Nuclear Magnetic Resonance Spectroscopy. Carbon-13 Chemical Shifts in Norbornyl Derivatives

Abstract

The ^(13)C chemical shifts for a number of norbornyl derivatives have been obtained in natural abundance. The compounds studied were the series of methylnorbornanes, methylnorbornenes, methyl-2-norbornanones, methyl-2,2-difluoronorbornanes, and a variety of exo- and endo-2-substituted norbornanes. The observed shifts have been interpreted in terms of inductive, bond length, and steric effects. The ɑ- and β-substituent effects are shown to be dependent on the extent of substitution at both the α and β carbon atoms and changes in bond lengths have been used to rationalize the shift changes. Steric interactions between the endo-2 and endo-6 positions, and the exo-2 and syn-7 positions have been shown to generate important chemical- shift changes and these changes can be used as a semiquantitative measure of steric interactions in norbornyl derivatives. Studies with electron-withdrawing groups have revealed a new long-range cmr y effect produced by exo-2 substituents on C-6. These shifts appear to provide a sensitive and quantitative measure of the electronic demands of a neighboring group as a function of stereochemistry. The substituent shifts obtained from the monosubstituted derivatives have been used to predict the shifts of each of the 1- and 3-methyl-2-norbornanols. The agreement between observed and calculated values is generally good and supports the general approach. Several ^(13)C-^(13)C coupling constants for norbornane, nortricyclene, and quadricyclene are reported and interpreted in terms of the s character of the C-C bonds.

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