Stability-reactivity relation on the reaction of beta,beta-disubstituted vinyl cations with ethanol

A family of beta,beta-disubstituted alpha-(p-methoxyphenyl)vinyl cations has been generated by the laser flash photolysis of the corresponding vinyl bromides, and the rates of reactions of the cations with ethanol in acetonitrile have been measured at 25 degrees C. The observed rate constants differ greatly depending on the substituents, ranging from 3.05 x 10(5) L mol(-1) s(-1) to 8.18 x 10(7) L mol(-1) s(-1). The thermodynamic stabilities of the vinyl cations have been estimated by means of ab initio MO calculations for model compounds, which reveals that their stability is almost unaffected by beta,beta-dialkyl substituents. In the present system, therefore, the stability-reactivity relation, in which a less stable cation is expected to show higher reactivity, breaks down. Several model transition structures have been considered in the MO calculations, and the results indicate that the beta substituents can move away from the incoming nucleophile to avoid steric congestion but that there still exists a large steric repulsion in the transition state; The calculations indicate that the reactivity of the vinyl cations is primarily controlled by this steric effect, which is the reason for the breakdown of the stability-reactivity relationship.