Direct synthesis of imines via solid state reactions of carbamates with aldehydes

Article abstract of DOI:10.1002/adsc.201200907

Various solid carbamates were prepared from the reactions of liquid amines with carbon dioxide in an autoclave and these carbamates were used as stable, efficient alternatives for toxic liquid amines. Solid-state grinding of carbamates and aldehydes, using a mortar and pestle, produced imines as the sole products in greater than 97% yields. Complete conversions were generally accomplished within a day at 25 °C without using solvents or additives. Reaction rates were drastically enhanced upon increasing the reaction temperature. In contrast, reactions of aldehydes with liquid amines in the presence of solvent or in neat conditions afforded imines in moderate yields along with by-products.

Full text of DOI:10.1002/adsc.201200907

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DOI: 10.1002/adsc.201200907
Direct Synthesis of Imines via Solid State Reactions of
Carbamates with Aldehydes
Byeongno Lee,a Kyu Hyung Lee,a Byung Wook Lim,a Jaeheung Cho,b
Wonwoo Nam,c and Nam Hwi Hura,*
a
Department of Chemistry, Sogang University, Seoul 121-742, Korea
Fax : (+82)-2-701-0967, phone: (+82)-2-705-8440; e-mail: nhhur@sogang.ac.kr
Department of Emerging Materials Science, DGIST, Daegu 711-873, Korea
Department of Bioinspired Science, Department of Chemistry and Nano Science, Ewha Womans University, Seoul
b
c
120-750, Korea
Received: October 10, 2012; Published online: January 25, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200907.
Abstract: Various solid carbamates were prepared
from the reactions of liquid amines with carbon di-
oxide in an autoclave and these carbamates were
used as stable, efficient alternatives for toxic liquid
amines. Solid-state grinding of carbamates and alde-
hydes, using a mortar and pestle, produced imines
as the sole products in greater than 97% yields.
Complete conversions were generally accomplished
within a day at 258C without using solvents or addi-
tives. Reaction rates were drastically enhanced
upon increasing the reaction temperature. In con-
trast, reactions of aldehydes with liquid amines in
the presence of solvent or in neat conditions afford-
ed imines in moderate yields along with by-prod-
ucts.
plications in organic syntheses. The majority of carba-
mates can generate amines by releasing CO2 gas,
which suggests they can be used as synthetic equiva-
lents to liquid amines.[4–7] Compared to toxic and vola-
tile amines, solid carbamates could prove to be better
alternatives for the efficient synthesis of amine-de-
rived chemicals largely because they are easy to store
and handle in the lab. Moreover, solid carbamates
would be appropriate precursors for the preparation
of imines via the solid-state grinding method, which
allows them to react readily with solid aldehydes.
The majority of organic synthetic methods proceeds
in liquid media and involve multiple steps, which then
require separation and purification processes. In con-
trast, organic solid-state reactions are able to synthe-
size useful chemicals in the absence of solvents. The
solid-state grinding method can therefore minimize
the synthetic steps and may thus allow the separation
process to be skipped, which in turn could allow for
an environmentally benign approach to synthesize
various organic materials.[10,11] However, in order to
apply this simple technique to a wide range of organic
Keywords: aldehydes; amines; grinding; imines;
solid carbamates; solid state reaction
It has long been known that amines readily absorb reactions, organic solids that can readily react with
carbon dioxide (CO2) to yield liquid or solid adducts other substrates in the solid state are necessary.
