GEOMETRICAL DEPENDENCE OF GAMMA-TRIMETHYLSILYL GROUPS ON NORBORNYL SOLVOLYSES - RAPID-INJECTION KINETIC METHODS FOR SOLVOLYSES OF UNSTABLE MESYLATES

Solvolytic rate constants in ethanol and aqueous ethanol mixtures are reported for solvolyses of unstable mesylates, prepared in situ from tert-butyl alcohol, and the following 6-trimethylsilyl (TMS) substituted 2-exo-norbornanols: 6-exo- and 6-endo-(trimethylsilyl)-substituted and 6,6-bis(trimethylsilyl)-substituted. These kinetic data for ethanol at 25 °C show similar relative rates to those observed for solvolyses of the corresponding p-nitrobenzoates in 97% w/w trifluoroethanol/water at 100 °C; there are up to ca. 100-fold larger rate enhancements due to 7-silicon than those previously reported for acyclic and monocyclic systems; e.g. in ethanol 3.3 × 104 for 6-exo-(tri-methylsilyl)-2-exo-norbornyl mesylate; these results support recent experimental and theoretical studies showing that a W conformation is preferred. In contrast, for solvolyses of the corresponding 2-endo-brosylates in 80% ethanol/water and in 97% trifluoroethanol, the 6-exo-TMS substituent shows only a 2-4-fold rate enhancement, and the 6-endo-TMS substituent shows rate retardation. Additional rate constants are reported for conventional solvolyses of mesylates of 1-adamantanol, 2-exo-norbornanol, and 6-exo-(trimethylsilyl)-2-endo-norbornanol. These data for 6-exo-TMS compounds establish a 2-exo-/2-endo-norbornyl rate ratio of <106, the largest observed for an unhindered secondary system. © 1990, American Chemical Society. All rights reserved.