Reactions of aryl 5-substituted-2-thiophenecarboxylates promoted by 4-Z-C6H4O-/4-Z-C6H4OH in 20 mol % DMSO(aq). Effect of nucleophile on acyl-transfer reaction

Article abstract of DOI:10.1002/bkcs.10567

Nucleophilic substitution reactions of 5-XC₄H₂(S)C(O)OC₆H₃-2-Y-4-NO₂ (1) promoted by 4-Z-C₆H₄O-/4-Z-C₆H₄OH in 20 mol % dimethyl sulfoxide (DMSO)(aq) have been studied kinetically. The reactions exhibited second-order kinetics with β_acyl = -2.52 to -2.83, ρ(x) = 2.81-3.16, β_nuc = 0.88-0.04 and β_lg = -0.94, respectively. The results have been interpreted with an addition-elimination mechanism in which the nucleophilic attack occurs in the rate-determining step. Comparison with existing data reveals that the ratedetermining step changes from the second to the first step by the change in the nucleophile from R₂NH/R₂NH₂⁺ to 4-Z-C₆H₄O-/4-Z-C₆H₄OH.

Full text of DOI:10.1002/bkcs.10567

Article
DOI: 10.1002/bkcs.10567
BULLETIN OF THE
S. Y. Pyun et al.
KOREAN CHEMICAL SOCIETY
Reactions of Aryl 5-substituted-2-Thiophenecarboxylates Promoted by
4-Z-C6H4O−/4-Z-C6H4OH in 20 mol % DMSO(aq). Effect of
Nucleophile on Acyl-Transfer Reaction
‡
‡
§
Sang Yong Pyun,†, Kyu Cheol Paik, Man So Han, and Bong Rae Cho
*
†Department of Chemistry, Pukyong National University, Pusan 608-737, Korea.
*E-mail: sypyun@pknu.ac.kr
‡Department of Chemistry, Daejin University, Pocheon-si 487-711, Korea
§Department of Chemistry, Korea University, Seoul 136-713, Korea
Received July 15, 2015, Accepted August 3, 2015, Published online November 25, 2015
Nucleophilic substitution reactions of 5-XC4H2(S)C(O)OC6H3-2-Y-4-NO2 (1) promoted by 4-Z-C6H4O−/4-
Z-C6H4OH in 20 mol % dimethyl sulfoxide (DMSO)(aq) have been studied kinetically. The reactions exhib-
ited second-order kinetics with βacyl = −2.52 to −2.83, ρ(x) = 2.81–3.16, βnuc = 0.88–0.04 and βlg = −0.94,
respectively. The results have been interpreted with an addition–elimination mechanism in which the nucle-
ophilic attack occurs in the rate-determining step. Comparison with existing data reveals that the rate-
determining step changes from the second to the first step by the change in the nucleophile from R2NH/
R2NH2+ to 4-Z-C6H4O−/4-Z-C6H4OH.
Keywords: Nucleophilic substitution, Bronsted and Hammett plots, Concerted and stepwise mechanism
Introduction
X
X
S
O
O
S
O
Nu-
20 mol % DMSO(aq)
NO2
Nucleophilic substitution reactions of XC6H4C(O)OC6H4Y
have been studied extensively under various conditions.1–4
The results of the structure-reactivity studies have led to a
qualitative understanding of the relationship between the reac-
tant structure and the reaction mechanism. One of the most
interesting results from these studies is the change in the
rate-determining step (RDS) from the second to the first
step by the change in the nucleophiles from amines to arylox-
ides. In contrast, much less is known about the corresponding
acyl-transfer reactions involving heterocyclic aromatic
compounds.
