Reactions of 4-Nitrophenyl 5-substituted Furan-2-carboxylates with R2NH/R2NH2+ in 20 mol% DMSO(aq): Effect of Aryl Group on the Acyl-Transfer Reaction

Article abstract of DOI:10.1002/bkcs.12296

Reactions of 4-nitrophenyl 2-furoates (1a–e) with R₂NH/R₂NH₂⁺ in 20 mol% DMSO(aq) have been studied. The reactions produced aminolysis products and exhibited second-order kinetics. The Br?nsted plots were linear with β_nuc values of 0.75–0.89, which remained nearly the same for all 5-furyl substituents. The rate data showed excellent correlations on the Yukawa-Tsuno plots with ρ(x)?=?0.72–1.1, and r?=?0.55–0.95, respectively. The ρ value increased and r value decreased with a stronger nucleophile, indicating an increase in the electron density at the C-O bond and a decrease in the resonance contribution. The results have been interpreted with the addition–elimination mechanism in which the second step is the rds. By comparing with the data for ArC(O)OC₆H₄-4-NO₂ (Ar?=?Ph, thienyl), the effect of the aryl group on the acyl transfer reaction was assessed.

Full text of DOI:10.1002/bkcs.12296

Article
DOI: 10.1002/bkcs.12296
BULLETIN OF THE
S. Y. Pyun et al.
KOREAN CHEMICAL SOCIETY
Reactions of 4-Nitrophenyl 5-substituted Furan-2-carboxylates with
R2NH/R2NH2+ in 20 mol% DMSO(aq): Effect of Aryl Group on the
Acyl-Transfer Reaction
‡
‡
§,
Sang Yong Pyun,†, Kyu Cheol Paik, Man So Han, and Bong Rae Cho
*
*
†Department of Chemistry, Pukyong National University, Busan 608-737, South Korea.
*E-mail: sypyun@pknu.ac.kr
‡Dvision of Life Science and Chemistry, Daejin University, Pochun-si 11159, South Korea
§Department of Chemistry, Korea University, Seoul 136-713, South Korea. *E-mail: chobr@korea.ac.kr
Accepted April 23, 2021
Reactions of 4-nitrophenyl 2-furoates (1a–e) with R2NH/R2NH2+ in 20 mol% DMSO(aq) have been stud-
ied. The reactions produced aminolysis products and exhibited second-order kinetics. The Brönsted plots
were linear with βnuc values of 0.75–0.89, which remained nearly the same for all 5-furyl substituents.
The rate data showed excellent correlations on the Yukawa-Tsuno plots with ρ(x) = 0.72–1.1, and
r = 0.55–0.95, respectively. The ρ value increased and r value decreased with a stronger nucleophile,
indicating an increase in the electron density at the C O bond and a decrease in the resonance contribu-
tion. The results have been interpreted with the addition–elimination mechanism in which the second step
is the rds. By comparing with the data for ArC(O)OC6H4-4-NO2 (Ar = Ph, thienyl), the effect of the aryl
group on the acyl transfer reaction was assessed.
Keywords: Aminolysis, Brönsted plot, Hammett plot, Yukawa-Tsuno plot, Addition–elimination mecha-
nism, 4-Nitrophenyl 5-substituted Furan-2-carboxylates
Introduction
transition states. However, the aryl group effect on the
aminolysis reaction was not elucidated.
