Oxidant-free synthesis of benzimidazoles from alcohols and aromatic diamines catalysed by new Ru(II)-PNS(O) pincer complexes

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ISSN 1477-9226

PAPER

Joseph T. Hupp, Omar K. Farha et al.

Eﬃcient extraction of sulfate from water using

framework

a Zr-metal–organic

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ARTICLE

Oxidant-free Synthesis of Benzimidazoles from Alcohols and

Aromatic Diamines Catalysed by New Ru(II)-PNS(O) Pincer

Complexes

Received 00th January 20xx,

Accepted 00th January 20xx

Qi Luo, Zengjin Dai, Hengjiang Cong, Renjie Li, Tianyou Peng, and Jing Zhang*

DOI: 10.1039/x0xx00000x

Benzimidazoles are chemically and pharmaceutically important, and an environmentally benign synthetic method based

on acceptorless dehydrogenative condensation of primary alcohols and benzene-1,2-diamine is developed in this work.

Three Ru(II) hydride complexes [RuHCl(CO)(PNS(O)] (containing two isomers 1a and 1b) and [RuHCl(CO)(PPh₃)(SNC^NHC)]PF₆

(2) based on two new quinoline-based ligands 2-(diphenylphosphanylmethyl)-8-phenylsulfinylquinoline (PNS(O)) and 1-

mesityl-3-(8-phenylthioquinolyl-2-methyl)-2-imidazole carbene (SNC^NHC) are prepared and fully characterized. These

complexes catalyse the condensation of benzyl alcohol and benzene-1,2-diamine to 2-phenylbenzimidazole with the

liberation of H₂, and the catalytic activity follows the order: 1a ≈ 1b > 2. When 0.2 mol% 1a and 2 mol% of NaBPh₄were

used, various 2-functionalized benzimidazoles were obtained in good yields (70 – 85%) and high turnover numbers (TONs

~ 425). This homogeneous system does not need oxidants or stoichiometric strong base (KOH or KO^tBu, etc.) that are

normally used in the reported homogeneous systems, and thus is a greener process.

www.rsc.org/

base is still necessary.

Introduction

Scheme 1 Catalytic systems for synthesizing benzimidazoles

Various methodologies have been developed to prepare

benzimidazole derivatives for their important applications in

pharmaceutical industry,¹PEM fuel cells,²textile and dyestuff

industry,³in which the heterogeneous⁴or homogeneous

catalysis⁵was used to make the progress much easier and

more alternative. The acceptorless dehydrogenative

condensation (ADC) of alcohol and benzene-1,2-diamine

catalysed by transition metal complexes is one of the ‘green’

procedures as the by-products are only water and hydrogen

(Scheme 1).^{4d, 6}However, homogeneous systems capable of

the conversion are scarce, and a stoichiometric amount of

strong base (KO^tBu, KOH etc.) is usually needed in reported

systems, which essentially produces equivalent amount of

inorganic salts. For instance, Kempe and co-workers⁶used a

Ir(I)-PNP pincer complex [Ir(cod)2,6-DiAmPy(ⁱPr)₂] to catalyse

the condensation of primary alcohols and benzene-1,2-

through condensation of primary alcohols and aromatic

diamines.

H

NH₂

N

Cat.

(2 H₂)

R₂

OH

-H₂O

N

NH₂

R₁

H

N

R

HN

(iPr)₂P

N

Ir

N

S

Ph₂P

Ru

C

P(iPr)₂

Cl

Ph₃P

O

Viswanathamurthi and co-workers

Open to air No H₂released

Kempe and co-workers

ADC H₂released

Cat./KOH/Substrate

0.5 200 100

diamine to the 2-functionalized benzimidazoles in 56

– 96%

24 h TON 94 ~ 198

Cat./KOtBu/Substrate

1.4 140 100

24 h TON 40 ~ 68

yields with the catalyst loading of 1.4 mol%. Nevertheless, one

equiv. of KO^tBu relative to the alcohol was still necessary in

this system. Therefore, developing new catalytic system for

oxidant-free synthesis of benzimidazole from alcohol and

amine without the use of stoichiometric amount of strong

N

Ru

C

H

O

S

PPh₂

Ph

Ir(2 wt%)/TiO₂-500

Cl

O

Inoue and co-workers

ADC H₂released

This work

ADC H₂released

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072,

P. R. China

† E-mail addresses: jzhang03@whu.edu.cn.

Electronic Supplementary Information (ESI) available: Hydrogen gas detection,

spectral data and CIF files for complexes 1a (CCDC 1524939), 1b (CCDC 1525821)

and 2 (CCDC 1524940). See DOI: 10.1039/x0xx00000x.

Cat./Substrate

Cat./NaBPh₄/Substrate

1

100

0.2

2

100

18 h TON ~ 100

12 h TON 350 ~ 425

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Transition metal complexes based on electron-rich, ‘bulky’ NaH (excess), 2-methyl-8-phenylthioquinoline was obtained in

pincer ligands have attracted much interest due to their good yield (80%). The sulfide moiety of this compound could

outstanding catalytic performance in organic synthesis, be easily oxidized by an excess amount of H₂O₂to form 2-

medicinal and material chemistry.⁷Adjustable ligands bearing methyl-8-phenylsulfinylquinoline in almost quantitative yield.

groups such as pyridine-,⁸dialkylamine-⁹or aryl-centred¹⁰The methyl group of this intermediate was deprotonated with

frameworks, and an alterable metal centre make the pincer LDA (lithium di-iso-propylamide) and then reacted with 1

complexes versatile in some important catalytic reactions: the equiv. of chlorodiphenylphosphine to form the new ligand 2-

hydrogenation of carboxylic acid derivatives^8b,11or CO₂,¹²the (diphenyl-phosphanylmethyl)-8-phenylsulfinylquinoline

acceptorless alcohol dehydrogenation (AAD),¹³the Heck (PNS(O), L1) in 81% isolated yield (Scheme

2) as a white solid.

reaction and Suzuki-Miyaura coupling reaction.¹⁴Compared The ³¹P{¹H} NMR spectrum of L1 shows a singlet at -12.04 ppm,

with pyridine-based pincer ligand, quinoline-based ligands which is much different to those of other reported ligands 2,6-

have a larger framework with more substitutable sites, which bis(diphenylphosphanylmethyl)pyridine (37.8 ppm)²²or bis(2-

may display more variable catalytic properties upon (diphenylphosphanyl)ethyl)amine (-20.5 ppm),^23aand is similar

coordinated to the metal centre. However, transition metal to that of 4,5-bis(diphenylphosphanyl)acridine (-14.7 ppm).^23b

1

complex based on the quinoline-centred pincer ligand is rarely In the H NMR spectrum, one singlet peak with integration of

reported to the best of our knowledge.¹⁵

two protons appears at 3.83 ppm, which can be assigned to

Usually, C, N and P are common donors of a pincer ligand, the methylene group of L1. Moreover, a doublet at 39.60 ppm

whereas S donors (normally functionalized as sulfide or with the coupling constants J_PC= 18.2 Hz was observed in its

sulfoxide moiety) are rarely used. Milstein and co-workers^{16,17 13}C NMR spectrum, indicating the methylene group is attached

reported several metals (Ir, Rh, Ru) complexes based on to the phosphorus atom of L1

.

