Conformational Study of SnCl4 Complexes of Aldehydes, α,β-Unsaturated Aldehydes, and Esters: Observation of Both s-Cis and s-Trans Isomers

A variable-temperature NMR study shows that 2-methylpropanal (1a), ethyl 2-pentenoate (2), ethyl 4-methyl-2-pentenoate (3a), 2-pentenal (4), and 4-methyl-2-pentenal (5a) prefer the C-CH3 eclipsed conformations, rather than the CH-eclipsed form. The preference is enhanced in the presence of SnCl4. This is opposite to consideration based on steric interactions but is consistent with hyperconjugative interaction, in which the C−H bond is aligned with the π*CC orbital. Thus, the current result suggests that the C−H bond is a better donor than the C−C bond in hyperconjugation with electron-deficient π orbital. On the other hand, 2-ethylbutanal (1b), ethyl 4-ethyl-2-hexenoate (3b), and 4-ethyl-2-hexenal (5b) prefer the CH-eclipsed form due to steric effects. At −50 °C, the SnCl4 complexes of 3a and 3b show two distinct sets of vinyl protons in a ratio of ∼10:1, which are assigned to the s-trans- and s-cis isomers, respectively.