posted on 1996-02-09, 00:00authored byBenjamin W. Gung, Mathew M. Yanik
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.