Table 1 IR spectroscopic data of the photoproduct A generated in Ar at 10 K
and of 3 and 8 calculated with the DFT Method
triplet biradicals which are expected to be formed by the bond
cleavage. It was found that the biradical 7 formed by the C–N
2
1
bond cleavage was more stable by 7.6 kcal mol than the
a
b
Experimental
Calculated
triplet biradical formed by the C–C bond cleavage. The triplet
21
21
n/cm
n/cm
biradical 7 can be readily captured by O
2
to afford 6, while in the
absence of O , the biradical undergoes the Curtius-like
rearrangement to yield 8 from its singlet state which would be
accessible from the triplet state.
In conclusion, it is established that on the irradiation of the
azirine 1, the C–N bond is selectively cleaved, in contrast to the
C–C bond cleavage of the normal 2H-azirines reported so far.
On the basis of the calculations, we propose that the selective
C–N bond cleavage is due to the participation of the excited
triplet state of 1 in the bond cleavage. Moreover, we have
demonstrated that acetonitrile oxide (2) is produced through the
2
A
3
8
Assignment
2
780 (7)
2795 (6)
2024 (100)
Me str
2046 s
CNCNN str
+
2
1
1
946 (81)
585 (11)
CNN NC str
1590 (9)
1585 (21)
1539 (10)
Ar ip, NO
Ar ip
2
unsym
1
1
597 m
539 m
1583 (32)
1534 (11)
1
1
Ar ip, NO
Ar ip
2
unsym
490 (4)
440 (6)
1412 (4)
1342 (68)
Me deform
NO sym
Ar ip
1340 s
1338 (100)
329 (8)
2
2
capture of 7 with O followed by the fragmentation, which
1
1
1
197 w
111 m
1180 (5)
1106 (12)
CH ip
would provide a convenient method of photochemical genera-
tion of 2.
This work was supported in part by grants from the Ministry
of Education, Science, Sports and Culture of Japan (No.
1108 (13)
027 (47)
56 (7)
851 (5)
ArCH ip
Me rock
ArCH ip
ArCH op
1
8
8
51 w
858 (4)
1
2020214) and from the Core Research for Evolutional Science
a
b
Measured in Ar at 10 K; s = strong; m = medium; w = weak.
Calculated frequencies are scaled by the use of a linear scaling function;
and Technology (CREST).
See ref. 7. Relative intensities are designated in parentheses. Frequencies
with relative intensities greater than 5% were given in the table.
Notes and references
1
The photochemistry of 2H-azirines has been reviewed: (a) A. Padwa,
Acc. Chem. Res., 1976, 9, 371. For recent developments in this field; see,
for example: (b) A. Padwa, R. J. Rosenthal, W. Dent, P. Filho, N. J.
Turro, D. A. Hrovat and I. R. Gould, J. Org. Chem., 1984, 49, 3174; (c)
R. L. Barcus, L. M. Hadel, L. J. Johnston, M. S. Platz, T. G. Aavino and
J. C. Scaiano, J. Am. Chem. Soc., 1986, 108, 3928.
ketene imine 8. The direct observation of 8 provides a strong
piece of evidence in support of the C–N bond cleavage in the
photolysis of 1, because the formation of 8 is rationalized in
terms of the Curtius-like rearrangement of the methyl group in
the biradical 7 having a vinyl nitrene character.9
2
(a) S. Murata, Y. Mori, Y. Satoh, R. Yoshidome and H. Tomioka, Chem.
Lett., 1999, 597; (b) T. Mizushima, S. Ikeda, S. Murata, K. Ishii and H.
Hamaguchi, Chem. Lett., 2000, 1282; see also, (c) H. Tomioka, K.
Tabayashi and Y. Izawa, J. Chem. Soc., Chem. Commun., 1985, 906; (d)
T.-Y. Liang and G. B. Schuster, J. Am. Chem. Soc., 1987, 109, 7803.
