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 = 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 R2P(BH3)Li with R‘2PCl followed by deprotection with
Et2NH. Diphosphanes 1a,b and 2a−d react with ZC⋮CZ (Z = CO2Me) to give the corresponding R2PCZCZPR2 (3a,b) or R2PCZCZPR‘2 (4a−d). The reaction of 2a,b with HC⋮CZ give the corresponding
R2PCHCZPR2 (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.