Curved Hammett Plot in Alkaline Hydrolysis of O-Aryl Thionobenzoates: Change in Rate-Determining Step versus Ground-State Stabilization

Article abstract of DOI:10.1021/jo035854r

Second-order rate constants have been measured for alkaline hydrolysis of O-aryl thionobenzoates (X-C₆H₄-CS-OC₆H ₄-Y) in 80 mol % H₂O-20 mol % DMSO at 25.0 ± 0.1 °C. The Hammett plot for the reaction of O-4-nitrophenyl X-substituted thionobenzoates (X-C₆H₄-CS-OC₆H ₄-NO₂, 1a-e) exhibits a downward curvature. However, a possible traditional explanation in terms of a change in the rate-determining step (RDS) has been considered but rejected. The proposed explanation involves stabilization of the ground-state (GS) through-resonance interaction between the electron-donating substituent X and the thionocarbonyl functionality on the basis of the linear Yukawa-Tsuno plot obtained for the same reaction. The Bronsted-type plot for the reaction of O-aryl thionobenzoates (C ₆H₅-CS-OC₆H₄-Y, 2a-i) is linear but exhibits many scattered points with a small β_lg, (-0.35). The Hammett plot for the same reaction shows rather poor correlation with σ^- constants but much better correlation with σ° constants. The alkaline hydrolysis of O-aryl thionobenzoates (1a-e and 2a-i) has been proposed to proceed through an addition intermediate in which bond formation is the RDS.

Full text of DOI:10.1021/jo035854r

Cu r ved Ha m m ett P lot in Alk a lin e Hyd r olysis of O-Ar yl
Th ion oben zoa tes: Ch a n ge in Ra te-Deter m in in g Step ver su s
Gr ou n d -Sta te Sta biliza tion
Ik-Hwan Um,* J i-Youn Lee, Han-Tae Kim,† and Sun-Kun Bae†
Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea
ihum@mm.ewha.ac.kr
Received December 22, 2003
Second-order rate constants have been measured for alkaline hydrolysis of O-aryl thionobenzoates
(X-C6H4-CS-OC6H4-Y) in 80 mol % H2O-20 mol % DMSO at 25.0 ( 0.1 °C. The Hammett plot for
the reaction of O-4-nitrophenyl X-substituted thionobenzoates (X-C6H4-CS-OC6H4-NO2, 1a -e)
exhibits a downward curvature. However, a possible traditional explanation in terms of a change
in the rate-determining step (RDS) has been considered but rejected. The proposed explanation
involves stabilization of the ground-state (GS) through-resonance interaction between the electron-
donating substituent X and the thionocarbonyl functionality on the basis of the linear Yukawa-
Tsuno plot obtained for the same reaction. The Brønsted-type plot for the reaction of O-aryl
thionobenzoates (C6H5-CS-OC6H4-Y, 2a -i) is linear but exhibits many scattered points with a small
âlg (-0.35). The Hammett plot for the same reaction shows rather poor correlation with σ- constants
but much better correlation with σ° constants. The alkaline hydrolysis of O-aryl thionobenzoates
(1a -e and 2a -i) has been proposed to proceed through an addition intermediate in which bond
formation is the RDS.
In tr od u ction
to proceed in a stepwise manner with one or two inter-
mediates.1-7 Castro et al. have reported that the reaction
of aryl 4-nitrophenyl thionocarbonates with weakly basic
amines (e.g., piperazinium ion and 1-formylpiperazine)
proceeds through two intermediates (e.g., a zwiterionic
T( and its deprotonated anionic T-), while the corre-
sponding reaction with strongly basic amines (e.g., pip-
eridine and piperazine) proceeds through one interme-
diate (T().1,2 Lee et al. have suggested that the nature of
reaction medium is also an important factor to determine
the reaction mechanism, since the deprotonation process
from T( to T-, which has often been observed as the rate-
determinining step for the reaction in H2O, is absent for
aminolysis of aryl dithioacetates and their related esters
in MeCN.3a However, we have recently reported that the
reaction of O-4-nitrophenyl thionobenzoate with alicyclic
secondary amines proceeds through two intermediates
regardless of amine basicity and the nature of the
reaction medium.4a
On the other hand, the reactions of thionocarbonyl
derivatives with anionic nucleophiles have been sug-
gested to proceed either in a stepwise manner with one
intermediate or in a concerted manner without an inter-
mediate.8,9 Castro et al. have proposed that the reactions
of bis(4-nitrophenyl) and methyl 4-nitrophenyl thiono-
carbonates with substituted phenoxides proceed in a
stepwise manner with a change in the rate-determining
Nucleophilic substitution reactions of thionocarbonyl
derivatives (e.g., R-CS-OR′, R-CS-SR′, RO-CS-OR′,
RO-CS-SR′ etc.) have intensively been investigated in
recent years.1-5 Reactions of thionocarbonyl compounds
with amine nucleophiles have generally been understood
† Department of Chemistry, Kunsan National University, Kunsan
573-701, Korea.
