Elimination reactions of (E)-2,4,6-trinitrobenzaldehyde O-benzoyloximes promoted by R2NH/R2NH2+ in 70 mol% MeCN (aq). Effects of the β-aryl group and leaving group on nitrile-forming transition states

Article abstract of DOI:10.1002/bkcs.10791

Elimination reactions of (E)-2,4,6-(NO₂)₃C₆H₂CH=NOC(O)C₆H₄X (3) promoted by R₂NH/R₂NH₂⁺ in 70 mol% MeCN (aq) have been studied. The reactions produced elimination products and exhibited second-order kinetics. The β and |β_1g| values remained nearly the same for all leaving groups and bases. The results can be described by the negligible p_xy interaction coefficient, p_xy = ?β/?pK_lg = ?β_lg/?pK_BH ≈ 0, which provides a strong support for the (E1cb)_irr mechanism. For eliminations from (E)-ArCH NOC=(O)C₆H₄X (1, 3) and (E)-2,4,6-(NO₂)₃C₆H₂CH=NOAr′ (2, 3), the change of the β-aryl group (Ar) from 2,4-dinitrophenyl (1) to 2,4,6-trinitrophenyl (3) increased the rate by 270-fold without appreciable change in the transition state structure. On the other hand, the leaving group (OAr′) variation from benzoate (3) to 4-nitrophenoxy (2) induced a change in reaction mechanism from (E1cb)_irr to E2. These results have been attributed to the cyclic transition state for the nitrile-forming eliminations involving the benzoate leaving group.

Full text of DOI:10.1002/bkcs.10791

Article
DOI: 10.1002/bkcs.10791
BULLETIN OF THE
S. Y. Pyun et al.
KOREAN CHEMICAL SOCIETY
Elimination Reactions of (E)-2,4,6-Trinitrobenzaldehyde
+
O-Benzoyloximes Promoted by R2NH/R2NH2 in 70 mol% MeCN (aq).
Effects of the β-Aryl Group and Leaving Group on Nitrile-Forming
Transition States
‡
‡
§,
Sang Yong Pyun,†, Kyu Cheol Paik, Man So Han, and Bong Rae Cho
*
*
†Department of Chemistry, Pukyong National University, Pusan 608-737, Korea.
*E-mail: sypyun@pknu.ac.kr
‡Department of Chemistry, Daejin University, Gyeonggi-do 487-711, Korea
§Department of Chemistry, Korea University, Seoul 136-713, Korea. *E-mail: chobr@korea.ac.kr
Received March 2, 2016, Accepted March 23, 2016, Published online May 27, 2016
+
Elimination reactions of (E)-2,4,6-(NO2)3C6H2CH NOC(O)C6H4X (3) promoted by R2NH/R2NH2 in
70 mol% MeCN (aq) have been studied. The reactions produced elimination products and exhibited
second-order kinetics. The β and |βlg| values remained nearly the same for all leaving groups and bases.
The results can be described by the negligible pxy interaction coefficient, pxy = ∂β/∂pKlg = ∂βlg/∂pKBH
≈
0, which provides a strong support for the (E1cb)irr mechanism. For eliminations from (E)-ArCH NOC
(O)C6H4X (1, 3) and (E)-2,4,6-(NO2)3C6H2CH NOAr0 (2, 3), the change of the β-aryl group (Ar) from
2,4-dinitrophenyl (1) to 2,4,6-trinitrophenyl (3) increased the rate by 270-fold without appreciable change
in the transition state structure. On the other hand, the leaving group (OAr0) variation from benzoate (3) to
4-nitrophenoxy (2) induced a change in reaction mechanism from (E1cb)irr to E2. These results have been
attributed to the cyclic transition state for the nitrile-forming eliminations involving the benzoate leaving
group.
Keywords: Elimination, E2 and (E1cb)irr, Mechanism, β-Aryl group and leaving group effect
Introduction
β-carbon in the transition state can be better stabilized by
the 2,4,6-trinitrophenyl group. However, such expectation
was not borne out by the experimental results; the reaction
proceeded by the E2 mechanism.
