Ion Pair Dynamics: Solvolyses of Chiral 1,3-Diarylallyl Carboxylates as a Case Study

摘要

Chiral ion pairs play a key role in modern enantioselective synthesis, though little is known about their properties. We have now used the special features of unsym-metrically substituted allyl derivatives to obtain unprecedented insight into ion pair dynamics. By employing chiral high-performance liquid chromatography, it was possible to follow the time-dependent concentrations of all four isomeric esters (two regioisomeric pairs of enantiomers) and all four isomeric alcohols generated during the hydrolysis of enantiopure l-(4-chlorophen-yl)-3-phenylallyl and 3-(4-chlorophenyl)-l-phenylallyl 4-nitro-benzoates. Combination of these results with the directly measured rate constant for the reaction of the laser-flash photolytically generated l-(4-chlorophenyl)-3-phenylallyl cation with water provided a complete mechanistic scheme for allyl carboxylate solvolysis. It is demonstrated that solvolysis and internal return can be explained by the same intermediates. The correlation equation log k = s_N(N + E) was used to elucidate the variable importance of external and internal return in the solvolysis reactions. This information will be crucial for the interpretation of the ultrafast dynamics of ion pairs generated by femtosecond laser pulses.