Ag(I) complexes of benzimidazol-2-ylidene ligands: A study of catalytic efficiency towards three-component coupling reactions

Article abstract of DOI:10.3906/kim-1510-6

The N -heterocyclic carbene (NHC)-Ag(I) complexes 3a-c were synthesized from benzimidazolium chlorides (2a-c = NHC.HCl), incorporating benzyl derivatives and a 2-methoxyethyl group (a: ³N = 2,4,6-trimethylbenzyl; b: ³N = 2,3,5,6-tetr

Full text of DOI:10.3906/kim-1510-6

Turk J Chem
Turkish Journal of Chemistry
(2016) 40: 681 – 687
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1510-6
Research Article
Ag(I) complexes of benzimidazol-2-ylidene ligands: a study of catalytic efficiency
towards three-component coupling reactions
1
2
Rafet KILINC¸ ARSLAN1,∗, Naim SADIC¸ , Bekir C¸ ETINKAYA
˙
1Department of Chemistry, Faculty of Science and Letters, Pamukkale University, Denizli, Turkey
2
˙
Department of Chemistry, Faculty of Science, Ege University, Izmir, Turkey
Received: 02.10.2015
•
Accepted/Published Online: 10.02.2015
•
Final Version: 21.06.2016
Abstract: The N -heterocyclic carbene (NHC)–Ag(I) complexes 3a–c were synthesized from benzimidazolium chlorides
(2a–c = NHC.HCl), incorporating benzyl derivatives and a 2-methoxyethyl group (a: 3 N = 2,4,6-trimethylbenzyl; b:
3 N = 2,3,5,6-tetramethylbenzyl; c: 3 N = pentamethylbenzyl). The compounds were characterized by spectroscopic
and elemental analyses. The synthesized NHC–Ag(I) complexes (3a–c) were tested as catalysts for the catalytic three-
component coupling reaction of aldehyde, alkyne, and amine to propargylamines in various solvents. All complexes were
active catalysts for catalytic three-component coupling reactions with good yields under neat and mild conditions (after
4 h, yields of up to 94%).
Key words: N -heterocyclic carbene, benzimidazol-2-ylidene, coupling reaction, catalyst
1. Introduction
N -heterocyclic carbenes (NHCs) are cyclic constructions that are generally derived from deprotonation of imi-
dazoli(ni)um, benzimidazolium, diazepinium, and pyrimidium salts. Since the isolation of the first free carbene
by Arduengo,1 transition metal carbene complexes have found wide application in organometallic chemistry.2−8
NHC–Ag(I) complexes have particular importance due to their structural variations and wide applications as op-
erative NHC transfer agents in transmetalation reactions to make other NHC–metal complexes.9−15 The notice-
able biological activity of NHC–silver complexes as anticancer and antimicrobial agents has been confirmed.16−22
In spite of carbene-free silver complexes having been used as catalysts in C–C and C–X (X: heteroatom) bond
formation reactions,23−25 the availability of NHC–Ag(I) complexes in chemical catalysis remains limited, and
only a few examples were published.26−36 In the course of our studies involving the use of benzyl substi-
tuted benzimidazol-2-ylidene ligands, we herein report the synthesis of a number of NHC–Ag(I) complexes and
compare their catalytic activities in formation of propargylamines by means of the three-component coupling
reaction with the present complexes.
2. Results and discussion
2.1. Synthesis and characterization
The benzimidazolium salts to be used as carbene precursors, 2a–c, were prepared by the published methods.37−46
These salts 2a–c are colorless, air-stable solids. The NHC–Ag(I) complexes (3a–c) were prepared from
∗Correspondence: rkilincarslan@pau.edu.tr
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KILINC¸ARSLAN et al./Turk J Chem
the reaction of benzimidazolium salts with silver(I) oxide as white powders in 56%–82% yield, as shown in
Scheme 1. The molecular and crystal structures of [1-(2-methoxyethyl)-3-(2,4,6-trimethylbenzyl)benzimidazol-
46
¨
2-ylidene]silver(I) chloride were previously characterized using X-ray diffraction by Ozdemir and co-workers.