During the last decades, there has been much interests in
the structure–property relationship studies on the base-
promoted
nitrile-forming
eliminations
from
(E)-
In order to understand the origin of such dichotomy, it
seemed crucial to conduct more systematic investigations
using substrates having the same β-aryl group and different
leaving group and the same leaving group and different
β-aryl group. We, therefore, have investigated the reactions
of (E)-2,4,6-(NO2)3C6H2CH NOC(O)C6H4X (3) with
benzaldehyde O-benzoyloximes.1–7 The main interest was
how the structural features of such compounds, including
syn-stereochemistry, poor leaving group, and an sp2 hybri-
dized β-carbon atom would influence the transition state
structure. Most of the reactions proceeded by the E2 mech-
anism despite the above-mentioned factors favor E1cb- or
E1cb-like transition state.8–11
+
R2NH/R2NH2 in 70 mol% MeCN (aq) (Eq. (1)). The
effect of the β-aryl group was studied by comparing the
transition state parameters for 1 and 3. The influence of the
leaving group was assessed by using the transition state
parameters for 1 and 2. The results of these studies are
reported here.
Earlier, we reported that the eliminations from (E)-2,4-
(NO2)2C6H3CH NOC(O)C6H4X (1) promoted by R2NH/
R2NH2+ in 70 mol% MeCN (aq).7 The (E1cb)irr mechanism
was proposed for this reaction based on the observation of
the nitriles and benzoates as the only products as well as
the 2nd-order kinetics and nearly identical values of the β
and |βlg| for the leaving group and base strength variations.
We also investigated the elimination reactions of (E)-2,4,6-
(NO2)3C6H2CH NOC6H4X (2) promoted by R3N/R3NH+
in 70 mol% MeCN (aq),5 with the expectation that the
reaction may proceed by the E1cb mechanism because the
pKa values of benzoic acid and 4-nitrophenol in MeCN are
identical12 and the partial negative charge developed at the
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Results
values remained nearly the same within experimental error
for all of the leaving groups (Table 2).
All compounds (3a–d) were prepared in 45−85% yields by
stirring the mixtures of appropriate benzoyl chlorides with
(E)-2,4,6-trinitrobenzaldoxime in aqueous NaOH solution
as before.1
Figure 2 shows the Brönsted βlg plots for eliminations
from 3a–d. The k2 values correlated reasonably well with
the leaving group pKlg values. Here again, the data for i-
Bu2NH were excluded (Figure 2). The |βlg| values are in the
range of 0.30−0.35 and (Table 3). The |βlg| values are
nearly the same for all of the leaving groups employed in
this study (Table 3).
+
When 3a was reacted with i-Bu2NH/i-Bu2NH2 in
70 mol% MeCN (aq), 2,4,6-dinitrobenzonitrile was
obtained in 96% yield. The hydrolysis product, (E)-2,4,6-
trinitrobenzaldehyde oxime, was not detected on TLC. The
yields of benzoates were determined by comparing the UV
absorptions of the infinity samples with those for the
authentic benzoates. The yields were in the range
of 95–97%.
The rates of the reactions were determined under
pseudo-first-order conditions employing at least tenfold
excess amount of the base. The decrease in the absorption
at the λmax of 3a–d was monitored. Excellent pseudo-first-
order kinetics plots were obtained over three half-lives. The
Discussion
Mechanism of Elimination from 3. Earlier, we reported
+
that the reactions of 2 with R2NH/R2NH2 in 70 mol%
MeCN (aq) proceeded by the (E1cb)irr mechanism.7 The
+
mechanism of the reactions of 3 with R2NH/R2NH2 in
70 mol% MeCN (aq) was also investigated by the product
observed rate constants (kobs
)
are summarized in
1.50
1.25
1.00
0.75
0.50
0.25
Tables S1–S4, Supporting Information. In all cases, the
plots of kobs vs. base concentration were straight lines pass-
ing through the origin. This outcome indicates that the reac-
tions are overall second-order, first order to the substrate
and first order to the base (Figure S1−S5). The second-
order rate constants (k2) were obtained from the slopes of
the plots. The k2 values are summarized in Table 1. The k2
value increased with basicity and the leaving group ability.
