Control of C-C and C-N bond cleavage of 2H-azirine by means of the excitation wavelength: Studies in matrices and in solutions

Article abstract of DOI:10.1246/cl.2001.832

The remarkable wavelengh-dependent photoreactions of 3-methyl-2-(1-naphthyl)-2H-azirine (1) were observed in matrices at 10 K and in solutions at room temperature. Irradiation of 1 with the long-wavelength light (366 nm) exclusively gave the products form

Full text of DOI:10.1246/cl.2001.832

832
Chemistry Letters 2001
Control of C–C and C–N Bond Cleavage of 2H-Azirine by Means of the Excitation Wavelength:
Studies in Matrices and in Solutions
Hiroshi Inui and Shigeru Murata*
Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo 153-8902
(Received May 28, 2001; CL-010497)
The remarkable wavelengh-dependent photoreactions of 3-
methyl-2-(1-naphthyl)-2H-azirine (1) were observed in matrices
at 10 K and in solutions at room temperature. Irradiation of 1
with the long-wavelength light (366 nm) exclusively gave the
products formed by the cleavage of the C–N bond of the 2H-
azirine ring, while the products derived from the C–C bond
cleavage were predominantly obtained in the irradiation with
the short-wavelength irradiation (>300 nm).
The photocycloaddition of 2H-azirines with alkenes pro-
vides a useful approach to a variety of five-membered hetero-
cyclic systems.1 It has been established that the irradiation of
2H-azirines gives nitrile ylides as reactive intermediates
through the C–C bond cleavage, which have been directly
observed by the use of a laser flash photolysis method2–4 and a
matrix-isolation technique.5 However, we have recently found
that irradiation of the 2H-azirine having 4-nitrophenyl group at
the 2-position causes the selective cleavage of the C–N bond of
the 2H-azirine ring.6 This finding has opened a new field in the
2H-azirine photochemistry. In this paper, we report the wave-
length effect on the photolysis of 3-methyl-2-(1-naphthyl)-2H-
azirine (1)2 in matrices and in solutions, which provides the
first example of the perfect control of the C–C and C–N bond
cleavage of 2H-azirines by means of the excitation wavelength.
The arizine 1 matrix-isolated in Ar showed an imine
stretching band having a medium intensity at 1775 cm–1 and
intense bands at 800 and 781 cm–1 assigned to the out-of-plain
deformation of C–H bonds of the naphthyl ring. Irradiation of
1 in an Ar matrix at 10 K with light of 366 nm of a high-pres-
sure mercury lamp caused a decrease in the intensities of the
bands due to 1.7 Simultaneously, a very intense peak appeared
at 2041 cm–1, which revealed the formation of product having a
cumulenic double bond (designated as A). On the other hand,
irradiation of 1 with the short-wavelength light (>300 nm) gave
another product having a cumulenic double bond at 1946 cm–1
and IR peaks with medium intensities at 1026, 794, and 774
cm–1 (designated as B), along with the product A. To identify
the structure of the products, we carried out the calculations of
vibrational frequencies for possible compounds having a cumu-
lenic double bond with the DFT method (B3LYP/6-31G(d)).8
As shown in Figure 1, we found that the vibrational frequencies
and their relative intensities calculated for the ketene imine 2
and the nitrile ylide 3 were in excellent agreement with those
observed for the products A and B, respectively. The formation
of 2 is interpreted in terms of the migration of the methyl group
in the biradical 4 generated from the cleavage of the C–N bond
(Scheme 1). On the other hand, as mentioned in an introducto-
ry part, nitrile ylides are well-known reactive intermediates
formed by the photolysis of 2H-azirines.1–5 Thus, we conclude
that the C–C bond cleavage of the azirine ring of 1 to produce
the nitrile ylide 3 is caused by the irradiation with the short-
wavelength light (>300 nm), while the selective C–N bond
cleavage can be achieved by the irradiation with the long-wave-
length light (366 nm).
The remarkable wavelength effect was also observed in the
photolysis of 1 in solutions at room temperature. Irradiation of
an O2-saturated solution of 1 containing 6% (v/v) of acryloni-
trile in acetonitrile with the long-wavelength light (366 nm)
caused a slow consumption of the starting azirine 1.9 From the
photoreaction mixture, the isoxazoline 510 and 1-naph-
thalenecarbaldehyde (6) were isolated in 57 and 61% yields,
respectively. On the other hand, the irradiation of 1 under iden-
tical conditions with the short-wavelength light (>300 nm)
afforded an E–Z mixture of the pyrroline 72 in 82% yield (E/Z =
1.0), along with a small amount of 6 (3%). No trace amounts of
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
833
state.11 The experimental and theoretical evidences supporting
this assumption are as follows: (i) the addition of benzophenone
as a triplet sensitizer to the O2-saturated solution of 1 contain-
