On the interpretation of the 13C NMR spectra of alkyl vinyl and alkyl propenyl ethers

Abstract

Abstract The 13 C chemical shifts of the E and Z forms of some alkyl propenyl ethers ROCH=CHMe (R = Me, Et, i-Pr, t-Bu) have been measured. The chemical shift of the α carbon of the olefinic system is equally dependent on the alkyl group in both isomers, whereas the shift of the β carbon of the Z isomer is essentially less sensitive to the bulkiness of R than that of the E isomer. The results achieved show that the relatively large variation with R in the chemical shifts of the olefinic carbons in alkyl vinyl ethers (ROCHCH 2 should not be ascribed to a changing p-π conjugation, as has been done in some reports in the literature, but preferably to a changing through-space shielding effect of R on the olefinic carbons. Only a small decrease in the conjugation occurs on going from R = Me to R= t-Bu, which results from a slightly nonplanar structure of the C-O-C=C system in the latter case.