Barrier to rotation around the C-sp(2)-C-sp(2) bond of the ketoaldehyde enol ether MeC(O)CH=CH-OEt as determined by C-13 NMR and ab initio calculations

NMR measurements and ab initio calculations were applied to determine the b arriers to rotation around formally single bonds of the title methyl-beta - ethoxyvinyl ketone, i.e., the vinylogue of the ethyl ester of acetic acid. For comparison, ab initio calculations were performed for alpha,beta -unsat urated, beta -N, and beta -S substituted ketones. The relative height of th e rotational barriers for C-sp2-C-sp2 and C-sp2-X bonds of the MeC(O)-CH=CH -Xalkyl(s) analogues was found to be reverse for X = N(alkyl)(2) vs X = O-a lkyl or X = S-alkyl. This finding is discussed in terms of differences in t he electron density distribution in these molecules, resulting from differe nces in electron-donating properties of the heteroatoms N, O, and S.