A kinetic study on nucleophilic displacement reactions of aryl benzenesulfonates with potassium ethoxide: Role of K+ ion and reaction mechanism deduced from analyses of LFERs and activation parameters

Article abstract of DOI:10.1021/jo302373y

Pseudofirst-order rate constants (k_obsd) have been measured spectrophotometrically for the nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates 4a-f and Y-substituted phenyl benzenesulfonates 5a-k with EtOK in anhydrous ethanol. Dissection of k _obsd into k_EtO^- and k_EtOK (i.e., the second-order rate constants for the reactions with the dissociated EtO ^- and ion-paired EtOK, respectively) shows that the ion-paired EtOK is more reactive than the dissociated EtO^-, indicating that K ⁺ ion catalyzes the reaction. The catalytic effect exerted by K ⁺ ion (e.g., the k_EtOK/k_EtO^- ratio) decreases linearly as the substituent X in the benzenesulfonyl moiety changes from an electron-donating group (EDG) to an electron-withdrawing group (EWG), but it is independent of the electronic nature of the substituent Y in the leaving group. The reactions have been concluded to proceed through a concerted mechanism from analyses of the kinetic data through linear free energy relationships (e.g., the Bronsted-type, Hammett, and Yukawa-Tsuno plots). K⁺ ion catalyzes the reactions by increasing the electrophilicity of the reaction center through a cyclic transition state (TS) rather than by increasing the nucleofugality of the leaving group. Activation parameters (e.g., ΔH^? and ΔS^?) determined from the reactions performed at five different temperatures further support the proposed mechanism and TS structures.

Full text of DOI:10.1021/jo302373y

Article
pubs.acs.org/joc
A Kinetic Study on Nucleophilic Displacement Reactions of Aryl
Benzenesulfonates with Potassium Ethoxide: Role of K+ Ion and
Reaction Mechanism Deduced from Analyses of LFERs and Activation
Parameters
Ik-Hwan Um,*,† Ji-Sun Kang,† Young-Hee Shin,† and Erwin Buncel‡
†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea
‡Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
ABSTRACT: Pseudofirst-order rate constants (kobsd) have
been measured spectrophotometrically for the nucleophilic
substitution reactions of 2,4-dinitrophenyl X-substituted
benzenesulfonates 4a−f and Y-substituted phenyl benzenesul-
fonates 5a−k with EtOK in anhydrous ethanol. Dissection of
−
k
obsd into kEtO and kEtOK (i.e., the second-order rate constants for the reactions with the dissociated EtO− and ion-paired EtOK,
respectively) shows that the ion-paired EtOK is more reactive than the dissociated EtO−, indicating that K+ ion catalyzes the
reaction. The catalytic effect exerted by K+ ion (e.g., the kEtOK/kEtO ratio) decreases linearly as the substituent X in the
−
benzenesulfonyl moiety changes from an electron-donating group (EDG) to an electron-withdrawing group (EWG), but it is
independent of the electronic nature of the substituent Y in the leaving group. The reactions have been concluded to proceed
through a concerted mechanism from analyses of the kinetic data through linear free energy relationships (e.g., the Brønsted-
type, Hammett, and Yukawa−Tsuno plots). K+ ion catalyzes the reactions by increasing the electrophilicity of the reaction center
through a cyclic transition state (TS) rather than by increasing the nucleofugality of the leaving group. Activation parameters
(e.g., ΔH‡ and ΔS‡) determined from the reactions performed at five different temperatures further support the proposed
mechanism and TS structures.
strongly dependent on the nature of the electrophilic center
(PO vs PS).
INTRODUCTION
■
Metal ions have often been reported to catalyze acyl-group
transfer reactions as a Lewis acid catalyst.1−10 Since Lewis
acidity increases as the charge density of metal ions increases,
studies have focused mostly on the reactions involving
multivalent metal ions (e.g., La3+, Eu3+, Co3+, Zn2+, Cu2+,
Mn2+, etc.).1−5 It is well-known that alkali-metal ions are
ubiquitous in nature and play important roles in biological
processes (e.g., the Na+/K+ pump to maintain high K+ and low
Na+ concentrations in mammalian cells).11 Nevertheless, the
effect of alkali-metal ions on acyl-group transfer reactions has
much less been investigated.
