Ketene-forming eliminations from aryl bis(4′-chlorophenyl)acetates promoted by R2NH-R2NH2+ in aqueous MeCN. Change of mechanism

Article abstract of DOI:10.1039/b303231k

Elimination reactions of (4′-ClC₆H₄) ₂CHCO₂C₆H₃-2-X-4-NO₂ promoted by R₂NH-R₂NH₂⁺ in 70 mol% MeCN (aq.) have been studied kinetically. The reactions are second-order and exhibit Broensted β= 0.44-0.86 and |β_lg| = 0.41-0.71. The Broensted β decreased with a poorer leaving group and |β _lg| increased with a weaker base. The results are consistent with an E2 mechanism. When X = H, the reaction proceeded by the concurrent E2 and E1cb mechanism.

Full text of DOI:10.1039/b303231k

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Ketene-forming eliminations from aryl bis(4Ј-chlorophenyl)acetates
promoted by R2NH–R2NH2؉ in aqueous MeCN.
Change of mechanism†
Sang Yong Pyun,a Dong Choon Lee,a Ju Chang Kima and Bong Rae Cho*b
a Department of Chemistry, Pukyong National University, Pusan 608-737, Korea
b Department of Chemistry, Korea University, 1-Anamdong, Seoul 136-701, Korea.
E-mail: chobr@korea.ac.kr
Received 25th March 2003, Accepted 11th June 2003
First published as an Advance Article on the web 23rd June 2003
Elimination reactions of (4Ј-ClC6H4)2CHCO2C6H3-2-X-4-NO2 promoted by R2NH–R2NH2ϩ in 70 mol% MeCN
(aq.) have been studied kinetically. The reactions are second-order and exhibit Brönsted β = 0.44–0.86 and |βlg| = 0.41–
0.71. The Brönsted β decreased with a poorer leaving group and |βlg| increased with a weaker base. The results are
consistent with an E2 mechanism. When X = H, the reaction proceeded by the concurrent E2 and E1cb mechanism.
We were interested in learning if a change to the E1cb mechan-
ism could be realized from the E2-central transition state by
introducing an electron withdrawing substituent at the β–aryl
group. Here we report that the elimination reactions from 1b–d
proceed by the E2 mechanism via an E2-central transition state.
When the leaving group was changed to X = H (1a), the mech-
anism changed to a competing E2 and E1cb. By comparing
Introduction
Ketene-forming eliminations from aryl phenylacetates have
been extensively investigated because of the mechanistic diver-
sity in the E2 and E1cb borderline. It is well known that base-
catalyzed hydrolysis of p-nitrophenylacetates and other esters
proceed by an E1cb mechanism to afford the ketene intermedi-
ate followed by the addition of water.1–14 When R2NH in MeCN
with the existing data for Ph2CHCO2Ar, the effect of β–aryl
was used as the base-solvent system, the reaction proceeded by
substituent on the ketene-forming elimination is assessed.
the E2 mechanism via an E1cb-like transition state.15,16 A com-
peting E2 and E1cb mechanism was observed in eliminations
Results
from p-nitrophenyl p-nitrophenylacetates promoted by R2NH–
R2NH2 in 70 mol% MeCN (aq.).15 Moreover, a gradual
ϩ
For reactions of 1a–d with R2NH–R2NH2ϩ in 70 mol% MeCN
(aq.), the yields of aryloxides as determined by comparison of
the UV absorption of the infinity sample of the kinetic runs
with those of the authentic aryloxides were in the range 90–
98%. The possibility of a competing aminolysis had been ruled
out in a previous study.18 The rates of elimination reactions
were followed by monitoring the increase in the absorption at
the λmax for the aryloxides in the range of 400–426 nm. In all
cases, clean isosbestic points were noted in the range 310–
312 nm. Excellent pseudo-first-order kinetics plots which
covered at least three half-lives were obtained.
change of the mechanism from E2 to E1cb via a competing E2
and E1cb mechanism has been demonstrated by systematically
varying the structure of 2-Y-4-NO2C6H3CH2CO2C6H3-2-X-4-
NO2.17 The transition state gradually became more E1cb-like as
a poorer leaving group was introduced, and the E1cb mechan-
ism emerged when X, Y = H. The E1cb mechanism became
predominant when X = H and Y = NO2, probably because the
carbanion intermediate was stabilized by the strongly electron-
withdrawing substituent and the E2 mechanism could no
longer compete.
