Regioselective Single and Double Hydrophosphination and Hydrophosphinylation of Unactivated Alkynes

A lanthanum-based N,N-dimethylbenzylamine complex was used as a precatalyst for both hydrophosphination and hydrophosphinylation of alkynes under mild conditions. In the case of hydrophosphination, the catalyst induced monoaddition with high regiospecificity, yielding only the anti-Markovnikov product, and stereoselectivity that could be controlled on the basis of the reaction conditions. Undertaking the catalysis with excess phosphine yielded the E isomer as the major product; however, using excess alkyne, the Z isomer was instead isolated as the major product. A brief investigation into the catalytic cycle suggested that a dimeric form of the lanthanum phosphide active catalyst provided the Z isomers as kinetic products that then underwent isomerization to yield the final E isomers. In the case of hydrophosphinylation, the chemoselectivity depended on the nature of the alkyne used. Terminal alkynes gave only double addition products while both single and double addition products were successfully isolated in the case of internal alkynes. The hydrophosphinylation also showed high chemo- and regioselectivity as only the anti-Markovnikov products were isolated. Monohydrophosphinylation of internal alkynes gave almost exclusively the E isomer, and double hydrophosphinylation of all alkynes led to 1,2-addition products.