Synthesis of rhodium complexes derived from benzimidazolin-2-ylidene ligands and first used for the addition of arylboron to benzonitriles

Article abstract of DOI:10.1016/j.jorganchem.2013.01.025

Benzimidazolium salts and their new rhodium complexes ([Rh(COD)(NHC)Cl]) were synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR, and IR spectroscopy. The addition of arylboron to benzonitriles in the presence of a catalytic amount of rhodium NHC complexes was examined. Corresponding carbonyl compounds were obtained in good yields. This method allows for the preparation of a wide variety of carbonyl compounds in moderate to excellent yields and displays a high level of activity for the addition of arylboron to nitriles.

Full text of DOI:10.1016/j.jorganchem.2013.01.025

Journal of Organometallic Chemistry 732 (2013) 21e26
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Synthesis of rhodium complexes derived from benzimidazolin-2-ylidene ligands
and first used for the addition of arylboron to benzonitriles
a
b
a,
_
*
ꢀ
Serpil Demir , Murat Yigit , Ismail Özdemir
a Inönü University, Faculty of Science and Art, Department of Chemistry, 44280 Malatya, Turkey
b Adiyaman University, Faculty of Science and Art, Department of Chemistry, 02040 Adiyaman, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzimidazolium salts and their new rhodium complexes ([Rh(COD)(NHC)Cl]) were synthesized and
characterized by elemental analysis, 1H NMR, 13C NMR, and IR spectroscopy. The addition of arylboron to
benzonitriles in the presence of a catalytic amount of rhodium NHC complexes was examined. Corre-
sponding carbonyl compounds were obtained in good yields. This method allows for the preparation of
a wide variety of carbonyl compounds in moderate to excellent yields and displays a high level of activity
for the addition of arylboron to nitriles.
Received 5 December 2012
Received in revised form
28 January 2013
Accepted 30 January 2013
Keywords:
Rh-NHC
Ó 2013 Elsevier B.V. All rights reserved.
Benzimidazolin-2-ylidene
Sodium tetraphenylborate
Ketone
Arylation of nitrile
1. Introduction
Besides its prominent role in pharmaceutical chemistry, the ben-
zimidazole moiety also serves as a basis for ionic liquids [17] and as
N-Heterocyclic carbenes (NHCs) and their metal complexes are
prevalent in modern organometallic chemistry [1]. The history of
the NHC family of ligands is rich but began with little fanfare in
1968 with simultaneous reports of new Cr-NHC and Hg-NHC
complexes by Öfele [2] and Wanzlick and Schonherr [3], respec-
tively. In 1991, Arduengo et al successfully isolated a free NHC [4],
but by that time the coordination chemistry of NHCs was already
well-developed. Lappert et al. exploited electron rich olefins (ero),
which act as masked NHCs, and significantly advanced the syn-
thetic methodologies for building complexes [5]. Key work by
Herrmann and co-workers has led to the widespread use of NHC
complexes in catalysis [6]. Different types of NHCs derived from
imidazole [7], benzimidazole [8], imidazolidine [9,10], and triazole
[11] have been reported during the last decade. Studies involving
NHCs are, in general, dominated by NHCs based on the imidazole
ring [12,13]. Benzimidazolin-2-ylidenes have received considerably
less interest, though in recent years this area has been developed by
Hahn and co-workers [14e16]. The benzimidazole skeleton is found
in a variety of natural products. Dimethylbenzimidazole, for
example, is a key component in vitamin B12 and its derivatives.
precursor for NHCs [18,19]. In addition, NHC ligands can be easily
derived by changing the substituents on nitrogen atoms and the
backbone of carbenes, which provides various ligands for metal-
organic materials. NHCs resemble the widely used donor phos-
phine ligands [20e22]. Binding a transition metal to the carbene
carbon atom of the NHC ligand leads to the formation of a very
strong metal-carbon bond, which endows the NHC metal com-
plexes to possess higher stability toward heat, moisture and oxy-
gen. So far, a variety of NHC transition metal complexes have been
synthesized, and some of them have been applied to a broad
spectrum of catalytic reactions including cross couplings, olefin
metathesis and hydroformylation of olefins [23e26].
