Geometrical Dependence of γ-Trimethylsilyl Groups on Norbornyl Solvolyses. Rapid-Injection Kinetic Methods for Solvolyses of Unstable Mesylates

Article abstract of DOI:10.1021/jo00292a030

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 deg C show similar relative rates to those observed for solvolyses of the corresponding p-nitrobenzoates in 97percent w/w trifluoroethanol/water at 100 deg C; there are up to ca. 100-fold larger rate enhancements due to γ-silicon than those previously reported for acyclic and monocyclic systems; e.g. in ethanol 3.3 1E4 for 6-exo-(trimethylsilyl)-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 80percent ethanol/water and in 97percent trifluroethanol, 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 cmpounds establish a 2-exo/2-endo-norbornyl rate ratio of > 1E6, the largest observed for an unhindered secondary system.