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: k_obs=(k_a+k_b[OH])/{1 +K_w/(K_a′[OH])}. The 4-nitrophenyl ester however hydrolysed according to a simple bimolecular rate law: k_obs=k_b[OH] which is shown to be a special case of the more complicated equation where k_a=k_b. K_w/K_a′. Spectroscopic titration of the esters yielded values of K_w/K_a′(for ionisation of NH) which when utilised in the above equation enabled values of k_a and k_b to be calculated. The ionisation constant (K_w/K_a′) 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 k_a(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 k_b, 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 k_b(the elimination reaction) but not in that for k_a.

The deuterium oxide solvent isotope effect on k_a was previously suggested to be zero for the general case for the E1cB mechanism. We provide evidence to show that this is not true for some esters known unequivocally to hydrolyse via the elimination mechanism.

The corresponding phosphyl oxygen esters also show evidence of a hydrolysis reaction proceeding via the dianion.