Ligand Effects on the Chemoselectivity of Ortho-Metalated Rhodium(II) Catalyzed α-Diazo Ketone Transformations

Abstract

Rh2(OOCR)2(PC)2 complexes (PC = orthometalated phosphines, OOCR = carboxylates) with very polarizable ligands, such as aromatic rings directly joined to the rhodium atoms, control chemoselectivity in competitive metal carbene transformations of α-diazo ketones. These catalysts have a mixed set of ligands that allows choosing among a big selection of ligands to gradually affect the electronic and steric properties of the catalyst. Their selectivity depends on the electrophilicity of the ligands and the polarizability of the metalated aromatic rings. Thus, Rh2(OOCR)2(PC)2 compounds [PC = (C6H4)P(CH3)(C6H5), (p-CH3C6H3)P(p-CH3C6H4)2, (C6H4)P(C6H5)2; R = C3F7 or CF3] exhibit an exceptional selectivity for aliphatic C−H insertion over aromatic substitution in catalytic decomposition of 1-diazo-5-methyl-3-phenyl-2-hexanone. Results obtained from these complexes in competitive intramolecular cyclopropanation versus tertiary C−H insertion, cyclopropanation versus aromatic substitution, and C−H insertion versus aromatic substitution are compared with those obtained for other rhodium(II) compounds, such as rhodium(II) caprolactamate (Rh2(cap)4), rhodium(II) perfluorobutyrate, and rhodium(II) acetate.