Phosphine-catalysed reductive coupling of Dihalophosphanes

Abstract

Classically, tetraorgano diphosphanes have been synthesized through Wurtz -type reductive coupling of halophosphanes R 2 PX or more recently, through the dehydrocoupling of phosphines R 2 PH. Catalytic variants of the dehydrocoupling reaction have been reported but are limited to R 2 PH compounds. Using PEt 3 as a catalyst, we now show that TipPBr 2 (Tip = 2,4,6-iPr 3 C 6 H 2 ) is selectively coupled to give the dibromodiphosphane (TipPBr) 2 (1), a compound not accessible using classic Mg reduction. Surprisingly, when using DipPBr 2 (Dip = 2,6-iPr 3 C 6 H 3 ) in the PEt 3 -catalysed reductive coupling the diphosphene (PDip) 2 (2) with a P=P double was formed selectively. In benzene solutions (PDip) 2 has a half life-time of ca. 28 days and can be utilized with NHCs to access NHC-phosphinidene adducts. Control experiments show that [BrPEt 3 ]Br is a potential oxidation product in the catalytic cycle, which can be then debrominated by using Zn dust as sacrificial reductant.