Photolysis of α-azidoacetophenones: Trapping of triplet alkyl nitrenes in solution

Article abstract of DOI:10.1021/ol0068750

(Matrix presented) Selective excitation of the ketone chromophore in α-azidoacetophenones, 1, leads to intramolecular triplet energy transfer to the azido group, which forms the corresponding triplet alkyl nitrene, 2. Azides 1 also undergo α-cleavage to form benzoyl and methyl azido radicals in competition with nitrene formation. Thus the major photoproduct, 2-benzoylamino-1-phenylethanone, 3, comes from trapping of 2 with a benzoyl radical. This appears to be the first observation of bimolecular trapping of triplet alkyl nitrenes in solution.

Full text of DOI:10.1021/ol0068750

ORGANIC
LETTERS
2001
Vol. 3, No. 4
523-526
Photolysis of r-Azidoacetophenones:
Trapping of Triplet Alkyl Nitrenes in
Solution
Sarah M. Mandel, Jeanette A. Krause Bauer, and Anna D. Gudmundsdottir*
Department of Chemistry, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0172
anna.gudmundsdottir@uc.edu
Received November 15, 2000
ABSTRACT
Selective excitation of the ketone chromophore in r-azidoacetophenones, 1, leads to intramolecular triplet energy transfer to the azido group,
which forms the corresponding triplet alkyl nitrene, 2. Azides 1 also undergo r-cleavage to form benzoyl and methyl azido radicals in competition
with nitrene formation. Thus the major photoproduct, 2-benzoylamino-1-phenylethanone, 3, comes from trapping of 2 with a benzoyl radical.
This appears to be the first observation of bimolecular trapping of triplet alkyl nitrenes in solution.
Upon irradiation or thermal activation, alkyl azides rearrange
with loss of nitrogen to form imine derivatives, possibly
through nitrene intermediates. However, there are only a few
examples where products have been isolated, which could
be attributed to the decomposition of alkyl azides into alkyl
nitrene intermediates.1 This has led to the view that the
rearrangement of the alkyl azide takes place not through a
singlet nitrene intermediate but directly from the excited state
of the azide.2 More recently, physical measurements have
been reported that support the existence of triplet alkyl
nitrenes in the gas phase and in matrices.3
Furthermore, a spin barrier prohibits the triplet nitrene from
rearranging to a singlet imine product, whereas there is no
such restriction on the singlet nitrene. Triplet alkyl nitrene
can therefore be expected to be longer-lived than the singlet
and thus easier to intercept by chemical reactions.
We selected R-azidoacetophenone derivatives for this study
since they contain an intramolecular triplet sensitizer, which
ensures efficient formation of the triplet alkyl nitrene by
bypassing the singlet nitrene intermediate (Figure 1). The
UV spectra of azides 1 trail out to 365 nm due to the
We set out to study the reactivity of triplet alkyl azides in
solution, with the intention of intercepting triplet alkyl
nitrenes in bimolecular reactions. Triplet alkyl nitrene is
presumably the ground state, as triplet methyl nitrene is 38.6
kcal/mol lower in energy than singlet methyl nitrene.4
(3) (a) Ferrante, R. F. J. Chem. Phys. 1987, 86, 25. (b) Chapell, E. L.;
Engelking, P. C. J. Chem. Phys. 1988, 89, 6007. (c) Carrick, P. G.; Brazier,
C. R.; Bernath, P. F.; Engelking, P. C. J. Am. Chem. Soc. 1987, 109, 5100.
(d) Carrick, P. G.; Engelking, P. C. J. Chem. Phys. 1984, 81, 1661. (e)
Radziszewski, J. G.; Downing, J. W.; Jawdosiuk, M.; Kovacic, P.; Michl,
J. J. Am. Chem. Soc. 1985, 107, 594. (f) Barash, L., Wasserman, E.; Yager,
W. A. J. Am. Chem. Soc. 1967, 89, 3931. (b) Wasserman, E.; Smolinsky,
G.; Yager, W. A. J. Am. Chem. Soc. 1964, 86, 3166. (g) Smolinsky, G.;
Wasserman, E.; Yager, W. A. J. Am. Chem. Soc. 1962, 84, 3220.
(4) (a) Richards, C., Jr.; Meredith, C.; Kim, S.-J.; Quelch, G. E.; Schaefer,
H. F., III. J. Chem. Phys. 1994, 100, 4819. (b) Demuynck, J.; Fox, D. J.;
Yamaguchi, Y.; Schaefer, H. F., III. J. Am. Chem. Soc. 1980, 102, 6204.
(c) Nguyen, M. T.; Sengupta, D.; Ha, T. K. J. Phys. Chem. 1996, 100,
6499. (d) Nguyen, M. T. Chem. Phys. Lett. 1985, 290.
(1) (a) Banks, R. E.; Berry, D.; McGlinchey, M. J.; Moore, G. J. J. Chem.
Soc. C 1970, 1017. (b) Pancrazi, A.; Khuong-Huu, Q. Tetrahedron 1975,
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(2) (a) Moriarty, R. M.; Reardon, R. C. Tetrahedron 1970, 26, 1379. (b)
Moriarty, R. M.; Serridge, P. J. Am. Chem. Soc. 1971, 93, 1534. (c)
Abramovitch, R. A.; Kyba, E. P. J. Am. Chem. Soc. 1971, 93, 1537.
10.1021/ol0068750 CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/23/2001

