Phosphine-catalysed reductive coupling of dihalophosphanes

Classically tetraaryl diphosphanes have been synthesized through Wurtz-type reductive coupling of halophosphanes R2PX or more recently, through the dehydrocoupling of phosphines R2PH. Catalytic variants of the dehydrocoupling reaction have been reported, but are limited to R2PH compounds. Using PEt3 as a catalyst, we now show that TipPBr(2) (Tip = 2,4,6-iPr(3)C(6)H2) 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 PEt3 catalysed reductive coupling the diphosphene (PDip)(2) (2) with a PvP 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. To show that this protocol is more widely applicable, we show that Ph2PCl and Mes(2)PX (X = Cl, Br) are efficiently coupled using 10 mol% of PEt3 to give (Ph2P)(2) and (Mes(2)P)(2), respectively. Control experiments show that [BrPEt3]Br is a potential oxidation product in the catalytic cycle, which can be debrominated by Zn dust as a sacrificial reductant.

Automated summary

In this study, the selective synthesis of different diaryl diphosphanes and diphosphenes in reductive coupling reactions using PEt3 as a catalyst was demonstrated. The results show the wide applicability of this method in the synthesis of phosphorus compounds. Control experiments revealed a potential oxidation product in the catalytic cycle that could be debrominated by Zn dust as a sacrificial reductant.