Decarbonylation of phenylacetic acids by high valent transition metal halides

Abstract

Triphenylacetic acid underwent unusual decarbonylation when allowed to react with a series of halides of group 4–6 metals in their highest oxidation state, in dichloromethane at ambient temperature. Thus, the reaction of CPh₃COOH with MoCl₅, in 1 : 1 molar ratio, afforded the trityl salt [CPh₃][MoOCl₄], 1, in 79% yield, while the 1 : 2 reaction of CPh₃COOH with NbF₅ afforded [CPh₃][NbF₆], 2, in 70% yield, NbOF₃ being the metal co-product. CPh₃COOH reacted with NbCl₅, TiF₄ and WOCl₄ to give mixtures of compounds, however the cation [CPh₃]⁺ was NMR identified in each case. [CPh₃][NbCl₆], 3, was isolated from NbCl₅ and CPh₃COCl, prior to being generated from CPh₃COOH and PCl₅. The reaction of CPh₃COOH with TiCl₄ was non-selective, and the salt [CPh₃][Ti₂Cl₈(μ-κ²-O₂CCPh₃)], 4, was obtained in 18% yield. The decarbonylation reactions of CMePh₂COCl and CMe₂PhCOCl by means of NbCl₅ led to the indanes 5a–b, which were isolated in 79–97% yields after hydrolysis of the mixtures and subsequent alumina filtration of the organic phases. The reactions of CH(Ph)₂COOH with NbCl₅ and WCl₆ afforded NbCl₄(OOCCHPh₂), 6, and CHPh₂COCl, respectively, as the prevalent species. CPh₂(CH₂CH₂Br)COOH did not undergo CO release when allowed to interact with WCl₆, instead selective intramolecular condensation to C(Ph)₂C(O)OCH₂CH₂, 7, occurred. MeCCCOOH underwent hydrochlorination by WCl₆ to give MeC(Cl)CHCOOH, 8, in 72% yield. All the products were fully characterized by elemental analysis, IR and multinuclear NMR spectroscopy. In addition, the solid state structures of 1, 2, 4, 7, and 8 were elucidated by X-ray diffraction.