THE CIEPLAK EFFECT - HYPERCONJUGATIVE INTERACTIONS AT FACIALLY DISSYMMETRIC PI-SYSTEMS

Molecular orbital calculations demonstrate that at the transition state for the addition of methanol to norbornanones, 1a-d, the bond between the methanol oxygen and the carbonyl carbon is substantially formed. The bonds antiperiplanar to the approach of the methanol are little changed, but those antiperiplanar to the carbonyl oxygen are somewhat shortened in going to the transition state. In the presence of electron withdrawing substituents at C2 and C3 the anti C7-OMe bond is shorter than for 1a, but does not change in the presence of electron donating ethyl groups. For the addition of methanol there appears to be little evidence that the Cieplak postulate is supported by semiempirical calculations.