The second-order rate constants for the reactions of Umemoto's reagents I (X=H) and II (X=F) with carbanions follow the correlation lg k ₂=s _N(N+E) and allow to calculate their electrophilic reactivity parameters E. The reactivities of Togni's hypervalent iodine-based CF₃ transfer reagents do not follow this correlation.

Abstract

Kinetics of the reactions of colored carbanions (reference nucleophiles) with S-(trifluoromethyl)dibenzothiophenium ions (Umemoto's reagents) and hypervalent trifluoromethyl-substituted iodine compounds (Togni's reagents) have been determined photometrically using stopped-flow techniques. The second-order rate constants k ₂(20 °C) for the reactions of Umemoto's sulfonium ions (generation I and II) with the reference nucleophiles in DMSO follow the correlation lg k ₂(20 °C)=s _N(N+E) and can be used to determine the electrophilicity parameters E of these trifluoromethylating reagents. It is shown that the conditions reported for the reactions of Umemoto's generation I reagents with a variety of C-nucleophiles are in line with the electrophilicity parameter E≈−13 determined for these reagents. Though Togni's hypervalent iodine-based trifluoromethylation reagents do not follow this linear free energy relationship, the kinetics of their reactions with carbanions indicate that they cover the same reactivity range as Umemoto's generation I and II reagents.