Aminolysis of 2,4-dinitrophenyl X-substituted benzoates and Y-substituted phenyl benzoates in MeCN: Effect of the reaction medium on rate and mechanism

Article abstract of DOI:10.1002/chem.200500647

Second-order rate constants (k_N) have been determined spectrophotometrically for the reactions of 2,4-di-nitrophenyl X-substituted benzoates (1a-f) and Y-substituted phenyl benzoates (2a-h) with a series of alicyclic secondary amines in MeCN at

Full text of DOI:10.1002/chem.200500647

FULL PAPER
DOI: 10.1002/chem.200500647
Aminolysis of 2,4-Dinitrophenyl X-Substituted Benzoates and Y-Substituted
Phenyl Benzoates in MeCN: Effect of the Reaction Medium on Rate and
Mechanism
[
a]
Ik-Hwan Um,* Sang-Eun Jeon, and Jin-Ah Seok
Abstract: Second-order rate constants
kN) have been determined spectropho-
tometrically for the reactions of 2,4-di-
nitrophenyl X-substituted benzoates
nature of the substituent X in the non-
leaving group does not affect the rate-
determining step (RDS) or reaction
mechanism. The Hammett correlation
yl benzoate (1c) results in a linear
Brønsted-type plot with a bnuc value of
0.40, suggesting that bond formation
between the attacking amine and the
carbonyl carbon atom of 1c is little ad-
vanced in the transition state (TS). A
concerted mechanism is proposed for
the aminolysis of 1a–f in MeCN. The
(
ꢁ
(
1a–f) and Y-substituted phenyl ben-
with s constants also exhibits good
zoates (2a–h) with a series of alicyclic
secondary amines in MeCN at 25.0ꢀ
linearity with a large slope (1 =3.54)
Y
for the reactions of 2a–h with piperi-
dine, implying that the leaving-group
departure occurs at the rate-determin-
ing step. Aminolysis of 2,4-dinitrophen-
0
.18C. The k values are only slightly
N
larger in MeCN than in H O, although
medium change from H O to MeCN
2
2
the amines studied are approximately
appears to force the reaction to pro-
ceed concertedly by decreasing the sta-
bility of the zwitterionic tetrahedral in-
8
pK units more basic in the aprotic
a
solvent than in H O. The Yukawa–
2
Keywords: aminolysis · Brønsted ·
Hammett · kinetics · solvent effects
ꢀ
Tsuno plot for the aminolysis of 1a–f is
linear, indicating that the electronic
termediate (T ) in aprotic solvent.
[
1–6]
Introduction
ing step (RDS) for stepwise reactions.
The center of the
curve in curved Brønsted-type plots has been defined as
pK , for which the rate of breakdown of the zwitterionic tet-
rahedral intermediate (T ) is the same as the rate of its for-
The rate-determining step has been suggested to
change at pK from breakdown of T to its formation as the
attacking amine becomes more basic than the leaving group
by 4 to 5 pK units, indicating that the pK value is governed
0
The mechanism of ester aminolysis has been intensively in-
vestigated as a result of its importance in biological process-
a
ꢀ
[1–12]
[1–2]
es and synthetic methodology.
Linear free-energy rela-
mation.
0
a
ꢀ
tionships, such as Brønsted-type and Hammett equations,
have been most commonly used to determine reaction
mechanisms. Curved Brønsted-type plots, often found for
the aminolysis of esters with a good leaving group, have
been interpreted in terms of a change in the rate-determin-
0
a
a
by the basicity of the attacking amine and the leaving
[1–6]
group.
However, aminolysis of esters with a poor leaving
group or of thiono esters has been suggested to proceed
ꢀ
through one or two intermediates (T and its deprotonated
ꢁ
counterpart T ), depending on the basicity and nature of
the amines.
