Revisiting the Phospha-Wittig–Horner Reaction

Abstract

P,P-Dichlorophosphines 2a–c (RPCl2, R = Ph (a), t-Bu (b), 2,4,6-Me3Ph (c)) and P,P-dibromophosphines 4d,e (RPBr2, R = (i-Pr)3SiC≡C (d) and H2C═CH (e)) react with triethylphosphite under Michaelis–Arbuzov conditions to give phosphinodiphosphonates 3a–e in quantitative yields. After complexation to W(CO)5 and treatment with CH3ONa, phospha-Wittig–Horner reagents 9a,b are obtained on a multigram scale in good overall yield. Phospha-Wittig–Horner reagents with unsaturated substituents at IIIP (10d,e) can be prepared in analogous procedures; however, they prevail in an unusual ylide form that allows conjugation between the lone pair and the acetylene and vinyl π-systems, respectively. Phosphinophosphonate 9a has been characterized by X-ray crystallography and is shown to react smoothly with acetone within minutes. The resulting W(CO)5-coordinated phosphaalkene is shown to dimerize to a 1,2-diphosphitane or to undergo a 1,3-proton shift depending on the reaction conditions. In addition, a one-pot synthetic sequence starting from W(CO)5-coordinated phosphinodiphosphonates 5d,e has been developed to engage compounds with vinyl and acetylene substituents in phospha-Wittig–Horner reactions.