Towards a complete account of the exchange chemistry of a diastereotopic proton pair. I. Base-catalyzed enolization–exchange of 2-norbornanones; on the rate controlling factors
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abstract
We report the rates of base-catalyzed hydrogen isotope exchange of the exo and endo protons in a series of 2-norbornanones. The exo:enclo rate ratios for 4-methyl-substituted 2-norbornanones 3 and 6 are less than the rate ratios of the 4-demethyl analogs 2 and 5. Therefore, a methyl group is not a useful torsional-effect probe. This is supported by an analysis which established that the small difference in the torsional strain (0.45 kcal) between eclipsed vicinal hydrogens and an eclipsed vicinal methyl and hydrogen would only resuit in a maximum twofold increase in the rate ratio. The decrease in the exo:endo rate ratio is a consequence of the electron-donating effect of a methyl group which affects the exo rate to a greater degree. Introduction of a strain-inducing exo methylene group at C(5) and C(6) in the norbornyl system does not produce a significant change in the exchange rates. The average increase of 7.9 and 2.6 in the exo and endo rates, respectively, in enones 4, 5, and 6 over the ketones 1, 2, and 3 are explicable by an inductive effect. Thus, homoallylic stabilization of the developing carbanion is not important.Through application of a new mechanistic criterion which involves a log–log plot of the exo and endo exchange rate constants, we establish, for the bicyclic ketones, that charge transfer to oxygen is greater in the transition state for endo deprotonation.