(1) Castro, E. A. Chem. Rev. 1999, 99, 3505-3524.
(2) (a) Castro, E. A.; Bessolo, J .; Aguayo, R.; Santos, J . G. J . Org.
Chem. 2003, 68, 8157-8161. (b) Casto, E. A.; Galvez, A.; Leandro, L.;
Santos, J . G. J . Org. Chem. 2002, 67, 4309-4315. (c) Castro, E. A.;
Leandro, L.; Quesieh, N.; Santos, J . G. J . Org. Chem. 2001, 66, 6130-
6135. (d) Castro, E. A.; Garcia, P.; Leandro, L.; Quesieh, N.; Rebolledo,
A.; Santos, J . G. J . Org. Chem. 2000, 65, 9047-9053. (e) Castro, E. A.;
Leandro, L.; Santos, J . G. Int. J . Chem. Kinet. 1999, 31, 839-845. (f)
Castro, E. A.; Saavedra, C.; Santos, J . G.; Umana, M. I. J . Org. Chem.
1999, 64, 5401-5407.
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Chem. 1997, 62, 5780-5784. (b) Song, H. B.; Choi, M. H.; Koo, I. S.;
Oh, H. K.; Lee, I. Bull. Korean Chem. Soc. 2003, 24, 91-94. (c) Oh, H.
K.; Ku, M. H.; Lee, H. W.; Lee. I. J . Org. Chem. 2002, 67, 8995-8998.
(d) Oh, H. K.; Ku, M. H.; Lee, H. W.; Lee, I. J . Org. Chem. 2002, 67,
3874-3877. (e) Oh, H. K.; Lee, Y. J .; Lee, H. W.; Lee, I. New J . Chem.
2002, 26, 473-476. (f) Lee, I.; Lee, H. W.; Lee. B. C.; Choi, J . H. Bull.
Korean Chem. Soc. 2002, 23, 201-204. (g) Oh, H. K.; Kim, S. K.; Lee,
H. W.; Lee, I. J . Chem. Soc., Perkin Trans. 2 2001, 1753-1757. (h)
Oh, H. K.; Kim, S. K.; Lee, H. W.; Lee, I. New J . Chem. 2001, 25, 313-
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Soc., Perkin Trans. 2 2000, 2306-2310. (j) Oh, H. K.; Lee, Y. H.; Lee,
I. Int. J . Chem. Kinet. 2000, 32, 131-135.
(4) (a) Um, I. H.; Seok, J . A.; Kim, H. T.; Bae, S. K. J . Org. Chem.
2003, 68, 7742-7746. (b) Um, I. H.; Lee, S. E.; Kwon, H. J . J . Org.
Chem. 2002, 67, 8999-9005. (c) Um, I. H.; Kwon, H. J .; Kwon, D. S.
J . Chem. Res. Synop. 1995, 301.
