A kinetic study on nucleophilic displacement reactions of phenyl Y-substituted-phenyl carbonates with alkali metal ethoxides: Metal ion effect and reaction mechanism

Article abstract of DOI:10.1246/bcsj.20120104

Pseudo-first-order rate constants (k_obsd) have been measured for reactions of phenyl Y-substituted-phenyl carbonates with alkali metal ethoxides (EtOM, M = Li, Na, and K). The plot of k_obsd vs. [EtOM] curves upward for the reaction of diphenyl carbonate with EtOM but is linear for that with EtOK in the presence of 18-crown-6-ether (18C6), indicating that the reaction is catalyzed by M⁺ ions and the catalytic effect disappears in the presence of 18C6. The kobsd values for the reactions with EtOK have been dissected into f_EtO- and k_EtOK, i.e., the second-order rate constants for the reactions with dissociated EtO^- and ion-paired EtOK, respectively. The Hammett plots correlated with σ^- and σ^-0 constants exhibit highly scattered points, while the Yukawa-Tsuno plots result in an excellent linear correlation with p = 2.11 and r = 0.21 for k_EtO-, and P = 1.62 and r = 0.26 for k_EtOK, implying that the reaction proceeds through a concerted mechanism. The catalytic effect (i.e., the k_EtOK/k_EtOr ratio) is independent of the electronic nature of the substituent Y. Thus, it has been concluded that K⁺ ion catalyzes the reaction by increasing the electrophilicity of the reaction center.

Full text of DOI:10.1246/bcsj.20120104

© 2012 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 85, No. 9, 1007-1013 (2012) 1007
A Kinetic Study on Nucleophilic Displacement Reactions of Phenyl
Y-Substituted-Phenyl Carbonates with Alkali Metal Ethoxides:
Metal Ion Effect and Reaction Mechanism#
Ik-Hwan Um,*1 Ji-Yoon Seo,1 Ji-Sun Kang,1 and Jun-Sung An2
1Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea
2Hansung Science High School, Seoul 120-080, Korea
Received April 4, 2012; E-mail: ihum@ewha.ac.kr
Pseudo-first-order rate constants (kobsd) have been measured for reactions of phenyl Y-substituted-phenyl carbonates
with alkali metal ethoxides (EtOM, M = Li, Na, and K). The plot of kobsd vs. [EtOM] curves upward for the reaction of
diphenyl carbonate with EtOM but is linear for that with EtOK in the presence of 18-crown-6-ether (18C6), indicating
that the reaction is catalyzed by M+ ions and the catalytic effect disappears in the presence of 18C6. The kobsd values for
ꢀ
the reactions with EtOK have been dissected into kEtO and kEtOK, i.e., the second-order rate constants for the reactions
¹
¹
with dissociated EtO and ion-paired EtOK, respectively. The Hammett plots correlated with · and ·○ constants exhibit
highly scattered points, while the Yukawa-Tsuno plots result in an excellent linear correlation with µ = 2.11 and r = 0.21
ꢀ
for kEtO , and µ = 1.62 and r = 0.26 for kEtOK, implying that the reaction proceeds through a concerted mechanism. The
ꢀ
catalytic effect (i.e., the kEtOK/kEtO ratio) is independent of the electronic nature of the substituent Y. Thus, it has been
concluded that K+ ion catalyzes the reaction by increasing the electrophilicity of the reaction center.
