Scheme 2. Photochemical Mechanism
photoproduct whose structure was established by NMR and
X-ray analyses to be that of compound 6. Note eq 2.
Consideration of the photoproduct structure, together with
the known electrophilic reactivity of aza-enone 1, suggested
that this product arises from two fragments: reactant 1 and
zwitterion 7. Hence, the photochemistry must initially lead
to this zwitterion. Further insight came from unpublished
heterocyclic photochemistry in our hands,4 which suggests
a general tendency for fission of bond 4-5 in heterocycles
of this type.
A further point is that this type of photochemical reactivity
arises as a consequence of stabilization of the valence at C-5
by the adjacent nitrogen lone pair. This has analogy where
C-5 is substituted by other stabilizing substituents.6
Acknowledgment. Support of this research by the
National Science Foundation is gratefully acknowledged with
special appreciation for its support of basic research. Ad-
ditionally, we acknowledge initial efforts by Dr. Vladimir
Tyurin. Finally, we dedicate this paper to Prof. Nikolai
Zefirov on the occasion of his 70th birthday.
Supporting Information Available: Experimental de-
tails, X-ray data, and computational details. This material is
OL048438H
The mechanism for formation of 7 is thus suggested in
Scheme 2. The reaction is characteristic of a singlet process.4
Thus, species 7z and 7d are both singlets. However, the
reaction begins on the singlet (i.e., S1) hypersurface. Radia-
tionless decay to S0 seems likely to occur via a conical
intersection or avoided crossing at this point, and this is being
explored computationally.
At this point, our computations5 utilizing CASSCF and
natural orbital analysis suggest that S1 species 7d is best
described as a singlet diradical, while 7z is an S0 zwitterion.
(5) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(6) (a) Zimmerman, H. E.; Solomon, R. D. J. Am. Chem. Soc. 1986,
108, 6276-6289. (b) Note also ref 7.
(7) Canovas, A.; Fonrodona, J.; Bonet, J.-J.; Brianso, M. C.; Brianso, J.
L. HelV. Chim. Acta 1980, 63, 2380-2389.
(3) Compounds 3 and 5 were characterized by NMR and high-resolution
MS. The sulfone 2 was characterized by NMR and elemental analysis.
(4) Studies with Dr. Oleg Mitkin to be published.
3780
Org. Lett., Vol. 6, No. 21, 2004