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Abstract
The Carpenter model for predicting the qualitative effects of substituents on the rates of pericyclic reactions is extended to include the Claisen rearrangement. Simple Huckel and MINDO/3 calculations of the five cyano-substituted and the five methoxy-substituted allyl vinyl ethers and their pericyclic transition states are shown to predict similar substituent effects.
The syntheses of the five cyano-substituted allyl vinyl ethers are reported. Activation parameters for the Claisen rearrangements of these compounds as well as for unsubstituted allyl vinyl ether are obtained from kinetic studies in di-n-butyl ether solutions. The effects of the cyano-substituent on the experimental activation enthalpies are observed to be qualitatively the same as the predicted effects in all cases but one. In this case, the observed lowering of (DELTA)H('=) can be explained in terms of a pseudo-pericyclic transition state in which an oxygen lone pair contributes to the stabilization of a zwitterionic structure.
The results are compared to previous reports of substituent effects on {3,3} sigmatropic reactions as well as other theoretical models for predicting substituent effects.
