Catalytic synthesis of vinylphosphanes via calcium-mediated intermolecular hydrophosphanylation of alkynes and butadiynes

The calcium complex [(thf)4Ca(PPh2)2] (1) is a very effective catalyst for the hydrophosphanylation of substituted alkynes of the type R–Ctriple bond; length of mdashC–R (R = Me, Ph) yielding (E)-1,2-diphenyl-1-diphenylphosphanylethene (2a) and (Z)-1-phenyl-2-diphenylphosphanyl-1-propene (2b). The calcium-mediated hydrophosphanylation of butadiynes of the type R–Ctriple bond; length of mdashC–Ctriple bond; length of mdashC-R (R = Me, SiMe3, Ph, Mes, tBu) proceeds less selectively and diverse products are obtained such as 1,4-substituted 1,4-bis(diphenylphosphanyl)-1,3-butadienes (3), 1,4-diphenyl-1,2-bis(diphenylphosphanyl)-1,3-butadiene (4), and 1,4-di(tert-butyl)-1,4-bis(diphenylphosphanyl)buta-1,2-diene (5). Besides these regioisomers also several configuration isomers with respect to the C=C double bonds [(E)/(Z) isomerism] are obtained. A catalytic cycle can be formulated with the first addition of a Ca–P bond of the catalyst 1 to a Ctriple bond; length of mdashC triple bond always leading to the formation of an intermediate with the newly formed C–P bond in 1-position whereas the remaining phosphanido calcium fragment binds to the carbon in 2-position. The addition of a second diphenylphosphane is much faster and therefore, only two-fold hydrophosphanylated butadiynes are observed. Neither addition products with only one HPPh2 group nor those with more than two PPh2 substituents are obtained.