Chiral linker-bridged bis-N-heterocyclic carbenes: Design, synthesis, palladium complexes, and catalytic properties

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Chiral Linker - Bridged Bis-N-Heterocyclic Carbenes: Design,

Synthesis, Palladium Complexes, and Catalytic Property†

Dao Zhang,^*Yu He and Junkai Tang

Received 00th January 20xx,

Accepted 00th January 20xx

A

series of chiral bis(benzimidazolium) salts 10-19 with (1R,2R)-cyclohexene, (1R,2R)-diphenylethylene and (aR)-

DOI: 10.1039/x0xx00000x

binaphthylene linker have been designed and synthesized in 30-94 % yield. Ten chiral bis(NHC) palladium complexes 20-28

have been synthesized and characterized by NMR, HRMS, elemental analysis and further confirmed by X-ray single crystal

analysis. These bis(NHC)-Pd complexes showed obviously different catalytic properties in the asymmetric Suzuki-Miyaura

coupling reactions. The (1R,2R)-cyclohexene-bridged bis(NHC)-Pd complex (R,R)-23 achieved the highest yield of 90%,

while complex (aR)-28, with a binaphthylene linker, showed the best enantioselectivity of 60 ee%. The structural analysis

of these complexes suggested that such difference of catalytic performance has the close relationships with their

coordination surroundings around metal centres.

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ester, amide and phosphine oxide substituents in the ortho-

position.^2bFurthermore, the interaction of the two aryl

moieties in antiparallel between chiral ligands and substrates,

Introduction

The palladium-catalyzed asymmetric Suzuki-Miyaura

coupling of aryl boronic acids and aryl halides to axially chiral

biaryls continues to be an area of high research interest due to

the wide existence of biaryl cores in numerous natural

products and chiral ligands.¹A typical Pd-catalyzed coupling

with good enantioselectivity usually results from its suitable

chiral auxiliary ligand that was first believed to govern the

spatial configuration of the palladium active center. Thus,

much effort has been devoted to search for more efficient

ligands that can produce unique palladium catalytic systems.

Several kinds of chiral ligands, including chiral mono- or

diphosphine ligands under homogeneous or heterogeneous

conditions,^2,3bishydrazone or phosphine−hydrazone chiral

ligands,⁴chiral diene ligands,⁵or a pyridylmethylamine ligand

with a stereogenic center,⁶have been designed for palladium-

catalyzed asymmetric Suzuki−Miyaura reactions, and some of

them showed evident asymmetric induction activity. For most

excellent chiral ligands, the suitable steric effect of the

substituent near the chiral moiety was thought to be the first

factor that influences ee values of the obtained biaryl

compounds. For example, The well-known chiral phosphine

existing in the oxidative addition and transmetalation

intermediates can enhance the enantioselectivity of the

coupling reaction. On this basis, the palladium catalyst with

bis-hydrazones ligand II (Figure 1) indeed provided excellent

enantioselectivities of up to >98% ee in the asymmetric

coupling of substituted naphthylene bromides with aryl

boronic acid at room temperature.^4aBesides, the chiral biaryl

backbone of phosphine-containing ligands seemed to be a

great benefit to the high ee values of biaryl products due to its

template effect.²

Ph

N

Ph

N

NMe₂

PCy₂

N

Ph

I

II

Fernandez & Lassaletta et al

4a

Buchwald et al ^2b

Figure 1. Two representative examples of chiral ligands in Pd-

catalyzed asymmetric Suzuki-Miyaura coupling

ligand I (Figure 1), later called (S)-KenPhos afforded the highest

ee value of up to 94 % for the coupling reaction between aryl

boronic acid and aryl bromide incorporating phosphonate

As strong

σ donor ligands, N-heterocyclic carbenes (NHCs)

have received much attention in the field of metal based

asymmetric catalysis.⁷The easy introduction of chiral elements

and the facile preparation of their precursors have made them

promising chiral auxiliary ligands in catalytic reactions with

high enantioselectivity. Although several research groups have

engaged in the study during the past few years, the use of

chiral NHC ligands

(

III

-VI,

Figure 2) in asymmetric

Suzuki−Miyaura coupling reacꢀons is rare and only limited

progress has been made.^8-11In 2010 Labande et al. firstly

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reported a kind of planar chiral ferrocenyl phosphine-NHC

We are currently interested in the design and application

ligand III supported palladium catalysts that gave an of NHCs to the synthesis of early and late transition metal

enantioselectivity of up to 46% ee for the Suzuki-Miyaura catalysts.^11,12Recently, we presented

series of chiral

a

coupling.^8aNotably, the best ee value of 80% was achieved cyclohexane-1,2-bridged bis(NHC)-Pd pre-catalysts (R,R)-VI

from a type of PEPPSI-Pd catalysts bearing chiral bulky (Figure 2) that was able to catalyse the asymmetric Suzuki-

monodentate NHC ligand IV by Kündig et al. in 2014.⁹Although Miyaura couplings between 1-halido-2-R-naphthalene and

this result did not reach the levels of literature precedent, naphthylboronic acid in good yields and moderate

chiral NHCs no doubt revealed their promising prospects of enantioselectivities.¹¹To further understand the structural

application in the Pd-catalyzed Suzuki-Miyaura coupling effect of chiral bis(NHC) ligands on the asymmetric catalysis,

reaction.

we undertook the modification of above bis(NHC) ligand

(Figure 3) by increasing steric hindrance and electronic effect

on X positions adjacent to the chiral source bridge, changing

the chiral cyclohexane-1,2-linker with (R,R)-diphenylethylene-

1,2-linker at the Y position, and introducing more chiral

element, such as R_p-cyclophane group on R substituents.

Moreover, considering that the axially chiral biaryl bridged

bis(NHC) ligand, reported Shi et al. has good catalytic activity in

metal-catalyzed enantioselective reactions,¹³we prepared

a

similar (aR)-binaphthylene bridged bis(NHC) palladium catalyst

for comparison's sake. We herein describe the design,

synthesis of these novel chiral bis(NHC) ligands and their

palladium complexes, as well as their behaviours in

Suzuki−Miyaura coupling reacꢀons.

Results and discussion

Synthesis of Chiral Bis-Benzimidazoles

Figure 2. Chiral NHC palladium complexes reported in

We first attempted to synthesize (1R,2R)-cyclohexene-

bridged bis-benzimidazoles, with one or two alkyl substituents

on the X positions of benzimidazolium moieties, respectively

(Figure 3). According to an improved literature method for

asymmetric Suzuki-Miyaura coupling reactions

(R,R)-1a ¹¹

, the reaction of (1R,2R)-diaminocyclohexane (DACH)

L-tartrate with 2-fluoro-1-R-3-nitrobenzene (R = Me, CF₃) in

the presence of 4 equiv of K₂CO₃at 80 °C for 24 h afforded

nitrobenzene derivatives 2b and 2d, which were smoothly

reduced by hydrogen gas at atmosphere pressure using Pd/C

as catalyst to give tetraamines 3b and 3d in almost

quantitative yields (Scheme 1 & 2). Similar methods could be

adopted in the synthesis of the unsymmetrical compounds 2c

and 3c using two different 2-fluoro-1-R-3-nitrobenzenes (R = H,

Me etc). Compounds 3b, 3c and 3d underwent cyclization to

produce the corresponding 1,2-cyclohexene-bridged bis-

benzimidazoles (R,R)-1b, 1c and 1d in 60-80 % yield using

excess triethyl orthoformate. It should be noted that, besides

1d, two by-products 1d′ and 1d′′ usually were found in the

reaction mixture by TLC. They could then be conveniently

separated from each other by gel chromatography and further

identified by NMR and high-resolution mass spectra (HRMS).

Next we tried to prepare similar bis-benzimidazoles from

(1R,2R)-diphenylethylenediamine

aforementioned method (Scheme 3). As expected, the chiral

bis-benzimidazole (R,R)- was successfully obtained from o-

(DPEN)

using

the

4

fluoro-nitrobenzene through three continuous steps in high

yield. However, when 2-fluoro-1-CF₃-3 -nitrobenzene was used

as starting material, the result was not in line with our

prediction. The simple and environmental friendly reduction of

Figure 3. Targeted chiral linker - bridged bis(NHC) Ligands.

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5b with H₂gas failed to give the desired CF₃substituted binaphthylene-linked bis-benzimidazole (aR)-

tetraamine product which is similar to ; several broken from (aR)-[1,1'-binaphthalene]-2,2'-diamine according to the

fragments 7a

7b and 7c were isolated and identified by NMR literature method.¹³

8 was prepared

6

,

and HRMS after workup. Moreover, the axially chiral

Scheme 1. Synthesis of (1R,2R)-cyclohexene-bridged bis-benzimidazoles 1a-c

Scheme 2. Synthesis of (1R,2R)-cyclohexene-bridged bis-benzimidazole 1d

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Scheme 3. Synthesis of (1R,2R)-diphenylethylene-bridged bis-benzimidazole 4

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Scheme 4. Synthesis of (1R,2R)-cyclohexene-bridged bis(benzimidazolium) salts 10-14 and their palladium complexes 20-24

Br

R

N

2 R-Br

N

Ph

Br

Ph

N

Ph

N

9A,C-E

Pd(OAc)₂, DMSO

（

）

R

Pd

N

50 ^oC 2h;100 ^oC 2h

O

Br

R

Br

O

4

R = A, 15, 94 %

R = C, 16, 85 %

R = D, 17, 80 %

R = E, 18, 75 %

R = A, 25, 59 %

R = C, 26, 50 %

R = D, 27, 55 %

Scheme 5. Synthesis of (1R,2R)-diphenylethylene-bridged bis-benzimidazolium salts 15-18 and their palladium complexes 25-27

Scheme 6. Synthesis of (aR)-binaphthylene-bridged bis(benzimidazolium) salt 19 and its palladium complex 28

Synthesis of Chiral Bis(Benzimidazolium) Salts.

week; the yield (less than 5%) is still very low and no satisfied

NMR could be obtained. The reason for this was attributed to

the two CF₃groups that greatly decrease the nucleophilicity

of nitrogen atoms of bis-imidazoles. Moreover, the common

purification of the crude by gel chromatograph made thing

worse and more unidentified impurities appeared after

workup.

