Self-associated, "distillable" ionic media

Article abstract of DOI:10.3390/90600387

Liquid or low melting association products of carbon dioxide and a secondary amine, both neutral molecules that may be gaseous, are recognised as "distillable" ionic media.

Full text of DOI:10.3390/90600387

Molecules 2004, 9, 387–393
molecules
ISSN 1420-3049
http://www.mdpi.org
Self-associated, “Distillable” Ionic Media
Ulf P. Kreher *, Anthony E. Rosamilia, Colin L. Raston, Janet L. Scott and Christopher R.
Strauss *
Centre for Green Chemistry, Monash University, PO Box 23, Victoria 3800, Australia. Fax: +61 3
8558 8001; Tel: +61 3 8558 8007.
* Authors to whom correspondence should be addressed; E-mail:UKreher@boronmolecular.com,
Chris.Strauss@sci.monash.edu.au.
Received 18 February 2004 / Accepted: 22 February 2004 / Published: 31 May 2004
Abstract: Liquid or low melting association products of carbon dioxide and a secondary
amine, both neutral molecules that may be gaseous, are recognised as “distillable” ionic
media.
Keywords: DIMCARB, distillable, ionic liquid, ionic media, carbon dioxide,
dimethylamine.
Introduction
Increased environmental awareness has led to demands and regulations for the control, at least,
and preferably the avoidance of hazardous compounds. Spent volatile organic compounds (VOCs),
particularly solvents, are major components of the Toxics Release Inventory [1]. Potential
replacements for VOCs include ionic liquids, supercritical carbon dioxide, and water or solvent-
free conditions [2].
Ionic liquids have attracted interest through their negligible vapour pressure, high solubility with
many organic compounds, immiscibility with some organic solvents, and lack of flammability [3].
However, they tend to be expensive and are often available in only limited quantities. Also,
toxicological data are limited for ionic liquids, their potential metabolites and their decomposition
products. Furthermore, the isolation of non-volatile products from ionic liquids can present
logistical problems through requirements for the application of bi- or multi-phase systems, an
extraction step, or the use of pervaporation [4].

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Products from the addition of carbon dioxide and a secondary amine, neutral molecules that both
may be gaseous, herein are recognised as self-associated, “distillable” ionic media. This is
significantly different from the recent report of a pre-existing ionic liquid (N-butyl, N’-(3-
aminopropyl)-imidazolium tetrafluoroborate) the cation of which incorporated a primary amine that
could react with carbon dioxide to form a carbamate [5]. In that case the ionic liquid was used as a
carbon dioxide scavenger.
The present communication deals with a class of “distillable” ionic media, the N,N-dialkyl-
ammonium N’,N’-dialkylcarbamates (herein referred to as dialcarbs) that although not new, have
not been recognised or explored previously in this context.
Dialcarbs can be readily and inexpensively produced in bulk (litres can be prepared on the
laboratory-scale within a few hours), merely by mixing the reactants in the appropriate ratio in the
gas phase, with cooling [6].
Many dialcarbs melt below 100°C: N,N-dimethylammonium N’,N’-dimethylcarbamate
(DIMCARB) and N,N-diethylammonium N’,N’-diethylcarbamate (DIETCARB) are liquids even at
ambient temperature. By definition, ionic liquids have almost no vapour pressure [7]. This
introduces a clear distinction between dialcarbs and conventional ionic liquids. At temperatures as
low as 60°C, DIMCARB and DIETCARB undergo dissociation to the respective amine and carbon
dioxide, which can then be re-associated by condensation [8]. Thus, through association-
dissociation-reassociation, with cooling, heating and re-cooling respectively, the dialcarbs can be
recovered by “distillation”. Although the process does not involve distillation of a single molecular
entity, it affords a comparable result. Thus dialcarbs could be regarded as a separate category of
solvents hierarchically located between VOCs and ionic liquids, based on their properties and
behaviour.
Although dialcarbs, and particularly DIMCARB, have been investigated by Schroth et al [6] and
more recently by Hess and coworkers [9], their potential applicability appears to have been
overlooked by others. The major focus of the aforementioned researchers [6,9] was on the utility of
DIMCARB as a convenient and safe precursor of dimethylamine, carbon dioxide and
dimethylcarbamate in synthetic chemistry.
DIMCARB is a clear colourless viscous liquid, existing as a stoichiometric salt (2:1
dimethylamine:carbon dioxide on a molar basis), with a melting point of 29°C and as a non-
stoichiometric compound (1.6-1.9:1 dimethylamine:carbon dioxide on a molar basis). Both forms
are stable at ambient pressure at temperatures up to 60°C.
The single crystal X-ray structure [10] shows that the DIMCARB salt is stabilised by 4
hydrogen bonds forming a 12-membered ring between two N,N-dimethylcarbamate anions and two
N,N-dimethylammonium cations (Figure 1).
Several inorganic salts are moderately soluble in DIMCARB, which in turn is highly soluble in
water, where it suffers minimal depletion of carbon dioxide [11]. The solubility of some inorganic
salts in DIMCARB [11] lies between that of water [12], and other organic solvents.
Except for hydrocarbons, organic solvents e.g. benzene, dichloromethane, diethyl ether and
methanol, are highly miscible with DIMCARB, whose solvating power presumably results from its
low molecular weight and large number of coordinating centres. As with conventional ionic liquids,
these properties allow reactions to be carried out on a multigram scale in only a few mL of dialcarb.

