13C nuclear magnetic resonance studies. 58. 13C spectra of a variety of bicyclo[2.2.2]octane derivatives. Further definition of the deshielding δ effect

Abstract

The 13C nmr spectra of 35 bicyclo[2.2.2]octane and -octene derivatives have been determined to extend our examinations of the effects of stereochemistry on the shieldings of closely neighboring carbons. This series includes a variety of methyl substituted bicyclooctanols and -octenols as well as the corresponding hydrocarbons and some bicyclooctanones. With the bicyclo[2.2.2]octane skeleton it is possible to examine an array of systems having substituents separated by three and four bonds in a variety of orientations. The interactions, termed γ and δ effects, respectively, produce distinctive shielding variations which are useful for stereochemical elucidations. Particularly interesting are the pronounced shifts observed for the carbons bearing closely neighboring substituents. Characteristically, for vicinal substituents, these carbons are shielded while for syn-axial δ interactions, these carbons are deshielded by as much as 8.6 ppm. The results are compared with the trends found in other sterically crowded Systems.