Cu-catalyzed controllable C-H mono-/di-/triarylations of imidazolium salts for ionic functional materials

Article abstract of DOI:10.1039/c7cc01076a

Herein the Cu-catalyzed direct C-H mono-, di- and triarylations of imidazolium salts with aryl iodides/bromides are accomplished for the first time. The unprecedented alkenylation and alkynylation are also realized using alkenyl and alkynyl iodides, respectively. Moreover, triarylated imidazolium salts with different substituents can be accessed in a modular and one-pot manner. This protocol provides an efficient tool for the assembly of diverse imidazolium-based ionic functional materials. As applicable examples, an electrochromic bisbenzimidazolium salt 7 and a photochromic triarylimidazolium salt 8 are easily obtained.

Full text of DOI:10.1039/c7cc01076a

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DOI: 10.1039/C7CC01076A
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Cu-Catalyzed Controllable C−H Mono-/Di-/Triarylations of
Imidazolium Salts for Ionic Functional Materials
Shiqing Li, Junbin Tang, Yinsong Zhao, Ruyong Jiang, Tianbao Wang, Ge Gao* and Jingsong You*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Herein the Cu-catalyzed direct C−H mono-, di- and triarylations of Although the direct C−H functionalization of azole heterocycles has
imidazolium salts with aryl iodides/bromides are accomplished for been well established by Miura and Daugulis,9 that of imidazolium
the first time. The unprecedented alkenylation and alkynylation are salts is considered difficult because the deprotonation of
also realized using alkenyl and alkynyl iodides, respectively. imdiazolium salts under alkaline conditions generates NHCs or
Moreover, triarylated imidazolium salts with different substituents aNHCs,2,4 which are robust ligands to stabilize transition-metal
can be accessed in a modular and one-pot manner. This protocol complexes, and thus the reductive elimination usually does not occur
provides an efficient tool for the assembly of diverse imidazolium- on the metal center. In 1998, Cavell et al found that the complexes
based ionic functional materials. As the applicable examples, an of NiII and PdII containing NHCs and aryl or alkyl ligands could undergo
electrochromic bisbenzimidazolium salt 7 and a photochromic reductive elimination to give C2-substituted imidazolium salts.10
triarylimidazolium salt 8 are easily obtained.
They later reported the C2‒H alkylation of imidazolium salts with
alkenes under the catalysis of a Ni(cod)2/PPh3 catalytic system
(scheme 1a).11 To the best of our knowledge, this is the first and only
example of transition-metal-catalyzed direct C2‒H functionalization
of imidazolium salts so far. Efforts along this direction have recently
progressed with very specific but enlightening results. A stable NHC
with bulky N-substituents, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene (IPr), could couple with aryl iodides using Pd2(dba)3 as the
catalyst with limited substrate scope.12 The stoichiometric reactions
of two air and moisture sensitive CuI(NHC)Br complexes with
iodobenzene gave the corresponding 2-phenylbenzimidazolium
salts.13 Nevertheless, catalytic direct C2-H arylation of imidazolium
salts has not been reported yet. Moreover, further arylation at C4
and C5 positions are also unavailable.
Imidazolium salts are a well-known subclass of ionic liquids (ILs)1 and
N-heterocyclic carbene (NHC)2 precursors. Recently, the C-arylated
imidazolium salts have attracted considerable attention. For
example, Goossens et al synthesized a series of 2-aryl imidazolium
salts as T-shaped rigid core ionic liquid crystals, which showed
interesting mesogenic properties.3 Abnormal N-heterocyclic
carbenes (aNHCs) can be generated from 2-aryl and 2,4(5)-diaryl
imidazolium salts by alkaline deprotonation.4 Certain aNHC-metal
complexes in some cases showed higher catalytic ability than the
normal NHC congeners.5 When the C4 and C5 positions are
substituted by thiazoles or thiophenes, ionic photochromic materials
were obtained.6 Holdcroft group and Coates group independently
found 2-aryl benzimidazolium salts and fully substituted imidazolium
salts exhibit substantial base stability, respectively, which makes
them very promising ionic moiety candidates in high-performance
hydroxide conducting membranes.7 It is noticed that all these
arylated imidazolium skeletons were de novo built up in several steps.
