Effect of different substituents on the one-pot formation of 3,4,5-substituted furan-2(5H)-ones: a kinetics and mechanism study

Article abstract of DOI:10.1039/c5ra09717g

The kinetics of reactions between benzaldehyde and diethyl acetylenedicarboxylate with para-substituted anilines for the one-pot formation of 3,4,5-substituted furan-2(5H)-ones have been studied spectrally at different temperatures in formic acid. For all

Full text of DOI:10.1039/c5ra09717g

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Effect of different substituents on the one-pot
formation of 3,4,5-substituted furan-2(5H)-ones: a
kinetics and mechanism study†
Cite this: RSC Adv., 2015, 5, 52508
*
Mehdi Shahraki, Sayyed Mostafa Habibi-Khorassani and Maryam Dehdab
The kinetics of reactions between benzaldehyde and diethyl acetylenedicarboxylate with para-substituted
anilines for the one-pot formation of 3,4,5-substituted furan-2(5H)-ones have been studied spectrally at
different temperatures in formic acid. For all substituents at the para position of the aniline ring, the
reaction followed second-order kinetics. The partial orders with respect to substituted aniline and diethyl
acetylenedicarboxylate were one and one, and the reactions also revealed zero-order kinetics in relation
to benzaldehyde. Para-electron donating substituents on the aniline ring increased the rate of reaction.
On the basis of experimental data, a three step mechanism has been proposed. Kinetic values (k and Ea)
and associated activation parameters (DHs, DGs and DSs) of the reactions were determined.
Received 23rd May 2015
Accepted 9th June 2015
DOI: 10.1039/c5ra09717g
www.rsc.org/advances
ultraviolet-visible (UV-visible) spectroscopy. Several research
studies have documented over a large area of different reactions
1. Introduction
which have previously been reported using the UV-visible
spectrophotometry technique.22–30 In our previous work, exper-
imental and theoretical kinetics studies and mechanism
investigation have fully been discussed for various reac-
tions.31–37 In continuing our investigations on the kinetic of the
organic reactions, we focused on 3,4,5-substituted furan-2(5H)-
ones by one-pot multicomponent reaction that was reported by
MT Maghsoodlou and coworkers.38 Depending upon the stoi-
chiometry and conditions, other products have also been
reported for this reaction.39–42 However, under optimized
conditions reported by M. T. Maghsoodlou and et al. (Fig. 1),38
3,4,5-substituted furan-2(5H)-ones are obtained in high ylides
A multicomponent reaction (MCR) is a process in which three or
more accessible components combine to produce a nal
product that displays the features of all inputs and thus offers
greater possibilities for molecular diversity per step with a
minimum of synthetic time and effort in a single reaction.
Synthesis of furan-2(5H)-ones as subunits in a large number
of natural products1–5 is a signicant target for organic and
medicinal chemists. Highly substituted furans are a structural
component of a vast number of biologically active natural and
synthetic compounds such as anticancer,6,7 anti-inammatory,8
antifungal,9,10 antimicrobial,11,12 and antiviral HIV-1.13–15 The
reactions leading to the formation of furanones are frequently
reported by several research groups, however, they have
proposed the reactions mechanism without offering evidence to
support them.16–20 There are a number of reasons for tendency
to know reaction mechanisms. Knowledge of mechanisms aids
the synthetic chemist in predicting by products and improving
reaction conditions, the most powerful tool for the experi-
mental study of reaction mechanisms is chemical kinetics.
Kinetics is the study of the speed with which a chemical reac-
tion occurs, and with all of the factors which inuence these
rates.21
The spectroscopic technique is the largest and important
techniques used in chemistry. The most widely method used for
kinetic and mechanistic studies of chemical reactions is
Department of Chemistry, Faculty of Science, University of Sistan and Baluchestan, PO
Box 98135-674, Zahedan, Iran. E-mail: smhabibi@chem.usb.ac.ir; Fax: +98 541
2446565; Tel: +98 541 2446565
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra09717g
Fig. 1 The three-component synthesis of furan-2(5H)-ones.38
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(%90–98). In the current work for the rst time, we describe
kinetic results together with the detailed mechanistic studies
for the mentioned reactions based on a global kinetic analysis
methodology using the UV-vis spectrophotometry apparatus.
Also, this work focused especially on the effects of temperature,
different para-substituted anilines on the activation parameters
of preparation reaction of 3,4,5-substituted furan-2(5H)-ones.
2. Experimental
2.1. Chemicals and apparatus used
All chemicals were used without further purication.
