Mechanistic Study Reveals Hypoboric Acid Catecholate (HAC): An Enhanced Diboron Reagent for Decarboxylative Borylation

The decarboxylative borylation of redox‐active esters promoted by bis(catecholato)diboron (B 2 cat 2 ) and blue light enables the synthesis of versatile organoboron building blocks from carboxylic acid materials. An LED‐illuminated NMR kinetic study revealed that the reaction is surprisingly accelerated by water, and this hidden catalysis can be attributed to a novel intermediate formed in situ, namely a hypoboric acid catecholate (HAC). This unsymmetrical diboron species can alternatively be formed by hydrolysis of B 2 cat 2 or transesterification between B 2 cat 2 and hypoboric acid (B 2 (OH) 4 ). Experimental and computational studies revealed and rationalized the regioselective capture of alkyl radicals by the HAC species, and the concomitant formation of a more reductive boryl radical chain carrier. The regioselectivity is characteristic of the HAC species studied, and is controlled by electronic rather than steric effects.