Phenylacetylene hydrophosphination

The inteimolecular hydrophosphination of phenylacetylene (76, equation 20) with di-phenylphosphine using Ni(acac)2 (47) and (Et0)2P(0)H has been reported to give aUcenyl-phosphine (77) in high yields . Alkenylphosphines are an important class of ligands whose applications have been limited by the lack of a simple and convenient synthetic procedure. This reaction gave the corresponding iyn-addition products with a mixture of a- and / -adducts. Selectivities could be fine-tuned by judicious choice of catalyst precursor used to facilitate this reaction. 

The same reaction was performed in the presence of a source of Pd and a bidentate phosphine in toluene at 50 °C. Methylphenylphosphine borane (41a) was reacted with 1-octyne in the presence of palladium acetate (5%) and dppp (10%) for 35 minutes. Interestingly, the Markovnikov product 43a is obtained in 85% yield, with no trace of 42, meaning that in this case the phosphorus attacks the internal carbon of 1-octyne. The same reaction using bdpp (2,4-dw(diphenylphosphino)pentane) and phenylacetylene (39, R = Ph) also yields regioisomer of 43 with 53% yield. Several experiments with diphenylphosphine borane under different reaction conditions (palladium source, temperature, alkyne) showed that the reaction does not occur in the absence of palladium (0) and that hydrophosphination of 1-ethynylcyclohexene (42) undergoes selective hydrophosphination at the triple bond, with the alkeneic bond untouched. 

Calcium-based catalysts have also shown promise in hydrophosphination reactions (Scheme 4.299) [62]. Treatment of phenylacetylene with diphenylphosphine in the presence of a p-diketiminate-ligated calcium complex afforded an excellent yield of the vinylphosphine. Furthermore, the reaction was highly selective for the formation of the F-stereoisomer. This chemistry provided a transition metal-free route to the synthesis of these valuable targets. 

The first example of Pd and Ni-catalyzed hydrophosphination of terminal and internal alkynes was reported in 2001 (Scheme 8.51) [134]. Regioselectivity of the reaction depended on the catalytic system used and the nature of the alkyne. Hydrophosphination of phenylacetylene in acetonitrile solution in the presence of Pd(0) complexes led to the -adduct, while Ni(acac)2 in the presence of (EtO)2 P(0)0H gave nearly pure a-adduct (a / = 95 5). 

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