 ×
10-2 M) was irradiated for 5 min. UV and 1H NMR
spectroscopic analysis of the resulting solution showed
a 24% consumption of (Z)-6. In addition to the doublet
at δ 3.10 due to residual (Z)-6, the 1H NMR spectrum
showed a second doublet at δ 2.86 due to the formation
of (E)-6 in 51% yield and a singlet at δ 3.70, confirming
the formation of imidazole 4 in 28% yield. These results
The permutation patterns for these products were
confirmed by studying the phototransposition chemistry
of 4-deuterio-1-methyl-5-phenylpyrazole (3-4-d1) prepared
by the acid-catalyzed deuterium exchange of 3. Deute-
rium incorporation was confirmed by the mass spectrum,
which exhibited a molecular ion at m/e 159. Further-
more, the 1H NMR spectrum exhibited a one proton
singlet at δ 7.53 for the C-3 proton but no signal for the
C-4 proton at δ 6.33.
A solution of 3-4-d1 in methanol (50 mL, 2.0 × 10-2 M)
was irradiated for 1.5 h. 1H NMR analysis of the
individual phototransposition products, separated by
preparative-layer chromatography, showed that H-3 of
the reactant had transposed to position 2 in 1-methyl-
5-phenylimidazole (7-4-d1), to position 4 in 1-methyl-2-
phenylimidazole (8-5-d1), and to position 2 in 1-methyl-
4-phenylimidazole (4-5-d1) confirming that the trans-
positions had occurred via the P4, P6, and P7 permutation
pathways.
The photoreaction of 3 was also monitored by 1H NMR
spectroscopy after shorter irradiation times. Thus, after
irradiation of a solution of 3 in methanol (3.0 mL, 2.0 ×
clearly show that isocyanide (Z)-6 undergoes (Z) f (E)
isomerization and photocyclization to imidazole 4. In
addition, cyclization of (Z)-6 to 4 was also observed when
the former was heated to 80 °C. No evidence, however,
could be detected for the photochemical or thermal
isomerization of isocyanide 6 to enaminonitrile 5.
1
10-2 M) for 8 min, the H NMR spectrum of the residue
showed the appearance of three singlets at δ 3.62, 3.56
and 3.60 due to the formation of the P4, P6, and P7
N-methylimidazoles 7, 8, and 4. In addition, the spec-
The direct irradiation of enaminonitrile 5 was also
investigated. When a solution of 5 in methanol (97% Z,
3.0 mL, 2.0 × 10-2 M) was irradiated for 20 min, 1H NMR
analysis of the residue showed that 39% of (Z)-5 had been
consumed and that an almost quantitative yield of (E)-5
had been formed. A very small singlet could also be
discerned at δ 3.76 due to the formation of a trace
(10) For additional examples of the photocyclization of enamino
nitriles see: (a) Ferris, J . P.; Kuder, J . E.; Cantalano, A. W. Science
1969, 1966, 765. (b) Ferris, J . P.; Kuder, J . E.; J . Am. Chem. Soc. 1970,
92, 2527. (c) Koch, T. H.; Rodehorst, R. M. J . Am. Chem. Soc. 1974,
96, 6707. (d) Ferris, J . P.; Trimmer, R. W. J . Org. Chem. 1976, 41, 19.
(e) Ferris, J . P.; Narang, R. S.; Newton, T. A.; Rao, V. R. J . Org. Chem.
1979, 44, 1273.

Photochemistry of Phenyl-Substituted 1-Methylpyrazoles
J . Org. Chem., Vol. 62, No. 24, 1997 8327
Ta ble 1. Qu a n tu m Yield s for P yr a zoles 1-3
reactant
φ(consumption)
φ(P4)
φ(isocyanide)
φ(cyanide)
φ(P6)
φ(P7)
1
2
3
0
0.22 ( 0.01
0.041 ( 0.003
0.037 ( 0.002
0.0080 ( 0.0001
0.11 ( 0.01
<10-3
0.0080 ( 0.001
<10-3
0.014 ( 0.001
0.022 ( 0.007
Ta ble 2. Qu a n tu m Yield s for Nitr ile a n d Isocya n id e
P h otolysis
perature and phosphorescence at 77 K are shown in Table
3 and reveal that the unreactive 3-phenyl isomer 1 has
a fluorescence-to-phosphorescence ratio of 2.6, whereas
the more reactive 4- and 5-phenylisomers 2 and 3 have
larger proportions of phosphorescence with fluorescence-
to-phosphorescence ratios of only 0.8. The measured
fluorescence and phosphorescence lifetimes at room tem-
perature and at 77 K, respectively, are also shown in
Table 3.13 The 5-phenyl isomer 3 was observed to exhibit
double fluorescence decay with lifetimes of 2.82 and 7.14
ns. This may be due to emission from conformations with
different angles of twist between the phenyl and pyrazole
rings. The forced single-exponential decay gave a lifetime
of 4.68 ns, which is the value shown in Table 3. These
lifetimes of S1 and T1 states and the energy gaps are
consistent with π,π* configurations. The values of the
radiative rate constants for fluorescence and the rate
constants for the formation of various products have been
calculated from kf ) φf/τf and kr ) φproduct/τf and are shown
in Table 4.
reactant
φ(consumption)
φ(E-isomer)
φ(P4 imidazole)
(Z-5
(Z)-6
0.26 ( 0.01
0.42 ( 0.01
0.26 ( 0.01
0.22 ( 0.01
<10-3
0.12 ( 0.01
Ta ble 3. Sp ectr oscop ic Da ta
abs,
compd λ (nm)
log
ꢀ
fl(0,0)
kcal/mol kcal/mol φf
ph(0,0)
τf
τp
φp
(ns) (s)
1
2
3
248.3 4.20 104.4
249.7 4.24 104.8
238.0 4.24 103.7
73.9
77.3
72.9
0.36 0.19 13.10 7.67
0.31 0.47 7.47 8.17
0.30 0.37 4.68 8.75
trum also revealed one pair of doublets (J ) 5.3 Hz) at δ
2.73 and 2.84 and a second pair at δ 2.78 (J ) 4.9 Hz)
and 3.28 (J ) 3.5 Hz). Although the compounds respon-
sible for these signals were not isolated, the first pair of
doublets was assigned to the E and Z isomers of 3-(N-
methylamino)-3-phenylpropenenitrile (9) by comparison
1
with the H NMR spectrum of (E)-9 synthesized in this
laboratory. The second pair of doublets is consistent with
Gen er a lity of Isocya n id e P a th w a y. These results
reveal the operation of two different photocleavage-
photocyclization pathways for the P4 transposition. The
quantum yields for geometrical isomerization and pho-
tocyclization of enaminonitrile (Z)-5 given in Tables 1 and
2 confirm our earlier suggestion that at short irradiation
times this pathway cannot account for a significant
portion of the P4 imidazole formed.5 In contrast, the
quantum yields for the same reactions of (Z)-6 show that
even at short irradiation times transposition via an
isocyanide intermediate can account for formation of a
significant fraction of the imidazole product.
Although isocyanides have been spectroscopically de-
tected as intermediates in the analogous P4 isoxazole-
to-oxazole phototransposition,14 this is the first example
of the involvement of an isocyanide in the P4 pyrazole-
to-imidazole reaction. Furthermore, the chemical and
quantum yields observed show that the photocleavage-
photocyclization pathway via an isocyanide intermediate
constitutes a major P4 transposition pathway.
the E- and Z-isomers of isocyanide 10. As expected from
these assignments, after 20 min of irradiation the
doublets due to the (E)- and (Z)-nitrile 9 continued to
grow in intensity while those assigned to isocyanide 10
were substantially diminished.
Qu a n tu m Yield s. The quantum yields for reactant
consumption and product formation from irradiation of
phenyl-substituted 1-methylpyrazoles 1-3 and enamino-
nitrile 5 were determined in triplicate using 1,3-di-
methyluracil11 as the actinometer while the quantum
yields for the reaction of isocyanide 6 were determined
using 1,3-cycloheptadiene11 as the actinometer and are
tabulated in Table 1 and 2.
Sen sitized Ir r a d ia tion s. Sensitized irradiations of
phenyl-substituted pyrazoles 1-3 (ET ) 72.9-77.3 kcal/
mol based on the 0,0 band of phosphorescence) were
carried out in acetone (ET ) 79-82 kcal/mol).12 All of
these phenyl-substituted pyrazoles were found to be
unreactive as triplets.
