Addition of Nucleophiles to Silenes. A Theoretical Study of the Effect of Substituents on Their Kinetic Stability

The addition of water to nine silenes (H<sub>2</sub>SiCH<sub>2</sub> (<b>1</b>), Me<sub>2</sub>SiC(SiH<sub>3</sub>)<sub>2</sub> (<b>2</b>), Cl<sub>2</sub>SiCH<sub>2</sub> (<b>3</b>), Me<sub>2</sub>SiCMe<sub>2</sub> (<b>4</b>), (H<sub>3</sub>Si)<sub>2</sub>SiCMe<sub>2</sub> (<b>5</b>), (H<sub>3</sub>Si)<sub>2</sub>SiC(Me)OSiH<sub>3</sub> (<b>6</b>), Me<sub>2</sub>SiC(SiMe<sub>3</sub>)H (<b>7</b>), Me(HCC)SiCH<sub>2</sub> (<b>8</b>), and Me(Me<sub>3</sub>Si)SiCH<sub>2</sub> (<b>9</b>)) was studied with ab initio (MP4/6-31+G(d,p)) and DFT (B3LYP/6-31G(d)) methods. The energy barriers for addition, which denote the kinetic stability of the silene, strongly depend on the substituents. Silenes (<b>1</b>−<b>4</b>) exhibit low and even negative activation energies (−3 to 8 kcal/mol). Substituents that strongly reduce the polarity of the silene, as in <b>5</b> and <b>6</b>, increase significantly the activation energy for the nucleophilic addition of H<sub>2</sub>O to ca. 16 kcal/mol. The calculated activation energies show a good correlation with Δ<i>t</i> (Δ<i>t</i> = the difference in the total NBO charge between Si and C), i.e., the higher the polarity of the silene the lower is the activation barrier for water addition.