1560
J . Org. Chem. 1996, 61, 1560-1561
Sch em e 1. P r op osed Mech a n ism for th e
Gen er a tion of Ca r ben es via th e
P h otosen sitiza tion of Sp ir od ia zir id in es
w ith Keton es1
F or m a tion of Ca r ben es by th e
Keton e-P h otosen sitized F r a gm en ta tion of
Sp ir od ia zir id in es
Alan J . Post and Harry Morrison*
Department of Chemistry, Purdue University,
Brown Building, West Lafayette, Indiana 47907-1393
Received September 22, 1995
We recently reported on the aryl-sensitized isomeriza-
tion2 and the ketyl radical-initiated fragmentation3 of
N-alkylspirooxaziridines. As part of our continuing
interest in the photosensitization of small ring hetero-
cycles, we have extended these studies to an examination
of the photosensitized chemistry of spirodiaziridines. We
now report that the irradiation of these diaza heterocycles
with ketone sensitizers leads to the formation of carbenes.
1,2-Diazaspiro[2.5]octane (1) was initially chosen as a
prototypical diaziridine because its ring system is similar
to that of the previously studied oxaziridines. The UV
spectrum of 1 in acetonitrile indicates a high singlet
energy with only end absorption past 200 nm and an
extinction coefficient of 12 M-1 cm-1 at 254 nm (similar
to oxaziridines). As anticipated, direct irradiation4 of 1
in acetonitrile with 254 nm light for extended periods did
not produce photoproducts in any significant quantities.
Nor did photosensitization with several potential aro-
matic sensitizers, such as toluene, N,N-dimethylaniline,
p-dicyanobenzene, and triphenylene.
with p-methoxyacetophenone in hexane, nortricyclene (3)
was formed as the only photoproduct (eq 2). It is known7
This was not the case, however, when a ketone
sensitizer was employed. Here, facile photochemistry
was observed. Thus, photolysis of 1 with acetophenone
in hexane yielded cyclohexene as the major photoproduct
(eq 1). Since we had observed earlier that the carbonyl
that a carbene at C2 of norbornane preferentially inserts
into C6 to form 3 as opposed to undergoing a 1,2 H shift
to give norbornene. Likewise, analogous photolyses of 4
in either methanol or methanol-d4 produced the carbene
O-H insertion product, 2-methoxyadamantane (cf. eq
3),8 in agreement with previous reports.9
photosensitized cleavage of N-alkyl spirooxaziridines was
restricted to sensitizers with lowest lying 3n,π* states
(such as acetophenone in hexane)5 we also explored the
efficacy of sensitizers with lowest lying 3π,π* states, such
as acetophenone in acetonitrile and p-methoxyacetophe-
none in hexane.5 Both were also able to efficiently
produce cyclohexene upon photolysis.
Given the facile photochemical fragmentation of diaz-
irines to carbenes,6 we considered a carbene to be the
most likely intermediate in the spirodiaziridine reaction.
Confirmation was provided by the photosensitization of
spiro(bicyclo[2.2.1]heptane-2,3′-diaziridine) (2) and spiro-
(adamantane-2,3′-diaziridine) (4). When 2 was irradiated
A possible mechanism for the formation of carbenes
from the spiro diaziridines by ketone photosensitization
is outlined in Scheme 1. The initial step in this mech-
anism is electron transfer from the diaziridine to the
carbonyl triplet excited state to form the diaziridine
radical cation 5 and a ketyl. This step is well precedented
in the extensive studies of the photoreduction of ketones
by amines.10 In the nonpolar hydrocarbon solvent one
would expect these species to be closely associated as a
tight ion pair. Homolysis within 5 would give a ring-
opened radical cation 6 which might then be deproto-
nated by the ketyl to give 7. Hydrogen atom abstraction
by the hydroxycarbinyl radical, and loss of molecular
nitrogen, would afford the carbene and the reduced
ketone. In fact, reduced p-methoxyacetophenone was
(1) Organic Photochemistry. 111. Part 110: Mohammad, T.; Mor-
rison, H. J . Am. Chem. Soc., in press.
(2) Post, A. J .; Nwaukwa, S.; Morrison, H. J . Am. Chem. Soc. 1994,
116, 6439-6440.
(3) Post, A. J .; Morrison, H. J . Am. Chem. Soc. 1995, 117, 7812-
7813
(4) All photolyses were carried out using a Rayonet Reactor (New
England Ultraviolet Co.) equipped with a rotating turntable. The
reactor was fitted with 16 254 or 300 nm lamps. Argon-degassed
solutions were irradiated in quartz (254 nm) or Pyrex (300 nm)
photolysis tubes.
(7) Freeman, P. K.; George, D. E.; Rao, V. N. M. J . Org. Chem. 1964,
29, 1682-1684.
(5) Turro, N. J . Modern Molecular Photochemistry; University
Science Books: Mill Valler, CA, 1991; pp 380-381.
(6) For reviews, see: (a) Baron, W. J .; DeCamp, M. R.; Hendrick,
M. E.; J ones, J ., J r.; Levin, R. H.; Sohn, M. B. In Carbenes; Moss, R.
A., J ones, J ., J r., Eds.; Wiley: New York, 1973; Vol. 1. (b) Kirmse, W.
Carbene Chemistry; Academic Press: New York, 1968.
(8) Specific deuteration at C2 was readily determined by the absence
of the characteristic proton resonance in the NMR spectrum.
(9) Morgan, S.; J ackson, J . E.; Platz, M. S. J . Am. Chem. Soc. 1991,
113, 2782-2783 and references therein.
(10) For a review, see: Cohen, S. G.; Parola, A.; Parsons, G. H., J r.
Chem. Rev. 1973, 73, 141-161.
0022-3263/96/1961-1560$12.00/0 © 1996 American Chemical Society
Post, Alan J.