Sp ect r oscop ic P r op er t ies. Pyrazoles 1-3 exhibit
structureless absorption spectra in methanol solvent with
absorption maxima (Table 3) ranging from 238 to 250 nm
with extinction coefficients consistent with π f π*
transitions. 1-3 exhibit both fluorescence and phospho-
rescence in methanol/ethanol. At 77 K, discernible 0,0
bands in the fluorescence spectra are at approximately
104 kcal/mol while the 0,0 bands in the phosphorescence
indicate that the triplet states are located at 73-77 kcal/
mol. The quantum yields of fluorescence at room tem-
To determine the generality of this pathway, the
photoreactions of a variety of N-substituted pyrazoles
were reinvestigated. After irradiation of a solution of
1-methylpyrazole (11) in acetonitrile (3.0 mL, 2.0 × 10-2
M) for 5 min, 1H NMR analysis of the crude reaction
mixture showed the appearance of the N-methyl singlet
at δ 3.68 due to the formation of 1-methylimidazole (12)
and to a pair of doublets at δ 2.71 (J ) 5.1 Hz) and 2.97
(J ) 4.9 Hz) due to the N-methyl groups of (E)- and (Z)-
enaminonitriles 13. In addition to these previously
1
reported products,5 the H NMR spectrum also showed
a second pair of doublets at δ 2.59 (J ) 5.1 Hz) and 2.89
(J ) 5.0 Hz) that decreased in intensities upon more
prolonged irradiation times consistent with the formation
and subsequent consumption of isocyanides (E)-14 and
(Z)-14. In addition, the solution had the characteristic
penetrating odor associated with an isocyanide. Simi-
(13) Fluorescence lifetimes were measured by Dr. A. M. Halpern,
Department of Chemistry, Indiana State University.
(14) (a) Ferris, J . P.; Antonucci, F. R. J . Am. Chem. Soc. 1974, 96,
2014. (b) Ferris, J . P.; Trimmer, R. W. J . Org. Chem. 1976, 41, 13.
(11) Numao, N.; Hamada, T.; Yonemitsu, O. Tetrahedron Lett. 1977,
1661.
(12) Schmidt, M. W.; Lee, E. K. C. J . Am. Chem. Soc. 1970, 92, 3579.

8328 J . Org. Chem., Vol. 62, No. 24, 1997
Pavlik and Kebede
Ta ble 4. Ra te Con sta n ts
10-6 kr(isocyanide) (s-1 10-6 kr(cyanide) (s-1
compd
10-6 kf (s-1
)
10-6kr(P4) (s-1
)
)
)
10-6kr(P6) (s-1
)
10-6kr(P7) (s-1
)
1
2
3
27.48
41.50
64.10
4.04
1.83
15.73
<0.2
0.87
<0.2
3.24
5.04
of an isocyanide. Furthermore, whereas the band at 2195
cm-1 increased after irradiation for 30 min, the band at
2103 cm-1 due to the isocyanide was substantially
decreased in intensity, consistent with the transient
nature of the isocyanide. Finally, although 1,3-dimeth-
ylpyrazole (28) is known to transpose to 1,2-dimethylimi-
dazole (19) and 1,4-dimethylimidazole (17) via the P4 and
P6 pathways,5 1H NMR analysis of the crude reaction
mixtures obtained from irradiation of an acetonitrile
solution of 28 (30 mL, 2 × 10-2 M) showed the clean
conversion of the reactant pyrazole 28 to the imidazole
products 19 and 17 without the formation of either
enaminonitrile or isocyanide intermediates.
larly, in addition to N-methylimidazoles 17 or 17-19, 1H
NMR examination of the crude reaction mixtures ob-
tained after irradiation of 1,4-dimethylpyrazole (15) or
1,5-dimethylpyrazole (16) showed pairs of doublets (J )
5.0 Hz) at δ 2.81 and 2.92 or at δ 2.69 and 2.95 due to
the N-methyl groups of the (E)- and (Z)-enamino nitriles
20 or 22 and a second pair of doublets (J ) 5.0 Hz) at δ
2.78 and 2.88 or at δ 2.58 and 2.86, indicating the
presence of the (E)- and (Z)-isocyanides 21 or 23. 4-Meth-
Mech a n istic Discu ssion . The photochemistry of
pyrazoles has been suggested to involve a competition
between electrocyclic ring closure leading to P6 and P7
transpositions and cleavage of the N-N bond resulting
in the formation of the precursor of the P4 transposition
product.5 1-Methyl-4-phenylpyrazole (2) transposes re-
giospecifically via the latter pathway. Thus, excitation
of 2 is suggested to result in cleavage of the N-N bond
resulting in the formation of a species that can be viewed
as diradical 2a or zwitterion 2b , which are resonance
forms of â-iminovinyl nitrene 2c.
AM1 calculations are consistent with cleavage of the
N1-N2 bond upon photochemical excitation of 2. These
calculations reveal that the N1-N2 bond length increases
from 1.35 Å in the ground state of 2 to 4.75 Å in the
energy-minimized excited singlet state of 2. This corre-
sponds to a change in the N1-N2 bond order from 1.13
to ∼0 indicating that cleavage of the N1-N2 bond is
essentially complete by the time the molecule reaches the
relaxed excited single state. These same calculations,
however, also indicate that excitation is also accompanied
by substantial breaking of the C3-C4 bond and an
increase in the N2-C3 bond order, suggesting that
H-C3-N2 is being expelled from the molecule.15 These
calculations also suggest that in the relaxed excited
singlet state the departing H-CtN molecule is almost
perpendicular to the plane of the N1-C5-C4 portion of
the molecule, indicating substantial twisting in the S1
state. These calculations, however, describe reactions
yl-1-phenylpyrazole (24) reacted similarly. After 15 min
of irradiation of a solution of 24 in acetonitrile (3.0 mL,
1
2.0 × 10-2 M), H NMR analysis of the crude reaction
mixture showed two pairs of doublets at δ 1.82 (J ) 1.3
Hz) and 1.87 (J ) 1.8 Hz) due to the allylic R methyl
group coupling with the â-proton of the enaminonitriles
(E)- and (Z)-26 and a second pair of doublets of lesser
intensity at δ 1.59 (J ) 1.3 Hz) and 1.66 (J ) 1.4 Hz)
suggesting the presence of isocyanides (E)- and (Z)-27.
Interestingly, IR examination of this residue showed
absorption bands at 2195 cm-1 due to the nitrile func-
tional group and at 2103 cm-1 confirming the presence
(15) It is interesting to note that loss of HCN is a major fragmenta-
tion pathway in the mass spectrum of 2.

Photochemistry of Phenyl-Substituted 1-Methylpyrazoles
J . Org. Chem., Vol. 62, No. 24, 1997 8329
occurring in the gas phase and may not accurately reflect
bonding changes upon excitation in solution.
In addition to phototransposition via the P4 pathway
and photocleavage to nitriles and isocyanides, reactions
that are mechanistically analogous to those already
discussed for 1-methyl-4-phenylpyrazole (2), the 5-phenyl
isomer 3 also phototransposes via the P6 and P7 path-
ways. These pathways have previously been rationalized
by assuming that photocleavage of the N-N bond, the
initial step on the P4 reaction pathway, competes with
electrocyclic ring closure, which would yield 1,5-diazabi-
cyclopentene 35, which can undergo one or two consecu-
tive 1,3-sigmatropic shifts of nitrogen.5 Rearomatization
of the isomeric 2,5-diazabicyclopentene intermediates 36
and 37 would thus provide the P6 and P7 imidazoles 8
and 9. Although the electrocyclic ring closure-heteroatom
In addition to recyclizing to the pyrazole reactant,
rearrangement of â-imino vinyl nitrene 2c to nitrile 5,
either by direct transfer of hydrogen from C-3 of the
original pyrazole to nitrogen, or possibly by way of ketene
imine 29, is an expected pathway for a terminal vinyl
nitrene.16 Alternatively, 2c would also be expected to
undergo intramolecular cyclization to yield iminoazirine
30,17 a plausible precursor of nitrile 5, isocyanide 6, and
imidazole 3. Although azirines unsubstituted at C-2 are
generally not thermally stable, one such azirine has been
found to rearrange photochemically to both a nitrile and
an isocyanide.18 Although it is plausible that nitrile
formation may involve the vinyl nitrene formed by C-N
bond fission, azirines are also known to undergo photo-
chemical ring opening of the C-C bond, which in the
present case would yield nitrile ylide 31, a likely precur-
sor of imidazole 3 and isocyanide 6. Indeed, a number
migration mechanism adequately rationalizes formation
of the P6 and P7 imidazole products, and although similar
bicyclic intermediates have been chemically trapped in
the analogous phototransposition reactions of cyanopy-
rroles,21 no evidence for such intermediates in pyrazole
photochemistry has been presented.
of â-acyl nitrile ylides and one example of a â-imino
nitrile ylide have been generated by irradiation of acyl
or iminoazirines and shown to cyclize to oxazoles or to
an imidazole.19,20 In these cases, the azirines were
substituted with a phenyl group at C-2, and thus isocya-
nides and nitriles were not possible products.