There was an exception, however, the k2 value determined
with i-Bu2NH was faster than those measured with Bz(i-Pr)
NH, i-Pr2NH, and 2,6-DMP. This outcome can be attribu-
ted to the smaller steric effect of i-Bu2NH than those of the
former.
16.5
17.0
17.5
18.0
18.5
19.0
pKa
Figure 1 shows the Brönsted plots for eliminations from
+
3a–d promoted by R2NH/R2NH2 in 70 mol% MeCN
Figure 1. Brönsted plots for eliminations from (E)-2,4,6-
(aq) at 25.0ꢀC. The plots are straight lines, if the data for i-
Bu2NH are excluded. The β values calculated from the
slopes of these plots are in the range of 0.25−0.29. The
+
(NO2)3C6H2CH NOC(O)C6H4Y promoted by R2NH/R2NH2 in
70 mol% MeCN (aq) at 25.0ꢀC [Y = H (3a, ●), p-OMe (3b, ■),
m-Cl (3c, ▲), p-CF3 (3d, ▼)].
Table 1. Rate constants for eliminations from (E)-2,4,6-(NO2)3C6H2CH NOC(O)C6H4Ya promoted by R2NH/R2NH2+ in 70 mol%
MeCN (aq)b,c at 25.0ꢀC.
k2, M−1 s-1f,g
R2NHd
pKa
Y = H
Y = p-OMe
Y = m-Br
Y = p-CF3
e
Bz(i-Pr)NH
i-Bu2NH
i-Pr2NH
2,6-DMPh
16.8
18.2
18.5
18.9
2.79
10.9
7.29
12.3
1.43
5.80
3.66
5.01
6.29
24.3
13.9
23.0
7.12
28.0
17.2
32.1
a
[Substrate] = 8.0 × 10−5 M.
b
[R2NH]/[R2NH2+] = 1.0.
c
μ = 0.1(Bu4N+Br).
d
[R2NH] = 1.0 × 10−3 − 2.0 × 10−2 M.
Ref. 12.
e
f
Average of three or more rate constants.
Estimated uncertainty, ꢁ3%.
cis-2,6-Dimethylpiperidine.
g
h
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Table 2. Brönsted β values for eliminations from (E)-2,4,6-
(NO2)3C6H2CH NOC(O)C6H4Y promoted by R2NH/R2NH2+ in
70 mol% MeCN (aq) at 25.0ꢀC.
B
kB
¡Õ + BH+
+
-OC(O)Ar
ArC
ArCH=NOC(O)Ar
k1
k2'
k-1
k2
Y = p-OMe
Y = H
20.7
Y = m-Br
Y = p-CF3
a
BH+
19.2b
-
ArC=NOC(O)Ar
kd
pKlg
21.3
19.5
-
+
BH+
ArC=NOC(O)Ar
β
a
0.25 ꢁ 0.02 0.29 ꢁ 0.05 0.25 ꢁ 0.05
0.29 ꢁ 0.07
k-d
Ref. 12.
Scheme 1.
b
Determined from the slope of the plot of σ vs. pKa.
can be expressed as kobs = k1k2[B]/(k−1[BH+] + k2)
(Scheme 1).11 For the (E1cb)R mechanism, in which the
first step must be reversible, i.e., k−1[BH+] ꢂ k2, the rate
expression can be simplified to kobs = k1k2[B]/k−1[BH+]. If
[B]/[BH+] = 1.0, the equation can further be simplified to
kobs = k1k2/k−1. Because [B]/[BH+] = 1.0 was maintained
throughout the reaction, kobs should be a constant. In con-
trast, kobs was linearly dependent on the base concentration
(Figure S1–S5). Hence, the possibility of the (Elcb)R mech-
anism is clearly ruled out. On the other hand, the (E1cb)irr
mechanism requires that the k−1 step is rate limiting, i.e.,
1.5
1.2
0.9
0.6
0.3
k−1[BH+] ꢃ k2. The rate equation should become kobs
=
k1[B], which is identical to that of the E2 mechanism.13
Hence, the distinction between the E2 and (E1cb)irr
mechanisms cannot be made by the rate equation.