Substituted-aniline 1, diethyl acetylenedicarboxylate 2 and
benzaldehyde 3 were supplied by Merck (Darmstadt, Germany),
Acros (Geel, Belgium) and Fluka (Buchs, Switzerland). Extra
pure formic acid and acetic acid were also obtained from Merck
(Darmstadt, Germany).
Fig. 3 The original experimental absorbance curve versus time at
selected wavelength of 420 nm for the reaction between 1 (10ꢀ2 M)
and 2 (10ꢀ2 M) with 3 (10ꢀ2 M) in formic acid. Dotted line is experi-
mental values and the solid line is a fit curve.
A Cary UV-vis spectrophotometer model Bio-300 with a 10
mm light-path quartz spectrophotometer cell equipped with
thermostated housing cell was used to record absorption
spectra in order to follow reaction kinetics.
Then the second-order rate constant (kovr ¼ 3.50 minꢀ1 Mꢀ2
)
is automatically calculated using the standard equations within
the program at 20 C.34
ꢁ
3. Results and discussion
3.1. Effects of concentration
2.2. General procedure
The rst experiment is performed with 10ꢀ2 M solutions of
aniline 1, diethyl acetylenedicarboxylate 2 and benzaldehyde 3
in formic acid solvent. The progress of reaction is monitored by
the absorbance changes as a function of wavelengths (Fig. 2).
Since at wavelength of 420 nm, reactants 1, 2 and 3 have no
relatively absorbance values, it provides the opportunity to fully
investigate the kinetics and mechanism of the reaction. Hence,
the product absorbance changes with time are undertaken
under the same condition at wavelength of 420 nm (Fig. 3) at
20 ꢁC. As can be seen in Fig. 3, the original experimental
absorbance curve versus time (dotted line) is exactly tted to
second order t curve (solid line). So the reaction is second
order (a + b + g ¼ 2), the overall reaction rate is described by the
kinetic equation:
In order to obtain partial order with respect to aniline 1, pseudo-
order condition is performed for the reaction. So in the second
experiment, we followed the reaction kinetics by plotting the
UV-vis absorbance versus time at wavelength of 420 nm with 1
(10ꢀ3 M), 2 (10ꢀ2 M) and 3 (10ꢀ2 M) at 20.0 ꢁC in formic acid. For
this case the rate law can be expressed:
Rate ¼ kovr[1]a[2]b[3]g
Rate ¼ kobs[3]g
(2)
Rate ¼ kovr[1]g[2]b[3]a
(1)
kobs ¼ kovr[1]a[2]b
All kinetic mathematical ts (the zero, rst or second t
curve) are done using the soware43 associated with the UV-Vis
spectrophotometer instrument. The original experimental
absorbance against time data make a pseudo-rst-order avail-
able t curve at 420 nm, which exactly ts the experimental
curve, Fig. 4A. Herein, observation rate constant (kobs) is auto-
matically calculated for the eqn (2) by the soware associated
within the UV-vis instrument. It is obviously that the reaction is
of the rst order type with respect to aniline 1 (a ¼ 1).
Also, to gain the partial order of reaction with respect to
reactant 3, third experiment is done under pseudo-order
condition [(10ꢀ2 M, 1), [(10ꢀ2 M, 2) and [(10ꢀ3 M, 3)]. In this
experiment the original experimental absorbance versus time
provides a second-order curve (kovr ¼ 3.49 minꢀ1 Mꢀ2, Fig. 4B)
like the rst experiment. Due to the concentration changes of
Fig.
2 Absorption changes versus wavelength for the reaction
between 1 (10ꢀ2 M), 2 (10ꢀ2 M) and 3 (10ꢀ2 M) in formic acid for
generation of product 4.
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According to the obtained results from the four experiments,
the rate law of reaction can be written:
From the first experiment: a + b + g ¼ 2
From the second experiment: a ¼ 1
From the third experiment: a + b ¼ 2 and g ¼ 0
From the fourth experiment: b ¼ 1
Rate ¼ kovr[1][2]
3.2. Effect of solvents and temperature
The solvent dielectric constant and polarity are important
parameters that inuence rate constants. Solvent effects on the
rate of reaction depend on the relative stabilization of the
starting materials and the corresponding transition state
through solvation.44,45 For investigation of the effect of medium
dielectric constant on the rate of reaction, the same kinetic
ꢁ
procedure is followed in the presence of acetic acid in 20 C.
It is found that as medium dielectric constant decreased, the
reaction rate reduced from formic acid (kovr ¼ 3.50 minꢀ1 Mꢀ2
)
to acetic acid (kovr ¼ 0.713 minꢀ1 Mꢀ2).