Acyl transfer reactions have been extensively studied due
to their importance in biology and chemistry.1-7One of the
most critical mechanistic questions on this reaction was
how the reaction mechanism changes from a stepwise to a
concerted mechanism. It is well established that the reac-
tions between XC6H4C(O)OC6H4Y with various nucleo-
philes proceed through an addition–elimination mechanism
in which the rate determining step (rds) changes depending
on the basicity of nucleophile and leaving group.6,7 Earlier,
It was reported that the rds of the aminolysis of esters chan-
ged from the second to the first step when the basicity of
the nucleophile was stronger than that of the leaving group
by 4–5 pKa units.5o,8-10 The kꢀ1/k2 values for the reactions
of 2-XC6H4C(O)OC6H4Y with R2NH remained nearly the
same for all X, indicating that the rds is not affected by the
aryl substituent.5h Similarly, the aminolysis of
4-nitrophenyl thiophene-2-carboxylate (2) proceeded by the
second order kinetic with large values of βnuc and leaving
group ρꢀ, a result consistent with the second step as the
To provide a better insight into the effect of the aryl
group on the aminolysis, we have now investigated the
reactions of 4-nitrophenyl furan-2-carboxylates with R2NH/
+
R2NH2 in 20 mol% DMSO(aq) (Eq. 1). Since the aro-
matic resonance energy of furan (16 kcal/mol) is smaller
than those of thiophene and benzene,12 the effect of aroma-
ticity of the aryl group on the reaction mechanism could be
elucidated. We have employed different 5-furyl substituents
+
(1a–e) to study the electronic effect and R2NH/R2NH2
buffer as the nucleophile to keep the pH constant.
Results
5j
rds. Since the X group of 2 is closer to the carbonyl car-
bon than that of phenyl11, and the aromatic resonance
energy of thiophene (29 kcal/mol) is smaller than that of
benzene (36 kcal/mol)12 the electronic effect of the X group
in 2 should be more efficiently transmitted to the reaction
site than that in the phenyl derivatives. These properties
would provide different stabilization of the respective
4-Nitrophenyl furan-2-carboxylates (1a–e) were prepared
by reacting 5-substituted furan-2-carbonyl chloride with
4-nitrophenol in the presence of Et3N in CH2Cl2 as
reported.13 The reactions of 1a–e with R2NH/R2NH2 in
+
20 mol% DMSO(aq) produced 4-nitrophenoxide in 96–
98% yields, as determined by comparing the absorbance of
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Table 1. Rate constants for the aminolysis of 5-XC4H2(O)C(O)OC6H4-4-NO2 a promoted by R2NH/R2NH2+ b in 20 mol% DMSO(aq)
at 25.0ꢂC.
kN (Mꢀ1 sꢀ1 e,f When X is
)
Aminec
pKa
H (1a)
OCH3 (1b)
CH3 (1c)
Br (1d)
NO2 (1e)
d
1-formylpiperazine
Morpholine
N-(2-hydroxylethyl)- piperazine
Piperazine
3-Methylpiperidine
Piperidine
7.98
8.65
9.38
9.85
10.8
11.02
0.056
0.414
1.57
4.99
18.2
24.8
0.0165
0.110
0.426
1.38
2.86
3.99
0.0282
0.204
0.785
2.68
6.74
8.41
0.064
0.622
2.14
7.03
27.6
33.7
0.200
1.79
8.91
31.9
107
117
a [Substrate] = 5.0 ꢃ 10ꢀ5 M.
b [R2NH]/[R2NH2+] = 1.0.
c [R2NH] = (5.0 ꢃ 10ꢀ4 ꢀ 0.02) M.
d Ref. 5i.
e Average of three or more rate constants.
f Estimated uncertainty, ꢁ3%.
nucleophile concentration because the kN values calculated
from the slopes of the plots were in excellent agreement
(within ꢁ3%) with the kN calculated by this method. The
kN values for the aminolysis of 1a–e are l in Table 1.
The rate increased with the increase in the electron-
withdrawing ability of X and the basicity of the amine.
The log(kN/q) values for the aminolysis of 1a, 1b, and 1e
showed good correlations with the pKa + log(p/q) values of
the promoting nucleophiles on the Brönsted plots, where
p and q represent the number of equivalent protons in the
acid HA and the number of equivalent basic sites in
the conjugated base Aꢀ, respectively (Figure 1).14 For
R2NH-promoted reactions from 1a–e, the values of βnuc
=
0.75–0.89 were determined (Table 2). Figure 2 shows the
plots of log(kN/q) vs. σ values for the R2NH-promoted
aminolysis of 1a–1e. The influence of the 5-furyl substitu-
ents on the aminolysis rates correlated well with the Ham-
mett σ values except for 1b (X = OCH3), which showed
negative deviation from the straight line defined by other
substituents (Figure 2). In contrast, the rate data showed
excellent correlations with the Yukawa-Tsuno plots using
σ + r(σ+ ꢀ σ) (Figure 3). The ρ and r values are listed in
Table 3. The reactions exhibit ρ = 0.71–1.13 and
r = 0.55–0.95. The ρ value increased, and r value
decreased with a stronger base.