depending on the nature of the amine.[1–7] Usually, Within this context, we reported our studies of the
two equivalents of an amine (RNH2) combine with solid state reactivity of hydrazinium carboxylate
À
a CO2 molecule to form an alkyl ammonium carba- (H3N+NHCO2 ), isolated from the reaction of liquid
mate salt (RNHCO2À H3N+R). Although it is stable, hydrazine (H2NNH2) with supercritical CO2, towards
the carbamate can revert to the parent amine and carbonyl compounds.[12] The solid state reactions, per-
CO2 by external stimuli, such as heating or pres- formed by grinding a mixture of two reactants with
sure.[3–7] This reversible interaction between amines a mortar and a pestle at ambient conditions, yielded
and CO2 is utilized in industry to capture and store azines as the sole products in yields greater than
CO2 from flue gases.[6] A wide range of amines has 97%.[12] This single-step process involves CO2 evolu-
been extensively investigated to develop new liquid tion, which appears to trigger the close contact of hy-
absorbents for CO2 capture.[4–9] In contrast, only a few drazine and the aldehyde at the molecular level and
studies on the reactivity of the carbamate salts have favours the forward reaction for azine formation.
been performed,[1] due presumably to the lack of ap-
Adv. Synth. Catal. 2013, 355, 389 – 394
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389

Byeongno Lee et al.
Table 1. Reactions of aldehydes with solid carbamates (2a, 2e) in the solid state and with amines (1a, 1e) in solution.[a]
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Entry Carbamates/
Aldehyde Solvent
Product
Time Conver- Selectivity Yield Remarks
Amine (mmol) (mmol)
[h]
sion [%] [%]
[%]
1
2a (5)
1a (10)
3a (10)
none
neat
3
>98
~100
>98[b] grinding
2
3
3a (10)
3a (10)
4aa
3
3
>98
~86
<77
~90
75[c]
solution
1a (10)
MeOH[e] 4aa
~77[c] solution
4
2e (5)
3c (10)[d] none
3c (10)[d] neat
<1
>98
~100
>98[b] grinding
5
6
1e (5)
1e (5)
4ec[d]
3
1
<30
>98
96
~90
29[c]
solution[f]
3c (10)[d] MeOH[e] 4ec[d]
~81[c] solution
[a]
[b]
[c]
[d]
[e]
[f]
Reaction conditions: 20-mL vial, 508C.
Isolated yield based on 3.
1
Yield was determined by H NMR.
3c: 4-chlorobenzaldehyde, 4ec: N1,N2-bis(4-chlorobenzylidene)ethane-1,2-diamine.
MeOH: 10 mL.
The volume of ethylenediamine (1e, 0.3 g, ~0.33 mL) seemed to be too small to react with the solid aldehyde (3c, 1.4 g)
in the absence of a solvent.
As a continuing effort to understand the nature of
To explore the reactivity of the carbamate salt to-
organic reactions in the solid state, we synthesized wards an aldehyde, the reaction between benzylamine
solid carbamate salts by reacting liquid amines with carbamate (2a) and 3-hydroxybenzaldehyde (3a),
CO2 and investigated their solid-state reactivity to- both in the solid state, was initially investigated.
wards aldehydes. Remarkably, the solid state grinding Grinding 2a with two molar equivalents of 3a, using
of carbamates and aldehydes produced various imine a mortar and pestle, and then storing the mixed
derivatives, which were isolated as sole products in powder at 508C produced the white crystalline
over 97% yields. This is the first example to demon- powder 3-benzyliminomethylphenol (4aa) as the sole
strate that solid carbamates can be used as synthetic product. The solid-state reaction occurred rapidly
alternatives to toxic liquid amines.
(Table 1, entry 1) and complete conversion was
In addition, the grinding method shows higher se- achieved within 3 h without intermediate grinding of
lectivity towards imine formation than the conven- the mixture. Moreover, the selectivity was almost
tional solution-based synthesis.