Metal ions have often been reported to catalyze nucleophilic
displacement reactions of esters as a Lewis acid catalyst.1-10
Since the Lewis acidity increases with increasing the charge
density of metal ions, most studies have focused on reactions
(2b), methylparaoxon (3a), and methylparathion (3b).9 M+
ions catalyze the reactions of 2a and 3a in the order K+ <
Na+ < Li+ while the reactions of 2b and 3b are catalyzed by
K+ and Na+ but are inhibited by Li+, indicating that the effect
of M+ ions (catalysis or inhibition) is dependent on the nature
of the electrophilic centers.9
The effect of alkali metal ions has also been investigated in
nucleophilic displacement reactions of carboxylic esters such
as 4-nitrophenyl picolinate (4) and 2-pyridyl benzoate (5) with
EtOM.10a,10b Alkali metal ions have been reported to catalyze
the reactions of 4 and 5, and the catalytic effects are in the
order Na+ > K+ > Li+ for the reaction of 410a and Na+ >
Li+ > K+ for that of 5.10b An increase in the electrophilicity of
the reaction site through a five-membered cyclic complex 4M+
or a six-membered cyclic complex 5M+ has been suggested
to be responsible for the catalytic effect shown by M+ ions
(Chart 2).10a,10b
involved multivalent metal ions (e.g., La3+, Eu3+, Co3+, Zn2+
,
Cu2+, Mn2+, etc.).1-4 The effect of alkali metal ions on
nucleophilic displacement reactions has been much less inves-
tigated, although alkali metal ions are ubiquitous and are
known to play important roles in biological processes (e.g., the
Na+/K+ pump to maintain high K+ and low Na+ concentration
in mammalian cells).5-10 The first study on alkali metal ions has
been performed by Buncel et al. for the nucleophilic displace-
ment reaction of 4-nitrophenyl diphenylphosphinate (1a)
(Chart 1) with alkali metal ethoxides (EtOM, where M = Li,
Na, and K) in anhydrous ethanol.5a They found that alkali metal
ions catalyze the reaction of 1a and the catalytic effect
increases as the charge density of M+ ions increases, i.e.,
K+ < Na+ < Li+.5a
Similarly, it has been expected that M+ ions would catalyze
the reaction of 4-nitrophenyl 2-hydroxybenzoate (6) with
EtOM through a six-membered cyclic complex 6M+. Un-
expectedly, we have found that M+ ions strongly inhibit the
reaction through formation of 6M+.10c We have shown that
the reaction of 6 with EtOM proceeds rapidly through an ¡-
In contrast, we have recently reported that Li+ ion inhibits
the corresponding reaction of 4-nitrophenyl diphenylphos-
phinothioate (1b) while K+ and Na+ behave as a catalyst.8 A
similar result has been reported for the reactions of various
organophosphorus insecticides, e.g., paraoxon (2a), parathion
X
Ph P O
Ph
X
NO2 EtO P O
OEt
X
NO2 MeO P O
OMe
NO2
X = O (1a), S (1b)
X = O (2a), S (2b)
Chart 1.
X = O (3a), S (3b)
Published on the web September 1, 2012; doi:10.1246/bcsj.20120104

1008 Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
Metal Ion Effect and Reaction Mechanism
O
NO2
O
M
O
NO2
C
O
C
N
C
O
O
N
M
O
M
6M+
4M+
5M+
Chart 2.
O
O
EtOM
O C OEt MO
O C O
Y
Y
7a-h
Y = H (a), 4-Cl (b), 3-Cl (c), 4-CHO (d), 4-CO2Et (e), 4-COMe (f), 4-CN (g), 4-NO2 (h).
M = Li, Na, K, K/18C6.
Scheme 1.
oxoketene intermediate in the presence of complexing agents
0.06
0.05
0.04
0.03
0.02
0.01
0.00
for M+ ions (e.g., 18-crown-6-ether for K+ ion and 15-crown-
5-ether for Na+ ion), but in the absence of such complex-
ing agents, the reaction is extremely slow.10c Thus, we have
concluded that M+ ions prevent the ¡-oxoketene process by
forming 6M+, which would be highly stable.10c
We have now extended our study to the reaction of phenyl
Y-substituted-phenyl carbonates 7a-7h with EtOM in anhy-
drous ethanol to get further information on M+ ion effects and
reaction mechanisms (Scheme 1). We wish to report that alkali
metal ions catalyze the reaction by increasing the electro-
philicity of the reaction center, and the reaction proceeds
through a concerted mechanism, in which expulsion of the
leaving group in the transition state is advanced only a little.
We have also demonstrated convincingly that the Yukawa-
Tsuno equation is highly effective in deduction of the reaction
mechanism.
