On the Stability of Six-Membered-Ring Carbenes and Silylenes

The stability of the recently synthesized six-membered-ring silylene (1) and its eight structural isomers was investigated by quantum chemical methods. The structure and stability of the three most stable isomers (a, b, c) were compared to the well-known stable Denk-silylene (3). The related analogous carbenes and germylenes were also studied. According to the calculated isodesmic reaction energies and NICS(0) values, 1 is nonarmatic, but since the missing aromaticity is compensated by the reduced ring strain, it is nearly as stable its 3. The more stable five-membered-ring isomer (b) shows considerable aromatic character. The analysis of the molecular orbitals indicate that I is less reactive in both nucleophilic and electrophilic reactions than 3. The high kinetic stability found in studying the dimerization and the complex formation with NH3 and the 1,2H-migration reaction suggest the synthesizability of b and c. On the other hand, the analogous carbene derivatives seem to be less stable in both thermodynamic and kinetic aspects.