FORMATION AND REACTIONS OF THE PHOSPHANYLIDENE-PHOSPHORANE (TBU)2P-P=PX(TBU)2 (X = BR, CL)

The formation of (tBu)2P-P = P(Br)tBu2 1 from [(tBu)2P]2PLi and BrH2C-CH2Br begins with an exchange of Li against Br and is then determined by the migration of Br from the secondary P atom in [(tBu)2P]2PBr 6 to the primary P in 1. Similarly, (tBu)2P-P = PCl(tBu)2 2 is obtained starting from PCl3 and LiP(tBu)2. The formation of phosphanylidene-phosphorane is not influenced by the choice o the halogene substituent, but the presence of the tBu groups is strongly required. (tBu)2P-P(Li)-P(SiMe3)2 e.g., yields (tBu)2P-P(Br)-P(SiMe3)2 with BrH2C-CH2Br; however neither this nor (tBu)2P-P(Cl)-P(SiMe3)2 do rearrange to a phosphanylidene-phosphorane. The F3C substituent could be neglected in this investigations as [(F3C)2P]2P-SiMe3 cannot be lithiated by means of BuLi. Compounds 1 and 2 display a characteristic temperature dependent behaviour. While 1 at + 20-degrees-C decomposes via the reactive intermediate (tBu)2P-P to form the cyclophosphanes P3[P(tBu)2]3 and P4[P(tBu)2]4, it gives crystals of [(tBu)2P]2P-P[P(tBu)2]2 at -20-degrees-C (from a solution in toluene). Reacting 1 with tBuLi poduces (tBu)2P-P = P(H)tBu2 20 and (tBu)2P-P(H)-P(tBu)2 14. Initially, a transmetallation yields tBuBr and (tBu)2P-P = PLi(tBu)2 21, then LiBr and isobutene are eliminated and 20 is formed which can rearrange to produce 14. Without the elimination of isobutene, 1 reacts with nBuLi to give 21 which can be trapped with Me3SiCl as (tBu)2P-P(SiMe3)-P(tBu)2 23. The main product in this reaction is however [(tBu)2P]2P-nBu 22.