How big is the pinacol boronic ester as a substituent

The synthetically versatile pinacol boronic ester group (Bpin) is generally thought of as a bulky moiety due to the two adjacent quaternary sp 3 -hydribized carbon atoms in its diol backbone. However, recent diastereoselective reactions reported in the literature have cast doubt on this perception. Herein, we report a detailed experimental and computational analysis of Bpin and structurally related boronic esters which allows us to determine three different steric parameters for the Bpin group: the A-value, ligand cone angle, and percent buried volume. All three parameters suggest that the Bpin moiety is remarkably small, with the planarity of the oxygen-boron-oxygen motif playing an important role in minimising steric interactions. Of the three steric parameters, percent buried volume provides the best correlation between steric size and diastereoselectivity in a Diels-Alder reaction.