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

Abstract

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.