Hydrolysis of 8-(pinacolboranyl)quinoline: where is the 8-quinolylboronic acid?

Abstract

The compound 8-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-quinoline was prepared and found to hydrolyze rapidly in air; however, the expected product (quinolin-8-yl)boronic acid was not observed. Instead, the (quinolinium-8-yl)trihydroxyborate zwitterion or an anhydride were observed depending on the conditions of hydrolysis. The two products are related to one another in the degree of hydration, and the two forms could be interconverted. Both hydrolysis products were structurally characterized. Additionally, a commercial sample of '8-quinolylboronic acid' was actually found to be the anhydride. The results call into question whether monomeric (quinolin-8-yl)boronic acid can actually be isolated in the neutral Lewis base-free form.