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

Abstract

Abstract Detailed kinetic and product 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°C is characterized by (1) a complete 1,2-SiMe3 rearrangement in the products, (2) m (sensitivity to the solvent ionizing power Yx) values of 0.98 (X=Cl) and 0.91 (X=Br) for the solvolyses in aq acetone, (3) α-deuterium isotope effects of 1.167 and 1.163 for 1a-Cl in 97% aq 2,2,2-trifluoroethanol (TFE) and 40% aq ethanol respectively, and (4) substituent effect expressed by a Yukawa–Tsuno LArSR equation: logk⁄kH=−3.71 (σ°+1.16 Δ\barσR+) for the solvolysis of 1a-Cl and the p-Me, m-Me, p-Cl, and m-Cl derivatives in 40% aq ethanol. These findings are consistent with kc mechanism. The relative rates for 1a-Br, α-(trimethylsilyl)benzyl bromide, and α-(t-butyldimethylsilyl)benzyl bromide are 1.07×105 : 0.54 : 1.0 in 97% aq TFE at 25°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-l-yl chloride in 97% aq TFE indicating that a β-Si-Si σ bond orthogonal to the leaving group does not enhance the rate of ionization at all.