Frustrated N-heterocyclic carbene–silylium ion Lewis pairs

Abstract

The reaction of the N-heterocyclic carbene 1,3-di-tert-butyl-4,5-dimethylimidazolin-2-ylidene (1b) with trimethylsilyl iodide, triflate and triflimidate [Me₃SiX, X = I, CF₃SO₃ (OTf), (CF₃SO₂)₂N (NTf₂)] by mixing the neat, liquid starting materials afforded the corresponding 2-(trimethylsilyl)imidazolium salts [(1b)SiMe₃]X as highly reactive, white crystalline solids. Only the triflimidate (X = NTf₂) proved to be stable in solution and could be characterized by means of NMR spectroscopy (in C₆D₅Br) and X-ray diffraction analysis, whereas dissociation into free 1b and Me₃SiOTf was observed for the triflate system, in agreement with the trend derived by DFT calculations; the iodide was too insoluble for characterization. The compounds [(1b)SiMe₃]X showed the reactivity expected for frustrated carbene–silylium pairs, and treatment with carbon dioxide, tert-butyl isocyanate and diphenylbutadiyne gave the 1,2-addition products [(1b)CO₂SiMe₃]X (X = I, OTf, NTf₂), [(1b)C(NtBu)OSiMe₃]OTf and [(1b)C(Ph)C(SiMe₃)CCPh]OTf, respectively. Upon reaction with [AuCl(PPh₃)], metal–chloride bond activation was observed, with formation of the cationic gold(I) complexes [(1b)Au(PPh₃)]X (X = OTf, NTf₂).