4915
trans-stilbene (10a) and styrene did not react with 1 under the same reaction conditions, more
electron-donating or electron-de®cient stilbene derivatives such as 1,2-bis(4-methoxyphenyl)-
ethene (10b) and 1,2-bis(4-cyanophenyl)ethene (10c) stereospeci®cally aorded the corresponding
(2p+2p) photocycloadducts in 14 and 13% yields, respectively. Electron-de®cient alkenes having
no aryl substituent such as acrylonitrile and methyl acrylate, and electron-donating alkenes such
as 2,3-dimethyl-2-butene and ethyl vinyl ether did not add to 1 under the same conditions.
From the mechanistic viewpoints, the triplet sensitized photoreaction and the ¯uorescence
quenching experiments were carried out as follows: The photocycloaddition of t-2a or t-2b to 1
was not sensitized by triplet sensitizers such as benzophenone (69 kcal/mol) and Michler's ketone
(61 kcal/mol). The monomer ¯uorescence of 1 (1Â10^5 mol/dm3) in benzene was eciently quenched
by t-2a,b, accompanying the appearance of a weak exciplex emission at longer wavelength (lmax
ꢁ450 nm) than the former emission of 1. The excimer ¯uorescence of 1 (1Â10^3 mol/dm3, lmax
ꢁ500 nm) was also eciently quenched by t-2a, accompanying the appearance of a weak exciplex
emission at a shorter wavelength than the excimer emission. These results were reasonably elucidated
by the singlet exciplex mechanism for the stereospeci®c and endo-selective photocycloaddition of
arylalkenes to 1. Under the present reaction conditions ([1] >0.01 mol/dm3), the primary process
1
1
...
may be the formation of pyrene excimer 12* followed by the formation of exciplex [1 2]* via
exciplex (excimer) substitution.8,9 This exciplex produces the photocycloadduct eciently. The
endo-selectivity can be explained by the p±p overlap interaction between 1 and the styryl
chromophores of the arylalkenes via sandwich-type exciplexes as previously reported.1a,10 It is
notable that the photoisomerization of arylalkenes was eectively suppressed by the presence of 1
under the reaction conditions, because the triplet energy of 1 is quite a bit lower than those of
arylalkenes. Scope and detailed mechanism are now under investigation.
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scienti®c Research from the Ministry
of Education, Science, Sports and Culture of Japan. Work at the University of Texas at Dallas
was supported by Grant AT-532 from the Robert A. Welch Foundation.
References
1. (a) Caldwell, R. A.; Creed, D. Acc. Chem. Res. 1980, 13, 45±50. (b) Gilbert, A. In Synthetic Organic
Photochemistry; Horspool, W. M., Ed.; Plenum Press: London, 1984; pp. 1±60. (c) McCullough, J. J. Chem. Rev.
1987, 87, 811±860. (d) Wender, P. A.; Dore, T. M. In CRC Handbook of Organic Photochemistry and
Photobiology; Horspool, W. M.; Song, P.-S., Eds. CRC: London, 1994; pp. 280±290.
2. (a) Mizuno, K.; Caldwell, R. A.; Tachibana, A.; Otsuji, Y. Tetrahedron Lett. 1992, 33, 5779±5782. (b) Mizuno, K.;
Konishi, S.; Takata, T.; Inoue, H. ibid. 1996, 37, 7775±7778, and references cited therein.
3. (a) Forster, Th. Angew. Chem., Int. Ed. Engl. 1969, 8, 333±343. (b) DeSchryver, F. C.; Collart, P.;
Vandendriessche, J.; Goedeweeck, R.; Swinnen, A. M.; Van der Auweraer, M. Acc. Chem. Res. 1987, 20, 159±
166. (c) Zachariasse, K.; Kuhnle, W. Z. Phys. Chem. 1976, 101, 267±276. (d) Staab, H. A.; Riegler, N.; Diederich,
F.; Krieger, C.; Schweitzer, D. Chem. Ber. 1984, 117, 246±259. (e) Winnik, F. M. Chem. Rev. 1993, 93, 587±614.
4. (a) Fages, F.; Bodenant, B.; Weil, T. J. Org. Chem. 1996, 61, 3956±3961. (b) Robertson, M. A. F.; Yeager, H. L.
Macromolecules 1996, 29, 5166±5171. (c) Lewis, F. D.; Zhang, Y.; Letsinger, R. L. J. Org. Chem. 1997, 62, 8565±
8568. (d) Szajdzinska±Pietek, E.; Wolszczak, M.; Plonka, A.; Schlick, S. J. Am. Chem. Soc. 1998, 120, 4215±4221,
and references cited therein.