Influence of Bond Angle Distortion and σ-π-σ-Delocalization on the Stability and Chemistry of Allylic Cations

The rate and products of solvolysis of a number of exocyclic primary allylic 3,5-dinitrobenzoates and their tertiary isomers have been investigated. The results obtained prompt the following generalizations. (1) Constriction of the endocyclic C-C-C angle at the γ position of a primary allylic ester diminishes both the rate of solvolysis of the ester and the positive charge density at the γ position of the resulting allylic cation. (2) If the appropriate orbital orientation can be achieved, and in particular if it is rigidly maintained by the geometrical constraints of a bicyclic system, σ-π-σ-delocalization may lead to the stabilization of an incipient allylic cation. In fact, solvolysis of suitably selected allylic esters may be one of the most sensitive probes for participation of this type. (3) Stabilization of an incipient allylic cation may also result from π-π-σdelocalization from suitably oriented π-electron nucleophiles. Participation of this type is favored when the nucleophile can interact in a symmetrical sense with the p orbital at the γ position of the allylic ester.