Methoxycarbonylation of aliphatic diamines with dimethyl carbonate promoted by in situ generated hydroxide ion: A mechanistic consideration

Article abstract of DOI:10.1002/adsc.200900699

The methoxycarbonylation reactions of aliphatic diamines with dimethyl carbonate are accelerated greatly in the presence of water. Theoretical investigations on the mechanistic aspects of the methoxycarbonylation of 1,6-hexanediamine strongly suggest that the hydroxide ion, generated in situ from the interaction of 1,6-hexanediamine with water, is an active catalytic species and plays a pivotal role in the rate-determining hydrogen abstraction step from the amino group.

Full text of DOI:10.1002/adsc.200900699

FULL PAPERS
DOI: 10.1002/adsc.200900699
Methoxycarbonylation of Aliphatic Diamines with Dimethyl
Carbonate Promoted by in situ Generated Hydroxide Ion:
A Mechanistic Consideration
Dae Won Kim,a Eun Soo Huh,a Sang Do Park,b Ly Vinh Nguyen,a
Mai Dao Nguyen,a Hoon Sik Kim,a,* Minserk Cheong,a,*
and Dinh Quan Nguyena,*
a
Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University, Seoul 130-701, Korea
Fax : (+82)-2-965-4408; phone: (+82)-2-961-0432, (+82)-2-961-0239; e-mail: khs2004@khu.ac.kr or mcheong@khu.ac.kr
Korea Institute of Energy Research, Daejeon 305-343, Korea
b
Received: October 8, 2009; Published online: February 9, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900699.
Abstract: The methoxycarbonylation reactions of ali- water, is an active catalytic species and plays a pivo-
phatic diamines with dimethyl carbonate are acceler- tal role in the rate-determining hydrogen abstraction
ated greatly in the presence of water. Theoretical in- step from the amino group.
vestigations on the mechanistic aspects of the me-
thoxycarbonylation of 1,6-hexanediamine strongly Keywords: 1,6-di(methyl carbamate)hexane; dimeth-
suggest that the hydroxide ion, generated in situ yl carbonate; isocyanates; methoxycarbonylation;
from the interaction of 1,6-hexanediamine with non-phosgene process
Introduction
applications. In the patent literature, although it was
stated that water was found to facilitate the methoxy-
Non-phosgene processes for the synthesis of carba- carbonylation of aliphatic monoamines such as am-
mates, precursors of isocyanates, have attracted in- monia, ethylamine, and diethylamine in the presence
creasing interest in recent years because of the envi- of a solvent, the role of water was not discussed in
ronmental concern on the use of highly toxic phos- detail.[14] Moreover, the methoxycarbonylation reac-
gene.[1–9] Of the various non-phosgene processes so far tions of diamines using water as a promoter or a cata-
reported, methoxycarbonylation of amines by dimeth- lyst have never been attempted.
yl carbonate (DMC)[10–13] has been considered the
We report here the water-assisted methoxycarbony-
most environmentally benign route to the carbamates lation of 1,6-hexanediamine (HDA) with DMC as
because methoxycarbonylation reactions can be car- well as a theoretical investigation on the role of
ried out under relatively mild conditions, producing water. We focused on the methoxycarbonylation of
methanol as the sole by-product as shown in HDA because the methoxycarbonylated product, 1,6-
Scheme 1.
di(methyl carbamate)hexane (HDC), can be easily
Various substances including inorganic salts[5] and transformed into 1,6-hexanediisocyanate (HDI), one
ionic liquids[6] were used as catalysts for the methoxy- of the most important aliphatic diisocyanates.
carbonylation of amines, but the processes using these
catalysts need to be greatly improved in terms of cata-
lytic activities and product purification for practical Results and Discussion
Effect of Water on the Methoxycarbonylation of
HDA
The methoxycarbonylation of HDA with DMC was
Scheme 1. Methoxycarbonylation of a diamine with DMC.
significantly facilitated by the presence of water.