The transition metal-catalyzed addition of aryl groups to a car-
bon-heteroatom double bond remains a relatively undeveloped
process compared to the related addition to carbonecarbon double
bonds [27]. In the past decades, addition of carbonemetal species to
carbon-heteroatom multiple bonds, such as the carbonyl, imino, and
nitrile groups, is an important reaction for organic synthesis, and the
addition products are important precursors for the synthesis of
biologically active compounds [28,29]. In recent years, the transition
metal catalyzed addition reactions provide for the synthesis of aryl
ketones from nitriles. Larock and Zhou indicated synthesis of aryl
ketones by the palladium catalyzed CeH activation of arenes and
intermolecular carbopalladation of nitriles [30]. Later, Lu and Zhao
* Corresponding author.
_
E-mail address: ismail.ozdemir@inonu.edu.tr (I. Özdemir).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.01.025

22
S. Demir et al. / Journal of Organometallic Chemistry 732 (2013) 21e26
developed a cationic palladium catalyzed addition of arylboronic
acids to nitriles [31]. In 2010, Nickel catalyzed addition of arylbor-
onic acids to nitriles has reported by Cheng et al. [32]. Recent pub-
lications describing the rhodium catalyzed addition of organoboron
reagents to aldehydes have attracted many chemists’ interest [33e
38]. In 2001 Fürstner and Krause developed an efficient protocol
to perform the arylation of aldehydes using catalytic systems based
on N-heterocyclic ligands and rhodium complexes [39]. The
rhodium-catalyzed nucleophilic addition reactions of organoboron
and -stannane reagents to carbon-heteroatom multiple bonds are
now recognized to be highly useful tools for CeC bond formation
[40e43]. Recently, arylboron compounds including arylboronic
acids and tetraaryl borates have proved to be effective arylating
reagents in rhodium catalysis. The treatment of benzonitrile with
sodium tetraphenylborate was reported by Miura et al. [36]. In this
type reaction, the use of NHC as a ligand are not specified to the
present. We previously reported catalytic reactions that hydro-
silylation and the addition to aldehyde of phenylboronic acid in the
presence of a catalytic amount of rhodium NHC complex [44,45]. In
this letter, we report the first use of Rh-NHC complex as a catalyst
for the efficient addition of the sodium tetraphenylborate to the Ce
N triple bond to produce diaryl ketones. For this purpose, our team
has focused on the synthesis, characterization of a series benzimi-
dazolium salts as NHC precursor and their rhodium complexes.
2. Results and discussion
2.1. Synthesis and characterization
Benzimidazolium salts (1) as NHC precursors were readily
accessible by reaction of alkyl halides with 1-alkyl benzimidazole
derivatives in heating DMF (Scheme 1-Path A). These salts were
characterized by 1H NMR, 13C NMR, IR, elemental analysis tech-
niques which support the proposed structures. All compounds
showed good solubility in water and common organic solvents,
such as dichloromethane, chloroform, methanol, acetonitrile, and
N,N-dimethylformamide. The 1H NMR spectra of the benzimida-
zolium salts (1aee) in CDCl3 exhibit as a singlet in the range
Scheme 1. Synthesis of rhodium NHC complexes.

S. Demir et al. / Journal of Organometallic Chemistry 732 (2013) 21e26
23
THF to give the RhCl(COD)(NHC) complexes as crystalline solids in
59e82% yields.
New Rh complexes which are very stable in the solid state have
been characterized by analytical and spectroscopic techniques. The
complexes appear to be spectroscopically pure, and exhibit signals
slightly upfield in comparison with the parent carbene precursors
(1); as expected, the C2eH signal is absent. 13C chemical shifts,
which provide a useful diagnostic tool for metal carbene com-
plexes, show that Ccarbene is substantially deshielded. The signal for
the carbene carbon of Rh-NHC complexes was doubled with peaks
Scheme 2. The reaction of sodium tetraphenylborate with benzonitrile.
d
10.44e11.93 ppm, characteristic of the NCHN benzimidazolium
proton. The chemical shift of the C2 carbon and IR spectra showed
strong CeN bands are in agreement with data reported for other
benzimidazolium salts [46e49].
observed in the
d
196.1e198.0 ppm range (JRh-Ccarbene ¼ 50.0e
To form carbene rhodium (COD) complexes three different syn-
thesis routes are well established [50]. First the reaction of a free
carbene with the dimeric precursor [Rh(COD)Cl]2. This method is
used for both imidazolium/imidazolidinium and benzimidazolium
groups to give carbene rhodium complexes by cleaving the chloro
bridge of the dimeric COD complexes during the reaction [51e55].