In an earlier paper, we reported theacyl-transfer reactions of
40-nitrophenyl 5-substituted-2-thiophenecarboxylates [5-
XC4H2(S)C(O)OC6H4-4-NO2 (1)] promoted by secondary
amines.5 The reactions proceeded via an addition–elimination
mechanism in which the break-down of the intermediate is
the RDS. The electronic effects of the 5-thienyl substituent
showed a much better correlation in the Yukawa–Tsuno
plots than in the Hammett plots. Moreover, the ρ value
increased but r value decreased as the nucleophile was chan-
ged to a better one. This outcome has been attributed to an
increase in the electron density at the C O bond and
a decrease in the resonance demand. In this work, we have
studied the acyl-transfer reactions of 1 promoted by 4-Z-
C6H4O−/4-Z-C6H4OH in 20 mol % dimethyl sulfoxide
(DMSO)(aq) (Eq. (1)). We were interested in learning the
effects of the change in the nucleophile from amines to aryl-
oxides on the reactions of 1.
Nu
1
Y
+
N
O
O2
X = H (a), MeO (b), Me (c), Cl (d), NO2 (e);
Y = H (a'), MeO (b'), Cl (c'), CF3 (d'), NO2 (e')
Nu- = ZC6H4O-; Z = H, MeO, Me, Cl, CN
Y
(1)
Results
Compounds 1aa0-ea0 were available from previous study.5
The reaction of 1aa0 with C6H5O−/C6H5OH in 20 mol %
DMSO(aq) produced 4-nitrophenoxide as indicated by the
gradual increase in the absorption at λmax = 410 nm (4-
NO2C6H4O−). For all reactions, the yields of aryloxides deter-
mined by comparing the infinity absorption of the samples
from the kinetic studies with those of the authentic aryloxides
were in the range of 97−99%.
The rates of the reactions were determined by monitoring
the increase in the absorption at the λmax for the aryloxides
in the range of 400–434 nm. Excellent pseudo first-order
kinetics plots, which covered at least three half-lives were
obtained. The rate constants are summarized in Tables S1–
S6 in the Supporting Information. For all reactions, the plots
of kobs against base concentration were straight lines passing
through the origin. This outcome indicates that the reactions
are second-order, first-order to the substrate, and first-order
to the nucleophile (Figures S1–S10). The second-order rate
Bull. Korean Chem. Soc. 2015, Vol. 36, 2810–2814
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constants (kN) were obtained either from the slopes of straight
lines or by dividing the kobs values by the nucleophile concen-
trations. The kN values are summarized in Tables 1 and 2. The
kN value increased with increasing electron-withdrawing abil-
ity of the 5-thienyl substituents, the leaving group ability, and
the basicity of the nucleophile.
involved in resonance effects of its own.9 The Brönsted βacyl
value, whichcorrelates theeffect ofsubstituent attheacylmoi-
ety on the reaction rate, indicates the extent of the charge
development and the hybridization change in going from
Table 2. Rate constants for the reactions of C4H3(S)C(O)OC6H3-2-
Y-4-NO2a promoted by 4-Z-C6H4O−/4-Z-C6H4OHb,c in 20 mol %
DMSO(aq) at 25.0 ꢁC.
The plots of log kN values for 1aa0-1ea0 against the pKa
values of X-thiophenecarboxylic acid are depicted in
Figure 1. The plots are linear with reasonable correlations.
The βacyl values obtained from the slopes of the plots are in
therange of−2.52to−2.83(Table 3). In addition, thekN values
for phenolysis from 1aa0-1ea0 showed good correlation with
the pKa values of the promoting nucleophiles on the Brönsted
plots (Figure 2). The βnuc values are in the range of 0.88–1.04
(Table 4). Similarly, the plots of log kN values for 1aa0-1ae0
vs. the pKlg values of leaving groups correlated reasonably
on the Brönsted plot (Figure 3). The slope of the plot is
−0.94 ꢀ 0.22.
kN, M−1 s−1d,e When Y is
OCH3
(1ab0)
Cl
(1ac0)
CF3 (1ad0),
NO2 (1ae0)
Y
H (1aa0)
pKa(Y-ArOH)7
kN
7.14
0.266
7.05
0.0290
5.43
0.719
5.28, 4.11
5.75, 73.8
a [Substrate] = 5.0 × 10−5 M.
b [4-Z-C6H4O−]/[4-Z-C6H4OH] = 1.
c [4-Z-C6H4O−] = 6.0 × 10−4 − 0.1 M.
d Average of three or more rate constants.
e Estimated uncertainty, ꢀ3%.
