KINETIC AND SPECTROSCOPIC CHARACTERIZATION OF HIGHLY REACTIVE METHANESULFONATES - LEAVING GROUP EFFECTS FOR SOLVOLYSES AND COMMENTS ON GEMINAL ELECTRONIC EFFECTS INFLUENCING S(N)1 REACTIVITY

Highly reactive methanesulfonates (mesylates, ROMs) have been prepared from 1-phenylethanol. cyclohex-2-en-1-ol. diphenylmethanol and p-methoxybenzyl alcohol by treatment with methane-sulfonyl chloride and triethylamine in dichloro- or trichloro-methane at -20 to 0 degrees C. The mesylates, characterised in solution by H-1 and C-13 NMR at -20 degrees C, were obtained in satisfactory purity (ca. 95%) in cold solutions but they decomposed by reaction with chloride, triethylamine or the parent alcohol. Rate constants for solvolyses in aqueous acetone and aqueous ethanol have been determined by a fast response conductimetric method. Product selectivities for solvolyses of p-methoxybenzyl mesylate in aqueous ethanol and methanol at 0 degrees C have been determined by HPLC. From additional new or literature kinetic data for solvolyses of corresponding bromides, chlorides and p-nitrobenzoates (OPNB). Br/Cl, OMs/Br and OMs/OPNB rate ratios were calculated; the results are consistent with electronic effects stabilising the carbocationic transition states and increasing OMs/Br rate ratios for these S(N)1 solvolyses; none of the evidence supports a geminal electronic effect on Br/Cl rate ratios (e.g. caused by stabilisation of the initial state in p-methoxybenzyl chloride). Steric effects on ester/halide rate ratios for solvolyses of tertiary substrates are confirmed. Relative rates over a 10(16) range for ester and halide leaving groups are evaluated for solvolyses of 1-phenylethyl substrates in 80% ethanol-water, updating previous work by Noyce et al. (1972).