Air-stable Ruthenium(II)-NNN Pincer Complexes for the Efficient Coupling of Aromatic Diamines and Alcohols to 1H-benzo[d]imidazoles with the Liberation of H2

Article abstract of DOI:10.1002/cctc.201800017

Two new phosphine-free Ru^II-NNN pincer complexes ([RuCl(L1)(CH₃CN)₂]Cl (1) and [RuCl(L2)(CH₃CN)₂]Cl (2) [L1=2,6-bis(1H-imidazole-2-yl)pyridine, L2=2,6-bis(1-hexyl-1H-imidazole-2-yl)pyridine] were synthesized to catalyze the condensation of benzyl alcohol and benzene-1,2-diamine homogeneously to 2-pheny-1H-benzo[d]imidazole and H₂. The reactivity in the order of 1>2 is lower than that of the phosphine-containing Ru^II-NNN pincer complex [RuCl₂(L1)(PPh₃)₃] (3), and thus a homogeneous system that contained 1, 1 equivalent of 1,2-bis(diphenylphosphanyl)ethane, and 10 equivalents of NaBPh₄ was developed to improve the catalytic efficiency for the condensation of primary alcohols and benzene-1,2-diamine (or its derivatives) to 2-substituted 1H-benzo[d]imidazoles in excellent yields (up to 97 %) and turnover numbers (388). This system can be used to realize the facile one-step synthesis of 1H-benzo[d]imidazole derivatives from alcohols without the use of an oxidant and/or a stoichiometric amount of inorganic base that is usually necessary in homogeneous systems reported previously.

Full text of DOI:10.1002/cctc.201800017

Accepted Article
Title: Air-stable Ru(II)-NNN Pincer Complexes for Efficient Coupling of
Aromatic Diamines and Alcohols to 1H-benzo[d]imidazoles with
liberation of H2
Authors: Lin Li, Qi Luo, Huahua Cui, Renjie Li, Jing Zhang, and
Tianyou Peng
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10.1002/cctc.201800017
ChemCatChem
FULL PAPER
Air-stable Ru(II)-NNN Pincer Complexes for Efficient Coupling of
Aromatic Diamines and Alcohols to 1H-benzo[d]imidazoles with
liberation of H2
Lin Li, Qi Luo, Huahua Cui, Renjie Li, Jing Zhang* and Tianyou Peng*
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China
Abstract: Two new phosphine-free Ru(II)-NNN pincer complexes
([RuCl(L1)(CH3CN)2]Cl (1) and [RuCl(L2)(CH3CN)2]Cl (2), L1 = 2,6-
bis(1H-imidazole-2-yl)pyridine, L2 = 2,6-bis(1-hexyl-1H-imidazole-2-
yl)pyridine) were synthesized for homogeneously catalyzing the
condensation of benzyl alcohol and benzene-1,2-diamine to 2-
pheny-1H-benzo[d]imidazole and H2. It was found that the reactivity
with an order of 1 > 2 is lower than that of the phosphine-containing
Ru(II)-NNN pincer complex [RuCl2(L1)(PPh3)3] (3), and thus a
homogeneous system containing complex 1, 1 equiv. of 1,2-
bis(diphenyl-phosphanyl)ethane (dppe), and 10 equiv. of NaBPh4
was developed to improve the catalytic efficiency for the
condensation of primary alcohols and benzene-1,2-diamine (or its
derivatives) to 2-substituted 1H-benzo[d]imidazoles in excellent
yields (up to 97%) and turnover number (TONs ~ 388). The present
system realizes facile one-step synthesis of 1H-benzo[d]imidazole
derivatives from alcohols without using the oxidant and/or the
stoichiometric amount of inorganic bases that is usually necessary in
homogeneous systems reported previously.
carbene ligand, it is worthy to develop new pincer system
containing alternative donors such as N or O atoms instead.
