Level 1 – Case 12
84
X
X
C N
H
OCO
C N
H
OCO
Y
DBU
DBU
X
X
C
H
OCO
DBU
C N
H
OCO
DBU
Y
G
G
1 (E-isomer) E E
3
2 (Z-isomer)
4
G
G
Y
E D
E D
Y
N
Figure 12.1
Although many examples of both anti and syn E2 eliminations have been described, it is generally accepted that the anti-elimination requires less energy, proceeds via a more symmetrical transition state and is usually greatly favored over
the syn elimination. All these reasons, together with the relief of the steric strain
during the process, could explain the much faster rate of anti-elimination from Z- Z Z
isomer 2.
2
Other structural features of the transition states 3 and 4, like the extent of the
C E -H and the N-benzoate bond cleavage can be deduced from a careful interpretaN N
tion of the experimental data in Table 12.1.
Even though E2 reactions are concerted processes, it is rare to find a pure (exactly synchronous) E2 case. There is a broad spectrum of E2 mechanisms from
those on which the departure of the leaving group is clearly ahead the C-H bond
breaking, to those that have undergone more C-H bond cleavage in the transition
state. To discuss the degree of synchronicity of the E2 eliminations in oximes 1
and 2 we should consider the |E lg | values, the Hammett constant U
t and the k H
k k /k
H H D
k k
values simultaneously. The |E lg | values give an idea about the extent of the N-leat
ving group bond breaking. As expected for an E2 reaction, the observed values are
in the middle of the range (around 0.5), indicating that the reaction is sensitive to
the leaving group pK a
K K . The observed |E lg | for the anti-elimination in compound 2
(|E lg | = 0.40) is smaller than the one observed for the syn-elimination in compound
1 (|E lg | = 0.49), indicating a smaller degree of N-leaving group bond cleavage in
the E2 anti transition state 4.
2 The much faster rate of anti-eliminations has been attributed to the steric strain in the Z- Z Z
isomer and the favorable overlap between the developing p orbitals at the E-carbon and
D-nitrogen atoms in the transition state.
84
X
X
C N
H
OCO
C N
H
OCO
Y
DBU
DBU
X
X
C
H
OCO
DBU
C N
H
OCO
DBU
Y
G
G
1 (E-isomer) E E
3
2 (Z-isomer)
4
G
G
Y
E D
E D
Y
N
Figure 12.1
Although many examples of both anti and syn E2 eliminations have been described, it is generally accepted that the anti-elimination requires less energy, proceeds via a more symmetrical transition state and is usually greatly favored over
the syn elimination. All these reasons, together with the relief of the steric strain
during the process, could explain the much faster rate of anti-elimination from Z- Z Z
isomer 2.
2
Other structural features of the transition states 3 and 4, like the extent of the
C E -H and the N-benzoate bond cleavage can be deduced from a careful interpretaN N
tion of the experimental data in Table 12.1.
Even though E2 reactions are concerted processes, it is rare to find a pure (exactly synchronous) E2 case. There is a broad spectrum of E2 mechanisms from
those on which the departure of the leaving group is clearly ahead the C-H bond
breaking, to those that have undergone more C-H bond cleavage in the transition
state. To discuss the degree of synchronicity of the E2 eliminations in oximes 1
and 2 we should consider the |E lg | values, the Hammett constant U
t and the k H
k k /k
H H D
k k
values simultaneously. The |E lg | values give an idea about the extent of the N-leat
ving group bond breaking. As expected for an E2 reaction, the observed values are
in the middle of the range (around 0.5), indicating that the reaction is sensitive to
the leaving group pK a
K K . The observed |E lg | for the anti-elimination in compound 2
(|E lg | = 0.40) is smaller than the one observed for the syn-elimination in compound
1 (|E lg | = 0.49), indicating a smaller degree of N-leaving group bond cleavage in
the E2 anti transition state 4.
2 The much faster rate of anti-eliminations has been attributed to the steric strain in the Z- Z Z
isomer and the favorable overlap between the developing p orbitals at the E-carbon and
D-nitrogen atoms in the transition state.