Figure 1. Brönsted plots for the aminolyses of 5-XC4H2(O)C(O)
OC6H4-4-NO2 promoted by R2NH/R2NH2 in 20 mol% DMSO
(aq) at 25.0ꢂC [X = H (1a, ●), OCH3 (1b, ■), NO2 (1e, ▲)].
+
the infinity samples from the kinetic studies with that of
4-nitrophenoxide.
The rates of aminolysis were determined by monitoring
the increase in the absorption at the λmax for the
4-nitrophenoxide at 400 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. The plots of kobs vs. base
concentration were straight lines passing through the origin,
indicating that the reactions are second-order, first order to
1a–e and first order to the amine bases (Supporting Infor-
mation Figures S1–S10). The slopes are the overall second-
order rate constants kN.
Discussion
Reaction Mechanism. The reaction mechanism was deter-
mined by the results of the product and kinetic studies. The
+
reactions of 1 with R2NH/R2NH2 in 20 mol% DMSO(aq)
produced aminolysis products almost quantitatively and
showed second-order kinetics, first order to the substrate,
and first order to the nucleophile. The rate increased with
the increase in the electron-withdrawing ability of X group
and the basicity of the amine bases (Table 1). The most
likely mechanism for this reaction is the addition–
When R2NH = 1-formylpiperizine, however, the rate
was too slow to follow to completion. The kN values for
this reaction were calculated by dividing the kobs by the
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Acyl Transfer Reactions of 4-Nitro 2-Furoates
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Table 2. Brönsted βnuc values for the aminolyses of 5-XC4H2(O)C(O)OC6H4-4-NO2 promoted by R2NH/R2NH2+ in 20 mol% DMSO(aq)
at 25.0ꢂC.
X
σa
OCH3
CH3
H
X Cl
X NO2
0.78
0.89 ꢁ 0.08
ꢀ0.27
ꢀ0.17
0
0.23
0.86 ꢁ 0.06
βnuc
a Ref. 15.
0.75 ꢁ 0.07
0.78 ꢁ 0.07
0.84 ꢁ 0.05
Table 3. The ρ and r values for the aminolysis of 5-XC4H2(O)C
(O)OC6H4-4-NO2 promoted by R2NH/R2NH2+ in 20 mol%
DMSO(aq) at 25.0ꢂC.
a
Amine
pKa
ρ
r
1
2
3
4
5
6
Piperidine
3-Methyl piperidine
Piperazine
N-(2-hydroxyethyl)piperazine
Morpholine
1-Formylpiperazine
11.02 1.11 ꢁ 0.1
0.55
10.80 1.13 ꢁ 0.06 0.70
9.85 0.93 ꢁ 0.04 0.83
9.38 0.90 ꢁ 0.04 0.83
8.65 0.82 ꢁ 0.04 0.90
7.98 0.71 ꢁ 0.04 0.95
a Ref.5i.
tetrahedral intermediate as the RDS.1d,2k,l On the other
hand, the large βnuc values (0.8 ꢁ 0.2) were observed for
reactions where the breakdown of the tetrahedral intermedi-
ate is rds.1d,2k,l Hence, the large βnuc values of 0.75–0.89
determined for this reaction are entirely consistent with the
stepwise mechanism in which the second step is rds. Also,
the βnuc value increased with a stronger electron-withdraw-
ing X group (Table 3). If the first step is rds (kꢀ1 << k2), an
electron-withdrawing group would destabilize the reactant
more than the product of the first step (T), shift the transi-
tion state toward the reactant (1), and decrease the N C
bond formation. If the second step is rds (kꢀ1 >> k2), how-
ever, the βnuc value should be identical for all substituents
because there is no change in the extent of the N C bond
formation inT and P H+(Scheme 1). Hence, the nearly
identical βnuc values measured for all substituents provide
additional support for this mechanism.
