Pinacol Boronates by Direct Arene Borylation with Borenium Cations

摘要

Aryl boronate esters are essential synthetic building blocks, their ubiquity arising from ease of handling combined with high efficacy in C-X (X = C, N, O) bond-forming reactions. Aryl boronate esters are commonly synthesized from arenes by a multistep process involving haloarene intermediates and either stoichiometric hard organometallic reagents or metal-catalyzed cross-coupling (e.g., Cu, Ni, Co, or Pd). Recent alternative approaches avoiding haloarenes include: [4+2] cycloadditions, aryl-CH deprotonation, diazonium ions, and direct arene borylation with sterically hindered iridium catalysts. The latter represents a considerable advance, being highly generic and proceeding in one step from the parent aryl CH. It is however still subject to a number of restrictions: 1) borylation is dominated by steric factors; 2) in low steric environments regioselectivities are poor, e.g., anisole reacts with 74% meta and 25% para borylation; 3) iridium catalysts are required making larger scale syntheses uneconomical.