Another method for prepare carbene complexes of rhodium is in situ
deprotonation of the azolium salts [56e59]. Substituting the halide
bridge intheprecursordimerbyanalkoxybridge, this‘‘internalbase”
deprotonates the azolium salt in situ, leading to the desired com-
plexes [60]. The third preparation pathway for carbene substituted
COD complexes is transmetallation from the silver intermediate [61].
We chose the second preparation pathway that we obtained excel-
lent yieldsby in situ deprotonationwith [Rh(OMe)COD]2in this work.
(Scheme 1-Path B) The reaction of benzimidazolium salts with the
binuclear [Rh(OMe)COD]2 complex proceeded smoothly in refluxing
51.1 Hz), upfield relative to the free carbene and with the charac-
teristic doublet from coupling to Rh. Coupling constants J(103Rhe
13C) for the new rhodium complexes (2aee) are comparable with
those found for carbene rhodium(I) complexes [62]. In 2012,
Herrmann et al. reported a water soluble carbene complexes of Rh,
Ir and Ru from benzimidazolin-2-ylidene ligand [63]. Rhodium
complex exhibits peak in
d 195.5 ppm, which was reported in the
same paper. These new complexes show typical spectroscopic
signatures which are in line with those recently reported for other
RhCl(COD)(1,3-dialkylbenzimidazole-2-ylidene) complexes.
2.2. Catalytic application of Rh-NHC complexes
In 2006 Miura et al. developed an efficient protocol to perform
the arylation of benzonitrile using rhodium complexes. They first
Table 1
Rhodium-NHC catalyzed reaction of sodium tetraphenylborate with nitriles.
Entry
1
ArCN
Product
Catalyst
2a
Yield %a
95
2
3
4
5
2b
2c
2d
2e
97
94
97
96
6
2a
96
7
8
9
10
2b
2c
2d
2e
98
93
97
98
11
2a
83
12
13
14
15
2b
2c
2d
2e
83
72
81
78
16
2a
93
17
18
19
20
2b
2c
2d
2e
86
84
89
96
a
Reaction conditions: benzonitrile (2 mmol), sodium tetraphenylborate (0.5 mmol), [RhClCOD(NHC)] (0.005 mmol), o-xylene/H2O (9/1.5 mL), 120 ꢀC, 1 h. isolated yield
(purity of yield checked by NMR and GC and GCeMS). Yields are based on sodium tetraphenylborate.

24
S. Demir et al. / Journal of Organometallic Chemistry 732 (2013) 21e26
indicated the reaction using phenylboronic acid, potassium phe-
nyl(trifluoro)borate and sodium tetraphenylborate as phenylating
reagents in the presence of acetic anhydride [64]. Miura has
reported that the treatment of sodium tetraphenylborate with
benzonitrile in the presence of [RhCl(cod)]2 edppp catalytic system
(cod ¼ 1,5-cyclooctadiene, and dppp (1,3-bis(diphenylphosphino)
propane, in o-xylene at 120 ꢀC for 44 h under nitrogen gave ben-
zophenone imine in 21% yield. The addition of H2O (0.5 mL) com-
pletely suppressed the ortho-phenylation and significantly
promoted the reaction to afford benzophenone imine in 91% yield
within 1 h. Under the conditions with H2O, part of benzophenone
imine was hydrolyzed to benzophenone. The omission of dppp or
the use of PPh3 in place of it resulted in low efficiency of the phe-
nylation. These results have shown that ligand is important this
catalytic system. N-heterocyclic carbenes have emerged as an
extremely useful class of ligands for use in transition-metal com-
plexes. It is well recognized that replacement of phosphines by
NHCs can provide complexes with enhanced stability and catalytic
performance due to the high electron-donor ability. With the
introduction of N-heterocyclic carbenes novel class of ligands have
been developed that display similar binding properties than
Table 1). Under the same reaction conditions, Miura investigated
treatment of 4-methylbenzonitrile with sodium tetraphenylborate
to give the related ketone in 98% yield. We have obtained same
conversion for 4-methylbenzonitrile while achieving better results
for 4-chlorobenzonitrile [36]. We have found that arylboron com-
pound can be additioned efficiently with various nitriles in the
presence of an appropriate rhodium-NHC based catalyst. Corre-
sponding ketone was observed in o-xylene/water reaction mixture.
The good yields were observed in the reaction using all Rh-NHC
complexes which has substituted phenyl groups on N-atoms.
These results imply that Rh complex with benzimidazolin-2-
ylidene ligand shows the excellent catalytic activity.