Using 8-fluoro-2-methylquinoline as the starting material,

the imidazolium salt 1-mesityl-3-(8-phenylthioquinolyl-2-

methyl)imidazolium bromide (HL2Br) was synthesized through

pyridine-based PNS (sulfide) and NNS(O)) (sulfoxide) pincer

ligands, in which [RuHCl(CO)(PNS)] (PNS 2-((tert-

butylthio)methyl)-6-((di-tert-butylphosphanyl)methyl)

pyridine) was applied to the dehydrogenative coupling of 1-

=

the procedure depicted in Scheme

3. Refluxing 8-fluoro-2-

methylquinoline with 1.2 equivalents of SeO₂in dry pyridine

resulted in the formation of 8-fluoroquinoline-2-carboxylic

acid, in which the fluoride group could be easily substituted by

the phenylthiol to form 8-phenylthio-quinoline-2-carboxylic

acid. This compound was esterified with ethanol and then

reduced by an excess amount of NaBH₄to form 2-

hydroxymethyl-8- phenylthioquinoline in excellent yield (93%

for two steps). After this intermediate was reacted with conc.

HBr/AcOH and then treated with 1-mesityl-imidazole, the N-

heterocycle carbene precursor HL2Br was obtained in good

yield (80% for two steps). An alternative procedure involving

the direct bromination of 2-methyl-8-phenylthioquinoline with

N-bromosuccinimide to form the 2-bromomethyl-8-

phenylthioquinoline was unsuccessful, resulting in a mixture of

muti-brominated products and no desired product was

hexanol and benzylamine to form

a mixture of hexyl

hexanoate and N-benzylhexanamide in a neutral system.¹⁶In

2013, Gusev and co-workers¹⁸demonstrated a Ru(II)-SNS type

complex [RuCl(PPh₃)(SNS)] (SNS

= bis(2-(ethylthio)ethyl)-

amine) as an efficient catalyst for the hydrogenation of esters

and imine. Meanwhile, several Ru(II) complexes based on

other tridentate¹⁹or bidentate²⁰ligand containing sulfide or

sulfoxide groups were also structurally characterized.

Herein, we illustrate a new homogeneous system with Ru(II)

hydride complexes bearing new quinoline-based pincer ligand

2-(diphenylphosphino)methyl-8-phenylsulfinyl-quinoline

(PNS(O)). This system efficiently catalyses the condensation of

primary alcohols and aromatic diamines to 2-substituted

benzimidazoles and H₂with a low catalyst loading (0.2 mol%)

in the presence of a catalytic amount of NaBPh₄(2 mol% to the

substrate). Although Viswanathamurthi and co-workers

reported Ru(II)-PNS (thiosemicarbazones) complexes for the

condensation of alcohols and benzene-1,2-diamine to 2-

substituted benzimidazoles in 2014 (Scheme 1), KOH (two

equiv. relative to the alcohol) accompanied with O₂as the

oxidant was used.^5bThe present homogeneous system need

neither oxidants nor stoichiometric strong base (KOH or KO^tBu,

etc.) to promote the reaction, and thus is a more ‘green’

process.

1

isolated from the mixture. In the H NMR spectrum of HL2Br,

one singlet peak with integration of two protons appears at

6.45 ppm, which can be assigned to the methylene group of

H

L2Br. Moreover, a characteristic singlet at 10.22 ppm in its

¹HNMR spectrum, associated with the imidazolium proton that

is deprotonated upon the formation of the free NHC. These

observations are very similar to those of a pyridine-NHC pincer

ligand reported by Milstein and co-workers.²⁴

Scheme 2 Synthesis of L1

Results and discussion

Synthesis of Pincer Ligands PNS(O) (L1) and SNC(^NHC) (L2)

The starting material 8-fluoro-2-methylquinoline was

synthesized according to the reported method.²¹When 8-

fluoro-2-methylquinoline and two equivalents of thiophenol

were heated in dry DMF at 150°C for 12 h in the presence of

2 | J. Name., 2012, 00, 1-3

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Scheme 3 Synthesis of NHC Precursor HL2Br(SNC(^NHC))

The crystal structure of complex 1a displays a distorted

octahedral geometry around the metal centre. The CO ligand is

bound to the Ru(II) centre trans to the nitrogen atom of L1

while the hydride is located trans to the chloride ligand.

Looking from the top of H(hydride)-Ru-Cl axis, the coordinated

atoms of S, N, P, C (carbonyl) are sequentially anti-clockwise

arranged (Fig. 2, left). The bond distance of Ru1-S1 (2.2782(6)

Å) is slightly shorter than that of Ru1-P1 (2.2883(7) Å) while

the bond distance of Ru1-N1(quinolyl) (2.123(2) Å) is longer

than the reported Ru-N(pyridyl) complexes [RuCl(H)(CO)(PNN)]

(2.103(2) Å) and [RuCl(H)(CO)(PNP)] (2.087(2) Å).²⁵Due to the

meridional coordination geometry of the L1 framework and

the lack of a plane of symmetry involving the P, N, S atoms, the

two protons of the methylene group are inequivalent.

Synthesis and Characterization of Ru(II) Complexes.

When a THF solution of L1 and 1 equiv. of [RuHCl(CO)(PPh₃)₃]

was refluxed for 3 h under a nitrogen atmosphere, a yellow

solid of [RuHCl(CO)(L1)] (containing the mixture isomers of 1a

and 1b, 1:1 mol/mol) was precipitated (determined by ¹H NMR

and ³¹P{¹H} NMR). The pure complexes 1a and 1b were

isolated by the flash column chromatography in 40% and 45%

With the similar method to that for complex 1a, bright

yellow single crystals of complex 1b suitable for the X-ray

diffraction analysis were obtained by slow diffusion of the

diethyl ether to a concentrated CH₂Cl₂solution of 1b. The

yields, respectively (Scheme 4). The IR spectrum of 1a exhibits

a strong absorption at 1940 cm^-1assigned to the coordinated

CO. The ³¹P{¹H} NMR spectrum exhibits a singlet peak at 55.47

ppm, representing a downfield of 67.5 ppm compared to the

free ligand L1. The presence of the hydride coordinated to the

Ru(II) centre is confirmed by the ¹H NMR spectrum, which

shows a doublet peak at -13.07 ppm with J_PH= 22.8 Hz.

Furthermore, the ¹H NMR of complex 1a shows the methylene

group as two doublets of doublets at 4.80 and 5.08 ppm,

indicating the lack of a symmetry plane involving the P, N, S

atoms. Similarly, the ³¹P{¹H} NMR spectrum of complex 1b

shows a singlet peak at 58.14 ppm, which represents a

structure of 1b is shown in Figure

1

while the selected bond

. The crystal

distances and bond angles are listed in Table

1

structure of 1b displays the Ru(II) centre adopts a distorted

octahedral geometry with the CO ligand is bound to the metal

centre trans to the nitrogen atom of L1 while the hydride is

located trans to the chloride ligand. Different to that of

complex 1a, the coordinated atoms of S, N, P, C (carbonyl) are

sequentially clockwise arranged (Fig. 2, right, looking from the

top of H(hydride)-Ru-Cl axis).

The bond angle of Ru1-S1-O1 (119.48 (6)

than that of complex 1a (124.20 (7) ), while the bond angle of

S1-Ru1-Cl1 (93.69 (2) ) of 1a is smaller than that of complex 1b

(99.64 (2) ). Furthermore, the bond distances of Ru1-S (2.2850

°) of 1b is smaller

°

1

downfield of 70.2 ppm compared to the free ligand L1. The H

°

NMR of complex 1b shows the hydride ligand as a doublet

peak at -12.90 ppm with J_PH= 21.6 Hz and the two magnetic

nonequivalent protons of methylene moiety as two groups of

double doublet peaks at 4.73 and 5.15 ppm. The chemical

shifts of the hydride ligand in 1a (-13.07 ppm) and 1b (-12.90

ppm) indicate the hydride is located trans to the chloride or to

the pyridine nitrogen atom rather than to CO. This observation

is consistent to other reported PNP-Ru(II) and PNN-Ru(II)

hydride complexes.²⁵The ¹H NMR and ¹³C NMR spectra of

complex 1a are similar to these of complex 1b, indicating they

are stereoisomers. Both complexes 1a and complex 1b can be

kept in air for weeks at room temperature in the solid state

without obviously decomposition.

