Using frustrated Lewis pairs to explore C–F bond activation

Abstract

Our interest in C–F bond activation prompted an investigation of the reactions of PhC(O)CF₃ with a superbasic proazaphosphatrane (Verkade's base) and a corresponding FLP. The reaction of PhC(O)CF₃ with P(MeNCH₂CH₂)₃N in a 2 : 1 ratio generates [FP(NMeCH₂CH₂)₃N][PhC(O)CF₂COCF₃Ph]. While this salt was not isolable, the anion could be derivatized to allow the isolation of [PhC(O)CF₂C(OSiMe₂tBu)CF₃Ph], 1. To further probe this C–F bond activation, the FLP P(MeNCH₂CH₂)₃N/BPh₃ was allowed to react with PhC(O)CF₃, which afforded the salt [PhC(CF₂)OP(MeNCH₂CH₂)₃N][FBPh₃], 2. Insights into the mechanism of the reaction of the proazaphosphatrane with PhC(O)CF₃ have emerged from the reactivity of 2 and these have been augmented by DFT computations. Salt 2 could be readily converted to the triflate derivative 3, which was a viable synthon for further C–F bond reactivity. Sequential reaction with Na[HBEt₃] afforded the salts [PhC(CF(H))OP(MeNCH₂CH₂)₃N][O₃SCF₃], 4 and [PhC(CH₂)OP(MeNCH₂CH₂)₃N][O₃SCF₃], 5. Interestingly, the formation of 5 contrasts with the formation of [HP(MeNCH₂CH₂)₃N][PhC(CH₂)OB(C₆F₅)₃], 6 observed following the reaction of the FLP P(MeNCH₂CH₂)₃N/B(C₆F₅)₃ with PhC(O)Me. Thus these differing FLP protocols provide avenues to salts containing either an enolate derived cation or anion.