The first kinetic study on the effect of alkali-metal ions was
performed by our group for the phosphinyl-transfer reaction of
4-nitrophenyl diphenylphosphinate 1a with alkali metal
ethoxides (EtOM, M = Li, Na, and K) in anhydrous ethanol.6a
The study has shown that M+ ions catalyze the reaction but the
catalytic effect disappears in the presence of complexing agents
for M+ ions, e.g., 18-crown-6-ether (18C6) or cryptands.6a
Besides, the catalytic effect has been found to increase as the
size of M+ ions decreases (i.e., K+ < Na+ < Li+).6a In contrast,
we have recently reported that the reaction of 4-nitrophenyl
diphenylphosphinothioate 1b with EtOM is inhibited by Li+
ion but is catalyzed by Na+ and K+ ions.9 More interestingly,
the K+ ion complexed by 18C6 exhibited stronger catalytic
effect than Na+ or K+ ion,9 indicating that the role of M+ ions is
A similar result has been reported for the phosphoryl-transfer
reactions of ethyl paraoxon 2a and methyl paraoxon 3a with
EtOM and the corresponding reactions of ethyl parathion 2b
and methyl parathion 3b; i.e., (1) the reaction of the PO
compounds 2a and 3a is catalyzed by M+ ions in the order K+ <
Na+ < Li+, (2) the reaction of the PS compound 2b is
catalyzed by K+ ion but inhibited by Na+ and Li+ ions, and (3)
the reaction of the PS compound 3b is strongly catalyzed by
18C6-complexed K+ ion but inhibited by Na+ and Li+ ions.10
The contrasting M+ ion effects were discussed on the basis of
competing electrostatic effects and solvational requirements as
function of anionic electrophilic field strength and cationic size
(Eisenman’s theory).12
The effect of M+ ions on sulfonyl-transfer reactions has also
been investigated. We have reported that the reaction of Y-
Received: October 25, 2012
© XXXX American Chemical Society
A
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substituted phenyl benzenesulfonates (Y = 4-NO2, 3-NO2, 4-
CF3, 3-Br, 4-Cl, and H) with EtOM is catalyzed by K+ ion but
inhibited by Li+ ion.13 The reaction was suggested to proceed
through a stepwise mechanism, in which formation of the
EtO−S bond is well advanced in the rate-determining transition
state (TS), while breakdown of the S−OAr bond is negligible
on the basis of the kinetic results that σ° constants result in
much better Hammett correlation than σ− constants.13
Accordingly, it has been concluded that K+ ion catalyzes the
reaction by stabilizing the TS through TS1, in which K+ ion
increases the electrophilicity of the sulfonyl sulfur.13 However, a
possibility that K+ ion catalyzes the reaction by increasing the
nucleofugality of the leaving group cannot be excluded, if
breakdown of the S−OAr bond is involved in the rate-
determining step (RDS). Thus, more detailed information on
the reaction mechanism including the TS structure is necessary
to understand the role of K+ ion.
were higher than 0.9995. The uncertainty in the kobsd values is
estimated to be less than 3% from replicate runs. The kobsd
values and detailed kinetic conditions for the reactions of 2,4-
dinitrophenyl benzenesulfonates (4a−f) and Y-substituted
phenyl benzenesulfonates (5a−k) are summarized in Tables
S1−S23 in the Supporting Information (SI).
As shown in Figure 1, the plot of kobsd vs [EtOK] for the
reaction of 2,4-dinitrophenyl 4-methoxybenzenesulfonate (4a)
We have now carried out a systematic study on the reaction
of 2,4-dinitrophenyl X-substituted benzenesulfonates 4a−f and
Y-substituted phenyl benzenesulfonates 5a−k with EtOK to
revisit the role of K+ ion as well as the reaction mechanism
(Scheme 1). We have introduced various substituents on the
Figure 1. Plot of kobsd vs [EtOK] for the reaction of 2,4-dinitrophenyl
4-methoxybenzenesulfonate 4a with EtOK in anhydrous EtOH at 25.0
0.1 °C. The curved line was calculated by eq 2.
Scheme 1
with EtOK curves upward. Similarly curved plots are illustrated
in Figures S6A−S10A in the SI for the reactions of 2,4-
dinitrophenyl X-substituted benzenesulfonates (4b−f), and in
Figures S11A−S27A (SI) for those of Y-substituted phenyl
benzenesulfonates (5a−k). It is noted that such upward
curvature illustrated in the plots of kobsd vs [EtOK] is typical
of reactions of esters with EtOM (M = K, Na, and Li), in which
the ion-paired EtOM was reported to be more reactive than the
dissociated EtO−.6
−
Dissection of kobsd into kEtO and kEtOK. To confirm that
the ion-paired EtOK is more reactive than the dissociated
EtO−, the kobsd values for the reactions of 4a−f have been
nonleaving sulfonyl moiety as well as on the leaving aryloxide.
Moreover, activation parameters (i.e., ΔH‡ and ΔS‡) have been
measured to get further information on the TS structures.