In the past decades, transition-metal catalyzed direct C−H bond
functionalization has been a spotlight in organic synthesis, which is
regarded as an idea strategy for the formation of C−C and
C−heteroatom bonds with excellent atom- and step-economy.8
Key Laboratory of Green Chemistry and Technology of Ministry of Education,
College of Chemistry, Sichuan University 29 Wangjiang Road, Chengdu 610064, P.
R. China. E-mail: gg2b@scu.edu.cn; jsyou@scu.edu.cn; Fax: (+86) 28-85412203
†Electronic Supplementary Information (ESI) available: Experimental procedures,
characterization data and copies of NMR spectra, see DOI: 10.1039/x0xx00000x
Scheme 1 Catalytic direct C−H functionalization of imidazolium salts.
J. Name., 2013, 00, 1-3 | 1
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Due to our continuous interest in the synthesis and application of 2). Aryl iodides possessing either an electron-donating or electron-
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imidazolium derivatives,14 we present herein the first Cu-catalyzed withdrawing functionality could smoothly arDylOatI:e101.a10t3o9g/Ce7nCeCra0t1e07th6Ae
direct C−H arylation of imidazolium salts, which can sequentially corresponding products in moderate to excellent yields (3aa−ak). For
provide mono-, di- and triaryl imidazolium salts under controlled example, both NMe2 and NO2 groups are compatible to afford 3ac
conditions. The practical simplicity and high efficiency of this and 3ad in high yields of 72% and 88%, respectively. In contrast, they
protocol are demonstrated by the one-pot accesses to an were not tolerated in the Pd-catalyzed coupling of IPr with aryl
electrochromic bisbenzimidazolium salt and a photochromic 2,4,5- iodides.12 For disubstituted benzene with different halogens, the
trisubstituted imidazolium salt.
more reactive iodide arylated 1a to give 3ah and 3ai in 83% and 90%
At the outset of our study, the coupling of 1,3- yields, respectively. The intact bromide and chloride could be
dimethylimidazolium iodide (1a) with iodobenzene (2a, 1 equiv) at subjected to further functionality installment. The sterically hindered
120 °C using two known catalytic systems, Pd2(dba)3 and mesityl iodide 2l could also give 3al in an acceptable yield of 62%. 2-
Ni(cod)2/PPh3, did not give any arylated product at all (Table 1, Iodothiophene 2m efficiently coupled with 1a to afford 3am in a high
entries 1 and 2). To our delight, 2-phenylimidazolium salt 3aa was yield of 90%. Notably, both 2-alkenyl and 2-alkynyl imidazolium salts
solely generated in 23% yield by using Cu(OAc)2 as the catalyst (Table were both successfully obtained by treatment of 1a with alkenyl and
1, entry 3). Cu2O15 gave 3aa in an improved yield of 55% (Table 1, alkynyl iodides, respectively. The 2-alkynyl imidazolium salt 3ao was
entry 4). The base screening resulted in about 85% yield of 3aa by isolated in 32% yield due to the inevitable decomposition and
using 1 equiv of NaOAc or K2CO3, while K3PO4 gave a lower yield homocoupling of (iodoethynyl)benzene. It is notable that aryl
(Table 1, entries 5−7). Increasing the amount of 2a to 2 equiv bromides could also be used for the arylation, but gave diminished
delivered 3aa in 95% yield with 1 equiv of NaOAc as the base (Table yields after anion exchange. For instance, the BF4 salts of 3aa and 3ae
1, entry 8). Reducing the amount of Cu2O decreased the yield, but a were isolated in 67% and 82% yields, respectively.