Recently, we reported that the ketene-forming elimination
The plots of kobs vs. base concentration for the reaction of
1b–d are straight lines passing through the origin, indicating
that the reactions are second-order, first order to the substrate
and first order to the base (Fig. 1 and Figs S1 and S2 in the
ESI†). The slopes are the overall second-order rate constants
ϩ
from Ph2CHCO2Ar promoted by R2NH–R2NH2 in 70 mol%
MeCN (aq.) proceeded via an E2-central transition state, with
similar extents of Cβ–H and Cα–OAr bond cleavage.18 Com-
parison of the transition state parameters revealed that the
extent of proton transfer decreased and the degree of the leav-
ing group bond cleavage increased by the change of the sub-
strate from PhCH2CO2Ar to Ph2CHCO2Ar. The result was
attributed to the double bond stabilizing effect of the β-Ph
group. To further expand our understanding of ketene-forming
eliminations, we have now studied the reactions of aryl bis(4Ј-
chlorophenyl)acetates 1a–d under the same condition (eqn. 1).
E
k2 . Values of k2E for eliminations from 1b–d are summarized in
Table 1.
On the other hand, the plots of kobs vs [base] for the reactions
ϩ
of 1a with R2NH–R2NH2 in 70 mol% MeCN (aq.) showed
curvilinear relationships (Fig. 2 and S3–S6†). The data were
analyzed by assuming that the reaction proceeds by concurrent
E
E2 and E1cb mechanisms (eqn. 2).15,17 The k2 , k1, and kϪ1/k2
values that best fit with eqn. 2 were calculated, and the plots
were dissected into the E2 and E1cb reaction components.20
In all cases, the correlations between the calculated and the
(1)
† Electronic supplementary information (ESI) available: Rate constants
ϩ
for eliminations from 1a–d promoted by R2NH–R2NH2 in 70 mol%
MeCN (aq.), plots of kobs vs base concentration, and plots of log k1 and
log kϪ1/k2 vs pKa. See http://www.rsc.org/suppdata/ob/b3/b303231k/
(2)
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 3 4 – 2 7 3 8
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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a
ϩ
Table 1 Rate constants for ketene-forming eliminations from (4Ј-ClC6H4)2CHCO2C6H3-2-X-4-NO2 (1a–d) Promoted by R2NH–R2NH2 in 70
mol% MeCN (aq.) b, c at 25.0 ЊC
k2E/MϪ1 sϪ1 f, g
e
R2NHd
pKa
X = H (1a)h
X = Cl (1b)
X = CF3 (1c)
X = NO2 (1d)
Morpholine
THIQi
Piperidine
Pyrrolidine
16.6
17.1
18.9
19.6
0.0111
0.0165
0.310j
5.53
0.531
1.08
10.7
2.57
5.10
39.4
17.6
36.9
198
67.9
212
410
a [Substrate] = 3.0 × 10Ϫ5 M. b [R2NH]/[R2NH2ϩ] = 1.0. c µ = 0.10 M (Bu4NϩBrϪ). d [R2NH] = 6.0 × 10Ϫ4 to 2.0 × 10Ϫ1 M. e Ref. 18. f Average of three or
more rate constants. g Estimated uncertainty, 5%. h Calculated from the kobs by using eqn. 2. i Tetrahydroisoquinoline. j k2E = 0.325 MϪ1 sϪ1 when
[R2NH]/[R2NH2ϩ] = 0.5.
