Substituent effects on the stereochemistry of gas-phase intracomplex nucleophilic substitutions.

The mechanism and stereochemistry of the intracomplex solvolysis of proton-bound complexes [Y...H...M]+ between M = CH3 (18)OH and Y = 1-arylethanol [(S)-1-(para-tolyl)ethanol (1S), (S)-1-(para-chlorophenyl)ethanol (2S), (S)-1-(meta-alpha,alpha,alpha-trifluoromethylphenyl)ethanol (3S), (S)-1-(para-alpha,alpha,alpha-trifluoromethylphenyl)ethanol (4S), (R)-1-(pentafluorophenyl)ethanol (5R), (R)-alpha-(trifluoromethyl)benzyl alcohol (6R), and (R)-1-phenylethanol (7R)] have been investigated in the gas phase (CH3F; 720 Torr) in the 25-140 degrees C temperature range. Gas-phase solvolysis of [Y...H...M]+ (Y=2S, 3S, 4S, and 7R) leads to extensive racemization above a characteristic temperature t(#) (e.g. at t(#)>60 degrees C for 7R), whereas below that temperature the reaction displays a preferential retention of configuration. Predominant retention of configuration is instead observed in the intracomplex solvolysis of [Y...H...M]+ (Y=1S, 4S, 5R, and 6R) with the temperature range investigated (25 <or= T <or= 120 degrees C). These results indicate that the intracomplex solvolysis proceeds through the intermediacy of the relevant benzylic cations (Bz+), which is electrostatically coordinated to a H2O and a CH3 (18)OH molecule (a pure SN1 mechanism). The obtained gas-phase mechanism is discussed in the light of related solution data. It is concluded that the stereochemistry of unimolecular solvolytic reactions is determined by the lifetime and the dynamics of the species involved and, if occurring in solution, by the nature and the dynamics of the solvent cage as well.