Mechanism of Anion-Catalyzed C–H Silylation Using TMSCF₃: Kinetically-Controlled CF₃‑Anionoid Partitioning As a Key Parameter

The mechanism of anion-catalyzed C–H silylation by R₃SiCF₃ reagents has been investigated using homogeneous TBAT-initiation, in situ and stopped-flow ¹⁹F NMR spectroscopy ²H-KIE, LFER, deuterium-labeled crossover, structure-selectivity quantitation (TMSCF₃/TESCF₃), carbene trapping, and DFT-calculations. Analysis of the kinetics of reactions of 1,3-difluorobenzenes (2), and the generation of ArSiMe₃ and Me₃SiF as a function of the concentration of [2], [TMSCF₃], and [TBAT], show that a CF₃-anionoid is the active intermediate. The CF₃-anionoid is reversibly released from siliconate [(CF₃)₂SiMe₃]⁻ and undergoes partitioning through rate-limiting arene deprotonation (¹H/²H KIE 9.5) to generate ArSiMe₃ (via a transient aryl anionoid) and fluoroform (CF₃H), in competition with F-anion transfer to TMSCF₃ to generate CF₂ and TMSF. The [2]/[TMSCF₃] concentration ratio directly and proportionally controls the kinetics of the partition, in favor of C–H deprotonation. Higher concentrations of TBAT and lower concentrations of TMSCF₃ lead to faster rates of ArSiMe₃ generation. Use of the homologous TESCF₃ reagent leads to faster rates of anion catalysis and an increased selectivity toward C–H deprotonation. Perfluoroalkenes, generated in situ from CF₂, capture the CF₃-anionoid leading to progressive inhibition of the anion-catalysis. Inhibition is suppressed by using a styrene additive to trap the CF₂ and the efficiency of the process enhanced by slow-addition of TMSCF₃ (1) to maintain a high concentration ratio [2]/[1].