Organocatalytic Trans Semireduction of Primary and Secondary Propiolamides: Substrate Scope and Mechanistic Studies

We report a chemoselective, phosphine-catalyzed semireduction of primary and secondary propiolamides. In the presence of stoichiometric pinacolborane and catalytic n-tributylphosphine, a variety of propiolamides were successfully converted to the corresponding acrylamides in excellent yield with (E)-stereoselectivity. The reaction condition is tolerant of various functional groups including alkene, alkyne, ketone, or ester. Deuterium labeling studies established that the hydride from activated pinacolborane is added to the alpha-carbon and the proton on the amide nitrogen is abstracted by the ss-carbon to furnish the (E)-acrylamides. DFT calculations revealed a clear energetic driving force for the (E)- over the (Z)-isomer.