[
a] Prof. Dr. I.-H. Um, S.-E. Jeon, J.-A. Seok
Department of Chemistry
[
7–9]
Ewha Womans University, Seoul 120–750 (Korea)
Fax : (+82)2-3277-2349
E-mail: ihum@mm.ewha.ac.kr
In contrast to the effects of amine basicity and leaving-
group substituents, the effect of nonleaving-group substitu-
ents on the reaction mechanism is not yet completely under-
stood. For the pyridinolysis of 2,4-dinitophenyl X-substitut-
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. Please see: Fig-
ure S1 for Hammett plots for the reactions of 2a–h with piperidine in
MeCN; Figure S2 for a plot illustrating the dependence of Dlog
ed benzoates (X=H, 4-Cl, and 4-NO ) in 44% aqueous eth-
2
anol, Castro et al. demonstrated that the Brønsted-type plot
is downwardly curved for the reactions of unsubstituted ben-
zoate, but linear for the corresponding reactions of 4-chloro
k
N
(=logk
in H O) for the aminolysis of 2,4-dinitrophenyl benzoate (1c) in
MeCN and H
N
in MeCNꢁlogk
N
in H
2
O) on DpK
a
(=pK
a
in MeCNꢁpK
a
2
2
O (contains 20 mol% DMSO) at 25.0ꢀ0.18C; and Ta-
[10]
0
a
and 4-nitro derivatives. More recently, the pK value has
bles S1–S23 for the kinetic data for the aminolysis of 1a–f and 2a–h
in MeCN.
been reported to increase from 9.7 to 9.9 and >10 as the
Chem. Eur. J. 2006, 12, 1237 – 1243
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1237

substituent X changes from H to 4-Cl and 4-NO , respec-
2
tively, for the pyridinolysis of (S)-4-nitrophenyl X-substitut-
[11]
ed thiobenzoates in 44% aqueous ethanol. Thus, Castro
0
a
et al. concluded that the pK value increases as the substitu-
ent X in the nonleaving group is substituted for a stronger
[
11]
electron-withdrawing group.
with the conclusion drawn by Gresser and Jencks from reac-
This argument is consistent
clic secondary amines in MeCN [Eq. (1)]. Although scat-
tered information on the aminolyses of esters in aprotic sol-
[12]
tions of diaryl carbonates with quinuclidines. Gresser and
[
4a–c]
Jencks concluded that the electron-withdrawing substituent
vent is available,
the reaction mechanisms are not yet
0
in the nonleaving group increased the pK value by decreas-
clearly understood due to a lack of systematic studies. Until
recently, mechanistic studies in MeCN have been limited to
the reactions of esters with substituents at the acyl position
and/or on the leaving group. Structurally similar amines
a
ing the k /k ratio, that is, an electron-withdrawing substitu-
2
-1
ent in the nonleaving group favors amine expulsion, but re-
ꢀ
tards the departure of the leaving group from T (e.g., k
ꢁ
1
[12]
and k , respectively, in Eq. (1)).
with a wide pK range have not been used due to an ab-
2
a
sence of pKa data for these
compounds in aprotic solvent.
In the present study, we have
employed not only the substitu-
ents X and Y on the nonleaving
benzoyl moiety and the leaving
phenoxide group, but also alter-
native substituents at the Z-po-
sition in the attacking alicyclic
amines, for which pK values in
a
MeCN have recently been re-
[18]
ported.
Such a systematic
variation of X, Y, and Z has al-
lowed us to investigate the re-
action mechanism systematical-
ly. Herein, we report a detailed
mechanism for the reaction in
aprotic solvent as well as a
study into the effect of the reac-
tion medium on the reaction
rate and mechanism by compar-
In contrast, we have shown that the k /k ratio is inde-
pendent of the electronic nature of the substituent X in the
nonleaving group for aminolyses and alkaline hydrolyses of
ing the data obtained in this study with those reported for
the corresponding reactions performed in aqueous media.