1H-benzo[d]imidazole and its derivatives are essential
components of biologically active molecules, which have led to
extensive applications in pharmaceutical,[6] agrochemicals,[7] and
functional materials (Scheme 1),[8] and therefore much attention
has been devoted to the synthesis of 1H-benzo[d]imidazole
derivatives using various protocols. Among which, one-pot
synthesis of 1H-benzo[d]imidazole derivatives through the ADC
process of alcohols and aromatic diamines by using well-defined
transition metal complexes as catalyst is much attractive since
its by-products are water and H2 that can be used elsewhere.[9]
Although several heterogeneous systems developed are
capable of this conversion,[10] only one homogeneous system
was reported by Kepe and co-workers in 2014,[9a] in which a Ir(I)
pincer complex [Ir(cod)2,6-DiAmPy(iPr)2] achieved the catalytic
acceptorless condensation of primary alcohols and benzene-1,2-
diamine to 2-functionalized 1H-benzo[d]imidazole derivatives
with yields of 56-96% in the presence of 1.4 mol% catalyst and 1
equiv. of KOH (relative to the substrate).[9a]
Based on our previous reports on Ru(II) and Ni(II) complexes
bearing pyridine-based NNN pincer ligands, which show high
reactivity for acceptorless dehydrogenation of primary alcohols
to carboxylic acid and hydrogenation of CO2 to formic acid in the
Introduction
Acceptorless dehydrogenative coupling (ADC) of alcohols with
amines, as an atom-economical and environmentally friendly
method for the construction of new C-N bond, has recently
emerged as an important process to afford many useful organic
compounds such as imines,[1] amides,[2] amines,[3] or nitrogen-
containing heterocyclic compounds.[4] Pioneered by Milstein and
co-workers, who reported a Ru-PNN complex as an efficient
catalyst for coupling primary alcohols and amines to amides,[2d,5]
several precious metal pincer complexes have been developed
for these conversion, in which the pincer ligand is usually
constructed by at least one strong electron-rich donor (such as
phosphine or N-heterocyclic carbene moiety).[5a-c] From the view
of normally relative comprehensive synthetic process and the
air- or moisture-sensitive nature of phosphine or N-heterocycle
homogeneous system,[11,12] here we illustrate
a
new
homogeneous system consisting of Ru(II)-NNN (NNN = 2,6-
bis(1H-imidazol-2-yl)pyridine) and 1,2-bis(diphenylphosphanyl)
ethane (dppe) to efficiently catalyze the condensation of various
primary alcohols and benzene-1,2-diamine to 2-substituted 1H-
benzo[d]imidazole derivatives with excellent yields (up to 97%)
and high turnover number (TON) of ~388. This system just need
2.5 mol% NaBPh4 (relative to the substrate) as an additive, and
avoids the utilization of the oxidant and/or the stoichiometric
Scheme 1. Examples of important molecules containing 1H-benzo[d]imidazole
moiety.
[a]
L. Li, Q. Luo, H. Cui, Prof. R. J. Li, Prof. J. Zhang, Prof. T. Y. Peng
College of Chemistry & Molecular Sciences
Wuhan University
Wuhan 430072, P. R. China
E-mail: jzhang03@whu.edu.cn (J. Zhang); typeng@whu.edu.cn (T.
Peng)
Supporting information for this article is given via a link at the end of the
document.
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10.1002/cctc.201800017
ChemCatChem
FULL PAPER
Scheme 2. Representative examples for 1H-benzo[d]imidazole synthesis from
alcohol.
Figure S3). This observation is consistent with the two
acetonitrile ligands coordinated to the Ru(II) center in axial and
meridional position,[16] respectively and similar to that observed
in our previous reported complexes.[11b,12]
1
The H NMR spectrum of complex 2 exhibits one triplet peak
with integration of four protons at 4.61 ppm, which is assigned to
the N-CH2- groups of L2, representing a downfield of 0.19 ppm
compared to that of the free ligand L2 (Supporting Information,
Figure S5). There also exist two singlet peaks at 2.16 and 2.87
ppm assigned to the two coordinated acetonitrile ligands,
indicating complexes 1 and 2 have the same coordination
geometry around the Ru(II) center. When ligand L1 and 1 equiv.
of [RuCl2(PPh3)3] were refluxed in anhydrous ethanol for 6 h
under N2 flow, complex [RuCl(L1)(PPh3)2]Cl (3) was obtained as
a yellow solid in 86% yield (Scheme 3). Due to its very poor
solubility in normal organic solvents (CHCl3, CH2Cl2, THF,
1
toluene, acetone, DMSO, etc.), there is no HNMR or 13C NMR
spectra of complex 3 was recorded. The FT-IR spectrum
(Supporting Information, Figure S11) of 3 shows a strong peak
at 3059 cm-1 assigned to the imidazolyl NH of L1, while the ESI-
MS exhibits the isotopic peaks (M/z) at 871.8 and 610.0 which
can be assigned to [RuCl(L1)(PPh3)2]+ and [RuCl(L1)(PPh3)]+,
respectively (Supporting Information, Figure S12). The
elemental analysis result (ref. Experimental Section) of complex
3 also confirm its structure.