mediate that undergoes a 1,2 H shift.8 We do not expect to
observe any 2-imino-1-phenyl-ethanone formation upon
irradiation, since 1,2 H shifts are not allowed in the triplet
manifold.
Azides 1 were photolyzed at ambient temperature in
toluene under argon. Preparative photolysis of azide 1a
yielded one major product, 2-benzoylamino-1-phenyletha-
none (3a, 73%), and acetophenone (9a, 15%) plus small
amounts of 2-benzylamino-1-phenylethanone (4a), N-benzyl-
N-(2-oxo-2-phenylethyl)benzamide (5a), benzaldehyde (6a),
1,2-diphenylethanone (7a), 1,2-diphenylethane-1,2-dione (8a),
and 1,4-diphenyl-1,4-butadione (10a) (Figure 2). The major
Figure 2. Products from photolysis of azide 1a.
Figure 1. Intramolecular sensitization in azide 1.
product, 3a, was isolated and characterized using spectros-
copy and crystallography.9 All of the other products were
characterized by injecting authentic samples in a GCMS.
From the product study, it is apparent that azide 1a undergoes
R-cleavage to form a benzoyl radical in competition with
nitrene formation. Benzoyl radicals do not readily abstract
H atoms from appropriate solvents but are quenched rapidly
by molecular oxygen.10 Similarly, interception of a benzoyl
radical by triplet nitrene 2a yields radical 12a (see Figure
3), which can abstract an H atom from toluene to form the
major product 3a and a benzyl radical. Product 5a is formed
when radical 12a combines with a benzyl radical. The minor
product 4a comes from nitrene 2a reacting with a benzyl
radical. Products 6a-8a result from the benzoyl radical
absorption of the aryl ketone chromophore, whereas the UV
absorption spectra of simple nonconjugated alkyl azides
exhibit only a weak band around 285 nm.5 Thus, by
irradiating azides 1 with a light source above 300 nm, it can
be ensured that the ketone chromophore absorbs most of the
light. We anticipate the intersystem crossing from the singlet
manifold to the triplet state in azides 1 to be similar to the
analogous acetophenone. The intersystem crossing from the
singlet to the triplet excited state in acetophenone is of the
order 1011 s-1, and the quantum efficiency approximates
unity.6 Furthermore, we expect to observe reactivity mainly
from the triplet manifold of the azido group since Wagner
et al. have shown that phenyl ketones are efficiently
quenched by intramolecular γ-azido groups.7
(6) (a) McGarry, P. F.; Doubleday, C. E., Jr.; Wu, C.-H.; Stabb, H. A.;
Turro, N. J. J. Photochem. Photobiol. A 1994, 77, 109. (b) Lamola, A. A.;
Hammond, G. S.; J. Chem. Phys. 1965, 43, 2129. (c) Borkman, R. F.;
Kearns, D. R. Chem. Commun. 1966, 14, 446.
(7) Wagner, P. J.; Scheve, B. J. J. Am. Chem. Soc. 1979, 101, 378.
(8) (a) Boyer, J. H.; Straw, D. J. Am. Chem. Soc. 1952, 74, 4506. (b)
Boyer, J. H.; Straw, D.; J. Am. Chem. Soc. 1953, 75, 1642.
(9) (a) The 1H NMR, IR, and MS spectra of 3a are identical to those
reported. Demeuov, N. B.; Akhmedzhanova, V. I.; Moldagulov, M. A.;
Shakirov, R. S.; Chem. Nat. Compd. (Engl. Transl.) 1998, 34, 484. (b)
Crystallographic data for the structure reported is available from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2
1EZ, U.K. as supplementary publication no. CCDC-1557621.
(10) (a) Neville, A. G.; Brown, C. E.; Rayner, D. M.; Lusztyk, J.; Ingold,
K. U. J. Am. Chem. Soc. 1991, 113, 1869. (b) Brown, C. E.; Neville, A.
G.; Rayner, D. M.; Ingold, K. U.; Lusztyk, J. Aust. J. Chem. 1995, 48,
363.
The thermal decomposition of azide 1a yields 2-imino-
1-phenyl-ethanone, presumably via a singlet nitrene inter-
(5) Reiser, A.; Wagner, H. M. Chemistry of the Azido Group; Patai, S.,
Ed.; John Wiley & Sons: London, 1971.
524
Org. Lett., Vol. 3, No. 4, 2001

