GENERATION OF ALPHA-KETO CATIONS - QUANTITATIVE ASPECTS

exo-3-Methylbicyclo[2.2.1]heptan-2-on-endo-3-yl tresylate (4) undergoes acetolysis to give two unrearranged products, ketone 5 and keto acetate 6, along with rearranged keto acetate 7 and ketone 8. These products are suggested to arise via the intermediacy of a discrete-keto cation 9. The α-methyl-d3isotope effect is 1.47 and suggests an extremely large demand for hyper onjugative stabilization. The rate-retarding effect of the a-keto group is estimated at 107.3in this system. exo-3-Arylbicyclo[2.2.1]heptan-2-on-endo-3-yl mesylates and trifluoroacetates 19-24 also give solvolysis products derived from α-keto cations. These benzylic α-keto cations are not prone to rearrange when the aryl group is phenyl, p-toluyl, p-thioanisyl, or p-anisyl, but rearrangement can occur with the p-trifluoromethyl substituent. The effect of aryl substituents on the rate of solvolysis suggests a ρ+value of about -7.1, again implying a large demand for aryl stabilization. Bicyclo[2.2.1]heptan-2-on-exo-3-yl triflate (34) also solvolyzes readily to give products consistent with neighboring σ participation and not via a discrete secondary α-keto cation. A similar kΔ process also accounts for the rapid solvolysis of the triflate derivative of pivaloin, 39. The triflate and mesylate derivatives of α-hydroxycyclohexanone are very labile and gave unrearranged acetolysis products. A deuterium-labeling study supports a mechanism involving enolization followed by solvolysis of an enol allylic triflate. © 1979, American Chemical Society. All rights reserved.