Catalytic condensation of alcohol and benzene-1,2-diamine
to 1H-benzo[d]imidazole derivatives and H2.
amount of strong base (KOtBu or KOH) that is normally
necessary in the previously reported homogeneous systems as
shown in Scheme 2.[13,14]
In the preliminary studies, benzyl alcohol and benzene-1,2-
diamine were selected as the substrates. A typical experiment
was carried out by using complex 1 (6.25 μmol) and benzene-
1,2-diamine (2.5 mmol) with additive (62.5 μmol) in an excess
amount of benzyl alcohol (7.5 mmol) at 165°C (oil bath) under
nitrogen atmosphere for 24 h. When using the neutral salt
NaBPh4 as an additive, 2-phenyl-1H-benzo[d]imidazole can be
produced in a yield of 31%, higher than that using strong bases
(KOtBu, KOH, Cs2CO3) as an additive (Table 1, entries 1-4). The
unreacted starting materials can be recovered (for example,
65% for benzene-1,2-diamine).
Results and Discussion
Synthesis of 2,6-bis(1-hexyl-1H-imidazol-2-yl)pyridine (L2)
and Ru(II) complexes 1~3.
The pincer ligand 2,6-bis(1H-imidazol-2-yl)pyridine (L1) was
prepared from pyridine-2,6-dicarbonitrile according to the
reported procedure.[15] By refluxing L1 and 2 equiv. of 1-
bromohexane in acetone for 24 h in the presence of 2 equiv. of
KOtBu, 2,6-bis(1-hexyl-1H-imidazol-2-yl)pyridine (L2) was
obtained in good yield as a yellow oil (80%). There is no signal
observed in the region of 9~14 ppm in 1H NMR, indicating the
two imidazolyl groups of L2 are alkylated (Supporting
Information, Figure S1). While a triplet peak at 4.42 ppm with
integration of four protons can be assigned to the two N-CH2-
C5H11 groups of L2. Upon refluxing the acetonitrile solution
containing L1 or L2 with 0.5 equiv. of [RuCl2(p-cymene)]2 for 12
Scheme 3. Synthesis of complexes 1-3.
h
under
N2
flow,
[RuCl(L1)(CH3CN)2]Cl
(1)
and
{RuCl(L2)(CH3CN)2}Cl (2) can be obtained in 88% and 85%
(Scheme 3), respectively. The 1H NMR spectrum of complex 1
shows two singlet peaks with integration of three protons each at
2.19 and 2.89 ppm, respectively, which are assigned to the CH3
groups of two acetonitrile ligands (Supporting Information,
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Table 1. Optimization of Catalytic Condition for the 2-phenyl-1H-
benzo[d]imidazole Synthesis.[a]
PPh3), indicating that the coordination of phosphorus donor to
the Ru center increases the catalytic reactivity (Table 1, entry
11).
It
is
noted
that
the
use
of
1,2-
bis(diphenylphosphanyl)ethane (dppe,
1
equiv. relative to
complex 1) afforded the excellent yield (92%) in a shorter time (6
h), which was also confirmed by the hydrogen production (87%
in 4 h) (Table 1, entry 13, and Supporting Information, Table S2
and Figure S13). These results again suggest that the more
electron-rich Ru center (upon chelated by dppe) exhibit a higher
catalytic reactivity. Using the RuCl2(PPh3)3 instead of complex 1
as the catalyst under the same condition, 2-phenyl-1H-
benzo[d]imidazole was isolated in 33% yield accompanied by
the formation of N-benzyl-2-phenyl-1H-benzo[d]imidazole in
37% (Table 1, entry 14), while addition of PPh3 (2 equiv. relative
to Ru(II) precursor) did not improve the yield (Table 1, entries 14
and 15). Although the system containing RuCl2(PPh3)3 /dppe
(1:1 mol) afforded the 2-phenyl-1H-benzo[d]imidazole in good
yield (70%), it is not efficient for conversion of benzene-1,2-
diamine with aliphatic alcohols. For instance, 2-cyclohexyl-1H-
benzo[d]imidazole was obtained in low yield (22%) (Table 2, 3j
and Supporting Information, Table S4), while the diamine was
Entry
1
Catalyst
PPh3 (mol %)
None
None
None
None
0.25
Base
Yield[b]
12
1
KOtBu
2
1
KOH
11
3
1
CeCO3
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
NaBPh4
none
11
4
1
31
5
1
82
6
1
0.50
88(82[c]
78
)
7
1
0.75
8
1
0.50
70[d]
51[e]
48
9
1
0.50
recovered in
~ 70%. These results indicated that the
coordination of NNN pincer ligand to the Ru(II) center led to the
higher yield and higher selectivity for 2-substituted 1H-
benzo[d]imidazole with a broader substrate scope. As expected,
there was almost no product observed without the use of Ru(II)
complex or NaB(Ph)4 (Table 1, entries 16, 17).
