Modulation of Lewis\nAcidity and Steric Parameters\nin Cyclic Boronates: Copper-Catalyzed Internal-Selective Borylation\nof Challenging Terminal Alkynes

The installation of a boron functionality into a more\nsubstituted\ncarbon of terminal alkynes in copper-catalyzed borylations with diborons\nis still a challenging subject, primarily due to the intrinsic Lewis\nacidity of boron moieties such as B(pin), which typically favor addition\nto a terminal carbon. We disclose herein that the copper-catalyzed\ninternal-selective hydroboration of a variety of challenging terminal\nalkynes, including sterically demanding and electron-rich aryl variants,\nis feasible using commercially available, six-membered bis(hexylene\nglycolato)diboron [B<sub>2</sub>(hex)<sub>2</sub>]. This observation\nunderscores that subtle changes in boronate-Lewis acidities associated\nwith ring size markedly influence the regioselectivities of the borylcupration\nstep. Furthermore, this study reveals that the reduced steric bulk\nof boronate moieties, as well as diminished Lewis acidities, serve\nas pivotal factors contributing to internal selectivity. This provides\ncrucial insights into the regiocontrol in the copper-catalyzed borylations\nof terminal alkynes.