Diels-Alder Reactivity of Allenylboronic Acid Pinacol Ester and Related Dienophiles: Mechanistic Studies and Distortion/Interaction-Activation Strain Model Analysis

Abstract

A DFT study of the Diels-Alder reactions of vinylboronic acid pinacol ester, allenylboronic acid pinacol ester, methyl acrylate, and methyl 2,3-butadienoate with cyclopentadiene has been performed. Competitive mechanisms for the reactions of the carboxylic esters have been fully investigated, and similar results to those previously found for the organoboron compounds were obtained. Moreover, reactivity and selectivity patterns were correctly reproduced. The distortion/interaction-activation strain energy model analysis provided a rationale to explain the experimental outcome of the studied [4 + 2] cycloadditions. While the regioselectivity of the allenyl compounds and the relative reactivity of the boronic and carboxylic esters appear to originate from electronic effects that arise from orbital overlap and correlate with the energies of the vacant orbitals of the dienophiles, the lower reactivity of the allenes relative to the vinyl compounds may be due to the distortion of the dienophiles.