Temperature is considered a major factor that affects the rate
of a chemical reaction. The inuences of temperatureꢁon the
ꢁ
reaction rate were studied in the range of 20 C to 60 C with
10 ꢁC intervals for each reaction and the values of second-order
rate constants are determined. Kinetic data are shown in
Table 1. Reaction rate constant has a temperature dependency,
which is usually given by the Arrhenius equation.
Most reactions are found to obey the Arrhenius equation:
ꢀ
ꢁ
ꢀEa
k ¼ A exp
(3)
RT
Taking the natural logarithm of both sides of the equation
provides:
Fig. 4 (A) Plot of absorbance versus time at 420 nm for the reaction
between 1 (10ꢀ2 M), 2 (10ꢀ2 M) and 3 (10ꢀ3 M) in formic acid. Dotted
line is experimental values, and the solid line is fit curve. (B) Plot of
absorbance versus time at 420 nm for the reaction between 1 (10ꢀ3 M),
2 (10ꢀ2 M) and 3 (10ꢀ2 M) in formic acid. Dotted line is experimental
values, and the solid line is fit curve. (C) Plot of absorbance versus time
Ea
ln k ¼ ln A ꢀ
(4)
RT
where Ea is the activation energy of reaction, R is universal gas
constant (8.314 J Kꢀ1 molꢀ1), and A is frequency factor.
The activation energy of reaction is usually determined
experimentally by plotting the graph of ln k versus the reciprocal
of the temperature 1/T, which will yield a straight line with a
Ea
at 420 nm for the reaction between 1 (10ꢀ2 mol Lꢀ1), 2 (10ꢀ3 mol Lꢀ1
)
and 3 (10ꢀ2 mol Lꢀ1) in formic acid, dotted line is experimental values,
and the solid line is fit curve.
slope of ꢀ
and an intercept ln A (Fig. 5). The highest acti-
reactant 3 had no effect on the reaction rate, then reaction is
RT
zero-order with respect to reactant 3 (g ¼ 0). Also, in fourth vation energy (Ea ¼ 80.12 kJ molꢀ1) is obtained for NO2-aniline,
experiment, we followed kinetics under pseudo order condition that means reaction will occur slower than alone aniline and
of reactant 2. As can be seen in Fig. 4C, the reaction shows rst para-donating substituent-aniline. Nevertheless, the differences
order kinetics that it means the partial order with respect to in activation energies are not much, because the substituents
reactant 2 is one (b ¼ 1). So, the reaction between aniline 1 and on aniline ring are far from the center of reaction. The Eyring
diethyl acetylenedicarboxylate 2 with benzaldehyde 3 follows equation is used in chemical kinetics to describe the variance of
second-order kinetics.
the rate of a chemical reaction with temperature and was used
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Table 1 Reaction rate constant (kovr (minꢀ1 Mꢀ2)) in different temperatures and different substituted-aniline under the same conditions for the
reaction between 1 (10ꢀ2 M) and 2 (10ꢀ2 M) with 3 (10ꢀ2 M) in formic acid
(l) (nm)
Substituted-aniline
20.0 ꢁ
C
30.0 ꢁ
C
40.0 ꢁC
50.0 ꢁ
C
60.0 ꢁ
C
420
420
420
420
420
OMe-aniline
Ethyl-aniline
Aniline
F-aniline
NO2-aniline
8.10 (0.0028)a
6.16 (0.0026)
3.50 (0.0046)
0.75 (0.0029)
0.44 (0.0022)
24.31 (0.0057)
15.9 (0.0068)
9.06 (0.0038)
2.10 (0.0057)
1.40 (0.0029)
52.53 (0.0063)
34.88 (0.0074)
20.40 (0.0038)
5.12 (0.0037)
3.42 (0.0026)
115.42 (0.0023)
91.97 (0.0064)
48.45 (0.0025)
12.87 (0.0010)
9.33 (0.0021)
210.31 (0.0039)
174.10 (0.0075)
115.35 (0.0037)
32.938 (0.0037)
23.84 (0.0072)
a
Standard deviation (SD).
kovr
T
Fig. 5 Dependence of second order (ln kovr ¼ ln k1) on reciprocal
temperature for the reaction between reactants 1, 2 and 3 in formic
acid measured at wavelength 420 nm in accordance with Arrhenius
equation.
Fig. 6 Eyring plot (ln
versus T) according to eqn (5) for the
reaction between 1, 2 and 3 in formic acid.
