THEORETICAL AND EXPERIMENTAL STUDIES OF REGIOSELECTIVITY IN SELENOXIDE ELIMINATION

In order to gain insight into the mechanism of selenoxide elimination, the transition state structures have been determined by applying the ab initio MO method to the elimination for systems having a simplified skeleton, CH2(Y)CH(Se(O)CH3)CH3 (Y OCH3, OH, CN and Cl). The calculations show that an allylic alkyl ether and an allylic alcohol should be produced preferentially in the case of Y = OCH3 and OH, respectively, whereas a vinylic cyanide should be the major product in the case of Y = CN. The chloro-substituted model has given a result most consistent with the methoxy-substituted model. The elimination occurs via a transition state with a five-membered ring structure, and the breaking of the C-beta-H bond takes place earlier than that of the Se-C-alpha bond, in the favorable path. The asynchronous nature of bond breaking is quite conspicuous in the cyano-substituted model, relative to the other systems. For comparison with the theoretical results, beta-methoxy-, beta-hydroxy-, beta-cyano-, and beta-chloro-substituted selenides were newly synthesized from trans-7-tetradecene, respectively, and were oxidized by H2O2 in tetrahydrofuran or in carbon tetrachloride. The regioselectivity observed for the elimination, i.e., the formation of a vinylic compound from the beta-cyano selenide and a mixture of allylic and vinylic products from the beta-chloro selenide, agrees well with the theoretical calculations. The difference in the activation energies of the two paths calculated for the model species is not large enough in the mthoxy- and hydroxy-substituted selenides to explain the almost; complete formation of an allylic compound in the physical experiments.