Alternative Si–F Elimination and Fluoride Abstraction Pathways in Selective Monodefluorination of Me₃SiCF₃ and Me₃SiCF₂H

The silyl reagents Me3SiCF3 (Ruppert–Prakash reagent) and Me3SiCF2H are commonly activated via desilylation to install CF3 and CF2H groups. We explore the C–F activation of these reagents using phosphine and pyridine Lewis bases in combination with boron and/or silicon based Lewis acids to give products of the form [Me3SiCFR(LB)][NTf2] or [HCFR(LB)][NTf2] (R = F or H; LB = P(o-Tol)3, PPh3, PPh2(C6F5), 2,4,6-Ph3-NC5H2). The reaction mechanism is investigated, and we find that it is likely that Me3SiCF3 decomposes to Me3SiF and difluorocarbene, the latter of which reacts with the LB to form a difluoromethylide that reacts with Me3SiNTf2 or adventitious protons. However, Me3SiCF2H undergoes fluoride abstraction and transfer of the [Me3SiCFH]+ fragment to the LB, in a mechanism reminiscent of frustrated Lewis pair activation of fluoroalkanes. The C–F activated silyldifluoromethyl phosphonium salts are shown to facilitate further derivatization through the silyl and phosphino motifs, while the difluoromethylpyridinium salt is shown to be a latent source of difluoromethyl in nucleophilic transfer and redox alkylation reactions.