When 1 was photolyzed ( > 300 nm, 10 K) in the Ar matrix
doped with a large amount of O (20%), we could observe no IR
2
peaks assigned to 8. Instead, the irradiation afforded 4-nitro-
benzaldehyde (4), the structure of which was readily confirmed
by comparison of the IR spectrum with that of the authentic
1
3 Light yellow granules: mp 100–102 °C;
s), 2.96 (1H, s), 7.19 (2H, d, J = 9.0 Hz), 8.15 (2H, d, J = 9.0 Hz); 13
NMR (CDCl ) d 12.6, 32.9, 123.5, 126.0, 146.6, 149.1, 163.3; IR (KBr)
3
H NMR (CDCl ) d 2.56 (3H,
sample matrix-isolated in Ar at 10 K, together with a species
C
having IR bands at 2334 and 1315 cm21 (designated as B) as
3
21
1
770, 1594, 1516, 1345, 1104, 853, 697 cm ; UV (MeCN) lmax (log
primary photoproducts. The species B could be identified as
acetonitrile oxide (2) on the basis of the agreement of the
vibrational frequencies with those reported for 2.6 Isotopic
e) 301 (4.10) nm.
4
5
(a) T. Mukaiyama and T. Hoshino, J. Am. Chem. Soc., 1960, 82, 5339;
,10
(
b) H. Krawczyk and A. Gryff-Keller, J. Chem. Res. S, 1996, 452.
A number of experimental and theoretical studies on regiochemistry of
,3-dipolar cycloaddition of nitrile oxides with dipolarophiles have been
labeling supports this identification of B. With use of 18O
(97%
doubly labeled), the intense band at 2334 cm was slightly
2
2
1
1
2
1
21
shifted (1 cm ), while a large isotopic shift of 28 cm was
reported: A. Rastelli, R. Gandolfi and M. S. Amade, J. Org. Chem.,
1998, 63, 7425, and references therein.
2
1
observed in the band at 1315 cm , which is assigned to the N–
O stretch. These values are in fair agreement with those reported
6 Z. Mielke, M. Hawkins and L. Andrews, J. Phys. Chem., 1989, 93,
,11
for the isotopic shifts of the bands assigned to 2.6 Conse-
558.
7
Calculated vibrational frequencies are scaled by the use of a linear
scaling function of 1.009446 2 0.0000306n, which is determined
experimentally by comparison of frequencies calculated by B3LYP/
quently, the generation of 2 in the photolysis of 1 in the presence
of O , which is presumed on the basis of the characterization of
2
reaction products in solutions, is unambiguously confirmed by
the direct observation using a low-temperature matrix-isolation
technique.
6
-31G(d) with those observed for the authentic sample of 1, 4, and
4-nitrophenyl)diazomethane matrix-isolated in Ar at 10 K. Using this
scaling function, the agreement between calculated and observed
(
It is generally accepted that the photochemical C–C bond
cleavage of 2H-azirines to produce nitrile ylides proceeds from
21
frequencies of these three compounds is within ±14 cm in the range
of 2100 to 750 cm21
.
1
the excited singlet state having an n–p* character. However,
8 The DFT calculations were carried out by the Gaussian 98 program
package.
9
0 W. G. Isner and G. L. Humphrey, J. Am. Chem. Soc., 1967, 89, 6442.
1 The vibrational frequencies for 2 (16O), which are calculated with the
DFT method (B3LYP/6-31G(d)) and scaled by the use of the function
the theoretical calculation using the INDO/S method12 pre-
G. Smolinsky and A. P. Pryde, J. Org. Chem., 1968, 33, 2411.
1
dicted that the lowest excited singlet state of 1, S , could be
1
1
roughly described as a local p–p* excitation of its 4-nitrophenyl
group. Moreover, it is reasonable to think that a large spin-orbit
interaction in the nitro group can accelerate the intersystem-
2
1
18
described in ref. 7, are 2319 and 1368 cm . The O isotopic shifts in
crossing from the S
we propose that the selective C–N bond cleavage observed in
the photolysis of 1 results from the participation of its T state.
This assumption is supported by the DFT calculations on the
1
state to the excited triplet state, T
1
. Thus,
21
these bands are calculated to be 3 and 27 cm , respectively, which are
in agreement with the experimental values.
1
12 The INDO/S calculations were carried out by the WinMOPAC program
package.
Chem. Commun., 2001, 1036–1037
1037