Table 2 The k1 and kϪ1/k2 values for ketene-forming eliminations from
a
ϩ
(4Ј-ClC6H4)2CHCO2C6H4-4-NO2 (1a) Promoted by R2NH–R2NH2
buffers in 70 mol% MeCN (aq.) b, c at 25.0 ЊC
R2NHd
k1/MϪ1 sϪ1 e, f
k
Ϫ1/k2/MϪ1 e, g
Morpholine
THIQh
0.123
0.168
0.792
2.02
1590
730
74.6
26.7
Piperidinei
Pyrrolidine
a [Substrate] = 3.0 × 10Ϫ5 M. b [R2NH]/[R2NH2ϩ] = 1.0 except otherwise
noted. c µ = 0.10 M (Bu4NϩBrϪ). d [R2NH] = 6.0 × 10Ϫ4 to 2.0 × 10Ϫ1 M.
e Calculated from the kobs by using eqn. 2. f The slope of the plot of log
k1 vs pKa of the base is 0.40 0.03. g The slope of the plot of log kϪ1/k2
vs pKa of the base is Ϫ0.58 0.01. h Tetrahydroisoquinoline. i k1 = 0.755
MϪ1 sϪ1 and kϪ1/k2 = 77.9 MϪ1, respectively, when [R2NH]/[R2NH2ϩ] =
0.5.
Fig. 1 Plots of kobs vs base concentration for eliminations from (4Ј-
ϩ
ClC6H4)2CHCO2C6H3-2-Cl-4-NO2 (1b) promoted by R2NH–R2NH2
in 70 mol% MeCN (aq.) at 25.0 ЊC, [R2NH]/[R2NH2ϩ] = 1.0, µ = 0.10 M
(Bu4NϩBrϪ). R2NH = morpholine (᭿), tetrahydroisoquinoline (᭹),
piperidine (ꢀ), pyrrolidine (᭢).
the kobs is always smaller at a lower buffer ratio, as expected
from eqn. 2 (Table S1†). Moreover, the rate data show excellent
E
correlation with eqn. 2 and the calculated values of k2 , k1, and
kϪ1/k2 for the piperidine-promoted eliminations from 1a are
nearly identical regardless of the buffer ratio (see footnote in
Tables 1 and 2). These results provide additional evidence in
support of the kinetic analysis given above.
The Brönsted plots for the R2NH–promoted eliminations
from 1a–d are linear with excellent correlations (Fig. 3). The
β values are in the range of 0.44–0.86 and increase with poorer
leaving groups and as the electron-withdrawing ability of the
E
β-aryl substituent increases (Table 3). Similarly, the k2 values
correlated satisfactorily with the leaving group pKlg values
(Fig. 4). The |βlg| values are in the range of 0.41–0.71 and
increase with a weaker base and remain nearly the same with
the variation of the β-aryl substituent (Tables 4 and 5).
For reactions of 1a with R2NH–R2NH2ϩ in 70 mol% MeCN
(aq.), the k1 increases and the kϪ1/k2 decreases with a stronger
Fig. 2 Plot of kobs vs buffer concentration for eliminations from (4Ј-
ϩ
ClC6H4)2CHCO2C6H4-4-NO2 (1a) promoted by R2NH–R2NH2 in 70
mol% MeCN (aq.) at 25.0 ЊC, [R2NH]/[R2NH2ϩ] = 1.0, µ = 0.10 M
(Bu4NϩBrϪ), R2NH
= Piperidine. The closed circles are the
experimental data and the solid line shows the computer fitted curve by
using eqn. 2. The curve is dissected into the E2 and E1cb reaction
components (dashed lines).
E
experimental data are excellent. Calculated values of k2 , k1,
and kϪ1/k2 for the eliminations from 1a are summarized in
Tables 1 and 2.
The possibility of the buffer association as the cause of the
curvature was ruled out by the straight lines observed in the
plots of kobs vs [base] for the reaction of 1b–d (Fig 1, S1 and
S2†). Also, the possibility that the salt effect may have caused the
increase in the kobs at higher base concentration is negated
because the ionic strength is maintained to be 0.1 M with
Bu4NϩBrϪ. To assess the effect of buffer ratio, the kobs values for
the ketene-forming eliminations from 1a have also been meas-
Fig. 3 Brönsted plots for the ketene-forming eliminations from (4Ј-
ϩ
ClC6H4)2CHCO2C6H3-2-X-4-NO2 (1a–d) promoted by R2NH–R2NH2
in 70 mol% MeCN (aq.) at 25.0 ЊC, [R2NH]/[R2NH2ϩ] = 1.0, µ = 0.10 M
(Bu4NϩBrϪ). [X = H (1a, ᭿), Cl (1b, ᭹), CF3 (1c, ꢀ), NO2 (1d, ᭢)].