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10.1021/jo035854r CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/03/2004
2436
J . Org. Chem. 2004, 69, 2436-2441

Alkaline Hydrolysis of O-Aryl Thionobenzoates
TABLE 1. Su m m a r y of Secon d -Or d er Ra te Con sta n ts
for Rea ction s of O-4-Nitr op h en yl Th ion oben zoa tes
(X-C6H4-CS-OC6H4-4-NO2, 1a -e) w ith OH- in 80 m ol %
H2O-20 m ol % DMSO a t 25.0 ( 0.1 °C
step (RDS) from breakdown of an addition intermediate
to its formation upon increasing the basicity of the
attacking phenoxides on the basis of biphasic Brønsted-
type plots, e.g., ânuc ) 1.25 and 1.60, respectively, at pKa
< 7.1 and ânuc ) 0.30 and 0.44, respectively, at pKa >
7.1.8a In contrast, the reactions of thiol or dithiocarbonate
esters with substituted phenoxides and thiophenoxides
have been suggested to proceed through a concerted
mechanism on the basis of linear Brønsted-type plots
with a ânuc value ca. 0.65.8b-e Similarly, the absence of a
break in the Brønsted-type plot for the reaction of
4-nitrophenyl acetate and related esters with substituted
phenoxides has been proposed as evidence of a concerted
mechanism by Williams et al.10 In contrast, Buncel et al.
have argued against a concerted mechanism for the
reaction of substituted phenyl acetates and diphenylphoph-
inates with phenoxide, on the basis of the Hammett plot
exhibiting rather poor correlation with σ- but signifi-
cantly better correlation with σo constants.11
One of the most common criteria used to determine
reaction mechanism is the presence or absence of a break
in a Hammett- or Brønsted-type plot.12 However, the
controversy concerning the reaction mechanism arises
mainly from the Hammett- or Brønsted-type investiga-
tion.10,11 The study performed by changing the substitu-
ent only in the nucleophile or in the leaving group
appears to be inconclusive. We believe that properly
balanced information of substituent effects is necessary
to determine the reaction mechanism more conclusively.
Thus, we have synthesized two series of O-aryl thion-
obenzoates (1a -e and 2a -i) and investigated the effect
of substituents in the nonleaving and leaving groups on
reaction rates and mechanism for their alkaline hydroly-
ses. We report that deduction of a reaction mechanism
based just on a linear or a nonlinear Hammett or
Brønsted-type plot can be misleading.
-1
-
X
kOH /M s-1
1a , 4-OCH3
1b, 4-CH3
1c, H
1d , 3-OCH3
1e, 3-Cl
0.408 ( 0.001
0.830 ( 0.003
1.87 ( 0.03
2.71 ( 0.02
7.97 ( 0.07
TABLE 2. Su m m a r y of Secon d -Or d er Ra te Con sta n ts
for Rea ction s of O-Y-Su bstitu ted P h en yl
Th ion oben zoa tes (C6H5-CS-OC6H4-Y, 2a -i) w ith OH- in 80
m ol % H2O-20 m ol % DMSO a t 25.0 ( 0.1 °C
-
Y
pKa (Y-C6H5OH)a
kOH /M-1s-1
2a , 4-OCH3
2b, 4-CH3
2c, H
2d , 3-COCH3
2e, 3-NO2
2f, 4-COCH3
2g, 4-CHO
2h , 4-NO2
2i, 3,4-(NO2)2
10.20
10.19
9.95
9.19
8.35
8.05
7.66
7.14
5.42
0.172 ( 0.003
0.161 ( 0.007
0.210 ( 0.004
0.489 ( 0.006
1.46 ( 0.01
0.650 ( 0.004
0.846 ( 0.007
1.87 ( 0.03
10.5 ( 0.1
a
pKa data taken from: J encks, W. P.; Regenstein, J . In
Handbook of Biochemistry, 2nd ed.; Sober, H. A., Ed.; Chemical
Rubber Publishing Co.: Cleveland, OH, 1970; p J -195.
(kobs) were measured spectrophotometrically for the al-
kaline hydrolyses of O-4-nitrophenyl X-substituted thion-
obenzoates (1a -e) and O-Y-substituted phenyl thion-
obenzoates (2a -i) in 80 mol % H2O-20 mol % DMSO at
25.0 ( 0.1°C. The kobs values were determined from the
slope of linear plots of ln(A∞ - At) vs time. Generally five
different OH- concentrations were used to determine the
-
second-order rate constants (kOH ) from the slope of the
linear plots of kobs vs OH- concentration. Correlation
coefficients of the plots were usually higher than 0.9995.
It is estimated from replicate runs that the uncertainty
in the rate constants is less than (3%. The second-order
rate constants determined in this way are summarized
in Tables 1 and 2.
