A Semiempirical Study of the Effect of Geometry and Methyl Substituents on the Stabilities of Protonated Cyclopropylcarbinyl Ketones: Resolution of the 7-Nortricyclanone Anomaly.
A long-standing anomaly regarding the difference in stabilities of a series of protonated cyclopropylcarbinyl ketones has been resolved. The enthalpies of protonation of the ketones to the oxocarbenium ions (DeltaH(i)) were calculated from the PM3 heats of formation of the ketones and ions using isodesmic cycles. The values of DeltaH(i) correlate well with the experimentally determined values of pK(R+) for the ketones in concentrated sulfuric acid. Methyl substitution on the cyclopropyl ring uniformly stabilizes the resulting ions. Orientation of the -OH(+) to the cyclopropyl affects stability as well: a syn (periplanar) orientation lowers stability, while an anti (antiperiplanar) orientation increases it, apparently because of charge repulsion between the protonated carbonyl and the partial positive charges on the ring carbons caused by carbinyl "resonance" stabilization. Decreasing the angle about the carbonyl carbon leads to a decrease in stability for all systems tested and appears to be a major source of destabilization of the nortricycle system relative to skeletal analogs with more open carbonyl bond angles. The more stable ions have pi HOMOs, while the nortricycle and related ions have sigma HOMOs, which is another source of destabilization of the nortricycle system. For comparison, the corresponding hydrocarbon carbenium ions were examined; similar orders of stability were identified.
Buckley
Neil
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Department of Pharmaceutical Chemistry, School of Pharmacy, S-926, Box 0446, The University of California, San Francisco, California 94143-0446.