Chiral bis(benzimidazolium) salts (R,R)-10 - 14, 15 - 18 and

19 have been efficiently synthesized through direct alkylation

reactions of the bis-benzimidazoles with different benzyl

bromides (Scheme 4-6). It is worthy to note that the reaction

should be carried out under an inert atmosphere and the

solvent 1,4-dioxane must be dried by standard method.

Otherwise the reaction was not clean and more by-products

were found. A mixture of the bis-imidazole (1b-c, 4 and 8)

and 2.2 equiv. of R-Br (9A, R_p-9B, or 9C-E) was refluxed for 18

hours in 1,4-dioxane to afford the crude imidazolium salts.

After purification by silica gel column chromatography, these

bis(benzimidazolium) salts were isolated as white solids in

high yields. All these bis(benzimidazolium) salts were

characterized by ¹H and ¹³C NMR and HRMS.

However, we have been bothered with the synthesis of

CF₃-containing bis(benzimidazolium) salt (Scheme 7) for a

long time. We ever tried to increase the amount (up to 5

Scheme 7. Attempt to alkylation of CF₃-containing bis-

benzimidazole 1d with benzyl bromide 9A

equiv. to 1d) of R-Br (9A) or extend the reaction time to a

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Synthesis of Chiral Bis(NHC) Palladium Complexes

NMR and HRMS data of bis(benzimidazolium) salts and

corresponding palladium complexes are listed in Table 1. In

the NMR spectra of these bis(NHC)-Pd(II) complexes the

signals of the NCHN protons at 11.37 - 13.38 ppm for the

bis(imidazolium) salts disappeared completely, while the

characteristic peaks of carbene carbon (Pd-C) were observed

at 169.6 - 177.3 ppm. Notably, from their ¹³C NMR spectra

there existed two types of carbene carbon atoms in these

With the expected bis(benzimidazolium) salts in hand, the

metalation reactions were attempted to prepare the

corresponding chiral palladium carbene complexes.

Previously we have found that the coligands, including Br⁻, Cl⁻,

AcO⁻, CF₃COO⁻, and water, have little influence on the

catalytic results of this kind of bis(NHC)-Pd catalysts in Suzuki-

Miyaura coupling reactions.^11bAccordingly, only neutral

halide complexes (for example, Scheme 4, (R,R)-20) were

taken into account in the synthesis of metal carbene

compounds. The in-situ deprotonation of imidazolium salts

by Pd(OAc)₂proved to be a reliable synthetic pathway for

these chelating bis(NHC) palladium complexes. The synthetic

route for chiral 1,2-cyclohexene-bridged bis(NHC) palladium

complexes is depicted in Scheme 4. Treatment of the above

bis(NHC) precursors with 1.0 equivalent of Pd(OAc)₂in DMSO

at 50 ^oC for 2 h and 100 ^oC for another 3 h afforded the

cyclometalated palladium complexes except complex 28 (δ

175.9 ppm) with a chiral binaphylene linker. For example, the

signals for the C_NHC-Pd of compounds 23 and 25 appear at

177.3, 170.7 ppm and 175.8, 170.4 ppm, respectively. The

result indicates that these complexes are C₁-symmetric and

two distinct ligand resonances could be found in their ¹H

NMR spectra, which is similar to the chiral trans-9,10-

dihydro-9,10-ethanoanthracene-11,12-diyl (DEA) bridged bis-

NHC palladium complex reported by Veige et al.^15d

All bis(NHC) palladium complexes were in good agreement

with the results by HRMS (Table 1). Although no peak was

observed at the position corresponding to the parent

molecule M, there was a dominant fragment ion [M − Br]⁺

that showed the most dominant intensity (100%) as the base

peak in their mass spectra. In some case, the fragment ion

formed by bond cleavage between the alkyl carbon and

nitrogen could be observed.

desired NHC metal complexes (R,R)-21

- 24 in moderate to

high yields. Following this route, the synthesis of chiral 1,2-

diphenylethylene- and binaphthylene-bridged bis(NHC)-Pd

complexes (R,R)-25-27 and (aR)-28 have been also developed

(Scheme 5-6). The purification of these NHC palladium

complexes was performed by gel chromatograph and

recrystallization from a mixture of dichloromethane and

hexane. However, the reaction of 1,2-diphenylethylene-

bis(benzimidazolium) salt (R,R)-18 with Pd(OAc)₂gave a

completely insoluble solid, which hindered us to characterize

it further.

Structural Analysis of Chiral Linker - Bridged Bis(NHC) Palladium

Complexes

The obtained bis(NHC) palladium complexes 20 - 28 are air-

and moisture-stable in the solid state and even in common

organic solvents. The introduction of bulky t-Bu groups

greatly improved the solubility of NHC metal compounds,

which brings convenience to their characterization and the

follow-up catalytic application. For example, the palladium

complexes with R groups A or C (Figure 3) more easily

dissolve in organic solvents (for example, THF, CH₂Cl₂and

CH₃Cl) than others do. Especially, no satisfied ¹³C NMR data

could be obtained for complexes 21 and 24 with two

Crystals of complexes 23 and 24 (Figure S64 and Table S1)

suitable for X-ray structure determination were developed

from dichloromethane/hexane solution. As shown in Figure 4,

complex 23 features a distorted square planar coordination

of the palladium with the two carbene carbon atoms in a cis

disposition, which is a good orientation for Suzuki-Miyaura

coupling reactions. The seven-membered palladacycle

adopted a pseudo-boatlike conformation (Figure 5), and the

selected bond lengths, angles and dihedral angles of 23 are

denoted in the caption of Figure 4, which fall comfortably

within the range found other chelating or cis bis(NHC)-Pd(II)

complexes.^11,13-15

paracyclophane (R_p-B) substituents.

All palladium NHC complexes were fully characterized by

NMR, HRMS and elemental analysis. The selected ¹H & ¹³C

Table 1. Selected ¹H and ¹³C NMR and HRMS data of bis(benzimidazolium) salts 10-19 and bis(NHC) palladium complex 20

-28

Bis(benzimidazolium)

Salt

Bis(NHC)-Pd

Complex

m/z ([M-Br]⁺) ^c

¹H (NCHN) ^a

¹³C (C_carbene-Pd) ^b

Calcd.

Found

10

11

12

13

14

15

16

17

19

11.96

20

21

22

23

24

25

26

27

28

175.6, 169.8

N.D. ^d

1209.3892

943.2414

1223.4024

1237.4205

971.2727

1307.4024

1065.3721

871.0707

1209.3929

943.2434

1223.4094

1237.4226

971.2734

1307.4128

1065.3736

871.0726

1379.4031

12.49

12.81,12.59

11.90

9.79 ^e

176.1, 169.7

177.3, 170.7

N.D. ^d

13.38

175.8, 170.4

176.5, 171.5

175.4, 169.6 ^e

175.9

13.06

13.29

11.37

1379.4024

d

^aIn ppm (400 MHz) in CDCl₃at 20 ^oC. ^bIn ppm (100.5MHz) in CDCl₃at 20 ^oC. ^cHRMS (positive ions). Not determined due to its

low solubility. ^eIn DMSO-d₆.

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conformation in complexes 29 and 30 without methyl groups.

Among the three dihedral angles of two carbene rings [NHC

(C7) and NHC(C37)] and the PdC(7)C(37)Br(1)Br(2)

coordination plane around palladium [for short, P(Pd)], the

NHC(C7)/P(Pd) dihedral angle [61.9°] is much smaller than

that [71.3°] in 29, while the other two are similar to each

other (92.7ºand 63.9º for 23, 92.5ºand 64.4º for 29).

Moreover, the C-Pd-C bite angle [85.53°] in 23 is bigger than

that [83.2(4)°] in 29. (2) Although the R substituent on N-

atom of NHCs has little effect on the bond distances, their

influence on bond angles and dihedral angles is significant (30

vs 29, Figure 4). The C-Pd-C bite angle is decreased from

84.3(3)° in 30 to 83.2(4)° in 29, while the Br-Pd-Br bite angles

is increased from 92.51(4)° in 30 to 94.52(6)° in 29 along with

the increase of the volume augmentation of the R groups. At

the same time one of the NHC/P(Pd) dihedral angles radically

enlarged nearly ten degrees (from 62.3ºin 30 to 71.3º in 29)

Figure 4. Molecular structure of complex (R,R)-23 with ellipsoids

drawn at the 50% probability level. All hydrogen atoms and tert-

butyl groups are omitted for clarity. Selected bond lengths (Å) and

bond angles (deg): Pd – C(7) = 1.960(8), Pd – C(37) = 2.004(7), Pd –

Br(2) = 2.4683(18), Pd – Br(1) = 2.4517(19), C(7) – Pd – C(37) =

85.5(3), C(37) – Pd – Br(1) = 89.1(2), C(7) – Pd – Br(2) = 91.3(2),

Br(1) – Pd – Br(2) = 94.04(5), C(7) – Pd – Br(1) = 174.5(2), C(37) –

Pd – Br(2) = 175.8(2). Dihedral angels (deg): NHC(C7)/NHC (C37) =

92.7, NHC(C7)/P(Pd)* = 61.9, NHC(C37)/P(Pd)* = 63.9 (Pd). Note:

P(Pd)* represents the palladium coordination plane defined by

[PdC(7)C(37)Br(1)Br(2)].

for the same reason. Moreover, the weak co-ligands (such as

H₂O molecule) can reinforce the bonding of carbene carbon

with the metal center, which shorten the two Pd-C bonds to

ca. 1.96 Å.^11b

Asymmetric Suzuki−Miyaura Coupling using Chiral Bis(NHC)-Pd

Complexes

An initial screening revealed that complex (aR)-28

performed best in the asymmetric coupling of 1-bromo-2-

methoxynaphthalene and naphthylboronic acid (Figure 6).