ϩ
ured at [R2NH]/[R2NH2 ] = 0.5. At a given base concentration,
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 3 4 – 2 7 3 8
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Table 3 Brönsted β values for ketene-forming eliminations from (4Ј-
Table
5
Effect of β-aryl substituent on the E2 reactions of
ϩ
ϩ
ClC6H4)2CHCO2C6H3-2-X-4-NO2 (1a–d) promoted by R2NH–R2NH2
(4Ј-YC6H4)2CHCO2C6H4-2-X-4-NO2 promoted by R2NH–R2NH2 in
70 mol% MeCN (aq.) at 25.0 ЊC
in 70 mol% MeCN (aq.) at 25.0 ЊC
X
H
Cl
CF3
17.0
NO2
16.0
Y = Ha
Y = Cl (1a)
a
pKlg
β
20.7
0.86 0.14
18.1
0.67 0.07
Rel. rateb
1
3
0.60 0.06
0.44 0.02
βc
βlg
0.67 0.04
Ϫ0.57 0.04
a Ref. 18. b R2NH = piperidine. c X = H.
0.86 0.14
Ϫ0.59 0.02
a Ref. 17.
p-nitrophenylacetates under the same conditions and were
ascribed to the concurrent E2 and E1cb mechanisms, i.e., kobs
=
E
k2 ϩ k1k2[B]/(kϪ1[BHϩ] ϩ k2) (eqn. 2).15,17 Hence, the present
result can also be attributed to the same mechanism. There is
convincing evidence in support of this mechanism. (i) All of
the rate data show excellent correlation with eqn. 2. Also, the
shapes of the dissected curves are typical for the E2 and E1cb
mechanisms. (ii) The kobs decreases when buffer ratio is changed
from 1.0 to 0.5, as expected from eqn. 2 (Table S1†). Moreover,
E
the calculated values of k2 , k1, and kϪ1/k2 are nearly the same
regardless of buffer ratio, indicating that the same mechanism is
operating under these conditions (see footnotes for Tables 1 and
2). (iii) The k2E values for 1a nicely fit in the trend observed for
1b–d, i.e., they increase gradually with a better leaving group
and a stronger base (Table 1). They show excellent correlations
in the Brönsted plots (Figs 3 and 4), and the calculated values
of β and |βlg| are in good agreement with the E2 mechanism
(Tables 3 and 4). (iv) The k1 increases with a stronger base, as
would be expected for a deprotonation step. In addition, the
slope of the plot of log k1 vs pKa of the base is 0.40 0.03,
which is again consistent because the transition state for this
step should be reactant-like (see footnote in Table 2). (v) The
increase in the kϪ1/k2 ratios with a weaker base is also consist-
E
Fig. 4 Plots of log k2 vs pKlg values of the leaving group for
eliminations from (4Ј-ClC6H4)2CHCO2C6H3-2-X-4-NO2 (1a–d)
ϩ
promoted by R2NH–R2NH2 in 70 mol% MeCN (aq.) at 25.0 ЊC,
[R2NH]/[R2NH2ϩ] = 1.0, µ = 0.10 M (Bu4NϩBrϪ). [R2NH = morpholine
(᭿), tetrahydroisoquinoline (᭹), piperidine (ꢀ), pyrrolidine (᭢)].
base (Table 2). The plots of log k1 and log kϪ1/k2 vs the pKa
values of R2NH2 are straight lines with excellent correlation
(Figs S7, S8†). The slopes of the plots are 0.40
Ϫ0.58 0.01, respectively.
To provide additional evidence for the competing E1cb
mechanism, an H–D exchange experiment was carried out by
mixing 1a with R2NH–R2NH2 in 70 mol% MeCN–30% D2O
at Ϫ10 ЊC. The reactant was recovered immediately after mix-
ing. The NMR spectrum indicated that approximately 10% of
the benzylic C–H bond of 1a was converted to a C–D bond.