10
11
12
13
14
15
16
17
2
0.50
3
None
0.50
84
4[f]
34
1
0.25[g] (dppe)
None
0.50
92[h](87[i])
33[j]
Using the optimized reaction conditions (165°C , 0.25 mol%
complex 1, 0.25 mol% dppe, 2.5 mol% NaBPh4), the substrate
scope of the reaction was then examined, and results are listed
in Table 2. Both aromatic alcohols and aliphatic alcohols can be
dehydrogenative coupled with benzene-1,2-diamine to
corresponding 2-substituted 1H-benzo[d]imidazole in excellent
yields (up to 97%). Aromatic alcohols functionalized by both
electron-donating groups (methyl, methoxy) and weak electron-
withdrawing groups (Cl, Br) afforded the 2-substituted 1H-
benzo[d]imidazoles in 90-96% yields (Table 2, 3d, 3e, 3g, 3i). It
is noted that both (2-chlorophenyl)methanol and (4-
chlorophenyl)methanol gave the corresponding products in
almost the same yields (96%, Table 2, 3f and 3g). The excellent
yields for products with the halogen substitution on the aromatic
ring of alcohol or diamine indicate the halogens (F, Cl, Br) are
tolerant to the catalytic reaction condition (Table 2, 3c, 3f-3i, 3s).
When aliphatic alcohol with high boiling point (>135°C ) was used,
the corresponding 2-alkyl-1H-benzo[d]imidazole was obtained in
almost quantitative isolated yield (95%) (Table 2, 3j-3l, 3r). But
the 1-butanol (bp ~110°C ) gave 2-propyl-1H-benzo[d]imidazole
in a relative lower yield (75%) even though the mesitylene was
RuCl2(PPh3)3
RuCl2(PPh3)3
None
35[k]
0.0
None
0.50
1
0.0
[a] Reaction conditions: benzene-1,2-diamine (2.5 mmol), benzyl alcohol (7.5
mmol), base (2.5 mol%), catalyst loading (0.25 mol%), heated at 165°C for 24
h
under nitrogen; [b] isolated yield; [c] yield of H2
=
100%

n(H2)/(2n(diamine); [d] Catalyst (0.25 mol%) in 1 mL of mesitylene at 165°C.
[e] Catalyst (0.25 mol%) in 1 mL of p-xylene at 150°C. [f] complex 4,
[Ru(Cl(L3)(CH3CN)2)Cl, L3 = 2,6-bis(benzimidazole-2-yl)-4-hydroxypyridine;[12]
[g] Using dppe instead of PPh3, heated at 165°C for 12 h; [h] The reaction
hour is 6 h; [i] yield of H2= 100%  n(H2)/(2n(diamine); [j], [k] yield of 1-
benzyl-2-phenyl-1H-benzo-[d]imidazole as the main by-product in 37% and
36%, respectively.
To our delighted, when PPh3 was loaded with NaBPh4 under
the same condition, 2-phenyl-1H-benzo[d]imidazole was
obtained in good yields (Table 1, entries 5-7), in which the use of
2 equiv. of PPh3 (relative to complex 1) gave the best yield
(88%). A hydrogen production test exhibited the H2 was obtained
in 82% (Table 1, entry 6 and Supporting Information, Table S1).
It was well consistent with the production of 2-phenyl-1H-
benzo[d]imidazole, indicating the oxidant or hydrogen acceptor
is not necessary in our catalytic system.
used
as
the
solvent.
Unfortunately,
2-tert-butyl-1H-
benzo[d]imidazole was obtained in very low yield (18%) when
2,2-dimethyl-1-propanol was loaded, probably due to the steric
reason (Table 2, 3n). As expected, 1,4-bis(1H-benzo[d]imidazol-
2-yl)benzene was formed in good yield (87%, Table 2, 3t) when
1,4-phenylenedimethanol was coupled with benzene-1,2-
diamine.
Upon lowering the catalyst loading to 0.025 mol%, 2-phenyl-
1H-benzo[d]imidazole was obtained in yield of 90% with TONs
of ~3600 when the reaction was carried out for 24 h (Scheme 4).