Many transition-metal complexes catalyze the addition re-
actions of main metal reagents, but their mechanism has not yet
been studied in detail. However, the transmetalation between the
main metal reagents and transition-metal complexes has been
proposed as a key step of the catalytic cycle. A plausible mechanism
for the reaction of nitrile with borate which reported by Miura has
shown in Scheme 3 [36].
3. Conclusion
phosphines. They act also as very strong s-donors and very poor p-
acceptors but form at the same time much more stable metale
NHC-bonds in many cases. Two nitrogen atoms within the het-
erocycle stabilize the carbene carbon through inductive and
In summary, a series of NHC precursors and their rhodium
complexes have been synthesized and characterized. All complexes
are stable toward light and air in the both solid state and solution.
We have developed a first Rh-NHC catalyst for the addition reaction
of sodium tetraphenylborate to nitriles. Further studies on this
chemistry and its application of rhodium catalyzed addition pro-
tocol are currently underway.
mesomeric effects, allocating strong
s-donation to a metal ion. As
a
result, M-NHC complexes are often thermally robust and
electron-rich, ideal for catalytic application. So far, NHC ligand has
not used in the above mentioned reaction, therefore we firstly used
Rh-NHC catalyst at the arylation of benzonitrile with sodium tet-
raphenylborate (Scheme 2).
4. Experimental
In an initial attempt, benzonitrile (2 mmol) was treated with
sodium tetraphenylborate (0.5 mmol) in the presence of
[RhClCOD(NHC)] (0.005 mmol) in o-xylene/H2O at 120 ꢀC for 1 h
under nitrogen. All the four aryl groups of the NaBPh4 were added
to the nitrile. As a result, benzophenone was formed in good yields
for all Rh complexes (Entries 1e5 in Table 1). The reaction using
other nitriles with sodium tetraphenylborate as phenyl source was
next examined. 4-Methylbenzonitrile under went phenylation to
give the corresponding ketone in 84e96% isolated yield (Entries 6e
10 in Table 1). The reaction of 4-chlorobenzonitrile also proceeded
efficiently (Entries 11e15 in Table 1). The reactions of 4-pyridine
carbonitrile with sodium tetraphenylborate also proceeded
smoothly o-xylene/H2O at 120 ꢀC for 1 h, affording the corre-
sponding ketone in good isolated yields (93e98%; Entries 16e20 in
4.1. General procedures
All reactions for the preparation of benzimidazolium salts and
their rhodium complexes were carried out under argon in flame-
dried glassware using standard Schlenk techniques. Chemicals
were obtained from Sigma Aldrich and Fluka. [Rh(OMe)(cod)]2 was
prepared according to the published method [65]. Melting points
were determined in glass capillaries under air with an
Electrothermal-9200 melting point apparatus. FT-IR spectra were
recorded as KBr pellets in the range 400e4000 cmꢁ1 on a Perkin
Elmer Spectrum 100. 1H- and 13C NMR spectra were recorded with
a Varian AS 400 Merkur spectrometer operating at 400 MHz (1H),
100 MHz (13C) in CDCl3 with tetramethylsilane as an internal ref-
erence. Coupling constants (J values) are given in hertz. NMR
multiplicities are abbreviated as follows: s ¼ singlet, d ¼ doublet,
hept ¼ heptet and m ¼ multiplet signal. All catalytic reactions were
monitored on an Agilent 6890N GC system by GC-FID with an HP-5
column of 30 m length, 0.32 mm diameter and 0.25
mm film
thickness. Column chromatography was performed using silica gel
60 (70e230 mesh). Solvent ratios are given as v/v.
4.2. Synthesis and characterization of benzimidazolium salts
Benzimidazolium salt (1a) was prepared according to the liter-
ature [46]. Other N-heterocyclic carbene precursors (1be1e) were
prepared according to general pathway depicted in Scheme 1. 1-
alkyl benzimidazole (1.0 mol) was treated with an alkyl halide
(1.0 mol) in DMF at 80 ꢀC. Upon cooling to room temperature,
a colorless solid precipitated when was added diethylether. Crude
product was filtered, dried under vacuum and then was recrystal-
lized from absolute ethanol to give colorless needles, and the solid
was washed with diethylether (2 ꢂ 10 mL) and dried under vacuum
led to the formation of the corresponding benzimidazolium salts.
Scheme 3. The mechanism proposed by Miura.