ϩ
ϩ
0.03 and
ent. The kϪ1 should increase with the acidity of R2NH2 ,
although little change is expected for k2 by the base strength
variation. This predicts an increase in the kϪ1/k2 ratio with a
weaker base, as observed. Furthermore, the slope of the plot of
log kϪ1/k2 vs pKa of the base is Ϫ0.58 0.03. Note that the sum
of the absolute values of the slopes of the plots for the k1 and
kϪ1 steps are close to unity. This is as expected, if the β value
for the complete deprotonation is unity and if k2 is a constant
(vide supra). (vi) The NMR spectrum of the recovered reactant
ϩ
ϩ
from the reaction of 1a with R2NH–R2NH2 in 70 mol%
Discussion
MeCN–30% D2O at Ϫ10 ЊC indicated that approximately 10%
of the benzylic C–H bonds of 1a underwent H–D exchange.
Mechanism of eliminations from 1a–d
Results of kinetic investigations and product studies reveal that
the reactions of 1b–d with R2NH–R2NH2ϩ in 70 mol% MeCN
(aq.) proceed by the E2 mechanism. Because the possibility of
competing aminolysis is ruled out,18 and the ketene-forming
elimination reactions exhibit second-order kinetics, all but
bimolecular pathways can be ruled out. Moreover, an E1cb
mechanism is negated by the substantial values of β and
|βlg|.21–23 The positive interaction coefficients, i.e., pxy = ∂β/∂pKlg =
∂βlg/∂pKBH > 0, provide additional evidence for the E2 mechan-
ism (vide infra).23–25
Mapping of the E2 transition state
The structures of the transition states may be assessed by the
Brönsted β and |βlg| values. For R2NH-promoted eliminations
from 1d, the value of β = 0.44 was determined (Table 3). This
indicates less than half proton transfer in the transition state.
The |βlg| values are usually taken as a measure of the extent of
the leaving group cleavage. However, it should be noted that the
values of |βlg| are not restricted to 0–1. In fact, for reactions
involving loss of arenesulfonate in the rate-determining step,
they range up to nearly 3.0.26 For ketene-forming eliminations
from aryl p-nitrophenylacetates, the maximum value of |βlg| =
1.34 was reported.8,15 Therefore, the measured values of |βlg| =
0.41 for pyrrolidine-promoted eliminations from 1d can most
On the other hand, the plots of kobs vs base concentration for
the reactions of 1a are curves at low buffer concentration and
straight lines at [base] > 0.01–0.05 M (Fig. 2 and S3–S6†). Simi-
lar results were observed in eliminations from p-nitrophenyl
Table 4 Brönsted βlg values for ketene-forming eliminations from (4Ј-ClC6H4)2CHCO2C6H3-2-X-4-NO2 (1a–d) promoted by R2NH–R2NH2ϩ in 70
mol% MeCN (aq.) at 25.0 ЊC
R2NH
Morpholine
THIQa
Piperidine
Pyrrolidine
b
pKa
βlg
16.6
Ϫ0.71 0.02
17.1
Ϫ0.70 0.02
18.9
Ϫ0.59 0.02
19.6
Ϫ0.41 0.02
a Tetrahydroisoquinoline. b Ref. 18.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 3 4 – 2 7 3 8
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reasonably be interpreted with a moderate Cα–OAr bond cleav-
age in the transition state. Taken together, the transition state
for the ketene-forming elimination from 1d appears to be react-
ant-like, with less than half proton transfer and a smaller
degree of the leaving group bond cleavage. Therefore, it seems
reasonable to locate the transition state slightly toward the
reactant from the E2-central in the More-O’Ferrall–Jencks
reaction coordinate diagram (D in Fig. 5).
state for the ketene-forming elimination from 1a is more skewed
toward the proton transfer with similar extent of the leaving
group bond cleavage. It appears that the electron-withdrawing
4Ј-Cl group enhances the acidity of the Cβ–H bond to increase
the extent of proton transfer, and stabilize the negative charge
at the β-carbon, so that there is little change in the Cα–OAr
bond cleavage.