2
-1
[
5,6,13,14]
various esters.
mett plots are nonlinear for the aminolyses of 2,4-dinitro-
Recently we have found that the Ham-
Results and Discussion
[5c]
phenyl X-substituted benzoates (1a–e) and benzenesulfo-
nates, and for alkaline hydrolyses of 1a–e
thiono analogues in H O (contains 20 mol% DMSO). For
[6]
[13]
and their
All the reactions in this study obeyed pseudo-first-order ki-
netics with excess amine up to over 90% of the total reac-
tion. Pseudo-first-order rate constants (kobsd) were deter-
mined from the equation ln(A ꢁA )=ꢁkobsdt+C. Correla-
[14]
2
all cases, p-electron-donating substituents (e.g., 4-MeO and
-Me) exhibit negative deviations from the Hammett plot,
4
1
t
and the degree of the negative deviation becomes more sig-
nificant with increasing electron-donating ability. Tradition-
ally, such a curved Hammett plot has been interpreted as a
tion coefficients were usually higher than 0.9995. It is esti-
mated from replicate runs that the uncertainty in rate con-
stants is less than ꢀ3%. All the plots of k
versus amine
obsd
[15]
change in the rate-determining step.
However, we have
concentration were linear, passing through the origin, indi-
cating that general base catalysis by a second amine mole-
cule is absent in this study. Thus, the apparent second-order
attributed the negative deviation exhibited by the electron-
donating substituents to the stabilization of the ground-state
of the substrate through resonance interactions between the
electron-donating substituent X and the electrophilic center
rate constants (k ) were determined from the slope of the
N
linear plots of kobsd versus amine concentration. Generally
five different amine concentrations were used to determine
the k values. The k values determined in this way are sum-
(
I$II) in view of the fact that the corresponding Yukawa–
[5c,6,13,14]
Tsuno plots are linear.
N
N
We have extended our study to the reactions of 1a–f and
Y-substituted phenyl benzoates 2a–h with a series of alicy-
marized in Tables 1–3. The kinetic conditions and results are
summarized in Tables S1–S23 in the Supporting Information.
1238
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1237 – 1243

Aminolysis of X- and Y-Substituted Benzoates
FULL PAPER
Effect of nonleaving-group substituents X on the reaction
rate and mechanism: The second-order rate constant (kN)
increases as the substituent X in the nonleaving group of
1
a–f changes from a strong electron-donating group to a
strong electron-withdrawing group (Table 1), that is, the kN
Table 1. Summary of second-order rate constants (k
N
) for the reactions
of 2,4-dinitrophenyl X-substituted benzoates (1a–f) with piperidine and
[
a]
morpholine at 25ꢀ0.18C in MeCN and H
2
O.
ꢁ
1
ꢁ1
k
N
m
s
X
Piperidine
Morpholine
1
1
1
1
1
1
a, 4-NO
b, 4-Cl
c, H
2
3120ꢀ70 (2880ꢀ20)
518ꢀ2 (371ꢀ4)
287ꢀ1 (174ꢀ2)
157ꢀ2 (100ꢀ2)
68.8ꢀ0.7 (56.2ꢀ0.9)
8.98ꢀ0.06
133ꢀ1 (138ꢀ3)
35.7ꢀ0.1 (32.7ꢀ0.2)
21.7ꢀ0.2 (19.6ꢀ0.3)
12.7ꢀ0.1 (11.2ꢀ0.1)
5.66ꢀ0.16 (5.77ꢀ0.06)
0.832ꢀ0.011
d, 4-Me
e, 4-MeO
f, 4-Me
2
N
[
a] Data in the parenthesis are rate constants for the corresponding reac-
tions performed in H
reference [5a].
2
O containing 20 mol% DMSO. Data taken from
value for the reactions with piperidine increases from 8.98
ꢁ
1
ꢁ1
to 287 and 3120m
s
as the substituent X changes from
Figure 1. Hammett and Yukawa–Tsuno plots (inset) for the reactions of
4
-Me N to H and then 4-NO , respectively. A similar result
2
2
1
a–f with piperidine (*) and morpholine (*) in MeCN at 25.0ꢀ0.18C.
is obtained for the corresponding reactions with morpholine.