The influence of substituents in the acyl moiety on the phe-
nolysis rates gave excellent correlations with σ values on the
Hammett plot (Figure 4 and Figures S11 and S12). The rate
data correlated better with σ than with σ− parameters. The
Hammett ρ values are in the range of 2.81–2.93 (Figure S11).
3.0
1.5
Discussion
0.0
Mechanism of Acyl-Transfer Reactions of 1. The mechan-
ism of the reaction between 1 and aryloxides has been assessed
by the kinetic results. The reactions of 1 with 4-Z-C6H4O−/4-
Z-C6H4OH in 20 mol % DMSO(aq) are second-order, first-
order to the substrate, and first-order to the nucleophile
(Figure S1–S10). The kN value increased with increasing
electron-withdrawing ability of the 5-thienyl substituents,
the leaving group ability, and the basicity of the nucleophile
(Table 1).
–1.5
–3.0
2.8
3.2
3.6
4.0
pKa(XArCOOH)
The reaction mechanism and RDS have been elucidated by
the Brönsted and Hammett plots. It is widely accepted that the
β value can be regarded as a measure of the degree of charge
transfer in the transition state, as long as the base is not
Figure 1. Brönsted plots of log kN vs. pKa (XArCOOH) values for the
reactions of 5-XC4H2(S)C(O)OC6H4-4-NO2 promoted by 4-Z-
C6H4O−/4-Z-C6H4OH in 20 mol % DMSO(aq) at 25.0 ꢁC [Z = CN
( ), Cl ( ), H ( ), CH3 ( ), OCH3 ( )].
Table 1. Rate constants for the reactions of 5-XC4H2(S)C(O)OC6H4-4-NO2a promoted by 4-Z-C6H4O−/4-Z-C6H4OHb in 20 mol % DMSO(aq)
at 25.0 ꢁC.
kN, (M−1 s−1 e,f
) When X is
Z/Xc
pKa
H (1aa0)
OCH3 (1ba0)
CH3 (1ca0)
Cl (1da0)
NO2 (1ea0)
d
4-CN
4-Cl
H
4-CH3
4-OCH3
7.95
9.38
9.95
10.19
10.20
0.00774
0.266
0.540
1.20
0.000837
0.0531
0.0879
0.192
0.00383
0.0960
0.225
0.420
0.562
0.0749
1.47
3.55
8.88
11.1
0.983
59.0
195
225
317
1.75
0.352
a [Substrate] = 5.0 × 10−5 M.
b [4-Z-C6H4O−]/[4-Z-C6H4OH] = 1.
c [4-Z-C6H4O−] = 6.0 × 10−4–0.1 M.
d pKa data in 20 mol % DMSO(aq) taken from Refs. 4f and 6.
e Average of three or more rate constants.
f Estimated uncertainty, e rat.
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Nucleophilic Substitution, Bronsted and Hammett Plots,
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Table 3. Brönsted βacyl values for the reactions of 5-XC4H2(S)C(O)OC6H4-4-NO2 promoted by 4-Z-C6H4O−/4-Z-C6H4OH in 20 mol %
DMSO(aq) at 25.0 ꢁC.
Z
4-OCH3
10.20
2.56 ꢀ 0.26
4-CH3
10.19
−2.63 ꢀ 0.15
H
4-Cl
9.38
−2.62 ꢀ 0.18
4-CN
7.95
−2.52 ꢀ 0.18
a
pKa
βacyl
a Ref. 7.