440
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Adv. Synth. Catal. 2010, 352, 440 – 446

Methoxycarbonylation of Aliphatic Diamines with Dimethyl Carbonate
Table 1. Effect of water on the reaction of HDA with
Table 2. Effect of molar ratio of DMC/HAD.[a]
DMC.[a]
Run Molar ratio
(DMC: HDA)
HDA conversion [%]
Yield [%]
Run Molar ratio
(H2O:HDA)
HDA conversion
[%]
Yield [%]
ACHTUNGTRENNUNG
HMC HDC
HMC HDC
1
2
3
4
5
6
1:1
2:1
3:1
4:1
5:1
6:1
79.8
99.8
100
99.6
99.1
98.6
62.5
34.7
34.2
33.2
34.1
34.2
17.2
65.0
65.8
66.3
64.8
64.2
1
2
3
4
5
6
7
8
9
10
0:1
8.2
8.1
0.1
0.25:1
0.5:1
1:1
2:1
3:1
4:1
5:1
6:1
41.8
66.1
75.1
89.6
95.5
99.8
99.6
99.8
99.4
47.3
39.6 2.1
55.1 10.6
56.0 19.0
51.0 38.5
40.2 55.3
37.2 62.5
34.7 64.6
34.7 65.0
34.6 64.6
40.7 6.5
[a]
The reaction was conducted for 4 h at 258C and at the
molar ratio of H2O/HDA=6 (HDA: 0.04 mol).
8:1
11[b] 0:1
while the HMC yield fell from 62.5% to 34.7%. The
HDC yield remained almost constant on further in-
creases of the molar ratio.
[a]
The reaction was conducted for 4 h at 258C and at the
molar ratio of DMC/HDA=2 (HDA: 0.04 mol).
The reaction was carried out in the presence of
[BMim]OH at the molar ratio of [BMim]OH/HDA=0.2.
[b]
The methylated side products, H2N
ACHTUGNERTN(NUNG CH2)6NHMe
and MeNH(CH2)6NHCO2Me, were also produced,
AHCTUNGTRENNUNG
but in yields of around 0.2% only at higher molar
ratios of DMC/HDA.
Table 1 shows the results of the methoxycarbonylation
of HDA with DMC carried out at 258C with and
without the presence of water. The methoxycarbony- Effect of Reaction Time
lation of HDA proceeded very slowly in the absence
water, producing HMC and HDC in yields of 8.1 and The change of product composition with time is
0.1%, respectively. The yields of HMC and HDC, shown in Figure 1. The HDC yield and the HDA con-
however, started to rise with the addition of water, version rose gradually with time up to 6 h and then
implying that a new active catalytic species, presuma- remained almost constant thereafter at around 66%
bly OHÀ, is generated by the presence of water, possi- and 100%, respectively. However, the HMC yield
bly through the interaction between HDA and water. reached a maximum at 0.5 h and then started to fall
Both HDA conversion and HDC yield increased to about 33%, implying that HMC is an intermediate
gradually with increasing molar ratio of H2O/HDA up on the way to HDC. The HDC yield, however, never
to 4 and then remained almost unchanged on further exceeded 67%, clearly indicating that an equlibrium
increases of the molar ratio.
exists among HDC, HMC, and HDA, possibly be-
To test the possible hydrolysis of HDC to HMC cause of the presence of the co-produced CH3OH.
and HDA, HDC was reacted with a large excess of
water (molar ratio of water/HDC=20) at 258C, but
1
GC and H NMR analysis did not reveal the forma-
tion of HMC and HDA. This finding is a clear indica-
tion that the hydrolysis of HDC hardly takes place
under the experimental conditions.
Effect of DMC/HDA Molar Ratio
The effect of the molar ratio of DMC/HDA was also
investigated at 258C at the molar ratio of H2O/
HDA=6 and the results are listed in Table 2. The
conversion of HDA was almost quantitative in the
DMC/HDA molar ratio range 2–6. However, as was
predicted, the formation of HMC prevails at the
molar ratio of DMC/HDA=1, indicating that HDA is
methoxycarbonylated to HMC more readily than
HMC to HDC. The HDC yield increased to about 4-
fold when the molar ratio was raised from 1 to 2, Figure 1. Change of product composition with time.
Adv. Synth. Catal. 2010, 352, 440 – 446
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441

Dae Won Kim et al.
