Steric effects in photoinduced electron transfer reaction of halogenated 1,4-benzoquinones with donor olefins

Article abstract of DOI:10.1021/ol006080t

(Equation presented) The photoinduced electron transfer reaction of halogenated 1,4-benzoquinones with 2,3-dimethyl-2-butene or 3,4-dimethyl-2-pentene gave the primary, secondary, and tertiary monoallyl ethers of hydroquinones, with the product distributi

Full text of DOI:10.1021/ol006080t

ORGANIC
LETTERS
2000
Vol. 2, No. 13
1979-1981
Steric Effects in Photoinduced Electron
Transfer Reaction of Halogenated
1,4-Benzoquinones with Donor Olefins
Ken Kokubo, Takayoshi Masaki, and Takumi Oshima*
Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity,
Toyonaka, Osaka 560-0043, Japan
oshima@ch.wani.osaka-u.ac.jp
Received May 18, 2000
ABSTRACT
The photoinduced electron transfer reaction of halogenated 1,4-benzoquinones with 2,3-dimethyl-2-butene or 3,4-dimethyl-2-pentene gave the
primary, secondary, and tertiary monoallyl ethers of hydroquinones, with the product distributions highly dependent on the steric nature of
quinones and olefins.
The photoinduced electron transfer (PET) reaction has
attracted much attention from a synthetic and theoretical
viewpoint.1 Irradiation of quinones with olefins is commonly
utilized for the synthesis of [2 + 2] adducts, cyclobutanes
and oxetanes.2 Such a reaction of halogenated quinones with
electron-rich olefins is expected to proceed through a PET
reaction in view of the redox potentials of these components.
Recently, Kochi et al. proposed3 that the photoreaction of
chloranil with stilbene in benzene proceeds via an electron-
transfer mechanism to give the corresponding oxetanes.
However, Xu et al. reported that the irradiation of chloranil
with cyclohexene affords a hydroquinone monoallyl ether
instead of oxetane.4 Another aliphatic donor olefin, i.e., 2,3-
dimethyl-2-butene, is also known to undergo PET with
various acceptors, such as dicyanobenzene,5 benzoyl cya-
nide,6 1,2-diketone,7 and ketoester,8 to give the corresponding
mixture of primary (1°) and tertiary (3°) radical coupling
products with acceptors in each case. In the present study,
we found that photoreaction of unsubstituted 1a and halo-
genated 1,4-benzoquinones 1b-d with 2,3-dimethyl-2-butene
2a or cis- and trans-3,4-dimethyl-2-pentene 2b gave oxetane
5a (only for 1a), with the mixture of 1°, 2°, and 3° monoallyl
ethers of hydroquinones markedly dependent on the steric
nature of the quinones as well as the olefins.
The photoreaction of 1a-d (100 mM for 1a and 1b, 50
mM for 1c and 1d) with a 5 equiv excess of 2a was carried
(5) Borg, R. M.; Arnold, D. R.; Cameron, T. S. Can. J. Chem. 1984, 62,
1785.
(6) Cantrell, T. S. J. Chem. Soc., Chem. Commun. 1975, 637.
(7) Turro, N. J.; Shima, K.; Chung, C.-J.; Tanielian, C.; Kanfer, S.
Tetrahedron Lett. 1980, 21, 2775.
(8) Shima, K.; Sawada, T.; Yoshinaga, H. Bull. Chem. Soc. Jpn. 1978,
51, 608.
(9) Filtration (>390 nm) causes the selective excitation of quinones and
possible CT complexes with 2a. For example, the mixture of 1c (10 mM)
and a 100 equiv excess of 2a in acetonitrile showed a definite CT absorption
at around λ ) 450 nm by digital subtraction of 1c.
(10) Gilbert et al. reported that a similar reaction gave only spiro-
oxetane: Bryce-Smith, D.; Evans, E. H.; Gilbert, A.; McNeill, H. S. J.
Chem. Soc., Perkin Trans. 2 1991, 1587.
(1) (a) Kavarnos, G. J., Ed. Fundamentals of Photoinduced Electron
Transfer; Wiley: New York, 1993. (b) Fox, M. A.; Chanon, M., Eds.
Photoinduced Electron Transfer; Elsevier: Amsterdam, 1988.
(2) Maruyama, K.; Osuka, A. In The chemistry of the quinonoid
compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1988; Volume
2, Chapter 13, p 759.
(3) (a) Sun, D.; Hubig, S. M.; Kochi, J. K. J. Org. Chem. 1999, 64,
2250. (b) Hubig, S. M.; Sun, D.; Kochi, J. K. J. Chem. Soc., Perkin Trans.
2 1999, 781.
(4) Xu, J.-H.; Song, Y.-L.; Zhang, Z.-G.; Wang, L.-C.; Xu, J.-W.
Tetrahedron 1994, 50, 1199.
10.1021/ol006080t CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/07/2000