The effect of the electronic nature of the substituent X on
the reaction rate is illustrated in Figure 1. The Hammett
plots are not linear, that is, the electron-donating substitu-
The identities of the points are given in Table 1.
ents, such as 4-Me N and 4-MeO, exhibit negative deviations
does not affect the reaction mechanism in the aminolysis
and hydrolysis of 1a–e and their related esters.
2
[5,6,13,14]
from the linearity, and the degree of deviation is more sig-
nificant for the stronger electron-donating substituent. How-
ever, the corresponding Yukawa–Tsuno plots shown in the
An electron-withdrawing substituent X on the nonleaving
group of 1a–f would accelerate the rate of nucleophilic
attack, but would retard the rate of leaving-group departure.
In contrast, an electron-donating substituent X would inhibit
nucleophilic attack, but would increase the rate at which the
leaving group departs. Thus, one might expect a large 1X
value for reactions in which the nucleophilic attack is the
rate-determining step, but a small one for reactions in which
leaving-group departure is involved in the rate-determining
step; this is due to the opposite substituent effect. In fact, a
remarkable decrease in the 1X value has been reported for
the reactions of semicarbazide with X-substituted benzalde-
hydes in weakly acidic medium (e.g., pH 3.9). The 1X has
been shown to decrease from 0.91 to nearly zero as the sub-
stituent X changes from electron-donating to electron-with-
inset of Figure 1 exhibit good linearity with 1 values of 1.30
X
and 0.98, and r values of 0.59 and 0.82 for the reactions with
piperidine and morpholine, respectively [Eq. (2)].
X
H
0
þ
0
logðk =k Þ ¼ 1½s þ rðs ꢁs Þꢂ
ð2Þ
The r value in the Yukawa–Tsuno equation represents the
resonance demand of the reaction center or the extent of
+
0
resonance contribution, while the term (s ꢁs ) is the reso-
nance substituent constant, which is a measure of the capaci-
ty for p-delocalization in the p-electron-donating substitu-
[
16,17]
ent.
Equation (2) becomes the Hammett equation when
r=0 or the Brown–Okamoto equation when r=1. As the r
value in the present system is neither 0 nor 1, the Yukawa–
Tsuno plots exhibit better linearity than the Hammett or
Brown–Okamoto plots. Thus, the nonlinear Hammett plots
shown in Figure 1 are not due to a change in the rate-deter-
mining step upon changing the substituent X in the nonleav-
ing group, but due to the ground-state stabilization induced
by the resonance interaction between the electron-donating
substituent X and the carbonyl group of the substrate (as il-
lustrated by the resonance structures I and II). This argu-
ment is consistent with our recent proposal that the elec-
tronic nature of the substituent X in the nonleaving group
[15a]
drawing groups.
Jencks has suggested that the reactions
proceed through an addition–elimination pathway and at-
tributed the remarkable decrease in the 1 value to a
X
[15a]
change in the rate-determining step.
The 1 values obtained in this study are 1.30 and 0.98 for
X
the reactions of 1a–f with piperidine and morpholine, re-
spectively. The magnitude of the 1 values is slightly smaller
X
for the reactions with weakly basic morpholine than for
those with strongly basic piperidine. However, one cannot
attribute such a small decrease in the 1X value to a change
in the rate-determining step. To obtain more information on
Chem. Eur. J. 2006, 12, 1237 – 1243
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1239

I.-H. Um et al.
o
the reaction mechanism, the effect of the substituent Y in
the leaving group on reactions rates has been investigated.