Discu ssion
Effect of Non lea vin g Gr ou p Su bstitu en t on Ra te
a n d Mech a n ism . As shown in Table 1, the second-order
-
rate constant (kOH ) increases as the substituent X in the
thionobenzoyl moiety changes from an electron-donating
group (EDG) to an electron-withdrawing group (EWG)
for the reaction of 1a -e with OH-. The effect of the
substituent X on the reactivity is illustrated in Figure 1.
The Hammett plot exhibits distinct curvature with
two intersecting linear portions. Such a curved Hammett
plot has traditionally been interpreted as a change in
the RDS for a reaction proceeding through a stepwise
mechanism.12,13 In fact, a change in the RDS has been
Resu lts
All the reactions in the present study obeyed pseudo-
first-order kinetics. Pseudo-first-order rate constants
(8) (a) Castro, E. A.; Angel, M.; Arellano, D.; Santos, J . G. J . Org.
Chem. 2001, 66, 6571-6575. (b) Castro, E. A.; Arellano, D.; Pavez, P.;
Santos, J . G. J . Org. Chem. 2003, 68, 6192-6196. (c) Castro, E. A.;
Pavez, P.; Santos, J . G. J . Org. Chem. 1999, 64, 2310-2313. (d) Castro,
E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 2003, 68, 9034-9039. (e)
Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 2003, 68, 3640-
3645.
(11) (a) Buncel, E.; Um, I. H.; Hoz, S. J . Am. Chem. Soc. 1989, 111,
971-975. (b) Pregel, M.; Dunn, E. J .; Buncel, E. J . Am. Chem. Soc.
1991, 113, 3545-3550. (c) Tarkka, R. M.; Buncel, E. J . Am. Chem.
Soc. 1995, 117, 1503-1507.
(9) Um, I. H.; Park, H. J .; Kwon, D. S. Bull. Korean Chem. Soc. 1991,
12, 93-97.
(10) (a) Williams, A. Acc. Chem. Res. 1989, 22, 387-392. (b) Ba-
Saif, S.; Luthra, A. K.; Williams, A. J . Am. Chem. Soc. 1987, 109,
6362-6368. (c) Bourne, N.; Chrystiuk, E.; Davis, A. M.; Williams, A.
J . Am. Chem. Soc. 1988, 110, 1890-1895. (d) Deacom, T. C.; Farra,
R.; Sikkel, B. J .; Williams, A. J . Am. Chem. Soc. 1978, 100, 2625-
2634.
(12) (a) Carroll, F. A. Perspectives on Structure and Mechanism in
Organic Chemistry; Brooks/Cole: New York, 1998; pp 371-386. (b)
Carey, F. A.; Sunderberg, R. Advanced Organic Chemistry Part A;
Kluwer Academic/Plenum Publishers: New York, 2000; pp 204-215.
(c) Page, M.; Williams, A. Organic & Bio-organic Mechanisms; Long-
man: Singapore, 1997.
J . Org. Chem, Vol. 69, No. 7, 2004 2437

Um et al.
SCHEME 1
of the addition intermediate (the k2 step in Scheme 1) to
its formation (the k1 step in Scheme 1) upon changing
the substituent X in the nonleaving group from EWG to
EDG.