Optimization of the reaction conditions revealed a toluene

solution associated with Cs₂CO₃as base and 2 mol % of (R,R)-

20 to be an efficient combination for this reaction. On the

whole, these chiral bis(NHC)-Pd complexes were able to

sufficiently catalyse the Suzuki−Miyaura coupling: the

(1R,2R)-cyclohexene-bridged bis(NHC)-Pd complex (R,R)-23

Previously crystal structures of cyclohexane-1,2-based

11a

11b

bis(NHC) palladium complexes (R,R)-29

and (R,R)-30

(Figure 5) have been determined by X-ray diffractions. By

comparing 23 with them, we can draw some findings listed as

follows: (1) The introduce of methyl group on the fragment of

NHC ligands has led to the obvious structural change of

bis(NHC)-Pd(II) complexes (23 vs 29). The cyclohexane-1,2-

achieved the highest yield of 90%, while complex (aR)-28

with binaphthylene linker, showed the best

enantioselectivity of 60 ee%.

,

a

linker shows

a boat conformation in 23 but a chair

N-R =

N

N-R

N-R =

N

=

N

NO₂

O

Me

Bu^t

^tBu

Bu^t

^tBu

Figure 5. The side views of (1R,2R)-cyclohexane-bridged bis(NHC) palladium complex (R,R)-23 (left) and compounds (R,R)-29 ^11a

(middle), and (R,R)-30 ^11b(right).

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palladium complexes in mediating the coupling of aryl

boronic acids with low active aryl chlorides. These palladium

complexes revealed a high isolated yield (66-93%) of biaryls

at elevated temperature of 110 ^oC (Figure S65), which

suggested that they are high stabilities to the harsh reaction

conditions.

a)

(

)-28 (2 mol %)

aR

Ar¹

Ar²

(HO)₂B

Ar²

Ar¹

Br

Cs₂CO₃(2.4 equiv.)

toluene, 80 ^oC, 24 h

R

1.0 equiv

1.2 equiv

R = OMe, 53% yield, 60% ee

R = OCH₂Ph, 41% yield, 30% ee

b)

(R,R)-20 (2 mol %)

Ar¹

Ar²

(HO)₂B

Ar²

Ar¹

X

Cs₂CO₃(2.4 equiv.)

toluene, 80 ^oC, 24 h

MeO

1.0 equiv

1.2 equiv

X = Br, 65% yield, 43% ee

X =Cl, 52% yield, 64% ee

Figure 6. Suzuki−Miyaura coupling using chiral bis(NHC)-Pd

complexes 20-28

Scheme 8. Asymmetric Suzuki−Miyaura Coupling Using 20 and 28

As shown in Figure 6, the structure characteristics of these

chiral bis(NHC)-Pd complexes significantly affected their

catalytic behaviours. Complexes (R,R)-23 and (R,R)-24 with

two Me groups on the NHC fragments near to the chiral linker

showed slightly higher yields but lower ee values than their

analogues (R,R)-20 and (R,R)-21. Considering that complex 20

has a similar structure and catalytic property to 29 ^11a, it is

acceptable to draw a direct comparison of their coordination

surroundings around palladium centres in 23 and 29. The

biggest difference is that in 23, one of the NHC/P(Pd) dihedral

Conclusions

In conclusion we have designed and prepared a series of

novel chiral-linker bridged bis(bebzimidazolium) salts by

changing the chiral bridged linker, increasing steric hindrance

and introducing more chiral elements. The formed chiral

bis(NHC) palladium complexes have been full characterized

1

by H & ¹³C NMR and HRMS, and further confirmed by X-ray

single crystal analysis. These palladium complexes could

sufficiently catalyze the Suzuki−Miyaura coupling reactions of

aryl halides with substituted arylboronic acids in good yields

(up to 90 %) and moderate enantioselectivities (up to 60 %

ee), depending on their structure characteristics. Compared

with the higher ee % value of other palladium catalytic

angles (61.9º) is much smaller than that (71.3°) in complex 29.

For this particular reaction, the latter is benefit to good

enantioselectivity of the produced biaryls while the former

ready access to high yield of couplings. In addition to the

aforementioned chiral linker-related efforts, the planar chiral

R_p-cyclophane group has also been used to modify bis(NHC)

systems,

such

as

chiral

monophosphines,

the

ligands on their N-positions [(R,R,R_p,R_p)-21 and (R,R,R_p,R_p)-24

]

enantioselectivity produced by bi(NHC) palladium system has

plenty of room to move up.

for the sake of the high ee values; unfortunately, the effect in

practice is not as exception.

We anticipated that a bulkier substituent at the 2-position

of the aryl boronic acid would lead to higher selectivities. As

such, the use of 2-benzyloxyl-naphthylboronic acid was

investigated (Scheme 8a). The benzyloxyl group set close to

the Ar¹−Ar²bond generates more steric hindrance in the

transition state in comparison to the previously used

methoxy group. This hypothesis proved correct; only 30 % ee

value could be obtained using complex (aR)-28 under the

previously optimized conditions. Besides, these bis(NHC)-Pd

catalysts were sensitive to aryl halide (Scheme 8b). Using

chloronaphthalene to replace its bromide analogue as a

coupling partner led to an improved selectivity from 43 % to

64 % ee, a slightly low yield of 52% was observed for complex

(R,R)-20. Based on this result, we explored other bis(NHC)

Experimental

General Methods

All manipulations were performed under an inert

atmosphere of dry argon by using vacuum line and Schlenk

tube techniques. The solvents were dried and distilled prior

to use by literature methods. All chemical reagents were

purchased from commercial sources (Acros, Aldrich, Alfa

Aesar, or Fluka) and used as received unless otherwise

indicated.

Starting Materials and Reagents

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ARTICLE

Bis(NHC) palladium complex (R,R)-20 ^11b

Journal Name

,

bis-benzimidazole

9C ^11b9D ^11b9E^11b

(R,R)-3a^11awere

Bis-

A mixture of bis-imidazole (0.50 mmol) and benzyl bromide

o

(1.33 mmol) was stirred at 100 C in 1,4-dioxane (5.0 mL) for

(aR)-8 ¹³benzyl bromides 9A ^11bR_p-9B ¹⁶

and compounds (R,R)-1a ^11a(R,R)-2a ^11a

synthesized following literature procedures.

benzimidazoles (R,R)-1b-d, (R,R)- and relevant compounds

(R,R)-1d , (R,R)-1d′′, (R,R)-2b-d, (R,R)-3b-d, (R,R)-5a-b, (R,R)-

,

18 h. Then the solvent was removed under reduced pressure

and the residue was purified by

a silica gel column

4

chromatography (eluent: EtOAc/CH₂Cl₂/CH₃OH/Et₃N

8/2/1/0.2) to afford the bis(benzimidazolium) salt.

=

′

6,

7a-b and (R)-7c were prepared and their characterization

data, including NMR full spectra were list in Supporting

Information.

(R,R,R_p,R_p)-11. Compound 11 was prepared from (R,R)-1a

(158 mg, 0.50 mmol) and R_p-9B (400 mg, 1.33 mmol) as a

white solid. Yield: 234 mg (51 %). Anal. Calcd for C₅₄H₅₄Br₂N₄

(M = 918.8414 g mol^–1): C, 70.59; H, 5.92; N, 6.10; Found: C,

70.44; H, 5.75; N, 6.12. HRMS (positive ions): m/z 839.3524,

757.4270 (calcd for [M–Br]⁺839.3511, [M–2Br-H]⁺757.4261).

¹H NMR (400 MHz, CDCl₃): δ 12.50 (s, 2H, NCHN), 8.94 (s, 2H,

ArH), 7.57(s, 2H, ArH), 7.26 - 7.23(m, 4H, ArH), 6.89 - 6.81 (m,

14H, ArH), 6.47 - 6.33 (m, 2H, CH), 5.60 (d, J = 15.0 Hz, 2H,

CH₂), 5.20 (d, J = 14.1 Hz, 2H, CH₂), 3.13 - 2.92 (m, 6H, CH₂),

2.69 (m, 4H, CH₂), 2.49 - 2.39 (m, 8H, CH₂), 2.08 (s, 4H, CH₂),

1.80(s, 2H, CH₂). ¹³C-NMR (100 MHz, CDCl₃): δ 141.4 (CH

_imidazolium), 139.4 (C_Ar), 139.0 (C_Ar), 138.2 (C_Ar), 135.7 (C_Ar),

134.2 (C_Ar), 134.1 (C_Ar), 133.2 (C_Ar), 132.4 (CH_Ar), 131.7 (CH_Ar),

130.3 (CH_Ar), 129.3 (CH_Ar), 128.0 (CH_Ar), 127.4 (CH_Ar), 114.7

(CH_Ar), 113.5 (CH_Ar), 60.5 (CH), 51.8 (CH₂), 46.2 (CH₂), 35.1

(CH₂N), 34.8 (CH₂), 34.4 (CH₂), 34.1 (CH₂), 24.3 (CH₂).