Analysis of the current kinetic data through linear free energy
relationships (e.g., Brønsted-type, Hammett, and Yukawa−
Tsuno plots) has led us to conclude that the reaction proceeds
through a concerted mechanism, in which K+ ion catalyzes the
reaction by increasing the elctrophilicity of the reaction center
rather than by increasing the nucleofugality of the leaving
aryloxide. The activation parameters (i.e., ΔH‡ and ΔS‡)
determined from the reactions performed at 5 different
temperatures further support the proposed mechanism and
TS structures.
−
dissected into kEtO and kEtOK, i.e., the second-order rate
constants for the reactions with the dissociated EtO− and ion-
paired EtOK, respectively. EtOM was reported to exist as a
dimer or other aggregates in a high concentration region (e.g.,
[EtOM] > 0.1 M) but was suggested to exist as the dissociated
and ion-paired species in concentrations below 0.1 M.14 Since
the reactions in this study were carried out in a low
concentration region (e.g., [EtOK] < 0.1 M), EtOK would
exist as the dissociated EtO− and ion-paired EtOK. Accordingly,
both the dissociated EtO− and ion-paired EtOK would react
−
with the substrate with rate constants kEtO and kEtOK
,
respectively, as shown in Scheme 2.
A rate eq 1 can be derived on the basis of the reactions
proposed in Scheme 2. Under a pseudofirst-order kinetic
condition, kobsd can be expressed as eq 2. Since the dissociation
RESULTS AND DISCUSSION
■
The reactions were followed spectrophotometrically by
monitoring the appearance of the leaving Y-substituted
phenoxide ion under pseudofirst-order conditions with large
excess of EtOK. All reactions in the current study obeyed
pseudofirst-order kinetics. Pseudofirst-order rate constant
(kobsd) was calculated from the slope of the linear plot of ln
(A∞ − At) vs t. The correlation coefficients of the linear plots
constant Kd = [EtO−]eq[K+]eq/[EtOK]eq, and [EtO−]eq
=
[K+]eq at equilibrium, eq 2 becomes eq 3. The concentrations
of [EtO−]eq and [EtOK]eq can be calculated from the previously
reported Kd value (i.e., Kd = 1.11 × 10−2 M for EtOK)15 and
the total concentration of EtOK (i.e., [EtOK]) using eqs 4 and
5.
B
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Scheme 2
Table 1. Summary of the Kinetic Data for the Reactions of
2,4-Dinitrophenyl X-Substituted Benzenesulfonates 4a−f
with EtOK in Anhydrous EtOH at 25.0 0.1 °C
−1
kEtO /M s−1
kEtOK/M−1 s−1
kEtOK/kEtO
−
−
entry
X
4a
4b
4c
4d
4e
4f
4-MeO
4-Me
H
0.624 0.100
1.53 0.20
5.82 0.40
19.9 3.0
505 20
9.16 0.10
20.2 0.2
40.6 0.4
14.7
13.2
6.98
5.48
2.91
2.23
4-Cl
109
3
3-NO2
4-NO2
1470 19
1740 27
−
780 30
−
rate = kEtO [EtO ]eq [4] + kEtOK[EtOK]eq [4]
(1)
(2)
(3)
(4)
−
the substituent X changes from 4-MeO to H and 4-NO2, in
turn. A similar result is demonstrated for the reactions with the
ion-paired EtOK, although kEtOK > kEtO regardless of the
−
kobsd = kEtO [EtO ]eq + kEtOK[EtOK]eq
−
−
k
obsd/[EtO−]eq = kEtO +kEtOK[EtO−]eq /Kd
[EtOK] = [EtO−]eq + [EtOK]eq
electronic nature of the substituent X. This confirms that the
ion-paired EtOK is more reactive than the dissociated EtO−. It
is noted that the catalytic effect exerted by K+ ion (i.e., the
[EtO−]eq = [−Kd + (Kd + 4Kd[EtOK])1/2]/2
One might expect that the plot of kobsd/[EtO−]eq vs [EtO−]eq
is linear with a positive intercept, if the reaction proceeds as
proposed in Scheme 2. In fact, Figure 2 shows that the plot is
2
−
k
EtOK/kEtO ratio) decreases linearly as the substituent X
changes from an EDG to an EWG (see also Figure S1 in the
SI).
(5)
The effect of substituent X on the reactivity of the dissociated
EtO− and ion-paired EtOK is illustrated in Figure 3. The
Figure 2. Plot of kobsd/[EtO−]eq vs [EtO−]eq for the reaction of 2,4-
dinitrophenyl 4-methoxybenzenesulfonate 4a with EtOK in anhydrous
EtOH at 25.0 0.1 °C.
