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

The symmetrical diphosphanes R 2 P-PR 2 {where R = Ph (la), Cy (1b), or Bu t (1c)} are made by the reaction of R 2 PLi with R 2 PCl. The unsymmetrical diphosphanes R 2 P-PR' 2 {where R = But and R' = Ph (2a); R = But and R' = o-Tol (2b); R = Cy and R' = Ph (2c); R = Cy and R' = o-Tol (2d); R = Cy and R' = But (2e)} are made by the reaction of R 2 P(BH 3 )Li with R' 2 PCl followed by deprotection with Et 2 NH. Diphosphanes la,b and 2a-d react with ZC=CZ (Z = CO 2 Me) 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 R 2 PCH=CZPR 2 (5a,b). A mechanism is proposed that accounts for the cis stereospecificity and the regioselectivity of these diphosphinations. Treatment of [PtCl 2 (1,5-cod)] or [PdCl 2 (NCPh) 2 ] with a selection of the diphosphines (3-5) gave the expected chelate complexes, four of which, [PtCl 2 (3b)], [PtCl 2 (4a)], [PtCl 2 (4c)], and [PdCl 2 (4a)], have had their crystal structures determined.