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 [Me 3 SiX, X = I, CF 3 SO 3 (OTf), (CF 3 SO 2) 2 N (NTf 2)] by mixing the neat, liquid starting materials afforded the corresponding 2-(trimethylsilyl)imidazolium salts [(1b)SiMe 3 ]X as highly reactive, white crystalline solids. Only the triflimidate (X = NTf 2) proved to be stable in solution and could be characterized by means of NMR spectroscopy (in C 6 D 5 Br) and X-ray diffraction analysis, whereas dissociation into free 1b and Me 3 SiOTf 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)SiMe 3 ]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)CO 2 SiMe 3 ]X (X = I, OTf, NTf 2), [(1b)C(NtBu)OSiMe 3 ]OTf and [(1b)C(Ph)C(SiMe 3)CCPh]OTf, respectively. Upon reaction with [AuCl(PPh 3)], metal-chloride bond activation was observed, with formation of the cationic gold(i) complexes [(1b)Au(PPh 3)]X (X = OTf, NTf 2).