Figure 3. Hammett plots for the reactions of 2,4-dinitrophenyl X-
−
○
substituted benzenesulfonates 4a−f with the dissociated EtO ( ) and
●
ion-paired EtOK ( ) in anhydrous EtOH at 25.0
0.1 °C. The
identity of the points is given in Table 1.
linear with a positive intercept for the reaction of 4a. Similarly
linear plots are illustrated for the reactions of 4b−f in Figures
S6B−S10B in the SI, indicating that the above equations
derived from the reactions proposed in Scheme 2 are indeed
correct. Thus, the kEtO and kEtOK/Kd values have been
calculated from the intercept and the slope of the linear
Hammett plots for the reactions of 4a−f with the dissociated
EtO− and ion-paired EtOK exhibit excellent linear correlations
with large slopes, i.e., ρX = 2.89 and 2.15 for the reactions with
EtO− and EtOK, respectively. The ρX values obtained for the
current reactions are comparable with those reported
previously for reactions of esters with anionic nucleophiles
(e.g., ρX = 2.90 0.05 for the reactions of 4-nitrophenyl X-
substituted benzoates with EtO− and C6H5O− ions,16 and ρX =
2.28 for the reactions of O-4-nitrophenyl X-substituted
−
plots, respectively. The kEtOK value can be calculated from the
k
EtOK/Kd ratios determined above and the previously reported
Kd value.15 The kEtO and kEtOK values calculated in this way are
summarized in Table 1 for the reaction of 4a−f together with
−
−
the kEtOK/kEtO ratios. The effect of substituent X on the
thionobenzoates with N3 ion17) but are much larger than
−
reactivity and reaction mechanism will be discussed sub-
sequently.
those reported for the reactions with neutral amines (e.g., ρX =
0.58 for the reactions of 4a−f with piperidine,18 and ρX = 0.42−
0.75 for the reactions of 4-nitrophenyl X-substituted benzoates
with various amines19). The large positive ρX values obtained in
the current reactions suggest that the negative charge
developed in the sulfonyl moiety of the TS is significant.
However, the current Hammett plots alone are not sufficient to
Effect of Substituent X on Reactivity and Reaction
Mechanism. As shown in Table 1, the reactivity of 4a−f
toward the dissociated EtO− increases as the substituent X in
the benzenesulfonyl moiety changes from an electron-donating
group (EDG) to an electron-withdrawing group (EWG); e.g.,
kEtO increases from 0.624 M−1 s−1 to 5.82 and 780 M−1 s−1 as
−
C
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conclude whether the reactions proceed through a concerted
mechanism or through a stepwise pathway.
Effect of Substituent Y on Reactivity and Reaction
Mechanism. To obtain more conclusive information on the
reaction mechanism, the kinetic study has been extended to the
reactions of Y-substituted phenyl benzenesulfonates 5a−k. The
−
second-order rate constants kEtO and kEtOK are summarized in
−
Table 2 together with the kEtOK/kEtO ratios. It can be observed
Table 2. Summary of the Kinetic Data for the Reactions of Y-
Substituted Phenyl Benzenesulfonates 5a−k with the
Dissociated EtO− and Ion-Paired EtOK in Anhydrous EtOH
a
at 25.0 0.1 °C
103kEtO
/
103kEtOK
/
kEtOK/
−
−
entry
Y
pKa
M−1 s−1
M−1 s−1
kEtO
5a
5b
5c
5d
5e
5f
2,4-(NO2)2 8.04
3,4-(NO2)2 9.75
5820 400
1400 110
40600 400
5700 100
6.98
4.07
4.77
6.83
12.7
9.76
5.23
6.17
7.45
7.78
10.2
Figure 4. Brønsted-type plots for the reactions of Y-substituted phenyl
b
b
−
4-NO2
4-CHO
2,4-Cl2
4-CN
4-COMe
4-CF3
3-Br
11.98
12.66
12.91
13.04
13.26
13.76
14.54
14.90
15.76
28.7
137
●
benzenesulfonates 5a−k with the dissociated EtO ( ) and ion-paired
○
3.28 0.70
4.30 1.00
6.79 0.60
2.56 0.20
22.4 0.4
54.6 0.9
66.3 0.3
13.4 0.1
EtOK ( ) in anhydrous EtOH at 25.0 0.1 °C. The identity of the
points is given in Table 2.