A higher TONs (6000) was obtained when 0.01 mol% complex 1
Upon using the mesitylene or p-xylene as solvent, the yields
of 2-phenyl-1H-benzo[d]imidazole dropped to 70% or 51%
(Table 1, entries 8 and 9), respectively. The use of complex 2
instead of
1 resulted in the formation of 2-phenyl-1H-
benzo[d]imidazole in 48% under the same condition, indicating
the imidazolyl N-H groups of complex 1 play an important role in
the catalytic cycle (Table 1, entry 10). Complex 3 gave almost
the same yield (84%) as complex 1 (accompanied by 2 equiv. of
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Scheme 4. The synthesis of 2-phenyl-1H-benzo[d]imidazole with lower
rationalize this transformation is illustrated in Scheme 5. Initially,
complex 1 catalyzes the dehydrogenation of primary alcohol
promoted by the borate salts, leading to the formation of
aldehyde and H2. The aldehyde subsequently reacts with
benzene-1,2-diamine to generate the imine intermediate, which
can be in equilibrium with dihydrobenzimidazole. The
dihydrobenzimidazole is then dehydrogenated to form the final
product and liberate the second H2 catalyzed by the Ru(II)
complex and borate salts.[9f,10a] Since the NaBPh4 is necessary
in the catalytic reaction (Table 1, entry 15), it is worthy to
illustrate the role of NaBPh4 in the catalytic cycle. We initially
think the excess amount of NaBPh4 relative to Ru(II) complex
just help to remove the two chloride ligands from the Ru(II)
catalyst loading.
Table 2. Acceptoless Dehydrogenative Condensation of Primary Alcohols and
Functionalized benzene-1,2-diamine.[a],[b]
center.
However,
upon
using
the
complex
[17]
[Ru(L1)(MeCN)2(solvent)](BPh4)2
with dppe as the catalyst
precursor (both 0.25 mol% relative to the diamine), 2-phenyl-1H-
benzo[d]imidazole was obtained in only 8% yield when the
reaction was carried out at 165°C for 12 h under N2 flow.
However, the yield dramatically increased to 93% when 2 mol%
NaBPh4 was added to this system under the same condition
(Supporting Information, Figure S14). After the catalytic reaction
was complete, the borate was collected and determined by the
11B NMR spectrum, which exhibits only one broad singlet peak
at 2.93 ppm (Supporting Information, Figure S10). This signal is
located at the region of B(OR4)- anion and is very different to that
of B(Ph4)- anion (-6.62 ppm, Supporting Information, Figure
S8).[18] This observation indicates that NaB(Ph)4 is probably
decomposed to NaB(OR)4 during the catalytic cycle. When
B(OH)3 and 1 equiv. of NaOH were heated in benzyl alcohol at
165°C for 2 h, the 11B NMR spectrum shows only one broad
singlet signal at 3.18 ppm (Supporting Information, Figure S9),
which is similar to that observed in our catalytic cycle, again
suggesting the decomposition of NaB(Ph)4 to NaB(OR)4 (R =
alkyl or H). Since the B(OH)3 was reported as an efficient
dehydration catalyst for coupling the carboxylic acid and amine
(or alcohol) to form the amide (or ester),[19] the formation of
-
B(OR)4 anion would also help the dehydrogenation or
dehydration during the catalytic cycle because it can easily bind
to protonic H of alcohol, aldehyde or dihydrobenzimidazole
through hydrogen bond. Furthermore, when NaB(OH)4 (formed
Scheme 5. The plausible mechanism of catalytic dehydrogenative
condensation of benzene-1,2-diamine with alcohol.
[a] Reaction conditions: benzene-1,2-diamine derivatives (2.5 mmol),
alcohol (7.5 mmol), complex 1 (0.25 mol%), dppe (0.25 mol%), NaBPh4
(2.5 mol%), at 165°C for 12 h; [b] Isolated yield; [c], [d] The reaction time is
6 h; [e], RuCl2(PPh3)3 (0.25 mol%) was used instead of complex 1; [f], [g]
Catalyst (0.25 mol%) in 1 mL of mesitylene at 150°C; [g] The reaction hour
is 24 h; [h] benzene-1,2-diamine (2.5 mmol), benzene-1,4-dimethanol (1
mmol), 1 mL mesitylene.
was used with longer reaction time (48 h). These results indicate
that complex 1 is an excellent catalyst precursor for acceptorless
dehydrogenative coupling of alcohol and diamine to 2-
substituted 1H-benzo[d]imidazole (Scheme 4).