FULL PAPERS
To confirm that HMC is an intermediate of HDC, Equilibrium Considerations
HMC was methoxycarbonylated at the molar ratio of
HMC/DMC/H2O=1/2/6 at 258C. Contrary to our pre- To shift the equilibrium of HDA methoxycarbonyla-
diction, the methoxycarbonylation of HMC was ex- tion toward the formation of HDC, the reaction was
tremely slow, resulting in conversion of less than 2% carried out using a Dean–Stark apparatus in the pres-
HMC to HDC over 4 h. This was a rather unexpected ence of excess quantities of DMC (DMC/HDA=10).
result because HDC was produced in a yield of 65% CH3OH was continuously withdrawn from the reac-
from the methoxycarbonylation of HDA conducted at tion system as an azeotropic mixture with DMC as
258C for 4 h at the molar ratio of HDA/DMC/H2O= soon as it formed. To reduce the formation of methy-
1/2/6. One possible explanation for this is that the me- lated products at elevated temperatures, the reaction
thoxycarbonylation of HMC occurs at a reasonable was performed at 258C for 2 h then at a reflux tem-
rate only in the presence of HDA as a hydroxide ion perature for an additional 2 h. The reaction, carried
(OHÀ) supplier or proceeds at a much higher temper- out in the presence of water, produced HDC in a
ature. The activation of HMC by OHÀ and the subse- yield of over 98% along with a small amount of me-
quent hydrogen abstraction from the NH2 group seem thylated side product, H2NACHTNUGRTNEUNG(CH2)6NHMe. On the con-
to require a much higher energy than that required trary, the reaction performed in the absence of water
for HDA, possibly due to the presence of an electron- produced HDC, HMC, and a series of methylated
withdrawing carbamate group in HMC (see Section products [H2N
ACHTUNTGRENNGU(CH2)6NHMe, MeHNACHTUNGTRNE(NUGN CH2)6NHMe,
on Computational Calculations). In fact, the conver- and MeHN(CH2)6NHCO2Me] in yields of 41, 39, and
AHCTUNGTRENNUNG
sion of HMC to HDC increased significantly to 23% 13%, respectively. From these results, it is obvious
when the methoxycarboylation of HMC was carried that the presence of water suppresses the formation
out in the presence of small amounts of HDA at the of methylated side products while increasing the yield
molar ratio of HMC/DMC/H2O/HDA=1/2/6/0.1.
and selectivity of HDC. The presence of water seems
To find out the reason for the significantly lower re- to provide an unfavourable environment for DMC to
activity of HMC than that of HDA toward the me- function as a methylating agent.
thoxycarbonylation, the interactions of OHÀ with
Interestingly, the formation of methylated side
HDA and HMC were compared by means of compu- products was negligible when the HMC was methoxy-
tational calculations (vide infra).
carbonylated instead of HDA at reflux for 4 h at the
molar ratio of HMC/DMC/water=1/10/3. The pres-
ence of an electron-withdrawing carbamate group at
the other end of HMC seems to greatly restrict the
methylation of the HMC amino group. For this
Effect of Reaction Temperature
As shown in Figure 2, the HDA conversion did not reason, to avoid the formation of methylated side
change much with the temperature increases, but the products, HDA should be methoxycarbonylated first
HMC yield increased by about 14% with the rise of at a low temperature just enough to convert all HDA
temperature from 40 to 608C at the expense of the to HMC and HDC, and then at reflux to convert
HDC yield. This finding implies that the equilibrium HMC to HDC while simultaneously removing
between HMC and HDC is shifted slightly towards CH3OH.
the formation of HMC at elevated temperatures.
Computational Calculations
To elucidate the role of water, the interactions be-
tween HDA and DMC with and without the presence
of water were theoretically investigated at the
B3LYP/6-31+G* level of the theory using the Gaussi-
an 03 program.[15] Four cases were calculated: (i) in
the absence of water, (ii) in the presence of water,
(iii) in the presence of OHÀ, and (iv) in the co-pres-
ence of both water and OHÀ. For each case, as in
Figure 3, the methoxycarbonylation was seen to pro-
ceed through a tetrahedral intermediate (I*). In the
absence of water and OHÀ, the free energy of activa-
tion (DG°) for the first transition state (TS1) and the
free energy of formation for the tetrahedral inter-
mediate (DGi) were calculated as 38.1 and 20.7 kcal
Figure 2. Change of product composition with temperature. molÀ1, respectively (Figure 3a). DG° and DGi were
442
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Methoxycarbonylation of Aliphatic Diamines with Dimethyl Carbonate
H bond by OHÀ and the stabilization of the resulting
negative charge on the electrophile (DMC) by water
molecules may be achieved in a cooperative network
of hydrogen bonds. With such strong interactions,
À
both the cleavage of the C O bond and the formation
of C N bond seem to be accelerated to form carba-
mates.
