Multigram Copper-Catalyzed Borylation of Functionalized Terminal Alkynes with B2Pin2

A comprehensive study of the commercially available ligands/additives in the formal copper-catalyzed hydroboration reaction of functionalized terminal alkynes is described. Based on the common reaction system (CuCl − B₂Pin₂), a set of common and inexpensive ligands or additives − namely, rac-BINAP, PPh₃, [(t-Bu)₃PH]BF₄, and LiCl – was tested for its effects on the borylation regioselectivity and overall reaction efficiency. The results observed demonstrated correlation between the substrate nature (propargylic, homopropargylic, or higher homolog) and the ligand, which could be used for the preferential preparation of linear or branched isomer of the target alkenyl boronic species. It was found, that LiCl and (t-Bu)3P could be used for preparation of β-substituted alkenyl boronates, whereas α-substituted isomers were preferentially obtained in the presence of triarylphopsphine ligands. The scope of the transformations covered derivatives of protected (homo)propargylic alcohols and amines and tolerated free alcohol, ether, silyl ether, ketal, ester, and alkene moieties. The practical utility of the discussed methodologies was proven by the multigram preparation of target building blocks (10...100 g for over 20 boronates, up to 300 g – for a few selected representatives).