Regio- and chemoselective hydroboration of terminal alkynes with pinacolborane catalyzed by organo rare earth metal complexes

We herein report the regio- and chemoselective hydroboration of terminal alkynes with pinacolborane by a series of constrained-geometry catalyst-type or CGC-type rare earth metal monoalkyl complexes. This protocol offers a 100% atom-efficient and straightforward route for the synthesis of a variety of (E)-1-alkenylborons with the advantages such as high yield, high E-selectivity up to >99.9%, multiple metal center, broad substrate scope, broad functional-group tolerance, and no necessity for a solvent. A gram-scale synthesis and subsequent transformations of (E)-1-alkenylboron to several functional compounds suggest the practical applications of this synthetic methodology. The reaction mechanism has been explored through ESI-MS, isotopic labeling experiments, and in situ¹H NMR spectroscopy.