7167
competition in the case of 1,10-dimethyl 2-phenylacyl radical derived from DBK 6 whose rate of
decarbonylation is estimated to be near 1.5Â108 s^1.7 The cation eect, yield of rearranged
products and the rate of decarbonylation of 1-methyl 2-phenylacyl radical in the case of DBK 5 is
intermediary between those in DBK 4 and 6 (Table 1).
The results of the above study on DBK allows us to understand the selectivity observed during
the photolysis of naphthyl esters 1±3. Comparing the behavior of DBK 4 and naphthyl ester 1
both generating the phenyl acyl radical brings out a dierence in behavior when included in NaY.
While 1 quantitatively rearranges to 7 (Scheme 2), 4 gives only 10% of the rearranged products 8
and 9, the dierence which we believe is the result of the spin state of the geminate radical pair.
The possibility to spin interconversion provided by TlY and PbY alters the yield of the
rearranged products from 10 (NaY) to 89% (PbY) suggesting that the spin barrier is the main
reason for the lack of rearrangement in 4 in NaY. An insight into the process is gained by
comparing the behavior of naphthyl ester 3 and DBK 6. While the former gives quantitatively the
rearranged product 7, the latter does not give any rearrangement product. In this case, due to the
presence of a-methyl groups, the decarbonylation occurs 50 times faster than in unsubstituted
phenyl acyl radical.7 Since we would have detected 1% of the rearranged product by GC, we
believe that the rate of cation (Tl+ and Pb2+) induced ISC in the geminate pair (A3 to A1; Scheme
3) in the presence of heavy cations must be ꢀ106 s^1. Thus, a comparison of the results observed
with DBK systems and naphthyl esters suggests the lack of decarbonylation products in the case
of naphthyl esters 1±3 to be due to the formation of primary radical pair A in the singlet state.
The results reported here unequivocally establish that heavy cations present within zeolites can
have a remarkable in¯uence on ISC of geminate radical pairs. Use of this technique to control
product distribution during photochemical reactions involving triplet radical pairs, radical ion
pairs and diradicals is a possibility that is currently being explored by us.
Acknowledgements
V.R. thanks the Division of Chemical Sciences, Oce of Basic Energy Sciences, Oce of
Energy Research, and the US Department of Energy for support of this program. N.J.T.
acknowledges support of the National Science Foundation and the Department of Energy under
Grant No. NSF CHE 9810367 to the Environmental Molecular Sciences Institute (EMSI) at
Columbia University. He also thanks the National Science Foundation grant (CHE-98-12676) for
their support.
References
1. (a) Pitchumani, K.; Warrier, M.; Cui, C.; Weiss, R. G.; Ramamurthy, V. Tetrahedron Lett. 1996, 37, 6251. (b) For
a study on phenyl esters, see: Pitchumani, K.; Warrier, M.; Ramamurthy, V. J. Am. Chem. Soc. 1996, 118, 9428.
2. Vasenkov, S.; Frei, H. J. Am. Chem. Soc. 1998, 120, 4031.
3. (a) Gu, W.; Warrier, M.; Ramamurthy, V.; Weiss, R. G. J. Am. Chem. Soc. 1999, 121, 9467. (b) For a related
study on phenyl esters, see: Tung, C.-H.; Ying, Y.-M. J. Chem. Soc., Perkin Trans. 2 1997, 1319.
4. Turro, N. J.; Kraeutler, B. Acc. Chem. Res. 1980, 13, 369.
5. Turro, N. J.; Zhang, Z. Tetrahedron Lett. 1987, 28, 5637.
6. (a) Olson, D. H. Zeolites 1995, 15, 439. (b) Grey, C. P.; Poshni, F. I.; Gualtieri, A. F.; Norby, P.; Hanson, J. C.;
Corbin, D. R. J. Am. Chem. Soc. 1997, 119, 1981.