Organophosphorus intermediates. VI. The acid-catalysed reaction of trialkyl orthoformates with phosphinic acid

Abstract

Trialkyl orthoformates react with phosphinic acid to give the corresponding alkyl phosphinate and alkyl mono- or bis- (dialkoxymethyl)phosphinates; dialkyl phosphonites and phosphine, PH₃, are also formed. The formation of P-alkylated products is acid- catalysed and is believed to proceed by alkylation of the trivalent tautomer of phosphinic acid (or its ester) by the dialkoxymethyl carbonium ion arising from the ortho ester in the presence of acid. A comparison with the corresponding reactions of phosphonic acid suggests that the trivalent tautomer of phosphinic acid [HP(OH)₂] is formed much more readily than the trivalent form of phosphonic acid [P(OH)₃] and this is accounted for by proposing that the equilibria

RPH(O)OR′ +H⁺ ↔ RP⁺H(OH)OR' ↔ RP(OH)OR'+H⁺ (R = H, OH; R' = H, alkyl)

lie further to the right for R = H than for R = OH because of inductive effects. ³¹P and ¹H n.m.r. data are reported.