9.95
−2.83 ꢀ 0.15
Table4. Brönstedβvaluesforthereactionsof5-XC4H2(S)C(O)OC6H4-4-NO2 promotedby4-Z-C6H4O−/ 4-Z-C6H4OHin20 mol%DMSO(aq)
at 25.0 ꢁC.
X
OCH3
3.90
1.04 ꢀ 0.08
CH3
3.71
0.89 ꢀ 0.04
H
Cl
NO2
2.69
1.02 ꢀ 0.08
pKaa (X ArCOOH)
βnuc
a pKa values of XC4H2(S)COOH. Ref. 8.
3.51
3.32
0.88 ꢀ 0.03
0.95 ꢀ 0.05
3.0
1.5
2
1
β = –0.94 + 0.22 (r = 0.926)
lg
0.0
0
–1.5
–3.0
–1
–2
8.4
9.1
9.8
10.5
4.2
4.9
5.6
6.3
7.0
pKa(ZArOH)
pKa(YArOH)
Figure 2. Brönsted plots of log kN vs. pKa(ZArOH) values for the
reactions of 5-X-C4H2(S)C(O)OC6H4-4-NO2 promoted by 4-Z-
C6H4O−/4-Z-C6H4OH in 20 mol % DMSO(aq) at 25.0 ꢁC [X =
OCH3 ( ), CH3 ( ), H ( ), Cl ( ), NO2 ( )].
Figure 3. Brönsted plots of log kN vs. pKa(Y-ArOH) values for reac-
tions of C4H3(S)C(O)OC6H3-2-Y-4-NO2 promoted by 4-Cl-
C6H4O−/4-Cl-C6H4OH in 20mol% DMSO(aq) at 25.0 ꢁC.
the reactant to the transition state.3,10 If the nucleophile attacks
the carbonyl group in the RDS, the βacyl value should be a large
negative value. If, on the other hand, the leaving group depar-
ture is involved in the RDS, a small negative or positive βacyl
value is predicted. Thus, theobservedvalues of βacyl = −2.52ꢂ
−2.83 for ArO− promoted reactions from 1 is consistent with
the nucleophilic attack by aryloxides being the RDS. Moreo-
ver, βacyl values are much larger than |βlg| = 0.94, which is usu-
ally taken as the extent of the leaving group cleavage in the
transition state. Further, the Brönsted plots for βacyl showed
much better correlation than the |βlg| plot (Figures 1 and 3).
These results indicate a greater extent of C O bond formation
than the Cα OAr bond cleavage in the transition state.
The βnuc value can be used to assess the extent of the bond
formation between the nucleophile and the substrate in the
rate-limiting transition state.11 Earlier, Um et al. reported that
the βnuc values for the reactions of 4-nitrophenyl phenylcarbo-
nate and 4-nitrophenyl benzoate with aryloxides in 20 mol %
X = NO2
X
= NO2
2
1
2
1
ρ
= 2.10 0.19
X
=
=
Cl
=
0
X
H
ρ = 3.16 0.08
X
CH3
–1
X
=
OCH3
–0.3
0.0
0.3
0.6
0.9
1.2
−
σ
X = Cl
0
X = H
X = CH3
X = OCH3
–1
–0.4
–0.2
0.0
0.2
0.4
0.6
0.8
σ
Figure 4. Hammett plot of log kN vs. σ values for the reactions of 5-
XC4H2(S)C(O)OC6H4-4-NO2 promoted by C6H5O−/C6H5OH in 20
mol % DMSO(aq) at 25.0 ꢁC. Inset: Plot of log kN vs. σ− for the same
reaction.
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Nucleophilic Substitution, Bronsted and Hammett Plots,
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X
Table 5. Effect of the nucleophiles variation on the acyl-transfer
reactions from 5-XC4H2(S)C(O)OC6H3-2-Y-4-NO2 in 20 mol %
DMSO(aq) at 25.0 ꢁC.