moderate yield of 47% was still obtained when 2 mol% of Cu2O was
used (Table S1 in the ESI†). However, both 3aa and 2,4-
diphenylimidazolium salt 4aa were obtained in similar yields around
40% when 1 equiv of K2CO3 was employed (Table 1, entry 9). Using 2
equiv of K2CO3 and reacting for 36 hours afforded 4aa as the single
product in a high yield of 87% (Table 1, entries 10 and 11). Further
adjustment of the amounts of 2a and K2CO3 to 5 equiv resulted in
2,4,5-triphenylimidazolium salt 5aa as the sole product in 89% yield
(Table 1, entries 12 and 13).
Table 2 Substrate scope of various azolium salts and iodidesa,b,c
Table 1 Optimization of the multiple arylation reactionsa
2a/
x equiv
Cat./
20 mol%
Base/
y equiv
Yieldb
3aa:4aa:5aa
Entry
1
2
1
1
1
1
1
1
1
2
2
2
2
3
5
Pd2(dba)3
Ni(cod)2/PPh3
Cu(OAc)2
Cu2O
None
None
0:0:0
0:0:0
3
None
23:0:0
4
None
55:0:0
5
Cu2O
K3PO4/1
K2CO3/1
NaOAc/1
NaOAc/1
K2CO3/1
K2CO3/2
K2CO3/2
K2CO3/3
K2CO3/5
69:trace:0
85:trace:0
86:0:0
6
Cu2O
a Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), Cu2O (20 mol%) and NaOAc
(1.0 equiv) were stirred in DMF (1 mL) at 120 °C for 24 h. b Isolated yield. c The
yield in the parenthesis represents the yield of a BF4 salt after anion exchange
for the reaction using ArBr instead of ArI.
7
Cu2O
8
Cu2O
95:0:0
9
Cu2O
38:40:0
-:80:0
10
11c
12
13c
Cu2O
On the other hand, imidazolium salts with both N-alkyl and N-aryl
substituents were suitable substrates in this reaction. For example,
the direct phenylation of room-temperature ILs (RTILs) 1b−c and
1g−h gave 2-phenyl imidazolium salts 3ba−ca and 3ga−ha as new
RTILs in fairly high yields. This result implies that special caution
should be taken when running transition-metal-catalyzed coupling
reactions in imidazolium-based ionic liquids. The counteranion of the
imidazolium substrate showed little influence on this coupling
reaction. The coupling between a BF4 salt 1e and iodobenzene
delivered 1,2,3-triphenylimidazolium 3ea as a BF4 salt in 88% yield.
Cu2O
0:87:0
Cu2O
-:51:42
Cu2O
0:0:89
a Reaction conditions: 1a (0.2 mmol), 2a (x equiv), catalyst (20 mol%)
and base (y equiv) in DMF (1 mL) at 120 °C for 24 h under a N2
atmosphere. b Isolated yield. c 36 h.
With the optimal conditions in hand, the scopes of iodides and
azolium salts were firstly examined for the C2 mono-arylation (Table
2 | J. Name., 2012, 00, 1-3
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Fully-substituted imidazolium salts 3ja and 3jp were obtained in 93% the addition sequence of the aryl iodides, the other two isomers 6b
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and 97% yields, respectively. It is noted that 3ja was reported with and 6c were obtained in 62% and 52% yieldDs,OrIe: 1s0p.e10ct3i9v/eCly7C. ICt0n1e0e76dAs
substantial alkaline stability.7b Finally, a benzo-fused imidazolium salt, to point out that no isolation is required until the final purification
2-ethylimidazo[1,5-a]pyridin-2-ium, could also couple with for the desired product. Diarylated products with different
iodobenzene to provide the corresponding salt 3ka in 77% yield.
substituents could also be separated after the first two arylation if
required.