À
The involvement of in situ generated OHÀ is indi-
rectly supported by a separate experiment using 3-
butyl-1-methylimidazolium hydroxide ([BMim]OH)
as the catalyst. The conversion of HDA surged to
47.3% from 8.2% when the methoxycarbonylation
was performed in the presence of [BMim]OH at 258C
and at the molar ratio of [BMim]OH/HDA=0.2, im-
plying that OHÀ catalyzed the methoxycarbonylation
of HDA (see Table 1, runs 1 and 13).
To identify the reason for the different reactivities
of HDA and HMC to methoxycarbonylation, the in-
teractions of OHÀ with HDA and HMC were com-
pared based on theoretical calculations. As can be
seen from the optimized structure of HDA (HDA-
OHÀ) in Figure 4 a, the distances between bonds in
the oxygen atom of OHÀ and the hydrogen atoms of
two NH2 were calculated as 1.84 and 1.87 ꢁ, respec-
tively, indicating that OHÀ interacts strongly with two
amino groups of HDA. Such a strong interaction as
this would facilitate the rate-determining hydrogen
abstraction from NH2.
Figure 3. Optimized geometries of the first transition states
(TS1) and tetrahedral intermediates (I*) for the methoxy-
carbonylation of HDA with DMC: a) in the absence of
Interestingly, two optimized structures, keto
water and OHÀ, b) in the presence of water, c) in the pres- (HMCk-OHÀ) and enol (HMCe-OHÀ) forms, are ob-
ence of OHÀ. The numbers in parentheses refer to activa-
tion energy or free energy of formation in kcalmolÀ1.
tained from the interaction of OHÀ with HMC as
shown in Figure 4 b and c. Caculations of free energy
of formation (DG) show that HMCe-OHÀ is more
stable than HMCk-OHÀ and HDA-OHÀ by 1 and
reduced to 29.6 and 18.8 kcalmolÀ1, respectively when 17 kcalmolÀ1, respectively. For the activation of
a water molecule was intentionally included in the HMC, OHÀ should interact with the amino group
calculation (Figure 3b). However, these values are
still too high to explain the experimental results ob-
tained in the presence of water. Surprisingly, DG°
and DGi were greatly reduced to 12 and
À2.9 kcalmolÀ1, respectively, when the water mole-
cule was replaced by OHÀ, clearly implying that OHÀ
is an active species in the methoxycarbonylation of
HDA (Figure 3c). This is a reasonable deduction be-
cause amine is easily transformed into ammonium hy-
droxide on contact with water. The involvement of an
additional water molecule in the transition state was
not helpful in reducing the activation energy further
(see Supporting Information). Details of the calcula-
tion results of the mechanisms for four different cases
are provided in the Supporting Information.
Based on the computational and experimental re-
sults, it is concluded that OHÀ, in situ generated by
the interaction of HDA with water, is involved in the
rate-determining hydrogen abstraction step in which
hydrogen is abstracted from the amino group. Activa-
tion of the nucleophile through the cleavage of an N
Figure 4. Optimized structures showing the interactions of
HDA and HMC with OHÀ: a) HDA-OHÀ, b) HMCk-OHÀ,
c) HMCe-OHÀ.
À
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Dae Won Kim et al.
