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.