The relative importance of ground-state conformation and orbital orientation in intramolecular catalysis: hydrolysis of aryl hydrogen biphenyl-2,2′-dicarboxylates

Abstract

The pH-rate profile for the hydrolysis of 4-nitrophenyl hydrogen biphenyl-2,2′-dicarboxylate (diphenate) shows a pH-independent region in the pH range 5–9, and obeys rate law (i). Compared with 4-nitro-k_obs=k_aa_H//_Ka+aH+K_w//_aH·k_OH+k_bK_a//_Ka+aH(i) phenyl benzoate, the hydrolysis of 4-nitrophenyl hydrogen diphenate proceeds 800 times faster at pH 6.0. This is explained by invoking intramolecular nucleophilic catalysis by the neighbouring carboxylate ion. The negligible solvent isotope effect at pH 6.0 (k_H2O/k_D2O 1.22) and the low negative entropy of activation (-7.2 cal K^–1 mol^–1), the high value of the Hammett reaction constant (ρ 2.19) and Brönsted constant (β–0.95) for variations in the leaving group provide evidence for nucleophilic catalysis. The effective molarity for the intramolecular cyclisation of 4-nitrophenyl hydrogen diphenate has been calculated to be ca. 200, which is close to that observed for this phenomenon in glutarates and quite low in comparison with that obtained for succinates. This has been taken to indicate that the reactivity of diphenic acid esters is governed more by steric factors which affect the proper orientation of the orbitals than the ground-state conformation arising out of restriction of rotamer distribution.