Similarly, 1,3-dimethylpyrazole (28) would undergo
photocleavage to vinyl nitrene 28a , which could rear-
range to an unstable N-methylketene imine 32 or un-
dergo ring closure to azirine 33 which bears a C-2 methyl
substituent. Photochemical ring opening of 33 would
yield nitrile ylide 34, which could cyclize to 1,2-dimeth-
ylimidazole 19 but could not yield as isocyanide. This is
consistent with our observation that 28 transposes to 19
without the formation of nitrile or isocyanide photocleav-
age products.
AM1 calculations, however, are consistent with elec-
trocyclic ring closure in the first excited state of 3. These
calculations thus reveal that upon relaxation the excited
singlet molecule begins to undergo disrotatory ring
deformation with N1 moving out of the plane by 14.4°.
In addition, excitation is also accompanied by decreases
in the N2-C3 and C4-C5 bond orders and an increase in
the C3-C4 bond order. These calculations thus suggest
that the geometry and bond orders of the S1 relaxed
pyrazole 3 are approaching the structure of 1,5-diazabi-
cyclo[2.1.0] intermediate 35 and are consistent with
electrocyclic ring closure. These predicted changes are
quite different from those suggested for 4-phenylpyrazole
2, which was not predicted to undergo disrotatory defor-
mation and which indeed was not observed to react by
the electroyclic ring closure-heteroatom migration path-
way.
In an attempt to trap such a bicyclic intermediate, the
photochemistry of 1-methyl-5-phenylpyrazole (3) was also
studied in furan solvent. Under these conditions pho-
(16) Hassner, A.; Wiegand, N. H.; Gottlieb, H. E. J . Org. Chem. 1986,
51, 3176.
(17) (a) Smolinsky, G. J . Org. Chem. 1962, 27, 3557. (b) Horner, L.;
Christman, A.; Gross, A. Chem. Ber. 1963, 96, 399. (c) Hassner, A.;
Fowler, F. W. Tetrahedron Lett. 1967, 1545. (d) Fowler, F. W.; Hassner,
A.; Levy, L. A. J . Am. Chem. Soc. 1967, 89, 2077.
(18) Bauer, W.; Hafner, K. Angew. Chem., Int. Engl. Ed. 1969, 8,
772.
(19) Padwa, A.; Smolanoff, J .; Tremper, A. Tetrahedron Lett. 1974,
29; J . Am. Chem. Soc. 1975, 97, 4682.
(20) (a) Padwa, A. Acc. Chem. Res. 1976, 9, 371. (b) Griffin, G. W.;
Padwa, A. In Photochemistry of Heterocyclic Compounds; Buchardt,
O., Ed.; J ohn Wiley and Sons: New York, 1976.
(21) Barltrop, J . A.; Day, A. C.; Ward, R. W. J . Chem. Soc., Chem.
Commun. 1978, 131.

8330 J . Org. Chem., Vol. 62, No. 24, 1997
Pavlik and Kebede
totransposition was not observed. Instead, irradiation
led to the formation of a thermally labile oil that was
identified as a mixture of the endo and exo-pyrazole-
furan [4 + 2] adducts 38 and 39. Thus, the 13C NMR
system crossing, φT ) 0.68, and reaction, φr ) 0.041.
Thus, only 4% of the excitation energy results in chemical
reaction, and essentially all of this is channeled into the
electrocyclic ring closure-heteroatom migration pathway.
This is consistent with the predictions of AM1 calcula-
tions, which show that energy minimization of the excited
state of 3 is accompanied by disrotatory ring deformation
with N1 moving out of the plane by 14.4°.
When the location of the phenyl ring is changed to
position 4 of the pyrazole ring, several significant changes
are observed. First, the rates of fluorescence and inter-
system crossing both decrease. In contrast, the rate of
photochemical reactivity is enhanced by a factor of 3.4.
Taken together, these kinetic changes result in a decrease
in the quantum yield of triplet formation from 0.68 for 3
to 0.40 for 2 and in an increase in the quantum yield for
fluorescence from 0.28 to 0.38. The largest change,
however, is the increase in the quantum yield for reaction
from 0.041 for 3 to 0.22 for 2. Furthermore, this
reactivity is now completely channeled into the P4 reac-
tion pathway. Thus, the total P4 rate constant changes
from less than 2.23 × 106 s-1 for 3 to 20.6 × 106 s-1 for 2.
This rate enhancement is presumably due to the effect
that the C-4 phenyl group has on breaking the N-N bond
due to stabilization of the initially formed diradical 2a
or zwitterion 2b, which is possible when the phenyl ring
is located at the 4 position but is not possible when it is
on position 5 on the pyrazole ring. This effect is also
supported by calculations that predict that the N-N bond
is essentially broken in the energy-minimized excited
singlet state of 1-methyl-4-phenylpyrazole (2) but is
intact in the corresponding excited singlet state of
1-methyl-5-phenylpyrazole (3).
Neither the recorded nor the calculated electronic
spectra provide an obvious reason for the lack of reactiv-
ity of 1-methyl-3-phenylpyrazole (1). Placing the phenyl
group at ring position 3 is again accompanied by de-
creases in the rate constants for both fluorescence and
intersystem crossing. In the absence of photochemical
reactivity, these changes account for the increased life-
time of the singlet state. This isomer is also the most
luminescent of the three isomers with 68% of the
absorbed light returned as fluorescence and phosphores-
cence. The relatively low quantum yield for phospho-
rescence of 0.19 for this isomer, as compared to 0.47 and
0.37 for 2 and 3, is not due to a low triplet yield but rather
to a relatively low quantum efficiency for phosphores-
cence of 0.37. It is also interesting to note that AM1
calculations predict that photochemical excitation of 1
is not accompanied by either disrotatory deformation, as
in the case of 3, or by breaking of the N-N bond, as in
the case of 2. This is consistent with its lack of reactivity.
Furthermore, it is relevant to point out that a similar
trend in reactivity has also been observed with phenyl-
isothiazoles and phenylisoxazoles. Thus, current work
in our laboratory has shown that 3-phenylisothiazole and
3-phenylisoxazole are also substantially less reactive
than their 4- and 5-phenyl isomers.
spectrum shows two sets of signals characteristic of endo
and exo isomers of an unsymmetrically substituted
7-oxabicyclo[2.2.1]hept-2-ene ring system.22,23 The more
intense set of signals was assigned to the endo isomer
38 since this was presumed to be the major product.
Thus, the spectrum exhibits signals for the nonequivalent
vinyl carbons at δ 147.4 and 137.4 for the major isomer
and at δ 149.6 and 135.7 for the minor isomer. Similarly,
signals for the two nonequivalent C-1 and C-4 carbons
of the major isomer were observed at δ 103.5 and 80.0
and at δ 102.3 and 79.4 for the minor isomer. In this
case, the upfield signals at δ 80.0 and 79.4 were assigned
to the bridgehead carbon on the same side of the molecule
as the phenyl ring. Inspection of molecular models
indicates that this carbon would indeed be in the shield-
ing zone of the phenyl ring. The spectrum also shows
two sets of signals for the nonequivalent C-1 and C-2
carbon atoms at δ 62.1 and 56.5 for the major isomer and
at δ 63.0 and 55.3 for the minor isomer. Finally, in
addition to signals for the phenyl and quarternary benzyl
carbons in the δ 126-128 region, signals at δ 37.5 and
36.7 due to the N-methyl carbons of the major and minor
isomer were also present.
The structures of these adducts are consistent with
furan trapping of a photochemically generated 1,5-
diazabicyclopentene intermediate 35. Indeed, heating of
adducts 38 and 39 at 100 °C resulted only in the
formation of the original pyrazole, 1-methyl-5-phenylpyra-
zole (3). Neither imidazole 8 nor 9 could be detected by
1H NMR or GLC analysis. This indicates that [4 + 2]
trapping of the initially formed 1,5-diazabicyclopentene
35 is faster than 1,3-sigmatropic shift of nitrogen. It also
indicates that 1,5-diazabicyclopentene 35, generated on
the ground state by the retro-Diels-Alder reaction of 38
and 39, aromatizes to pyrazole 3 rather than undergoing
sigmatropic shift of nitrogen to a 2,5-diazabicyclic species.