out at 20 °C under an argon atmosphere in benzene-d6 by
irradiation with a high-pressure mercury lamp through a
>390 nm filter9 (Scheme 1). Unsubstituted 1a provided the
methane.12 It is also noted that the reaction of fluoranil 1b
exclusively gave the 3° adduct 3b (entry 2), while the
reaction of bromanil 1d gave a mixture of 3d and 4d,
increasing the 4/3 ratio compared to that with 1c (entry 4).
Thus, the product distributions were very dependent on the
identity of 1,4-benzoquinones. The reaction of 1a with 2a
was also examined in acetonitrile to give a comparable
amount of 5a (24%) and 3a (54%) for a 1 h reaction.13
Mechanistically, the present photoreaction of 1 can be
envisaged to proceed via PET because a free energy change
of an electron transfer ∆GET deduced from the Rehm-Weller
equation14 is very negative for 1c (-18.7 kcal) with 2a,
although the relevant value for 1a is slightly positive (+2.5
kcal).15 The radical ion pairs thus generated by the PET
would lead to the phenoxy and allyl radical with (1°, 3°)
termini via a proton transfer (PT)and finally collapse to the
hydroquinone monoallyl ethers 3 and 4 by way of 3° and 1°
attack, respectively (Scheme 2).
Scheme 1
Scheme 2
3° adduct, hydroquinone monoallyl ether 3a (53% by NMR),
along with spirooxetane 5a (30%) upon 1 h irradiation (Table
1, entry 1).10 However, the reaction of chloranil 1c did not
Table 1. Photoreaction of 1 with 2a in Benzene-d6
yield/%a
entry
1
time/h
3
4
5
1
2
3
4
1a
1b
1c
1d
1
2
1
1
53 (54)b
89
30 (24)b
Considering the increasing atomic radii of the quinone
substituent, H (0.3 Å) < F (0.6) < Cl (0.99) < Br (1.14), as
well as the calculated spin density of the allyl radical,16 the
smaller H or F substituted phenoxy radical would tend to
exclusively attack the 3° carbon atom of high spin density
and provide the corresponding monoallyl ether 3. However,
the larger Cl or Br substituted radical would exhibit the
duality in radical attack associated with a critical balance of
the steric repulsion and the spin density. Therefore, the
bulkiest compound, 1d, provided a significant amount of 4
due to the enhanced competitive attack on the less hindered
1° carbon atom possessing a low spin density. In regard to
the formation of oxetane 5a, it is not clear whether the PET
mechanism3 can explain the [ 2+ 2] addition or not because
of the subtle value of ∆GET.
71
54
24
40
a Determined by 1H NMR. b Values in parentheses are obtained in
acetonitrile-d3.
afford the corresponding oxetane but a mixture of 3° and 1°
monoallyl ethers 3c and 4c in a ratio of 3:1 (entry 3).
Compounds 3 and 4 were isolated by using HPLC, and the
structures were determined by H and 13C NMR.11 Since
compound 3c gradually decomposed, the structure was also
confirmed as the O-methylated derivative 3c′ with diazo-
1
(11) 3c: oil; 1H NMR (C6D6) δ 1.27 (s, 6H), 2.02 (s, 3H), 4.72 (s, 1H),
4.78 (s, 1H). 4c: colorless needles; mp 153.5-154.0 °C; 1H NMR (CDCl3)
δ 1.77 (s, 3H), 1.87 (s, 3H), 1.97 (s, 3H), 4.47 (s, 2H), 6.03 (s, -OH); 13C
NMR δ 17.3, 20.7, 21.9, 74.6, 118.8, 123.4, 127.7, 133.1, 145.2, 146.0;
MS (EI) m/z 328 (M+, Cl ) 35). Anal. Calcd for C12H12O2Cl4: C, 43.67;
H, 3.66. Found: C, 43.44; H, 3.65.
Scheme 3
(12) 3c′: oil; 1H NMR (CDCl3) δ 1.57 (s, 6H), 2.05 (s, 3H), 3.89 (s,
3H), 4.82 (s, 1H), 4.90 (s, 1H); 13C NMR δ 19.2, 26.8, 60.8, 88.3, 109.2,
127.1, 129.6, 148.2, 149.6, 150.0; HRMS (CI) m/z ((M+H)+) calcd for
C13H15O2Cl4 344.9797, found 344.9794.
(13) However, similar reaction of 1c and 1d failed because of the very
poor solubility in acetonitrile.
(14) See the calculation for ∆GET for benzene solution: Xue, J.; Xu,
J.-W.; Yang, L.; Xu, J.-H. J. Org. Chem. 2000, 65, 30.
(15) Ered of 1a (-0.50 V vs SCE) 1c (0.02) and Eox of 2a (1.53) were
T
obtained from ref 1b. E00 of 1a and 1c are 2.3 and 2.7 eV, respectively.
(16) Calculations using the PM3 method were performed with the
MOPAC program using CS MOPAC Pro software (ver 4.0).
1980
Org. Lett., Vol. 2, No. 13, 2000