Results and Discussion
The reaction was followed spectrophotometrically by mon-
itoring the appearance of the leaving Y-substituted phenoxide
anion under pseudo-first-order conditions with large excess
EtOM. All reactions in the current study obeyed pseudo-first-
order kinetics. Pseudo-first-order rate constant (kobsd) was
calculated from the slope of the linear plot of ln(A¨ ¹ At)
vs. t. The correlation coefficients of the linear plots are higher
than 0.9995. From replicate runs, the uncertainty in the kobsd
values is estimated to be less than «3%. The kinetic results
for the reactions of diphenyl carbonate (7a) with EtOM are
shown in Figure 1 and are summarized in Table 1. The kobsd
values and detailed kinetic conditions for the reactions of
phenyl Y-substituted-phenyl carbonates 7b-7h with EtOK are
summarized in Tables S1-S7 and are illustrated graphically in
Figures S1-S7 in the Supporting Information.
Effect of Alkali Metal Ions on Reactivity. As shown in
Figure 1, the reactivity of EtOM is highly dependent on the
nature of the M+ ions, i.e., it decreases in the order EtOK >
EtONa > EtOLi > EtOK/18C6. Besides, the plots of kobsd vs.
[EtOM] for reactions of 7a with EtOM curve upward while
the plot for the reaction with EtOK in the presence of 18-
crown-6-ether (18C6), a complexing agent for K+ ion, is linear
with decreased kobsd values. Such curved plots are typical for
0.005 0.010 0.015 0.020 0.025
0.000
[EtOM] / M
Figure 1. Plots of kobsd vs. [EtOM] for the reactions of
diphenyl carbonate (7a) with EtOLi ( ), EtONa ( ), and
EtOK ( ) and with EtOK in the presence of 18C6 ( ) in
anhydrous EtOH at 25.0 « 0.1 °C. [18C6]/[EtOK] = 2.0.
Table 1. Summary of Kinetic Data for the Reactions of
Diphenyl Carbonate (7a) with EtOM in Anhydrous EtOH
at 25.0 « 0.1 °C
¹1
¹1
ꢀ
ꢀ
EtOM
kEtO /M¹1 s
k
EtOM/M¹1 s
kEtOM/kEtO
EtOLi
EtONa
EtOK
EtOK/18C6
1.00
1.01
0.955
1.02
1.36
2.08
4.41
®
1.36
2.06
4.62
®
reactions in which alkali-metal ions were reported to behave
as a Lewis acid catalyst. Thus, one can suggest that M+ ions
catalyze the reaction of 7a in the order K+ > Na+ > Li+ and
the catalytic effect shown by K+ ion disappears in the presence
of the complexing agent.

I.-H. Um et al.
0.06
Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012) 1009
6
EtOK
EtONa
EtOLi
0.05
0.04
0.03
0.02
0.01
0.00
5
4
3
2
1
0
0
1
2
3
4
0.000
0.003
0.006
0.009
0.012
[18C6]/[EtOK]
[EtO−]eq / M
Figure 2. Plot showing the effect of added 18C6 on kobsd
for the reaction of diphenyl carbonate (7a) with EtOK in
anhydrous EtOH at 25.0 « 0.1 °C. [EtOK] = 19.4 mM.
ꢀ
Figure 3. Plots illustrating dissection of kobsd into kEtO and
kEtOM for the reactions of diphenyl carbonate (7a) with
EtOLi ( ), EtONa ( ), and EtOK ( ) in anhydrous EtOH
at 25.0 « 0.1 °C.
-
kEtO
-
7 + EtO + M+
Products
One can derive a rate equation as eq 1 on the basis of
the reactions proposed in Scheme 2. Under a pseudo-first-
order kinetic condition, kobsd can be expressed as eq 2. Since
Kd
¹
the dissociation constant Kd = [EtO ]eq[M+]eq/[EtOM]eq, and
kEtOM
¹
[EtO ]eq = [M+]eq at equilibrium, eq 2 becomes eq 3. The
7 +
EtOM
Products
¹
concentrations of [EtO ]eq and [EtOM]eq can be calculated
¹3
Scheme 2.
from the Kd value reported previously (i.e., Kd = 4.72 © 10
,
9.80 © 10¹3, and 1.11 © 10¹2 M for EtOLi, EtONa, and EtOK,
in turn)12 and the initial concentration of EtOM (i.e., [EtOM])
using eqs 4 and 5.