This is consistent with our previous suggestion that the
first sigmatropic shift of nitrogen in the pyrazole f
imidazole transposition occurs on the S1 surface and that
internal conversion to the ground-state 1,5-diazabicyclo-
pentene results only in its return to the starting pyra-
zole.24
It is of interest to understand how changing the
position of the phenyl substituent on the pyrazole ring
alters the manner in which the excitation energy is
partitioned in the three isomeric phenyl-substituted
1-methylpyrazoles.
For 1-methyl-5-phenylpyrazole (3), the excited singlet
state partitions between fluorescence, φf ) 0.30, inter-
Con clu sion
Chemical and quantum yields show that the photo-
cleavage-photocyclization pathway via an isocyanide
intermediate is a major route for the P4 phototransposi-
tion of 4-phenyl-1-methylpyrazole (2). Furthermore, it
has been shown that this pathway is general to a variety
of other pyrazoles that bear hydrogen at the C-3 ring
position.
(22) Senda, Y.; Ohno, A.; Ishiyama, J .; Imaizumi, S.; Kamiyama, S.
Bull. Chem. Soc. J pn. 1987, 60, 613.
(23) Iwase, S.; Maeda, T.; Hamanaka, S.; Ogawa, M. Nipppon
Kagaku Kaishi 1975, 1934; Chem. Abstr. 1976, 84, 29989Z.
(24) Connors, R. E.; Pavlik, J . W.; Burns, D. S.; Kurzweil, E. M. J .
Org. Chem. 1991, 56, 6321.

Photochemistry of Phenyl-Substituted 1-Methylpyrazoles
J . Org. Chem., Vol. 62, No. 24, 1997 8331
mL were collected, and fractions 67-89 were concentrated to
yield 1-methyl-3-phenylpyrazole (1) as a white solid: mp 54-
55 °C (lit.36 mp 56 °C); 1.4 g (9.1 mmol, 24%); 1H NMR (CDCl3)
δ 3.89 (s, 3H), 6.52 (d, J ) 2.3 Hz, 1H), 7.31-7.38 (m, 4H),
7.77 (d, J ) 2.2 Hz, 1H), 7.80 (m, 1H);37 13C NMR (CDCl3) δ
39.0, 102.8, 125.4, 125.5, 127.5, 128.6, 131.3;38 IR (KBr) 3112,
2936, 1704, 1603, 1503, 1451, 1353, 1306, 1232, 1147, 1080,
1030, 948, 918, 763 cm-1; UV (CH3CN) λmax (log ꢀ) 253 (4.24);
MS m/e 158 (100), 157 (30), 130 (13), 117 (9), 103, (5), 89 (8),
77 (21), 55 (13), 51 (11), 42 (17).
These results also show that a 1,5-diazabicyclo[2.1.0]hex-
2-ene intermediate can be trapped by photolysis of
5-phenyl-1-methylpyrazole (3) in furan solvent. This
constitutes the first direct evidence for the intermediacy
of a 1,5-diazabicyclo[2.1.0]hex-2-ene in the P6 and P7
pyrazole-to-imidazole phototransposition.
Exp er im en ta l Section
Gen er a l P r oced u r es. 1H and 13C NMR spectra were
recorded at 200 and 50.3 MHz on a Bruker FT-NMR system.
1H and 13C chemical shifts were measured relative to internal
Me4Si and CHCl3, respectively. Infrared spectra were recorded
on a PE-1620 FT spectrometer. GLC was performed on a PE-
9000 FID instrument equipped with 15 m × 3 µm 50%
methylphenylsilicon phase capillary column. Mass spectra
were recorded with an HP 5970B mass-selective detector
interfaced to an HP 5880 capillary gas chromatograph. Ul-
traviolet absorption spectra were recorded on a Hitachi U-2000
spectrometer. Luminescence spectra were recorded on a PE-
LS 50 spectrometer. Flash column chromatography was
carried out on silica gel, 40 µm average particle size (J . T.
Baker, Inc.). Preparative-layer chromatography was carried
out on 20 × 20 cm glass plates coated with 2 mm of Kieselgel
60 F254 (Merck).
Syn th eses of Rea cta n ts a n d P r od u cts. Compounds
previously described in the literature were prepared as fol-
lows: 3-(N-methylamino)-2-phenylpropenenitrile (5, 97.1% Z)
by condensation of phenylformylacetonitrile with N-methyl-
formamide;25,26 3-(N-methylamino)-3-phenylpropenenitrile (9,
93.9% E) by methylation of 3-amino-3-phenylpropenenitrile
with methylamine hydrochloride;27,28 4-phenyl-1-H-imidazole
in three steps from acetophenone;29 1-methylpyrazole (11) by
methylation of pyrazole using trimethyl phosphate;30 1,3- and
1,5-dimethylpyrazole (28) and (16) as a mixture by condensa-
tion of methylhydrazine with 4,4-dimethoxy-2-butanone and
each isomer purified to greater than 99.6% purity by repeated
spinning-band distillation;31 1,4-dimethylpyrazole (15) by con-
densation of 1,1,3,3-tetraethoxy-2-methylpropane32 and me-
thylhydrazine dihydrogen sulfate;33 1,4-and 1,5-dimethylimi-
dazole (17), and (18) as a mixture by methylation of 4-meth-
ylimidazole using methyl iodide,34 separated by reduced-
pressure spinning band distillation.
1-Meth yl-3-p h en ylp yr a zole (1). A solution of phenyl
3-chlorovinyl ketone35 (20.3 g, 121.9 mmol) in diethyl ether
(200 mL) was kept in an ice bath, and N-methylhydrazine (24.0
mL, 451.0 mmol) was added dropwise over 1 h with continuous
stirring. The mixture was refluxed for 3 h and cooled in an
ice bath. Saturated NaHCO3 (80 mL) was added dropwise and
the organic phase separated. The aqueous layer was extracted
with CH2Cl2 (4 × 50 mL), and the combined organic phase
was dried (MgSO4) and concentrated by rotary evaporation to
afford a crude product (19.6 g, 89.4% yield) that was analyzed
by GLC and found to contain two major components in a 3.4:
5.5 ratio. A portion of the crude product (6.0 g) was subjected
to silica gel (150 g) flash column chromatography. The column
(18 cm long × 8 cm diameter) was eluted with CH2Cl2 (500
mL) and CH2Cl2:EtOAc (95:5, 300 mL). Fractions of about 10
1-Meth yl-4-p h en ylp yr a zole (2). Methylhydrazine (8.0 g,
173.6 mmol) was added dropwise to a mechanicallly stirred
solution of 3-chloro-2-phenylpropenal39 (8.4 g, 50.5 mmol) in
ether (25 mL) maintained under a N2 atmosphere at ∼0 °C.
The resulting mixture was heated at reflux for 3 h, cooled in
an ice bath, and treated by dropwise addition of aqueous
saturated NaHCO3 (80 mL). The aqueous phase was extracted
with CH2Cl2 (4 × 50 mL) and the combined organic phase dried
(MgSO4) and concentated by rotary evaporation. The resulting
yellow solid (8.8 g) was subjected to silica gel (300 g) flash
column chromatography. The column (36.0 × 5.5 cm) was
eluted with CH2Cl2, and 16 fractions (100-150 mL) were
collected. Fractions 9-15 afforded 1-methyl-4-phenylpyrazole
(2) as a white solid, 4.2 g (46.2% isolated yield). Sublimation
(110-112.5 °C, 0.2 Torr) gave 2 as a white crystalline solid:
mp 100-101 °C (lit.40 mp 101-103 °C); 1H NMR (CDCl3) δ
4.00 (s, 3H), 7.43-7.22 (m, 5H), 7.60 (s, 1H), 7.80 (s, 1H); 13
C
NMR (CDCl3) δ 39.1, 133.0, 125.5, 126.3, 126.9, 128.8, 136.7;38
IR (KBr) 3035, 1604, 1560, 1451, 1415, 1363, 1324, 1213, 1195,
1075, 991, 955, 905, 849, 812, 754 cm-1; UV λmax (CH3CN) (log
ꢀ) 254 (4.12); MS m/e 158 (100) 143 (1), 130 (20), 117 (15), 116
(20), 103 (7), 77 (7), 63 (19), 42 (24), 38 (12).
5-Deu ter io-1-m eth yl-4-p h en ylp yr a zole (2-5-d 1). To a
solution of 2 (0.32 g, 2.03 mmol) in freshly dried THF (10 mL)
at -77 °C was added n-BuLi (2.5 mL, 4.0 mmol), and the
mixture stirred for 30 min.41 The temperature of the solution
was brought to 0 °C, and D2O (0.1 mL, 5.0 mmol) was added.