FULL PAPERS
rather than with the carbamate group, but the com- yield was considerably lower than that obtained in the
parison of the bond distances in the optimized struc- presence of water only. In contrast to aprotic solvents,
ture of HMC-OHÀ clearly reveals that OHÀ favours protic solvents such as methanol and D2O promoted
the interaction with the hydrogen atom of the more the methoxycarbonylation reaction although the
acidic carbamate group over the amino group. For effect was not as pronounced as that with H2O. It is
HMCe-OHÀ, the bond distance between the oxygen likely that the deprotonation of D2O by an amino
atom of OHÀ and the hydrogen atom of enol OH is group to generate an active species like ODÀ is
calculated as 1.00 ꢁ, which is shorter than that be- slower than that of H2O. The difference in the pro-
tween the oxygen atom of OHÀ and the hydrogen moting effect between H2O and D2O on the methoxy-
atom of NH2 (1.06 ꢁ). From these computational re- carbonylation could also be rationalized by the deute-
sults, it is believed that the lower activity of HMC rium effect.
than that of HDA toward the methoxycarbonylation
can be largely ascribed to the lengthening of the bond
distance between OHÀ and NH2 from 1.84 ꢁ in Plausible Mechanism for the Methoxycarbonylation
HDA-OHÀ to 2.06 ꢁ in HMCe-OHÀ. Consequently, of HDA
the rate of hydrogen abstraction from the NH2 group
of HMC would be much slower.
Based on these theoretical and experimental results, a
plausible mechanism for the methoxycarbonylation of
HDA is suggested. As shown in Figure 5, HDA is
likely to be activated first by OHÀ, generated in situ
from the interaction of HDA and H2O, through hy-
Effect of Solvent
The effect of solvents on the methoxycarbonylation drogen bonds (see also Supporting Information, c).
was also studied at 258C with the molar ratio of Subsequent abstraction of hydrogen from one of the
HDA/DMC/solvent at 1/2/6. As noted in Table 3, the amino groups would result in the generation of amido
reaction did not proceed at all in aprotic non-polar anion, A with the concomitant loss of H2O. Nucleo-
solvents such as tetrahydrofuran (THF) or dioxane in philic attack of the anion on the carbon atom of
the absence of water. In aprotic solvents, the genera- DMC produces an intermediate species, B, which in
tion of an active anionic species like OHÀ and the turn transforms into HMC on interaction with H2O,
subsequent activation of HDA do not seem to take simultaneously with the loss of CH3OH and the re-
place. THF and dioxane do not contain acidic hydro- generation of OHÀ. Further methoxycarbonylation of
gen(s) that could interact with the negative charge de- HMC to produce HDC would proceed in a similar
veloping on the carbonyl oxygen of the DMC and manner to that of HDA.
therefore fail to stabilize the tetrahedral intermediate.
Alternatively, the oxygen atom of THF or dioxane
seems to interact strongly with the H atoms of the
amino groups of HDA through hydrogen bonds,
thereby diminishing the ability of amino groups to in-
teract with the tetrahedral intermediate. This absence
of a stabilizing interaction might completely quench
the reaction.
As expected, the methoxycarbonylation proceeded
by the use of water coupled with THF, but the HDC
Table 3. Effect of solvent on the methoxycarbonylation of
HDA with DMC.[a]
Run Solvent
HDA conversion [%] HDC yield [%]
1
THF
0
0
2
3
dioxane
MeOH
D2O
0
42.1
75.2
0
3.1
42.4
5.0
4
5[b]
THF/H2O 52.5
[a]
The reaction was conducted for 4 h at 258C and at the
molar ratio of HDA/DMC/solvent=1/2/6 (HDA: 0.04
mol).
Figure 5. Plausible mechanism for the hydroxide ion-cata-
lyzed methoxycarbonylation of HDA. Cation (HDAH+) was
omitted for clarity.
[b]
Molar ratio of HDA/DMC/THF/H2O=1/2/3/3.
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Methoxycarbonylation of Aliphatic Diamines with Dimethyl Carbonate
Table 4. Effect of water on the methoxycarbonylation of var-
We hope that our findings on the methoxycarbony-
lation of diamines will be of great help in the devel-
opment of benign and economical non-phosgene com-
mercial processes for manufacturing isocyanates.
ious diamines.[a]
Run Diamine Molar ratio
Diamine con- Carbamate
(H2O:Diamine) version [%]
yield (%)
Mono Di
1
2
3
4
5
6
7
8
EDA
EDA
BDA
BDA
HDA
HDA
ODA
ODA
0:1
6:1
0:1
6:1
0:1
6:1
0:1
6:1
51.6
100
43.1
98.8
8.2
99.8
1.5
91.0
43.4
38.4
40.0
38.1
8.1
34.5
1.5
47.0
8.1
61.5
3.0
60.6
0.1
65.0
0
Experimental Section
Chemicals
Unless otherwise stated, all reagents were purchased from
Aldrich Chemical Co. and used as received.
