Synthesis, structure, and ligand-promoted reductive elimination in an acylrhodium ethyl complex

Article abstract of DOI:10.1021/om00125a028

8-Quinolinecarboxaldehyde and [(C₂H₄)₂RhCl]₂ reacted to give a polymeric acylrhodium ethyl compound which was solubilized by pyridine to give chloroethyl(8-quinolinecarbonyl-C,N)(pyridine)rhodium. This compound was stable in the presence of amine ligands, but phosphine ligands caused rapid reductive elimination. Intermediates in the reductive elimination were observed at -40°C by using ¹H, ¹³C, and ³¹P NMR in the case of the ligand PPh₃. In the first-formed intermediate PPh₃ displaced pyridine. The resulting five-coordinated Rh(III) complex reductively eliminated (with an observed first-order rate constant of 3.7 × 10^-4 s^-1 at -40°C) to give an η²-ketone Rh(I) intermediate. With excess phosphine RhCl(PPh₃)₃ and 8-quinolinyl ethyl ketone were the final products. Recrystallization of the starting pyridine complex from pyridine-ether gave Cl(C₂H₅)Rh(C₁₀H₆NO)(C ₅H₅N)₂·¹/₂C ₄H₁₀O (chloroethyl(8-quinolinecarbonyl-C,N)bis(pyridine)rhodium-hemi(diethyl ether)), whose structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the triclinic space group P1 with two molecules in the unit cell a = 8.933 (3) A?, b = 17.573 (8) A?, c = 7.760 (2) A?, α = 97.57 (3)°, β = 98.04 (2)°, and γ = 79.98 (3)°. The least-squares refinement with anisotropic thermal parameters for all non-hydrogen atoms converged at R_F = 0.046 (R_wF = 0.055) for 2595 observed reflections and 262 parameters refined.