Cyclohexyl trifluoroacetates, relative

Over the past few years, the debate over the origin of the p-silicon effect on carbocations has narrowed to one of the relative magnitudes of inductive and hyperconjugative factors. Theory and experiment are finally in agreement that hyperconjugation is by far the dominant factor—29 kcal/mol calculated to be from P-stabilization ( ) versus 9 kcal/mol from induction and polarization. The realization of these effects is dramatically revealed in the SnI solvolyses of the conformationally locked cyclohexyl trifluoroacetates (OTFA) (3-5), The relative solvolysis rates at 25 °C for compounds 3-5 are 1, 4 X 10, and 2.4 X 10, respectively. Compound 4 cannot attain the necessary anti-coplanar relationship of the Si-C and C-O bonds, which is present in 5 and required for full hyperconjugative interaction with the cation formed as the C-O bond suffers heterolysis. 

Scheme 3-1 First order rate constants for ionisation, relative to cyclohexyl trifluoroacetate 97% CF3CH2OH...

In another significant study, Lambert et al. reported the following relative rates for a series of conformationally constrained cyclohexyl trifluoroacetates (Lambert, J. B., et al. 

Aluminum chloride, used either as a stoichiometric reagent or as a catalyst with gaseous hydrogen chloride, may be used to promote silane reductions of secondary alkyl alcohols that otherwise resist reduction by the action of weaker acids.136 For example, cyclohexanol is not reduced by organosilicon hydrides in the presence of trifluoroacetic acid in dichloromethane, presumably because of the relative instability and difficult formation of the secondary cyclohexyl carbocation. By contrast, treatment of cyclohexanol with an excess of hydrogen chloride gas in the presence of a three-to-four-fold excess of triethylsilane and 1.5 equivalents of aluminum chloride in anhydrous dichloromethane produces 70% of cyclohexane and 7% of methylcyclopentane after a reaction time of 3.5 hours at... 

Explain the following observation The trifluoroacetoxy group on trans-2-triflueroacetoxycyclohexyl tosylate (5) is 30 times more rate-retarding (relative to cyclohexyl tosylate) in strongly ionizing trifluoroacetic acid than in formic acid. 

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