E1cB mechanisms. Part IV. Base hydrolysis of substituted phenyl phosphoro- and phosphorothio-diamidates

Abstract

Hydrolysis of substituted phenyl NN′-diphenylphosphorothiodiamidates has been measured over a range of hydroxide ion concentration (up to 2M) in 50% ethanol–water (v/v). The pseudo-first-order rate constants in ester obeyed the equation: kobs=(ka+kb[OH])/{1 +Kw/(Ka′[OH])}. The 4-nitrophenyl ester however hydrolysed according to a simple bimolecular rate law: kobs=kb[OH] which is shown to be a special case of the more complicated equation where ka=kb. Kw/Ka′. Spectroscopic titration of the esters yielded values of Kw/Ka′(for ionisation of NH) which when utilised in the above equation enabled values of ka and kb to be calculated. The ionisation constant (Kw/Ka′) has a high Hammett selectivity (ρ=–1·47, r= 0·828) in accord with considerable linkage between the substituent and NH via the O–P bond. The parameter ka(ρ=+1·37, r= 0·977) is argued to represent a unimolecular elimination of phenol from the conjugate base (monoanion) of the ester to form a phosphorothioimidate which rapidly decomposes to acid. The parameter kb, on account of its high Hammett selectivity and its dependence on σ–(ρ=+3·08, r= 0·987), represents the ionisation of the monoanion to yield a conjugate base (dianion) which eliminates a phenolate anion to give a phosphorothioimidate anion. Phenolateion character is displayed in the transition-state of the rate-determining step in kb(the elimination reaction) but not in that for ka.