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

The addition of water to nine silenes (H₂SiCH₂ (1), Me₂SiC(SiH₃)₂ (2), Cl₂SiCH₂ (3), Me₂SiCMe₂ (4), (H₃Si)₂SiCMe₂ (5), (H₃Si)₂SiC(Me)OSiH₃ (6), Me₂SiC(SiMe₃)H (7), Me(HCC)SiCH₂ (8), and Me(Me₃Si)SiCH₂ (9)) 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 (1−4) exhibit low and even negative activation energies (−3 to 8 kcal/mol). Substituents that strongly reduce the polarity of the silene, as in 5 and 6, increase significantly the activation energy for the nucleophilic addition of H₂O to ca. 16 kcal/mol. The calculated activation energies show a good correlation with Δt (Δt = 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.