Silicon Effects. III. Rates and Products for Solvolysis of α-(Pentamethyldisilanyl)benzyl Halides and 1,1,2,2-Tetramethyl-1,2-disilaindan-3-yl Chloride. Structure and Stability of α-(Pentamethyldisilanyl)benzyl Cation in Solution

Article abstract of DOI:10.1246/bcsj.64.3280

Detailed kinetic and products studies have been made on solvolysis of α-(pentamethyldisilanyl)benzyl halides (1a-X; X=Cl and Br) and 1,1,2,2-tetramethyl-1,2-disilaindan-3-yl chloride (2-Cl) in various solvents.The solvolysis of 1a-X at 25 deg C is characterized by (1) a complete 1,2-SiMe3 rearrangement in the products, (2) m (sensitivity to the solvent ionizing power Y_x) values of 0,98 (X=Cl) and 0,91 (X=Br) for the solvolysis in aq acetone, (3) α-deuterium isotope effects of 1,167 and 1,163 for 1a-Cl in 97percent aq 2,2,2-trifluoroethanol (TFE) and 40percent aq ethanol respectively, and (4) substituent effect expressed by a Yukawa-Tsuno LArSR equation: log k/k^H=-3,71(?⁰+1,16Δ?+_R) for the solvolysis of 1a-Cl and the p-Me, m-Me, p-Cl, and m-Cl derivatives in 40percent aq ethanol.These findings are consistent with k_c mechnism.The relative rates for 1a-Br, α-(trimethylsilyl)benzyl bromide, and α-(1-butyldimethylsilyl)benzyl bromide are 1,07*1E5:0,54:1.0 in 97percent aq TFE at 25 deg C indicating solvolytic generation of α-(pentamethyldisilanyl)benzyl cation to be ca. 7 kcal mol^-1 energetically more favorable than that of the corresponding α-(trialkylsilyl)benzyl cations.In contrast, 2-Cl solvolyzes 0,209 times less rapidly than does 2,2-dimethyl-2-silaindan -1-yl chloride in 97percent aq TFE indicating that a β-Si-Si ? bond orthogonal to the leaving group does not enhance the rate of ionization at all.