Evidence for the spontaneous formation of N-heterocyclic carbenes in imidazolium based ionic liquids

Article abstract of DOI:10.1039/c7cc06112a

We present a study of the reactions of aldehydes in ionic liquids which gives evidence for the spontaneous formation of N-heterocyclic carbenes in ionic liquids based on 1,3-dialkyl substituted imidazolium cations from the lack of a deuterium isotope effect on the reaction of these ionic liquids with aldehydes.

Full text of DOI:10.1039/c7cc06112a

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Evidence for the spontaneous formation of
N-heterocyclic carbenes in imidazolium based
ionic liquids†
Cite this: Chem. Commun., 2017,
3, 11154
5
Received 4th August 2017,
Accepted 4th September 2017
a
a
b
ac
N. M. A. N. Daud, E. Bakis,
J. P. Hallett,
C. C. Weber
and
a
T. Welton
*
DOI: 10.1039/c7cc06112a
rsc.li/chemcomm
1
1
12
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We present a study of the reactions of aldehydes in ionic liquids which in ionic liquids, including with Ag, Pd, and Zn. While
gives evidence for the spontaneous formation of N-heterocyclic these and similar observations have been used to infer the
carbenes in ionic liquids based on 1,3-dialkyl substituted imidazolium presence of NHCs in acetate ionic liquids, they do not confirm
cations from the lack of a deuterium isotope effect on the reaction of their spontaneous formation in the ionic liquids, but rather
these ionic liquids with aldehydes.
just indicate their accessibility.
Although 1-ethyl-3-methylimidazolium-2-ylidene has been
Ionic liquids are versatile systems that have generated significant shown to exist together with [C
2
C
1
im][OAc] and H[OAc] under
1
14
interest for a wide range of applications including synthesis,
cryogenic conditions (9 K), no direct observation of the
2
3
4
electrochemistry, catalysis, biomass processing, and CO2 spontaneous formation of a NHC in an ionic liquid under
5
capture. A large number of ionic liquids have been prepared ambient conditions has been achieved. In addition, for all of the
and investigated, but those containing 1,3-dialkylimidazolium reactions in which a ‘NHC-derived’ product has been formed, an
cations are by far the most commonly used. While these reliably alternative mechanism in which the same product is derived
give ionic liquids with relatively low melting points and excellent directly from the imidazolium cation itself can be proposed. For
thermal and electrochemical stabilities, they are not entirely example, a recent computational investigation of the reaction of
1
,6
inert.
2 2 1
CO with [C C im][OAc] has suggested that this follows a concerted
7
As early as 2002, Aggarwal et al., noted the low product reaction in which proton transfer to the acetate ion does not take
yields in the base-catalysed Baylis–Hillman reaction of methyl place until the bond between the C -proton of the imidazolium
acrylate and benzaldehyde in the presence of 1-butyl-3- ring and the CO has begun to form. After a detailed theoretical
methylimidazolium chloride ([C C im]Cl) and explained this study of this reaction Holl o´ czki et al., suggested that the presence
2
1
5
2
16
4
1
+
4 1
by a reaction of the benzaldehyde with [C C im] via a of neutral electrophiles at the very least facilitates the formation of
N-heterocyclic carbene (NHC) formed by deprotonation of the the carbene and possibly the presence of ‘carbene traps’ is
imidazolium cation. More recently, we have demonstrated the required for the apparently carbene derived products to form.
17
reaction of the ionic liquid 1-ethyl-3-methylimidazolium acetate Kelemen et al., demonstrated that [C
2 1
C im][OAc] is an organo-
(
[C im][OAc]) with cellulose and proposed a similar initiation catalyst for the benzoin condensation reaction and invoked the
2 1
C
8
18
of the reaction by the equivalent NHC. Rogers et al., observed formation of a NHC to explain this. Chiarotto et al., have
the reaction of CO2 with 1,3-dialkylimidazolium acetates to observed the same for [C C im][OAc]. At the same time they used
4
1
9
give the corresponding 1,3-dialkylimidazolium-2-carboxylate, as cyclic voltammetry to detect an oxidation peak in [C C im][OAc] at
4
1
well as reactions between elemental sulphur or selenium and 120–150 1C ascribed to the presence of NHC in the ionic liquid.
10
X 1
[C C im][OAc] (X = 2 or 4) to form imidazole-2-chalcogenones.
However, they did not observe this peak at the temperature at
A number of transition metal NHC complexes have also formed which they conducted the reaction (60 1C). It also remains possible
that the electrode itself was behaving as a carbene trap at these
a
b
c
elevated temperatures. Hence, their evidence for the spontaneous
formation of NHCs remains inconclusive.
Department of Chemistry, Imperial College London, London, SW7 2AZ, UK.
