Anion-Initiated Trifluoromethylation by TMSCF₃: Deconvolution of the Siliconate-Carbanion Dichotomy by Stopped-Flow...

The mechanism of CF₃ transfer from R₃SiCF₃ (R = Me, Et, iPr) to ketones and aldehydes, initiated by M⁺X^- (<0.004 to 10 mol %), has been investigated by analysis of kinetics (variable-ratio stopped-flow NMR and IR), ¹³C/²H KIEs, LFER, addition of ligands (18-c-6, crypt-222), and density functional theory calculations. The kinetics, reaction orders, and selectivity vary substantially with reagent (R₃SiCF₃) and initiator (M⁺X^-). Traces of exogenous inhibitors present in the R₃SiCF₃ reagents, which vary substantially in proportion and identity between batches and suppliers, also affect the kinetics. Some reactions are complete in milliseconds, others take hours, and others stall before completion. Despite these differences, a general mechanism has been elucidated in which the product alkoxide and CF₃^- anion act as chain carriers in an anionic chain reaction. Silyl enol ether generation competes with 1,2-addition and involves protonation of CF₃^- by the α-C-H of the ketone and the OH of the enol. The overarching mechanism for trifluoromethylation by R₃SiCF₃, in which pentacoordinate siliconate intermediates are unable to directly transfer CF₃^- as a nucleophile or base, rationalizes why the turnover rate (per M⁺X^- initiator) depends on the initial concentration (but not identity) of X^-, the identity (but not concentration) of M⁺, the identity of the R₃SiCF₃ reagent, and the carbonyl/R₃SiCF₃ ratio. It also rationalizes which R₃SiCF₃ reagent effects the most rapid trifluoromethylation, for a specific M⁺X^- initiator.