Heating 0.25 mol% complex 1 and 2.5 mol% NaBPh4 in
benzyl alcohol at 165°C for 12 h under nitrogen atmosphere,
benzaldehyde was obtained in yield of 6% (based on the
alcohol) accompanied by the liberation of hydrogen (confirmed
by the GC), indicating the benzaldehyde is probably the first
intermediate in the catalytic cycle. A possible mechanism to
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Synthesis and characterization of 2,6-bis(1-hexyl-1H-imidazole-2-
yl)pyridine (L2) and Ru(II) complexes 1-3.
in situ from B(OH)3 (2.5 mol%) and NaOH (2.5 mol%)) was used
as additives instead of NaBPh4 under the same condition listed
in Table 2 (3a), 2-phenyl-1H-benzo[d]omidazole was obtained in
93%, indicating the Na[B(OR)4] is probably the real additive
which play an important role in the catalytic cycle (Scheme 5, a).
Synthesis of 2,6-bis(1-hexyl-1H-imidazol-2-yl) pyridine (L2)
A solution of 2,6-bis(1H-imidazol-2-yl)pyridine (0.211 g, 1 mmol) and
KOtBu (0.224 g, 2 mmol) in acetone (50 mL) was stirred at room
temperature for 2 h. Then the 1-bromohexane (0.330 g, 2 mmol) was
added and the mixture was refluxed for 24 h. Upon cooling to the room
temperature, the solvent was removed in vacuum and the residue was
extracted with a mixed solvent of CH2Cl2 / CH3OH (10:1, V: V, 3×50 mL).
The combined organic solution was evaporated under reduced pressure
and the crude product of was further purified by the column
chromatography using silica gel (elute: CH2Cl2/ethyl acetate, 1/1 (v/v)) to
obtain the pure ligand L2 as a yellow oil. Yield: 0.304 g (80%). 1H NMR
(400 MHz, CDCl3) δ (ppm): 8.01 (d, J=8.0 Hz, 2H), 7.87 (t, J=8.0 Hz, 1H),
7.14 (d, J=1.2 Hz, 2H), 7.04 (d, J=1.2 Hz, 2H), 4.48 (t, J=7.4 Hz 4H),
1.67(m, 4H), 1.13 (m, 12H), 0.77(t, J=8.0 Hz, 6H). 13C NMR (100 MHz,
CDCl3) δ (ppm): 13.92 (s), 22.45 (s), 26.14 (s), 31.23 (s), 47.54 (s),
122.48 (s), 122.91 (s), 128.51 (s), 137.66 (d), 144.99 (s), 149.71 (s).
Elemental Anal. Calcd for C23H33N5 (%): C, 72.78; H, 8.76; N, 18.45.
Found: C, 72.73; H, 8.64; N 18.63.
Further experiment was carried out by using benzaldehyde
and benzene-1,2-diamine with complex 1 (0.25 mol%) and 2.5
mol% Na[B(OH)4] at 165C under nitrogen for 2 h (Scheme 5, b).
2-phenyl-1H-benzo[d]imidazole was obtained in excellent yield
(96%) while H2 was obtained in 94% (based on benzaldehyde,
Supporting Information, Table S3). This observation again
supports the plausible mechanism shown in Scheme 5.
Conclusions
In conclusion, we have illustrated two fully characterized
Ru(II)-NNN complexes as the catalyst precursor for
dehydrogenative condensation of primary alcohol and aromatic
diamine to the 2-substituted 1H-benzo[d]imidazole and H2 in the
presence of catalytic amount of NaBPh4. The catalytic reactivity
follows the order: 1 > 2. The coordination of strong electron-
donating ligand dppe to the Ru center greatly improved the
catalytic reactivity. Decomposition of NaB(Ph)4 to NaB(OR)4 play
an important role during the catalytic cycle. Examination of
substrate scope indicates that the electron-donating groups or
weak electron-withdrawing groups on alcohols or diamines gave
the excellent yield (~ 97%). A high turnover numbers (~ 6000)
was achieved with lower catalyst loadings and longer reaction
time. This homogeneous system is a good example for one-step
synthesis of 1H-benzo[d]imidazole derivatives from alcohols
using neither the oxidant nor the stoichiometric amount of
inorganic bases, thus is a greener method compared with other
reported homogeneous system.