E-mail: t.welton@imperial.ac.uk
Department of Chemical Engineering, Imperial College London, London, SW7 2AZ,
UK
We have also been studying the benzoin reaction in ionic
17,18
liquids. As with the above mentioned studies,
we have
School of Science, Auckland University of Technology, Auckland 1010,
New Zealand
shown that this reaction is catalysed by dialkylimidazolium
based ionic liquids and that this requires that the anion of
the ionic liquid be sufficiently basic and that methylation of the
†
Electronic supplementary information (ESI) available: Experimental details. See
DOI: 10.1039/c7cc06112a
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Table 1 Benzoin reaction in ionic liquids
Ionic liquid
Condition
Yield (%)
[C
2
[C
2
[C
2
[C
2
[C
2
C
1
C
1
C
1
C
1
C
1
im][OAc]
60 1C, under N
80 1C, under N
80 1C, under N
80 1C, under N
80 1C, under N
2
, 24 h
2
, 24 h
2
, 24 h
2
, 24 h
2
, 24 h
47
96
0
0
0
im][OAc]
2
1
C
im][OAc]
im][NTf
im][OTf]
2
]
2 1
Fig. 1 Kinetic plots of the benzoin reaction (1 : 2 mol ratio [C C im][OAc] :
2
2 1 1
benzaldehyde in [C C C im][NTf2], 80 1C, under nitrogen).
the Breslow intermediate. Further studies showed that the
reaction of anisaldehyde with [C im][OAc] at 30 1C in
C C C im][NTf ] led to the formation of the Breslow inter-
2 1
C
2
[
2
1
1
2
mediate, but not the final benzoin condensation product after
4 hours, so facilitating the investigation of the reaction. The
2
formation of the Breslow intermediate was also slow enough
under these conditions to enable us to study the kinetics of the
reaction.
Scheme 1 Mechanism of the benzoin reaction in ionic liquids via the
formation of a NHC.
2 1 2
Reaction of [C C im][OAc] with D O leads to the deuteration
C -position of the imidazolium ring prevents the reaction from of the C –H of the imdazolium ring. This allows for the investigation
2
2
occurring (Table 1).
of the potential deuterium isotope effect on this reaction. The rate of
Breslow et al. proposed a mechanism for this reaction using the reactions of anisaldehyde with [C C im][OAc] or [d-C C im][OAc]
2
1
2 1
thiazolium salts as the catalyst, which proposed the formation in solution in [C C C im][NTf ] were directly compared. It can
1
1
2
2
1
9
of a NHC. This mechanism can be straight-forwardly reinter- clearly be seen that there is no effect on the reaction when
preted for [C im][OAc] (Scheme 1), which accords with the conducted with [C C im][OAc] or [d-C C im][OAc] (Fig. 2).
available information to date. In this mechanism, the proton is
2 1
C
2
1
2 1
The absence of a deuterium isotope effect in the formation
2
abstracted from the C -position of the imidazolium ring by of the Breslow intermediate for anisaldehyde clearly demon-
the acetate ion, forming the NHC, followed by addition of strates that the C –H(D) bond cannot be involved in the rate-
2
the aldehyde to the NHC to yield the so-called Breslow inter- determining step of this reaction. This, in turn, negates the
1
mediate, which can be detected by H NMR (Fig. S1, ESI†) and possibility of a concerted reaction mechanism for this process
MS (Fig. S2, ESI†) in the reaction mixture, further supporting this (Scheme 2) and it can be concluded that the reaction mechanism
mechanism. However, it is also possible to postulate a different for this process is the one in which the NHC is formed first and
mechanism in which the Breslow intermediate is formed in a then subsequently reacts with the anisaldehyde (Scheme 1).
concerted substitution of the proton by the aldehyde, which Consequently, the NHC must have been present in the reaction
equally well matches the available data (Scheme 2).
Initial investigations of the reaction of benzaldehyde in pure
[C
2
C
1
im][OAc] gave a solid mass. By diluting the reactants in
1
,2-dimethyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide
2
([C C C im][NTf ]) the reaction mixture remained homogeneous
2 1 1 2
throughout the reaction, allowing its kinetics to be studied. This
showed that formation of the Breslow intermediate (blue circles)
is rapid and that the addition of the second benzaldehyde is the
rate-determining step of the reaction (Fig. 1). Consequently,
this reaction cannot be used to investigate the formation of
Fig. 2 Anisaldehyde loss in the reactions with [C
2
C
1
im][OAc] or [d-C
2
C
1
im][OAc]
2
2 1 2 1 1 2
(
1 : 2 mol ratio [C
C im][OAc] : anisaldehyde in [C C C im][NTf ], 30 1C,
À5
= À1.99 Â 10À5).
Scheme 2 Concerted mechanism of Breslow intermediate formation.
under N , k
2
H
= À1.92 Â 10 , k
D
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Conflicts of interest
There are no conflicts to declare.
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1
1156 | Chem. Commun., 2017, 53, 11154--11156
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