that s constants result in much better Hammett correlation
ꢁ
than s or s constants for the alkaline hydrolyses of 2a–h
and their thiono analogues,
[13,14]
in which the leaving-group
Effect of the leaving-group substituent Y on the reaction
rate and mechanism: The reactivity of 2a–h increases as the
substituent Y on the leaving group becomes a stronger elec-
tron-withdrawing group (Table 2), that is, the kN value for
departure has been suggested to be little advanced (if at all)
in the transition state of the rate-determining step. Howev-
0
er, in the present system, s (or s) constants exhibit much
ꢁ
ꢁ
poorer correlation than s constants. The fact that s con-
0
stants give a much better correlation than s (or s) constants
implies that the oxygen atom in the leaving aryloxide bears
a partial negative charge, which can be delocalized on sub-
stituent Y through resonance interactions in the transition
state. Thus, one can suggest that the leaving-group departure
occurs at the rate-determining step for the reactions of 2a–h
with piperidine in MeCN.
Table 2. Summary of second-order rate constants (k
of Y-substituted phenyl benzoates (2a–h) with piperidine in MeCN at
5ꢀ0.18C.
N
) for the reactions
2
2
ꢁ1 ꢁ1
2
ꢁ1 ꢁ1
Y
10 k
N
m
s
Y
10 k
N
m
s
2
2
2
2
a, 3-COMe
b, 3-CHO
c, 4-COOEt
d, 4-COMe
0.00791ꢀ0.00014
0.0220ꢀ0.0015
0.224ꢀ0.004
2e, 4-CN
2 f, 4-CHO
2g, 4-NO
2h, 3,4-(NO )
2 2
3.66ꢀ0.11
2.46ꢀ0.03
53.9ꢀ1.1
6920ꢀ70
2
0.334ꢀ0.003
the reactions of 2a–h with piperidine in MeCN increases
ꢁ
5
ꢁ2
ꢁ1 ꢁ1
from 7.91”10 to 3.66”10 and 69.2m
ent Y changes from 3-COMe to 4-CN and then to 3,4-
NO ) , respectively. The effect of substituent Y on the reac-
s
as the substitu-
(
2
2
tivity is illustrated in Figure 2. The Hammett plot is linear
Three transition states can be proposed to account for the
present result: 1) TS represents the transition-state struc-
1
ture for the reaction in which the nucleophilic attack and
the departure of the leaving group occur simultaneously (a
concerted mechanism). 2) TS represents the transition-state
2
structure for the stepwise mechanism in which leaving-group
departure occurs at the rate-determining step. 3) TS applies
3
to the transition-state structure for the stepwise mechanism
in which the leaving-group departure occurs after the rate-
determining step. Accordingly, one can exclude TS in the
3
ꢁ
present system on the basis of the fact that s constants
0
result in a much better Hammett correlation than s or s
constants.
Menger et al. obtained 1X and 1Y values of 1.02 and 6.24
for the reactions of pyrrolidine with 4-nitrophenyl X-substi-
tuted benzoates and Y-substituted phenyl acetates in MeCN,
Figure 2. Hammett plot for the reactions of 2a–h with piperidine in
MeCN at 25.0ꢀ0.18C. The identities of the points are given in Table 2.
[4a]
respectively. Although this result implies that in the RDS
the leaving group departs in either a concerted or a stepwise
mechanism, Menger et al. chose to favor a stepwise mecha-
nism, as addition intermediates are common in acyl-group
with a 1Y value of 3.54. This is much larger than the 1X
value of 1.30 obtained for the reactions of 1a–f with piperi-
dine, indicating that the rate of the reaction is significantly
more dependent on the electronic nature of the substituent
in the leaving group than in the nonleaving group.
[4a]
transfer reactions in H O.
