The Role of the Apical Donor in the Decomposition of Copper(II) Benzoate under DOW-Phenol Conditions

Abstract

The synthesis and characterization of copper(II) benzoates with the apical donors pyridine, 2-CH3-pyridine, 2,4-(CH3)2-pyridine, 2,6-(CH3)2-pyridine, 2-fluoropyridine, 2-chloropyridine, 2-bromopyridine, 3-bromopyridine, 2,5-dibromopyridine, 3,5-dibromopyridine, and aniline, starting from copper(II) benzoate, is reported. Single-crystal X-ray structures of the products with four apical ligands show the usual paddle-wheel structure of copper(II) carboxylates; in the case of aniline no paddle-wheel dicopper(II) benzoate could be isolated. The products of thermal decomposition of the pure copper(II) compounds were analyzed by HPLC, LC-MS, and GCFID, and the expected DOW-phenol products were found in all cases other than that of aniline. This supports the assumption that a paddle-wheel dicopper(II) benzoate is required for the DOW-phenol reaction. Generally, high ortho-selectivities (to phenyl benzoate and phenol; the selectivity increases with increasing basicity) are obtained, in good agreement with earlier findings on the role of the base. Small but significant steric effects are observed in the series of methylated pyridine donors and the monohalogenated pyridine donors used as apical ligands; with the two dibromopyridine donors there are large steric effects and the DOW-phenol reaction is partially suppressed. With halogenated pyridine donors as apical ligands, a Cu[I]-catalyzed process occurs, leading to dehalogenation.