Characterizations

¹H and ¹³C NMR spectra were recorded on JEOL ECA-400

1

and Bruker AV500 spectrometers. H and ¹³C chemical shifts

(δ) are given in ppm (the residual peak of deuterated solvents

was used as reference). Elemental and high-resolution mass

spectral (HRMS) analyses were carried out by the Analysis

Center, Fudan University. Optical rotations were measured in

a 100 mm cell with a Perkin-Elmer 241 photopolarimeter.

Flash column chromatography was performed on silica gel

(300−400 mesh). Analyꢀcal HPLC was performed using an

Agilent 1100 series chromatograph with a JASCO PU-980

pump and Agilent 1100 Series detection system, together

equipped with a Chiralcel OJ-H column. GC chromatograms

were recorded on a HP 4890A GC equipped with a DB-5 MS

UI capillary column, and the products were identified by

comparison with authentic samples.

(R,R)-12. Compound 12 was prepared from (R,R)-1c (158

mg, 0.50 mmol) and benzyl bromide 9A (577 mg, 1.33 mmol)

as a white solid. Yield: 534 mg (89 %). Anal. Calcd for

C₆₅H₇₈Br₂N₆O₆(M = 1199.1549 g mol^–1): C, 65.10; H, 6.56; N,

7.01; Found: C, 64.99; H, 6.48; N, 7.05. HRMS (positive ions):

m/z 1119.5165, 1037.5905 (calcd for [M–Br]⁺1119.5146, [M–

2Br-H]⁺1037.5917). ¹H NMR (400 MHz, CDCl₃): δ 12.81 (s, 1H,

NCHN), 12.59 (s, 1H, NCHN), 9.27 (d, J = 7.2 Hz, 1H, ArH), 8.27

(d, J = 12.8 Hz, 4H, ArH), 7.73 (t, J = 8.4 Hz, 4H, ArH), 7.62 (t, J

= 8.0 Hz, 1H, ArH), 7.40 (t, J = 8.0 Hz, 1H, ArH), 7.33 – 7.26 (m,

6H, ArH), 7.16 (t, J = 4.8 Hz, 1H, ArH), 6.94 (d, J = 2.4 Hz, 1H),

6.79 (br, 1H), 6.67 – 6.61 (m, 1H), 6.12 (d, J = 14.8 Hz, 1H),

5.99 – 5.93 (m, 1H), 5.82 (d, J = 14.8 Hz, 1H, CH₂), 5.71 (d, J =

14.8 Hz, 1H, CH₂), 5.54 (d, J = 14.8 Hz, 1H, CH₂), 5.17 (t, J =

12.8 Hz, 1H, CH₂), 5.06 – 4.99 (m, 2H), 3.18(s, 3H), 2.68 – 2.63

(m, 1H), 2.94 – 2.31 (m, 3H), 2.17 – 2.05 (m, 7H), 1.40 (s, 18H,

CH₃), 0.92 (d, J = 13.2 Hz, 18H, CH₃). ¹³C NMR (100 MHz, CDCl₃)

δ 153.0 (C_Ar-O), 152.9 (C_Ar-O), 148.2 (C_Ar-NO2), 148.0 (C_Ar-NO2),

147.6 (C_Ar-NO2), 143.8 (C_Ar-CH2), 143.7 (C_Ar-CH2), 142.7 (C_Ar-CH3),

141.9 (C_Ar), 141.6 (C_Ar), 131.7 (C_Ar-Bu_t), 131.2 (C_Ar-But), 130.8,

130.3 (C_Ar-But), 130.0 (C_Ar-But), 128.3 (CH_Ar), 127.7 (CH_Ar), 127.5

(CH_Ar), 127.2 (CH_Ar), 127.1 (CH_Ar), 126.0 (C_Ar-CH3), 125.9 (CH_Ar),

125.4 (CH_Ar), 125.1 (CH_Ar), 124.4 (CH_Ar), 124.2 (CH_Ar), 124.0

(CH_Ar), 115.0 (CH_Ar-NO2-m), 113.3 (CH_Ar), 111.1 (CH_Ar), 75.9

(CH₂O), 75.8 (CH₂O), 63.0 (CH), 60.0 (CH), 46.8 (CH₂N), 46.7

(CH₂N), 35.9 (C_But), 35.5 (C_But), 34.9 (C_But), 31.0 (CH_{3 But}), 29.7

(CH₂), 24.1 (CH₂), 21.1 (CH₃).

X-ray Crystallographic Studies

Single crystals of 23 and 24 suitable for X-ray analysis were

sealed into a glass capillary, and the intensity data of the

single crystals were collected on the CCD Bruker Smart APEX

system. Data obtained with the ω−2θ scan mode were

collected on a Bruker SMART 1000 CCD diffractometer with

graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at

293 K. The structures were solved using direct methods,

while further refinement with full-matrix least squares on F²

was obtained with the SHELXTL program package.¹⁷All non-

hydrogen atoms were refined anisotropically. Hydrogen

atoms were introduced in calculated positions with the

displacement factors of the host carbon atoms.

Crystal Data for 23: C₆₆H₇₈Br₂N₆O₆Pd, Mr = 1317.56,

colorless lamellar crystal, 0.18 × 0.10 × 0.01 mm, monoclinic,

space group P 21/c, a = 20.197(17) Å, b = 18.197(15) Å, c =

18.417(15) Å, β = 96.093(14)°, V = 6730(10) Å³, Z = 4, ρ_calcd

=

1.300 g cm⁻³, μ = 1.514 mm⁻¹, F(000) =2720, T = 293(2) K; I >

2σ(I), R₁= 0.0690 and wR₂= 0.1446; all data, R₁= 0.1766 and

wR₂= 0.1612; GOF 0.925, 11844 independent reflections (2θ

≤ 50.02°) and 768 parameters, 206 restraints. The CCDC file

1447381 contains the supplementary crystallographic data

for this paper. This data can be obtained free of charge from

the Cambridge Crystallographic Data Centre via

ccdc.cam.ac.uk/products/csd/request.

(R,R)-13. Compound 13 was prepared from (R,R)-1b (172

mg, 0.50 mmol) and 9A (577 mg, 1.33 mmol) as a white solid.

Yield: 558 mg (92 %). Anal. Calcd for C₆₆H₈₀Br₂N₆O₆(M =

1213.1860 g mol^–1): C, 65.34; H, 6.65; N, 6.93; Found: C, 65.33;

H, 6.63; N, 7.00. HRMS (positive ions): m/z 1133.5327,

1051.6071 (calcd for [M–Br]⁺1133.5302, [M–2Br–H]⁺

1051.6061). ¹H-NMR (400 MHz, CDCl₃): δ 12.06 (s, NCHN, 2H),

General Procedure for the Synthesis of Chiral Linker-Bridged

Bis(Benzimidazolium) Salts.

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

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Journal Name

ARTICLE

8.26 (d, J = 8.8 Hz, 4H, ArH), 7.68 (d, J = 8.8Hz, 4H, ArH), 7.39-

7.33 (m, J = 8.4 Hz, 4H, ArH), 7.27 (2H, ArH), 7.23 (d, J = 2.0 Hz,

2H, ArH), 6.78 (d, J = 2.4 Hz, 2H, ArH), 6.21 (dd, J = 9.2, 4.4 Hz,

2H, CH_Cy), 5.96 (d, J = 14.8 Hz, 4H, CH₂), 5.75 (d, J = 14.8 Hz,

4H, CH₂), 5.10 (d, J = 14.0 Hz, 4H, CH₂), 4.96 (d, J = 14.0 Hz, 4H,

CH₂), 3.34 (s, 6H, CH₃), 2.93 (m, 2H, CH₂), 1.46 (m, 2H, CH₂),

2.04 - 1.93 (m, 4H, CH₂), 1.38 (s, 18H, CH₃), 0.82 (s, 18H, CH₃).

¹³C NMR (100 MHz, CDCl₃): δ 152.9 (C_Ar-O), 147.8 (C_Ar-NO2),

147.6 (C_Ar-NO2), 143.8 (C_Ar-CH2), 142.6 (C_Ar-CH3), 140.5 (C_Ar),

131.7 (C_Ar-But), 131.1 (C_Ar-But), 130.0 (C_Ar-But), 127.7 (CH_Ar),

127.1 (CH_Ar), 126.3 (C_Ar-CH3), 125.6 (CH_Ar), 125.5 (CH_Ar), 124.2

(CH_Ar), 123.9 (CH_Ar), 111.8 (CH_Ar), 111.1 (CH_Ar),75.8 (CH₂O),

61.1 (CH), 46.5 (CH₂N), 36.1 (C_Bu_t), 35.5 (C_But), 34.3 (C_But), 31.2

(CH_{3 But}), 31.0 (CH₂), 23.9 (CH₂), 21.0 (CH₃).

