Ligand coupling and neighbouring-group effects in the alkaline hydrolysis of arylphosphonium salts: new stable tetraarylphosphonium benzimidazolate betaines

Abstract

The reactions with aqueous alkali of a series of 2-(o- and p-triphenylphosphoniophenyl)-benzimidazoles and -benzothiazoles have been investigated. Evidence of a neighbouring-group hypervalent interaction between the pyridine-like nitrogen of the benzazole and the 2-(o-triphenylphosphoniophenyl) substituent is presented from the results of a study of the course of alkaline hydrolysis of the salts and a comparison with that of the related 2-(p-triphenylphosphoniophenyl) systems in which such hypervalent interactions are not possible. Hydrolysis of the latter proceeds abnormally, in some cases, with the formation of biaryl coupling products in addition to the expected hydrolysis products. Similar results have been obtained from a comparison of the products of hydrolysis of o- and p-(benzoylphenyl)triphenylphosphonium salts, hydrolysis of the latter also giving rise to a biaryl coupling product, 4-benzoylbiphenyl. Traces of 4-cyanobiphenyl were also detected in the products of hydrolysis of p-cyanophenyl(triphenyl)phosphonium bromide. The kinetics of hydrolysis of the above range of salts has also been investigated in an ethanol–water solvent system (80 : 20, v/v). Whereas the benzoylphenylphosphonium salts undergo hydrolysis according to the usual third-order rate law, both series of salts derived from 1-methyl-2-phenylbenzimidazole and 2-phenylbenzothiazole are hydrolysed according to a second-order rate law, perhaps attributable to the interaction of the azole nitrogen with the solvent. Treatment of both 2-(o- and p-triphenylphosphoniophenyl)benzimidazoles with 1 equivalent of aqueous alkali results in the formation of the related tetraarylphosphonium benzimidazolate betaines. The ortho-isomer exhibits a marked shielding of the ³¹P nucleus (δ_P– 14.7) compared with the para-isomer (δ_P 22.6), consistent with a hypervalent interaction of the phosphonium centre with the benzimidazolate moiety.