N-Heterocyclic Carbenes with Polyfluorinated Groups at the 4- and 5-Positions from [3 + 2] Cycloadditions between...

We herein report the formation of fluorinated N-heterocyclic carbenes (NHC^Fs) that bear fluorine atoms at the 4- and 5-positions of the imidazol-2-ylidene ring. Treatment of sodium N,N'-bis(aryl)formamidinates with tetrafluoroethylene followed by the addition of LiBF₄ induced a [3 + 2] cycloaddition to afford 4,5-difluorinated imidazolium salts, which served as the precursors for 4,5-difluorinated NHCs. A key feature of this procedure is its applicability to other perfluorinated compounds, which enabled us to incorporate polyfluorinated functional groups at 4- and 5-positions on the imidazol-2-ylidene skeleton. Thus, employing octafluorocyclopentene and hexafluorobenzene led to the formation of 4,4,5,5,6,6-hexafluoro-1,3-diaryl-3,4,5,6-tetrahydrocyclopenta[d]imidazolium (CypIPr^F·HBF₄) and 4,5,6,7-tetrafluoro-1,3-diarylbenzimidazolium (BIPr^F·HBF₄) salts, respectively. A thorough NMR analysis of these NHC^Fs, their selenium adducts, and their tricarbonyl nickel complexes, (NHC^F)Ni(CO)₃, demonstrated that the fluorine substituents, contrary to expectations, tend to act as electron donors owing to the considerable positive mesomeric effect, while the perfluorocyclopentene-fused and tetrafluorobenzo-fused rings are pure electron acceptors due to their strong negative inductive effect. The unique and increased π-accepting character of the perfluorocyclopentene-fused and tetrafluorobenzo-fused NHC^Fs in both stoichiometric and catalytic reactions is further demonstrated by employing (NHC^F)Ni(CO)₃ and (NHC^F)AuCl species, respectively. Moreover, an analysis of the % buried volume (%V_bur) values clearly suggests that the modification of the NHC backbone with polyfluorinated groups can drastically alter the electronic properties of the NHC ligand without substantially changing its steric properties. Our experimental results were further corroborated by a series of computational calculations.