Barrier to rotation around the C_sp2-C_sp2 bond of the ketoaldehyde enol ether MeC(o)CH=CH-OEt 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 C_sp2-C_sp2 and C_sp2-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.