The above argument appears to be consistent with
the proposal that the electronic nature of the sub-
stituent in the nonleaving group influences the pKa°,
the center of the curvature on a curved Brønsted-type
plot (log kNuc vs pKa of the conjugate acid of nucleo-
phile), where a change in the RDS of a stepwise reaction
occurs.2a,14,15 Castro et al. have suggested that the pKa°
value increases as the substituent in the nonleaving
group becomes a stronger EWG, i.e., pyridinolysis of
2,4-dinitrophenyl X-substituted benzoates in 44%
aqueous EtOH results in a curved Brønsted-type plot
when X ) H (pKa° ) 9.5) but linear plots when X ) 4-Cl
and 4-NO2 (pKa° > 9.5).14a-c More recently, the pKa°
for the reaction of S-4-nitrophenyl X-substituted thio-
benzoates with a series of secondary alicyclic amines
in the same medium has been found to increase from
10.0 to 10.4 and > 10.8 as the substituent X changes
from
H
to 4-Cl and 4-NO2, respectively.2a Gresser
and J encks have also found a similar result for the
reaction of aryl 3,4-dinitrophenyl carbonates with qui-
nuclidines.15
Or igin of th e Cu r ved Ha m m ett P lot. However, one
can propose a different explanation for the curved
Hammett plot, other than due to a change in the
RDS, on the basis of the following argument. First, the
RDS cannot be determined by the magnitude of the
k1 and k2 values. Because a direct comparison of the
k1 with the k2 value is not possible due to the difference
in their units, e.g., the first-order (k2) vs second-order rate
constant (k1). Thus, the RDS should be determined by
the ratio of the nucleofugality of the leaving group and
that of the nucleophile from the addition intermediate,
k2/k-1. As mentioned in the preceding section, the mag-
nitude of k2 and k-1 would be increased by an EDG
but decreased by an EWG in the nonleaving thiono-
benzoyl moiety. Therefore, the ratio of k2/k-1 would
not be significantly influenced by the electronic nature
of the substituent in the nonleaving group. In fact, we
have recently shown that the k2/k-1 ratio is nearly
constant upon changing the substituent in the nonleaving
group for aminolysis of 2,4-dinitrophenyl substituted
benzoates.16 Second, OH- ion would be always a poorer
leaving group than 4-nitrophenoxide anion due to the
large difference in the basicity of the two anions.
Accordingly, nucleophilic attack by OH- ion to form
an addition intermediate is expected to be always
the RDS and a change in the RDS would not occur
F IGURE 1. Hammett plot for alkaline hydrolysis of O-4-
nitrophenyl X-substituted thionobenzoates (1a -e) in 80 mol
% H2O-20 mol % DMSO at 25.0 ( 0.1 °C. F ) 2.41 (σ e 0)
and 1.73 (σ g 0). The same type of Hammett plot was found
in the reactions of 2,4-dinitrophenyl substituted benzoates
with OH-, CN-, and N3 in the same reaction medium.19c
-
suggested to be responsible for the curved Hammett
plot obtained for the reaction of substituted benzalde-
hydes with semicarbazide in weakly acidic solution
(pH ) 3.9), i.e., from a large positive F to a small one
upon changing the substituent in the phenyl ring of
benzaldehyde from electron-donating to electron-with-
drawing.13
Since the electrophilicity of the thionocarbonyl carbon
atom of 1a -e would be increased by an EWG in the
nonleaving thionobenzoyl moiety but decreased by an
EDG, one might expect a large F value for the reaction
in which the nucleophilic attack by OH- occurs in the
RDS of a stepwise reaction. In contrast, the nucleofugal-
ity of the negatively charged leaving group, 4-nitrophe-
noxide, would be decreased by an EWG but increased by
an EDG in the nonleaving thionobenzoyl moiety. Thus,
due to the opposite substituent effect, one might expect
a small F value for the reaction in which the leaving
group departure is involved in the RDS, whether the
reaction proceeds concertedly or stepwisely. As shown in
Figure 1, the Hammett F value has been determined to
decrease from 2.41 to 1.73 as the substituent X changes
from EDG to EWG. Thus, one might suggest that the
RDS of the present reaction changes from the breakdown
(14) (a) Castro, E. A.; Steinfort, G. B. J . Chem. Soc., Perkin Trans.
2 1983, 453-457. (b) Castro, E. A.; Santander, C. L. J . Org. Chem.
1985, 50, 3595-3600. (c) Castro, E. A.; Valdivia, J . L. J . Org. Chem.
1986, 51, 1668-1672.
(15) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963-
6970.
(13) J encks, W. P. Catalysis in Chemistry and Enzymology; McGraw-
Hill: New York, 1969; pp 480-483.
(16) Um, I. H.; Min, J . S.; Lee, H. W. Can. J . Chem. 1999, 77, 659-
666.
2438 J . Org. Chem., Vol. 69, No. 7, 2004

Alkaline Hydrolysis of O-Aryl Thionobenzoates
upon changing the substituent X in the thionobenzoyl
moiety.
