Hyperconjugation Involving Strained Carbon-Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond C-13-C-13 Coupling Constants of alpha- and beta-Nopinol

Abstract

σ(C-C)-σ*(C-O) interactions involving the strained carbon-carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C-OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C-C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the (13)C-(13)C one-bond coupling constants. For the donor C-C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C-C bond coupling..