The benzimidazolium salts and silver complexes were determined from their characteristic spectroscopic data
and elemental analyses. 13 C and 1 H NMR chemical shifts were consistent with the proposed structures; in the
1 H NMR spectrum, the formation of NHC–Ag(I) complexes was determined by the absence of the resonance
for C–H acidic protons around 9–11 ppm. In the 13 C NMR spectrum, peaks for carbene carbons were not
observed, which is consistent with the observation described in the literature. This observation was attributed
to the fluxional behavior of the NHC–Ag(I) complexes.46−51
Me
Me
Me
O
O
O
+
N
N
N
ArCH2Cl
1/2 Ag2O
-
Cl
Ag Cl
N
N
N
3
2
1
NHC-Ag(I)
R
R
a
c
b
2,3,5,6-(CH )
2,3,4,5,6-(CH )
3 5
2,4,6-(CH )
R =
3 4
3 3
Scheme 1. The synthesis of benzimidazolium salts 2a–c and NHC–Ag(I) complexes 3a–c.
For the complexes of 3b and 3c, crystals could not be obtained despite the use of different solvent
systems.
2.2. Catalytic activity of the NHC silver complexes
Recently, the formation of propargylamines by means of the three-component coupling reaction (Scheme 2) has
attracted much attention, due to their importance in the construction of nitrogen-containing biologically active
substances.52 The complexes of transition metals, especially silver, are known to show highly effective catalytic
activity for this three-component coupling reaction.34 However, only a limited number of NHC–Ag(I) complexes
have been tested as catalysts,33−35 and generally these complexes were mononuclear NHC–Ag(I) complexes.
Additionally, it was observed the monomeric NHC–Ag(I) complexes exhibit higher activity than the dimeric
forms {[Ag(NHC)X]2 } and the ionic NHC–Ag(I) complexes.35 However, the m-xyly bridging tetradentate
cationic bis-NHCs silver complexes have been described by Cheng,36 and it is interesting that, when these
dinuclear complexes were used, the catalyst in the coupling reaction progressed remarkably faster.
Our work has demonstrated the preparation of monomeric (NHC)Ag–Cl 3a–c and catalytic activities
of the synthesized catalysts for the three-component coupling reaction of piperidine, p-formaldehyde, and
phenylacetylene (model reaction). The results are summarized in the Table. When using the catalyst 3a
(3 mol%) for the three-component coupling reaction of benzaldehyde (entry 5) moderate yield was achieved.
However, using the same molar ratio of this complex in the coupling of cyclohexanecarboxaldehyde or p-
formaldehyde good catalytic activity was observed (entries 1 and 6). Alteration of the benzyl substituent of
the NHC ligand did not have a strong influence on the catalytic performance of the derived complexes. A poor
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KILINC¸ARSLAN et al./Turk J Chem
result was observed when only Ag2 O was used as catalyst (Table, entry 4); moreover, when the reaction was
repeated in the absence of any silver source, no desired product was detected.
O
N
NH
+
+
R
H
R
Me
O
N
Ag Cl
Cat. =
N
R
NHC-Ag(I)
Scheme 2. Three-component coupling reaction.
The influence of solvents on the three-component coupling reaction using 3a as catalyst was examined
(Table, entries 6–13). Among the solvents in the Table, dioxane, CH3 CN, DMSO, and acetone were successful
in the coupling reaction (Table, entries 5 and 7–9). DMF and toluene gave moderate yields of the desired
products (Table, entries 10 and 11). Low yields were observed when the reactions were performed with water
and ethanol (Table, entries 12 and 13). The three-component coupling reaction was also carried out under neat
reaction conditions at 80 ◦ C and high yields of isolated product were obtained (Table, entries 14–19). These
results indicate that the use of volatile solvents in the three-component coupling reactions could be avoided and
thus it can be considered to provide an important contribution to reducing environmental pollution. A tentative
mechanism for the three-component coupling reaction was suggested by Chao and co-workers:53 initially by
exchange of H of the C–H bond of alkyne and Ag(I) species, a silver acetylide, and an iminium ion between
aldehyde and amine forms. In the second step, the silver acetylide intermediate and the iminium ion generated
in situ react to give the corresponding propargylamines and regenerate the silver(I) catalyst for further reactions.