Therefore, we propose that the origin of the curved
Hammett plot in Figure 1 is stabilization of the GS
through resonance between the electron donating sub-
stituent and the thionocarbonyl group as illustrated by
resonance structures I and II. The presence of such a
resonance structure stabilizes the GS of the substrate
(but not the TS) and would cause a decrease in reactivity.
The decrease in the reactivity of the substrates by the
contribution of the resonance structure II would be more
significant for the substrate with a stronger EDG. This
argument is consistent with the result that the points
for 1a and 1b deviate negatively from the Hammett plot
in Figure 1 and the degree of negative deviation is more
significant for the reaction of 1a (X ) 4-MeO) than for
the reaction of 1b (X ) 4-Me). A similar explanation has
recently been advanced by Neuvonen et al.17 A kinetic
study together with spectroscopic measurements of 13C
NMR chemical shifts and υ(CdO) frequencies has shown
that presence of an EWG in the nonleaving (or leaving)
group of esters increases the reactivity toward imidazole
and results in an upfield shift of the carbonyl carbon
resonance.17 Neuvonen et al. have attributed the en-
hanced reactivity of esters with an EWG in the nonleav-
ing (or leaving) group to a decrease in the resonance
stabilization of the GS of esters.17
F IGURE 2. Yukawa-Tsuno plot for alkaline hydrolysis of
O-4-nitrophenyl X-substituted thionobenzoates (1a -e) in 80
mol % H2O-20 mol % DMSO at 25.0 ( 0.1 °C. The same type
of Hammett plot was found in the reactions of 2,4-dinitrophe-
nyl-substituted benzoates with OH-, CN-, and N3- in the same
reaction medium.19c
benzensulfonates19b as well as alkaline hydrolysis of 2,4-
dinitrophenyl-substituted benzoates.19c
log kX/kH ) F[σ° + r(σ+ - σ°)]
(1)
The F value in the present system has been determined
to be 1.72. A similar F value has been reported for the
reaction of 4-nitrophenyl-substituted benzoates with OH-
performed in 33% MeCN.20 One would not expect such a
large F value for a reaction in which the leaving group
departure is involved in the RDS of a stepwise reaction
due to the opposite substituent effect as mentioned above.
Similarly, if the reaction of 1a -e with OH- were a direct
SN2-type displacement with a TS illustrated as TS1, the
F value would not be so large because little charge would
develop on the thionobenzoyl moiety. In fact, the F values
(either positive or negative) have been reported to be
small in SN2 reactions, e.g., F ) -0.3 and +0.8 for SN2
reactions of benzyl chlorides with OH- in H2O and with
I- in acetone, respectively.12a Thus, the large positive F
value obtained in the present system is consistent with
the argument that the reaction of 1a -e with OH-
proceeds through an addition intermediate in which
formation of the intermediate is the RDS.
To ascertain the validity of our proposal, the kinetic
data in Table 1 have been treated by the Yukawa-Tsuno
equation (eq 1).18 The term (σ+ - σ°) is the resonance
substituent constant measuring the capability for π-de-
localization of the π-electron donor substituent, while the
r value is a parameter characteristic of the given reaction,
representing the extent of resonance contribution.18 As
illustrated in Figure 2, the Yukawa-Tsuno plot exhibits
a good linearity for the reaction of 1a -e with OH-. Such
a good linear Yukawa-Tsuno plot clearly indicates that
the downward curvature in Figure 1 is not due to a
change in the RDS but due to stabilization of the GS of
the substrate through resonance structure II. Thus, the
present study suggests that deduction of a reaction
mechanism based just on a linear or a nonlinear Ham-
mett- or Brønsted-type plot can be misleading, as re-
ported previously for aminolyses of 4-nitrophenyl-sub-
stituted benzoates19a and 2,4-dinitrophenyl-substituted
(17) (a) Neuvonen, H.; Neuvonen, K.; Koch, A.; Kleinpeter, E.;
Pasanen, P. J . Org. Chem. 2002, 67, 6995-7003. (b) Neuvonen, H.;
Neuvonen, K. J . Chem. Soc., Perkin Trans. 2 1999, 1497-1502.