After further stirring for 30 min, the solution was brought to
room temperature and stirred for 8 h. To the mixture was
added water (4.0 mL) and the organic layer separated. The
aqueous layer was extracted with CH2Cl2 (4 × 25 mL), and
the combined organic phase was washed with brine (5 mL)
followed by water (10 mL). The organic phase was dried
(MgSO4), and the solvent was removed by rotary evaporation
to yield 5-deuterio-1-methyl-4-phenylpyrazole (2-5-d1) as a
white solid: mp 90-92 °C; 0.27 g (1.70 mmol, 83.7% with
83.2% D-incorporation); 1H NMR (CDCl3) δ 4.00 (s, 3H), 7.43-
7.22 (m, 5H), 7.80 (s, 1H); MS m/e 159 (100), 158 (34), 131
(17), 118 (14), 117 (25), 104 (6), 103 (11), 77 (8), 43 (25), 39
(11).
1-Meth yl-5-p h en ylp yr a zole (3). Methylhydrazine (22.5
g, 489.1 mmol) was added dropwise to a mechanically stirred
solution of 3-chloro-3-phenylpropenal42 (23.0 g, 138.1 mmol)
in ether (125 mL) maintained under a N2 atmosphere at ∼0
°C. The resulting mixture was heated at reflux for 3 h, cooled
in an ice bath, and neutralized with saturated NaHCO3. The
resulting solution was extracted with CH2Cl2 (4 × 50 mL),
dried (MgSO4), and concentrated to afford a red liquid (23.0
g) that contained 3 and 1 in a 6.6:2.8 ratio. A portion of the
crude product (19.0 g) was subjected to steam distillation, and
the distillate (1 L) was extracted with ether (4 × 100 mL).
The etheral extract was dried (Na2SO4) and concentrated to
give an oily residue (10.2 g). A portion (2.0 g) of this was
subjected to silica gel (100 g) flash column chromatography.
The column (14.0 cm long × 5.5 cm diameter) was eluted with
(25) Bonafede, J . D.; Matuda, R. R. Tetrahedron 1972, 28, 2377.
(26) J ulian, P. L.; Oliver, J . J .; Kimball, R. H.; Pike, A. B.; J efferson,
G. D. Organic Syntheses; Wiley: New York, 1943; Collect. Vol. II, p
487.
(27) Yamamoto, T.; Muraoka, M. OPPI Briefs 1984, 16, 130.
(28) Kunthan, J .; J ehlicka, V.; Hakn, E. Collect. Czech. Chem.
Commun. 1967, 32, 4309.
(29) Novelli, A.; De Santis, A. Tetrahedron Lett. 1967, 265.
(30) Yamauchi, K.; Kinoshita, M. J . Chem. Soc., Perkin Trans. 1
1973, 2506.
(31) Butler, D. E.; Alexander, S. M. J . Org Chem. 1972, 37, 215.
(32) Klimko, V. T.; Skoldinov, A. P. Zh. Obshch. Khim. 1959, 29,
4027; Chem. Abstr. 1960, 54, 20870a.
(36) Auwers, K.v.; Breyhan, T. J . Prakt. Chem. 1935, 143, 259.
(37) Aubagnac, J . L.; Elguero, J .; J acquier, R. Bull. Chim. Soc. Fr.
1969, 3306.
(38) Cabildo, P.; Claramunt, R. M.; Elguero, J . Org. Magn. Reson.
1984, 22, 603.
(33) Bystrov, V. F.; Grandberg, I. I.; Sharova, G. I. Zh. Obshch.
Khim. 1965, 35, 293; Chem. Abstr. 1965, 62, 166226f.
(34) Pyman, F. L.; J . Chem. Soc. 1922, 121, 2616.
(35) Nesmeyanov, A. N.; Kochetkov, N. K.; Rybinskaya, M. I. Izv.
Akad. Nauk. SSSR, Otdel Khim. Nauk. 1950, 350; Chem. Abstr. 1951,
45, 1585c.
(39) Arnold, Z; Zemlika, J . Proc. Chem Soc. 1958, 227.
(40) Auwers, K. v.; Cauer, E. J . Prakt. Chem. 1930, 126, 192.
(41) Katritzky, A. R.; J ayaram, C.; Vassilatos, S. N. Tetrahedron
1983, 39, 2023.
(42) Arnold, Z; Zemlika, J . Collect. Czech. Chem. Commun. 1959,
24, 2385.

8332 J . Org. Chem., Vol. 62, No. 24, 1997
Pavlik and Kebede
dichloromethane:ethyl acetate (95:5), and fractions of 8-10 mL
were collected. Fractions 22-35 provided 1-methyl-5-phe-
nylpyrazole (3) as a colorless oil: bp 90-95 °C (1.5 Torr) (lit.43
bp 105 °C (1.0 Torr)) 557.0 mg (3.5 mmol, 27.9%); 1H NMR
(CDCl3) δ 4.00 (s, 3H), 6.33 (d, J ) 1.6 Hz, 1H), 7.45 (s, 5H),
7.53 (d, J ) 1.6 Hz, 1H); 13C NMR δ 37.3, 105.8, 128.3, 128.5,
128.6, 130.0, 139.0, 143.3;43 IR (KBr) 3100, 3059, 1606, 1576,
1542, 1484, 1472, 1440, 1275, 1228, 1179, 979, 783 cm-1; UV
(CH3CN) λmax (log ꢀ) 245 (4.03); MS m/e 158 (100), 130 (24),
115 (19), 110 (22), 91 (14), 77 (20), 63 (18), 51 (18), 50 (17).
4-Deu ter io-1-m eth yl-5-ph en ylpyr azole (3-4-d1). 1-Meth-
yl-5-phenylpyrazole (3) (0.832 g, 5.27 mmol) was dissolved in
70% D2SO4 in D2O (2.0 mL, 14.3 mmol) and kept at 70-74 °C
for 3.5 days while the H-D exchange was monitored using
1H NMR spectroscopy. After neutralization by dropwise
addition of saturated NaHCO3, the resulting mixture was
extracted with dichloromethane (4 × 50 mL). The organic
extract was dried (MgSO4) and concentrated to give a light
yellow oily product (0.824 g) that was distilled to give 4-deu-
terio-1-methyl-5-phenylpyrazole (3-4-d1) as colorless oil: bp 70
°C (0.40 Torr); 0.735 g (4.74 mmol, 88.4% yield); 1H NMR
(CDCl3) δ 4.00 (s, 3H), 7.50 (m, 5H), 7.53 (s, 1H); MS m/e 159
(100), 131 (20), 130 (9), 116 (15), 104 (17), 103 (3), 91 (15), 77
(22), 51 (19).
1-Meth yl-2-p h en ylim id a zole (8). Dimethyl sulfate (2.7
g, 21.4 mmol) was added dropwise to 2-phenyl-1H-imidazole
(2.0 g, 13.9 mmol). A vigorous reaction set in after 10 min at
room temperature. The resulting mixture was heated over a
steam bath for 30 min, and the residue was made alkaline
with concentrated ammonia (5 mL) and extracted with dichlo-
romethane (3 × 25 mL). The organic extract was dried
(MgSO4) and concentrated to give a brown oily residue (1.7
g). Distillation gave a colorless oil: bp (Kugelrohr oven temp)
70-100 °C (0.2 Torr) (lit.44 bp 175 °C (15 Torr)). The distillate
(1.5 g) was subjected to flash column chromatography over
silica gel (110 g) that was washed with hexane:triethylamine
(5:1) prior to sample application. The column (15 cm long ×
5.0 cm diameter) was eluted with dichloromethane, and 40
fractions of 10 mL were collected. Concentration of fractions
15-24 afforded 8 as a light yellow oil: 0.41 g (2.59 mmol,
18.6% yield); 1H NMR (CDCl3) δ 3.56, (s, 3H), 6.90 (s, 1H),
7.10 (s, 1H), 7.20-7.80 (m, 5H); UV (MeOH) λmax (log ꢀ) 257
(4.08); MS m/e 158 (75), 157 (100), 130 (8), 116 (12), 104 (12),
103 (10), 90 (6) 89 (14), 77 (15), 54.1 (17), 51 (15). Concentra-
tion of fractions 28-40 gave unreacted 2-phenyl-1H-imida-
zole: 1.01 g (50.5%).
1-Meth yl-4- a n d 1-Meth yl-5-p h en ylim id a zole (4 a n d 7).
Dimethyl sulfate (1.9 g, 14.8 mmol) was added to 4-phenyl-
1H-imidazole29 (2.0 g, 13.9 mmol) at room temperature. After
the solid dissolved, the light brown liquid was heated over a
steam bath for 30 min and left overnight at room temperature.