19.0
19.5
20.0
20.5
21.0
21.5
pKlg
The |βlg| values have often been used to distinguish the
two mechanisms. The E2 and (E1cb)irr mechanisms predict
significant and negligible degrees of the Nα-OC(O)Ar bond
cleavage, respectively. Hence, the |βlg| value should be lar-
ger for the E2 than for the (E1cb)irr mechanism. However,
the moderate |βlg| values of 0.30−0.35 determined for this
reaction do not provide a clear-cut evidence for either of
these mechanisms.
Figure 2. Plots of log k2 vs. pKlg values of the leaving group for
the elimination from (E)-2,4,6-(NO2)3C6H2CH NOC(O)C6H4Y
+
(3a–d). Promoted by R2NH/R2NH2 in 70 mol% MeCN (aq) at
25.0ꢀC [R2NH = Bz(i-Pr)NH (■), i-Bu2NH (●), i-Pr2NH (▲),
2,6-DMP (▼)].
Table 3. Brönsted βlg values for eliminations from (E)-2,4,6-
(NO2)3C6H2CH NOC(O)C6H4Y promoted by R2NH/R2NH2+ in
70 mol% MeCN (aq) at 25.0ꢀC.
The distinction between the two mechanisms has been
made using the interaction coefficients, pxy = ∂β/∂pKlg
=
R2NH
Bz(i-Pr)NH
i-Bu2NH
i-Pr2NH
2,6-DMPa
∂βlg/∂pKBH, which describes the interaction between the
base catalyst and the leaving group.11,14–17 According to
this formalism, the E2 mechanism predicts pxy > 0, while
the (E1cb)irr mechanism predicts pxy ≈ 0.
b
pKa
βlg
16.8
−0.32
ꢁ0.03
18.2
−0.32
ꢁ0.03
18.5
−0.30
ꢁ0.04
18.9
−0.35
ꢁ0.06
As shown in Table 2, the β values for eliminations from
3 are almost the same for all leaving groups. This effect
can be ascribed to a negligible pxy interaction coefficient,
pxy = ∂β/∂pKlg ≈ 0.11,14–17 The similar |βlg| values for all
bases can also be interpreted with pxy = ∂βlg/∂pKBH ≈ 0
(Table 3). The negligible pxy interaction coefficient provides
a strong evidence for the (E1cb)irr mechanism.
The structures of the transition states have been assessed
using the Brönsted β and |βlg| values. The Brönsted β values
indicate the extent of proton transfer in the transition state.
For eliminations from 3a–d, values of β = 0.25−0.29 were
determined (Table 2). This indicates a small degree of pro-
ton transfer in the transition state. The |βlg| values are usu-
ally taken as the extent of the leaving group bond cleavage.
Hence, the observed values of |βlg| = 0.30–0.35 (Table 3)
are consistent with limited extent of the Nα-OC(O)Ar bond
rupture. Taken together, the elimination reactions of 3 pro-
moted by R2NH/R2NH2+ in 70 mol% MeCN (aq) appear to
a
cis-2,6-Dimethylpiperidine.
Ref. 12.
b
and kinetic studies. The reactions produced nitriles and
benzoates as the only products and exhibited 2nd-order
kinetics, first order to the reactant, and first order to the
base. The observation of the elimination products and 2nd-
order kinetics rules out all but bimolecular β-elimination
pathways (Scheme 1). The most likely mechanism for such
a process is either E2 or E1cb. Of the three variants of the
E1cb mechanisms, (E1cb)ip, (E1cb)R, and (E1cb)irr, the
(E1cb)ip mechanism can be ruled out because it was
observed only in aprotic solvent, not in highly aqueous
medium such as 70 mol% MeCN (aq) employed in this
study. The distinction between (E1cb)R and (E1cb)irr can be
made using the rate equation. Assuming steady state for the
intermediate, the rate equation for the E1cb mechanisms
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proceed by the (E1cb)irr mechanism via a reactant-like tran-
sition state with limited extents of proton transfer and Nα-
OC(O)Ar bond cleavage.13
reaction increased by 270-fold without appreciable changes
in the β and |βlg| values (Table 4). This outcome indicates
that transition state structures are similar for both reactions,
despite large rate difference. This result can be attributed to
the cyclic transition state (TS1), in which the negative
charge is delocalized through the intramolecular hydrogen
bonding. This would stabilize the TS1, thereby rendering it
less insensitive to the reactant structure variation. On the
other hand, the partial negative charge developed at the
β-carbon can be better stabilized by the more strongly
electron-withdrawing 2,4,6-trinitrophenyl group, thereby
increasing the rate.
