Studies of nucleophilic displacement at phosphorus in heteroarylphosphorus compounds.

The course of the alkaline hydrolysis of methyl-2-(1-methylpyrrolyl)-diphenylphosphonium iodide proceeds with exclusive loss of 1-methylpyrrole whilst in the corresponding alkaline hydrolysis of benzyltri-2-(1-methylpyrrolyl) phosphonium bromide cleavage of the heterocyclic substituent occurs to a minor extent; both toluene and 1-methylpyrrole are formed in a 3:2 molar ratio. These results indicate that the stabilities of the forming carbanions are in the order benzyl > 2-(1-methylpyrrolyl) > phenyl, in contrast with earlier studies on related 2-furyl and 2-thienyl systems which have shown the heteroaryl carbanions to be more stable than either benzyl or phenyl carbanions. The ability of the heterocyclic systems to stabilise the forming negative charge in the 2-position is discussed in terms of the sigma-inductive effect of the heteroatom and the electron moment of each ring system and is further related to the direction of the dipole moment of each heteroaryl group. The rates of alkaline hydrolysis of methyltri-2-(1-methylpyrrolyl)- phosphonium iodide and triphenyl(1-methylpyrrol-2-yl)methylphosphonium iodide have been determined and compared with the corresponding data for 2-furyl- and 2-thienylphosphonium salts. The findings are discussed in terms of the stabilities of the forming carbanions in the rate-determining step of the reaction, and also with regard to the electronic effect of the substituent on the position of the equilibria which precede the rate-determining step. Studies of the [31]Pn.m.r. spectra of the salts have been made, and the results are discussed in relation to the apparent electronic effects of the heteroaryl substituents on the adjacent phosphonium centre. The effect of the 2-(1-methylpyrrolyl) substituent at phosphorus on the course of the Uittig reaction in pro tic solvents has been investigated. The reaction of me thyltri-2-(1-methylpyrrolyl) phosphonium iodide and methy1-2-(1-methylpyrrolyl)diphenylphosphonium iodide with benzaldehyde in alcoholic alkoxide media proceed with loss of 1-methylpyrrole and the formation of di-2-(1-methylpyrrolyl)-beta-styrylphosphine oxide and diphenyl-beta-styrylphosphine oxide respectively, indicating that these reactions involve vinylphosphonium intermediates. In contrast, the reactions of the corresponding 2-furyl-phosphonium salts proceed via collapse of the cyclic oxaphosphetan and yield normal Wittig products. Since loss of the 1-methylpyrrolyl substituent from the intermediate vinylphosphonium salt takes place rather than migration from phosphorus to ex-carbon, a steric effect by the 1-methyl group is suspected. This is supported by results obtained for related t-butyl and o-tolyl systems. Diethyl and Diphenyl esters of 2-furyl-, 2-thienyl-, 2-(1-methylpyrrolyl)- and phenylphosphonic acids have been synthesised and a study of the kinetics of alkaline hydro lysis lias been made. The results are discussed in relation to the electronic effect of the heteroaryl substituent on the approach of the nucleophile and also on the departure of the leaving group in the reaction. The results for the hydrolysis of the corresponding carboxylic acid esters are introduced into the discussion together with infrared and [31]P n.m.r. spectroscopic data for the phosphonate esters.