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 because of the two adjacent quaternary sp3 -hydribized carbon atoms in its diol backbone. However, recent diastereoselective reactions reported in the literature have cast doubt on this perception. Reported herein is a detailed experimental and computational analysis of Bpin and structurally related boronic esters which allows determination of 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.
['Fasano|Valerio|V|0000-0003-1819-4483', 'McFord|Aidan W|AW|0000-0001-8661-2742', 'Butts|Craig P|CP|0000-0001-6678-8839', 'Collins|Beatrice S L|BSL|0000-0002-2545-3458', 'Fey|Natalie|N|0000-0003-0609-475X', 'Alder|Roger W|RW|0000-0002-3675-1186', 'Aggarwal|Varinder K|VK|0000-0003-0344-6430']
boronic esters A-values conformational analysis density-functional calculations steric hinderance