(18) (a) Tsuno, Y.; Fujio, M. Adv. Phys. Org. Chem. 1999, 32, 267-
385. (b) Tsuno, Y.; Fujio, M. Chem. Soc. Rev. 1996, 25, 129-139. (c)
Yukawa, Y.; Tsuno, Y. Bull. Chem. Soc. J pn. 1959, 32, 965-970.
(19) (a) Um. I. H.; Min, J . S.; Ahn, J . A.; Hahn, H. J . J . Org. Chem.
2000, 65, 5659-5663. (b) Um, I. H.; Hong, J . Y.; Kim, J . J . Chae, O.
M.; Bae, S. K. J . Org. Chem. 2003, 68, 5180-5185. (c) Um, I. H.; Han,
H. J .; Ahn, J . A.; Kang, S.; Buncel, E. J . Org. Chem. 2002, 67, 8475-
8480.
Effect of Lea vin g Gr ou p Su bstitu en t on Ra te a n d
Rea ction Mech a n ism . As a test of the preceding
(20) (a) Caplow, M.; J encks, W. P. Biochemistry 1962, 1, 883-893.
(b) Kirsch, J . F.; Clwell, W.; Simon, A. J . Org. Chem. 1968, 33, 127-
132.
J . Org. Chem, Vol. 69, No. 7, 2004 2439

Um et al.
F IGURE 3. Brønsted-type plot for alkaline hydrolysis of O-Y-
substituted phenyl thionobenzoates (2a -i) in 80 mol % H2O-
20 mol % DMSO at 25.0 ( 0.1 °C.
F IGURE 4. Hammett plots for alkaline hydrolysis of O-Y-
substituted phenyl thionobenzoates (2a -i) in 80 mol % H2O-
20 mol % DMSO at 25.0 ( 0.1 °C.
argument, the study has been extended to the alkaline
hydrolysis of 2a -i. The second-order rate constants for
alkaline hydrolysis of 2a -i are summarized in Table 2.
-
It is shown that the kOH value increases as the basicity
(r2 ) 0.996). However, much poorer correlation is ob-
tained when σ- constants are used (r2 ) 0.956, the inset
of Figure 4).
of the leaving group decreases except 3-NO2C6H4O-. The
effect of the leaving group basicity on the reactivity is
illustrated in Figure 3. The Brønsted-type plot is linear
but involves many scattered points. Besides, the âlg value
If the leaving group departure occurs in the RDS
(either in a concerted or stepwise mechanism), a partial
negative charge would develop on the O atom of the
leaving aryl oxides. Thus, one might expect good correla-
tion with σ- constants for the reaction in which the
leaving group departure is involved in the RDS. In fact,
σ- constants have often reported to give the best correla-
tion with a large F value for ester aminolysis in which
the RDS was suggested to be the leaving group departure
from an addition intermediate.22 The fact that σ° con-
stants give much better correlation than σ- constants
(Figure 4) suggests that no negative charge develops on
the O atom of the leaving aryl oxides in the rate-
determining TS. Thus, one can propose that the alkaline
hydrolysis in this study proceeds through an addition
intermediate in which its formation is the RDS.
has been determined to be rather a small value (âlg
)
-0.35). Such a small âlg value has often been reported
for reactions which proceed through rate-determining
formation of an addition intermediate, e.g., âlg ) -0.3 (
0.1 for alkaline hydrolysis of aryl acetates,21a,b and
cinnamates,21c and for aminolyses of aryl acetates, car-
bonates, and their thio analogues.2,22 However, signifi-
cantly large âlg values have been reported for reactions
involving leaving group departure as the RDS, e.g.,
-
reactions of aryl cinnamates with N3 ion (âlg ) -0.95)
and aminolysis of aryl acetates and benzoates with a poor
leaving group (âlg ) -1.4).21c,22
The magnitude of âlg value has been understood as a
measure of the extent of leaving group departure in the
rate-determining TS. Thus, one can suggest that the
leaving group departure is not advanced in the TS of the
RDS on the basis of the small âlg obtained in the present
system.
