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)(2)Si=CMe2 (5), (H3Si)(2)Si=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 Deltat (Deltat = 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.