HRMS (positive ions): m/z 961.4820, 879.5567 (calcd for [M–

Br]⁺961.4818, [M–2Br-H]⁺879.5577). ¹H NMR (400 MHz,

CDCl₃): δ 13.05 (s, 2H, NCHN), 9.47 (d, J = 8.4 Hz , 2H, ArH),

8.54 (s, 2H, ArH), 8.26 (d, J = 7.4 Hz, 4H, ArH), 7.70 (t, J = 5.0

Hz, 2H, ArH), 7.57 (d, J = 4.4 Hz, 2H, ArH) 7.44 (t, J = 7.8 Hz,

2H, ArH), 7.30 - 7.20 (m, 8H, ArH), 7.16 - 7.14 (m, 2H, CH),

5.74 (d, J = 14.4 Hz, 2H, CH₂), 5.63 (d, J = 10.4 Hz, 2H, CH₂),

3.79 (s, 6H, CH₃), 1.21 (s, 18H, CH₃), 1.05 (s, 18H, CH₃). ¹³C

NMR (100 MHz, CDCl₃): δ 155.5 (C_Ar-O), 147.3 (C_Ar), 142.5

(CH_imidazolium), 133.6 (C_Ar), 129.5 (CH_Ar), 128.8 (CH_Ar), 128.1

(CH_Ar), 127.5 (CH_Ar), 125.9 (CH_Ar), 125.3 (C_Ar-CH2), 116.8 (CH_Ar),

113.1 (CH_Ar), 64.6 (CH), 64.0 (CH₂O), 47.4 (CH₂N), 35.1 (C_But),

34.4 (C_Bu_t), 31.2 (CH_{3 But}), 30.9 (CH_{3 But}).

(R,R)-17. Compound 17 was prepared from (R,R)-4 (207 mg,

(R,R,R_p,R_p)-14. Compound 14 was prepared from (R,R)-1b

(172 mg, 0.50 mmol) and R_p-9B (400 mg, 1.33 mmol) as a

white solid. Yield: 142 mg (30%). Anal. Calcd for C₅₆H₅₈Br₂N₄

(M = 946.8945 g mol^–1): C, 71.03; H, 6.17; N, 5.92; Found: C,

70.94; H, 6.11; N, 6.03. HRMS (positive ions): m/z 865.3850,

785.4596 (calcd for [M – Br]⁺865.3845, [M–2Br-H]⁺785.4583.)

¹H NMR (400 MHz, (CD₃)SO): δ 9.78 (s, 2H, NCHN), 7.72 (d, J =

8.40 Hz, 2H, ArH), 7.60 - 7.56 (m, 2H, ArH), 7.42 (d, J = 7.44 Hz,

2H, ArH), 6.70 - 6.68 (m, 2H, ArH), 6.43 - 6.42 (m, 4H), 6.25 (d,

J = 7.0 Hz, 2H), 5.85 - 5.84 (m, 2H, CH), 5.40 (d, J =1 4.8 Hz, 2H,

CH₂), 5.18 (d, J = 14.4 Hz, 2H, CH₂), 2.93 - 2.69 (m, 18H), 2.50 -

2 .41 (m, 4H, CH₂), 1.67 - 1.62 (m, 2H, CH₂). ¹³C NMR (100MHz,

(CD₃)SO): δ 141.0 (CH _imidazolium), 139.6 (C_Ar), 139.2 (C_Ar), 138.4

(C_Ar), 135.7 (C_Ar), 134.8 (C_Ar), 133.7 (C_Ar), 133.3 (C_Ar), 132.6

(C_Ar), 132.0 (C_Ar), 131.9 (C_Ar), 130.8 (CH_Ar), 129.9 (CH_Ar), 129.7

(CH_Ar), 127.6 (CH_Ar), 125.3 (CH_Ar), 114.5 (CH_Ar), 60.6 (CH), 49.7

(CH₂N), 34.9 (CH₂), 34.7 (CH₂), 34.1 (CH₂), 24.2 (CH₂), 19.2

(CH₃).

0.50 mmol) and benzyl bromide 9D (306 mg, 1.33 mmol) as a

white solid (eluent: EtOAc/CH₂Cl₂/CH₃OH/PE = 8/2/1/2). Yield:

339 mg (80%). Anal. Calcd for C₄₂H₃₄Br₂N₆O₄(M = 846.5652 g

mol^–1): C, 59.59; H, 4.05; N, 9.93; Found: C, 59.55; H, 4.01; N,

10.02. HRMS (positive ions): m/z 765.1826, 685.2567 (calcd

for [M–Br]⁺765.1825, [M–2Br-H]⁺685.2563). ¹H NMR (400

MHz, CDCl₃): δ 13.27 (s, 2H, NCHN), 9.61 (d, J = 8.6 Hz, 2H,

ArH), 8.35 (s, 2H, ArH), 8.26 (d, J = 7.5 Hz, 4H, ArH), 7.82 -

7.77 (m, 6H, ArH), 7.63 (t, J = 7.8 Hz, 2H, ArH), 7.47 (d, J = 8.4

Hz, 2H, ArH), 7.40 (d, J = 8.6 Hz, 4H, ArH), 7.34 - 7.30 (m, 4H,

ArH), 7.25 (t, J = 7.4 Hz, 2H, CH), 5.85 (d, J = 15.2 Hz, 2H, CH₂),

5.66 (d, J=15.2 Hz, 2H, CH₂). ¹³C NMR (100 MHz, CDCl₃): δ

148.1 (C_Ar-NO2), 141.8 (CH_imidazolium), 138.8 (C_Ar), 132.7 (C_Ar-CH2),

131.6 (C_Ar), 130.2 (C_Ar), 129.9 (C_Ar), 129.0 (C_Ar), 128.8 (CH_Ar),

128.7 (CH_Ar), 128.4 (CH_Ar), 124.2 (CH_Ar), 117.2 (CH_Ar), 112.0

(CH_Ar), 65.8 (CH), 50.1 (CH₂N).

(R,R)-18. Compound 18 was prepared from (R,R)-8e (207

mg, 0.50 mmol) and 9E (294 mg, 1.33 mmol) as a white solid

(eluent: EtOAc/CH₂Cl₂/CH₃OH/PE = 8/2/1/2). Yield: 321 mg

(75%). Anal. Calcd for C₅₀H₄₀Br₂N₄(M = 856.6874 g mol^–1): C,

70.10; H, 4.71; N, 6.54; Found: C, 69.93; H, 4.65; N, 6.62.

HRMS (positive ions): m/z 775.2450, 695.3216 (calcd for [M–

Br]⁺775.2436, [M–2Br-H]⁺695.3175). ¹H NMR (400 MHz,

CDCl₃): δ 13.04 (s, 2H, NCHN), 9.62 (d, J = 8.6 Hz, 2H, ArH),

8.44 - 8.40(m, 4H, ArH), 7.77 (br s, 16H, ArH), 7.61 - 7.59 (m,

4H, ArH), 7.55 - 7.51 (m, 2H, ArH), 7.46 - 7.27 (m, 2H, ArH),

7.23 - 7.19 (m, 2H, ArH), 6.96 (d, J = 8.5 Hz, 2H, CH₂), 5.65 (d, J

= 14.9 Hz, 2H, CH₂). ¹³C NMR (100 MHz, CDCl₃): δ 141.6

(CH_imidazolium), 133.2 (C_Ar), 133.1 (C_Ar), 132.8 (C_Ar), 131.8 (C_Ar),

130.1 (C_Ar), 129.8 (C_Ar), 129.6 (C_Ar), 129.3 (C_Ar), 129.2 (C_Ar),

128.3 (CH_Ar), 128.0 (CH_Ar), 127.8 (CH_Ar), 127.6 (CH_Ar), 127.6

(CH_Ar), 127.0 (CH_Ar), 126.8 (CH_Ar), 124.2 (CH_Ar), 116.7(CH_Ar),

112.4 (CH_Ar), 65.7 (CH), 51.8 (CH₂N).

(R,R)-15. Compound 15 was prepared from (R,R)-

0.50 mmol) and 9A (577 mg, 1.33 mmol) as a white solid

(eluent: EtOAc/CH₂Cl₂/CH₃OH 8/2/1). Anal. Calcd for

4 (207 mg,

=

C₇₂H₇₈Br₂N₆O₆(M = 1283.2343 g mol^–1): C, 67.39; H, 6.13; N,

6.55; Found: C, 67.32; H, 6.10; N, 6.57. Yield: 603 mg (94 %).

HRMS (positive ions): m/z 1203.5128, 1121.5905 (calcd for

[M–Br]⁺1203.5146, [M–2Br-H]⁺1121.5911. ¹H NMR (400 MHz,

CDCl₃): δ 13.37 (s, 2H, NCHN), 9.52 - 9.50 (d, J = 8.4 Hz ,2H,

ArH), 8.46 (s, 2H, ArH), 8.28 - 8.20 (m, 8H, ArH), 7.72 - 7.68 (m,

6H, ArH), 7.50 - 7.42 (m, 4H, ArH), 7.27 - 7.21 (m, 6H, ArH),

7.15 - 7.11 (m, 2H, ArH), 7.04 (s, 2H, CH), 5.76 (d, J = 14.3 Hz,

2H, CH₂), 5.65 (d, J = 8.4 Hz, 2H, CH₂), 5.10 - 4.96 (m, 4H, CH₂),

1.40 (s, 18H, CH₃), 0.87(s, 18H, CH₃). ¹³C NMR (100 MHz,

CDCl₃) δ 151.9 (C_Ar-O), 147.3 (C_Ar-NO2), 146.7 (C_Ar-NO2), 142.6

(C_Ar), 141.5 (CH_imidazolium), 140.4 (C_Ar), 132.8 (C_Ar-But), 130.9

(CH_Ar), 129.4 (CH_Ar), 128.6 (CH_Ar-NO2-m), 127.7 (CH_Ar), 127.5

(CH_Ar), 126.8 (CH_Ar), 126.0 (CH_Ar), 125.0 (C_Ar-CH2), 124.3 (CH_Ar),

123.8 (CH_Ar), 123.1(CH_Ar), 115.8 (CH_Ar), 111.8 (CH_Ar), 63.9

(CH₂O), 45.2 (CH₂N), 34.5 (C_But), 33.4 (C_But), 30.1 (CH_{3 But}), 30.0

(CH_{3 But}).

