Bimetallic Activation in Homogeneous Catalysis. 3. Mechanistic Study of the Methoxycarbonylation of Tricarbonyl(chlorobenzene)chromium: Isolation and Reactivity of Possible Reaction Intermediates

Veronique Dufaud, Jean Thivolle-Cazat, Jean-Marie Maurice Basset*, René Mathieu, Joël Jaud, Jacqueline Waissermann

*Corresponding author for this work

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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 OC′OXO- amides. The mechanism of methoxycarbonylation of tricarbonyl(chlorobenzene)chromium, (776-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[(jj6-C6H5)Cr(CO)3](Cl)Ly|2/>, 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-CeH5)Cr(CO)3]2(μ-Cl)2(PPh3)2 to give the monomers (L = PEl2Ph (4), PPh2Et (5); L2 = dppe (6), dpph (7)) and the dimer (L = PCy3 (3)). The X-ray structures of complexes trans-Pd26-C6H6)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) Å, b = 20.401 (7) Å, c = 17.255 (6) Å, V = 6508 Å3, Z = 8. Crystal data for 6: monoclinic with the space group P21/n, a = 9.841 (2) Å, b = 12.783 (5) Å, c = 28.377 (2) Å, β = 92.23 (1)°, V - 3567 Å3, 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)Ly}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-C6H6CO)Cr(CO)3]Cl(dppe) (13). Methanolysis of complexes Pd[(η6-C6H6C0)Cr(CO)3]Cl(PPh3)2 (8) and Pd2[(η)6-C6H6CO)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-C6H5C00Me)Cr(CO)3, which is the product of methoxycarbonylation of (η6-C6H5Cl)Cr(CO)3. Complexes {Pd[(η6-C6H5)Cr(CO)3]Cl(PPh3)y}2/y (1,2) and {Pd[(η6-C6H5CO)Cr(CO)3]Cl(PPh3}2/yU (8,11) then appear to be reasonable intermediates in the catalytic cycle of methoxycarbonylation of (η6-C6H6Cl)Cr(CO)3. A possible mechanism for this reaction is discussed.

Original languageEnglish (US)
Pages (from-to)4005-4015
Number of pages11
Issue number12
StatePublished - Dec 1 1991

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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