Transition Metal-Free <i>Trans</i> Hydroboration of Alkynoic Acid Derivatives: Experimental and Theoretical Studies

We report a phosphine-catalyzed <i>trans</i> hydroboration of alkynoate esters and amides. The reaction proceeds under mild conditions with exclusive (<i>E</i>)-selectivity to afford (<i>E</i>)-β-boryl acrylates and (<i>E</i>)-β-boryl acrylamides in good to excellent yields. The reaction is tolerant of a variety of functional groups and allows efficient access to novel oxaboroles as well as a pargyline derivative (MAO inhibitor). Theoretical calculations suggest an internal hydride generates a phosphonium allenoxyborane followed by the formation of a key phosphonocyclobutene intermediate that collapses in a stereoselective, rate-limiting step