Formation of diaryls by thermal decomposition of rhodium(I) triarylphosphine complexes

Abstract

Chlorotris(triphenylphosphine)rhodium, RhCl(PPh 3) 3 undergoes thermal decomposition above 140°C in solution and in the solid state to yield biphenyl. Rhodium complexes with para-substituted triphenylphosphine ligands form exclusively 4,4′-disubstituted biphenyls. Mixtures of two complexes RhCl(PAr 3) 3 and RhCl(PAr′ 3) 3 form the crossed diaryl ArAr′ and two non-crossed compounds (Ar) 2 and (Ar′) 2. The mechanism is assumed to involve dissociation of RhCl(PPh 3) 3 into PPh 3 and solvated RhCl(Pph 3) 2 followed by oxidative addition of the liberated phosphine to the latter bis-triphenylphosphine compound. Evidence for a phenyl-rhodium intermediate has been provided by phenylation of styrene by RhCl(PPh 3) 3. In an independent process, RhCl(PPh 3) 3 forms benzene by an ortho-metallation route.