°

(5) Å) and Ru1-N(quinolyl) (2.136(2) Å) in 1b are slightly longer

than those in complex 1a (2.2782(6) Å) and (2.123(2) Å,

respectively).

Scheme 4 Preparation of Complexes 1a and 1b

For further characterization of complex 1a, single crystals

suitable for X-ray diffraction analysis were obtained by slow

diffusion of the diethyl ether to a concentrated CH₂Cl₂solution

Fig. 1 The structures of complexes 1a (top) and 1b (bottom)

with thermal ellipsoids drawn at the 30% level. All hydrogen

atoms excepting Ru-H and the solvent are omitted for clarity.

of complex 1a. The structure of 1a is shown in Fig.

1 and the

selected bond distances and bond angles are listed in Table 1.

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Fig. 2 The coordination geometries of complexes 1a (left) and

1b (right)

Metal complexes bearing NHC ligands were typically

prepared from imidazolium salts via the silver carbene transfer

reaction,²⁶or the free carbene route.²⁷In the former method,

the carbenic proton of an imidazolium salt was deprotonated

by Ag₂O followed by the transmetallation of the metal

precursor with the Ag(I)-NHC intermediate; while the latter

method involves in the use of a strong base for the acidic

proton abstraction and the free carbene generated then being

trapped, after isolation or in situ, by a metal precursors. In this

work, the Ru(II)-L2 hydride complexes was synthesized by the

Fig. 3 The structure of complex

2

with thermal ellipsoids drawn

-

at the 30% level. All hydrogen atoms excepting Ru-H1 and PF₆

anion are omitted for clarity.

The bright yellow single crystals suitable for the X-ray

diffraction analysis were obtained by the slow diffusion of the

diethyl ether to a concentrated CH₂Cl₂solution of complex

The structure of is shown in Figure while the selected bond

distances and bond angles are listed in Table 1. The structure

of complex exhibits the CO ligand is coordinated to the Ru(II)

2.

2

3

2

former method (Scheme

5). The intermediate silver(I)-NHC

centre trans to the N atom of the pincer system while the

location of the hydride is trans to the phosphine. The bond

distance of Ru1-C19 (NHC) (2.034(2) Å) is slightly longer than

that in the reported Ru-NHC complexes containing a bipyridine

nitrogen trans to the NHC ligand (1.995 ~ 2.014Å).²⁴The bond

distance of Ru1-P1 (2.4402(6) Å) is significantly longer than

those in complexes 1a (2.2883(7) Å) and 1b (2.3070(5) Å), as a

result of the strong trans effect of the hydride ligand. The

(Ag-L2^NHC) was obtained as a pale yellow solid by the reaction

of the imidazolium salt (HL2Br) with Ag₂O in CH₂Cl₂at room

temperature. The absence of imidazolium (NCHN) resonance

1

at 10.40 ppm in the H NMR spectrum indicates the formation

of the Ag(I)-C(NHC) bond. When a THF solution containing the

in situ formed Ag-NHC intermediate and

1 equiv. of

[RuHCl(CO)(PPh₃)₃] was refluxed for 12 h, followed by the

extraction of the halogen ligand from the Ru(II)-NHC complex

with an excess amount of NH₄PF₆in methanol, the pure

bond distance of Ru1-S1 in complex

2 (2.3698(6) Å) is also

longer than those in complexes 1a (2.2782(6) Å) and 1b

(2.2850(5) Å). This observation is also consistent to the strong

trans effect of NHC carbene relative to the phosphine ligand.

complex [RuHCl(CO)(PPh₃)(L2)](PF₆) (

isolated yield as a yellow microcrystals. Complex

air for weeks at room temperature in the solid state.

The fully characterized complex gives rise to a doublet

2) was obtained in 41%

2

is stable in

2

Table 1 Selected Bond Lengths (Å) and Angles (deg) of

peak at -7.26 ppm with J_PH= 107.4 Hz in the ¹H NMR spectrum.

This large coupling constant clearly indicates the hydride is

coordinated to the Ru(II) centre trans to the P atom of

triphenylphosphine ligand, which is consistent to other

reported Ru(II) complexes.²⁸In the ¹³C{¹H} NMR spectrum the

carbonyl ligand gives rise to a broad singlet at 203.26 ppm, and

the NHC carbon appears at 184.45 ppm as a broad singlet. The

³¹P{¹H} NMR exhibits a singlet at 23.7 ppm for the coordinated

PPh₃ligand, which is similar to that of the reported complex

[RuHCl(CO)(PPh₃)(NNC^NHC)] (24.03 ppm).²⁴

Complexes 1a, 1b and 2

Selected bond lengths (Å ) and angles (deg) of complex 1a

Ru1-N1

2.123(2)

2.2883(7)

1.861(3)

164.25(2)

81.72 (6)

90.99(2)

171.3(11)

116.87(8)

Ru1-S1

2.2782(6)

2.5189(6)

1.58(3)

Ru1-P1

Ru1-Cl1

Ru1-C29

Ru1-H1

P1-Ru1-S1

N1-Ru1-P1

P1-Ru1-Cl1

H1-Ru1-Cl1

C1-S1-Ru1

N1-Ru1-C29

N1-Ru1-S1

S1-Ru1-Cl1

O1-S1-Ru1

Cl1-Ru1-S1-O1

175.42(9)

83.89(6)

93.69(2)

124.20(7)

-31.42

Scheme 5. Synthesis of Complex

2

Selected bond lengths (Å ) and angles (deg) of complex 1b

Ru1-N1

2.136(2)

2.3070(5)

1.852(2)

162.99(2)

81.44 (4)

88.62(2)

172.4(10)

121.85(6)

Ru1-S1

2.2850(5)

2.5003(5)

1.56(3)

Ru1-P1

Ru1-Cl02

Ru1-C29

Ru1-H15

P1-Ru1-S1

N1-Ru1-P1

P1-Ru1-Cl02

H15-Ru1-Cl02

C1-S1-Ru1

N1-Ru1-C29

N1-Ru1-S1

S1-Ru1-Cl02

O1-S1-Ru1

Cl02-Ru1-S1-O1

171.35(7)

83.69(4)

99.64(2)

119.48(6)

-39.45

4 | J. Name., 2012, 00, 1-3

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Yield(%)^a,b

Selected bond lengths (Å ) and angles (deg) of complex

2

Entry

Catalyst

Additive

Ru1-N1

2.164(2)

1.850(2)

2.4402(6)

165.77(7)

94.41(5)

94.7(1)

Ru1-C19

2.034(2)

1

1a

Cs₂CO₃

KO^tBu

78

77

53

62

60

50

37

80

64

2

Ru1-C47

Ru1-S1

2.3698(6)

2

1b

Ru1-P1

Ru1-H1

3

2

S1-Ru1-C19

N1-Ru1-P1

C19-Ru1-C47

N1-Ru1-C19

N1-Ru1-C47

S1-Ru1-P1

S1-Ru1-C47

C19-Ru1-P1

170.25(9)

4

1a

89.15(2)

5

1a

CsOH·H₂O

K₂CO₃

DBU^d

94.36(7)

6

1a

87.84(8)

101.03(7)

7

1a

8

1a

NaBPh₄

e

CsBPh₄

Catalytic dehydrogenative condensation of alcohol and benzene-

1,2diamine.