The mixture (3.0 g) was dissolved in H2O (5.0 mL) and made
alkaline by the addition of aqueous saturated NaHCO3. This
was extracted with dichloromethane (3 × 25 mL), and the
combined organic phase was dried (MgSO4) and concentrated.
The residual oil (2.1 g) was subjected to silica gel (100 g washed
with hexane:triethylamine, 5:1) flash column chromatography.
The column (14.0 cm long × 5.5 cm diameter) was eluted with
dichloromethane (350 mL), and 47 fractions (6-10 mL) were
collected. Fractions 35-39 were combined and concentrated
to give 1-methyl-4-phenylimidazole (4) (25.4 mg), while frac-
tions 40-43 were combined and concentrated to give a mixture
of 1-methyl-5-phenylimidazole (7) and 1-methyl-4-phenylimi-
dazole (4) (320.0 mg). This mixture was subjected to prepara-
tive layer chromatography (silica gel washed with hexane:
triethylamine, 5:1) and eluted with dichloromethane:methanol
(9:1) to give three bands. Band 1 (lowest Rf) gave unreacted
4-phenyl-1H-imidazole; band 2 (intermediate Rf) gave 1-meth-
yl-5-phenylimidazole (7) as a white solid (32.0 mg); band 3
(highest Rf) contained 1-methyl-4-phenylimidazole (4) (101.0
mg), which was recrystallized from ether to give a pale yellow
crystalline solid, 58.1 mg (0.37 mmol 2.5% yield). The total
yield of 4 was 73.5 mg (0.46 mmol, 3.1% yield) obtained as a
light yellow solid: mp 108-110 °C (lit.45 mp 110-111 °C); 1H
NMR (CDCl3) δ 3.60 (s, 3H), 7.10 (s, 1H) 7.2-7.5 (m, 4H), 7.7
(s, 1H), 7.8 (s, 1H); UV (MeOH) λmax (log ꢀ) 256 (4.08); MS m/e
158 (100), 157 (12), 131 (6), 130 (12), 117 (16), 116 (38), 89
(28), 77 (7), 63 (18), 51 (9). 1-Methyl-5-phenylimidazole (7)
was obtained as a white solid: mp 94.5-96.0 °C (lit.45 mp 96-
1
97 °C); H NMR (CDCl3) δ 3.62 (s, H), 7.39 (m, 5H), 7.50 (s,
1H); UV (MeOH) λmax (log ꢀ) 251 (4.11); MS m/e 158 (100), 157
(46), 131 (5), 130 (25), 115 (15), 104 (6), 89 (15), 77 (21), 63
(18), 51 (18).
Ir r a d ia tion a n d An a lysis P r oced u r es. To monitor pho-
toreactions on an analytical scale, 3.0 mL of a solution of the
appropriate reactant in methanol or acetonitrile was placed
in a quartz tube (7 mm i.d. × 15 cm long), sealed with a rubber
septum, and purged with nitrogen for 2-5 min prior to
irradiation. In the case of phenyl-substituted pyrazoles the
solutions were irradiated at 254 nm in a Rayonet photochemi-
cal reactor with 16 low-pressure Hg lamps. Pyrazole without
a phenyl substituent was irradiated using a 450-W high-
pressure Hg lamp in a water-cooled quartz immersion well.
Preparative-scale reactions were carried out by irradiating 50
mL of a nitrogen-purged solution of the appropriate reactant
in a quartz tube (2.2 cm i.d. × 20 cm long).
The formation of phototransposition products was monitored
by GLC analysis at 180 °C without concentrating the solutions.
The retentions of the products at 180 °C are given relative to
the appropriate starting compound. Quantitative GLC analy-
sis of reactant consumption and product formation was ac-
complished using calibration curves constructed for 4, 7, and
8 by plotting the detector response vs 10 standards of known
concentration. Correlation coefficients ranged from 0.989 to
1.000. The formation of photocleavage products were moni-
tored by UV absorption spectroscopy of the diluted solution
(1:500) and 1H NMR spectroscopy of the residue obtained by
concentrating the irradiated solution to dryness.
1-Meth yl-3-p h en ylp yr a zole (1). A solution of 1 (3.0 mL,
2.0 × 10-2 M) in methanol was irradiated for total of 42 h.
After 11.25 h and 42 h of irradiation GLC analysis showed
that 21% and 46% of 1 had been consumed but that no volatile
products were formed.
1-Meth yl-4-p h en ylp yr a zole (2). A solution of 2 (50 mL,
2.0 × 10-2 M) in methanol was irradiated for 1 h. The
resulting solution was concentrated, and the residue (0.156 g,
98.7% recovery) was subjected to silica gel (60 g) flash
chromatography. The column (25 cm long × 2.2 cm diameter)
was eluted with dichloromethane (200 mL) followed by dichlo-
romethane:ethyl acetate (9:1, 300 mL), and 10 mL fractions
were collected. Fraction 7 was concentrated to give (Z)-2-(N-
methylamino)-1-phenylethenyl isocyanide (6) as
a white
solid: mp 97.5-98.0 °C; 0.018 g (1.14 × 10-4 mol, 17.8% yield);
1H NMR (CDCl3) δ 3.1 (d, J ) 4.5 Hz, 3H), 4.2 (br s, 1H), 6.8
(d, J ) 12.6 Hz, 1H), 7.1-7.3 (m, 5H); 13C NMR δ (CDCl3) 34.4,
121.4, 125.3, 128.9, 132.0, 137.2; IR (NaCl window) 3319, 2105
(-NtC), 1650, 748, 638 cm-1; UV λmax (CH3OH) nm (log ꢀ)
225 (3.98), 263 (3.86), 298 (4.15), 313 (4.14); MS m/e 158.2
(100), 157.2 (11.2), 131.1 (7.9), 130.1 (10.2), 117.2 (18.1), 116.0
(35.4), 103.0 (6.4), 89.1 (27.1), 63.1 (15.3), 51.1 (10.0). Anal.
Calcd for C10H10N2: C, 75.75; H, 6.38; N, 17.71. Found: C:
75.76, H: 6.26, N: 17.73.
Fractions 10-18 were concentrated to give a mixture of (E)-
and (Z)-3-(N-methylamino)-2-phenylpropenenitrile (5),26,27 0.009
g (5.70 × 10-5 mol, 8.9% yield).
Fractions 26-33 were concentrated to give unreacted 1-meth-
yl-4-phenylpyrazole (2), 0.057 g (3.6 × 10-4 mol, 36.5%
recovery). Fraction 34 was concentrated to give 1-methyl-4-
phenylimidazole (4),45 0.011 g (6.96 × 10-5 mol, 10.9% yield).
5-Deu ter io-1-m eth yl-4-p h en ylp yr a zole (2-5-d 1). A solu-
tion of 2-5-d1 (20.0 mL, 2.0 × 10-2 M) in methanol was placed
in a quartz tube (2.2 cm i.d. × 12 cm long), sealed with a
septum, and purged continuously with nitrogen while being
irradiated for 3.0 h. The resulting solution was concentrated
to dryness, and the residue (0.053 g, 85% recovery) was
subjected to preparative layer chromatography (silica gel,
(43) Springer, J . P.; Arison, B. H. Can. J . Chem. 1986, 64, 2211.
(44) Balban, I. E.; King, H. J . Chem. Soc. 1925, 127, 2701.
(45) Hazeldine, C. E.; Pyman, F. L.; Winchester, J . J . Chem. Soc.
1924, 125, 1431.

Photochemistry of Phenyl-Substituted 1-Methylpyrazoles
J . Org. Chem., Vol. 62, No. 24, 1997 8333
acetonitrile). The band at Rf ) 0.12 gave 5-deuterio-1-methyl-
4-phenylimidazole (4-5-d1) as a white solid: mp 107-110 °C;
0.020 g (1.26 × 10-4 mol, 32%); 1H NMR (CDCl3) δ 3.60 (s,
3H), 7.2-7.5 (m, 4H), 7.75 (s, 1H), 7.81 (s, 1H); MS m/e 159
(100), 158 (24), 131 (14), 118 (16), 117 (42), 104 (6), 90 (18), 89
(18), 63 (19), 51 (11).
(Z)-2-(N-Meth ylam in o)-1-ph en yleth en yl Isocyan ide (6).