Figure 2. Hammett plots of log(kN/q) vs. σ values for the
aminolysis of 5-XC4H2(O)C(O)OC6H4-4-NO2 promoted by
+
R2NH/R2NH2
in
20 mol%
DMSO(aq)
at
25.0ꢂC
[R2N = piperidine (■), piperazine (●), 1-formylpiperazine (▲)]
(Δ and □ are points for log(kN/q) vs. σ and σ+, respectively).
Effect of 5-Furyl Substituent. The rate of aminolysis
increased with a stronger electron-withdrawing 5-furyl sub-
stituent. The Hammett plots were linear except for
X = MeO, which showed negative deviation when plotted
against σ value, and positive deviation when plotted against
σ+ value (Figure 2). A similar result was reported for reac-
tions of thienyl derivatives, and attributed to the extra stabi-
lization of the carbonyl carbon by the MeO group by
through resonance (I, II), albeit not as effectively as in the
cumyl cation (III, IV) used to define the σ+ value
(Scheme 2).17
On the other hand, the rate data showed excellent corre-
lation with the Yukawa-Tsuno equation (Figure 3), where ρ
and r values are the extents of negative charge development
and resonance demand at the reaction site, respectively
(Eq. 2).18
Figure 3. Yukawa-Tsuno plots for the aminolyses of 5-XC4H2(O)
+
C(O)OC6H4-4-NO2 promoted by R2NH/R2NH2 in 20 mol%
DMSO(aq) at 25.0ꢂC [R2N = piperidine (■), piperazine (●),
1-formylpiperazine (▲)].
elimination mechanism, in which either the first or second
step is rds. The rds was determined using the Brönsted βnuc
and Hammett ρ values. Brönsted βnuc value is a measure of
the bond formation between the nucleophile and carbonyl
carbon (N C) in the transition state.10,16b Small βnuc values
(0.2 ꢁ 0.1) have been interpreted with the formation of the
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Acyl Transfer Reactions of 4-Nitro 2-Furoates
KOREAN CHEMICAL SOCIETY
Scheme 1. Stepwise mechanism for the acyl transfer reaction.
Scheme 2. Extra resonance stabilization by the π-donor substituent.
value should become less negative and C O bond should
have less double bond character. This would predict a
larger ρ value and a smaller r value. This result provides
additional support for the second step as the rds.
Table 4. Effect of aryl group on the aminolysis of ArC(O)OC6H4-
4-NO2 promoted by R2NH/R2NH2+ in 20 mol% DMSO(aq)
at 25.0ꢂC.
Ar = phenyla
Ar = thienylb
Ar = furylc
pKa (ArCO2H)d
4.20
1.0
0.81
0.51
1.2
3.51
0.8
0.89 ꢁ 0.05
1.0 ꢁ 0.04
0.85
3.16
7.7
0.84 ꢁ 0.05
0.91 ꢁ 0.04
0.83
Rel. ratee
Effect of Aryl Group. The rates and transition state
parameters for the aminolysis of ArC(O)OC6H4-4-NO2 are
compared in Table 4. The relative rate changed from 1.0 to
0.8 to 7.7 by the change in the aryl group from phenyl
to thienyl to furyl. It is difficult to explain the similar rates
for the phenyl and thienyl derivatives, because of the sig-
nificant differences in the transition state parameters. When
Ar = furyl and thienyl for which the transition state struc-
tures are similar, however, the 10-fold faster rate for the
furyl derivative can readily be attributed to the stronger
electron withdrawing ability of the furyl group. All reac-
tions proceeded through an addition–elimination mecha-
nism in which the second step is rds.