9

1a

10

11

12

13

14^c

15^g

16

1a

NaBF₄

NaPF₆

1a

2

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 1a (5 μmol) and benzene-

1,2-diamine (2.5 mmol) with Cs₂CO₃(50 μmol) in an excess

amount of benzyl alcohol (7.5 mmol) at 150°C (oil bath) under

nitrogen atmosphere for 12 h. The 2-phenyl-benzimidazole

was obtained in 78% isolated yield (Table 2, entry 1). Upon

1a

none

0

none

NaBPh₄

KOBu^t

NaBPh₄

2

1a

85(82^f)

21

33

RuHCl(CO)(PPh₃)₃

a

Reaction conditions: benzene-1,2-diamine (2.5 mmol),

using complexes 1b or 2 under the same condition, the desired

benzyl alcohol (7.5 mmol), additive (0.05 mmol), catalyst

loading (0.005 mmol), 150°C, 12 h. ^bIsolated yield. ^cThe

reaction temperature is 165°C. ^dDBU = 1,8-diazabicyclo

[5.4.0] undec-7-ene. ^ePrepared by the reaction of NaBPh₄

product of 2-phenyl-benzimidazole was obtained in 77% and

53% yields, respectively (Table 2, entries 2 and 3). These

results indicate the catalytic activity follows the order: 1a

≈ 1b

f

g

and CsOH·H₂O in EtOH. Yield of H₂. 0.005 mmol KOBu^t

>

2. With complex 1a as the catalyst, using the strong base

(KO^tBu or CsOH) resulted in the formation of 2-

phenylbenzimidazole in relatively lower yields (62% and 60%,

respectively) accompanied by the formation of several kinds of

N-alkylation products. An alternative procedure involving the

use of the stoichiometric amount of CsOH or KO^tBu according

to the reported literature²⁰was much less successful, giving a

mixture contains a number of N-alkylation products and the 2-

phenyl-benzimidazole was obtained in low yield (~ 15%). It is

noted that the high yield (80%) of 2-phenyl-benzimidazole was

obtained when a catalytic amount of the neutral salt NaBPh₄

(10 equivalents relative to complex 1a) was used as the

additive under the same condition, while the NaPF₆or NaBF₄

gave almost no product (Table 2, entries 10, 11). Interestingly,

there is only 33% yield of desired product obtained when the

precursor RuHCl(CO)(PPh₃)₃was used (Table 2, entry 16),

which clearly indicates that the coordination of the pincer

ligand to the Ru(II) centre greatly improve the catalytic

reaction. As expected, there is no 2-phenyl-benzimidazole

obtained in the absence of complex 1a or additives (Table 2,

entries 12, 13). Increasing the reaction temperature to 165°C

(oil bath) afforded the 2-phenyl-benzimidazole in 85% yield

(Table 2, entry 14), which is consistent to the hydrogen gas

production (82%, seeing the supporting information, S4-5). The

hydrogen gas is double mole relative to the 2-

phenylbenzimidazole, which clearly indicates the hydrogen

acceptor is not necessary in our catalytic system.

(1 equiv. to 1a) was used

Table

3 Acceptorless Dehydrogenative Condensation of

Primary Alcohols and Functionalized benzene-1,2-diamine. ^{a, b}

3a, 85%

3c, 84%

3a, 85%

CF₃

H

N

3f, 77%

3i, 83%

3d, 74%

3g, 70%

3e, 80%

3h, 79% (64%)^c

^aReaction condition: benzene-1,2-diamine derivatives (2.5

mmol), alcohol (7.5 mmol), complex 1a (0.2 mol%),

c

NaBPh₄(2 mol%), 165°C, 12 h. ^bIsolated yield. Cs₂CO₃was

used.

With the optimized reaction condition in hand, we next

examined the substrate scopes of the reaction, the results are

listed in Table 3. Both aromatic alcohols and aliphatic alcohols

can be dehydrogenative coupled with benzene-1,2-diamine to

corresponding 2-substituted benzimidazoles in good yields (70

Table

2 Optimization of Catalytic Condition for the

Benzimidazole Synthesis.

– 85%). It is noted that the electronic effect of aromatic

alcohol gives very little difference between the strong

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electron-withdrawing group and the strong electron-donating nitrogen atmosphere, the 2-phenylbenzimidazole was

group for the desired coupling products. For instance, when 3- obtained in 21% (table 2, entry 15), indicating a slightly excess

trifluoromethylbenzylic alcohol was used, the product 3e was of additive (10 equiv. to 1a) favour to formation of the desired

obtained in 80% yield while the 4-methyloxybenzyl alcohol product (table 2, entry 4). Heating 1a and 1 equiv. of KO^tBu in

gave the corresponding coupling product in 77% yield (Table 3, DMSO-d₆at 80

°C for 2 h resulted in the formation of the

3e,

3f). Although the use of Cs₂CO₃instead of NaBPh₄resulted dearomatic species (confirmed by ¹H NMR, S6–7) which is

in a similar yield (78%) when benzyl alcohol was used (table 1, similar to other reported Ru(II)-PNN²⁵or Ru(II)-PNP³⁰

entry 1), a lower yield of 2-pentyl-benzimidazole was obtained complexes. The aromatization-dearomatization is a commonly

(64%) while the NaBPh₄gave 79% yield under the same process in bond activation and the detail was fully studied by

condition (Table 3, 3h).

Traditionally, the plausible mechanism for the synthesis of 2- out by using benzaldehyde and benzene-1,2-diamine

substituted benzimidazoles from benzene-1,2-diamine and with/without complex 1a or NaBPh₄at 165 C under nitrogen.

alcohols involves the in situ alcohol dehydrogenation to The results are listed in Table 4. When benzaldehyde and 1

aldehyde catalysed by a Ru(II) hydride complex with (or equiv. of benzene-1,2-diamine were heated at 165 C for 12 h,

without) a hydrogen acceptor, followed by the condensation 2-phenylbenzimidazole was obtained in 21% yield

of the aldehyde with benzene-1,2-diamine to generate the accompanied by the formation of 1-benzyl-2-

Milstein and co-workers.³¹Further experiments were carried

°

imine/2,3-dihydro-benzimidazole, this intermediate is further phenylbenzimidazole in 19% (Table 4, entry 1). Interestingly,

dehydrogenated to form the 2-substituted benzimidazole with the loading of NaBPh₄(2 mol%) give a higher selectivity for the

(or without) the Ru(II) catalyst.²⁹In order to study the 2-phenylbenzimidazole (41%) relative to the 1-benzyl-2-

mechanism of this catalytic system, the controlled phenylbenzimidazole (5%) (Table 4, entry 2). It is noted that

experiments were carried out without the complex 1a or complex 1a can efficiently catalyse the acceptorless

NaBPh₄, which resulted in almost no desired product condensation of benzaldehyde and benzene-1,2-diamine to 2-

formation (Table 2, entries 12, 13), indicating both the Ru(II) phenylbenzimidazole in excellent yield (95%) with high

hydride complex and NaBPh₄are necessary for the catalytic selectivity (Table 4, entry 4). A hydrogen production test

reaction.

exhibits the hydrogen was obtained in 86% yield (based on the

benzaldehyde, supporting information, S4-5 when the

reaction was carried out at 165 C for 1 h, which is well

)

Table 4 Condensation of benzaldehyde and benzene-1,2-

diamine catalysed by 1a. ^a

°

consistent to the formation of 2-phenylbenzimidazole (90%).

These results confirm the condensation of aldehyde and

benzene-1,2-diamine is much faster than the dehydrogenation

of alcohol to the aldehyde. The formation of the stoichiometric

amounts of hydrogen (two equivalents to the desired product)

also supports the plausible mechanism shown in Scheme 6.

