Modulation of Lewis Acidity and Steric Parameters in Cyclic Boronates: Copper-Catalyzed Internal-Selective Borylation of Challenging Terminal Alkynes

Abstract

The installation of a boron functionality into a more substituted carbon of terminal alkynes in copper-catalyzed borylations with diborons is still a challenging subject, primarily due to the intrinsic Lewis acidity of boron moieties such as B(pin), which typically favor addition to a terminal carbon. We disclose herein that the copper-catalyzed internal-selective hydroboration of a variety of challenging terminal alkynes, including sterically demanding and electron-rich aryl variants, is feasible using commercially available, six-membered bis(hexylene glycolato)diboron [B2(hex)2]. This observation underscores that subtle changes in boronate-Lewis acidities associated with ring size markedly influence the regioselectivities of the borylcupration step. Furthermore, this study reveals that the reduced steric bulk of boronate moieties, as well as diminished Lewis acidities, serve as pivotal factors contributing to internal selectivity. This provides crucial insights into the regiocontrol in the copper-catalyzed borylations of terminal alkynes.