(aR)-19. Compound 19 was prepared from (aR)-8 (243 mg,

0.50 mmol) and 9A (577 mg, 1.33 mmol) as a white solid

(eluent: EtOAc/CH₂Cl₂/CH₃OH/PE = 8/2/1/1). Yield: 528 mg

(78%). Anal. Calcd for C₇₈H₇₈Br₂N₆O₆(M = 1355.2985 g mol^–1):

C, 69.12; H, 5.80; N, 6.20; Found: C, 69.09; H, 5.75; N, 6.24.

HRMS(positive ions): m/z 1275.5178, 1193.5917 (calcd for

[M–Br]⁺1275.5166, [M-2Br-H]⁺1193.5905). ¹H NMR (400

MHz, CDCl₃): δ 11.37 (s,2H,NCHN), 8.22 (d, J = 8.4 Hz, 6H,

ArH), 8.03 - 8.02 (m, 2H, ArH), 7.92 (d, J = 8.2, 2H, ArH), 7.76

(d, J = 7.8 Hz, 4H, ArH), 7.76 (d, J = 7.8 Hz, 4H, ArH), 7.53 -

(R,R)-16. Compound 16 was prepared from (R,R)-4 (207 mg,

0.50 mmol) and 9C (415 mg, 1.33 mmol) as a white solid

(eluent: EtOAc/CH₂Cl₂/CH₃OH = 8/2/1). Yield: 442 mg (85%).

Anal. Calcd for C₆₀H₇₂Br₂N₄O₂(M = 1041.0473 g mol^–1): C,

69.22; H, 6.97; N, 5.38; Found: C, 69.16; H, 6.87; N, 5.43.

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

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Journal Name

7.42 (m, 4H, ArH), 7.31 - 7.12 (m, 14 H, ArH), 5.47 (s, 4H, CH₂),

5.34 (d, J = 11.2 Hz, 2H, CH₂), 5.00 (d, J = 13.8 Hz, 2H, CH₂),

1.32 (s, 18H, CH₃), 1.18 (s, 18H, CH₃). ¹³C NMR (100 MHz,

CDCl₃) δ 152.9 (C_Ar-O), 147.5 (C_Ar-NO2), 144.5 (C_Ar-NO2), 143.0

(C_Ar), 142.0 (CH_imidazolium), 133.6 (C_Ar-But), 132.4 (C_Ar-But), 131.4

(C_Ar), 129.3 (C_Ar), 128.6 (CH_Ar), 128.1 (CH_Ar), 127.9 (CH_Ar),

127.6 (CH_Ar), 125.6 (C_Ar-CH2), 125.2 (C_Ar-CH2), 123.6 (CH_Ar), 115.1

(CH_Ar), 112.7 (CH_Ar), 77.9 (CH₂O), 77.3 (CH₂O), 76.8 (CH₂O),

74.9 (CH₂O), 47.8 (CH₂N), 35.3 (C_But), 34.6 (C_But), 31.4 (CH_{3 But}),

30.9 (CH_{3 But}), 29.6 (CH_{3 But}).

36.4 (C _But), 35.5 (C _But), 34.5 (C _But), 34.4 (C _But), 31.2 (CH_{3 But}),

31.0 (CH_{3 Bu}_t), 29.8 (CH₂), 25.7 (CH₂), 24.7 (CH₂), 20.0 (CH₃).

(R,R)-23. Complex 23 was prepared from (R,R)-13 (239.8

mg, 0.2 mmol) as a white solid. Yield: 159 mg (65%). Anal.

Calcd for C₆₆H₇₈Br₂N₆O₆Pd (M = 1317.5901 g mol^–1): C, 60.16;

H, 5.97; N, 6.38; Found: C, 60.13; H, 5.95; N, 6.41. HRMS

(positive ions): m/z 1237.4226 (calcd for [M–Br]⁺1237.4205).

¹H NMR (400 MHz, CDCl₃): δ 8.32 (t, J = 11.2 Hz, 2H), 8.14 (t, J

= 8.8 Hz, 2H), 7.82 (br s, 1H), 7.59 (br s, 2H), 7.23 (s, 1H), 7.18

(s, 1H), 7.00 (d, J = 7.6 Hz, 1H), 6.94 -6.87 (m, 3H), 6.48 (d, J =

8.0 Hz, 3H), 5.30 – 5.25 (m, 2H), 5.06 – 5.03 (m, 2H), 4.78 (brs,

1H), 3.00 (s, 1H), 2.81 (s, 3H), 2.76 (s, 3H), 2.56 – 2.46 (m, 3H),

2.19 (m, 2H), 1.77 (br s, 2H), 1.41 (s, 9H, CH₃), 1.37 (s, 9H,

CH₃), 0.97 (s, 9H, CH₃), 0.76 (s, 9H, CH₃). ¹³C NMR (100 MHz,

CDCl₃) δ 177.3 (NCN), 170.7 (NCN), 153.0 (C_Ar-O), 152.9 (C_Ar-O),

147.5 (C_Ar-NO2), 147.4 (C_Ar-NO2), 147.0 (C_Ar-NO2), 146.6(C_Ar-CH2),

144.4 (C_Ar), 144.3(C_Ar), 135.6 (C_Ar), 134.3 (C_Ar), 132.9 (C_Ar),

132.3 (C_Ar), 128.2 (CH_Ar), 127.9 (CH_Ar), 127.6 (CH_Ar), 127.0

(CH_Ar), 124.1 (C_Ar-CH3), 123.9 (CH_Ar), 123.8 (CH_Ar), 123.7 (CH_Ar),

123.4 (CH_Ar), 120.7 (CH_Ar), 120.1 (CH_Ar), 110.8 (CH_Ar), 110.5

(CH_Ar), 74.8 (CH₂O), 74.6 (CH₂O), 64.8 (CH), 62.1 (CH), 50.7

(CH₂N), 42.8, 36.4 (C_Bu_t), 35.4 (C_But), 35.3 (C_But), 34.5 (C_But),

34.2 (C_But), 31.2 (CH_{3 But}), 31.0 (CH_{3 But}), 30.9 (CH_{3 But}), 26.0

(CH₂), 24.7 (CH₂), 24.0 (CH₂), 20.0 (CH₃).

General Procedure for the Synthesis of Chiral Linker-Bridged

Bis(NHC) Palladium Complexes

A mixture of chiral bis(benzimidazolium) salt (0.2 mmol)

and 98% Pd(OAc)₂(44.8 mg, 0.2 mmol) in DMSO (3.0 mL) was

stirred at 50 °C for 2 h and then at 110 °C for 3 h. After the

solvent was removed by reduced pressure while heating, the

residue was purified by silica gel column chromatography

(eluent: 1/10 EtOAc/CH₂Cl₂) to afford a brown crude product.

The crude product was then washed with toluene to afford

the pure palladium complex

(R,R,Rp)-21. Complex 21 was prepared from (R,R)-11 (167.8

mg, 0.2 mmol) as a light yellow solid. Yield: 153 mg (75%).

Anal. Calcd for C₅₄H₅₂Br₂N₄Pd (M = 1023.2455 g mol^–1): C,

63.38; H, 5.12; N, 5.48; Found: C, 63.25; H, 5.07; N, 5.60.

HRMS (positive ions): m/z 943.2434 (calcd for [M–Br]⁺

943.2414). ¹H NMR (400 MHz, (CD₃)₂SO): δ 7.81(d, J = 8.6 Hz,

1H, ArH), 7.74 (d, J = 8.2 Hz, 2H, ArH), 7.66 (d, J = 8.2 Hz, 1H,

ArH), 7.46 - 7.43 (m, 1H, ArH), 7.38 - 7.35 (m, 1H, ArH), 7.30 -

7.25 (m, 2H, ArH), 7.07 - 7.05 (m, 1H, ArH), 6.88 - 6.86 (m, 2H,

(R,R,R_p,R_p)-24. Complex 24 was prepared from (R,R,R_p,R_p)-

14 (189.4 mg, 0.2 mmol) as a light yellow solid. Yield: 120 mg

(57%). Anal. Calcd for C₅₆H₅₆Br₂N₄Pd (M = 1051.2986 g mol^–1):

C, 63.98; H, 5.37; N, 5.33; Found: C, 63.88; H, 5.33; N, 5.35.

HRMS (positive ions): m/z 971.2734 (calcd for [M–Br]⁺

1

971.2727). H NMR (400 MHz, (CD₃)₂SO): δ 8.13 (s, 1H, ArH),

7.55 (s, 1H, ArH), 7.33 - 7.13 (m, 3H, ArH), 6.98 - 6.29 (m,

14H), 6.09 (s, 1H), 5.75 - 5.61 (m, 3H), 6.80 - 6.78 (m, 1H),

5.25 (m, 2H, CH₂), 3.69 (m, 2H, CH₂), 3.25 - 3.15 (m, 3H, CH₂),

3.06 - 2.70 (m, 17H, CH₂), 2.18 - 1.64 (m, 4H, CH₂), 1.33 - 1.22

(m, 4H, CH₂). No satisfied ¹³C NMR spectrum could be

obtained due to its low solubility.

ArH), 6.70 (s, 3H), 6.85 - 6.39 (m, 8H), 6.32 (d, J = 7.7 Hz

，1H),

6.13 (s, 1H, CH), 5.70 - 5.52 (m, 4H, CH₂), 5.27 - 5.22 (m, 4H,

CH₂), 3.81 - 3.78 (m, 1H, CH₂), 3.33 - 2.76 (m, 12H, CH₂), 2.54 -

2.35(m, 5H, CH₂), 2.13 (d, J = 12.44 Hz, 1H, CH₂), 1.94 (s, 2H,

CH₂), 1.82 - 1.80 (m, 1H, CH₂), 1.66 (s, 1H, CH₂). No satisfied

¹³C NMR spectrum could be obtained due to its low solubility.