100%. The reaction products were carefully analyzed
1
A wide range of solid carbamates was prepared by by H NMR and powder XRD. No by-product, be-
the reactions of liquid amines with CO2. All products sides the imine, was identified from the reaction mix-
were isolated as white solids, as shown in Table S1 in ture. In the course of the solid-state reaction, 2a ap-
the Supporting Information. Their compositions and pears to dissociate gradually into benzylamine (1a)
structures were initially identified by elemental analy- and CO2. It is therefore conceivable that the solid car-
1
sis, melting point, H NMR, and 13C NMR. In addi- bamate (2a) could be useful for the preparation of
tion, thermal gravimetric analysis, IR spectroscopy, imines and could also be used for other organic syn-
UV-visible spectroscopy, and X-ray powder diffrac- thetic techniques as an alternative source to liquid
tion (XRD) were employed to characterize the com- amine (1a).[13–15] An additional advantage of the grind-
pounds and confirm their structures. Generally two ing method is that it allows the solid reactants to
equivalents of monoamines (1a–d, see the Supporting react efficiently at ambient conditions and leads to
Information) react with one CO2 molecule to yield the formation of the product without producing any
the corresponding carbamate salts (2a–d, see the Sup- waste.
porting Information). Meanwhile, the diamines (1e–h,
To confirm the solid state structure of 4aa, we grew
see the Supporting Information) produced the corre- single crystals of 4aa from a chloroform/ether (1:1)
sponding carbamate salts (2e–h, see the Supporting solution. The crystal structure of 4aa was determined
Information) using a 1:1 ratio of amine and CO2. All by single crystal X-ray diffraction, and it indicated
of the solid carbamates obtained from the reactions that the structure was composed of two crystalograph-
above are quite stable and do not show deterioration ically independent imine molecules. The two mole-
even after storing the solids for a long period (>6 cules are virtually identical although they have subtle
months).
differences in their bond distances and angles.
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Adv. Synth. Catal. 2013, 355, 389 – 394

Direct Synthesis of Imines via Solid State Reactions of Carbamates with Aldehydes
typically requires reaction temperatures over 1,0008C.
This difficulty is largely due to the presence of strong
ionic or covalent bonds in the solids. In molecular
solids like ice and 2a, however, the molecules are
held together by van der Waals forces, which are
weak intermolecular bonds. Accordingly, freestanding
molecules can be released from the solid matrix
simply by imposing external forces, and can therefore
react with other molecular solids at the molecular
level. The carbamate (2a) has important characteris-
tics as a solid reagent because of the orderly packing
of the molecules of benzylamine carbamate; this
allows 2a to be as reactive as benzylamine, even in
the solid state. Moreover, 2a has the ability to sublime
at elevated temperatures and can easily produce pure
amines by releasing CO2 gas.
Figure 1. ORTEP plot of the asymmetric unit of 3-benzyl-
iminomethylphenol (4aa), at 30% probability level of the
thermal ellipsoids. Hydrogen atoms are not labelled for
clarity. CCDC 882727 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
ACHTUNGTRENNUNG
Thus, these characteristics may exert advantageous
effects on the reactivity of 2a, which could in turn
contribute to the high selectivity of 2a in the solid
Figure 1 shows the ORTEP diagram of 4aa, which is state reaction.
based on the amine and aldehyde units linked by an To understand how the solid-state reaction occurs
À
À
À
imine ( N=C ) group. The bond distance of N1 C8 at the interface to yield the product selectively, the
in the imine group is 1.266 ꢁ, and it is similar to simple grinding method was further explored. We
those of typical N=C double bonds.[15,16] The bond chose the reaction between 2a and 3a as a model
À
À
À
À
angles of C8 N1 C7 and N1 C8 C9 are 116.508 and system mainly because the product (4aa) is well char-
124.008, respectively (see Table S1, Table S2 and acterized by single crystal X-ray diffraction, which
Table S3, in the Supporting Information). Detailed allows for easy monitoring of the reaction products by
data are presented in Table S2 in the Supporting In- XRD. Solid state reactions between the two reactants
formation.