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389
Figure 1. The single crystal X-ray structure of DIMCARB
The viscosity [9] and the conductance [13] of DIMCARB depend strongly on the temperature.
NMR investigations have indicated that this may result from rapid interchange between
components in complex equilibria (Scheme 1) [14]. These properties make it an interesting medium
for synthetic electrochemistry [13].
Scheme 1 Dynamic equilibrium of DIMCARB [14].
Table 1 Other known dialcarbs and their dissociation temperature [8].
mp range
(°C)
30-45
-
17-33
-
55-80
48-63
99-133
Dissociation
temperature (°C)
bp amine
(°C)
7.4
34-35
55
62
87-88
106
R1
R2
Me
Me
Et
Me
Et
Et
n-Pr
-
-
H
60-61
62
62-63
70-71
105
-
Me
-(CH2)4-
-(CH2)5-
Me
-
-6.5

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390
The above properties of dialcarbs suggest that they could serve as a solvent in reactions, where
an ionic liquid would be beneficial (e.g. in improving the chemoselectivity or yield of a reaction)
and in particular, when products are formed that are either non-volatile or thermally labile. Thus,
subsequent extraction of the products from the ionic liquid or the application of multiphase systems
can be avoided.
Results and Discussion
In this work, DIMCARB has been employed as a medium for aldol-condensation reactions
(Scheme 2) [15], mainly between aryl aldehydes and symmetrical ketones i.e. cyclohexanone,
acetone and cyclopentanone. When carried out in other media (except water), these reactions
frequently produce bis arylidenecycloalkanones [16]. With DIMCARB, mono-
arylidenecycloalkanones were formed preferentially when a 1:1 ratio of aldehyde : ketone was used
and the yields were moderate to excellent.
Scheme 2 Aldol-condensation type reactions performed in DIMCARB.
O
O
O
H
+
R2 R2
R2 R2
DIMCARB
R1
R1
Bis-arylidene products were only minor. Reactions using cyclopentanone, resulted in greater
selectivity toward the mono-arylidenes alkanones and greater yields were obtained.
Figure 2. Mannich adduct seen in the reaction
N
O
We have also detected Mannich adducts among the products, suggesting that dimethylamine
may play an important role. Apparently the Mannich adducts are formed via addition to transient
iminium species. These then deaminate. It seems that, in the case of the aldol-condensation
reactions, the DIMCARB is acting as both a dimethylamine donor and as a solvent/catalyst as
previously observed [6,9].
An advantage of DIMCARB as the medium is the capacity for recovery by ‘distillation’ from
the reaction mixture (see Figure 3). We have been able to recover up to 85 % DIMCARB by
distillation.

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391
Figure 3. Distillation of DIMCARB.
Re-association
to the Ionic Liquid
Distillation
as HNMe2 and CO2
Reaction Mixture
DIMCARB
Conclusions
DIMCARB was found to be a useful medium for monoarylidene synthesis. High yields and
selectivity were achieved. The methodology for mono-condensation products was simpler than for
other literature methods (that require at least two steps). It involved the mixing of the aldehyde and
ketone in the DIMCARB and heating up to 50 °C. Various workup methods were employed,
including: distillation of the DIMCARB (both high vacuum and under a carbon dioxide
atmosphere), addition of aqueous acid followed by extraction with typical water immiscible organic
solvents, or quenching on silica gel followed by solvent elution. DIMCARB also has potential uses
as an alternative to traditional ionic liquids that can be difficult to purify and recycle.
Experimental
General Method for Performing Aldol Condensations in DIMCARB
Benzaldehyde (1.0 g, 9.4 mmol) was added to DIMCARB (10 mL) at ambient temperature with
stirring. Gas was evolved. The solution was heated to 52 °C and cyclopentanone (0.79 g, 9.4 mmol)
was added in a single portion. Stirring at 52 °C was continued for 3 h, after which the colorless
reaction mixture had become dark brown. DIMCARB (8.71 g, 83 %) was recovered from the
reaction mixture by distillation at 60 °C under high vacuum.
The undistilled residue was acidified with 0.5 M H2SO4 (25 mL) and extracted with ethyl acetate
(3 × 25 mL). The combined organic fraction was dried with anhydrous MgSO4, filtered and the
solvent removed in vacuo to afford a brown oil (1.21 g). Si gel chromatography with ethyl acetate
elution yielded E-2-benzylidenecyclopentanone (1.06 g, 74 %) as a yellow solid m.p. 54-57 °C.
1H-NMR (300 MHz, CDCl3): δ 2.06 (app p, J = 7.6 Hz, 2H, -CH2-), 2.44 (t, J = 8.0 Hz, 2H, -CH2-
C=O), 3.00 (td, J = 7.2, 2.7 Hz, 2H, C=CCH2), 7.48 – 7.30 (m, 4H, 3 × ArCH, 1 × Ar-CH=C), 7.59
13
– 7.56 (m, 2H, ArCH). C-NMR (75 MHz, CDCl3): δ 20.20 (-CH2-); 29.36 (-CH2-C=C); 37.79
(-CH2-C=O); 128.73 (o-Ar); 128.36 (p-Ar); 130.55 (m-Ar); 132.42 (Ar-CH); 135.57 (Ar-CH=C);
136.12 (CH=CC=O); 208.24 (C=O).

Molecules 2004, 9
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