(R,R)-22. Complex 22 was prepared from (R,R)-12 (239.8

mg, 0.2 mmol) as a white solid. Yield: 177 mg (68%). Anal.

Calcd for C₆₅H₇₆Br₂N₆O₆Pd (M = 1303.5635 g mol^–1): C, 59.89;

H, 5.88; N, 6.45; Found: C, 59.71; H, 5.75; N, 6.55. HRMS

(positive ions): m/z 1223.4094 (calcd for [M–Br]⁺1223.4024).

¹H NMR (400 MHz, CDCl₃): δ 8.23 – 8.11 (m, 5H, ArH), 7.89 (s,

1H, ArH), 7.69 - 7.55 (m, 6H, ArH), 7.25 – 7.18 (m, 3H, ArH),

6.99 (t, J = 8.0 Hz, 1H), 6.91 -6.87 (m, 2H), 6.61 (d, J = 8.0 Hz,

2H), 6.50 – 6.48 (m, 1H), 6.11 (brs, 1H), 5.30 – 4.97 (m, 4H),

4.62 (brs, 1H), 3.00 (s, 1H), 2.68 (s, 3H), 2.50 – 2.33 (m, 2H),

2.26 – 2.14 (m, 2H), 2.04 – 1.99 (m, 1H), 1.84 – 1.77 (m, 2H),

1.45 – 1.36 (m, 18H, CH₃), 0.88 (br, 18H, CH₃). ¹³C NMR (100

MHz, CDCl₃) δ 176.1 (NCN), 169.7 (NCN), 153.1 (C_Ar-O), 153.0

(C_Ar-O), 147.5 (C_Ar-NO2), 147.4 (C_Ar-NO2), 146.6 (C_Ar-NO2), 144.6

(C_Ar-CH2), 144.4(C_Ar-CH2), 142.09 (C_Ar-CH3), 135.1 (C_Ar), 134.5 (C_Ar),

133.0 (C_Ar), 132.5 (C_Ar), 128.4 (CH_Ar), 128.0 (CH_Ar), 127.6 (CH_Ar),

127.5 (CH_Ar), 126.8 (CH_Ar), 124.2 (C_Ar-CH3), 124.1 (C_Ar-CH3), 123.8

(CH_Ar), 121.2 (CH_Ar), 112.9 (CH_Ar), 111.2 (CH_Ar), 111.0 (CH_Ar),

74.9 (CH₂O), 74.8 (CH₂O), 63.8 (CH), 62.1 (CH), 50.7 (CH₂N),

(R,R)-25. Complex 25 was prepared from (R,R)-15 (256.6

mg, 0.2 mmol) as a light yellow solid (eluent: EtOAc/CH₂Cl₂=

1/10). Yield: 164 mg (59%). Anal. Calcd for C₇₂H₇₆Br₂N₆O₆Pd

(M = 1386.6384 g mol^–1): C, 62.32; H, 5.22; N, 6.06; Found: C,

62.23; H, 5.20; N, 6.10. HRMS (positive ions): m/z 1307.4128

1

(calcd for [M–Br]⁺1307.4024). H NMR (400 MHz, CDCl₃): δ

9.46 (d, J = 12.16 Hz, 4H), 8.48 (s, 2H, ArH), 8.18 - 8.11 (m, 6H,

ArH), 7.90 (s, 2H, ArH), 7.66 (d, J = 11.2 Hz, 6H, ArH), 7.40 -

7.14 (m, 4H, ArH), 6.96 - 6.73 (m, 6H), 6.58 - 6.52 (m, 2H, CH₂),

6.17 - 6.01 (m, 2H, CH₂), 5.07 - 4.86 (m, 4H, CH₂), 1.41 (d, J =

10.8 Hz, 18H, CH₃), 0.74 (s, 18H, CH₃). ¹³C NMR (100 MHz,

CDCl₃): δ 175.8 (NCN), 170.4 (NCN), 153.1 (C_Ar-O), 152.9 (C_Ar-O),

147.4 (C_Ar-NO2), 147.3 (C_Ar-NO2), 147.0 (C_Ar-NO2), 146.6 (C_Ar),

130.3 (C_Ar), 129.4 (C_Ar), 127.7 (CH_Ar), 124.1 (CH_Ar), 123.8 (CH_Ar),

123.7 (CH_Ar), 112.6 (CH_Ar), 112.2 (CH_Ar), 111.8 (CH_Ar), 111.0

(CH_Ar), 74.9 (CH), 65.3 (CH₂O), 62.0 (CH₂O), 50.2 (CH₂N), 48.0

(CH₂N), 35.3 (C_Bu_t), 34.2 (C_But), 31.0 (CH_{3 But}), 30.9 (CH_{3 But}),

30.5 (CH_{3 But}), 29.6 (CH_{3 But}).

(R,R)-26. Complex 26 was prepared from (R,R)-16 (208.2

mg, 0.2 mmol) as a light yellow solid (eluent: EtOAc/CH₂Cl₂=

1/10). Yield: 115 mg (50%). Anal. Calcd for C₆₀H₇₀Br₂N₄O₂Pd

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

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ARTICLE

(M = 1145.4514 g mol^–1): C, 62.91; H, 6.16; N, 4.89; Found: C,

and the mixture was stirred and heated at the indicated

temperature. After the reaction mixture was heated to reflux

(when THF was used as a reaction solvent) or maintained at

70 °C for 24 h, it was treated with distilled water (10 mL),

extracted with CH₂Cl₂(3 × 10 mL), dried (MgSO₄), and purified

by flash chromatography (1/50 EtOAc/hexane) to give the

corresponding product.

63.00; H, 6.10; N, 4.95. HRMS (positive ions): m/z 1065.3736

1

(calcd for [M–Br^-]⁺1065.3721). H NMR (400 MHz, CDCl₃): δ

9.50 (d, J = 8.1 Hz, 2H, ArH), 8.48 (d, J = 7.4 Hz, 2H, ArH), 7.90

(d, J = 7.4 Hz, 2H, ArH), 7.64 (d, J = 8.4 Hz, 1H, ArH), 7.42 -

7.40 (m, 6H, ArH), 7.26 - 7.18 (m, 4H, ArH), 7.04 - 7.00 (m, 1H),

6.89 - 6.87 (m, 2H), 6.86 - 6.81 (m, 2H, CH), 6.78 - 6.61 (m, 2H,

CH₂), 6.30 (s, 1H, CH₂), 6.16 (d, J = 16.3 Hz, 1H, CH₂), 5.94 (d, J

= 15.5 Hz, 1H, CH₂), 5.64 (s, 1H, CH₂), 3.91 (s, 3H, CH₃), 3.59 (s,

3H, CH₃) ,1.443 (d, J = 5.2 Hz, 18H, CH₃), 0.74 (m, 18H, CH₃).

¹³C NMR (100 MHz, CDCl₃) δ 176.4 (NCN), 171.7 (NCN), 155.4

(C_Ar-O), 155.2 (C_Ar-O), 146.2 (C_Ar), 145.7 (C_Ar), 141.8 (C_Ar), 141.4

(C_Ar), 140.3 (C_Ar), 130.2 (C_Ar), 129.7 (C_Ar), 129.4 (C_Ar), 129.2

(C_Ar), 129.0 (CH_Ar), 124.0 (C_Ar-CH2), 123.9 (CH_Ar), 123.6 (CH_Ar),

123.1 (CH_Ar), 122.9 (CH_Ar), 112.7 (CH_Ar), 112.5 (CH_Ar), 111.5

(CH_Ar), 110.7 (CH_Ar), 65,4 (CH), 62.9 (CH₂O), 62.5 (CH₂O), 62.0

(CH₂O), 50.1 (CH₂N), 47.4 (CH₂N), 35.2 (C_Bu_t), 34.2 (C_But), 34.2

(C_But), 31.0 (CH_{3 But}), 31.0 (CH_{3 But}), 29.6 (CH_{3 But}).

Acknowledgements

We gratefully acknowledge the National Natural Science

Foundation of China (No. 21272040) for the financial support.

Notes and references

1

(a) P. Loxq, E. Manoury, R. Poli, E. Deydier, A. Labande,

Coord. Chem. Rev., 2016, 308, 131; (b) D. Zhang, Q. Wang,

Coord. Chem. Rev., 2015, 286, 1; (c) J. Wencel-Delord, A.

Panossian, F. R. Leroux, F. Colobert, Chem. Soc.

Rev., 2015, 44, 3418; (d) M. C. Kozlowski, B. J. Morgan, E. C.

Linton, Chem. Soc. Rev., 2009, 38, 3193; (e) O. Baudoin, Eur.

J. Org. Chem., 2005, 4223; (f) G. Bringmann, A. J. P.

Mortimer, P. A. Keller, M. J. Gresser, J. Garner, M. Breuning,

Angew. Chem., Int. Ed., 2005, 44, 5384.

(R,R)-27. Complex 27 was prepared from (R,R)-17 (169.3

mg, 0.2 mmol) as a light yellow solid (eluent: EtOAc/CH₂Cl₂=

1/10). Yield: 105 mg (55%). Anal. Calcd for C₄₂H₃₂Br₂N₆O₄Pd

(M = 950.9693 g mol^–1): C, 53.05; H, 3.39; N, 8.84; Found: C,

52.87; H, 3.53; N, 8.94. HRMS (positive ions): m/z 871.0726

1

(calcd for [M–Br]⁺871.0707). H NMR (400 MHz, (CD₃)₂SO): δ

2

For asymmetric Suzuki−Miyaura coupling reacꢀons

involving chiral phosphines, see: (a) A. N. Cammidge, K. V.