Synthesis of [RuCl(L1)(MeCN)2]Cl (1) and [RuCl(L2)(MeCN)2]Cl (2)
Ligand L1 (0.422 g, 2 mmol) and [RuCl2 (p-cymene)]2 (0.616 g, 1 mmol)
were dissolved in acetonitrile (20 mL) and the solution was refluxed for
12 h under a nitrogen atmosphere. After cooling to room temperature, the
red-brown precipitate was filtered, washed with diethyl ether (3 × 10 mL)
and then dried under vacuum for 12 h. Complex 1 was obtained as a red-
brown power (0.82 g, 88%). 1H NMR (400 MHz, [D6]DMSO) δ (ppm):
2.17 (s, 3H); 2.86 (s, 3H); 7.42(d, J = 1.2 Hz, 2H); 7.69 (d, J = 1.2 Hz,
2H); 7.89 (t, J = 8.0 Hz, 1H); 8.21 (d, J = 8.0 Hz, 2H). 13C NMR (100 MHz,
DMSO-d6) δ (ppm): 3.70; 4.51; 117.02; 120.43; 122.20; 127.31; 131.12;
136.73;
147.30;
152.59.
352.9
ESI-MS
(M/z):
429.7
([RuCl(C11N5H9)(CNCH3)2]+),
([RuCl(C11N5H8)(CH3CN)]+);
Elemental Anal. Calcd. for C15H15N7Cl2Ru (%): C, 38.72; H, 3.24; N,
21.07. Found: C, 38.59; H, 3.32; N, 21.12.
The same procedure was used for the synthesis of complex 2 as a
1
yellowish - brown solid (1.07 g, 85%). H NMR (400 MHz, [D6]DMSO) δ
(ppm): 0.83 (s, 6H); 1.28 (m, 12H); 1.82 (m, 4H); 2.13 (s, 3H); 2.83 (s,
3H); 4.60 (t, J = 8.0 Hz, 4H); 7.43 (d, J = 7.3 Hz, 2H); 7.76 (t, J = 8.0 Hz,
2H); 7.97 (d, J = 8.0 Hz, 3H); 13C NMR (100 MHz, DMSO-d6) δ (ppm):
3.86; 4.71; 114.07; 118.64; 121.62; 122.44; 125.04; 126.11; 129.95;
134.57; 136.09; 143.29; 152.61; 153.45. Elemental Anal. Calcd. for
C27H39N7Cl2Ru (%): C, 51.18; H, 6.20; N, 15.47. Found: C, 51.26, H, 6.03,
15.59.
Experimental Section
General Information.
All experiments were carried out under an atmosphere of purified
nitrogen except other noted. All solvents were purified with the standard
procedure. Commercially available reagents were used as received. The
NMR spectra were received using Bruker Avance Ⅱ HD 400 MHz
spectrometer. The 1H NMR chemical shifts are referenced to the residual
hydrogen signals of the deuterated solvent or TMS, the 13C NMR
chemical shifts are referenced to the 13C signals of the deuterated
solvent, and the Boron trifluoride diethyl etherate is used as an external
reference for 11B NMR. All spectra were recorded at room temperature
unless otherwise noted. Elemental analysis and ESI-Ms was performed
by the Test Centre of Wuhan University. Fourier transform infrared
(FTIR) spectrum was recorded by using a Nicolet iS10 spectrometer
Synthesis of [RuCl(L1)(PPh3)2]Cl (3)
Ligand L1 (0.211 g, 1 mmol) and [RuCl2(PPh3)] (0.958 g, 1 mmol) were
dissolved in anhydrous ethanol (30 mL) and the solution was refluxed for
6 h under a nitrogen atmosphere. After cooling to room temperature, the
yellow precipitate was filtered, washed with diethyl ether (3 × 10 mL) and
then dried under vacuum for 12 h to obtain pure complex 3 as a yellow
power (1.0 g, 88%). FT-IR (KBr pellet): 3059 cm-1; ESI-MS (M/z): 871.8
([RuCl(L1)(PPh3)2]+), 610.0 ( [RuCl(L1)(PPh3)]+); Elemental Anal. Calcd.
for C47H39N5Cl2P2Ru (%): C, 62.19; H, 4.33; N, 7.71 Found: C, 62.03, H,
4.18, N 7.80.