To examine the validity of the above argument, the reac-
tion of 7a with EtOK has been performed at a fixed EtOK
concentration with varying the concentration of 18C6, i.e.,
[EtOK] = 19.4 mM and the [18C6]/[EtOK] = 0, 0.4, 0.7, 1.0,
1.4, 2.1, 2.8, and 3.6. As shown in Figure 2, the kobsd value
decreases rapidly upon addition of the complexing agent up
to [18C6]/[EtOK] = 1.0 and then remains constant thereafter.
This supports clearly the preceding argument that K+ ion
catalyzes the reaction of 7a and the catalytic effect disappears
completely when [18C6]/[EtOK] ² 1.0.
Dissection of kobsd into kEtO and kEtOM
catalytic effect exerted by M+ ions, kobsd has been dissected
into kEtO and kEtOM, i.e., the second-order rate constant for
the reaction with the dissociated EtO and ion-paired EtOM,
ꢀ
ꢀ
Rate ¼ kEtO ½EtO ꢁeq½7ꢁ þ kEtOM½EtOMꢁeq½7ꢁ
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ꢀ
ꢀ
kobsd ¼ kEtO ½EtO ꢁeq þ kEtOM½EtOMꢁeq
k
obsd=½EtOꢀꢁeq ¼ kEtO þ kEtOM½EtOꢀꢁeq=Kd
ꢀ
½EtOMꢁ ¼ ½EtOꢀꢁeq þ ½EtOMꢁeq
2
½EtOꢀꢁeq ¼ ½ꢀKd þ ðKd þ 4Kd½EtOMꢁÞ ꢁ=2
1=2
¹
¹
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 3 shows that the plots
for the reaction of 7a with EtOM are linear with a positive
intercept, indicating that the above equations derived from
the reactions proposed in Scheme 2 are correct. Thus, one can
À
. To quantify the
ꢀ
¹
ꢀ
respectively. EtOM was reported to exist as a dimer or other
aggregates in a high concentration region (e.g., [EtOM] >
0.1 M).11 However, EtOM was suggested to exist as dissociated
and ion-paired species in concentration below 0.1 M.11 Since
the current reactions are carried out in a low-concentration
region (e.g., [EtOM] < 0.1 M), EtOM would exist as the dis-
calculate the kEtO and kEtOM/Kd values from the intercept
and the slope of the linear plots, respectively. The kEtOM value
can be calculated from the kEtOM/Kd ratios determined above
and the Kd value reported previously. The calculated kEtO and
kEtOM values for the reaction of 7a together with the kEtOM
kEtO ratios are summarized in Table 1.
As shown in Table 1, the kEtO value calculated from the
intercept of the linear plots of kobsd/[EtO ]eq vs. [EtO ]eq is
practically identical to the kEtO value of 1.02 M¹1 s¹1, which is
ꢀ
/
ꢀ
¹
ꢀ
sociated EtO and ion-paired EtOM. Accordingly, one can
¹
¹
¹
suggest that both the dissociated EtO and the ion-paired
ꢀ
ꢀ
EtOM react with the substrate with a rate constant kEtO and
kEtOM, respectively, as shown in Scheme 2.
calculated from the slope of the linear plot of kobsd vs. [EtOK]

1010 Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
Metal Ion Effect and Reaction Mechanism
3
2
1
0
O
M
O
C
7h
7g
2
1
EtO
PhO
O
M
C
O
7d
Y
Y
7f
7c
7e
PhO
EtO
7b
7h
7a
TS1
TS2
0
7g
ρ = 2.37 0.20
Chart 3.
R2 = 0.978
-1
7d
0.0
0.5
1.0
Table 2. Summary of Kinetic Data for the Reactions of
Phenyl Y-Substituted-Phenyl Carbonates 7a-7h with EtOK
in Anhydrous EtOH at 25.0 « 0.1 °C
σο
7f
7e
7c
7b
ꢀ
kEtO
kEtOK
kEtOK
/kEtO
Compound
Y
/M¹1 s
/M¹1 s
¹1
¹1
ꢀ
7a
7b
7c
7d
7e
7f
H
4-Cl
3-Cl
4-CHO
4-CO2Et
4-COMe
4-CN
4-NO2
0.955
5.11
8.21
4.41
15.2
22.8
51.6
37.9
48.0
87.6
151
4.62
2.97
2.78
2.35
3.32
3.58
1.66
1.83
7a
ρ = 1.25 0.20
22.0
R2 = 0.933
11.4
13.4
52.7
82.5
7g
7h
0.0
0.5
1.0
1.5
σ−
Figure 4. Hammett plots of kEtO correlated with · and ·○
(inset) for the reactions of phenyl Y-substituted-phenyl
carbonates 7a-7h with EtOK in anhydrous EtOH at 25.0 «
0.1 °C.
