Pivotal Neighbouring Group Participation in Substitution vs Elimination Reactions: Computational Evidence for Ion Pairs in the Thionation of Alcohols with Lawesson's Reagent

The mechanism of the thionation of alcohols with Lawesson's reagent was explored through quantum chemical DFT methods. Evidence that carbocations are involved was found. The mechanism is completely different to that established recently for the thionation of carbonyls. The presence of a phenyl ring at the alcoholic carbon atom exerts a pivotal role in the stability of the carbocation and enables a S-pi interaction, which is crucial for favoring thionation instead of undesired elimination. The influence of the solvent was also studied, and elimination is preferred in toluene, whereas substitution leading to thionation is favored in acetonitrile. A clear correlation between solvent polarity and elimination was observed. The study is in complete agreement with the different behavior observed experimentally for primary, secondary, and tertiary alcohols (bearing a phenyl ring or not), and the best conditions for minimizing undesired elimination can be predicted.