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.