Bimetallic activation in homogeneous catalysis. 3. Mechanistic study of the methoxycarbonylation of tricarbonyl(chlorobenzene)chromium: Isolation and reactivity of possible reaction intermediates

Article abstract of DOI:10.1021/om00058a015

The coordination of the Cr(CO)₃ moiety to the aromatic ring allows the carbonylation of chlorobenzene, catalyzed by palladium complexes, to esters, aldehydes, amides or α-oxo-amides. The mechanism of methoxycarbonylation of tricarbonyl(chlorobenzene)chromium, (η⁶-C₆H₅Cl)Cr(CO)₃, and the role of the Cr(CO)₃ moiety are now investigated. Tricarbonyl(chlorobenzene)chromium reacts at room temperature with Pd(PPh₃)₄ or Pd(dba)₂ and PR₃ via an oxidative addition to give aryl-palladium complexes of the type {Pd[(η⁶-C₆H₅)Cr(CO)₃](Cl)L _y}_2/y, which are monomers (L = PPh₃ (1), PEt₂Ph (4), PPh₂Et (5)) or dimers (L = PPh₃ (2), PCy₃ (3)). By comparison, the oxidative addition of PhCl to Pd(PPh₃)₄ is known to proceed at 140 °C, indicating that the coordination of Cr(CO)₃ to PhCl greatly enhances the rate of this reaction. However, this reaction does not proceed with L₂ = dppe or dpph, but the same complexes can be prepared by Cl-bridge cleavage and/or phosphine exchange with PR₃ on Pd₂[(η⁶-C₆H₅)Cr(CO) ₃]₂(μ-Cl)₂(PPh₃)₂ to give the monomers (L = PEt₂Ph (4), PPh₂Et (5); L₂ = dppe (6), dpph (7)) and the dimer (L = PCy₃ (3)). The X-ray structures of complexes trans-Pd₂[(η⁶-C₆H₅)Cr(CO) ₃]₂(μ-Cl)₂(PCy₃)₂ (3) and cis-Pd[(η⁶-C₆H₅)Cr(CO)₃]Cl (dppe) (6) have been determined. Crystal data for 3: orthorhombic with the space group Pccm; a = 18.486 (8) A?, b = 20.401 (7) A?, c = 17.255 (6) A?, V = 6508 A?³, Z = 8. Crystal data for 6: monoclinic with the space group P2₁/n, a = 9.841 (2) A?, b = 12.783 (5) A?, c = 28.377 (2) A?, β = 92.23 (1)°, V = 3567 A?³, Z = 4. These aryl-palladium complexes react at room temperature with CO under atmospheric pressure (L = PPh₃, PEt₂Ph; L₂ = dpph) or 5 bar (L = PCy₃) to give the corresponding monomeric or dimeric acyl-palladium complexes of the type {Pd[(η⁶-C₆H₅CO)Cr(CO) ₃](Cl)L_y}_2/y (y = 2: L = PPh₃ (8), PEt₂Ph (9), L₂ = dpph (10); y = 1: L = PPh₃ (11), PCy₃ (12)). The reaction does not proceed with L₂ = dppe even under 30 bar of CO; Cl-bridge cleavage and phosphine exchange with dppe on Pd₂[(η⁶-C₆H₅CO)-Cr(CO) ₃]₂(μ-Cl)₂(PPh₃)₂ enable the formation of cis-Pd[(η⁶-C₆H₅CO)Cr(CO) ₃]Cl(dppe) (13). Methanolysis of complexes Pd[(η⁶-C₆H₅CO)Cr(CO) ₃]Cl(PPh₃)₂ (8) and Pd₂[(η⁶-C₆H₅CO)Cr(CO) ₃]₂(μ-Cl)₂(PPh₃)₂ (11) in the presence of triethylamine at room temperature proceeds rapidly and leads to the formation of (η⁶-C₆H₅COOMe)Cr(CO)₃, which is the product of methoxycarbonylation of (η⁶-C₆H₅Cl)Cr(CO)₃. Complexes {Pd[(η⁶-C₆H₅)Cr(CO)₃]Cl(PPh ₃)_y}_2/y (1, 2) and {Pd[(η⁶-C₆H₅CO)Cr(CO) ₃]Cl(PPh₃)_y}_2/y (8, 11) then appear to be reasonable intermediates in the catalytic cycle of methoxycarbonylation of (η⁶-C₆H₅Cl)Cr(CO)₃. A possible mechanism for this reaction is discussed.