A solution of 6 (3.0 mL, 1.44 × 10-2 M in CH3OH) was
irradiated for 5 min. Analysis by UV absorption spectroscopy
before and after irradiation after 1:500 dilution showed the
optical density at 300 nm decreased from 0.431 to 0.382,
showing that 24% of 6 was consumed. The resulting solution
was concentrated to dryness, and the residue (0.005 g, 3.16 ×
10-5 mol, 73.5% recovery) was analyzed by 1H NMR. The
spectrum showed signals for the N-methyl group of the E and
Z isomers of 6 at δ 3.02 (d, J ) 4.5 Hz) and 2.85 (d, J ) 4.5
Hz), respectively, and a singlet at δ 3.68 due to the N-methyl
group of 4.
(Z)/(E)-3-(N-Meth yla m in o)-2-p h en ylp r op en en itr ile (5).
A solution of 5 (97% Z, 3.0 mL, 2.0 × 10-2 M) in methanol
was irradiated for 20 min. The resulting solution was con-
centrated to dryness, and the residue (0.007 g, 74% recovery)
was analyzed by 1H NMR. The spectrum (CDCl3) showed a
singlet of very low intensity at δ 3.60 due to the N-methyl
group of 4 and two doublets at δ 3.11 and 2.97 (J ) 5.1 Hz)
for the N-methyl groups of the Z and E isomers of 5,
respectively. Integration indicated that the Z:E ratio has
changed to 58.1:41.9 (39% consumption of (Z)-5.
recovery). The band at Rf ) 0.75 gave 0.105 g of a pale yellow
oil that was subjected to a second preparative layer chroma-
tography (silica gel, acetonitrile). The band at Rf ) 0.75 gave
a mixture of adducts 38 and 39 as an almost colorless oil:
0.083 g (3.7 × 10-4 mol, 50.0% yield); 1H NMR (CDCl3) major
adduct δ 2.48, (s, 3H), 4.01 (m, 1H), 4.13 (m, 1H), 4.94 (dd,
1H, J ) 3.30, 6.75 Hz), 5.01 (dd, 1H, J ) 2.80, 2.80 Hz), 6.85
(dd, 1H, J ) 1.57, 2.82 Hz), minor adduct δ 2.78, (s, 3H), 4.13
(m, 1H), 4.26 (m, 1H), 5.17 (dd, 1H, J ) 8.67, 6.46 Hz), 5.22
(dd, 1H, J ) 2.94, 2.94 Hz), 6.45 (dd, 1H, J ) 1.74, 2.77 Hz),
6.57 (d, 1H, J ) 2.19). In addition, both adducts exhibited
overlapping multiplites for the phenyl protons from δ 7.18-
7.42: 13C NMR (CDCl3) major adduct δ 37.5, 56.5, 62.1, 80.0,
103.5, 137.4, 147.4; minor adduct δ 36.7, 55.3, 63.0, 79.4, 102.3,
135.7, 149.6. In addition, both adducts exhibited overlapping
signals for the phenyl and benzyl carbons from δ 126.7-
128.6: IR (neat) 3058, 3027, 2954, 2924, 2867, 2792, 1724,
1604, 1464, 1493, 1447, 1276, 1308, 1138, 1049, 940, 862, 755,
700, 641 cm-1
.
Th er m olysis of Ad d u cts 38 a n d 39. Adducts 38 and 39
(0.015 g, 6.63 × 10-5 mol) were dissolved in DMSO-d6. 1H
NMR analysis indicated only 38 and 39 with no 1-methyl-5-
phenylpyrazole (3) present in the solution. The resulting
solution was heated at 100 °C for 25 min. 1H NMR analysis
showed the decomposition of 38 and 39 and the formation of
3. Similarly, GLC analysis of 38 and 39 at 180 °C showed
only the presence of 3.
1-Meth ylp yr a zole (11). Three solutions of 11 (3.0 mL, 2.4
× 10-2 M) in acetonitrile were each irradiated for 5, 10, or 15
min. The resulting solutions were concentrated to dryness,
and each residue was analyzed by 1H NMR spectroscopy. The
spectra (CDCl3) showed a decrease in the signals at δ 3.70 (s,
3H), 6.22 (dd, J ) 2.24, 1.49 Hz, 1H), 7.34 (d, J ) 2.24 Hz,
1H),7.48 (d, J ) 1.49 Hz, 1H) due to the consumption of
1-methylpyrazole (11), a continuous increase in the signals at
δ 3.68 (s, 3H), 6.84 (s, 1H), 7.00 (s, 1H), 7.41 (s, 1H); at δ 2.59
(d, J ) 5.05 Hz, 3H), 3.72 (d, 13.67 Hz, 1H), 6.52 (dd, J )
12.88, 8.89 Hz, 1H); and at δ 2.71 (d, J ) 5.12 Hz, 3H) 3.91 (d,
J ) 8.57 Hz, 1H), 7.05 (dd, J ) 13.67, 8.57 Hz, 1H) due to the
continuous formation of 1-methylimidazole (12), (E)-3-(N-
methylamino)propenenitrile (13E), and (Z)-3-(N-methylami-
no)propenenitrile (13Z), respectively, and an increase and
subsequent decrease in the signals at δ 2.89 (d, J ) 4.96 Hz,
3H) and δ 2.97 (d, J ) 4.92 Hz, 3H) due to the N-methyl group
of (E)-2-(N-methylamino)ethenyl isocyanide (14E) and (Z)-2-
(N-methylamino)ethenyl isocyanide (14Z), respectively.
1,4-Dim eth ylp yr a zole (15). Five solutions of 15 (3.0 mL,
2.1 × 10-2 M) in acetonitrile were each irradiated for 5, 10,
15, 30, or 60 min. The resulting solutions were concentrated
1-Meth yl-5-p h en ylp yr a zole (3). A solution of 3 (3.0 mL,
2.0 × 10-2 M) in methanol was irradiated for 2.0 h. GLC
analysis of the irradiated solution showed 76% consumption
of 3 (retention time of 2.70 min) and the formation of 1-methyl-
2-phenylimidazole (8) (30%), 1-methyl-5-phenylimidazole (7)
(31%), and 1-methyl-4-phenylimidazole (4) (29%) with relative
retentions of 2.13, 4.04, and 2.53, respectively.
Three solutions of 3 (3.0 mL, 2.0 × 10-2 M) in methanol
were each irradiated for 8, 15, or 20 min. In each case the
resulting solution was concentrated to dryness, and the residue
was analyzed by 1H NMR spectroscopy. In addition to signals
due to imidazoles 4, 7, and 8, the 1H NMR spectra showed
signals at δ 2.73 (d, J ) 5.32 Hz), 2.78 (d, J ) 4.90 Hz), 2.84
(d, J ) 5.30 Hz), and 3.28 (d, J ) 3.50 Hz) due to the N-methyl
groups of (E)-3-(N-methylamino)-3-phenylpropenenitrile (9E),
(E)-2-(N-methylamino)-2-phenylethenyl isocyanide (10E), (Z)-
3-(N-methylamino)-3-phenylpropenenitrile (9Z), and (Z)-2-(N-
Methylamino)-2-phenylethenyl isocyanide (10Z).
4-Deu ter io-1-m eth yl-5-p h en ylp yr a zole (3-4-d 1). A solu-
tion of 3-4-d1 (50.0 mL, 2.1 × 10-2 M) in methanol was
irradiated for 1.5 h. The resulting solution was concentrated
to dryness to yield the crude product (0.163 g, 99% recovery).
A portion (0.092 g) of this residue was subjected to preparative
layer chromatography (silica gel, acetonitrile). The bands at
Rf ) 0.40 gave 5-deuterio-1-methyl-4-phenylimidazole (4-5-d1)
(0.026 g, 1.64 × 10-4 mol, 28% yield): 1H NMR (CDCl3) δ 3.60
(s, 3H), 7.2-7.5 (m, 4H), 7.7 (s, 1H), 7.8 (s, 1H); MS m/e 159
(100), 158 (19), 131 (12), 130 (4), 118 (18), 117 (40), 104 (5), 90
(28), 89 (15), 77 (8), 63 (16), 51 (9).
1
to dryness, and each residue was analyzed by H NMR. The
spectra (CDCl3) showed a decrease in the signals at δ 2.03 (s,
3H) and 3.83 (s, 3H) due to the consumption of 1,4-dimeth-
ylpyrazole (15), a continuous increase in the signals at δ 2.17
(s, 3H), 3.60 (s, 3H), at δ 2.80 (d, J ) 5.39 Hz), and at δ 2.92
(d, J ) 5.06 Hz) due to the continuous formation of 1,4-
dimethylimidazole (17), (E)-2-methyl-3-(N-methylamino)pro-
penenitrile (20E), and (Z)-2-methyl-3-(N-methylamino)pro-
penenitrile (20Z), respectively, and an increase and subsequent
decrease in signals at δ 2.78 (d, J ) 5.28 Hz) and at δ 2.88 (d,
J ) 5.39 Hz) due to the formation and subsequent consumption
of (E)-1-methyl-2-(N-methylamino)ethenyl isocyanide (21E)
and (Z)1-methyl-2-(N-methylamino)ethenyl isocyanide (21Z).
