Several α-(trifluoromethyl)- and α-(pentafluoroethyl)carbinols were synthesized by the addition of perfluoroalkylating agents (TMSCF3, TMSC2F5, and/or C2F5Li) to polycyclic ketones. An improved procedure for the preparation of α-(perfluoroalkyl)trimethylsilyl ethers from (perfluoroalkyl)- trimethylsilanes and ketones was developed to facilitate the synthesis of the compounds of interest. All of the alcohols featured the bicyclo[2.2.1]heptyl skeleton or some analog of this structure containing double bonds or cyclopropyl groups. Sulfonate esters of the carbinols were prepared, and these sulfonates were solvolyzed in different solvents to examine the competition between destabilization of the carbocationic intermediates by perfluoroalkyl groups and stabilization by neighboring-group participation. The extent of the destabilization of the cations was gauged by the difference in rates of solvolysis of the α-hydrogen and α-(perfluoroalkyl) derivatives. The kα-H/kα-RF ratios ranged from 8 to ~104, and the extent of anchimeric assistance that occurred in each system influenced the difference in rates. The pentafluoroethyl group exerted a slightly smaller rate-retarding effect when compared to the trifluoromethyl group (kα-C2F5/kα-CF3 = 1.1–8.0). The products of the solvolysis reactions revealed a general trend of destabilization of both localized and delocalized cations by perfluoroalkyl groups.
ASJC Scopus subject areas
- Organic Chemistry