The chemistry of heteroarylphosphorus compounds. Part II. The importance of inductive effects on pre-equilibria in the alkaline hydrolysis of heteroarylphosphonium salts; phosphorus-31 nuclear magnetic resonance studies

Abstract

The alkaline hydrolysis of the heteroarylphosphonium salts, tri-(2-furyl)methylphosphonium iodide (III; R = Me, X = I) and benzyltri-(2-furyl)phosphonium bromide (III; R = PhCH₂, X = Br) results in cleavage of a phosphorus–furan bond to produce di-(2-furyl)methylphosphine oxide (IV; R = Me) and benzyldi-(2-furyl)phosphine oxide (IV; R = PhCH₂), respectively. Preliminary rate studies show that these reactions proceed ca. 10²—10³ times faster than the hydrolysis of the corresponding tri-(2-thienyl)phosphonium salts, which have been shown previously to undergo hydrolysis ca. 10⁸ times faster than the phenyl analogues. The greater rate of hydrolysis of the heteroarylphosphonium salts compared to the phenyl analogues is attributed to the greater electron-withdrawing character of the heteroaryl substituent, which increases from 2-thienyl to 2-furyl. Evidence for this has been adduced from a comparison of pK_a data for the analogous carboxylic acids, and also from a study of ³¹P n.m.r. chemical shifts of the phosphonium salts and a related series of phosphonate esters. The electron-withdrawing heterocyclic ring systems cause increases in the equilibrium constants of the pre-equilibria involved in the hydrolysis reactions; in addition, the heteroaryl carbanions eliminated in the rate-determining step are more stable than the phenyl carbanion. The question of 3d orbital stabilisation of the 2-thienyl carbanion is discussed. The equilibria between a series of phosphonium salts (V) and methoxide ion, in absolute methanol, to form the methoxyphosphoranes (VI) have been studied by ³¹P n.m.r. techniques. The position of equilibrium has been shown to depend on the electron-withdrawing character of the groups attached to phosphorus.