were performed at room temperature. Ground pow-
Liquid benzylamine (1a) was reacted with 3a to de- ders were stored in a vial, without any additional agi-
termine whether the solution-based reaction also tation, at 508C. The products were carefully analyzed
1
yields the imine (4aa) selectively. The reaction was by powder XRD and H NMR as a function of reac-
performed in CH3OH as well as in neat conditions tion time. New peaks corresponding to 4aa appeared
using benzylamine as the solvent. Both reactions pro- in the XRD patterns after only 30 min, as illustrated
duced the imine (4aa) along with unknown by-prod- in Figure 2. Over 98% conversion of the reactants to
ucts in yields of less than 75% (Table 1, entries 2 and 4aa was accomplished in the solid state within 3 h in
3). The selectivities towards 4aa in CH3OH and neat the absence of intermediate grinding. These results
conditions are lower by 10% and 23% than that of suggest that free benzylamine molecules, produced
2a, respectively. These values are significantly lower via the dissociation of 2a, react directly with the sur-
than those corresponding to the solid-state reaction. face molecules of 3a to yield the imine as the sole
Excellent reactivity towards the aldehyde was also ob- product. It is worth mentioning that the CO2 gas that
served in the 2-ammonio ethyl carbamate (2e), pro- evolved upon grinding accelerated the forward reac-
vided in Table 1 (entry 4). As anticipated, the solu- tion. This could be the reason why solid carbamates
tion-state yield from the reaction between ethylenedi- are as reactive as liquid amines. The ease of product
amine and aldehyde (3c) is lower compared to that formation via grinding also suggests that the highly
from the solid-state reaction (Table 1, entries 5 and pure amines produced by the release of CO2 gas play
6).
Based on the results described above, the solid-
a major role in determining the reactivity.
To extend the scope of the solid-state reaction, we
state grinding approach provided better yields and reacted 2a with other benzaldehydes (3b–3e, Table 2,
higher selectivities for the imine formation than the entries 1–4). By grinding a mixture of two reactants
solution-based method. The solid-state reaction be- alone, complete conversion was typically accom-
tween ionic or covalent solids is generally poorly able plished in a 24 h period at room temperature. To
to produce new phases at ambient conditions al- reduce the reaction time, ground powders stored in
though solid-state transformations of metal oxides vials were allowed to react at elevated temperatures.
into metal-organic materials can be accomplished by Reaction rates were drastically enhanced and conver-
mechano-chemical milling near room temperature.[17] sions were complete within 3 h. Imines were obtained
This is because a conventional solid-state reaction as sole products in high yields and excellent selectivi-
Adv. Synth. Catal. 2013, 355, 389 – 394
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391

Byeongno Lee et al.
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Figure 2. (a) Powder XRD patterns of the products obtained by grinding a mixture of 2a and the aromatic aldehyde (3a,
Table 1, entry 1) at intervals of 30 min, 1 h, and 2.5 h at 258C. The vertical bars at the bottom of the spectrum correspond to
1
the theoretical diffraction peaks of 4aa. (b) Yields of 4aa as a function of reaction time were determined by H NMR spec-
troscopy.
ties (~100%), regardless of the substituents on the al- to be excellent precursors for the substitution of
dehydes. We also evaluated the scope of this remark- liquid amines.
able grinding method with diamine carbamates, pre-
pared by the reaction of diamines with CO2. The di- as sole products, in which a 1:2 mixture of a diamine
amine carbamates, listed in Table S1 in the Supporting carbamate and an aldehyde were ground at room
Information, were used as solid reagents, and found temperature and stored in a vial at 508C. Complete
As shown in Table 3, various imines were isolated
ACHTUNGTRENNUNG
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Direct Synthesis of Imines via Solid State Reactions of Carbamates with Aldehydes
Table 2. Solid-state reactions of four aldehydes with the
solid carbamate (2a).[a]
solid alternatives for toxic liquid amines. A remark-
able feature is that they readily react with aldehydes,
even in the solid state, to generate imines as the sole
products, besides the CO2 and H2O generated from
the reaction. The solid-state grinding method also has
the potential to synthesize organic solids that are un-
available by conventional solvent-assisted methods.
