Page 643 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

2+
2+
6.16. Metal ions such as Zn ,Ni , and Cu 2+ enhance the rate of general base- 625
catalyzed enolization of 2-acetylpyridine by a several orders of magnitude.
PROBLEMS
Account for this effect.
6.17. The C(2) equatorial hydrogen is selectively removed when 1,3-dithianes are
deprotonated. Furthermore, if the resulting carbanion is protonated, there is a
strong preference for equatorial protonation, even though it leads to the less stable
axial orientation for the 2-substituent. Discuss the relevance of these observations
to the structure of the sulfur-stabilized carbanion in MO terminology.

H
H ax – H ax + ax
CH 3 S base CH 3 S *H CH 3 S *H eq
S H eq S CH S
CH 3 CH 3 3

6.18. a. It is found that when 2-methyl-2-butene is converted to a dianion, it first
gives the 2-methylbutadiene dianion 18-A, but this is converted to the more
stable anion 18-B, which can be referred to as a “methyltrimethylenemethane
dianion. Does simple HMO theory offer an explanation for this result?

H – H
CH 3 CH 3 H
CH 3 H – H – CH 3
CH 3 – H H
H H H
18-A 18-B

b. The HMO diagrams of several conceivable dianions that might be formed
from double deprotonation of 2-methyl-1,5-hexadiene are given. On the basis
of these diagrams, which dianion would be expected to be most stable?

–
– 2 –
– 2 – – CH
CH 3
C D 3 E F
α – 1.8β α – 1.9β
α – 1.4β α – 1.7β α – 1.3β α – 1.2β
α – β α – 0.5β
α α α + 0.5β α
α + 1.4β α + β α + 1.3β α + 1.2β

α + 1.7β α + 1.8β α + 1.9β

6.19. Which of the two possible structures for the dimer of methylketene is in best
accord with the observed pK of 2.8? Explain.

CH CH CH 3 CH 3 OH
3
O
O O CH 3