Figure 4. Product ratio from photolysis of azide 1a at different
temperature.
photolysis of azide 1b at ambient temperature yielded
products 3b and 9b (see Figure 5) in the ratio of 9:1,
Figure 3. Possible reaction pathways for azide 1a in toluene.
reacting with the solvent or itself. The product ratio indicates
that nitrene 2a does not abstract H atoms efficiently from
the solvent and is therefore similar to triplet phenyl nitrenes
that dimerize to form azo compounds rather than abstract H
atoms from typical solvents or undergo rearrangement.11 We
did not isolate the corresponding azo dimer (11a, see Figure
3) of nitrene 2a, but we speculate that 9a and 10a are
secondary photoproducts from photolysis of 11a and that
under our experimental conditions 11a is not photostable. It
is, however, also possible that azide 1a undergoes â-cleavage
to form acetophenone and azide radicals, which can then
yield compounds 9a and 10a.
We investigated how temperature affects the product ratio.
As the temperature of the photolysis is lowered, 10a becomes
more predominant in the product mixture, as well as 9a,
whereas at higher temperatures 7a and benzaldehyde are
more prevalent. In Figure 4, the percent of products coming
from triplet nitrene 2a, benzoyl, and acetophenone radicals
is plotted at 100, 0, and -63 °C. Since benzoyl radical-based
products are favored at higher temperatures the R-cleavage
must have a larger activation energy than the energy transfer
leading to formation of nitrene 2a. At lower temperatures
when less benzoyl radical is formed to trap nitrene 2a, more
9a and 10a are formed, which suggests that these products
come from nitrene 2a, unless the direct â-cleavage of 1a
has an even lower activation energy than either the R-cleav-
age or internal energy transfer.
Figure 5. Products from photolysis of azide 1b.
indicating that the p-bromo group on the phenyl ring does
not affect the photochemistry significantly.
In conclusion, photolysis of R-azido acetophenone leads
to intramolecular energy transfer to the azido group as well
as R-cleavage to form benzoyl radicals. Presumably, the
triplet excited azido group fragments to a triplet alkyl nitrene
and a nitrogen molecule. Since neither the triplet alkyl nitrene
nor the benzoyl radical efficiently abstracts H atoms from
the solvent, the triplet alkyl nitrene is intercepted by the
benzoyl radicals. To the best of our knowledge this is the
first time that triplet alkyl nitrenes have been intercepted in
bimolecular reactions in solution. We are further investigating
the triplet reactivity of alkyl azides and are currently
undertaking studies to detect the triplet nitrene intermediates
directly with laser flash spectroscopy.
We studied the photochemistry of azide 1b to verify that
R-azidoacetophenone derivatives generally undergo R-cleav-
age in competition with nitrene formation. Preparative
(11) (a) Schrock, A. K.; Schuster, G. B. J. Am. Chem. Soc. 1984, 106,
5228. (b) Marcinek, A.; Levya, E.; Whitt, D.; Platz, M. S. J. Am. Chem.
Soc. 1993, 115, 8609. (c) Poe, R.; Schnapp, K.; Young, M. J. T.; Grayzar,
J.; Platz, M. S. J. Am. Chem. Soc. 1992, 114, 5054.
Acknowledgment. A.D.G. is grateful to the University
of Cincinnati Research Council for a Summer Research
Fellowship and Faculty Research Support. Financial support
Org. Lett., Vol. 3, No. 4, 2001
525

from the Petroleum Research Foundation (ACS-PRF 35809-
G4) is also gratefully acknowledged. X-ray data was col-
lected through The Ohio Crystallographic Consortium,
funded by the Ohio Board of Regents 1995 Investment Fund
(CAP-075) located at the University of Toledo, Instrumenta-
tion Center in Arts and Sciences, Toledo, OH 43606.
Supporting Information Available: Spectroscopic data
for characterization of 1 and 3, and the X-ray structure of
3a. This material is available free of charge via the Internet
at http://pubs.acs.org
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