Synthesis and properties of the first stable silylene - Isocyanide complexes

Article abstract of DOI:10.1002/chem.200204643

The first stable silylene - isocyanide complexes, [Tbt(Mes)SiCNAr] (5c: Ar = Tip, 5d: Ar = Tbt, 5e: Ar = Mes*; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = mesityl, Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-tert-butylphenyl) were successfully synthesized by the reaction of a kinetically stabilized disilene, [Tbt(Mes)Si=Si(Mes)Tbt] (1), with bulky isocyanides, ArNC (3c - e). The spectroscopic data of 5c - e and theoretical calculations for a model molecule indicated that 5c - e are not classical cumulative compounds but the first stable silylene-Lewis base complexes. The reactions of 5c - e with triethylsilane and 1,3-dienes gave the corresponding silylene adducts, and they underwent isocyanide-exchange reactions in the presence of another isocyanide at room temperature. These results indicate dissociation of complexes 5c - e to the corresponding silylene 2 and isocyanides 3c-e under very mild conditions. The reaction of 5c with methanol gave the MeOH adduct 16, [Tbt(Mes)SiHC(O-Me)NTip], which has a hydrogen atom on the silicon atom. This regioselectivity can be explained in terms of the contribution of zwitterionic resonance structures D and E, which have an anion on the silicon atom. This result indicates that 5c is not a classical cumulene having Si=C double bonds that should react with methanol to give adducts bearing a methoxyl group on the silicon atom. Although the reactions of 5c - e with electrophilic reagents such as methanol, hydrogen chloride, and methyl iodide gave the formal silylene adducts, the studies on the reaction mechanism by trapping experiments and the observation of the intermediate suggested that the reaction mainly or partially proceeds by initial nucleophilic attack of the silicon atom, as is the case in the formation of 16 in the reaction of 5c with methanol. It was revealed that 5c - e show the nucleophilicity of the silicon atom, most likely resulting from the contribution of the zwitterionic resonance structures D and E.