1,5-Dim eth ylp yr a zole (16). Four solutions of 16 (3.0 mL,
2.1 × 10-2 M) in acetonitrile were irradiated for 10, 15, 30,
and 60 min, respectively. The resulting solutions were con-
centrated to dryness, and each residue was analyzed by 1H
NMR. The spectra (CDCl3) showed a decrease in the signals
at δ 2.24 (s, 3H) and 3.76 (s, 3H) due to the consumption of
1,5-dimethylpyrazole (16), a continuous increase in the signals
at δ 2.35 (s, 3H), 3.54 (s, 3H), at δ 2.10 (s, 3H), 3.60 (s, 3H), at
δ 2.17 (s, 3H), 3.50 (s, 3H), at δ 2.68 (d, J ) 5.76 Hz, 3H), and
at δ 2.94 (d, J ) 5.26 Hz, 3H) due to the continuous formation
of 1,2-dimethylimidazole (19), 1,4-dimethylimidazole (17), 1,5-
dimethylimidazole (18), (E)-3-methyl-3-(N-methylamino)prop-
enenitrile(22E) and (Z)-3-methyl-3-(N-methylamino)propene-
nitrile(22Z), and an increase and subsequent decrease in the
The band at Rf ) 0.22 gave 5-deuterio-1-methyl-2-phe-
1
nylimidazole (8-5-d1) 0.013 g (8.2 × 10-5 mol, 14% yield); H
NMR (CDCl3) δ 3.65 (s, 3H), 7.10 (s, 1H), 7.2-7.8 (m, 5H);
MS m/e 159 (84), 158 (100), 131 (6), 116 (10), 104 (13), 103 (8),
90 (7), 89 (8), 77 (14), 55 (16), 51 (15). The band at Rf ) 0.07
gave 4-deuterio-1-methyl-5-phenylimidazole (7-4-d1): 0.009 g
(5.7 × 10-5 mol, 10% yield); 1H NMR (CDCl3 + a drop of (TFA))
δ 3.90 (s, 3H), 7.40 (s, 2H), 7.60 (s, 3H), 8.82 (s, 1H) MS m/e
159 (100), 158 (62), 131 (20), 118 (22), 117 (18), 116 (14), 104
(18), 103 (26), 91 (17), 90 (23), 89 (23), 77 (30), 63 (21), 56 (20),
55 (14), 51 (24). The band at Rf ) 0.60 gave unreacted
4-deuterio-1-methyl-5-phenylpyrazole (3-4-d1) 0.026 g (1.64 ×
10-4 mol, 28% recovery).
1-Meth yl-5-p h en ylp yr a zole (3) in F u r a n . A solution of
3 (50 mL, 2.0 × 10-2 M) in furan was irradiated for 1.0 h. The
resulting solution was concentrated to dryness, and the residue
(0.186 g) was subjected to preparative layer chromatography
(silica gel, acetonitrile). The band at Rf ) 0.70 gave unreacted
1-methyl-5-phenylpyrazole (3), 0.041 g (2.6 × 10-4 mol, 26%

8334 J . Org. Chem., Vol. 62, No. 24, 1997
Pavlik and Kebede
signals at δ 2.57 (d, J ) 4.20 Hz, 3H) and δ 2.85 (d, J ) 5.32
Hz, 3H) due to the formation and subsequent consumption of
(E)-2-methyl-2-(N-methylamino)-ethenyl isocyanide (23E) and
(Z)-2-methyl-2-(N-methylamino)ethenyl isocyanide (23Z).
4-Meth yl-1-p h en ylp yr a zole (24). Five solutions of 24 (3.0
mL, 2.1 × 10-2 M) in methanol were irradiated for 5, 10, 15,
30, and 60 min, respectively. The resulting solutions were
concentrated to dryness, and each residue was analyzed by
was used as the actinometer for 1-methyl-4-phenylpyrazole (2)
and 1-methyl-5-phenylpyrazole (3). Cyclohepta-1,3-diene11
was used as the actinometer for 2-(N-methylamino)-1-phe-
nylethenylisocyanide (6). The quantum yield of reaction for
the reaction of 3-(N-methylamino)-2-phenylpropenenitrile (5)
was measured relative to the reaction of (2).
Sen sitiza tion . Solutions of 1-methyl-4-phenylpyrazole (2)
and 1-methyl-5-phenylpyrazole (3) in methanol (3.0 mL, 1.0
× 10-2 M) and in acetone (3.0 mL, 1.2 × 10-2 M) were placed
in Pyrex tubes, sealed with rubber septa, purged with nitrogen
for 10 min, and irradiated in a Rayonet reactor equipped with
16 300 nm lamps. The solutions were monitored as a function
of irradiation time by GLC.
1
1H NMR and IR spectroscopy. The H NMR spectra (CDCl3)
showed a gradual decrease in the signal at δ 2.16 (s, 3H) due
to the consumption of 4-methyl-1-phenylpyrazole (24), a
continuous increase in the signals at δ 2.30 (s, 3H), at δ 1.59
(d, J ) 1.26 Hz, 3H), and at δ 1.66 (d, J ) 1.40 Hz, 3H) due to
the continuous formation of 4-methyl-1-phenylimidazole (25),
(E)-2-methyl-3-(N-phenylamino)propenenitrile (26E), and (Z)-
2-methyl-3-(N-phenylamino)propenenitrile (26Z), and an in-
crease and subsequent decrease in the signals at δ 1.82 (d, J
) 1.40 Hz, 3H) and d 1.87 (d, J ) 1.33 Hz, 3H) due to the
formation of (E)-1-methyl-2-(N-phenylamino)ethenylisocyanide
(27E) and (Z)-1-methyl-2-(N-phenylamino)ethenylisocyanide
(27Z). The IR spectra showed a continuous increase in an
absorption signal at 2195 cm-1 (-CtN) and an increase and
subsequent decrease in an absorption signal at 2103 cm-1
(-N+tC:-).
Lu m in escen ce Sp ectr a , En er gies, a n d Lifetim e of
Excited Sta tes. Fluorescence and phosphorescence spectra
were recorded at room temperature and at 77 K, respectively,
in methanol:ethanol (1:1) using solutions with optical densities
less than 0.25 at 250 nm. Excitation and emission wave-
lengths were set at 250 and 310 nm for fluorescence and at
250 and 435 nm for phosphorescence. Quantum yields of
fluorescence were determined relative to p-xylene.46 Decay
constants and lifetimes for triplet states were obtained from
plots of natural logarithm of the phosphorescence decay versus
time.
Meth od of Ca lcu la tion . All AM147 calculations were
performed using the MOPAC48 program with standard pa-
rameters. In general, structures obtained from PC-MODEL49
were the starting points for AM1 optimizations using standard
minimization techniques and internal coordinates.
1,3-Dim eth ylp yr a zole (28). Three solutions of 28 (3.0 mL,
2.3 × 10-2 M) in acetonitrile were irradiated for 5, 10, and 15
min, respectively. The resulting solutions were concentrated
1
to dryness, and each residue was analyzed by H NMR. The
spectra (CDCl3) showed a decrease in signals at δ 2.26 (s, 3H)
and 3.80 (s, 3H) due to the consumption of 1,3-dimethylpyra-
zole (28) and a continuous increase in the signals at δ 2.20 (s,
3H), 3.54 (s, 3H) and at δ 2.17 (s, 3H), 3.60 (s, 3H) due to the
formation of 1,2-dimethylimidazole (19) and 1,4-dimethylimi-
dazole (17), respectively.
Qu a n tu m Yield of Rea ction s. Quantum yields of reac-
tions were determined in triplicate by simultaneously irradiat-
ing solutions (3.0 mL each) of the reactants and the actinom-
eter in Rayonet a reactor. The consumption of the reactant
and the formation of products were monitored using GLC, UV
absorption, and 1H NMR spectroscopy. 1,3-Dimethyluracil11
J O970897R
(46) Shold, D. M. Chem. Phys. Lett. 1977, 49, 243.
(47) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P.
J . Am. Chem. Soc. 1985, 107, 3902.
(48) Available from the Quantum Chemistry Program Exchange:
MOPAC, version 5, QCPE 455.
(49) PCMODEL, force field claculation based on the MM2 method
(N. L. Allinger), Serena Software, Box 3076, Bloomington, IN 47402-
3076.