A Study of Solvent Effects on the Rates of Solvolyses of Pinacolyl Derivatives

Article abstract of DOI:10.1021/jo00246a032

The solvolysis rates of RCH(O3SAr)C(CH3)3 (4a, R = Et, Ar = p-BrPh; 4b, R = i-Pr, Ar = p-BrPh; 4c, R = t-Bu, Ar = p-Tol) and CF3CH(O3SR)C(CH3)3 (6-OBs, R = p-BrPh; 6-OTf, R = CF3) have been determined in mixtures of ethanol and water (the E-W solvent series) and acetic acid and formic acid (the A-F solvent series).Correlations of the rate data by eq 1 neophyl-OTs + c> showed that 4a,b responded similarly to pinacolyl brosylate (1) to the examined solvent effect, yielding separate E-W, A-F regression lines, but with decreased dispersion with increased steric bulk of R.For compound 4c a linear correlation with eq 1 was obtained.These results are interpreted in terms of steric hindrance to electrostatic solvation of the incipient carbocation.The reactivity of the CF3-substituted sulfonate 6-OBs is greatly depressed.The substrate failed to react in the E-W solvent series.Added salt produced enhanced rates of solvolysis of 6-OBs in 25percent AcOH-75percent HCOOH.These results suggest an S_N2-like mechanism with very strong electrophilic solvent assistance in the transition state.However, since the solvolysis reactions of 6-OBs are attended with kinetic complexities, the data do not allow a detailed mechanistic interpretation.The solvolytic behavior of 6-OTf stands in sharp contrast to that of 6-OBs.For example, added nucleophilic salts cause only small increases in the rates of solvolysis of 6-OTf in both 70percent EtOH-30percent H2O and 25percent AcOH-75percent HCOOH.Furthemore, the solvolysis rate constants of 6-OTf in all solvents examined correlate with those of 2-adamantyl triflate.These data support a k_Δ mechanism for 6-OTf and are discussed in terms of the decreased importance of electrostatic solvation of the forming carbocation from 6-OTf than from pinacolyl brosylate.