8430 Zhu et al.
Asian J. Chem.
Effect of reaction temperature: As the reactions were
Conclusion
carried out in autoclave, the reaction temperature higher than
the boiling point of dimethyl carbonate (90 ºC) was able to be
obtained. Fig. 8 shows the effect of reaction temperature on
conversion of morpholine, yield of N-methylmorpholine and
selectivity of N-methylmorpholine at a dimethyl carbonate/
morpholine molar ratio of 1.0. From Fig. 11, we can see that
the yield of N-methylmorpholine and the selectivity of N-
methylmorpholine gradually increased with the reaction
temperature increased, the high temperature (≥ 140 ºC) was
beneficial to the methylation reaction of morpholine with
dimethyl carbonate, low temperature was beneficial to N-
carboxymethylation. The result is consistent with that reported
by Tundo et al.6-8, high temperatures is beneficial to methylation
reaction of dimethyl carbonate.
Morpholine can be N-methylated by dimethyl carbonate
to yield heterocyclic N-methylmorpholine . The experimental
results revealed that the methylation performance of morpholine
with dimethyl carbonate which occurred in autoclave had
greatly improved than that in the reflux unit. Compared with
methanol, dimethyl carbonate was the effective methylating
agent under the reaction conditions. The reaction conditions
including molar ratio of starting materials and reaction tempe-
rature evidently influenced the formation of N-methylmorpholine.
When n(dimethyl carbonate)/n(morpholine) closes to the stoichio-
metric ratio (1:1) was regarded as optimum. The yield and the
selectivity of N-methylmorpholine gradually increased with
the reaction temperature increased, the high temperature (≥
140 ºC) was beneficial to the methylation reaction of morpholine
with dimethyl carbonate. Based on these experiments, the
plausible mechanism of formation of N-methylmorpholine by
using morpholine reacted with dimethyl carbonate were
proposed.
100
REFERENCES
50
1. X.Z. Chen, H. Luo, C. Qian and C.H. He, React. Kinet. Mechan. Catal.,
104, 163 (2011)
YNMM
CM
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(2003).
SNMM
0
80. 100 120 140 160 180 200 220
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Inorg. Chem., 3, 524 (2004).
Reaction temperature (ºC)
Fig. 11. Effect of reaction temperature on the property of reaction,
experimental conditions: morpholine 0.50 mol, without catalyst,
n(dimethyl carbonate)/n(morpholine) = 1:1, reaction time: 1 h
6. P. Tundo, Pure Appl. Chem., 73, 1117 (2001).
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Proposed reaction mechanism: The reactions mentioned
above were carried out without catalyst and the reactions were
also studied in the presence of catalysts (not shown here), but
the performance of reaction was not obviously improved. This
is mainly due to the self-catalysis of morpholine. Because
morpholine is a weak base, the lone pair electrons of the N
atom of morpholine is nucleophilic and it can attack the
methoxy carbon atom of dimethyl carbonate to form N-methyl-
morpholine or it can attack the methoxy carbonyl carbon atom
of dimethyl carbonate to form morpholine-4-carboxylic acid
methyl, then morpholine-4-carboxylic acid methyl can transfer
to N-methylmorpholine. The plausible mechanism of formation
of N-methylmorpholine by using morpholine reacted with
dimethyl carbonate could be represented as shown in Scheme-III.
10. G. Lamoureux and C. Agüero, ARKIVOC, 251 (2009).
11. A. Bomben, B.A. Marques, M. Selva and P. Tundo, Tetrahedron, 51,
11573 (1995).
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41 (2001).
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19 (2010).
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Patent 20040024205 A1 (2004).
CH OH
3
+ CO2
O
O
CH3 O C O CH3
O
O
N
CH3 +H2O C O CH3
1
2
O
H N
O
O
CH3 O C O CH3
N C O CH3 +
CH3OH
O
N CH3 + CO2
Scheme-III: The plausible mechanism of formation of N-methylmorpholine
by using morpholine reacted with dimethyl carbonate