Zwitterionic and Donor-Stabilized N-Heterocyclic Silylenes (NHSis) for Metal-Free Activation of Small Molecules

This account describes recent progress (>2006) in the synthesis and structural characterization of isolable N-heterocyclic silylenes (NHSi's) and their fascinating reactivities with respect to an emergent topic in main-group chemistry: metal-free small-molecule activation. Since the seminal discovery of stable N-heterocyclic silicon analogues of nucleophilic Wanzlick−Arduengo-type carbenes in 1994, new types of NHSi's have emerged with unique electronic features and strikingly different reactivities. Among them, the first zwitterionic (ylide-like) silylene LSi: (L = CH[(CCH2)CMe][NAr]2; Ar = 2,6-Pri2C6H3) and unprecedented N-heterocyclic bis(silylenes) with amidinate ligands and Si(I)−Si(I) bonds were synthesized. Their striking electronic structures open new doorways to metal-free activation of C−H, C−X, Si−X, E−H (E = group 15, group 16 elements), P−P, E−O (E = C, N), and E−E bonds (E = O, S, Se, Te).