(Thermo Electron). Ligand 2,6-bis(1H-imidazol-2-yl)pyridine (L1),[15]
Ru(II) precursors [RuCl2(PPh3)3],[20] and [RuCl2(p-cymene)]2
[21]
General procedure for catalytic reactions.
were
prepared according to the reported literature. Ligand 2,6-bis(1H-
benzo[d]imidazol-2-yl)pyridine-4-ol (L3) and complex
[RuCl(CH3CN)2(L3)]Cl were synthesized with the method reported by our
group previously.[12]
Method A: Benzyl alcohol (0.81 g, 7.5 mmol), benzene-1,2-diamine (0.27
g, 2.5 mmol), Ru(II) complex 1, 2, 3 or 4 (6.25 μmol, 0.25 mol%),
phosphine (0 ~ 18.75 μmol), and additive (62.5 μmol, 2.5 mol%) were
mixed in a 25 mL schlenk tube and the reaction mixture was heated at
165°C (oil bath) for 12 h in an open system under purified nitrogen. After
cooling to the room temperature, the unreacted alcohol was removed
under vacuum and the residue was purified by column chromatography
on silica gel with ethyl acetate/pentane (1/4, v/v) as eluent to yield pure
2-phenyl-benzimidazole as a white solid, which is characterized by 1H
NMR and 13C NMR in comparison with the standard sample.
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10.1002/cctc.201800017
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Method B: Alcohol (7.5 mmol), benzene-1,2-diamine derivatives (2.5
mmol), complex 1 (6.25 μmol, 0.25 mol%), dppe (6.25 μmol, 0.25 mol%),
and NaBPh4 (62.5 μmol, 2.5 mol%) were mixed in a 25 mL schlenk tube
and the reaction mixture was heated at 165 °C for 12 h in an open
system under purified nitrogen. After cooling to the room temperature,
the unreacted alcohol was removed under vacuum and the residue was
purified by column chromatography on silica gel with ethyl
acetate/pentane (1/4, v/v) as eluent to yield pure 2-substituted-
benzimidazole as a white solid, which is characterized by 1H NMR and
13C NMR in comparison with the standard sample.
Science Fund of Hubei Province (2011CDB441), and the Funds
for Creative Research Groups of Hubei Province (2014CFA007).
Keywords:
dehydrogenative
condensation 1H-
•
benzo[d]imidazole
complex • NaB(Ph)4
•
homogeneous catalysis • Ru(II)-NNN
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Method C: Benzyl alcohol (8.1 g, 75 mmol), benzene-1,2-diamine (2.7g,
25 mmol), complex 1 (6.25 μmol 0.025 mol%), dppe (6.25 μmol, 0.025
mol%), and NaBPh4 (625 μmol, 2.5 mol%) were mixed in a 100 mL
schlenk tube and the reaction mixture was heated at 165 °C for 24 h in
an open system under purified nitrogen. After cooling to the room
temperature, the unreacted alcohol was removed under vacuum and the
residue was purified by column chromatography on silica gel with ethyl
acetate/pentane (1/4, v/v) as eluent to yield pure 2-substituted-
benzimidazole as a white solid, which is characterized by 1H NMR and
13C NMR in comparison with the standard sample.
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Procedure for H2 gas production.
Under a nitrogen atmosphere, benzene-1,2-diamine (0.54 g, 5.0 mmol),
benzyl alcohol (1.62 g, 15 mmol), phosphine (25 μmol, 0.50 mol%) or
dppe (12.5 μmol, 0.25 mol%), NaBPh4 (125 μmol, 2.5 mol%), and
complex 1 (12.5 μmol, 0.25 mol%) were added to a 100 mL schlenk tube
which is connected with a gas collection instrument through gravity
drainage method. The reaction mixture was heated at 165°C (oil bath).
Over a period of time, the volume of the gas was recorded (Supporting
Information Table S1, S2). The hydrogen was confirmed by the GC. After
cooling to the room temperature, the mixture was treated with the same
procedure according to the Method A. A blank experiment without
catalyst was taken at the same condition.
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benzaldehyde
and
benzene-1,2-diamine.
Benzaldehyde (2.5 mmol), benzene-1,2-diamine (3.0 mmol), complex 1
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Lin Li, Qi, Luo, Huahua Cui, Renjie Li,
Jing Zhang* and Tianyou Peng*
Page No. – Page No.
Air-stable
Ru(II)-NNN
Pincer
complexes for Efficient Coupling of
aromatic diamines and alcohols to
1H-benzo[d]imidazoles
liberation of H2
with
the
We illustrate a homogeneous system with Ru(II)-NNN complex, which efficiently
catalyzed the dehydrogenative condensation of primary alcohol with benzene-1,2-
diamine to the 2-substituted benzimidazole with the liberation of H2 in the presence
of catalytic amount of NaBPh4. The addition of dppe and the decomposition of
NaBPh4 to NaB(OR)4 in the catalytic cycle are favour to this greener process.
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