Hydrophosphorylation of Alkynes Catalyzed by Palladium: Generality and Mechanism

We carried out a comprehensive study on the generality, scope, limitations, and mechanism of the palladium-catalyzed hydro­phosphorylation of alkynes with P­(O)–H compounds (i.e., H-phosphonates, H-phosphinates, secondary phosphine oxides, and hypo­phosphinic acid). For H-phosphonates, Pd/dppp was the best catalyst. Both aromatic and aliphatic alkynes, with a variety of functional groups, were applicable to produce the Markovnikov adducts in high yields with high regio­selectivity. Aromatic alkynes showed higher reactivity than aliphatic alkynes. Terminal alkynes reacted faster than internal alkynes. Sterically crowded H-phosphonates disfavored the addition. For H-phosphinates and secondary phosphine oxides, Pd/dppe/Ph2P­(O)­OH was the catalyst of choice, which led to highly regio­selective formation of the Markovnikov adducts. By using Pd­(PPh3)4 as the catalyst, hypo­phosphinic acid added to terminal alkynes to give the corresponding Markovnikov adducts. Phosphinic acids, phosphonic acid, and its monoester were not applicable to this palladium-catalyzed hydro­phosphorylation. Mechanistic studies showed that, with a terminal alkyne, (RO)2P­(O)H reacted, like a Brønsted acid, to selectively generate the α-alkenylpalladium intermediate via hydropalladation. On the other hand, Ph­(RO)­P­(O)H and Ph2P­(O)H gave a mixture of α- and β-alkenylpalladium complexes. In the presence of Ph2P­(O)­OH, hydropalladation with this acid took place first to selectively generate the α-alkenylpalladium intermediate. A subsequent ligand exchange with a P­(O)H compound gave the phosphorylpalladium intermediate which produced the Markovnikov adduct via reductive elimination. Related intermediates in the catalytic cycle were isolated and characterized.