S. Demir et al. / Journal of Organometallic Chemistry 732 (2013) 21e26
25
4.2.1. 1,3-Bis-(2,4,6-trimethylbenzyl)benzimidazolium chloride (1b)
Yield: 3.85 g (92%). M.p.: 200e201 ꢀC. FT-IR y(CN): 1592 cmꢁ1
Anal. Calc. for C27H31N2Cl: C, 77.40; H, 7.46; N, 6.69. Found: C, 77.42;
H, 7.45; N: 6.70%. 1H NMR (399.9 MHz, CDCl3)
(ppm): 2.03 and
4.3.1. 1,3-Bis[(3,4,5-trimethoxybenzyl)]-benzimidazolin-2-ylidene-
[(1,2,5,6-
4)-1,5-cyclooctadiene]chlororhodium(I)(2a)
Yield: 0.48 g (67%). M.p.: 215e216 ꢀC. FT-IR y(CN): 1592 cmꢁ1
Anal. Calc. for C35H42N2O6RhCl: C, 57.98; H, 5.84; N, 3.86. Found: C,
57.94; H, 5.86; N, 3.82%. 1H NMR (399.9 MHz, CDCl3)
(ppm): 1.96e
.
h
.
d
2.00 (s, 18H, CH2C6H2(CH3)3-2,4,6), 5.64 (s, 4H, CH2C6H2(CH3)3-
2,4,6), 6.64 (s, 4H, CH2C6H2(CH3)3-2,4,6), 7.16 (s, 4H, NC6H4N), 10.59
d
1.99 (m, 4H, CH2COD), 2.36e2.39 (m, 4H, CH2COD), 3.37 (m, 2H,
CHCOD), 3.79 and 3.81 (s, 18H, CH2C6H2(OCH3)3-3,4,5), 5.18 (m, 2H,
CHCOD), 5.58 (d, J ¼ 14.8 Hz, 2H, CH2C6H2(OCH3)3-3,4,5), 6.73 (d,
(s, 1H, NCHN). 13C NMR (100.5 MHz, CDCl3)
d (ppm): 20.0 and 20.9
(CH2C6H2(CH3)3-2,4,6), 47.2 (CH2C6H2(CH3)3-2,4,6), 125.1, 129.9,
137.6 and 139.4 (CH2C6H2(CH3)3-2,4,6), 113.7, 127.1 and 131.4
(NC6H4N), 142.8 (NCHN).
J
¼
15.2 Hz, 2H, CH2C6H2(OCH3)3-3,4,5), 6.86 (s, 4H,
CH2C6H2(OCH3)3-3,4,5), 7.05 (m, 4H, NC6H4N). 13C NMR (100.5 MHz,
CDCl3) (ppm): 28.9 and 33.1(CH2COD), 53.6 and 53.7
d
4.2.2. 1-(2,4,6-Trimethylbenzyl)-3-(4-ter-butylbenzyl)
benzimidazolium bromide (1c)
(CH2C6H2(OCH3)3-3,4,5), 56.8 and 61.0 (CH2C6H2(OCH3)3-3,4,5),
69.3 and 100.9 (d, J ¼ 13.7 Hz and J ¼ 6.1 Hz, CHCOD), 105.6, 111.2,
122.7, 131.6, 135.1, 138.1 and 153.9 (CH2C6H2(OCH3)3-3,4,5 and
NC6H4N), 197.4 (d, J ¼ 51.1 Hz, Rh-Ccarbene).
Yield: 4.23 g (89%). M.p.: 219e220 ꢀC. FT-IR y(CN): 1601 cmꢁ1. Anal.
Calc. for C28H33N2Br: C, 70.43; H, 6.97; N, 5.87. Found: C, 70.45; H,
6.94; N: 5.88. 1H NMR (399.9 MHz, CDCl3)
d (ppm): 1.24 (s, 9H,
CH2C6H4C(CH3)3-p), 2.27 and 2.30 (s, 9H, CH2C6H2(CH3)3-2,4,6), 5.85
(s, 4H, CH2C6H2(CH3)3-2,4,6 and CH2C6H4C(CH3)3-p), 6.91 (s, 2H,
CH2C6H2(CH3)3-2,4,6), 7.36 and 7.41 (m, 8H, NC6H4N and
CH2C6H4C(CH3)3-p),11.21 (s,1H, NCHN).13C NMR (100.5 MHz, CDCl3)
4.3.2. 1,3-Bis[(2,4,6-trimethylbenzyl)]-benzimidazolin-2-ylidene-
[(1,2,5,6-
Yield: 0.47 g (75%). M.p.: 184e185 ꢀC. FT-IR y(CN): 1681 cmꢁ1
Anal. Calc. for C35H42N2RhCl: C, 66.82; H, 6.73; N, 4.45. Found: C,
66.85; H, 6.76; N, 4.42%. 1H NMR (399.9 MHz, CDCl3)
(ppm): 1.96e
h
4)-1,5-cyclooctadiene]chlororhodium(I)(2b)
.
