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-Pr'2C6H3) 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 IS, group 16 elements), P-P, E-O (E = C, N), and E E bonds (E = O, S, Se, Te).