Table 2 Activation parameters (DSs, DHs, DGs and Ea) using the
Arrhenius eqn (3) and Eyring eqn (5) and (6) in 20 ꢁC
to evaluate activation parameters involving DHs (activation
enthalpy), DSs (activation entropy).46
DHs
DSs
DGs
Ea
Substituted-aniline (kJ molꢀ1
)
(J molꢀ1 Kꢀ1
)
(kJ molꢀ1
)
(kJ molꢀ1
)
The linear form of the Eyring equation is given below:
k
DHs DSs
kB
h
OMe-aniline
Ethyl-aniline
Aniline
F-aniline
NO2-aniline
63.08
65.91
68.17
73.46
77.53
ꢀ11.20
ꢀ4.67
ꢀ2.01
3.40
59.24
64.92
68.76
74.83
80.81
65.67
68.51
70.76
76.05
80.12
ln ¼ ꢀ
þ
þ ln
(5)
T
RT
R
that kB ¼ Boltzmann's constant, T ¼ temperature, h ¼ Planck's
constant and R ¼ universal gas constant.
13.10
The values for DHs (activation enthalpy), DSs (activation
k
T
entropy) can be determined from slopes and intercepts of ln
Table 3 Activation parameters (DSs, DHs, DGs and ln A) using the
Arrhenius eqn (3) and The transition state eqn (6)–(8) in 20 ꢁC
versus 1/T plot (Fig. 6). Obtained activation parameters for para-
substituted aniline are given in Tables 2 and 3.
DHs
DSs
DGs
Also, the transition state equation was used to calculate
activation parameters DHs, DSs in order to check the results
obtained from the two methods (Eyring and the transition state
theory). According to the transition state theory, activation Ethyl-aniline
energy can be related to activation enthalpy (eqn (6)) and also
Substituted-aniline (kJ molꢀ1
)
(J molꢀ1 Kꢀ1
)
(kJ molꢀ1
)
ln A
OMe-aniline
60.71
63.55
65.80
71.09
75.16
ꢀ18.87
ꢀ12.36
ꢀ9.68
ꢀ4.27
5.42
66.34
67.24
68.69
72.37
73.55
29.15
29.94
30.26
30.91
32.08
Aniline
F-aniline
NO2-aniline
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pre-exponential factor is relevant to activation entropy accord-
ing to eqn (8) and (9), respectively:46
Ea ¼ DHs + 2RT
ekBT
(6)
(7)
DSs
þ2
R
A ¼
e
h
ꢀ
ꢀ
ꢁꢁ
e2kBT
h
DSs ¼ R ln A ꢀ ln
(8)
The values obtained on the basis of transition state theory
inferred that the activation enthalpy is the lowest for donating
substituent-aniline that have the higher reaction rates and the
highest for withdrawing substituent-aniline with the lowest
reaction rates (Table 3). Note that the negative activation
entropy indicated a greater degree of ordering in the transition
state than in the initial state. These results suggest that the
transition state in rst step of reaction, Fig. 8, involving
donating substituent-aniline is ordered more effectively stabi-
lized than the transition state containing withdrawing
substituent-aniline (Tables 2 and 3). The activation Gibbs free
energy has also been evaluated from the following form of the
Gibbs–Helmholtz equation:
DGs ¼ DHs ꢀ TDSs
(9)
Fig. 7 (A) Changes in absorption spectra vs. wavelength in 20 ꢁC for
the reaction between aniline 1 and diethyl acetylenedicarboxylate 2 in
formic acid as reaction proceeds into a 10 mm light-path cell. The total
acquisition time was 45 min; the time increment was set to 5 min.
Herein, the upward of direction of the arrow indicate that the progress
of reaction. (B) Changes in absorption spectra vs. wavelength in 20 ꢁC
for the reaction between aniline 1 and benzaldehyde 3 in formic acid as
reaction proceeds into a 10 mm light-path cell. The total acquisition
time was 5 min; the time increment was set to 1 min. Herein, the
upward of direction of the arrow indicate that the progress of reaction.
The activation free energy is essentially the energy require-
ment to get a molecule (or a mole of them) to undergo the
reaction. It is of interest to note that activation free energies
DGs for the different substituents are positive. The value of the
activation free energy for donating substituent-aniline is lower
than withdrawing substituent-aniline that means the reaction
will occur harder for withdrawing substituent-aniline.