Change of mechanism
The kinetic results described above clearly indicate that the
reactant-like transition state for 1d changes toward E2-central
as a poorer leaving group is introduced (1b,c). When the leaving
group ability is further decreased to X = H (1a), the E2 transi-
tion state is destabilized and the E1cb mechanism emerges sim-
ultaneously. The mechanism change also occurs by the β–aryl
substituent, i.e., from E2 for Ph2CHCO2Ar to concurrent E2
and E1cb mechanisms for 1a. It is interesting to note that the
mechanistic changes occur by the minor variations in the
reactant structure.
We have previously reported that the mechanism of elimin-
ations from aryl phenylacetates changes from E2 to E1cb via a
competing E2 and E1cb mechanism.17 The E2 transition state
for this reaction was assumed to be E1cb-like and was located at
E on the energy diagram, which is far from the position of the
E1cb transition state (Fig. 2).17 On the other hand, the E2 tran-
sition state for 1a is similar to E2-central except that the extent
of Cβ–H bond cleavage is much larger. However, the position of
the transition state (A) is significantly different from that of the
E1cb transition state. To our knowledge, this is the first example
that shows the change in the elimination mechanism from
E2-central to a competing E2 and E1cb.
In conclusion, we have studied the ketene-forming elimin-
ϩ
ations from 1a–d promoted by R2NH–R2NH2 in 70 mol%
Fig. 5 Reaction coordinate diagram for the ketene-forming elimin-
ation. The effect of the change to a poorer leaving group and a weaker
base are shown by the shift of the transition state from D to B and D to
C, respectively. The transition states for eliminations from 1a,1d, and
p-nitrophenyl p-nitrophenylacetate are indicated as A, D, and E,
respectively.
MeCN (aq.). The reactions of 1b–d proceed by a concerted E2
mechanism via an E2-central transition state. As the leaving
group is made poorer, the E2 transition state becomes more
skewed toward the proton transfer and the E1cb mechanism
competes when X = H (1a). The mechanism of eliminations
from Ph2CHCO2Ar is also changed from E2 to a competing E2
and E1cb by the electron-withdrawing β–aryl substituent.
Noteworthy is the susceptibility of the ketene-forming transi-
tion state to the variation of the reactant structure.
This conclusion is supported by the interaction coefficients.
E
Table 3 shows that the β values of the k2 process for 1a–d
increase with a poorer leaving group. This effect corresponds to
a positive pxy interaction coefficient, pxy = ∂β/∂pKlg, which
describes the interaction between the base catalyst and the
leaving group.23–25 The observed increase in the |βlg| values with
a weaker base (Table 4) is another manifestation of this
effect, i.e., pxy = ∂βlg/∂pKBH > 0. On the More-O’Ferrall–Jencks
diagram in Fig. 5, a change to a poorer leaving group will raise
the energy of the bottom edge of the diagram shifting the tran-
sition state toward the product and the E1cb intermediate. The
transition state will then move toward the left as depicted by a
shift from D to B on the energy diagram, resulting in an
increase in β (Fig. 5).23–25 A further increase in the leaving group
basicity to X = H (1a) is expected to shift the transition state to
A, in agreement with the large β. Similarly, a weaker base will
raise the energy of the left side of the diagram and shift the
transition state from D to C to increase the extent of Cα–OAr
bond cleavage and |βlg| (Fig. 5).23–25 The positive pxy coefficients
are not consistent with an E1cb mechanisms for which pxy = 0 is
expected, but provide additional support for the concerted E2
mechanism.23–25
Experimental
Materials
Aryl bis(4Ј-chlorophenyl)acetates (1a–d) were prepared from
bis(4Ј-chlorophenyl)acetic acid and substituted phenols in the
presence of Et3N and 2-chloro-N-methylpyridinum iodide as
previously reported.15,19 The yield (%), melting points (ЊC), IR
(KBr, C᎐O, cmϪ1), NMR (300 MHz, CDCl ), and combustion
᎐
3
analysis data for the new compounds are as follows. Elemental
analysis for 1d could not be performed because it decomposed
after several hours.
