Stereospecific diphosphination of activated acetylenes: A general route to backbone-functionalized, chelating 1,2-diphosphinoethenes

The symmetrical diphosphanes R2P-PR2 {where R=Ph (1a), Cy (1b), or But (1c)} are made by the reaction of R2PLi with R2PCl. The unsymmetrical diphosphanes R2P-PR'(2) {where R=Bu-t and R'=Ph (2a); R = Bu-t and R' =o-Tol ( 2b); R=Cy and R'= Ph ( 2c); R=Cy and R' =o- Tol ( 2d); R=Cy and R'=Bu-t ( 2e)} are made by the reaction of R2P( BH3) Li with R'2PCl followed by deprotection with Et2NH. Diphosphanes 1a, b and 2a-d react with ZCtCZ ( Z=CO2Me) to give the corresponding R-2-PCZ=CZPR(2) (3a, b) or R(2)PCZ=CZPR'(2) (4a-d). The reaction of 2a, b with HC=CZ give the corresponding R2PCH=CZPR(2) ( 5a, b). A mechanism is proposed that accounts for the cis stereospecificity and the regioselectivity of these diphosphinations. Treatment of [PtCl2(1,5-cod)] or [PdCl2(NCPh)(2)] with a selection of the diphosphines (3-5) gave the expected chelate complexes, four of which, [PtCl2(3b)], [PtCl2(4a)], [PtCl2(4c)], and [PdCl2(4a)], have had their crystal structures determined.