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This preparation of 1,2-disubstituted vinylboranes is based on a directed borylation of terminal alkynes followed by Cu-catalyzed allylic alkylation with Grignard reagents.

Good to excellent reactivity and regiocontrol have been achieved in the CuI-catalyzed borylation of dialkyl internal alkynes with bis(pinacolato)diboron. The presence of a propargylic polar group (OH, OR, SAr, SO2Ar, or NHTs), in combination with PCy3 as ligand, allowed maximizing the reactivity and site-selectivity (β to the propargylic function). DFT calculations suggest a subtle orbitalic influence from the propargylic group, matched with ligand and substrate size effects, as key factors involved in the high β-selectivity. The vinylboronates allowed the stereoselective synthesis of trisubstituted olefins, while allylic substitution of the SO2Py group without affecting the boronate group provided access to formal hydroboration products of unbiased dialkylalkynes.

The asymmetric boration of unsaturated sulfones (I) and (III) in the presence of optically active Cu(I)-diphosphine complexes followed by oxidation of the addition products with NaBO3 leads to corresponding β-hydroxysulfones (II) and (IV) with high enantioselectivity.