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].