D. Sun et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 2 (2013)
281
consider that large amount of charged reactants generates large
reaction heat, which results in the increase of both the tem-
perature and the pressure, and that either high temperature or
high pressure could accelerate the reaction rate. In the reaction
using solvent, the pressure during the reaction was lower than
that of solvent-free reaction (Figure 5c), and no significant
increase in temperature was observed: the maximum temper-
ature was 128 °C (Figure 6c). It is reasonable that the reaction
heat was absorbed by the solvent, which does not lead to
increase in temperature and pressure that cause the high yields.
In addition, it is preferable that the closed system confines a
gaseous reactant such as 1,3-butanediene in the reactor.
As mentioned above, there is a problem with the control of
reaction heat in reactions which generate large exothermic heat.
When the reaction rate is high, the reaction heat generates
quickly. In a slow reaction that heat generates gradually, the
reaction temperature can be controlled through heat transfer.
In a fast reaction that the heat generates quickly, the use of
solvent is effective in absorbing the reaction heat. In addition,
an excess of diene or dienophile can be regarded as a solvent
to absorb the reaction heat. Since the excess of diene or
dienophile can provide high concentration of reactants, this
would be effective in achieving high yields. Moreover, such
pressured conditions are readily achieved only by controlling
the charged amount and the temperature in the closed batch
reactor.
example, in the reaction using 1,3-butadiene, which was
condensed in a steel cylinder and stored in a refrigerator at
¹17 °C, the condensed 1,3-butadiene was directly poured into
the container from the cylinder. Afterwards, the container was
sealed with a tapered cap made of poly(tetrafluoroethylene),
set in a stainless steel jacket, and placed in an oven controlled
at prescribed temperatures. After the prescribed period, the
jacket with the container was cooled at 0 °C in an ice bath.
The recovered reaction mixture was analyzed on a gas
chromatograph (FID-GC, Shimadzu GC-14A) using a 60-m
capillary column (Inertcap-1, GL Science, Japan). A gas chro-
matograph with a mass spectrometer (GCMS-QP-5050A,
Shimadzu, Japan) and a 30-m capillary column (DB-WAX,
Agilent Technologies, USA) was used for identification of the
recovered compounds. 1H and 13C NMR spectra were recorded
on a DPX-300 spectrometer (Bruker, Germany) at 300 and
75.15 MHz, respectively, in CDCl3.
The pressure during the reaction was independently mea-
sured by using a pressure gauge ranging from 0.1 to 1.1 MPa in
another pressured reactor with a container volume of 22.7 cm3.
The temperature during the reaction was also independently
monitored by a thermocouple inserted in an inner tube of
another pressured reactor with a volume of ca. 65 cm3.
Enthalpy change of reaction was calculated using Gaussian
03.29 The optimization of the modeled structure, frequency
analysis of the optimized structure, and enthalpy calculation
under experimental conditions were carried out. All the calcu-
lations were conducted by B3LYP30/6-31 g(d) integral=grid=
ultrafine. Enthalpy change was obtained without consideration
of scaling factor. Depending on the experimental reaction
conditions, temperature and pressure were set at 30 or 125 °C
and 0.101 MPa.
Conclusion
Diels-Alder reactions of several dienes, such as 1,3-buta-
diene, isoprene, and 2,3-dimethyl-1,3-butadiene, with several
kinds of dienophiles such as methyl vinyl ketone, methyl
acrylate, and maleic anhydride, were investigated in a closed
batch reactor under pressured conditions without using any
catalysts or solvents.
References
It was found that most of the reactions proceeded fast under
such pressured conditions even at an equimolar ratio of diene
to dienophile. The solvent-free reactions proceeded faster than
the reactions in solvent: solvent-free conditions realize high
temperature, pressure, and concentration of reactants. It is
probable that high concentration of reactants, appropriate tem-
peratures, and pressured conditions are important for preparing
a Diels-Alder product. In addition, it would be the most
beneficial that solvent-free Diels-Alder reaction in a closed
system is convenient for keeping high concentration of reac-
tants, and a solvent-free system has no problems with sepa-
rating the products from the solvents, which is friendly to the
environment.
1
2
3
4
5
R. W. M. Aben, L. Minuti, H. W. Scheeren, A. Taticchi,
T. R. Kelly, S. K. Maity, P. Meghani, N. S. Chandrakumar,
B. Biolatto, M. Kneeteman, E. Paredes, P. M. E. Mancini,
A. Marrocchi, L. Minuti, A. Taticchi, H. W. Scheeren,
M. G. B. Drew, A. Jahans, L. M. Harwood, S. A. B. H.
6
7
8
9
The reactivity order of the reactants is as follows: 2,3-
dimethyl-1,3-butadiene > isoprene > 1,3-butadiene in dienes;
maleic anhydride > methyl vinyl ketone > methyl acrylate in
dienophiles. Combination of the reactants with different reac-
tivity governs the reaction rate and the reaction temperature.
11 L. Minuti, A. Marrocchi, S. Landi, M. Seri, E. Gacs-Baitz,
13 D. Huertas, M. Florscher, V. Dragojlovic, Green Chem.
Experimental
Commercially available reagents were purchased from Wako
Pure Chemical Industries Ltd. or Tokyo Chemical Industries
Co., Ltd. Typically, a reactant diene (31.5 mmol) and a
dienophile (30 mmol) were poured into a container made
of poly(tetrafluoroethylene) with a volume of 20.0 cm3. For
15 F. Benito-López, R. J. M. Egberink, D. N. Reinhoudt, W.