5g
5h
5i
Hammett plots constructed using σ° and σ− constants are
useful to study the reaction mechanism. If the current reactions
proceed through a concerted mechanism as suggested above,
breakdown of the S−OAr bond occurs at the RDS. In this case,
a partial negative charge develops on the oxygen atom of the
leaving aryloxide in the rate-determining TS. Since such
negative charge can be delocalized to the substituent Y through
resonance interactions, σ− constants should give a better
Hammett correlation than σ° constants. On the contrary, if the
reaction proceeds through a stepwise mechanism, breakdown of
the S−OAr bond should occur after the RDS. This is because
EtO− is much more basic and a poorer nucleofuge than the Y-
substituted phenoxide ion. Accordingly, if the reaction proceeds
through a stepwise mechanism, no negative charge would
develop on the oxygen atom of the leaving aryloxide in the rate-
determining TS. In this case, σ° constants should result in a
better Hammett correlation than σ− constants.
b
b
2.69
16.6
b
b
1.10
8.20
b
b
5j
4-Cl
0.450
3.50
b
b
5k
H
0.0510
0.520
a
b
pKa data for Y-PhOH in EtOH were taken from ref 16. The kinetic
data for the reactions of 5c and 5h−k were taken from ref 13.
−
that kEtO decreases as the leaving aryloxide becomes more
basic; e.g., it decreases from 5.82 M−1 s−1 to 3.28 × 10−3 and
5.10 × 10−5 M−1 s−1 as the pKa of the conjugate acid of the
leaving aryloxide increases from 8.04 to 12.66 and 15.76, in
turn. A similar result is demonstrated for the reactions with the
ion-paired EtOK, although the ion-paired species is more
reactive than the dissociated EtO− in all cases. It is also noted
−
that the kEtOK/kEtO ratio shows no correlation with the
electronic nature of the substituent Y. This contrasts the
preceding result; i.e., the kEtOK/kEtO ratio is linearly dependent
−
−
Thus, Hammett plots have been constructed using σY and
on the electronic nature of the substituent X for the reactions of
4a−f (Table 1 and Figure S1 in the SI). The contrasting K+ ion
effect will be discussed in detail in the following section.
The effect of the leaving-group basicity on the rate constants
is illustrated in Figure 4. The Brønsted-type plots for the
reactions of 5a−k with the dissociated EtO− and ion-paired
EtOK are linear over 7 pKa units with βlg = −0.62 0.02. Such
linear Brønsted-type plots are contrasting to the curved
Brønsted-type plots reported previously for the piperidinolysis
of Y-substituted phenyl benzoates19b and quinuclidinolysis of
diaryl carbonates (e.g., βlg changes from −1.4 0.1 to −0.3
0.1 as the leaving group basicity decreases),20 which were
suggested to proceed through a stepwise mechanism with a
change in the RDS.20 The βlg value of −0.6 0.1 is typical of
reactions reported previously to proceed through a concerted
mechanism (e.g., βlg = −0.52 for the reactions of aryl
σY° constants to examine whether the S−OAr bond rupture
occurs in the RDS. As shown in Figure S2 in the SI, σY°
−
constants result in a better correlation than σY constants for
the reactions of 5b−d and 5f−k with the dissociated EtO− (e.g.,
R2 = 0.988 for σY° and R2 = 0.967 for σY− constants). A similar
result is shown in Figure S3 in the SI for the reactions of 5b−d
and 5f−k with the ion-paired EtOK (i.e., R2 = 0.989 for σY° and
R2 = 0.964 for σY− constants). This appears to indicate that no
negative charge develops on the oxygen atom of the leaving
aryloxide. Thus, one might suggest that the current reactions
proceed through a stepwise mechanism, in which expulsion of
the leaving group occurs after the RDS. Clearly, this is
inconsistent with the concerted mechanism proposed in the
preceding section on the basis of the linear Brønsted-type plots
with βlg = −0.6 but is consistent with our previous report that
alkaline ethanolysis of Y-substituted phenyl benzenesulfonates
(Y = 4-NO2, 3-NO2, 4-CF3, 3-Br, 4-Cl, and H) proceeds
through a stepwise mechanism.13
dimethylphosphinothioates with phenoxide anion,21 βlg
=
−0.54 for the alkaline ethanolysis of Y-substituted phenyl
diphenylphosphinates,22a and βlg = −0.66 for the reactions of
aryl diphenylphosphinates with piperidine22b). Thus, one might
suggest that the current reactions proceed through a concerted
mechanism on the basis of the magnitude of the βlg value
shown in Figure 4.