X
S
OAr
O
O
S
k1[ArO-]
Y
O
O-
k-1
N
O2
-
1
[
T
]
Nucleophiles
R2NHa
C6H5O−
Y
k2
N
O2
N
b
X
pKa
9.85
9.95
1
−2.83 ꢀ 0.15
0.95 ꢀ 0.05
0.94 ꢀ 0.22
3.16 ꢀ 0.08
S
O
-O
O2
+
Relative rate
1
—
OAr
βacyl
Y
c
βnuc
0.89 ꢀ 0.05
Scheme 1.
|βlg|c
1.29 ꢀ 0.05d,e
1.02 ꢀ 0.07
ρ(x)f
a R2NH = piperazine.
b Ref. 7.
DMSO(aq) were 0.54 and 0.72, respectively.12 This outcome
has been interpreted with the concerted mechanism. On the
other hand, the βnuc = 0.90 determined for reactions of substi-
tuted phenyl acetate with p-chlorophenoxide has been attrib-
uted to the addition–elimination mechanism in which the
formation of the tetrahedral intermediate is the RDS.13 There-
fore, the observed βnuc = 0.88−1.04 for 1 is consistent with the
addition–elimination mechanism with the first step being
the RDS.
The Hammett ρ(x) value is a measure of the extent of neg-
ative charge development in the transition state.11a,14 For
ArO− promoted reactions from 1, ρ(x) = 3.16 was determined
(Figure 4). The large ρ(x) value indicates significant extent of
the C O bond rehybridization in the transition state.4a,16
The combined results reveal that the aryloxide-promoted
acyl-transfer reactions from 1 proceed by an addition–
elimination mechanism in which the first step is RDS
(Scheme 1). The transition state structure is more skewed
toward the formation of C O bond than the cleavage of
Cα OAr bond, with a greater extent of C O bond formation
than the Cα OAr bond cleavage and more negative charge
development at the α-carbon.
c X = H.
d Ref. 15.
e R2NH = morpholine.
f Y = H.
R2NH, it is more likely for the first step being the RDS
for the ArO−-promoted substitution reactions. Exactly the
opposite result is expected for the R2NH-promoted reactions.
It is interesting to note that the change in the RDS occurs for
both XC6H4C(O)OC6H4Y and 1 by the same variation of the
nucleophile. It appears that it is the nature of the nucleophile,
not the α-aryl group, that is important for determining
the RDS.
Conclusions
The acyl-transfer reactions of 1 with 4-Z-C6H4O−/4-Z-
C6H4OH in 20 mol % DMSO(aq) proceed by the addition–
elimination mechanism in which the first step is RDS. As
the nucleophile was changed from R2NH to ArO−, the extent
of rehybridization atthe carbonyl carbon andthe extent of Cα–
O bond formation increased, and the degree of Cα–OAr bond
cleavage decreased. This outcome indicates that the nucleo-
philic attack is more advanced than the leaving group depar-
ture in the rate-limiting transition state. The most interesting
result from this study is the change of the RDS from second
to first step by the change of the nucleophile from R2NH/
R2NH2+ to 4-Z-C6H4O−/4-Z-C6H4OH.
Effect of Nucleophile on the Acyl-Transfer Reactions.
Comparison of the transition state parameters presented in
Table 5 reveals that the transition state structure changes sig-
nificantly by the variation of the nucleophile from R2NH/
R2NH2 to Z-C6H4O−/4-Z-C6H4OH. As the nucleophile is
+
changed from R2NH to ArO−, the Hammett ρ(x) value
increases by three-fold, indicating a large increase in the rehy-
bridization at the carbonyl carbon in the transition state. The
large positive Hammett ρ(x) value observed for the reactions
of 1 with ArO− as the nucleophile is in good agreement with
the large negative βacyl value for the same reactions; in both
case, the reaction rates are highly sensitive to the electronic
effects of the substituent X in the acyl moiety.
Experimental
Materials.
