Hydrolysis of secondary alkyl sulphonates. SN2 reactions with high carbocation character

Abstract

Rate constants for hydrolysis of various secondary methanesulphonates (mesylates, ROMs) in water have been determined, and rate constants for 2-adamantyl mesylate (I; X = OMs) in <0.1% v/v. acetone–water have been used to estimate a rate constant for 2-adamantyl mesylate in pure water at 25°. Rate ratios for selected tosylates and mesylates have been evaluated and range from ca. 0.8 in 50% v/v ethanol–water to 2.1 in water but do not appear to depend markedly on the nature of the secondary alkyl group. The estimated rate constant for 2-adamantyl tosylate in water is 3.1(5)× 10^–4 s^–1 and leads to values of the following solvent parameters: solvent ionizing power, Y_OTs(4.1) and solvent nucleophilicity, N_OTs(–0.44). The magnitudes of nucleophilic solvent assistance, calculated using [k_ROMs/k_2–AdOMs]_H2O/[k_ROTs/k_2–AdOTs]_CF3CO2H, range from 48 for propan-2-yl to 9 for cyclohexyl, and are very similar to those calculated for formic acid. The unbranched substrates (propan-2-yl, butan-2-yl, pentan-2-yl, pentan-3-yl, and heptan-4-yl) give a good correlation with ∑σ* with a slope (–4.03), slightly greater than that for formic acid (–3.35) but much less than that for trifluoroacetic acid (–7.21). These and other results suggest that these unbranched substrates hydrolyse via S_N2 like transition states with high carbocation character. The rate ratio (exo/endo) for hydrolysis of 2-norbornyl systems (IV) and (V) is 2 550, the highest reported at 25°. By comparison with cyclopentyl mesylate as a model system it is suggested that 2-exo-norbornyl systems are solvolysed anomalously rapidly and 2-endo-norbornyl systems anomalously slowly.