9.22 (s, 3H, ArH), 8.50 (s, 2H, ArH), 8.25 - 8.10(m, 6H, ArH),

7.61 - 7.10 (m, 17H, ArH), 6.26 - 6.18 (m, 2H, CH₂), 5.9 (d, J =

16.4 Hz, 1H, CH₂), 5.53 (s, 1H, CH₂). ¹³C NMR (100 MHz, CDCl₃):

δ 175.4 (NCN), 169.8 (NCN), 147.7 (C_Ar-NO2), 147.6 (C_Ar-NO2),

143.6 (C_Ar), 142.8 (C_Ar), 136.8 (C_Ar), 129.5 (C_Ar), 129.4 (C_Ar),

129.3 (C_Ar), 129.1 (C_Ar), 128.9 (CH_Ar), 124.6 (CH_Ar), 124.3 (CH_Ar),

124.0 (CH_Ar), 113.0 (CH_Ar), 112.6 (CH_Ar), 112.4 (CH_Ar), 66.1

(CH), 60.6 (CH), 53.0 (CH₂N), 51.0 (CH₂N).

Crépy, Tetrahedron, 2004, 60

Buchwald, J. Am. Chem. Soc., 2000, 122, 12051; (c) J. F.

Jensen, M. Johannsen, Org. Lett., 2003, , 3025; (d) K.

, 4377; (b) J. Yin, S. L.

5

Mikami, T. Miymoto, M. Hatano, Chem. Commun., 2004,

2082; (e) M. G. Genov, A. Almorin, P. Espinet, Chem. Eur. J.,

2006, 12, 9346; (f) W. Tang, N. D. Patel, G. Xu, X. Xu, J.

Savoie, S. Ma, M.-H. Hao, S. Keshipeddy, A. G. Capacci, X.

Wei, Y. Zhang, J. J. Gao, W. Li, S. Rodriguez, B. Z. Lu, N. K.

Yee, C. H. Senanayake, Org. Lett. 2012, 14, 2258; (g) S.

Wang, J. Li, T. Miao, W. Wu, Q. Li, Y. Zhuang, Z. Zhou, L. Qiu,

Org. Lett. 2012, 14, 1966.

(aR)-28. Complex 28 was prepared from (aR)-19 (255.3 mg,

0.2 mmol) as a light yellow solid (eluent: EtOAc/CH₂Cl₂

=

3

4

For asymmetric Suzuki−Miyaura coupling using supported

chiral phosphines, see: (a) K. Sawai, R. Tatumi, T. Nakahodo,

H. Fujihara, Angew. Chem., Int. Ed., 2008, 47, 6917; (b) Y.

Uozumi, Y. Matsuura, T. Arakawa, Y. M. A. Yamada, Angew.

Chem., Int. Ed., 2009, 48, 2708; (c) T. Yamamoto, Y. Akai, Y.

1/10). Yield: 160 mg (55%). Anal. Calcd for C₇₈H₇₆Br₂N₆O₆Pd

(M = 1459.7026 g mol^–1): C, 64.18; H, 5.25; N, 5.76; Found: C,

64.10; H, 5.21; N, 5.81. HRMS (positive ions): m/z 1379.4031

1

(calcd for [M–Br^-]⁺1379.4024). H NMR (400 MHz, CDCl₃): δ

Nagata, M. Suginome, Angew. Chem., Int. Ed., 2011, 50

8844.

,

8.48 (d, J = 8.4 Hz, 4H, ArH), 8.01 (d, J = 8.2 Hz, 2H, ArH), 7.66

- 7.55 (m, 6H, ArH), 7.26 - 7.22(m, 4H, ArH), 6.92 - 6.56 (m,

12H), 5.95 (d, J = 8.0 Hz, 2H), 5.14 (s, 4H), 4.83 (d, J = 15.7 Hz,

2H, CH₂), 1.49 (s, 18H, CH₃), 1.05 (s, 18H, CH₃).¹³C NMR (100

MHz, CDCl₃) δ 176.1 (NCN), 152.6 (C_Ar-O), 133.0 (C_Ar), 132.5

(C_Ar), 131.6 (C_Ar), 130.0 (C_Ar), 127.9 (CH_Ar), 127.5 (CH_Ar), 127.2

(CH_Ar), 126.7 (CH_Ar), 124.4 (C_Ar-CH2), 123.8 (CH_Ar), 123.3 (CH_Ar),

112.6 (CH_Ar), 112.0 (CH_Ar), 111.6 (CH_Ar), 74.9 (CH₂O), 50.3

(CH₂N), 35.4 (C_But), 34.6 (C_But), 31.9 (CH_{3 But}), 31.3 (CH_{3 But}),

30.8 (CH_{3 But}), 29.6 (CH_{3 But}).

(a) A. Bermejo, A. Ros, R. Fernández, J. M. Lassaletta, J. Am.

Chem. Soc., 2008, 130, 15798; (b) A. Ros, B. Estepa, A.

Bermejo, A. Eleuterio, R. Ferna, J. M. Lassaletta, J. Org.

Chem., 2012, 77, 4740.

S.-S. Zhang, Z.-Q. Wang, M.-H. Xu, G.-Q. Lin, Org. Lett.,

2010, 12, 5546-5549.

G. Grach, A. Dinut, V. Terrasson, R. Medimagh, A. Bridoux,

V. Razafimahaleo, A. Gaucher, S. Marque, D. Prim, R. Gil, G.

Planas, C. Vi, I. Thomas, J. Roblin, Y. Troin, Organometallics,

2011, 30, 4074.

5

6

7

For reviews on metal-catalyzed asymmetric reactions

involving chiral NHCs, see: (a) V. César, S. Bellemin-

Laponnaz, L. H. Gade, Chem. Soc. Rev., 2004, 33, 619; (b) F.

Wang, L. Liu, C. Wang, M. Shi, Coord. Chem. Rev., 2012,

256, 804.

General Procedure for Asymmetric Suzuki−Miyaura Coupling

Reactions

The desired aryl halides (0.2 mmol), the corresponding

bis(NHC) palladium complex (0.006 mmol), the corresponding

aryl boronic acid (0.3 mmol), and base (0.50 mmol) were

introduced into an oven-dried Schlenk tube (25 mL).

Deoxygenated solvent (2 mL) was added, the flask was sealed,

8

9

(a) N. Debono, A. Labande, E. Manoury, J.-C. Daran, R. Poli,

Organometallics, 2010, 29, 1879; (b) P. Loxq, N. Debono, S.

Gülcemal, J.-C. Daran, E. Manoury, R. Poli, B. Cetinkaya, A.

Lababde, New J. Chem., 2014, 38, 338.

L. Benhamou, C. Besnard, E. P. Kündig, Organometallics,

2014, 33, 260.

J. Name., 2013, 00, 1-3 | 11

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Journal Name

10 L. Wu, A. Salvador, A. Ou, M. W. Shi, B. W. Skelton, R.

Dorta, Synlett, 2013, 24, 1215.

11 (a) G. Shi, J. Tang, D. Zhang, Q. Wang, Z. Chen, L. Weng, J.

Organomet. Chem., 2012, 700, 223; (b) Y. Li, J. Tang, J. Gu,

Q. Wang, P. Sun, D. Zhang, Organometallics, 2014, 33, 876.

12 (a) D. Zhang, N. Liu, Organometallics, 2009, 28, 499; (b) D.

Zhang, S. Zhou, Z. Li, Q. Wang, L. Weng, Dalton Trans.,

2013, 42, 12020; (c) L. Wan, D. Zhang, Organometallics,

2016, 35, 138.

13 (a) T. Chen, J. Jiang, Q. Xu, M. Shi, Org. Lett., 2007,

(b) W. Wang, T. Zhang, M. Shi, Organometallics, 2009, 28

9, 865;

,

2640; (c) G. Ma, T. Zhang, M. Shi, Org. Lett., 2009, 11, 875;

(d) L. Liu, F. Wang, M. Shi, Organometallics, 2009, 28, 4416;

(e) P. Gu, Q. Xu, M. Shi, Organometallics, 2013, 32, 7575.

14 L. G. Bonnet, R. E. Douthwaite, R. Hodgson,

Organometallics, 2003, 22, 4384.

15 (a) W.A. Herrmann, M. Elison, J. Fischer, C. Köcher, G. R. J.

Artus, Angew. Chem., Int. Ed., 1995, 34, 2371; (b) J.

Schwarz, V. P. W. Böhm, M. G. Gardiner, M. Grosche, W. A.

Herrmann, W. Hieringer, G. Raudaschl-Sieber, Chem. Eur. J.,

2000, 6, 1773; (c) T. Scherg, S. K. Schneider, G. D. Frey, J.

Schwarz, E. Herdtweck, W. A. Herrmann, Synlett, 2006,

2894;(d) A. B. Mullick, M. S. Jeletic, A. R. Powers, I. Ghiviriga,

K. A. Abboud, A. S. Veign, Polyhedron, 2013, 52, 810.

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Graphic Abstract

Article Doi

Chiral linker-bridged bis-N-heterocyclic carbenes: Design, synthesis, palladium complexes, and catalytic properties

DOI: 10.1039/c6dt00984k

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Article abstract of DOI:10.1039/c6dt00984k

Full text of DOI:10.1039/c6dt00984k

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