3. Experimental
3.1. General considerations
All reactions and manipulations for the preparation of NHC ligands and their Ag(I) complexes were carried
out under argon in flame-dried glassware using standard Schlenk line techniques. Anhydrous solvents were
either distilled from appropriate drying agents or purchased from Merck and degassed prior to use by purging
with dry argon and kept over molecular sieves. All other reagents were commercially available and used
as received. NMR spectra were recorded at 297 K on a Varian Mercury AS 400 NMR instrument at 400
MHz (1 H) and 100.56 MHz (13 C). Elemental analyses were performed by the TUBITAK Microlab (Ankara,
Turkey). Melting points was determined using an Electrothermal 9100 melting point detection apparatus. The
unsymmetrical benzimidazolium chlorides to be used as carbene precursors (2a–c) were prepared according
to a procedure slightly modified from the literature31 as depicted in Scheme 1. [1-(2-Methoxyethyl)-3-(2,4,6-
trimethylbenzyl)benzimidazol-2-ylidene]silver(I) chloride, 3a, was prepared according to the literature.46
¨
˙
683

KILINC¸ARSLAN et al./Turk J Chem
Table. Catalytic effect of NHC–silver catalyst and solvent on coupling reaction.a
N
O
Catalyst
(3 mol%)
+
+
NH
R
H
R
Entry
1
2
3
4
5
6
7
8
Aldehyde
Catalyst Solvent
t/h Yield (%)b,c
C6H11–CHO
C6H11–CHO
C6H11–CHO
C6H11–CHO
C6H5–CHO
H–CHO
H–CHO
H–CHO
H–CHO
H–CHO
H–CHO
H–CHO
H–CHO
H–CHO
C6H11–CHO
C6H5–CHO
p-CH3C6H4–CHO
p-OCH3C6H4–CHO 3a
p-FC6H4–CHO 3a
3a
3b
3c
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
DMSO
Acetone
CH3CN
DMF
PhMe
H2O
EtOH
Neat
Neat
4
4
4
4
4
4
4
4
4
4
4
12
4
4
4
4
4
4
4
82
80
77
32
52
84
80
78d
79
72
45
24e
24f
94
88
67
70
66
68
Ag2O
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
9
10
11
12
13
14
15
16
17
18
19
Neat
Neat
Neat
Neat
a Reaction conditions: aldehyde (1.0 mmol), piperidine (1.2 mmol), phenylacetylene (1.5 mmol), NHC–Ag(I) catalyst (3
mol %), dioxane (2.0 mL), 80 ◦ C, argon atmosphere.
b Yield after purification by silica gel chromatography.
c Average of two runs.
d
At 56 ◦ C.
e
For 12 h.
f At 78 ◦ C.
3.2. Preparation of ligands
3.2.1. 1-(2-Methoxyethyl)-3-(2,3,5,6-tetramethylbenzyl)benzimidazolium chloride, 2b
1-(2-Methoxyethyl)benzimidazol (1.0 mmol) was dissolved in dried toluene (5.0 mL). 2,3,5,6-Tetramethyl benzyl
chloride (1.1 mmol) was added to the solution and the mixture was refluxed for 6 h. The reaction mixture was
then cooled to room temperature. Then n-hexane (10.0 mL) was added and the mixture was filtered. The
solid was recrystallized from MeOH/Et2 O. Yield: 68%, mp: 198 ◦ C. 1 H NMR (CDCl3): δ = 9.81 (s, 1 H,
NCH N), 7.96 (m, 1 H, Ar–H), 7.91 (m, 1 H, Ar–H), 7.60 (m, 1 H, Ar–H), 7.55 [s, 1 H, C6H (CH3)4 ], 7.49
(m, 1 H, Ar–H), 5.76 [s, 2 H, CH2 C6 H(CH3)4 ], 4.91 (t, J = 2.4 Hz, 2 H, NCH2 CH2 OCH3), 3.83 (t, J =
2.4 Hz, 2 H, NCH2 CH2 OCH3), 3.25 (s, 3 H, NCH2 CH2 OCH3), 2.24 [s, 6 H, m-C6 H(CH3)4 ], 2.22 [s, 6
H, o-C6 H(CH3)4 ] ppm. 13 C NMR (CDCl3): δ = 141.5 (NC HN), 135.0, 133.4, 133.7, 133.3, 131.8, 127.1,
127.4, 114.6, 113.2, 113.4 [Ar–C and C6 H(CH3)4 ], 69.9 (NCH2 CH2 OC H3), 58.8 (NCH2C H2 OCH3), 47.8
(NC H2 CH2 OCH3), 47.2 [NC H2 C6 H-(CH3)4 ], 20.5 [m-C6 H(C H3)4 ], 20.4 [o-C6 H(C H3)4 ] ppm.