The βnuc values are nearly the same, indicating similar
transition state structures. However, the ρ value increased
from 0.51 to 0.91–1.0, and the r value decreased from 1.2
to 0.83–0.85 by the change in the aryl group from Ph to
thienyl and furyl. This outcome can be interpreted with the
distance between the substituents and the reaction site and
the aromatic resonance energy. Since the substituents are
closer to the reaction site11 and the aromatic resonance
energy is smaller in the heterocyclic than in the phenyl
derivatives,12 the electronic effects of the aryl substituent
should be more efficiently transmitted to the reaction state
in the former than in the latter. Also, the π-orbital of the
C O group can better be stabilized by resonance with
those in the heterocycles, thereby decreasing the resonance
demand. The combined effects predict a larger ρ value and
a smaller r value for the heterocycles, as observed.
f
βnuc
ρe
re
a Ref. 5k.
b Ref. 6e.
c This work.
d Ref. 19.
e R2NH = 1-(2-hydroxyethyl)piperazine.
f X = H.
logðk=k0Þ ¼ ρfσþr ðσþ ꢀσÞg
ð2Þ
The ρ and r values calculated by nonlinear regression
analysis were 0.71–1.11 and 0.55–0.95, respectively
(Figure 3). The ρ value increased and r value decreased
with the pKa value of the promoting base, indicating an
increase in the electron density at the carbonyl carbon and
a decrease in the resonance demand with a stronger nucleo-
phile (Table 3). Since the electron density at the carbonyl
carbon increases in the k1 step and decreases in the k2 step,
the ρ value for the first step (ρ1) should be positive and that
of the second step (ρ2) should be negative. If the second
step is rds (kꢀ1 >> k2), a stronger nucleophile would stabi-
lize the product (PH+) more than the reactant (T) of the k2
step and shift the transition state toward T, that is, the ρ2
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Acyl Transfer Reactions of 4-Nitro 2-Furoates
KOREAN CHEMICAL SOCIETY
In conclusion, the aminolysis of 4-nitrophenyl 2-furoates
5-NO2C4H2(O)C(O)OC6H4-4-NO2 (1e).
Yield 69%;
mp:193.7ꢂC; IR (cmꢀ1): 1754 (C O); H NMR: δ 7.44 (d,
1H, J = 3.68), 7.46 (d, 2H, J = 9.16), 7.54 (d, 1H,
J = 3.68), 8.35 (d, 2H, J = 9.16 Hz); 13C NMR: δ 111.5,
121.1, 122.3, 125.5, 143.1, 146.0, 154.1, 154.3; LR-EIMS:
m/z [M+] = 278 (15), 140 (100), 94 (5), 82 (10), 66 (10),
54 (8).
+
1
(1a–e) promoted by R2NH/R2NH2 in 20 mol% DMSO
(aq) proceeded by the addition–elimination mechanism in
which the second step is the rds. The βnuc value increased
with the increase in the electron-withdrawing ability of the
5-furyl substituent. The rate data showed excellent correla-
tions on the Yukawa-Tsuno plots. The ρ value increased
and r value decreased with a stronger nucleophile, indicat-
ing an increase in the electron density at the C O bond
and a decrease in the resonance demand. Comparison with
existing data revealed that the ρ value increased and r value
decreased by the change in the aryl group from Ph to
thienyl and furyl without substantial change in the transi-
tion state structure. This outcome has been attributed to the
proximity of the aryl substituent to the reaction site and the
smaller aromatic resonance energy of the heterocyclic
compounds.
All reagent and chemicals were purchased from Sigma-
Aldrich (St.Louis, MO, USA). The stock solutions of sub-
strate (0.03 M) were dissolving the corresponding substrate
in absolute MeCN. The solvent was used by adding 20 mol
% DMSO to the aqueous solution in order to increase the
solubility of the substrate as described previously.5o,5p,6d
The solutions of R2N/R2NH+ in 20 mol% DMSO(aq) were
prepared by adding 1 equiv of standardized 5 N HCl solu-
tion to 2 equiv of amine solution in 20 mol% DMSO(aq).
Kinetic Studies. Reactions of 1 with R2N/R2NH+ in
20 mol% DMSO(aq) were followed by monitoring the
increase in the absorbance of the 4-nitrophenoxide at
400 nm with a UV-vis spectrophotometer as described.6e
Reactions of 1e with R2N/R2NH+ (R2NH = piperidine and
3-methylpiperidine) in 20 mol% DMSO(aq) were too fast
to be measured by this method, so the rate studies utilized a
stopped-flow spectrophotomer.