2
However, our results could not differentiate a stepwise
mechanism (TS ) from a concerted mechanism (TS ) as ad-
2
1
ꢁ
More importantly, Figure 2 shows that s constants result
ditional data was required. Thus, the kinetic study was ex-
tended to the reactions of 1c with a series of alicyclic secon-
0
in better Hammett correlation than s or s constants (for
comparison see Figure S1 in the Supporting Information,
dary amines whose pK values in MeCN have recently been
reported.
a
[18]
which illustrates the corresponding Hammett plots with s or
0
s constants). If the leaving-group departure were not in-
volved in the rate-determining step, the oxygen atom of the
leaving aryloxide should not bear any negative charge in the
transition state. In this case, the use of s constants should
Effect of amine basicity on reaction rate and mechanism:
The reactivity of amines toward 1c increases with the in-
creasing the basicity of amines (Table 3). The effect of
amine basicity on reactivity is illustrated in Figure 3. The
o
give the best correlation. In fact, we have recently shown
1240
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 1237 – 1243

Aminolysis of X- and Y-Substituted Benzoates
FULL PAPER
Table 3. Summary of second-order rate constants (k
with alicyclic secondary amines at 25ꢀ0.18C in MeCN and H
N
) for the reactions of 2,4-dinitrophenyl benzoate (1c) the present study is considered
[
a]
2
O.
to be too small for reactions
which proceed through TS2.
Such a small bnuc value is typical
for reactions in which bond for-
mation is not much advanced,
indicating that the reaction pro-
MeCN
2
H O
ꢁ
1
ꢁ1
ꢁ1 ꢁ1
Entry
Amines
pK
a
k
N
m
s
pK
a
k
N
m
s
[
[
[
[
b]
b]
b]
b]
[c]
[c]
[d]
[c]
[c]
1
2
3
4
piperidine
piperazine
1-(2-hydroxyethyl)piperazine
morpholine
18.8
18.2
17.6
16.0
287ꢀ1
287ꢀ2
11.02
9.85
9.38
8.65
174ꢀ2
[c]
[c]
82.1ꢀ0.8
27.4ꢀ0.2
19.6ꢀ0.3
64.7ꢀ0.7
21.7ꢀ0.2
ceeds through TS1 or TS , in
3
[
a
a] Contains 20 mol% DMSO. [b] The pK data in MeCN taken from reference [18]. [c] Data taken from ref-
which the degree of bond for-
mation is not significant. How-
erence [5a]. [d] Data taken from reference [9b].
ever, TS has already been ex-
3
cluded, as mentioned in the
ꢁ
preceding section, on the basis of the fact that s constants
0
exhibit much better Hammett correlation than s or s con-
stants. Thus, one can propose that the aminolysis of 1a–f in
MeCN proceeds through a concerted mechanism with a
transition state similar to TS , and the change in the
1
medium from H O to MeCN is responsible for the mecha-
2
nism change from a stepwise to a concerted process. Lee
et al. suggested a similar proposal, that is, the solvent
change from H O to MeCN causes a mechanistic change
2
from stepwise to concerted for the reactions of aryl chloro-
[20]
thionoformates with anilines.
Effect of medium on reaction rate and mechanism: Amines
are more reactive in MeCN than in H O (Tables 1 and 3).
2
Figure 3. Brønsted-type plot for the reactions of 1c with alicyclic secon-
dary amines in MeCN at 25.0ꢀ0.18C. The identities of the points are
given in Table 3.
However, the rate enhancement observed upon the medium
change from H O to MeCN is considered to be insignificant,
2
as these amines are approximately 8 pK units more basic in
a
[18]
the aprotic solvent than in H O. This argument can be fur-
2
ther supported by analyzing the effect of the reaction
Brønsted-type plot is linear with a bnuc value of 0.40ꢀ0.06.
medium on reactivity (Dlogk =logkN in MeCNꢁlogk in
N
N
The linear Brønsted-type plot suggests that the reactions of
H O) and on the amine basicity (DpK =pK in MeCNꢁpK
2
a
a
a
1
c with these amines proceeds through a common mecha-
in H O). The plot of Dlogk versus DpK is linear with a
2 N a
nism without changing the rate-determining step or the reac-
tion mechanism upon changing the basicity of the amines.