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KILINC¸ARSLAN et al./Turk J Chem
3.2.2. 1-(2-Methoxyethyl)-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazolium chloride, 2c
This compound was prepared in the same manner as 2b using 2,3,4,5,6-pentamethyl benzyl chloride (1.1 mmol)
and 1-(2-methoxyethyl)benzimidazol, 1 (1.0 mmol). Yield: 62.0%, mp: 197-198 ◦ C. 1 H NMR (CDCl3): δ
= 9.79 (s, 1 H, NCH N), 7.95 (m, 1 H, Ar–H), 7.64 (m, 1 H, Ar–H), 7.58 (m, 1 H, Ar–H), 7.54 (m, 1 H,
Ar–H), 5.70 [s, 2 H, CH2 C6 (CH3)5 ], 4.90 (t, J = 2.4 Hz, 2 H, NCH2 CH2 OCH3), 3.82 (t, J = 2.4 Hz, 2 H,
NCH2 CH2 OCH3), 3.24 (s, 3 H, NCH2 CH2 OCH3), 2.25 [s, 3 H, p-C6 (CH3)5 ], 2.24 [s, 6 H, m-C6 (CH3)5 ],
2.22 [s, 6H, o-C6 (CH3)5 ] ppm. 13 C NMR (CDCl3): δ = 141.2 (NC HN) 137.8, 133.4, 133.6, 132.1, 131.6,
128.1, 127.3, 124.5 114.0, 113.2 [Ar–C and C6 (CH3)5 ], 69.9 (NCH2 CH2 OC H3), 58.8 (NCH2C H2 OCH3), 47.7
(NC H2 CH2 OCH3), 47.6 [NC H2 C6 - (CH3)5 ], 17.2 [p-C6(C H3)5 ], 20.5 [m-C6(C H3)5 ], 20.4 [o-C6(C H3)5 ].
3.3. Preparation of silver–NHC complexes
˚
Benzimidazolium chloride (1.0 mmol), activated molecular sieves (4 A), and Ag2 O (0.5 mmol) in dichloromethane
(15.0 mL) were stirred at 25 ◦ C for 12 h in the dark, covered with aluminum foil under argon46,54 and filtered
using Celite. The solvent was removed under vacuum. The residue was recrystallized from DCM /n-hexane.
3.3.1. [1-(2-Methoxyethyl)-3-(2,3,5,6-tetramethylbenzyl)benzimidazolin-2-ylidene]chloro silver(I),
3b
Yield: 74%, mp: 176–177 ◦ C. Anal. Calc. for C21 H26 N2 OAgCl: C, 54.15; H, 5.63; N, 6.01. Found: C, 54.36; H,
5.54; N: 6.12. 1 H NMR (δ, CDCl3)= 2.07 [s, 6H, CH2 C6 H(CH3)4 -2,6]; 2.22 [s, 6H, CH2 C6 H(CH3)4 -3,5]; 3.20
[s, 3H, CH2 CH2 OCH3 ]; 3.69 [t, 2H, J = 5.1 Hz, CH2 CH2 OCH3 ]; 4.45 [t, 2H, J = 5.1 Hz CH2 CH2 OCH3 ];
5.39 [s, 2H, CH2 C6 H(CH3)4 -2,3,5,6]; 7.07 [s, 1H, CH2 C6 H(CH3)4 -2,3,5,6]; 7.36 [m, 3H, NC6 H4 N]; 7.53 [d,
1H, J = 8.1 Hz, NC6 H4 N]. 13 C NMR (CDCl3): 16.2 [CH2 C6 H(CH3)4 -2,6]; 20.8 [CH2 C6 H(CH3)4 -3,5]; 47.3
[CH2 CH2 OCH3 ]; 50.0 [CH2 C6 H(CH3)4 -2,3,5,6]; 59.1 [CH2 CH2 OCH3 ]; 72.2 [CH2 CH2 OCH3 ]; 129.6, 133.3,
133.5, 134.2 [CH2 C6 H(CH3)4 -2,3,5,6]; 111.1, 112.4, 124.1, 124.4, 134.6,135.4 [NC6 H4 N]; the C2 carbon was
not observed.