To assess this mechanism and the resulting steric effects,
we employed cis- or trans-3,4-dimethyl-2-pentene 2b in
place of 2a in a photoreaction similar to that of 1b-d
(Scheme 3). This olefin is interesting in that the possible
abstraction of three different protons, Ha, Hb, and Hc, of
[2b]•+ would give rise to the corresponding three radicals,
A, B, and C, respectively (Scheme 4). When 1b was
small amount of 6c (12%) and 6d (8%), respectively (entries
6 and 7). In these reactions, however, a careful NMR analysis
showed no indication of formation of intermingled adduct
from the most stable (2°, 3°) radical, C. Incidentally, it was
also found that the stereochemistry of 7 was retained at
g96% for the reaction with trans-2b (entries 6 and 7), while
considerable stereoisomerization was observed in the reaction
with cis-2b (entry 8). Such stereorandomization can be
accounted for by the geometrical isomerization of interme-
diacy [2b]•+. These results indicate that the reaction of
smaller 1b causes Ha abstraction followed by radical coupling
at the 2° center of A to produce 6. On the other hand, bulky
1c and 1d prefer Hb abstraction (for B) to Ha abstraction
(for A). The subsequent radical coupling is performed in such
a way that the resulting A undergoes 2° attack and B 1°
attack to afford 6 and 7, respectively.
Scheme 4
Why is the generation of A and B radicals dependent on
the quinone substituent and why is the most stable allyl
radical, C, not formed? This problem can be resolved by
considering the geometrical structure at the stage of proton
transfer from [2b]•+ to the counter semiquinone radical. As
depicted in Scheme 4, the calculated structure of [2b]•+ has
the highest spin density (0.40) on the 2° carbon atom and
the relatively larger positive charge (+0.1) on the 3° carbon
atom. The positive 3° carbon atom will be more stabilized
by electrostatic interaction with the closely located less
hindered semiquinone radical of 1b (Scheme 4, left). This
steric and electronic situation facilitates the adjacent Ha+
abstraction to give radical A. In contrast to 1b, on account
of the severe steric repulsion with the isopropyl group of
2b, the more separated bulky semiquinone radicals of 1c and
1d will induce the positive charge on the 2° carbon atom
(Scheme 4, right) and preferentially bring about Hb+ abstrac-
tion to yield radical B. Absence of the radical C product
can be explained by stereoelectronic effects which inhibit
the efficient σ-π interaction between the C-Hc bond and
the vacant p-orbital due to restricted orthogonal conforma-
tion.
irradiated with trans-2b, the corresponding 2° adduct 6b was
formed in low yield (19%) (Table 2, entry 5). Formation of
Table 2. Photoreaction of 1 with 2b in Benzene-d6
yield/%a
entry
1
2b
time/h
6
7
5
6
7
8
1b
1c
1d
1c
trans
trans
trans
cis
5
1
1
1
19
12
8
45 (96:4)b
45 (>99:1)b
10
40 (78:22)b
In conclusion, the photoinduced electron-transfer reaction
of halogenated 1,4-benzoquinones 1 with 2,3-dimethyl-2-
butene 2a or 3,4-dimethyl-2-pentene 2b was markedly
affected by the steric nature of the quinone substituents to
provide the corresponding 1°, 2°, and 3° hydroquinone
monoallyl ethers. The product distributions were rationalized
in terms of steric effects on both H+ abstraction from [2b]•+
and recombination of generated allyl and phenoxy radicals
as well as the spin density.
a Determined by H NMR. b Ratio of trans:cis.
1
1° adduct 7 could not be observed in an elaborate NMR
analysis. However, tetrafluoro-1,4-hydroquinone was also
obtained in a yield of 33%.17 However, the bulky 1c and 1d
yielded the 1° adducts 7c and 7d (both 45%) along with a
(17) In ref 4, the corresponding hydroquinone was similarly formed in
the photoreaction of 1c with cyclohexene in 46% yield.
OL006080T
Org. Lett., Vol. 2, No. 13, 2000
1981