We suggest that the discrepancies in the reaction
mechanisms may be due to the limited numbers of substituents
to construct a Hammett plot with σ− constants, because only 4-
NO2 has a σ− constant among the 6 substituents studied
previously.13 In fact, a careful examination of Figures S2 and S3
D
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Article
in the SI reveals that the Hammett plots correlated with σ°
constants also exhibit many scattered points. Thus, the
conclusion drawn from the traditional Hammett correlation
using σ° and σ− constants is considered not to be reliable.
To obtain more conclusive information on the reaction
mechanism, the dual-parameter Yukawa−Tsuno equation (eq
6) has been employed. Equation 6 was originally derived to
account for the results obtained from solvolysis of benzylic
systems, in which a partial positive charge develops in the TS.23
We have recently shown that eq 6 is highly effective in
elucidating ambiguities in the reaction mechanisms of
benzenesulfonyl,18 benzoyl19 and phosphinyl22 transfer reac-
tions with amines or anionic nucleophiles.
indicates that a negative charge develops partially on the oxygen
atom of the leaving aryloxide, which can be delocalized to the
substituent Y through resonance interactions. This is only
possible for reactions in which breakdown of the S−OAr bond
occurs in the RDS. As discussed above, no negative charge
would develop on the leaving group if the current reactions
proceed through a stepwise mechanism. Thus, one can
conclude that the current reactions proceed through a
concerted mechanism, in which expulsion of the leaving
group is advanced only slightly on the basis of the small r
values. This is consistent with the preceding suggestion that the
reactions of 5a−k proceed through a concerted mechanism on
the basis of the linear Brønsted-type plots with βlg = −0.6 0.1.
Role of K+ Ion: An Increase in Electrophilicity or
Nucleofugality. One might expect that the TS structure for
the reactions with the dissociated EtO− is similar to TS2, in
which formation of the EtO−S bond and breakdown of the S−
OAr bond occur simultaneously. It is noted that K+ ion is
absent in TS2. In contrast, K+ ion should be involved in the TS
for the reactions with the ion-paired EtOK, since K+ ion
catalyzes the reactions. Thus, one can suggest that K+ ion
catalyzes the current reactions by increasing the electrophilicity
of the reaction center through TS3 or by increasing the
nucleofugality of the leaving aryloxide through TS4.
log(kY/kH) = ρ [σY° + r(σY− − σY°)]
(6)
Y
Thus, Yukawa−Tsuno plots have been constructed in Figure
5 for the reactions of 5b−d and 5f−k with the dissociated EtO−
If the reactions with the ion-paired EtOK proceed through
TS3, one might expect that the catalytic effect shown by K+ ion
−
(i.e., the kEtOK/kEtO ratio) is dependent on the electronic
Figure 5. Yukawa−Tsuno plots for the reactions of Y-substituted
phenyl benzenesulfonates 5b−d and 5f−k with the dissociated EtO−
nature of the substituent X in the benzenesulfonyl moiety
(proximal) but independent of the substituent Y in the leaving
aryloxide (distal). On the contrary, if the reactions proceed
●
○
( ) and ion-paired EtOK ( ) in anhydrous EtOH at 25.0 0.1 °C.
The identity of the points is given in Table 2.
−
through TS4, the kEtOK/kEtO ratio would be independent of the
substituent X (distal) but dependent on the substituent Y
−
(proximal). In fact, the kEtOK/kEtO ratio decreases linearly as
and ion-paired EtOK. The Yukawa−Tsuno plots exhibit
excellent linear correlations with ρY = 2.61 and r = 0.29 for
the reactions with the dissociated EtO− and ρY = 2.42 and r =
0.25 for those with the ion-paired EtOK. Since the r value in
the Yukawa−Tsuno equation represents the resonance demand
of the reaction center or the extent of resonance contribu-
tions,23,24 the r value of 0.25 or 0.29 obtained in this study
the substituent X changes from an EDG to an EWG (Table 1
and Figure S1 in the SI) but is independent of the substituent Y
(Table 2). Thus, one can conclude that the reactions with the
ion-paired EtOK proceed through TS3, in which K+ ion
catalyzes the reactions by increasing the elctrophilicity of the
reaction center.