d
(ppm): 20.7 and 21.5 (CH2C6H2(CH3)3-2,4,6), 31.6
(
d
CH2C6H4C(CH3)3-p), 35.0 (CH2C6H4C(CH3)3-p), 47.8 (CH2C6H2(CH3)3-
2,4,6), 51.5 ( CH2C6H4C(CH3)3-p), 125.4, 130.2, 138.3 and 140.2
(CH2C6H2(CH3)3-2,4,6), 114.2, 114.3, 126.6, 127.5, 127.6, 128.3, 130.6,
130.8, 131.9, and 152.7 (CH2C6H4C(CH3)3-p and NC6H4N), 143.1
(NCHN).
2.02 (m, 4H, CH2COD), 2.31 and 2.32 (s, 18H, CH2C6H2(CH3)3-2,4,6),
2.42e2.46 (m, 4H, CH2COD), 3.51 (m, 2H, CHCOD), 5.18 (m, 2H,
CHCOD), 5.95 (d, J ¼ 14.8 Hz, 2H, CH2C6H2(CH3)3-2,4,6), 6.54 (d,
J ¼ 15.2 Hz, 2H, CH2C6H2(CH3)3-2,4,6), 6.28e6.31 (m, 2H, NC6H4N),
6.72e6.75 (m, 2H, NC6H4N), 6.90 (s, 4H, CH2C6H2(CH3)3-2,4,6). 13
C
NMR (100.5 MHz, CDCl3)
d (ppm): 21.0 and 21.2 (CH2C6H2(CH3)3-
4.2.3. 1-Izo propyl-3-(3,4,5-trimethoxybenzyl)benzimidazolium
2,4,6), 28.9 and 33.1(CH2COD), 50.3 (CH2C6H2(CH3)3-2,4,6), 69.1 and
100.1 (d, J ¼ 14.5 Hz and J ¼ 6.8 Hz, CHCOD), 110.8, 122.1, 128.4,
129.9, 135.4, 138.4 and 138.6 (CH2C6H2(CH3)3-2,4,6 and NC6H4N),
197.7 (d, J ¼ 50.4 Hz, Rh-Ccarbene).
chloride (1d)
Yield: 2.82 g (75%). M.p.: 198e199 ꢀC. FT-IR y(CN): 1568 cmꢁ1
.
Anal. Calc. for C20H25N2O3Cl: C, 63.74; H, 6.69; N, 7.43. Found: C,
63.76; H, 6.65; N, 7.42%. 1H NMR (399.9 MHz, CDCl3)
d
(ppm): 1.83
(d, J ¼ 6.4 Hz, 6H, CH(CH3)2), 4.97 (hept., J ¼ 6.4 Hz, 1H, CH(CH3)2),
3.79 and 3.81 (s, 9H, CH2C6H2(OCH3)3-3,4,5), 5.84 (s, 2H,
CH2C6H2(OCH3)3-3,4,5), 6.94 (s, 2H, CH2C6H2(OCH3)3-3,4,5), 7.59e
7.60 and 7.71-7.73 (m, 4H, NC6H4N), 11.93 (NCHN). 13C NMR
4.3.3. [1-(2,4,6-Trimethylbenzyl)-3-(4-ter-butylbenzyl)]-
benzimidazolin-2-ylidene-[(1,2,5,6-h
4)-1,5-cyclooctadiene]
chlororhodium(I)(2c)
Yield: 0.38 g (59%). M.p.: 171e172 ꢀC. FT-IR y(CN): 1471 cmꢁ1
.
(100.5 MHz, CDCl3)
d
(ppm): 22.5 (CH(CH3)2), 51.7
Anal. Calc. for C36H44N2RhCl: C, 67.23; H, 6.90; N, 4.36. Found: C,
(CH2C6H2(OCH3)3-3,4,5), 52.1 (CH(CH3)2), 56.9 and 60.9
(CH2C6H2(OCH3)3-3,4,5), 106.6, 113.6, 114.1, 127.1, 127.3, 128.9, 130.9,
131.8, 138.7 and 153.9 (CH2C6H2(OCH3)3-3,4,5 and NC6H4N), 142.5
(NCHN).