43.7
Catalysis
[a]
The reaction was conducted for 4 h at 258C and at the
molar ratio of DMC/diamine=2.
A typical methoxycarbonylation reaction was performed as
follows: HDA (40 mmol, 4.65 g), DMC (80 mmol, 7.20 g),
and water (240 mmol, 4.32 g) were loaded into a 50-mL two-
necked flask equipped with a condenser and a magnetic stir
bar. The flask was then heated to a specified temperature
with vigorous stirring. After the reaction was completed, the
product mixture was dissolved in methanol and analyzed
using an Agilent 6890 gas chromatograph equipped with a
thermal conductivity detector and a DB-5 capillary column
(30 mꢂ0.32 mmꢂ0.25 mm). Products were characterized
using a Bruker 400 NMR spectrometer and an Agilent
6890–5973 MSD GC-mass spectrometer equipped with an
HP-5MS capillary column (30 mꢂ0.32 mmꢂ0.25 mm).
1H NMR data of HDC and methyl-6-aminohexane-1-car-
bamate (HMC) obtained are listed below.
Methoxycarbonylation of Other Diamines
As illustrated in Table 4, the effect of water was also
investigated for the methoxycarbonylation of di-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
HDA, the presence of water also greatly accelerated
the methoxycarbonylation reactions of EDA, BDA,
and ODA, resulting in much higher conversions of di-
amines and yields of dicarbamates compared with
those from the reactions carried out in the absence of
water. In general, the diamine conversions and the di-
carbamate yields decrease with the increasing number
of CH2 groups between the two amino moieties. The
significantly reduced reactivities of HDA and ODA
with a longer alkylene chain in the absence of water
could be largely attributed to their low solubility in
DMC.
It is worth mentioning here that in the methoxycar-
bonylation of diamines, the process using water would
be more economical than those employing metal
salts[5,16] and oxides[17] in terms of catalyst and product
recovery.
1
HDC: H NMR (400 MHz, CD3OD, 258C): d=3.6 (s, 6H,
CH3), 3.1 (t, 4H, CH2), 1.5 (m, 4H, CH2), 1.4 (m, 4H, CH2).
1
HMC: H NMR (400 MHz, CD3OD, 258C): d=3.6 (s, 3H,
CH3), 3.1 (t, 2H, CH2), 2.8 (t, 2H, CH2), 1.6 (m, 2H, CH2),
1.5 (m, 2H, CH2), 1.4 (m, 4H, CH2).
Calculated elemental analysis (%) for C8H18N2O2: C 55.2,
H 10.4, O 18.4; found: C 56.3, H 9.9, O 18.7.
To enhance the yield of HDC, the methoxycarbonylation
of HDA was also carried out using a Dean–Stark apparatus
in the presence of excess of DMC (HDA/DMC/H2O=1/10/
6 or 1/10/0). The reaction was performed at 258C for 2 h
and then at a reflux temperature for an additional 2 h with a
continuous withdrawal of the co-product CH3OH as an
azeotrope with DMC from the reaction system.
Acknowledgements
Conclusions
We acknowledge financial support by a grant from Carbon
Dioxide Reduction & Sequestration Research Center (AC3-
101), one of the 21st Century Frontier Programs funded by
the Ministry of Science and Technology of Korean Govern-
ment and Kolon Industries Co.
In conclusion, the presence of water greatly acceler-
ates the methoxycarbonylation reactions of diamines,
including EDA, BDA, HDA, and ODA while sup-
pressing the formation of methylated side products
Theoretical calculations of the optimized transition
and intermediate structures reveal that the OHÀ, pre-
sumably generated from the interaction between
HDA and water, is an active species for the methoxy-
carbonylation of HDA and plays a pivotal role in the
rate-determining hydrogen abstraction step from the
amino group.
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