Troposelective substitutions in microsolvated systems

The mechanism and the stereochemistry of the intracomplex "solvolysis" of t he proton-bound complexes I-X between (CH3OH)-O-18 and (R)-(+)-1-aryl-ethan ol (1(R)(X); aryl = phenyl (X = H); pentafluorophenyl (X = F)) have been in vestigated in the gas phase in the 25-100 degreesC temperature range. The r esults point to intracomplex "solvolysis" as proceeding through the interme diacy of the relevant benzyl cation IIIX (a pure S(N)1 mechanism), "Solvoly sis" of I-H leads to complete racemization at T > 50 degreesC, whereas at T < 50 <degrees>C the reaction displays a preferential retention of configur ation. Predominant retention of configuration is also observed in the intra complex "solvolysis" of I-F. This picture is rationalized in terms of diffe rent intracomplex interactions between the benzylic ion IIIX and the nucleo phile/leaving group pair, which govern the timing of their reorientation wi thin the electrostatic complex. The obtained gas-phase picture is discussed in the light of related gas-phase and solution data. It is concluded that the solvolytic reactions are mostly governed by the lifetime and the dynami cs of the species involved and, if occurring in solution, by the nature of the solvent cage. Their rigid subdivision into the S(N)1 and S(N)2 mechanis tic categories appears inadequate, and the use of their stereochemistry as a mechanistic probe can be highly misleading.