The magnitude of the bnuc values represents the position
of the transition state along the reaction coordinate or the
relative degree of bond formation between nucleophile and
slope of 0.46 (see Figure S2 in the Supporting Information).
Such a linear plot implies that the increased basicity of
amines is responsible for their enhanced reactivity in
[21]
MeCN.
The effect of the reaction medium on reactivity has been
suggested to be highly dependent on the nature of reac-
[19]
electrophile at the rate-determining transition state.
A
[22]
large bnuc value (0.9ꢀ0.2) has often been observed for reac-
tants. Nucleophilic substitution reactions involving anionic
nucleophiles exhibit significant rate acceleration, whereas
reactions between neutral molecules passing through a parti-
ally charged transition state or a zwitterionic intermediate
tions in which the breakdown of the intermediate to the
[1–6]
products is the rate-determining step (TS ).
In contrast, a
2
small bnuc value (0.3ꢀ0.1) has been reported for reactions
which proceed in a stepwise manner through a transition-
result in rate retardation upon the medium change from
[1–6]
[22]
state structure similar to TS3.
reported biphasic Brønsted-type plots for the reactions of
c with a series of primary and secondary amines in H O,
In fact, we have recently
H O to dipolar aprotic solvents. This is consistent with the
2
fact that the reactivity of the amines in this study increases
only slightly, although their basicity increases about 8 pKa
1
2
that is, bnuc decreases from 0.76ꢀ0.02 to 0.35ꢀ0.01 as the
units upon the medium change from H O to MeCN.
As MeCN cannot solvate ionic species as strongly as H O,
2
attacking amine becomes more basic than the leaving group
2
[5a,b]
by approximately 5 pK units.
The biphasic Brønsted-
partially charged transition states and zwitterionic inter-
a
ꢀ
type plots have been attributed to a change in the rate-de-
termining step from the breakdown of a zwitterionic inter-
mediate T would be destabilized upon the medium change
from H O to MeCN. One can expect that the zwitterionic
2
ꢀ
ꢀ
mediate (e.g., T in Equation (1)) to its formation.
intermediate T would be more destabilized than partially
If the aminolysis of 1c in the present system proceeded
through TS , a b value of approximately 0.9ꢀ0.2 should
charged transition states in MeCN. Thus, one can suggest
that the medium change from H O to MeCN forces the ami-
2
nuc
2
have been obtained. Thus, the b value of 0.40 obtained in
nolysis of 1a–f to proceed concertedly without the involve-
nuc
Chem. Eur. J. 2006, 12, 1237 – 1243
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1241

I.-H. Um et al.
ꢀ
ment of the intermediate T due to its instability in the
aprotic solvent.
Acknowledgements
The authors are grateful for the financial support from the Korea Re-
search Foundation (KRF-2002–070-C00061).
Conclusion
Kinetic studies on the aminolyses of 2,4-dinitrophenyl
X-substituted benzoates (1a–f) and Y-substituted phenyl
benzoates (2a–h) have allowed us to conclude the follow-
ing:
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1
2
3
) The Yukawa–Tsuno plots for the reactions of 1a–f exhib-
it good linearity, indicating that the electronic nature of
the substituent X does not affect the reaction mecha-
nism.
Santos, J. Org. Chem. 2003, 68, 8157–8161; d) E. A. Castro, M. An-
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[
ꢁ
) The fact that s constants result in good Hammett corre-
lation with a 1Y value of 3.54 for the reactions of 2a–h,
suggests that the leaving-group departure occurs at the
rate-determining step.
) A bnuc value of 0.40 for the aminolysis of 1c suggests
that bond formation between the carbonyl carbon of 1c
and the attacking amine is little advanced in the transi-
tion state.
6
9, 3150–3153; d) H. K. Oh, J. M. Lee, D. D. Sung, I. Lee, Bull.