To investigate the impact of the different substituents on the
rate constant of reaction and activation parameters, we changed
the substituents R on aniline from the electron donating
(methoxy, ethyl) to the electron withdrawing (uorine, nitro)
group. The reaction provides respective Schiff base and is
reversible in absence of reactant 3. The rst step of reaction
consists of Michael addition. According to the experimental
observations, the rst step is slow (please see Mechanism
section 3.3). Then, electron donating groups cause the aniline
more nucleophilic and accelerate the reaction.
information about the reaction products is presented in ESI
le,† and also the about rst intermediate I1 was reported
previously.47,48 According to these references, the rst step of the
reaction consists of Michael addition and is slow which
respective diethyl 2-aryloamino-fumarate I1 can be readily
synthesize and characterized according to known protocols and
can be puried by convenient distillation, then, I1 can be used
for subsequent steps.49,50 As a result of these preliminary
investigations and experimental observations, the speculative
mechanism containing three steps with starting reaction
between reactants 1 and 2 (the more rapidly occurring reaction
amongst competing reactions) is proposed in Fig. 8.
3.3. Mechanism
In separate experiments, reactions between aniline 1 and
diethyl acetylenedicarboxylate 2 (Re. (1)), aniline 1 and benzal-
dehyde 3 (Re. (2)), and then diethyl acetylenedicarboxylate 2 and
benzaldehyde 3 (Re. (3)) are performed under the same
concentration of each reactant (10ꢀ2 M) at 20 ꢁC. The Re. (1) is
monitored by recording scans of the entire spectra with 5 min
For the reaction, a steady state treatment is coupled with the
kinetic observations. So, the rate law is written using the nal
step of the proposed mechanism in Fig. 8 for the generation of
product 4:
ꢁ
intervals in 45 min reaction time at 20 C (Fig. 7A). The Re. (2)
was monitored by recording scans of the entire spectra with 1
min intervals during the whole reaction time (5 min) at 20 C
ꢁ
Rate ¼ k5[I4]
(10)
(Fig. 7B). There are no reactions between reactants 2 and 3 due
Also, the overall reaction can be written as:
to the lack of any progress. Moreover, some 1HNMR
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Fig. 8 Speculative mechanism for the reaction between aniline 1 and diethyl acetylenedicarboxylate 2 with para-substituted benzaldehyde 3 for
generation of product 4 in formic acid.
highly solvated relative to the reactant molecules that have
no any charge.
Rate ¼ k1[1][2]
(11)
This makes it possible to accept that the rst step is the
rate-determining step (RDS) of the reaction and the overall
order of reaction is two since k1 is the only rate constant which
appeared in the eqn (11). This equation, obtained from the
proposed mechanism, is same with the rate law that resulted
from UV-vis experiment (rate ¼ kovr [1][2]). It is possible to
analyze the effect of the solvent on the reaction, because the
rst step (k1) is the rate determining step, in this case in
both solvents the transition state partially charged, in
solvent with higher dielectric constant the transition state is
If the second step to be a rate determining step, therefore,
the rst step is fast with forward rate constant (k1) and return
rate constant (kꢀ1). We can write the rate law:
d½I1ꢂ
Rate ¼ k2½I1ꢂ½3ꢂ;
¼ k1½1ꢂ½2ꢂ ꢀ k ½I1ꢂ ꢀ k2½I1ꢂ½3ꢂ ¼ 0 (12)
ꢀ1
dt
k1½1ꢂ½2ꢂ
½I1ꢂ ¼
(13)
k2½3ꢂ þ k
ꢀ1
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k2k1½1ꢂ½2ꢂ½3ꢂ
Rate ¼
(14)
k2½3ꢂ þ k
ꢀ1
According to the above assumption, k2 is a rate determining
step, therefore it is logical to express kꢀ1 [ k2[3].
As a result, rate law becomes:
k2k1½1ꢂ½2ꢂ½3ꢂ
Rate ¼
(15)
k
ꢀ1
herein, the overall order of recent rate law is three and is not
compatible with the rate law arising from UV-vis experiment
(rate ¼ kovr[1][2]), so the second step could not be a rate deter-
mining step.
4. Conclusions
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Kinetics and mechanism of the reactions between para-
substituted anilines 1 and diethyl acetylenedicarboxylate 2 with
benzaldehyde 3 were investigated in formic acid and the below
results were obtained:
A mechanism involving three steps was proposed for the
reactions. The overall order of reaction followed second-order
kinetics and the partial orders with respect to para-substituted
aniline, diethyl acetylenedicarboxylate and benzaldehyde were
one, one and zero, respectively. Also, the rst step of the reac-
tion is recognized as a rate-limiting step according to the
experimental data. The electron donating group on substituted
aniline more stabilized the transition state than the electron
withdrawing group and with decreasing the activation energy
the value of rate constant (k1) was speed up. The rate of reaction
was also accelerated by increasing the temperature and was in
agreement with Arrhenius, Eyring equations and the transition
state equations.
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Acknowledgements
We gratefully acknowledge nancial support from the Research
Council of the University of Sistan and Baluchestan.
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