Frustrated N-heterocyclic carbene-silylium ion Lewis pairs

摘要

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)(2)N (NTf2) by mixing the neat, liquid starting materials afforded the corresponding 2-(trimethylsilyl) imidazolium salts (1b)SiMe3X 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)SiMe3X 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) CO2SiMe3X (X = I, OTf, NTf2), (1b)C(NtBu)OSiMe3OTf and (1b)C(Ph)C(SiMe3)CCPhOTf, 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).