Table 3. Summary of the Kinetic Results for the Reactions of 3,4-Dinitrophenyl Benzenesulfonate 5b, 4-Nitrophenyl
Benzenesulfonate 5c, and 4-Cyanophenyl Benzenesulfonate 5f with the Dissociated EtO− and Ion-Paired EtOK in Anhydrous
EtOH at 5 Different Temperatures
−1
103kEtO /M s−1
−
15.0 °C
20.0 °C
25.0 °C
30.0 °C
35.0 °C
ΔH‡/kcal mol−1
ΔS‡/eu
5b
5c
5f
532 20
761 70
1400 110
28.7
2460 200
50.1 4.0
9.84 2.00
3310 100
84.2 10.0
20.7 2.0
16.5 1.0
21.1 1.4
23.7 1.6
−2.7 0.2
5.1 0.4
6.72 1.00
1.24 0.30
17.1 2.0
2.95 1.00
6.79 0.60
103kEtOK/M−1 s−1
10.2 0.8
15.0 °C
20.0 °C
25.0 °C
30.0 °C
35.0 °C
ΔH‡/kcal mol−1
ΔS‡/eu
5b
5c
5f
2680 22
55.2 1.0
27.6 0.2
4260 67
84.3 1.1
45.4 0.7
5700 100
137
8800 190
12000 156
12.6 0.5
15.0 0.4
15.3 0.4
−12.8 0.4
−12.2 0.4
−12.4 0.4
221
111
3
1
309
168
8
1
66.3 0.3
E
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The Journal of Organic Chemistry
Article
Activation Parameters and TS Structures. To further
probe the above conclusion, activation parameters (ΔH‡ and
ΔS‡) have been calculated from the rate constants measured at
5 different temperatures for the reactions of 3,4-dinitrophenyl
benzenesulfonate 5b, 4-nitrophenyl benzenesulfonate 5c, and
4-cyanophenyl benzenesulfonate 5f with the dissociated EtO−
and ion-paired EtOK. The kinetic results are summarized in
Table 3 and illustrated graphically in Figure 6. The Arrhenius
rotational and vibrational degrees of freedom due to its cyclic
structure, while such restrictions are absent in TS2. This idea is
in accord with the fact that the ΔS‡ values are significantly
more negative for the reactions with the ion-paired EtOK than
for those with the dissociated EtO−.
One might suggest that differential solvation of the GS and
TS is responsible for the result that ΔS‡ for the reactions with
the dissociated EtO− is linearly dependent on the nature of the
substituent Y (see also Figure S5 in the SI). It is expected that
the dissociated EtO− is strongly solvated in EtOH through H-
bonding interactions. Since the negative charge of the EtO− ion
in the GS is partially transferred to the electrophilic center of
the substrate in the TS, solvation of the TS (i.e., TS2) through
H-bonding interactions would decrease. Thus, one might
expect an increase in ΔS‡ due to a decrease in solvation of the
TS. Furthermore, the charge transfer from EtO− to the
electrophilic center in the TS would be more advanced for the
reaction in which formation of the EtO−S bond is more
advanced in the TS. Therefore, ΔS‡ is expected to increase
more significantly for the reaction that proceeds through a later
TS (or for the reaction of the substrate possessing a weaker
EWG on the basis of a normal Hammond effect).25 In fact, ΔS‡
increases from −2.7 eu to +5.1 and +10.2 eu as the substituent
Y changes from 3,4-(NO2)2 to 4-NO2 and 4-CN, in turn (Table
3).
However, such solvation effect is expected to be less
important for the reactions with the ion-paired EtOK than
for those with the dissociated EtO− since the ion-paired species
would be less strongly solvated than the dissociated one. Thus,
the ΔS‡ value would be influenced mainly by the degrees of
bond formation and bond rupture in the TS. One might expect
that the reaction of the substrate possessing a weaker EWG in
the leaving group (or the less reactive substrate) would proceed
through a later TS; i.e., both formation of the EtO−S bond and
breakdown of the S−OAr bond would be more advanced as the
substituent Y in the leaving aryloxide changes from 3,4-(NO2)2
to 4-NO2 or to 4-CN. It is obvious that ΔS‡ decreases with
increasing the degree of the EtO−S bond formation but
increases with increasing the degree of the S−OAr bond
rupture. Accordingly, decreased ΔS‡ upon formation of the
EtO−S bond would be compensated by increased ΔS‡ upon
breakdown of the S−OAr bond. This accounts nicely for the
result shown in Table 3 that the ΔS‡ values are practically the
same for the reactions 5b, 5c and 5f with the ion-paired EtOK.
Thus, the contrasting ΔS‡ values for the reactions with the
dissociated EtO− and ion-paired EtOK also support the
proposed TS structures (i.e., TS2 and TS3, respectively).
Figure 6. Arrhenius plots for the reactions of 3,4-dinitrophenyl
●
○
benzenesulfonate 5b ( ), 4-nitrophenyl benzenesulfonate 5c ( ), and
−
▲
4-cyanophenyl benzenesulfonate 5f ( ) with the dissociated EtO
(A) and ion-paired EtOK (B) in anhydrous EtOH at 25.0 0.1 °C.
plots exhibit excellent linear correlations for the reactions with
the dissociated EtO− and ion-paired EtOK, indicating that the
ΔH‡ and ΔS‡ values determined in this way are reliable.