Effect of Leaving Group on the Nitrile-Forming Transi-
tion State. The mechanism of eliminations from (E)-2,4,6-
(NO2)3CH NOX changed from E2 to (E1cb)irr by the leav-
ing group variation from 4-nitrophenoxide to benzoate,
despite their identical pKlg values12 (Table 4). Also, the rate
of elimination and Brönsted β value decreased by threefold,
whereas |βlg| values remained nearly the same. This out-
come indicates a smaller extent of proton transfer in the lat-
ter transition state and similar degree of the bond to the
leaving group rupture in both transition states (Table 5).
These results can be interpreted with a cyclic transition
state (TS1). In TS1, the negative charge developed at the
β-carbon can be delocalized between the β-carbon and
Mapping of the Transition State. The changes in the
transition state parameters can be described on the More-
O’Ferrall–Jencks reaction coordinate diagram (Figure 3).14
For reactions of 3a with 2,6-DMP, the values of β = 0.29
and |βlg| = 0.35 indicate limited extents of proton transfer
and Nα-OC(O)Ar bond cleavage. Hence, the transition state
can be located near the right corner on the More-O’Ferrall–
Jencks reaction coordinate diagram (A in Figure 3).
A change of the leaving group to a better one will
increase the energy of the top edge of the diagram. The
transition state (A) will remain at nearly the same position
resulting in a negligible in β because there is no diagonal
character on the horizontal coordinate.
Similarly, a change to a stronger base will raise the
energy of the right edge of the diagram. The transition state
(A) will then move toward the right edge resulting in a neg-
ligible in |βlg| value. The nearly identical values of β and
|βlg| values for the leaving group and base strength varia-
tions are in good agreement with this prediction. This out-
come provides strong support for the above conclusions.
Effect of β-Aryl Group on the Nitrile-Forming Transi-
tion State. When the β-aryl group was changed from 2,4-
dinitrophenyl to 2,4,6-trinitrophenyl, the rate of elimination
Table 4. Effect of β-aryl group on the transition state parameters
for eliminations from (E)-ArCH NOC(O)C6H4-4-NO2 promoted
O
O
+
by R2NH/R2NH2 in 70 mol% MeCN (aq) at 25.0ꢀC.
ArC=NOCAr
+
BH+
ꢀ
ArCH=NOCAr + B
a
Ar = 2,4-(NO2)2C6H3
Ar = 2,4,6-(NO2)3C6H2
A
Rel rateb
β
βlg
pxy
1
270
0.29 ꢁ 0.05
−0.35 ꢁ 0.06
~0
0.32 ꢁ 0.02
−0.31 ꢁ 0.01
~0
E1cb
C
a
Ref. 7.
R2NH = 2,6-DMP.
b
ꢀlg
D
B
Table 5. Effect of leaving group on the transition state parameters
for eliminations from (E)-2,4,6-(NO2)3CH NOY promoted by
R3N/R3NH+ and R2NH/R2NH2+ in 70 mol% MeCN (aq).
E2
Y
4-Nitrophenyla
Benzoyl
+
BH+
ArCH=N+
O
+
B
ArC
N
R3N/R3NH+
20.7b,c
3d
R2NH/R2NH2
20.7b,c
+
Base
pKlg
Rel rate
O
+
OCAr
+
OCAr
1e
Figure 3. Reaction coordinate diagram for nitrile-forming elimina-
tions from (E)-2,4,6-trinitrobenzaldehyde O-benzoyloximes. The
transition state for eliminations from 3a is indicated as A. A
change to a better leaving group will have negligible influence on
the position of the transition state resulting in a negligible β value.