ing acrylonitrile in acetonitrile considerably enhanced the rate
of photodecomposition of 1. Analogously to the direct irradia-
tion, the benzophenone-sensitized photolysis of 1 afforded the
equimolar mixture of the isoxazoline 5 and the aldehyde 6. No
trace amounts of the pyrroline 7 were detected. (ii) the DFT
calculations (B3LYP/6-31+G(d)//B3LYP/6-31G(d)) predicted
that the biradical 4 in the triplet state was more stable by 8.7
kcal mol–1 than the triplet biradical which was expected to be
formed by the C–C bond cleavage.
On the other hand, the higher excited singlet state, S3,
which can cause the C–C bond cleavage of the azirine ring, par-
ticipates in the photoreaction initiated by the short-wavelength
irradiation (>300 nm). It seems that the high quantum yield for
the formation of the product derived from the C–C bond cleav-
age, as well as the low efficiency of the C–N bond cleavage,9 is
responsible for the selective C–C bond cleavage observed by
the irradiation with the short-wavelength light in solutions.
References and Notes
1
2
A. Padwa, Acc. Chem. Res., 9, 371 (1976).
a) R. L. Barcus, B. B. Wright, M. S. Platz, and J. C.
Scaiano, Tetrahedron Lett., 24, 3955 (1983). b) R. L.
Barcus, L. M. Hadel, L. J. Johnston, M. S. Platz, T. G.
Savino, and J. C. Scaiano, J. Am. Chem. Soc., 108, 3928
(1986).
5 were detected in the photoreaction mixture obtained by the
irradiation with the short-wavelength light. We have recently
reported that the isoxazoline 5 is produced by a [2 + 3] cycload-
dition reaction of acrylonitrile and acetonitrile oxide (8), which
is generated in situ through the capture of the biradical 4 with
O2 (Scheme 1).6 On the other hand, it is well-known that pyrro-
lines are produced by the capture of nitrile ylides with acryloni-
trile.1,2 Thus, the selective C–N bond cleavage of the azirine
ring of 1 is observed by the irradiation with light of 366 nm,
which is consistent with the wavelength effect observed in
matrices. Moreover, the product derived from the C–C bond
cleavage, that is, 7 was exclusively obtained by the photolysis
with light of >300 nm. Thus, we can practically achieve the
perfect control of the C–C and C–N bond cleavage of the 2H-
azirine by means of the excitation wavelength in the photolysis
of 1 in solutions.
In order to have information about the excited state of 1,
we carried out molecular orbital calculations using the INDO/S
method. The calculations predicted that the lowest excited sin-
glet state, S1, as well as the second excited singlet state, S2, of 1
could be roughly described as a local π–π* excitation of its
naphthyl moiety. The transition energies for the S1 and S2 states
were calculated to be 3.96 and 4.47 eV, respectively. On the
other hand, an n–π* excitation of the azirine moiety contributed
largely to the third excited singlet state, S3, having the transition
energy of 4.90 eV.
3
4
A. Padwa, R. J. Rosenthal, W. Dent, P. Filho, N. J. Turro,
D. A. Hrovat, and I. R. Gould, J. Org. Chem., 49, 3174
(1984).
a) I. Naito, H. Morihara, A. Ishida, S. Takamuku, K.
Isomura, and H. Taniguchi, Bull. Chem. Soc. Jpn., 64,
2757 (1991). b) I. Naito, Y. Fujiwara, Y. Tanimoto, A.
Ishida, and S. Takamuku, Bull. Chem. Soc. Jpn., 68, 2905
(1995).
5
E. Orton, S. T. Collins, and G. C. Pimentel, J. Phys. Chem.,
90, 6139 (1986).
6
7
H. Inui and S. Murata, Chem. Commun., 2001, 1036.
The absorption spectrum of the azirine 1 shows a transition
with maximum at 290 nm (log ε = 3.90 in acetonitrile).
Having a weak tailing to ca. 400 nm, 1 can be photolyzed
with light of 366 nm.
The calculated vibrational frequencies are scaled by a fac-
tor of 0.96.
The azirine 1 was recovered quantitatively by the irradia-
tion in the absence of O2, which is probably due to an effi-
cient recombination between the radical centers of the
biradical 4.
8
9
10 Viscous oil: 1H NMR (CDCl3) δ 2.08 (3H, s), 3.28 (1H,
dd, J = 17.3, 6.0 Hz), 3.40 (1H, dd, J = 17.3, 11.0 Hz), 5.20
(1H, dd, J = 11.0, 6.0 Hz); 13C NMR (CDCl3) δ 12.2, 44.3,
65.6, 117.3, 155.3.
11 The formation of 2 observed in an Ar matrix is rationalized
in terms of the Curtius-like rearrangement of the methyl
group in the biradical 4 having a vinyl nitrene character. It
appears that this reaction proceeds through the biradical 4
in its singlet state which would be accessible from the
vibrationally excited triplet state.
Taking into account that the excited state having an n–π*
character participates in the C–C bond cleavage of the azirine
ring,1 the fact that no products derived from the C–C bond
cleavage were obtained by the long-wavelength irradiation (366
nm) can be explained in terms of the selective excitation into
the S1 state having no n–π* character. We propose that the
C–N bond cleavage is caused from the excited triplet state, T1,
which can be produced by the intersystem-crossing from the S1