¹
in the presence of 18C6. In contrast, kEtOM increases from
1.36 M¹1 s¹1 to 2.08 and 4.41 M¹1 s¹1 as M+ changes from Li+
to Na+ and K+ ions, in turn. Thus, one can suggest that the
ꢀ
¹
ion-paired EtOM is more reactive than the dissociated EtO ,
ꢀ
and the metal ion effect (i.e., the kEtOM/kEtO ratio) increases as
the size of M+ ions increases, i.e., Li+ < Na+ < K+.
One might suggest that M+ ions catalyze the reaction of
7a by increasing the electrophilicity of the reaction center
through formation of a four-membered cyclic transition state
as modeled by TS1 or by enhancing the nucleofugality of the
leaving aryloxide via TS2 (Chart 3). It is apparent that the
enhanced nucleofugality through TS2 is effective only when
expulsion of the leaving-group occurs in the rate-determining
step (RDS), but would be ineffective for reactions in which
the leaving-group departs after RDS. Thus, we have performed
reactions of phenyl Y-substituted-phenyl carbonates 7b-7h
with EtOK in anhydrous ethanol to investigate the reaction
mechanism. The kinetic results will be discussed in the follow-
ing section.
The reactions of 7a-7h with EtOK would proceed through
a concerted mechanism or through a stepwise pathway with
a tetrahedral intermediate. If the current reaction proceeds
through a concerted mechanism, a partial negative charge
would develop on the oxygen atom of the leaving group.
Such a negative charge can be delocalized to the substituent
Y through resonance interactions. Thus, one might expect that
¹
·
·
constants should give a better Hammett correlation than
constants, if the reaction proceeds through a concerted
○
mechanism. On the contrary, if the reaction proceeds through
a stepwise mechanism, formation of an intermediate should
¹
be RDS. This is because EtO is much more basic and a
poorer nucleofuge than Y-substituted phenoxide anion (i.e.,
k
ꢂ k2). Thus, if the reaction proceeds through a stepwise
ꢀ1
Reaction Mechanism. As shown in Figures S1-S7 in the
Supporting Information, the plots of kobsd vs. [EtOK] for the
reactions of 7b-7h with EtOK curve upward, while those of
mechanism, no negative charge would develop on the oxygen
atom of the leaving group. In this case, ·○ constants should
result in a better Hammett correlation than · constants.
¹
¹
¹
k
obsd/[EtO ]eq vs. [EtO ]eq are linear with a positive intercept
To investigate whether the reaction proceeds through a
concerted mechanism or through a stepwise pathway with
ꢀ
in all cases. Thus, the kEtO and kEtOK/Kd values were deter-
mined from the intercept and slope of the linear plots, respec-
tively, while the kEtOK values were calculated from the kEtOK/Kd
values determined above and the Kd value reported previously
formation of an intermediate being the RDS, Hammett plots
¹
for kEtO have been constructed in Figure 4 using · and ·○
ꢀ
constants. One can see that the Hammett plot correlated with
¹
ꢀ
for EtOK. The kEtO and kEtOK values calculated in this way are
· constants results in a poor correlation with highly scattered
summarized in Table 2.
points (R2 = 0.933). The corresponding plot correlated with ·○
constants results in a slightly better correlation but exhibits still
many scattered points (R2 = 0.978).
ꢀ
It is demonstrated that kEtO increases as the substituent Y
becomes a stronger electron-withdrawing group (EWG), e.g.,
¹1
¹1
it increases from 0.955 M¹1 s to 11.4 and 82.5 M¹1 s as
the substituent Y changes from H to 4-CO2Et and 4-NO2,
respectively. A similar result is shown for kEtOK, although the
A similar result is demonstrated in Figure 5 for kEtOK, i.e.,
¹
the Hammett plot correlated with · constants results in a poor
correlation (R2 = 0.949), while ·○ constants shows a slightly
better correlation (R2 = 0.976). However, both Hammett plots
¹
ion-paired EtOK is more reactive than the dissociated EtO .