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1
995, 691–696; c) A. B. Maude, A. Williams, J. Chem. Soc. Perkin
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4) Amines are slightly more reactive in MeCN than in H O,
2
although they are approximately 8 pK units more basic
a
in the aprotic solvent than in H O.
2
5) The aminolysis of 1a–f proceeds concertedly through TS1
[
5] a) I. H. Um, J. S. Min, H. W. Lee, Can. J. Chem. 1999, 77, 659–666;
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in MeCN. The medium change from H O to MeCN ap-
2
pears to force the reaction to proceed concertedly by de-
creasing the stability of the zwitterionic tetrahedral inter-
5
659–5663; c) I. H. Um, K. H. Kim, H. R. Park, M. Fujio, Y. Tsuno,
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ꢀ
mediate (T ) in aprotic solvent.
[
[
7] a) E. A. Casto, A. Galvez, L. Leandro, J. G. Santos, J. Org. Chem.
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002, 67, 4309–4315; b) E. A. Castro, M. Angel, D. Arellano, J. G.
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Experimental Section
Materials: Compounds 1a–f and 2a–h were readily prepared from the re-
action of X-substituted benzoyl chloride with Y-substituted phenol in the
presence of triethylamine in anhydrous ether as previously reported.
[
8] a) H. K. Oh, J. M. Lee, H. W. Lee, I. Lee, Int. J. Chem. Kinet. 2004,
[
4a,5]
3
6, 434–440; b) H. K. Oh, M. H. Ku, H. W. Lee, I. Lee, J. Org.
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1
and spectral data, such as H NMR and IR spectra. MeCN was distilled
over phosphorus pentaoxide under nitrogen. Amines and other chemicals
were of the highest quality available.
[
6
9, 6365–6370; b) I. H. Um, J. A. Seok, H. T. Kim, S. K. Bae, J. Org.
Kinetics: The kinetic study was performed by using a Scinco S-3150 PDA
UV-vis spectrophotometer for slow reactions (t1/2 ꢃ10 s) or an Applied
Photophysics SX-17 MV stopped-flow spectrophotometer for fast reac-
tions (t1/2 <10 s); both spectrophotometers were equipped with a Neslab
RTE-110 constant temperature circulating bath to keep the reaction tem-
perature at 25.0ꢀ0.18C. All the reactions were carried out under
pseudo-first-order conditions in which the amine concentration was at
least 20 times greater than the substrate concentration. Solutions were
transferred by using Hamilton gas-tight syringes under nitrogen. The re-
actions were followed by monitoring the aryloxide produced in the reac-
Chem. 2003, 68, 7742–7746; c) I. H. Um, S. E. Lee, H. J. Kwon, J.
Org. Chem. 2002, 67, 8999–9005; d) I. H. Um, H. J. Kwon, D. S.
Kwon, J. Chem. Res. Synop. 1995, 301.
[
[
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53–457.
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2
[
[
12] M. J. Gresser, W. P. Jencks, J. Am. Chem. Soc. 1977, 99, 6963–6970.
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tion at a fixed wavelength, corresponding to the maximum absorption of
ꢁ
Y-C
reported.
Product Analysis: Y-substituted phenoxide (Y-C
6
H
4
O
(lmax). Other detailed kinetic methods have been previously
2
002, 67, 8475–8480.
[
5a–c]
[
14] I. H. Um, J. Y. Lee, H. T. Kim, S. K. Bae, J. Org. Chem. 2004, 69,
ꢁ
6
H
4
O ) was liberated
2436–2441.
quantitatively and identified as one of the reaction products by compari-
son of the UV-vis spectra after completion of the reactions with those of
authentic samples under the same kinetic conditions.
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1242
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Chem. Eur. J. 2006, 12, 1237 – 1243

Aminolysis of X- and Y-Substituted Benzoates
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9
Received: June 7, 2005
Published online: November 3, 2005
Chem. Eur. J. 2006, 12, 1237 – 1243
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1243