The investigation of the detail catalytic mechanism involving

the isolation of the active Ru intermediates from the system is

currently underway.

Entry 1a (mol%)

NaBPh₄(mol%)

X

(%)

Y (%)

1

2

3

4

0

2

0

21

41

70

95

19

Scheme

6

The plausible mechanism of catalytic

0

<5

dehydrogenative condensation of benzene-1,2-diamine with

primary alcohol to 2-substituted benzimidazole

0.2

<5

trace

90(86^c) trace

5^b

a

Reaction condition: benzene-1,2-diamine (3.0 mmol),

benzaldehyde (2.5 mmol), NaBPh₄(0.05 mmol), 1a (0.005

b

mmol), mesitylene (1 mL), 165°C, 12 h, N₂atmosphere.

The reaction time is 1 h. ^cYields of H₂.

Heating 0.2 mol% complex 1a and 2 mol% NaBPh₄in benzyl

alcohol at 150°C for 12 h under a nitrogen atmosphere only

afforded a small amount of benzaldehyde (4% based on the

alcohol) accompanied by the liberation of hydrogen

(confirmed by the GC), indicating the benzaldehyde is probably

the intermediate and the formation of benzaldehyde from

alcohol is slower than the dehydrogenative condensation of

aldehyde with aromatic diamines. When complex 1a and 1

equiv. of KO^tBu were heated in a mixture of benzyl alcohol and

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. Compound 8-fluoro-2-

benzene-1,2-diamine (1:3 mol/mol) at 150°C for 12 h under

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methylquinoline²¹was prepared according to the reported stirred for

ARTICLE

5

h

at -78

°

C

and

a

solution of

method. The NMR spectra were received using a Mercury 300 chlorodiphenylphosphine (0.44 g, 2.0 mmol) in 10 mL dry THF

and 400 MHz spectrometer. The ¹H NMR chemical shifts are was added. The mixture was allowed slowly to warm up to

referenced to the residual hydrogen signals of the deuterated room temperature and stirred overnight. To this reaction

solvent or TMS, the ¹³C NMR chemical shifts are referenced to mixture was added 10 mL of degassed water and the organic

the ¹³C signals of the deuterated solvent. The ³¹P NMR phase was washed with degassed brine (3 × 10 mL) under N₂

chemical shifts are reported in ppm downfield from H₃PO₄and atmosphere. The THF phase was separated and dried over

referenced to an external 85% solution of phosphoric acid in anhydrous Na₂SO₄, filtered, and the solvent was removed

D₂O. J values are given in Hz. All spectra were recorded at under vacuum. The residue was purified through the column

room temperature unless otherwise noted. Elemental analysis chromatography using silica gel with ethyl acetate/hexane

was performed by the Test Centre of Wuhan University. Ru(II) (9/1) as eluent, L1 (0.73 g, 81%) was obtained as a white solid.

precursor [RuHCl(CO)(PPh₃)₃]³²was prepared according to the ³¹P{¹H} NMR (DMSO-D₆) δ (ppm): -12.04 (s). ¹H NMR (300 MHz,

reported literature.

CDCl₃, 298 K) δ: 3.81 (s, 2H, CH₂), 7.15 (d, J=8.1 Hz, 1H), 7.32 (s,

9H), 7.47-7.51 (m, 4H), 7.60 (t, J=7.8 Hz, 1H), 7.74 (d, J=8.1 Hz,

1H), 7.90 (d, J=8.7 Hz, 1H), 7.96 (m, 2H), 8.35 (d, J=7.5 Hz, 1H).

¹³C{¹H} NMR (101 MHz, CDCl₃, 298 K) δ (ppm): 39.60 (d, CH₂,

J=18.2 Hz), 122.73-122.85 (m), 124.73 (d, J=9.1 Hz), 125.55,

125.71, 125.92, 126.44, 128.39-128.51 (m), 128.62-128.74 (m),

128.91 (d, J=8.4 Hz), 129.80, 130.35 (d, J=8.3 Hz), 130.62 (d,

J=11.1 Hz), 132.39-132.57 (m), 133.10 (d, J=19.5 Hz), 135.93 (d,

J=5.6 Hz), 137.42 (d, J=15.4 Hz), 138.23 (d, J=15.4 Hz), 143.00,

143.61, 146.10, 159.03 (d, J=7.0 Hz). Found: C, 74.21; H, 4.68;

N, 3.02. Calc. for C₂₈H₂₂NOPS: C, 74.48; H, 4.91; N, 3.10.

Synthesis of 2-methyl-8-phenylthioquinoline. To a solution of

8-fluoro-2-methylquinoline (6.00 g, 37 mmol) in dry DMF (80

mL) was added NaH (1.80 g, 74 mmol) and Ph-SH (8.10 g, 74

mmol), and the mixture was stirred and heated at 150°C for 12

h. The solvent was removed under vacuum and the residue

was dispersed in 400 mL water. The solution was adjusted to

pH ~ 2 by using HCl (6 M) and the product was extracted with

ethyl acetate (3 × 100 mL). The combined organic solution was

dried over Na₂SO₄and then evaporated under vacuum. The

residue was passed through the column chromatography on

silica gel (eluent: ethyl acetate/pentane: 1/9) to give a yellow Synthesis of complexes [RuHCl(CO)(L1)] (1a) and (1b). To a

solid of product (7.4 g, 80%). suspension of RuHCl(CO)(PPh₃)₃(0.19 g, 0.2 mmol) in THF (5

¹H NMR (300 MHz, CDCl₃, 298 K) δ (ppm): 2.79 (s, 3H, CH₃), mL) was added L1 (90 mg, 0.2 mmol), and the mixture was

6.92 (d, J=7.2 Hz, 1H), 7.20 (t, J=7.8 Hz, 1H), 7.29 (d, J=8.7 Hz, heated at 70˚C for 12 h, and cooled to room temperature. The

1H), 7.43-7.47 (m, 4H), 7.60-7.68 (m, 2H), 7.97 (d, J=8.1 Hz, yellow solid thus obtained was filtered, washed with ether (3 ×

1H). ¹³C NMR (75.5 MHz, CDCl₃, 298 K) δ (ppm): 25.41, 122.63, 3 mL), and then dried under vacuum (89 mg in total, 85%). The

123.90, 124.75, 125.64, 126.20, 128.85, 129.60, 131.96, isomers of complexes 1a and 1b were separated by column

135.83, 136.17, 139.39, 144.12, 158.38. Found: C, 76.55; H, chromatography on silica gel with CH₂Cl₂/MeOH (1/1) as

5.07; N, 5.72. Calc. for C₁₆H₁₃NS: C, 76.46; H, 5.21; N, 5.57.

eluent. The complex 1a (42 mg, 40%) was obtained as a pale

yellow solid while complex 1b (47 mg, 45%) was a yellow solid.

Synthesis of 2-methyl-8-phenylsulfinylquinoline. To a solution

of 2-methyl-8-phenylthioquinoline (2.51 g, 10 mmol) in

CH₃COOH (50 mL) was added H₂O₂(1.13 g, 30 wt% in water, 15

Complex 1a:

³¹P{¹H} NMR (DMSO-D₆) δ (ppm): 55.47 (s). ¹H

NMR (300 MHz, DMSO-D₆, 298 K) δ: 4.80 (dd, J=17.1, 10.8 Hz,

1H, CH₂), 5.08 (dd, J=17.1, 10.8 Hz, 1H, CH₂), 7.44 (s, 3H), 7.54-

7.66 (m, 7H), 7.78-8.00 (m, 6H), 8.07 (d, J=8.4 Hz, 1H)), 8.34 (d,

J=7.5 Hz, 1H), 8.75 (d, J=9.1 Hz, 1H), -13.07 (d, J_PH=22.8 Hz, 1H).