4-Nitrophenyl bis(4Ј-chlorophenyl)acetate (1a)
1
Yield 52%; mp 87–88 ЊC; IR 1757 (C᎐O); H-NMR δ 5.21
᎐
(s, 1H), 7.25 (d, 2H, J = 9.2 Hz), 7.32 (d, 4H, J = 8.8 Hz), 7.38
(d, 4H, J = 8.8 Hz), 8.27 (d, 2H, J = 9.2 Hz). Anal. calcd for
C20H13Cl2NO4: C, 59.72; H, 3.26; N, 3.48. Found: C, 59.68; H,
3.16; N, 3.42%.
Effect of ꢀ–aryl substituent on the E2 transition state
2-Chloro-4-nitrophenyl bis(4Ј-chlorophenyl)acetate (1b)
Table 5 compares the rates and transition state parameters for
the E2 reactions of (4Ј-YC6H4)2CHCO2C6H4-4-NO2. The rate
of the E2 reaction is 3 fold faster when Y = Cl (1a) than when
Y = H, indicating a modest increase by the p-Cl groups. On the
other hand, Brönsted β increases and |βlg| remain nearly the
same by the same variation of the substituent. The transition
1
Yield 42%; mp 85–87 ЊC; IR 1759 (C᎐O); H-NMR δ 5.29
᎐
(s, 1H), 7.29 (d, 1H, J = 9.0 Hz), 7.35 (d, 4H, J = 8.8 Hz), 7.38
(d, 4H, J = 8.8 Hz), 8.17 (dd, 1H, J = 2.8 Hz and 9.0 Hz), 8.35
(d, 1H, J = 2.8 Hz). Anal. calcd for C20H12Cl3NO4: C, 55.01; H,
2.77; N, 3.21. Found: C, 55.08; H, 2.72; N, 3.12%.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 3 4 – 2 7 3 8
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2-Trifluoromethyl-4-nitrophenyl bis(4Ј-chlorophenyl)acetate (1c)
Control experiments
1
Yield 45%; mp 72–73 ЊC; IR 1765 (C᎐O); H NMR δ 5.26
The stabilities of 1a–d were determined as reported earlier.15–17
Solutions of 1a–c in MeCN were stable for at least 2 months in
solution at room temperature. However, MeCN solution of 1d
was stable for only several hours.
᎐
(s, 1H), 7.32 (d, 1H, J = 8.6 Hz), 7.38 (d, 4H, J = 8.6 Hz), 7.45
(d, 4H, J = 9.0 Hz), 8.46 (dd, 1H, J = 2.8 Hz and 9.0 Hz), 8.57
(d, 1H, J = 2.8 Hz). Anal. calcd for C21H12Cl2F3NO4: C, 53.64;
H, 2.57; N, 2.98. Found: C, 53.64; H, 2.53; N, 2.94%.
Acknowledgements
2-Nitro-4-nitrophenyl bis(4Ј-chlorophenyl)acetate (1d)
This work was supported by Pukyong National University
Research Foundation Grant in 2002.
1
Yield 42%; mp 85–87 ЊC; IR 1759 (C᎐O); H NMR δ 5.29
᎐
(s, 1H), 7.29 (d, 1H, J = 9.0 Hz), 7.35 (d, 4H, J = 8.8 Hz), 7.38
(d, 4H, J = 8.8 Hz), 8.17 (dd, 1H, J = 2.8 Hz and 9.0 Hz), 8.35
(d, 1H, J = 2.8 Hz).
References
Acetonitrile was purified as described previously.15 The buffer
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ϩ
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prepared by dissolving an equivalent amount of R2NH and
ϩ
R2NH2 ClϪ in 70 mol% MeCN (aq.). In all cases, the ionic
strength was maintained to 0.1 M with Bu4NϩBrϪ.
Kinetic studies
ϩ
Reactions of 1a–d with R2NH–R2NH2 buffer in 70 mol%
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spectrophotometer as described before.26,27 Due to the instabil-
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E
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O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 3 4 – 2 7 3 8
2738