JOURNAL OF CHEMICAL RESEARCH 2016 487
Conclusions
synthesis and has been intensively studied.8–11 Chen reported
that11 some by-products, including amino-nitriles, diimines
and homopiperazines could be detected during the catalytic
hydrogenation. The reaction temperature and the pressure
of hydrogen played a critical role in the formation of these
by-products. The influence of reaction temperature on the
hydrogenation reaction was first examined and the results
indicated that the selectivity in formation of compound
4 gradually increased with the increase in the reaction
temperature. Thus, 90 °C was selected as the suitable reaction
temperature. Furthermore, the effect of hydrogen pressures
was also evaluated and it was discovered that the conversion
of compound 3 and the selectivity of formation of compound
4 increased with the increase of hydrogen pressures. Thus, 2.0
MPa was found to be satisfactory.
The catalyst has a significant influence on the reactivity
of the hydrogenation reaction. Three hydrogenation catalysts
including W-3 Raney Ni, TLD-A and Al–Ni–Mo were studied
respectively and the results showed that W-3 Raney Ni
exhibited better catalytic performance compared to the others.
The 100% reaction of compound 3 and the 85% selectivity
of formation of compound 4 were obtained when W-3 Raney
Ni was used while 68% and 76% yields were respectively
achieved when TLD-A and Al-Ni-Mo were employed.
In addition, Liu reported that the presence of a base was
beneficial in increasing the activity of Raney Ni.8,9 Therefore,
sodium hydroxide in this reaction was added in order to
accelerate the reaction and increase the yield.
Finally, the influence of the solvents on the reactivity was
also examined. The 100% conversion of compound 3 and the
85% selectivity in formation of compound 4 were obtained
when absolute ethanol was used as the solvent, while the 67%
yield was achieved when methanol was employed. It suggests
that the solvent with weaker polarity was more favourable
for the catalytic hydrogenation. Nevertheless, methanol was
employed in the first step and only a moderate yield was
gained in the second step. However, a one-pot reaction can be
realised by using the same solvent which has the features of a
simple operation and the possibility of recycling in industrial
applications. Thus, methanol was selected as the optimal
solvent. Without further purification the resulting methanol
solution was used directly in the next methylation step.
The N–methylation of primary amines to the corresponding
dimethyl tertiary amines has been widely studied12–15 and the
three main methods including the Eschweiler–Clarke reaction,
methanol–hydrogenation, and formaldehyde–hydrogenation
have been employed in industrial production. Although
the Eschweiler–Clarke reaction had the advantage of mild
operation conditions, formic acid is inevitably used. This
method suffers from several defects such as low efficiency,
environmental pollution and severe corrosion of equipment.
Therefore, this method has been gradually eliminated in
industrial practice. For the methanol–hydrogenation of
primary amines, although excellent yields could be obtained,
excessive hydrogen pressures and high reaction temperatures
were required, leading to excessive demands of the equipment
as well as high costs. Compared with the above methods,
the formaldehyde–hydrogenation of primary amines has
incomparable advantages including high efficiency and little
pollution. Therefore, with the W-3 Raney Ni as a catalyst,
81.0% yield of PMDPTA was achieved by using compound 4
and an aqueous formaldehyde solution in methanol at 90 °C,
and 3.0 MPa.
In conclusion, a green and effective synthetic route to PMDPTA
was established. Under the optimised reaction conditions,
PMDPTA was obtained from methylamine, acrylonitrile
and formaldehyde in 65% total yield. Methanol was used as
the solvent in all the three steps, leading to the convenient
recovery of solvents. This synthetic route is thus, suitable for
the industrial production of PMDPTA.
Experimental
Methylamine methanol solution, ethanol, methanol, acrylonitrile and
aqueous formaldehyde solution were purchased from commercial
sources and used without further purification. 1H NMR and
13C NMR spectra were measured on a Bruker Avance (III) 400 MHz
spectrometer using CDCl3 as the solvent. High-resolution mass spectra
(HRMS) were performed on a Bruker Daltonics miorOTOF-QII
instrument. The composition of the reaction mixture was identified by
GC-400A equipped with a OV-1701 column (30 m × 0.5 mm).
Synthesis of N,N-bis(2-cyanoethyl)methylamine (3)
Acrylonitrile (11.66 g, 0.22 mol) was added drop by drop into the
methylamine methanol solution (12.7 mL, 0.1 mol) at 0 °C in 30 min
while magnetically stirring. The reaction mixture was stirred for 4 h
at 25 °C and then the volatile was evaporated to give 3: light yellow
1
liquid; 13.03 g, 95.0% yield; H NMR (CDCl3, 400 MHz) δ: 2.32 (s,
3H, CH3), 2.48 (t, J = 6.8 Hz, 4H, CH2), 2.76 (t, J = 7.0 Hz, 4H, CH2).
Preparation of W-3 Raney Ni catalyst
Nickel–aluminium alloy powder (25 g) was added in batches to the
sodium hydroxide aqueous solution (152 g, 20 wt%) at 40–50 °C whilst
constantly stirring in a 500 mL beaker. The mixture was stirred at
50 °C for 1 h and then stirred at 25 °C for 24 h. After that, the upper
aqueous phase of the stationary mixture was discarded. The catalyst
was washed each time with distilled water (100 mL) until the pH value
of the aqueous phase was 7 and then replaced with absolute ethanol
by 3 times. The prepared catalyst was kept in a conical flask full of
absolute ethanol.
Synthesis of N,N-bis(3-aminopropyl)methylamine (4)
Compound 3 (30 mL), ethanol (150 mL), sodium hydroxide (0.18 g)
and W-3 Raney Ni (6 g) were added into the 300 mL autoclave. The
air in the autoclave was replaced with nitrogen by 3 times and then
with hydrogen by 3 times. The dangers of hydrogen/oxygen contact
with the catalyst should always be given attention since a mixture of
hydrogen and oxygen is capable of leading to an explosion. After the
reaction mixture was stirred at 90 °C, 2.0 MPa, the reaction was not
stopped until the hydrogen pressure no longer dropped over 6 h and
then the mixture was filtered. When the mixture was filtered, Raney
Ni should always be kept wet with ethanol and should never have direct
contact with air since dry Raney Ni can easily burst into flames in the
air, which could result in significant hazards. After the filtration, the
solvent was evaporated to give 4: viscous, colourless liquid; 25.5 g,
85.0% yield; 1H NMR (CDCl3, 400 MHz) δ: 1.65 (m, J = 7.6 Hz, 4H,
CH2), 2.26 (s, 3H, CH3), 2.37 (t, J = 7.4, 4H, CH2), 2.48 (t, J = 7.4 Hz,
4H, CH2), 5.15 (s, 4H, NH2).
Synthesis of N,N,N',N',N"-pentamethyldipropylenetriamine (PMDPTA)
Compound 4 (14.5 g, 0.1 mol), methanol (150 mL), aqueous
formaldehyde solution (40.6 g, 0.5 mol, 37 wt%) and W-3 Raney
Ni (3 g) were added to the autoclave. The air in the autoclave was
replaced with nitrogen (3 times) and then with hydrogen (3 times). The
dangers of hydrogen/oxygen contact with the catalyst should always
be given attention since a mixture of hydrogen and oxygen is capable
to of leading to an explosion. Then the reaction mixture was stirred at
a rotation speed of 500 rpm at 90 °C, 3.0 MPa. The reaction was not
stopped until the hydrogen pressure no longer dropped over 5 h and
then the mixture was filtered. When the mixture was filtered, Raney Ni
should always be kept wet with ethanol and should never directly have