¹³C{¹H} NMR (101 MHz, DMSO-D₆, 298 K) δ (ppm): 44.84 (d,

J=28.0 Hz, CH₂), 123.23 (d, J=9.9 Hz), 125.94, 127.30, 128.47,

128.89 (d, J=10.7 Hz), 129.20 (d, J=9.9 Hz), 129.52, 130.86,

131.22, 131.70-131.94 (m), 132.16, 133.23, 133.52 (d, J=10.7

Hz), 137.01, 137.76, 139.32, 143.64, 149.35, 150.36, 164.84 (d,

J=7.4 Hz), 205.30 (d, J=13.2 Hz, CO). IR (KBr pellets): 1940 cm^-1

(CO). Found: C, 51.53; H, 3.59; N, 2.12. Calc. for

C₂₉H₂₄ClNO₂PRuS·CH₂Cl₂: C, 51.25; H, 3.73; N, 1.99;

mmol) dropwisely, and then heated at 50°C for 2 h. After

cooling to the room temperature, 200 mL water was added to

the mixture and the solution was then neutralized by Na₂CO₃.

The white precipitate was collected by filtration, washed with

water, and then dried under vacuum overnight at 50°C. The

crude product was recrystallized from CH₂Cl₂/hexane

a

solution to give colourless crystals of 2-methyl-8-

phenylsulfinylquinoline (2.60 g, 97%).

¹H NMR (300 MHz, CDCl₃, 298 K) δ (ppm): 2.72 (s, 3H, CH₃),

7.24-7.32 (m, 4H), 7.63 (t, J=7.2 Hz, 1H), 7.78 (d, J=7.2 Hz, 1H),

7.96-7.98 (m, 3H), 8.37 (d, J=6.3 Hz, 1H). ¹³C NMR (75.5 MHz,

CDCl₃, 298 K) δ (ppm): 25.02, 122.75, 124.63, 125.69, 126.29, Complex 1b: ³¹P{¹H} (DMSO-D₆) δ (ppm): 58.14 (s). ¹H NMR

128.52, 129.86, 130.43, 135.91, 142.50, 143.54, 146.08, (300 MHz, DMSO-D₆, 298 K) δ (ppm): 4.73 (dd, J=17.1, 11.1Hz,

159.01; Found: C, 69.61; H, 5.04; N, 5.23. Calc. for 1H, CH₂), 5.15 (dd, J=17.1, 10.8 Hz, 1H, CH₂), 7.43 (s, 3H), 7.53

C₁₆H₁₃NOS·0.5H₂O: C, 69.54; H, 5.10; N, 5.07.

(s, 6H), 7.65 (s, 2H), 7.83-7.89 (m, 5H), 8.06-8.11 (m, 1H)), 8.36

(d, J=7.2 Hz, 1H), 8.75 (d, J=8.7 Hz, 1H), -12.90 (d, J_PH=21.6 Hz,

1H). ¹³C{¹H} NMR (101 MHz, DMSO-D₆, 298 K) δ (ppm): 44.42

(d, J=28.0 Hz, CH₂), 123.31 (d, J=10.7 Hz), 127.00, 127.28,

128.56, 128.74, 128.91-129.20 (m), 130.81, 131.39, 131.66 (d,

J=11.5 Hz), 131.88 (d, J=5 Hz), 133.52, 133.62, 137.39, 137.87,

139.56, 144.34, 145.80 (d, J=7.4 Hz), 149.67, 164.86 (d, J=2.7

Synthesis of 2-(diphenylphosphinomethyl)-8-phenylsulfinyl-

quinoline (PNS(O)) (L1). To an oven-dried, nitrogen flashed, 3-

neck round bottom flask was charged with 2-methyl-8-

phenylsulfinylquinoline (0.53 g, 2.0 mmol) in 10 mL dry THF.

The solution was cooled to -78°C and LDA (1.2 mL, 2 M in THF,

2.4 mmol) was slowly added with a syringe. The mixture was

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Hz), 205.55 (d, J=13.2 Hz, CO). IR: 1945 cm^-1(CO). Found: C, J=7.5 Hz, 1H), 7.45-7.47 (m, 3H), 7.54 (d, J=7.8 Hz, 1H), 7.68-

51.48; H, 3.52; N, 2.05. Calc. for C₂₉H₂₄ClNO₂PRuS·CH₂Cl₂: C, 7.71 (m, 2H), 8.19-8.26 (m, 2H). ¹³C{¹H} NMR (75.5 MHz, CDCl₃,

51.25; H, 3.73; N, 1.99.

298 K) δ (ppm): 14.21, 62.01, 121.50, 123.51, 125.15, 128.52,

129.20, 129.73, 131.02, 134.84, 135.99, 137.32, 142.35,

143.68, 146.78, 165.13. Found: C, 70.23; H, 4.78; N, 4.74. Calc.

for C₁₈H₁₅NO₂S: C, 69.88; H, 4.89; N, 4.53.

Synthesis of 8-fluoroquinoline-2-carboxylic acid. To a solution

of 8-fluoro-2-methylquinoline (10 g, 62 mmol) in pyridine (100

mL) was added SeO₂(8.35 g, 75 mmol) and the solution was

refluxed at 120°C for 12 h. After cooling to the room Synthesis of (8(phenylthioquinoline-2-yl)-methanol. To a

temperature, the precipitate was filtrated off, and the solution solution of NaBH₄(1.52 g, 40 mmol) in dry EtOH (50 mL) was

was taken to dryness under vacuum and the residue was added a solution of ethyl 8-phenylthioquinoline-2-carboxylate

dispersed in 50 mL water, which was adjusted to the pH ~ 12 (3.10 g, in 50 mL THF, 10 mmol) slowly, and the mixture was

by using NaOH and then extracted with CHCl₃(3 × 20 mL). The stirred at room temperature for 12 h. After the reaction, the

aqueous phase was acidized by HCl (6 M) under stirring to solvent was removed under vacuum and the residue was

precipitate a brown solid, which was dried under vacuum to dispersed in 200 mL water, then the solution was adjusted to

give 8-fluoroquinoline-2-carboxylic acid (8.20 g, 70%).

pH ~ 6 by adding saturated NaHCO₃solution. And the mixture

¹H NMR (400 MHz, DMSO-D₆, 298 K) δ (ppm): 7.66-7.74 (m, was extracted with ethyl acetate (3 × 20 mL). The combined

2H), 7.91 (d, J=8.0 Hz, 1H), 8.18 (d, J=8.0 Hz, 1H), 8.62 (d, J=8.0 organic solution was dried over anhydrous Na₂SO₄and the

Hz, 1H), 13.61 (s, 1H, -COOH). ¹³C{¹H} NMR (101 MHz, DMSO- solvent was removed under vacuum. The residue was

D₆, 298 K) δ (ppm):144.58 (d, J=18.2 Hz), 121.95, 124.07, recrystallized from a CH₂Cl₂/hexane solution to give a yellow

128.73, 130.40, 136.97 (d, J=12.0 Hz), 136.67, 149.02, 157.62 crystalline solid of (8-(phenylthioquinoline-2-yl)-methanol

(d, J_CF=255.6 Hz), 166.15. Found: C, 2.94; H, 3.04; N, 7.48. Calc. (1.25 g, 97%).

for C₁₀H₆FNO₂: C, 62.83; H, 3.16; N, 7.33.