I.-H. Um et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012) 1011
exhibit many scattered points. Thus, one cannot get conclusive
information on reaction mechanism from these Hammett plots.
It is well known that the Yukawa-Tsuno eq 6 is a powerful
tool for investigation of resonance effects in solvolyses of
benzylic systems, in which a partial positive charge develops in
the transition state.13-15 We have also shown that eq 6 is highly
effective in clarifying ambiguities in reaction mechanisms for
nucleophilic displacement reactions of various esters16-19 as
well as Michael-type addition of amines to activated acety-
lenes.20 Thus, Yukawa-Tsuno plots have been constructed to
get more conclusive information on the reaction mechanism.
3
3
logðkY=kHÞ ¼ µ½·ꢃ þ rð·ꢀ ꢀ ·ꢃÞꢁ
As shown in Figure 6, the Yukawa-Tsuno plots exhibit
excellent linear correlations with µ = 2.11 and r = 0.21 for
ð6Þ
7h
7g
2
7d
7f
7c
7e
7b
7h
1
0
ꢀ
kEtO , and µ = 1.62 and r = 0.26 for kEtOK. Since the r value
7a
ρ = 1.86 0.17
2 = 0.976
7g
2
1
0
represents the extent of resonance contribution, one might
suggest that a small negative charge develops, which can be
delocalized on the substituent Y in the leaving group. Besides,
the µ values of 2.11 and 1.62 for the reactions with the
R
7d
7f
-0.5 0.0 0.5 1.0
71e.5
σο
7c
¹
dissociated EtO and ion-paired EtOK, respectively are larger
7b
than those reported for alkaline hydrolysis of Y-substi-
tuted phenyl benzoates (µ = 1.32)16f and thionobenzoates
(µ = 1.11),17c which were suggested to proceed through a
stepwise mechanism with formation of an intermediate being
the RDS on the basis of the fact that ·○ constants result in much
7a
¹
better correlation than · constants. Thus, one can conclude
ρ = 1.00 0.14
¹
that the reactions of 7a-7h with the dissociated EtO and
R2 = 0.949
ion-paired EtOK proceed through a concerted mechanism in
which expulsion of the leaving group in the transition state is
advanced only a little on the basis of the small r values.
Accordingly, one can suggest that K+ ion catalyzes the
reaction by increasing the electrophilicity of the reaction center
through TS1 or by enhancing the nucleofugality of the leaving
aryloxide via TS2 as mentioned in the preceding section. One
might expect that the catalytic effect shown by K+ ion is
dependent on the electronic nature of the substituent Y, if the
increased nucleofugality via TS2 is responsible for the K+ ion
0.0
0.5
1.0
1.5
σ−
Figure 5. Hammett plots of kEtOK correlated with · and ·○
(inset) for the reactions of phenyl Y-substituted-phenyl
carbonates 7a-7h with EtOK in anhydrous EtOH at
25.0 « 0.1 °C.
¹
3
7h
(b)
(a)
2
1
7g
7h
7d
7f
7g
2
7e
7d
7f
7c
7e
7b
7c
7b
1
7a
0
7a
ρ = 1.62 0.06
ρ = 2.11 0.12
r = 0.21
r = 0.26
R2 = 0.995
R2 = 0.991
-1
0
0.0
0.5
1.0
0.0
0.5
1.0
σο + r (σ− − σο)
σο + r (σ− − σο)
ꢀ
Figure 6. Yukawa-Tsuno plots of kEtO (a) and kEtOK (b) for the reactions of phenyl Y-substituted-phenyl carbonates 7a-7h with
EtOK in anhydrous EtOH at 25.0 « 0.1 °C.

1012 Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
Metal Ion Effect and Reaction Mechanism
catalysis (proximal). In contrast, if K+ ion catalysis is due to
the enhanced electrophilicity through TS1, the catalytic effect
would be little influenced by the electronic nature of the
substituent Y (distal). Table 2 shows that the catalytic effect
for the BK 21 Scholarship as well as the Scholarship provided
by the Lotte Foundation.
