LIGAND COUPLING AND NEIGHBORING-GROUP EFFECTS IN THE ALKALINE-HYDROLYSIS OF ARYLPHOSPHONIUM SALTS - NEW STABLE TETRAARYLPHOSPHONIUM BENZIMIDAZOLATE BETAINES

The reactions with aqueous alkali of a series of 2-(o- and p-triphenyl phosphoniophenyl)-benzimidazoles and -benzothiazoles have been investi gated. Evidence of a neighbouring-group hypervalent interaction betwee n the pyridine-like nitrogen of the benzazole and the 2-(o-triphenylph osphoniophenyl) substituent is presented from the results of a study o f 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 latt er proceeds abnormally, in some cases, with the formation of biaryl co upling products in addition to the expected hydrolysis products. Simil ar results have been obtained from a comparison of the products of hyd rolysis of o- and p-(benzoylphenyl)triphenylphosphonium salts, hydroly sis of the latter also giving rise to a biaryl coupling product, 4-ben zoylbiphenyl. Traces of 4-cyanobiphenyl were also detected in the prod ucts of hydrolysis of p-cyanophenyl(triphenyl)phosphonium bromide. The kinetics of hydrolysis of the above range Of salts has also been inve stigated in an ethanol-water solvent system (80:20, v/v). Whereas the benzoylphenylphosphonium salts undergo hydrolysis according to the usu al 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 o f the azole nitrogen with the solvent. Treatment of both 2-(o- and p-t riphenylphosphoniophenyl)benzimidazoles with 1 equivalent of aqueous a lkali results in the formation of the related tetraarylphosphonium ben zimidazolate betaines. The ortho-isomer exhibits a marked shielding of the P-31 nucleus (delta(p) - 14.7) compared with the para-isomer (del ta(p) 22.6), consistent with a hypervalent interaction of the phosphon ium centre with the benzimidazolate moiety.