Similarly, a change to a stronger base will shift the transition state
toward the right edge resulting in a negligible |βlg| value. For com-
parison, the effects of the change to a stronger base and a better
leaving group on the diagonal reaction coordinate are shown by
the shifts from B to C and B to D, respectively.
pxy
>0
E2
0
Mechanism
β
βlg
(E1cb)irr
0.29 ꢁ 0.05
−0.30 ꢁ 0.04
0.88 ꢁ 0.11
−0.34 ꢁ 0.04
a
Ref. 5.
b
pKa of the leaving group in MeCN.
Ref. 12.
R3N = Et3N (pKa = 18.5).
R2NH = i-Pr2NH (pKa = 18.5).
c
d
e
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carbonyl oxygen atoms, the latter of which can further sta-
bilize TS1 by forming intramolecular hydrogen bond with
the Cβ-H proton.
(E)-2,4,6-(O2N)3C6H2CH NOC(O)C6H5 (3a).
Yield
1
85%; mp 190ꢀC; IR 1762; H NMR δ 7.61–7.63 (m 2H),
7.73–7.75 (m,1H), 8.14–8.16 (m, 2H), 9.32 (s,2H), 9.39
(s,1H); 13C NMR δ 124.0, 125.0, 127.0, 129.1, 129.5,
134.3, 148.4, 149.2, 152.9, 162.2.
O2N
O2N
(E)-2,4,6-(O2N)3C6H2CH NOC(O)C6H4-p-OMe
(3b).
1
Yield 73%; mp 216ꢀC; IR 1772; H NMR δ 3.93 (s, 3H),
7.11 (d, J = 9.16, 2H), 8.10 (d, J = 9.16, 2H), 9.31 (s, 2H),
9.34 (s, 1H); 13C NMR δ 55.6, 114.4, 118.8, 124.0, 125.1,
131.7, 148.3, 149.2, 152.2, 161.8, 163.9.
NO2
NO2
ꢁ
–
ꢁ
O2N
O2N
H
O
O
H
OAr
ꢁ
ꢁ
B
ꢁ
B
(E)-2,4,6-(O2N)3C6H2CH NOC(O)C6H4-m-Br
(3c).
ꢁ
–
Ar
Yield 57%; mp 240ꢀC; IR 1782; 1H NMR δ 7.60 (t,
J = 8.08, 1H), 7.93 (d, J = 8.08, 1H), 8.15 (d, J = 7.68,
1H), 8.30 (s, 1H), 9.33 (s, 2H), 9.45 (s, 1H); 13C NMR δ
122.1, 124.0, 124.9, 128.6, 129.3, 131.2, 131.8, 137.0,
148.4, 149.2, 153.4, 161.0.
TS1
TS2
Hence, the transition state should be less sensitive to the
leaving group and base strength variations, and the
(E1cb)irr mechanism should be the favored. Because com-
pound 2 cannot attain such a cyclic transition state (TS2), it
is less likely for the negative charge at the β-carbon to be
stabilized in a similar way. This would predict that the rate
should be more sensitive to the base strength, a larger β is
expected, and the reaction should follow E2 mechanism
where the transition state energy can be minimized by
breaking the Cβ-H and Nα-OAr bonds and forming partial
C N bond concertedly.
(E)-2,4,6-(O2N)3C6H2CH NOC(O)C6H4-p-CF3
(3d).
Yield 45%; mp 288ꢀC; IR 1744; 1H NMR δ 7.83 (d,
J = 8.2, 2H), 8.19 (d, J = 7.88, 2H), 9.28 (s, 2H), 9.40 (s,
1H); 13C NMR δ 110.0, 124.3, 125.6, 130.1, 130.5, 132.3,
132.6, 134.6, 149.2, 151.2, 153.7, 166.2.
