Barrier to rotation around the Csp2-Csp2 bond of the ketoaldehyde enol ether meC(O)CH=CHOet as determined by 13C NMR and ab initio calculations

NMR measurements and ab initio calculations were applied to determine the barriers to rotation around formally single bonds of the title methyl-β-ethoxyvinyl ketone, i.e., the vinylogue of the ethyl ester of acetic acid. For comparison, ab initio calculations were performed for α,β-unsaturated, β-N, and β-S substituted ketones. The relative height of the rotational barriers for Csp2-Csp2 and Csp2-X bonds of the MeC(O)-CH=CH-X-alkyl(s) analogues was found to be reverse for X = N(alkyl)2 vs X = O-alkyl or X = S-alkyl. This finding is discussed in terms of differences in the electron density distribution in these molecules, resulting from differences in electron-donating properties of the heteroatoms N, O, and S