67.27; H, 6.93; N, 4.32%. 1H NMR (399.9 MHz, CDCl3)
d
(ppm): 1.31
(s, 9H, CH2C6H4C(CH3)3-4), 1.78e2.05 (m, 4H, CH2COD), 2.18e2.32
(m, 4H, CH2COD), 2.36 (s, 9H, CH2C6H2(CH3)3-2,4,6), 3.42 (m, 2H,
CHCOD), 5.21 (m, 2H, CHCOD), 6.01 (s, 2H, CH2C6H2(CH3)3-2,4,6),
6.28 (s, 2H, CH2C6H4C(CH3)3e4), 6.79e6.92 (m, 3H,
CH2C6H4C(CH3)3-4 and NC6H4N), 6.97 (s, 2H, CH2C6H2(CH3)3-2,4,6),
7.29e7.36 (m, 5H, CH2C6H4C(CH3)3-4 and NC6H4N). 13C NMR
4.2.4. 1-Izo propyl-3-(2,4,6-trimethylbenzyl)benzimidazolium
chloride (1e)
Yield: 2.69 g (82%). M.p.: 216e217 ꢀC. FT-IR y(CN): 1565 cmꢁ1
.
(100.5 MHz, CDCl3)
29.1, 29.4, 32.9 and 33.1(CH2COD), 31.7 (CH2C6H4C(CH3)3e4), 34.9
(CH2C6H4C(CH3)3e4), 50.3 (CH2C6H2(CH3)3-2,4,6), 53.3
d (ppm): 21.4 and 21.5 (CH2C6H2(CH3)3-2,4,6),
Anal. Calc. for C20H25N2Cl: C, 73.04; H, 7.66; N, 8.52. Found: C,
73.07; H, 7.63; N, 8.55%. 1H NMR (399.9 MHz, CDCl3)
d
(ppm): 1.77
(s, 6H, CH(CH3)2), 2.21 and 2.23 (s, 9H, CH2C6H2(CH3)3-2,4,6), 4.99
(hept., J ¼ 6.7 Hz, 1H, CH(CH3)2), 5.96 (s, 2H, CH2C6H2(CH3)3-2,4,6),
6.82 (s, 2H, CH2C6H2(CH3)3-2,4,6), 6.97 (d, J ¼ 8.4 Hz, 1H, NC6H4N),
7.31 (t, J ¼ 7.6 Hz, 1H, NC6H4N), 7.48 (t, J ¼ 7.6 Hz, 1H, NC6H4N), 7.71
(d, J ¼ 8.4 Hz, 1H, NC6H4N), 11.78 (NCHN). 13C NMR (100.5 MHz,
(CH2C6H4C(CH3)3e4), 69.7 and 71.1 (d, J ¼ 14.3 Hz and J ¼ 14.1 Hz,
CHCOD), 99.6 and 99.9 (d, J ¼ 6.5 Hz and J ¼ 6.7 Hz, CHCOD), 111.1,
111.3, 122.3, 122.8, 126.1, 127.1, 128.7, 130.1, 133.6, 135.4, 135.9, 138.8,
139.0 and 151.1 (CH2C6H2(CH3)3-2,4,6, CH2C6H4C(CH3)3-4 and
NC6H4N), 198.0 (d, J ¼ 50.4 Hz, Rh-Ccarbene).
CDCl3)
d (ppm): 20.5 and 21.4 (CH2C6H2(CH3)3-2,4,6), 22.7
(CH(CH3)2), 48.1 (CH2C6H2(CH3)3-2,4,6), 52.1 (CH(CH3)2), 114.5,
125.9, 127.1, 127.4, 130.4, 130.5, 131.2, 131.9, 138.2 and 139.7
(CH2C6H2(CH3)3-2,4,6 and NC6H4N), 143.1 (NCHN).
4.3.4. [1-(3,4,5-Trimethoxybenzyl)-3-(i-propyl)]-benzimidazolin-2-
ylidene-[(1,2,5,6-
Yield: 0.48 g (82%). M.p.: 154e155 ꢀC. FT-IR y(CN): 1600 cmꢁ1
Anal. Calc. for C28H36N2O 3RhCl: C, 57.30; H, 6.18; N, 4.77. Found: C,
57.27; H, 6.15; N, 4.82%. 1H NMR (399.9 MHz, CDCl3)
(ppm): 1.75
h
4)-1,5-cyclooctadiene]chlororhodium(I)(2d)
.