Reagent grade acetonitrile and secondary amine were
fractionally distilled from CaH2. The solutions of R2NH/
+
R2NH2 in 70 mol% MeCN (aq) were prepared by dissol-
+
ving equivalent amount of R2NH and R2NH2 in 70 mol%
MeCN (aq). In all cases, the ionic strength was maintained
Conclusion
to 0.1 M with Bu4N+Br−.
+
Kinetic Studies. Reactions of 3 with R2NH/R2NH2 in
We have studied the elimination reactions of (E)-2,4,6-
trinitrobenzaldehyde O-benzoyloximes promoted by R2NH/
R2NH2 in 70 mol% MeCN (aq). The reaction proceeded
by the (E1cb)irr mechanism via a cyclic transition state,
which is insensitive to the reactant structure variations and
favors the (E1cb)irr mechanism. The increase in the reaction
rate without affecting the transition state structure by the
change to a strongly electron-withdrawing β-aryl group pro-
vided additional support for the cyclic transition state.
70 mol% MeCN (aq) were followed by monitoring the
decrease in the absorbance of the substrate at 258–280 nm
with a UV–vis spectrophotometer as described previously.3,4
Product Studies. The product of eliminations from 3a
+
+
promoted by i-Bu2NH/i-Bu2NH2 in 70 mol % MeCN
(aq) was identified as described.6 A solution of 0.54 g
(1.51 mmol) of 3a and an excess amount of base in 70 mol
% MeCN (aq) (10 mL) was stirred for 5 h at room temper-
ature. The solvent was removed in vacuo and the residue
was extracted with CH2Cl2 and washed thoroughly with
water until all of the amine, ammonium salt, and aryloxide
were completely removed. The product was 2,4,6-
trinitrobenzonitrile with mp 133–134ꢀC (lit.18 mp
134–135ꢀC). The yield of 2,4,6-trinitrobenzonitrile was
96%. Also, the yields of aryloxides determined by compar-
ing the UV absorptions of the infinity samples with those
for the authentic aryloxides were in the range of 95–97%.
Experimental
Materials. (E)-2,4,6-Trinitrobenzaldoxime was synthe-
sized as reported previously.1 All of the (E)-2,4,6-
trinitrobenzaldehyde O-benzoyloximes 1a−d were prepared
in reasonable yields by adding appropriate benzoyl chlor-
ides (0.6 mmol) to the solution of (E)-2,4,6-
trinitrobenzaldoxime (0.14 g, 0.5 mmol) in 0.4 N NaOH
(aq) solution at 0ꢀC. The solution was stirred for 20
−30 min at 10ꢀC and poured into 10 mL of cold water. The
products were recrystallized from ethanol. It was difficult to
obtain HRMS and elemental analysis results of 1 because
the compounds decomposed upon standing. However, the
NMR data of the compounds were consistent with the pro-
posed structures. The yield (%), melting point (ꢀC), IR
(KBr, C O, cm−1), 1H NMR (400 MHz, acetone-d6,
J values are in Hz), and 13C NMR (100 MHz, DMSO-d6)
spectral data for the new compounds are as follows.
Control Experiments. The stabilities of 3a–d were deter-
mined as reported.4,6 The solutions of 3 in MeCN were sta-
ble for at least 5 weeks when stored in the refrigerator.
Acknowledgment. This work was supported by the
Pukyong National University Research Fund in 2016.
Supporting Information. Observed rate constants for
+
elimination from 3a–d promoted by R2NH/R2NH2 in
70 mol% MeCN (aq), plots of kobs vs. base concentration,
and NMR spectra for all compounds are available on
Bull. Korean Chem. Soc. 2016, Vol. 37, 871–876
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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BULLETIN OF THE
Article
Elimination Reactions of Nitrile-Forming
KOREAN CHEMICAL SOCIETY
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11. J. R. Gandler, In The Chemistry of Double Bonded Functional
Groups, Vol. 2, S. Patai Ed., John Wiley and Sons, Chiche-
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Bull. Korean Chem. Soc. 2016, Vol. 37, 871–876
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
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