¹H NMR (300 MHz, CDCl₃, 298 K) δ (ppm): 4.53 (t, J=4.8Hz, 1H,

OH), 4.93 (d, J=1.4 Hz, 2H, CH₂), 7.05 (d, J=7.8 Hz, 1H), 7.25-

7.31 (m, 2H), 7.41-7.43 (m, 3H), 7.53 (d, J=8.1 Hz, 1H), 7.59-

7.61 (m, 2H), 8.07 (d, J=8.1 Hz, 1H). ¹³C{¹H} NMR (75.5 MHz,

CDCl₃, 298 K) δ (ppm): 64.21, 118.93, 124.56, 127.48, 128.84,

129.73, 132.04, 135.19, 137.14, 138.96, 143.02, 158.35.

Found: C, 67.20; H, 5.43; N, 5.07. Calc. for C₁₆H₁₃NO_S·H₂O: C,

67.34; H, 5.30; N, 4.91.

Synthesis of 8-phenylthioquinoline-2-carboxylic acid. To a

solution of 8-fluoroquinoline-2-carboxylic acid (0.96 g, 5 mmol)

in dry DMF (20 mL) was added NaH (0.48 g, 20 mmol) under

stirring, then Ph-SH (1.65 g, 15 mmol) were added to the

mixture. The mixture was heated at 150°C for 12 h. After

removing the solvent under vacuum, the residue was

dispersed in 40 mL water, and then was adjusted to pH~2 by

using HCl (6 M). The mixture was extracted with ethyl acetate Synthesis of 2-bromomethyl-8-phenylthioquinoline. To

a

(3 × 20 mL). The combined organic solution was dried over sealed tube charged with (8-phenylthioquinolin-2-yl)-methanol

anhydrous Na₂SO₄and the solvent was removed under (1.88 g, 7 mmol) and HBr (7 mL, 33% solution in CH₃COOH)

vacuum. The residue was purified through column were heated under 100

chromatography on silica gel with ethyl acetate/pentane (1/4) temperature, the mixture was added to 100 mL ice water and

as eluent, give a yellow solid product (1.20 g, 86%). NaHCO₃was added to neutralize the acids. Then the product

°C for 5 h, after cooling to room

¹H NMR (300M Hz, CDCl₃, 298 K) δ (ppm): 7.18 (d, J=7.2 Hz, was extracted with CH₂Cl₂(3 × 30 mL). The organic solution

1H), 7.47 (s, 4H), 7.59 (s, 2H), 7.68 (d, J=8.1 Hz, 1H), 8.26 (d, was dried over anhydrous Na₂SO₄and the solvent was

J=8.4 Hz, 1H), 8.37 (d, J=8.1 Hz, 1H). ¹³C{¹H} NMR (75.5 MHz, removed under vacuum. The residue was purified through the

CDCl₃, 298 K) δ (ppm): 119.67, 124.64, 127.70, 129.29, 129.33, column chromatography on silica gel with EA/PE (1/4, V/V) as

130.00 130.27, 131.13, 139.27, 140.22, 142.33, 144.46, 164.07. eluent to give

a yellow solid of 2-bromomethyl- 8-

Found: C, 64.11; H, 4.45; N, 4.76. Calc. for C₁₆H₁₁NO₂S·H₂O: C, phenylthioquinoline (1.90 g, 80%).

64.21; H, 4.38; N, 4.68.

¹H NMR (300 MHz, CDCl₃, 298 K) δ (ppm): 4.76 (s, 2H, CH2),

6.93 (d, J=7.8 Hz, 1H), 7.25 (t, J=7.8 Hz, 1H), 7.43-7.49 (m, 4H),

7.58-7.65 (m, 3H), 8.08 (d, J=8.4 Hz, 1H). ¹³C{¹H} NMR (75.5

MHz, CDCl₃, 298 K) δ (ppm): 34.45 (s, CH₂), 121.92, 123.87,

125.31, 127.20, 129.14, 129.79, 131.43, 140.34, 143.51,

155.98. Found: C, 58.54; H, 3.49; N, 4.46. Calc. for C₁₆H₁₂BrNS:

C, 58.19; H, 3.66; N, 4.24.

Synthesis of ethyl 8-phenylthioquinoline-2-carboxylate. To a

solution of 8-phenylthioquinoline-2-carboxylic acid (1.13 g, 4

mmol) in dry EtOH (25 mL) was added 98% H₂SO₄(1.8 mL), and

the mixture was heated at 90°C for 5 h. After cooling to the

room temperature, the solvent was removed under vacuum

and the residue was dispersed in 200 mL water, then the

solution was adjusted to pH ~ 6 by adding saturated NaHCO₃, Synthesis of 1-mesityl-3-(8-(phenylthioquinoline-2-yl)methyl)

and the product was extracted with ethyl acetate (3 × 20 mL). -1H-imidazol-3-ium bromide (HL2Br). A suspension of 2-

The combined organic solution was dried over anhydrous bromomethyl-8-phenylthioquinoline (3.3 g, 10 mmol) and 1-

Na₂SO₄. After the solvent was removed under vacuum, the mesityl-1H-imidazole (2.23 g, 12 mmol) in toluene (55 mL)

residue was recrystallized by CH₂Cl₂/hexane to give a yellow were heated at 110°C for 20 h. After cooling to room

solid of ethyl 8-phenylthioquinoline-2-carboxylate (1.25 g, temperature, the precipitate was collected by filtration and

96%).

¹H NMR (300 MHz, CDCl₃, 298 K) δ (ppm): 1.50 (t, J=7.2 Hz, 3H, give a white solid of HL2Br (5.11 g, 98%).

CH₃), 4.53 (q, J=7.2 Hz, 2H, CH₂), 6.97 (d, J=6.9 Hz, 1H), 7.34 (t,

washed with Et₂O, and then dried under vacuum at 80°C to

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¹H NMR (300 MHz, CDCl₃, 298 K) δ (ppm): 2.12 (s, 6H, o-CH₃), reaction mixture was heated at 165

ARTICLE

°

C for 12 h in an open

2.32 (s, 3H, p-CH₃), 6.45 (s, 2H, CH₂), 6.98 (s, 2H), 7.03 (d, J = system under purified nitrogen. After cooling to the room

7.5 Hz, 1H), 7.23 (s, 1H), 7.33 (t, J = 7.2 Hz, 1H), 7.44-7.48 (m, temperature, the unreacted alcohol was removed under

5H), 7.59 (d, J = 7.8 Hz, 1H), 7.96 (d, J = 8.1 Hz, 1H), 8.20 (d, J = vacuum and the residue was purified by column

8.1 Hz, 1H), 8.25 (s, 1H), 10.22 (s, 1H, NHC-H). ¹³C {¹H} NMR chromatography on silica gel with ethyl acetate/pentane (1/4,

(75.5 MHz, CDCl₃, 298 K) δ (ppm): 17.68 (s, o-CH₃), 20.96 (s, p- v/v) as eluent to yield pure 2-substituted-benzimidazole as a

1

CH₃), 53.61 (s, CH₂), 121.29, 124.60, 127.09, 127.60, 128.69, white solid, which is characterized by H NMR and ¹³C NMR in

129.63, 130.59, 131.62, 134.19, 134.54, 137.73, 137.91, comparison with the standard sample.