Supporting Information
ꢀ
(i.e., the kEtOK/kEtO ratio) is independent of Y. Thus, one can
The kobsd values and detailed kinetic conditions for the reac-
tions of 7b-7h with EtOK are summarized in Tables S1-S7.
The plots of kobsd vs. [EtOK] and kobsd/[EtO ]eq vs. [EtO ]eq
are illustrated in Figures S1-S7. This material is available free
of charge on the web at http://www.csj.jp/journals/bcsj/.
conclude that M+ ions catalyze the reactions of 7a-7h with
EtOK by increasing the electrophilicity of the reaction center
through TS1.
¹
¹
Conclusion
The current study has allowed us to conclude the following:
(1) The plot of kobsd vs. [EtOM] curves upward for the reaction
of 7a with EtOM but is linear for that with EtOK in the presence
of 18C6, indicating that the reaction is catalyzed by M+ ions but
the catalytic effect disappears in the presence of the complex-
References
#
This paper is dedicated with respect and affection to the late
Professor Yuho Tsuno, a true gentleman and an inspiring mentor.
a) E. V. Anslyn, D. A. Dougherty, Modern Physical
1
Organic Chemistry, University Science Books, Sausalito, USA,
2006, pp. 500-502. b) F. A. Carroll, Perspectives on Structure and
Mechanism in Organic Chemistry, Brooks/Cole, New York, USA,
1998, p. 445. c) M. I. Page, A. Williams, Organic & Bio-organic
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¹
ing agent. (2) The · and ·○ constants result in poor Hammett
correlations with many scattered points, while the Yukawa-
Tsuno plots exhibit excellent linear correlations with µ = 2.11
ꢀ
and r = 0.21 for kEtO , and µ = 1.62 and r = 0.26 for kEtOK
.
2
a) M. F. Mohamed, I. Sánchez-Lombardo, A. A. Neverov,
Thus, the reaction proceeds through a concerted mechanism, in
which departure of the leaving group in the transition state is
R. S. Brown, Org. Biomol. Chem. 2012, 10, 631. b) R. S. Brown,
Z.-L. Lu, C. T. Liu, W. Y. Tsang, D. R. Edwards, A. A. Neverov,
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ꢀ
advanced only a little. (3) The kEtOK/kEtO ratio is independent
of the electronic nature of the substituent Y, indicating that K+
ion catalyzes the reaction by increasing the electrophilicity of
the reaction center through TS1.
Overall, our study has revealed the role of M+ ions and the
reaction mechanism through the Yukawa-Tsuno equation.
3
a) J. Suh, Acc. Chem. Res. 1992, 25, 273. b) J. Suh, S. J.
Experimental
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Materials. Compounds 7a-7h were prepared readily as
reported previously21 from the reaction of phenyl chloro-
formate and the respective Y-substituted phenol in the pres-
ence of triethylamine in anhydrous ether. The crude products
were purified by column chromatography and their purity
was checked by their melting points and 1H NMR spectra.
The EtOM stock solutions were prepared by dissolving the
respective alkali metal in anhydrous ethanol under N2 and
stored in a refrigerator. The concentration of EtOM was mea-
sured by titration with monopotassium phthalate. 18-Crown-6-
ether (18C6) was recrystallized from acetonitrile and dried
under vacuum. The anhydrous ethanol was further dried over
magnesium and distilled under N2 just before use.
Kinetics. Kinetic study was performed using a UV-vis
spectrophotometer equipped with a constant-temperature cir-
culating bath. The reactions were followed by monitoring the
appearance of Y-substituted phenoxide ion. Reactions were
followed generally for 9-10 half-lives and kobsd were calculated
using the equation, ln(A¨ ¹ At) vs. t. The plots of ln(A¨ ¹ At)
vs. time were linear over 90% of the total reaction.
4
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a) I.-H. Um, Y.-H. Shin, S.-E. Lee, K. Yang, E. Buncel,
Products Analysis. Y-Substituted phenoxide was liberated
quantitatively and identified as one of the products in the reac-
tions of 7a-7h by comparison of the UV-vis spectrum after
completion of the reaction with that of authentic sample under
the same reaction conditions.
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This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (No. 2009-0075488). Ji-Sun Kang is also grateful
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