Experimental Section
Materials. All of the 4-nitrophenyl 5-substituted furan-
2-carboxylates 1 were prepared by the reactions of
4-nitrophenol with 5-substituted furan-2-carbonyl chloride
in the presence of Et3N in methylene chloride.13 The spec-
tral and analytical data of the compounds were consistent
with the proposed structures. The yield (%), IR (KBr,
Product Studies. The products were identified by periodi-
cally monitoring the UV absorption of the reactions mix-
tures under the reaction condition. The yields of aryloxides
determined by comparing the UV absorptions of the infinity
samples with those for the authentic 4-nitrophenoxide were
in the range of 96–98%.
1
C O, cmꢀ1), H NMR (400 MHz, CDCl3, J values are in
Hz), and 13C NMR (100 MHz), and mass spectral data for
the new compounds are as follows.
C4H3(O)C(O)OC6H4-4-NO2 (1a). Yield 86%; mp: 155ꢂC;
IR (cmꢀ1): 1743 (C O); 1H NMR: δ 6.64 (d, 1H,
J = 0.72), 7.44 (d, 2H, J = 9.16), 7.45 (d, 1H, J = 0.72),
7.72 (d, 1H, J = 0.72), 8.32 (d, 2H, J = 9.52); 13C NMR:
δ 112.5, 120.6, 122.5, 125.3, 143.1, 145.5, 147.9, 155.0,
155.8; LR-EIMS: m/z [M+] = 233 (28), 139 (3), 122 (5),
95 (100), 67 (21), 51 (7).
Control Experiments. The stabilities of 1 were deter-
mined as reported earlier.6e The solutions of 1 in MeCN
were stable for at least 2 weeks when stored in the
refrigerator.
Acknowledgment. This work was supported by the
Pukyong National University Research Fund (PK-2021).
5-MeOC4H2(O)C(O)OC6H4-4-NO2 (1b). Yield 87%; mp:
1
115ꢂC; IR (cmꢀ1): 1752 (C O); H NMR: δ 4.02 (s, 3H),
Supporting Information
5.47 (d, 1H, J = 3.68), 7.39–7.42 (m, 1H), 8.31 (d, 2H,
J = 8.8); 2 more protons 13C NMR: δ 58.1, 84.6, 114.7,
122.5, 124.7, 125.2, 132.9, 145.2, 155.2, 164.9; LR-EIMS:
m/z [M+] = 263 (21), 125 (100), 82 (15), 69 (39), 54 (17).
Observed rate constants for the aminolysis of 1a–e pro-
moted by R2N/R2NH+ in 20 mol% DMSO(aq), plots of
kobs vs. base concentration, and NMR spectra for all com-
pounds are available on request from the correspondence
author (11 pages). E-mail: sypyun@pknu.ac.kr.
5-MeC4H2(O)C(O)OC6H4-4-NO2 (1c). Yield 85%; mp:
1
115ꢂC; IR (cmꢀ1): 1752 (C O); H NMR: δ 2.46 (s, 3H),
6.25 (d, 1H, J = 3.32), 7.35 (d, 1H, J = 3.32), 7.41 (d, 2H,
J = 9.14) 2 more protons; 13C NMR: δ 14.2, 109.2, 122.2,
122.5, 125.3, 141.4, 145.4, 155.2, 155.8, 159.3; LR-EIMS:
m/z [M+] = 247 (20), 109 (100), 81 (60), 53 (37), 40 (5).
Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
5-BrC4H2(O)C(O)OC6H4-4-NO2 (1d). Yield 79%; mp:
1
131ꢂC; IR (cmꢀ1): 1749 (C O); H NMR: δ 6.59 (d, 1H,
J = 3.68), 7.38 (d, 1H, J = 3.68), 7.42 (d, 2H, J = 9.16),
8.32 (d, 2H, J = 9.16); 13C NMR: δ 114.7, 122.4, 122.7,
125.3, 130.0, 144.6, 145.6, 154.6, 154.7; LR-EIMS: m/z
[M+] = 311 (12), 173 (100),145 (4), 117 (12), 102 (2),
75 (3).
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Acyl Transfer Reactions of 4-Nitro 2-Furoates
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Article
Acyl Transfer Reactions of 4-Nitro 2-Furoates
KOREAN CHEMICAL SOCIETY
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