Kinetics and mechanisms of the reactions of benzoyl derivatives of nucleophiles: dependence of the solvation requirement of the reaction on the structures of the nucleophile and the acyl group

The reactions of two series of benzoyl derivatives of nucleophiles were investigated and the results were compared with those for N-benzoylimidazole. The general structure is ArCO-Nu, where Ar = X-C 6 H 4 - and/or X 2 -C 6 H 3 -; X=4-Me, H, 4-Cl, 4-CN, 4-NO 2 and X 2 =3,5-dinitro; Nu=iodosobenzoate, ArCO-Iba, and phosphate dianion, ArCO-Phos. Catalytic rate constants, kinetic solvent isotope effects, kinetic substituent effects (Hammett equation) and the dependence of ΔH ≠ and ΔS ≠ on σ x were determined. For ArCO-Iba, the hydrolysis occurs via two pathways, uncatalyzed (k H 2 O ) and specific base-catalyzed (k O H ) water attack on the iodine atom of the iodosobenzoate ring. This conclusion is based on theoretical calculations of the partial charges on ArCO-Iba, and the small /) values calculated at 25 °C, -0.22 and 0.92 for k H 2 O and k O H , respectively. The data for the reaction of ArCO-Phos are consistent with a dissociative transition state, leading to elimination of the metaphosphate monoanion (PO - 3 ). The dependence of the mechanistic pathway on the nucleophile is discussed. Two results are relevant to the reactions of ArCO-Iba: (i) moderate to large substituent effects on the activation entropies suggest that solvation of the leaving benzoate anion and desolvation of the entering nucleophile contribute to the OH - -mediated reaction; (ii) the negative and positive signs of ΔS ≠ indicate large differences in solvation of the transition states of the k H 2 O and k O H pathways. For the spontaneous decomposition of ArCO-Phos, sizeable substituent effects on both ΔH ≠ and ΔS ≠ were observed. This shows the contribution of solvation of the leaving benzoate and substituent-induced shift of the structure of the transition state.