138.89, 140.97, 143.69, 151.96. Found: C, 58.13; H, 4.52; N,

Procedure for H₂gas production. Under

a nitrogen

7.12. Calc. for C₂₈H₂₆BrN₃S·CH₂Cl₂: C, 57.91; H, 4.69; N, 6.99.

Synthesis of complex [RuH(CO)(L2)](PF₆) (2). To a solution of alcohol (2.70 g, 25 mmol), NaBPh₄(0.05 mmol), and complex

L2Br (0.51 g, 1 mmol) in THF (30 mL) was added Ag₂O (0.14 g, 1a (0.005 mmol) were added to a 25 mL schlenk tube which is

atmosphere, benzene-1,2-diamine (0.27 g, 2.5 mmol), benzyl

H

0.6 mmol), and the mixture was stirred in dark for overnight to connected with a gas collection instrument through gravity

afford Ag-NHC intermediate. RuHCl(CO)(PPh₃)₃(0.95 g, 1 drainage method.^7cThe reaction mixture was heated at 150°C

mmol) was added to the solution and stirred at room (oil bath). Over a period of time, the volume of the gas was

temperature for 1 h, then heated at 65°C for 12 h. After recorded (seeing the supporting information, S4-5). The

cooling to room temperature, the precipitate was filtered off hydrogen was confirmed by the GC. After cooling to the room

through a celite pad. The brown filtrate was taken to dryness temperature, the mixture was treated with the same

under vacuum, and then 10 mL MeOH was added. To the procedure according to the Method

A. A blank experiment

solution was added NH₄PF₆(1.63 g, 10 mmol) and stirred at without catalyst was taken at the same condition.

room temperature for 12 h. The yellow precipitate was

Catalytic acceptorless dehydrogenation of the benzyl alcohol

collected by the filtration and was recrystallized from a

to benzaldehyde. Benzyl alcohol (7.5 mmol), complex 1a

CH₂Cl₂/Et₂O solution to give yellow crystals of complex

41%).

2

(0.4 g,

(0.005 mmol), NaBPh₄(0.05 mmol) 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 room temperature, the solution was subjected to GC-MS

and ¹HNMR analysis. The yield of benzaldehyde was

determined by ¹HNMR.

³¹P{¹H} (DMSO-D₆) δ (ppm): 23.74 (s), -144.21 (m, PF₆). ¹H

NMR (300 MHz, CDCl₃, 298 K) δ (ppm): 1.75 (s, 3H, o-CH₃), 1.91

(s, 3H, o-CH₃), 2.22 (s, 3H, p-CH₃), 4.63 (d, J = 15.9 Hz, 1H,

CHH), 5.57 (d, J = 15.9 Hz, 1H, CHH), 6.76 (s, 1H), 6.83-6.84 (m,

2H), 6.94-7.00 (m, 6H), 7.13-7.17 (m, 6H), 7.29-7.33 (m, 3H),

7.40-7.42 (m, 3H), 7.55-7.61 (m, 4H), 7.73-7.76 (m, 2H), 7.89

-

Condensation of benzaldehyde and benzene-1,2-diamine.

Benzaldehyde (3.0 mmol), benzene-1,2-diamine (2.5 mmol),

complex 1a (0.005 mmol), NaBPh₄(0.05 mmol) and mesitylene

(1 mL) were mixed in a 25 mL schlenk tube and the reaction

mixture was heated at 165°C for several hours in an open

system under purified nitrogen. After cooling to the room

temperature, the mixture was treated with the same

2

(d, J = 3.3 Hz, 1H), 8.32 (d, J = 7.8 Hz, 1H), -7.26 (d, J_PH=107.4

Hz, 1H) ppm. ¹³C{¹H} NMR (101 MHz, CDCl₃, 298 K) δ (ppm):

18.09 (s, o-CH₃), 18.80 (s, o-CH₃), 20.90 (s, p-CH₃), 54.86 (s,

CH₂), 122.76, 122.92, 124.19, 127.69, 128.49, 128.58, 128.84,

128.98, 129.62, 130.02, 130.58, 132.12, 132.44, 132.55,

133.41, 133.72, 134.54, 135.05, 135.71, 136.02, 136.35,

139.30, 139.82, 147.53, 157.66, 184.45 (s, NHC), 203.26 (s,

procedure according to the Method

Bruker KAPPA APEX DUO

N, 4.16. Calc. for C₄₇H₄₁F₆N₃OP₂RuS·CH₂Cl₂: C, 54.50; H, 4.10; N, diffractometer with graphite-monochromated Mo-Kα (λ=

A.

CO). IR (KBr pellets): 1953 cm^-1(CO). Found: C, 54.73; H, 3.97; X-ray crystallography.

A

3.97.

0.71073 Å) was employed to collect the intensity data for the

single crystal of complexes 1a 1b and . The data was

,

2

General procedure for catalytic reactions.

collected at about 100 K using ω-scan techniques. The

structure was solved by direct methods using SHELXL-2014.³³

Multi-scan empirical absorption corrections were applied to

the data set using the program SADABS.³⁴The structure was

refined with SHELXL-2014.³³Hydrogen atoms bound to carbon

were placed at calculated positions and refined using a riding

mode. All non-hydrogen atoms were refined by full-matrix

least squares on F²using the SHELXTL grogram package.³⁵Cell

refinement, data collection, and reduction were done by

Bruker SAINT.³⁶The crystallographic data is available in the SI

as CIF file.

Method A: Benzyl alcohol (0.81 g, 7.5 mmol), benzene-1,2-

diamine (0.27 g, 2.5 mmol), complex 1a

additive (0.05 mmol) were mixed in a 25 mL schlenk tube and

the reaction mixture was heated at 150 C (oil bath) for 12 h in

, 1b or 2 (0.005 mmol),

°

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.

Complex 1a + CH₂Cl₂: C₃₀H₂₅Cl₃NO₂PSRu, yellow prism, 0.07 x

Method B: Alcohol (7.5 mmol), benzene-1,2-diamine 0.06 x 0.05 mm³, monoclinic, s.g. P2₁/n, a = 12.868 (2) Å, b =

derivatives (2.5 mmol), complex 1a (0.005 mmol), NaBPh₄16.395 (3) Å, c = 13.619 (2) Å, β = 93.143 (3)

(0.05 mmol) were mixed in a 25 mL schlenk tube and the Z = 4, Fw = 701.96, F(000) = 1416, D_c= 1.625 Mg/m³, μ= 0.985

̊

, V = 2869.9 (6) Å³,

J. Name., 2013, 00, 1-3 | 9

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mm^-1. The final cycle of refinement based on F²gave an Science Fund of Hubei Province (2011CDB441), and the Funds

agreement factor R₁= 0.0316 for data with I>2σ (I) and wR2 = for Creative Research Groups of Hubei Province (2014CFA007).

0.0763 for all data (7433 reflections) with a goodness-of-fit of We also thank Prof. Xiaojun Wu for NMR supporting and

1.015. Idealized hydrogen atoms were placed and refined in analysis.

the riding mode, with the exception of H-Ru which was located

in the difference map and refined independently. The X-ray

References

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2

3

4

Complex

0.13 x 0.11 mm³, triclinic, s.g. P-1, a = 11.4078 (6) Å, b =

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Conclusions

In summary, we have illustrated three fully characterized

Ru(II) hydride complexes bearing new quinoline-based pincer

ligands. These complexes catalyse the dehydrogenative

condensation of primary alcohol and benzene-1,2-diamine to

the 2-substituted benzimidazole and H₂in the presence of

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order: 1a

≈ 1b > 2. The mechanistic studies demonstrate that

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the dehydrogenation of alcohol to aldehyde and H₂is slower

than the condensation of aldehyde and diamine. Also, complex

1a is an efficient catalyst precursor for the condensation of

benzaldehyde and benzene-1,2-diamine to 2-phenyl-

benzimidazole and H₂in high yield (95%). The present work

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Acknowledgements

This work was supported by the National Natural Science

Foundation of China (21171134, 21573166), the Natural

10 | J. Name., 2012, 00, 1-3

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Oxidant-free synthesis of benzimidazoles from alcohols and aromatic diamines catalysed by new Ru(II)-PNS(O) pincer complexes

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