Moreover, this method exhibits the possibility of
large-scale production with a very small vessel: for ex-
ample, we could produce more than 20 g of imine in
a 50-mL vial. This grinding methodology could thus
be exploited for industrial applications. It is worth
noting that similar methods such as accelerated aging
and vapour digestion have been developed for the
large-scale synthesis of metal-organic materials and
co-crystals.[18]
[a]
Reaction conditions: 3 (10 mmol), carbamate (2, 5 mmol)
in a 10-mL vial.
Isolated yield based on 3.
Mixing: powders in the same vial are shaken, grinding:
powders are ground with a mortar and pestle.
Experimental Section
[b]
[c]
Synthesis of Solid Carbamates
The glass liner of an autoclave (Parr 4760, 300 mL) was
charged with an amine (1, 200.0 mmol, see Table S1 in the
Supporting Information) and dry ice (100 g), and the gauge
and gauge block assembly were attached. Subsequently, the
autoclave was placed in an oil bath on a hot plate. The reac-
tion temperature was about 1308C and the CO2 pressure
was in the range of ca. 90–110 bar. After the appropriate
time, the autoclave was removed from the oil bath and
cooled to ambient temperature very slowly. The excess gas
was discharged and the system was disassembled. The white
crystalline powders (2) in the glass liner were filtered in air,
followed by washing with cold methanol (3ꢂ100 mL), cold
diethyl ether (3ꢂ100 mL), and pentane (3ꢂ100 mL). The re-
sulting solid was then dried under vacuum. The typical
yields of solid carbamates (2) based on the used amines
were over 95%. The solid carbamates were fully character-
ized by various analytical techniques, which are given in
Supporting Information.
Table 3. Solid state reactions of the aldehyde (3d) with three
solid carbamates obtained from the corresponding di-
ACHTUNGTRENNUNG
amines.[a]
[a]
Reaction conditions: 3d (10 mmol), carbamate (2,
5 mmol) in a 10-mL vial.
Isolated yield based on 3d.
[b]
[c]
General Procedure for the Preparation of Imines
from the Solid-State Reactions between Carbamates
and Solid Aldehydes
Mixing: powders in the same vial are shaken.
About 1:2 mixtures of carbamate (5.03 mmol, 2) and alde-
hyde (10.0 mmol, 3) were ground at room temperature using
conversion to the corresponding imines was achieved
within 3 h at 508C. The solid-state reactions yield a pestle and mortar for 5 min. The ground mixture was
placed in a vial and warmed to the desired temperature in
an oven. CO2 and water were gradually released from the
reaction mixture. Complete conversion to imine was strong-
ly dependent upon the nature of aldehydes. Both yield and
selectivity are typically over 97%. The resulting products
highly selective products without producing waste;
these are distinct advantages over conventional or-
ganic reactions performed in solvent. Accordingly,
this simple grinding method is in accordance with
a green reaction that proceeds at ambient conditions,
requires no solvent, produces no waste, is very selec-
tive, and shows high atom efficiency.
1
were identified by H NMR, 13C NMR, IR, and GC/MS. To
measure UV-visible, melting point, and elemental analysis,
a small portion of the product (ca. 0.2 g) was purified by
washing with cold ether (3ꢂ5 mL) and pentane (3ꢂ5 mL),
followed by drying under vacuum. Analytical data for the
In summary, we have demonstrated that a wide
range of solid carbamates, prepared by the reactions
of liquid amines with CO2, could be used as stable, products are given in Supporting Information.
Adv. Synth. Catal. 2013, 355, 389 – 394
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Byeongno Lee et al.
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Acknowledgements
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We thank the Converging Research Centre Program
(2012K001486) and the Korea CCS R&D Centre grant
(20120008890) funded by the Ministry of Education, Science,
and Technology through the National Research Foundation
of Korea. BL thanks the Research Fellow Program
(2012R1A12043256) funded by the Ministry of Education,
Science, and Technology through the National Research
Foundation of Korea.
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