Combination of 17O and 13C NMR spectra of isomeric allyl and (Z)-propenyl ethers with their relative thermodynamic stabilities

Abstract

The 17O NMR spectra of ten allyl ethers ROCH 2CHCH 2 and those of the isomeric (Z)-propenyl ethers ROCHCHMe, where R is an alkyl group or a MeO-substituted alkyl group, were recorded in CDCl 3 solution. The difference in 17O NMR chemical shift between a propenyl ether and the isomeric allyl ether, ca. 50 to 60 ppm, depends primarily on the number of MeO groups attached to the α carbon of the R moiety, and is related to the strength of p-π conjugation in the propenyl ether. Previous 13C NMR shift data for the β olefinic carbon of the propenyl ethers and the thermodynamic data of isomerization of allyl ethers to propenyl ethers are discussed in view of their relation to the 17O NMR shift data.