4.3. Synthesis and characterization of rhodium NHC complexes
d
and 1.84 (d, J ¼ 7.1 Hz, 6H, CH(CH3)2), 1.87e2.13 (m, 4H, CH2COD),
2.22e2.59 (m, 4H, CH2COD), 3.28 (m, 2H, CHCOD), 3.54 (hept.,
J ¼ 7.1 Hz, 1H, CH(CH3)2), 3.82 and 3.83 (s, 9H, CH2C6H2(OCH3)3-
To a solution of benzimidazolium salts (10 mmol) in THF (15 mL)
was added rhodium dimer [Rh(OMe)COD]2 (5 mmol) and the
resulting mixture was stirred at reflux for 5 h. After removal of the
solvent, the residue was washed twice with diethylether (5 mL) and
dried under vacuum. The crude product was recrystallized from
CH2Cl2/Et2O.
3,4,5), 5.15 (m, 2H, CHCOD), 5.58 (d,
J
¼
15.0 Hz, 2H,
CH2C6H2(OCH3)3-3,4,5), 6.79 (s, 2H, CH2C6H2(OCH3)3-3,4,5), 6.59e
7.53 (m, 4H, NC6H4N). 13C NMR (100.5 MHz, CDCl3)
(ppm): 28.6
and 32.6 (CH2COD), 29.9 and 34.0 (CH(CH3)2), 53.5
d

26
S. Demir et al. / Journal of Organometallic Chemistry 732 (2013) 21e26
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(CH2C6H2(OCH3)3-3,4,5), 54.7 (CH(CH3)2), 57.1 and 61.3
(CH2C6H2(OCH3)3-3,4,5), 68.8 and 69.2 (d, J ¼ 14.4 Hz, CHCOD),
100.1 and 100.7 (d, J ¼ 6.65 Hz, CHCOD), 105.5, 111.7, 112.2, 122.5,
122.6, 132.0, 133.4, 135.9, 138.1 and 153.9 (CH2C6H2(OCH3)3-3,4,5
and NC6H4N), 196.1 (d, J ¼ 50.45 Hz, Rh-Ccarbene).
4.3.5. [1-(2,4,6-Trimethylbenzyl)-3-(i-propyl)]-benzimidazolin-2-
ylidene-[(1,2,5,6-
Yield: 0.42 g (78%). M.p.: 231e232 ꢀC. FT-IR y(CN): 1477 cmꢁ1
Anal. Calc. for C28H36N2RhCl: C, 62.40; H, 6.73; N, 5.20. Found: C,
62.44; H, 6.76; N, 5.22%. 1H NMR (399.9 MHz, CDCl3)
(ppm): 1.75
h
4)-1,5-cyclooctadiene]chlororhodium(I)(2e)
.
d
and 1.84 (d, J ¼ 7.1 Hz, 6H, CH(CH3)2), 2.05e2.11 (m, 4H, CH2COD),
2.31 and 2.34 (s, 9H, CH2C6H2(CH3)3-2,4,6), 2.48e2.55 (m, 4H,
CH2COD), 3.47 (m, 2H, CHCOD), 3.52 (hept., J ¼ 7.1 Hz,1H, CH(CH3)2),
5.11 (m, 2H, CHCOD), 5.89 (d, J ¼ 14.9 Hz, 2H, CH2C6H2(CH3)3-2,4,6),
6.91 (s, 2H, CH2C6H2(CH3)3-2,4,6), 6.54e7.15 (m, 4H, NC6H4N). 13C
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d (ppm): 21.4 and 21.9 (CH2C6H2(CH3)3-
2,4,6), 28.3 and 32.3 (CH2COD), 30.0 and 34.3 (CH(CH3)2), 49.9
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4.4. General procedure for the rhodium-catalyzed reaction of
sodium tetraphenylborate with nitrile
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(2 mmol), Rh-NHC complex (0.005 mmol) was stirred in o-xylene/
H2O (9/1.5 mL) at 120 ꢀC under N2 for 1 h. After cooling, the reaction
mixture was extracted with Et2O and dried over anhydrous MgSO4
and filtered. Solvent was removed under reduced pressure and
concentrated in vacuum to give the crude product. The crude
product was purified by column chromatography on silica gel (60e
120 mesh) using ethyl acetate/n-hexane as eluent to afford the
ketone. The purity of the compounds was checked by NMR and
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This work was financially supported by Inönü University
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