3.3.2. [1-(2-Methoxyethyl)-3-(pentamethylbenzyl) benzimidazolin-2-ylidene]chloro silver(I), 3c
Yield: 56%, mp: 164–165 ◦ C. Anal. Calc. for C22 H28 N2 OAgCl: C, 55.07; H, 5.88; N, 5.84. Found: C, 55.10;
H, 5.92; N: 5.82. 1 H NMR (δ, CDCl3)= 2.12 [s, 6H, CH2 C6 (CH3)5 -2,6]; 2.21 [s, 6H, CH2 C6 (CH3)5 -3,5];
2.26 [s, 3H, CH2 C6 (CH3)5 -4]; 3.19 [s, 3H, CH2 CH2 OCH3 ]; 3.67 [t, 2H, J = 5.1 Hz, CH2 CH2 OCH3 ]; 4.42
[t, 2H, J = 5.1 Hz CH2 CH2 OCH3 ]; 5.39 [s, 2H, CH2 C6 (CH3)5 -2,3,4,5,6]; 7.38 [m, 3H, NC6 H4 N]; 7.54
[d, 1H, J = 8.1 Hz, NC6 H4 N]. 13 C NMR (CDCl3): 17.1 [CH2 C6 (CH3)5 -2,6]; 17.2 [CH2 C6 (CH3)5 -3,5];
17.4 [CH2 C6 (CH3)5 -4]; 47.7 [CH2 CH2 OCH3 ]; 50.1 [CH2 C6 (CH3)5 -2,3,4,5,6]; 59.1 [CH2 CH2 OCH3 ]; 72.2
[CH2 CH2 OCH3 ]; 126.5, 133.0, 134.5, 137.3 [CH2 C6 (CH3)5 -2,3,4,5,6]; 111.1, 112.3, 124.0 124.3, 134.3, 134.8
[NC6 H4 N]; the C2 carbon was not observed.
4. Typical procedure of the three-component coupling reaction catalyzed by NHC-Ag(I) catalyst
In a typical procedure, a mixture of phenylacetylene (1.5 mmol, 164.7 µL), aldehyde (1.0 mmol), piperidine
(1.2 mmol, 118.7 µL), and silver complex (3 mol%) was added to an oven-dried Schlenk tube (15 mL) with
1,4-dioxane (2.0 mL). The Schlenk tube was placed in a preheated oil bath (80 ◦ C). The mixture was stirred at
80 ◦ C for a given time under an argon atmosphere. After the reaction was completed, the mixture was cooled
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KILINC¸ARSLAN et al./Turk J Chem
to room temperature and diethyl ether was added. The organic portion was dried over MgSO4 and filtered.
After the volatile components were removed under vacuum, the residue was purified by column chromatography
on silica using ethyl acetate/hexane (1/2).
5. Conclusion
In summary, the facile synthesis and characterization of three mononuclear NHC-Ag(I) complexes (3a–c) derived
from 1-(2-methoxyethyl)-3-(alkyl)benzimidazol-2-ylidene is reported. The preliminary catalytic study revealed
that the silver complexes show good activity in a three-component coupling reaction under solvent-free reaction
conditions. The efficiency slightly depends on the NHC ligand and decreases with the number of methyl groups
on the benzyl substituent on the 3 N atom. Studies on the structural diversity of carbene precursors and the
modification–efficiency relationship of NHC–Ag(I) complexes are continuing in our research lab.
Acknowledgments
¨
This work was financially supported by the Pamukkale University Research Fund (PA.U.B.A.P: 010FBE078)
¨
and TUBA.
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