Aryl 5-substituted-2-thiophenecarboxylates
1 were available from previous investigations.5 DMSO was
distilled under reduced pressure from CaH2. The p-
chlorophenol and p-cresol were recrystallized from n-hexane.
Other phenols were used without further purification. The
buffer solutions of Z-C6H4O−/4-Z-C6H4OH in 20 mol %
DMSO(aq) were prepared by adding appropriate amount of
the conjugate acid to 5 N NaOH solutions.
The βnuc value increased only slightly by the same variation
of the nucleophile, indicating a small increase in the C O
bond formation. The smaller |βlg| value observed for ArO−
indicates a smaller degree of Cα OAr bondrupture in the tran-
sition state. Taken together, these results can most reasonably
be explained in terms of the change in the RDS from second to
first step by the nucleophile variation from R2NH/R2NH2+ to
4-Z-C6H4O−/4-Z-C6H4OH. The RDS of the addition–
elimination reaction depends on the relative rates of the k−1
and k2 steps. Since ArO− is a better leaving group than
Kinetic Studies. Reactions of 1 with Z-C6H4O−/4-Z-
C6H4OH in 20 mol % DMSO(aq) were followed by monitor-
ingtheincrease intheabsorbance ofthearyloxidesintherange
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Nucleophilic Substitution, Bronsted and Hammett Plots,
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(e) I. H. Um, H. J. Han, J. A. Ahn, J. S. Kang, E. Buncel,
J. Org. Chem. 2002, 67, 8475; (f ) I. H. Um, K. H. Kim,
H. R. Park, M. Fujin, Y. Tsuno, J. Org. Chem. 2004, 69,
3937; (g) I. H. Um, J. Y. Lee, S. H. Ko, S. K. Bae, J. Org. Chem.
2006, 71, 5800; (h) I. H. Um, S. J. Hwang, M. B. Beck, E. J. Park,
J. Org. Chem. 2006, 71, 9191; (i) S. W. Min, J. A. Seo, I. H. Um,
Bull. Korean Chem. Soc. 2009, 30, 2403; (j) I. H. Um, L. R. Im,
E. H. Kim, J. H. Shin, Org. Biomol. Chem. 2010, 8, 3801;
(k) J. S. Kang, I. H. Um, Bull. Korean Chem. Soc. 2012, 33,
2269; (l) J. Y. Lee, M. Y. Kim, I. H. Um, Bull. Korean Chem.
Soc. 2013, 34, 3795; (m) I. H. Um, A. R. Bae, T. L. Um,
J. Org. Chem. 2014, 79, 1206.
of 400–434 nm with a UV–Vis spectrophometer as described
before.5
Product Studies. The products of Z-C6H4O− promoted reac-
tions from 1 were identified by UV spectrophotometer. When
the reaction mixture was scanned periodically by monitoring
the UV absorption, the absorbance due to the aryl oxide
increased. The yields of aryloxides determined by comparing
the UV absorptions of the infinity samples with those for the
authentic aryloxides are in the range of 97–99%.
Control Experiments. The stability of 1 was determined as
reported earlier.5 The solutions of 1 inMeCNwere stable for at
least 2 weeks when stored in the refrigerator.
5. S. Y. Pyun, B. R. Cho, Bull. Korean Chem. Soc. 2013, 34, 2036.
6. I. H. Um, K. Akhtar, Y. H. Shin, I. Y. Han, J. Org. Chem. 2007,
72, 3823.
7. W. P. Jencks, J. Regenstein, In Handbook of Biochemistry.
Selected Data for Molecular Biology, H. A. Sober Ed., The
Chemical Rubber Co., Cleveland, OH, 1968.
Acknowledgments. This work was supported by the
Pukyong National University Research Fund in 2015.
SupportingInformation. Additionalsupportinginformation
is available in the online version of this article.
8. pKa values calculated using Advanced Chemistry
Development(ACD/Labs)software
ACD/Labs).
v11.02(©1994–2015
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