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

1973; Royal Society of Chemistry; Issue: 3 Linguagem: Inglês

10.1039/p29730000318

2050-8239

Andrew Williams, Kenneth T. Douglas,

Radioactive element chemistry and processing

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.The deuterium oxide solvent isotope effect on ka 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.