Conformational study of SnCl4 complexes of aldehydes, alpha,beta-unsaturated aldehydes, and esters: Observation of both s-cis and s-trans isomers

A variable-temperature NMR study shows that 2-methylpropanal (la), 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 pi*(C=C) 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 jr orbital. On the other hand, 2-ethylbutanal (Ib), ethyl 4-ethyl-2-hexenoate (3b), and 4-ethyl-2-hexenal (5b) prefer the CH-eclipsed form due to steric effects. At -50 degrees C, the SnCl4 complexes of 3a and 3b show two distinct sets of vinyl protons in a ratio of similar to 10:1, which are assigned to the s-trans- and s-cis isomers, respectively.