A new family of carbenerhodium(i) complexes: Ligand variation as the key to success

Article abstract of DOI:10.1002/chem.19970030826

A synthetic methodology to obtain square-planar carbenerhodium(I) complexes of the general composition trans-[RhCl(=CRR')(L)₂] where L is a tertiary phosphane, arsane, or stibane has been developed. The starting material trans-[RhCl(C₂H₄)(SbiPr₃)₂] (3) reacts with diazoalkanes RR'CN₂ [RR' = Ph₂, Ph(C₆H₄X), (C₆H₄X)₂, Ph(CF₃), C₁₂H₈] under mild conditions to give the compounds trans-[RhCl(=CRR')(SbiPr₃)'2] (4-11) almost quantitatively. On treatment of 3 with EtO₂CCHN₂ and PhC(N₂)C(O)R, the olefin rhodium and diazoalkanerhodium compounds trans[RhCl{(E)-C₂H₂(CO₂Et)₂}(SbiPr₃)₂] (12) and trans-[RhCl{N₂C(R)C(O)Ph}(Sb-iPr₃)₂] (13, 14) are obtained instead of carbene complexes. Displacement of the SbiPr₃ ligands in 4 (R=R'=Ph) by PiPr₃, PiPr₂Ph, PiPrPh₂, PPh₃, PPh₂Me, AsiPr₃, and SbEt₃ leads to the corresponding carbene complexes trans-[RhCl-(=CPh₂)(L)₂] (15-21) in high yields. The results of the X-ray crystal structure analyses of 4 and 15 (L = PiPr₃) illustrate that the different donor-acceptor properties of SbiPr₃ and PiPr₃ have little influence on the Rh-C bond length. The reactions of 4 and 15 with CO and CNtBu afford, by metal-assisted C-C coupling, diphenylketene Ph₂C=C=O (23) and the corresponding imine Ph₂C=C=NtBu (26). On treatment of 4 and 15 with ethene, however, two different olefinic products, 3,3-diphenyl-1-propene (31) and 1,1-diphenyl-1-propene (32), are formed. Compound 15 reacts with KBr, NaOPh, and NaC₅H₅ by substitution of the chloride to give trans-[RhBr(=CPh₂)-(PiPr₃)₂] (33), trans-[Rh(OPh)(=CPh₂)-(PiPr₃)₂] (34) and [C₅H₅Rh(=CPh₂)-(PiPr₃)] (35), and with HCl by oxidative addition toyield [RhCl₂(CHPh₂)-(PiPr₃)₂] (36).