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.