Regiodivergent Oxidative Cross-Coupling of Catechols with Persistent <i>tert</i>-Carbon Radicals
Masumi Sugawara, Rikako Ohnishi, Tetsuya Ezawa, Mai Akakabe, Miki Sawamura, Daiki Hojo, Daisuke Hashizume, Yoshihiro Sohtome, Mikiko Sodeoka
Abstract
Oxidative cross-coupling is a powerful synthetic strategy for forming a carbon–carbon bond from two nucleophiles having C–H bonds. However, controlling the coupling selectivity (homo- vs cross-coupling) and the chemo- (C–C vs C–O) and regioselectivity in the reaction of two distinct enolizable substrates under aerobic conditions is notoriously challenging. Here, we present a regiodivergent oxidative cross-coupling reaction between catechols and carbonyl compounds (2-oxindoles and benzofuranones). The oxidative cross-coupling proceeds at the C(6) position of 4-substituted catechols under catalyst-free conditions, while the Pd(II)-BINAP-μ-hydroxo catalyst promotes the reaction at the C(5) position. A series of mechanistic control experiments support a homolytic aromatic substitution mechanism for the carbon–carbon bond-forming processes in both the C(6)- and C(5)-selective coupling reactions. Furthermore, computational analyses suggest that Pd(II)–catecholate is a key catalytic active species, which serves as a SOMO-phile, to facilitate the endothermic C(5)-selective carbon–carbon bond formation and the exothermic aerobic oxidative aromatization.