Con clu sion s
To obtain further information on the TS structure,
Hammett plots are constructed for the reaction of 2a -i
with OH-. As shown in Figure 4, the Hammett plot
exhibits good correlation when σ° constants are used
The present study has allowed us to conclude the
following. (1) The nonlinear Hammett plot obtained from
the reaction of 1a -e with OH- is not due to a change in
the RDS since the Yukawa-Tsuno plot is linear. Thus,
determination of the reaction mechanism based just on
a nonlinear Hammett plot can be misleading. (2) The
origin of the curved Hammett plot is GS stabilization
through resonance between the electron donating sub-
stituent and the thionocarbonyl group as illustrated by
the resonance structures I and II. (3) The alkaline
hydrolysis of 1a -e and 2a -i proceeds through an addi-
(21) (a) Kirsch, J . F.; J encks, W. P. J . Am. Chem. Soc. 1964, 86,
837-846. (b) Bruice, T. C.; Fife, T. H.; Bruno, J . J .; Brandon, N. E.
Biochemistry 1962, 1, 7-12. (c) Suh, J .; Lee, B. H. J . Org. Chem. 1980,
45, 3103-3107.
(22) (a) J encks, W. P.; Gilchrist, M. J . Am. Chem. Soc. 1968, 90,
2622-2637. (b) Menger, F. M.; Smith, J . H. J . Am. Chem. Soc. 1972,
94, 3824-3829. (c) Lee, J . P.; Yoon, J . H.; Um, I. H. Bull. Korean Chem.
Soc. 1999, 20, 805-808.
2440 J . Org. Chem., Vol. 69, No. 7, 2004

Alkaline Hydrolysis of O-Aryl Thionobenzoates
while the substrate concentration was 2 × 10-5 M. Pseudo-
first-order rate constants (kobs) were calculated from the well-
known equation, ln(A∞ - At) ) - kobst + c. The plots of ln(A∞
- At) vs time were linear over ca. 90% reaction. Usually, five
different OH- concentrations were employed and replicate
values of kobs were determined to obtain the second-order rate
constants (kOH-) from the slope of linear plots of kobs versus
NaOH concentrations.
P r od u ct An a lysis. Y-substituted phenoxide was liberated
quantitatively and identified as one of the products in the
hydrolysis of 1a -e and 2a -i by comparison of the UV-vis
spectra after completion of the reactions with those of authen-
tic samples under the same reaction conditions.
tion intermediate in which formation of the intermediate
is the RDS.
Exp er im en ta l Section
Ma ter ia ls. Compound O-4-nitrophenyl thionobenzoate (1c
and 2h ) was prepared as reported previously.6 Other com-
pounds (1a ,b,d ,e and 2a -g,i) were synthesized easily by a
similar method used for preparing 1c (e.g., by treating the
respective substituted thionobenzoyl chloride and the substi-
tuted phenol under the presence of triethylamine in anhydrous
ether). The structure and purity of 1a -e and 2a -i were
checked by their melting points, 1H NMR spectra, and elemen-
tal analyses. The analytical data are summarized in the
Supporting Information.
Kin etics. The kinetic study was performed with a UV-vis
spectrophotometer equipped with a constant temperature
circulating bath at 25.0 ( 0.1°C. All the solutions were
transferred by Hamilton gastight syringes. The reactions were
followed by monitoring the appearance of the leaving aryl
oxide. All the reactions were carried out under pseudo-first-
order conditions in which NaOH concentrations were at least
100 times greater than the substrate concentration.
Ack n ow led gm en t. This work was supported by
Korea Research Foundation (KRF-2002-070-C00061).
We are also grateful to J u Won Hwang (Queen’s
University) for carrying out some measurements of rate
constants.
Su p p or tin g In for m a tion Ava ila ble: Tables S1-S14 for
the kinetic results for the reactions of 1a -e and 2a -i with
OH- in 80 mol % H2O-20 mol % DMSO at 25.0 ( 0.1 °C. The
melting points of compounds 1a -e and 2a -i together with
their IR and 1H NMR spectra and elemental analyses data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Typically, a reaction was initiated by adding 5 µL of a 0.01
M solution of O-4-nitrophenyl thionobenzoate in acetonitrile
by syringe to a 10-mm quartz UV cell containing 2.50 mL of
the thermostated reaction mixture made up of solvent and an
aliquot of the NaOH stock solution. Generally, the base
concentration was varied over the range (5-100) × 10-3 M,
J O035854R
J . Org. Chem, Vol. 69, No. 7, 2004 2441