Formamide-Catalyzed Nucleophilic Substitutions: Mechanistic Insight and Rationalization of Catalytic Activity

Herein, detailed mechanistic investigations into formamide-catalyzed nucleophilic substitution (SN) of alcohols are reported. Alkoxyiminium chlorides and hexafluorophosphates were synthesized and characterized as a key intermediate of the catalytic cycle. The determination of reaction orders and control experiments indicated that the nucleophilic attack of the formamide catalyst onto the reagent BzCl is the rate-determining step. Linear free energy relationship revealed a correlation between the quantified Lewis basicity strength of formamides by means of 11B NMR spectroscopy and their catalytic activity in SN-transformations. The observed difference in catalytic ability was attributed to the natural bond order charge, dipole moment, and Sterimol parameter B5. Importantly, this rationalization enables the prediction of the capacity of formamides to promote SN-type transformations in general.