It is apparent that the electronic nature of substituent Y in
the leaving group influences the bond dissociation energy of the
S−OAr bond. Furthermore, the energy required to break the
S−OAr bond is reflected in the ΔH‡. Thus, one might expect
that the ΔH‡ in the current reactions is dependent on the
electronic nature of the substituent Y (e.g., σY constants) if
breakdown of the S−OAr bond occurs in the RDS. In fact,
Table 3 and Figure S4 in the SI show that the ΔH‡ values for
the reactions with the dissociated EtO− and ion-paired EtOK
are strongly dependent on the electronic nature of the
substituent Y (e.g., slope = −7.17 and R2 = 0.995 for the
reactions with the dissociated EtO− and slope = −2.88 and R2 =
0.985 for the reactions with the ion-paired EtOK). Such strong
dependence of ΔH‡ on the electronic nature of the substituent
Y would be only possible for reactions in which breakdown of
the S−OAr bond occurs in the RDS. This idea further supports
the preceding conclusion that the current reactions proceed
through a concerted mechanism with TS structures similar to
TS2 and TS3 for the reactions with the dissociated EtO− and
ion-paired EtOK, respectively.
CONCLUSIONS
■
Analyses of the kinetic data through LFERs and activation
parameters have led us to draw the following conclusions: (1)
The Brønsted-type plots for the reactions of 5a−k with the
The above argument is further supported by the ΔS‡ values.
Table 3 shows that the ΔS‡ for the reactions with the
dissociated EtO− is strongly dependent on the electronic nature
of the substituent Y; i.e., it increases from −2.7 eu to +5.1 and
+10.2 eu as the substituent Y in the leaving group changes from
3,4-(NO2)2 to 4-NO2 and 4-CN, in turn. In contrast, the ΔS‡
for the reactions with the ion-paired EtOK remains nearly
constant at −12.5 0.4 eu regardless of the electronic nature of
the substituent Y. The contrasting ΔS‡ behaviors can be
attributed to the structural difference between TS2 and TS3. It
is evident that TS3 would experience some restrictions in the
dissociated EtO− and ion-paired EtOK are linear with βlg
=
−0.62
0.02, a typical βlg value for the reactions reported
previously to proceed through a concerted mechanism. The
linear Yukawa−Tsuno plots with ρY = 2.42−2.61 and r = 0.25−
0.29 also suggest that the reactions proceed through a
concerted mechanism, in which breakdown of the S−OAr
−
bond is advanced only slightly. (2) The kEtOK/kEtO ratio is
strongly dependent on the electronic nature of the substituent
X in the benzenesulfonyl moiety but is independent of the
substituent Y in the leaving group, implying that K+ ion
catalyzes the reactions by increasing the electrophilicity of the
F
dx.doi.org/10.1021/jo302373y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
reaction center through TS3 rather than by increasing the
nucleofugality of the leaving group through TS4. (3) The
concerted mechanism is further supported by the fact that the
ΔH‡ values for the reactions with the dissociated EtO− and ion-
paired EtOK are linearly dependent on the electronic nature of
the substituent Y. (4) The reactions with the ion-paired EtOK
result in more negative ΔS‡ values than those with the
dissociated EtO−. Restrictions of the rotational and vibrational
degrees of freedom in the cyclic TS3 are responsible for the
more negative ΔS‡ values. (5) The ΔS‡ value is strongly
dependent on the electronic nature of the substituent Y for the
reactions with the dissociated EtO− but remains nearly constant
for the reactions with the ion-paired EtOK. The difference in
structures between TS2 and TS3 is responsible for the
contrasting ΔS‡ behaviors.
S23). This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ihum@ewha.ac.kr.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (2012-R1A1B3001637), and by NSERC of Canada
(E.B.). Ji-Sun Kang is also grateful for the BK 21 Scholarship as
well as the scholarship provided by the Lotte Foundation.
EXPERIMENTAL SECTION
■
Materials. Compounds 4a−f were prepared readily from the
reaction of 2,4-dinitrophenol and X-substituted benzenesulfonyl
chloride in the presence of triethylamine in anhydrous ether, while
5a−k were synthesized from the reaction of benzenesulfonyl chloride
with the respective Y-substituted phenol, as reported previously.18 The
crude products were purified by column chromatography, and their
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ASSOCIATED CONTENT
■
S
* Supporting Information
−
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dinitrophenyl X-substituted benzenesulfonates 4a−f with EtOK
−
(Figure S1); Hammett plots correlated with σY and σY°
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