Table 3 Di- and triarylation of imidazolium saltsa,b
a Reaction condition A: 1 (0.2 mmol), 2 (2 equiv), Cu2O (20 mol%), K2CO3 (2
equiv) in DMF (1 ml) at 120 °C for 36 h. b Reaction condition B: 1 (0.2 mmol),
2 (5 equiv), Cu2O (20 mol %) and K2CO3 (5 equiv) were stirred in DMF (1 ml) at
120 °C for 36 h. c Ratio of the two regioisomers.
Scheme 3 Proposed catalytic cycle of the C2‒H arylation.
Based on the previous work,11-13 a proposed catalytic cycle of the
C2−H arylation is given in Scheme 3. With the aid of NaOAc, a Cu(I)-
NHC complex I is firstly formed between 1a and the Cu(I) species. The
oxidative addition with iodobenzene gives a Cu(III) intermediate II.
Finally, II undergoes reductive elimination to release the product 3aa
and regenerate the Cu(I) species for the next catalytic cycle. The
C4/C5−H arylations follow similar catalytic pathways as the C2−H
arylation.
The scopes for the diarylation and triarylation were then
investigated. As shown in Table 3, the electron-rich aryl iodide 2b
could smoothly diarylate 1a to give 4ab in 77% yield, while the
electron-poor aryl iodide 2e gave 4ae in a lowered yield of 41%. For
unsymmetrical imidazolium salts, regioselective products were
obtained for diarylation. For example, the diarylation of 1b afforded
2,4- and 2,5-diphenyl salts 4ba (4ba’) in a 1:2 ratio based on the
NOESY analysis. For 1d, however, 2,4-diphenyl salt 4da was
determined almost as a single regioisomer. Under the optimal
conditions for triarylation, the electron-poor aryl iodides 2e and 2i
could directly deliver 2,4,5-triarylated imidazolium salts in moderate
yields. However, the electron-rich aryl iodide 2b only gave diarylated
product 4ab, and no triarylated product was observed. It should be
mentioned that the employment of 4-iodopyridine afforded in one
pot the corresponding 2,4,5-tri(4-pyridyl)imidazolium salt 5aq in 48%
yield, which would be a very interesting tridirectional ionic building
block for metal-organic coordination assemblies.16
Figure 1 (a) Synthesis of
120 °C, 30 h. (b) Reversible electrochromic process of
without supporting electrolyte. (c) Synthesis of . Conditions: (i) Cu2O
(20 mol%), NaOAc (1 equiv), 120 °C, 24 h. (ii) Cu2O (10 mol%), K2CO3 (3
7
. Conditions: Cu2O (40 mol%), NaOAc (2 equiv),
Scheme 2 Sequential C2-C4-C5 arylation of imidazolium salts in one pot.
Reaction conditions: (i) Iodide (1 equiv), Cu2O (20 mol%), NaOAc (1 equiv), 24
h. (ii) Iodide (1 equiv), Cu2O (10 mol%), K2CO3 (1 equiv), 24 h. (iii) Iodide (3
equiv), Cu2O (10 mol%), K2CO3 (3 equiv), 24 h.
7
in acetonitrile
8
equiv), 120 °C, 24 h. (d) Reversible photochromic process of
acetonitrile.
8 in
Considering the structural diversity, C2-, C4- and C5-arylated
imidazolium salts with different substituents are highly demanded. A
sequential C–H arylation in one pot could be envisaged because of
the well-controlled stepwise arylation with different bases (Scheme
2). The imidazolium salt 1a was sequentially arylated with 2a, 2b and
2e to afford triarylated imidazolium salts 6a in 57% yield. By changing
By applying the present protocol, ionic functional materials with
imidazolium unit were easily synthesized. Bisbenzimidazolium salt 7
with a phenylene bridge was synthesized by the direct C−H coupling
of 1-ethyl-3-phenylbenzimidazolium salt 1i with 1,4-diiodobenzene
in 84% yield in one step (Figure 1a). 7 can be used to fabricate
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