The carboboration of Me3Si-substituted alkynes and allenes with boranes and borocations

Abstract

The 1,1-carboboration of 1-Me3Si-1-alkynes is the dominant reaction observed using [PhBCl(2-DMAP)][AlCl4], 1, and PhBCl2 electrophiles, with highly substituted vinyl pinacol boronate esters isolated post esterification. Other aryl and heteroaryl congeners of both 1 and PhBCl2 have a limited scope in the 1,1-carboboration of 1-Me3Si-1-alkynes, with desilylboration more prevalent. PhBCl2 converts Me3Si-substituted allenes to allylboranes via a formal 1,3-carboboration with Me3Si-migration. [Cl2B(2-DMAP)][AlCl4] reacts with a number of 1-Me3Si-1-alkynes by desilylboration, whilst with Me3Si-ethyne a 1,1-boroamination reaction proceeds, which with excess boron electrophile is followed by an intermolecular desilylboration to form a tricationic-borate. The use of excess 1-Me3Si-1-propyne relative to 1 (and a thienyl congener of 1) formed 2-boradienes in low yields from the reaction with two equivalents of alkyne. Vinyl borocations ligated by 2,6-lutidine of the general formula, [(vinyl)BCl(2,6-lutidine)][AlCl4] formed 1-boradienes with 1-Me3Si-1-alkynes.