Bridged imidazolium salts used as precursors for chelating carbene complexes of palladium in the Mizoroki-Heck reaction

2894

LETTER

Bridged Imidazolium Salts Used as Precursors for Chelating Carbene

Complexes of Palladium in the Mizoroki–Heck Reaction¹

Precursors for

C

o

helating Ca

r

b

bene Compl

i

exe

s

of

a

Palladium

s

inthe Mizoroki–

S

H

eckReactio

c

n

herg, Sabine K. Schneider, Guido D. Frey, Jürgen Schwarz, Eberhardt Herdtweck, Wolfgang A. Herrmann*

Department Chemie, Lehrstuhl für Anorganische Chemie, Technische Universität München, Lichtenbergstraße 4, 85747 Garching,

Germany

Fax +49(89)28913473; E-mail: lit@arthur.anorg.chemie.tu-muenchen.de

Received 1 June 2006

Dedicated to Prof. Richard Heck on the occasion of his 75^thbirthday

much attention.^20,2,1We have published palladium NHC

Abstract: A variety of chiral and achiral imidazolium salts is syn-

complexes which can be used in C–C coupling reac-

thesized. Methylene-, ethylene-, propylene- and pyridinyl-bridged

bis(imidazolium) halides are used to generate the respective free

chelating carbenes. The synthesis of palladium complexes of gener-

tions.^22–24For example, some methylene-bridged bis(imi-

dazolium) salts, in combination with Pd(OAc)₂, were

al formula [cis-CH₂{NC(H)=C(H)N(R)C}₂PdX₂] with these chelat- equally efficient in Suzuki coupling with aryl chlorides as

substrates,^15,25and they even achieve the C–H activation

ing N-heterocyclic carbene ligands is reported. Structural proofs of

of methane.²⁶

complexes 28 and 46 are represented by X-ray diffraction studies.

Catalytic applications in the Mizoroki–Heck reaction are presented.

In the first part of the present article, a variety of new im-

Key words: catalysis, chelating ligands, chiral ligands, free car-

benes, Mizoroki–Heck reaction, imidazolium salts, palladium

idazolium salts will be presented. We will focus on the

variation of the bridging group Y and the substituents R¹

(Figure 1). Additionally, the isolation of the free carbene

1,1¢-dibutyl-3,3¢-ethylene-diimidazolin-2,2¢-diylidene

In advance of the Fischer^2,3and prior to the Schrock⁴car-

(26) has been achieved. The ammonia method is not ap-

benes, N-heterocyclic carbenes (NHCs) were discovered

plicable to deprotonate the methylene-bridged salts be-

by Wanzlick⁵and Öfele⁶in 1968. It turned out that the

cause of the higher acidity of the bridging methylene

free species of these carbenes are poorly accessible. Their

protons in comparison to the imidazolium ring protons.²⁷

renaissance started with the work of Arduengo⁷in 1991

who discovered the free carbene 1,3-diadamantyl-imida-

2⁺

H

zolin-2-ylidene. The bond between the N-heterocyclic

carbene and the metal center of a complex can be de-

scribed best as a dative s-bond, as the bond length falls

comfortably in the range of typical single bond lengths

(X-ray diffraction).^8,9One main feature of this type of car-

bene complexes is their resistance towards nucleophilic

and electrophilic attacks.

H

Y

N

H

N

H

(X^–)₂

N

R¹

R¹= Me, i-Pr, t-Bu, Cy, Ad, Mes, 2,6-(i-Pr)₂C₆H₃

rac-phenylethyl, (R)-phenylethyl, rac-1'-naphthylethyl

X = Cl, Br, I

Y = CH₂, (CH₂)₂, pyridinyl, 2,6-lutidine, o-, m-, p-xylylene

Today, N-heterocyclic carbenes play a major role as

ligands in organometallic chemistry. They make metal

complexes suitable for a broad spectrum of catalytic ap-

plications.^10–12For example, ruthenium complexes cata-

lyze olefin metathesis,¹³rhodium complexes catalyze

hydroformylation¹⁴and palladium complexes are used in

the catalytic C–C bond formation^12,15,16(e.g. Mizoroki–

Heck reaction¹⁷). In the absence of p-acidic co-ligands,

the strongly s-donating NHCs are expected to stabilize

high-oxidation state metal centers or generate highly reac-

tive, electron-rich complexes. In this context, it seems to

be of major interest to discuss the synthesis of this ligand

class and to show its good accessibility. In this context,

quite a few chelated complexes and their ligands are al-

ready known.^18,19The preparation of chelating ligands of

N-heterocyclic carbenes, which would provide extra air

and moisture stability for palladium centers, are receiving

Figure 1 R¹-substituted imidazolium salts with bridging groups Y

For this purpose, the weak base K[N(SiMe₃)₂] has been

successfully used.²⁸The substituted imidazole precursors

are accessible via a one-pot synthesis route related to a

method reported by Gridnev et al. (Scheme 1).²⁹

O

H

O

H

O

reflux

NH₄Cl

R

NH₂

R

N

– 3 H₂O

– HCl

Scheme 1 Synthesis of substituted imidazoles

The products can be purified by sublimation, extraction,

recrystallization, distillation, bulb-to-bulb distillation or

column chromatography.^29,30This method of synthesis,

however, is only suitable for aliphatic substituted imida-

SYNLETT 2006, No. 18, pp 2894–2907

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3

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1

.2

0

6

Advanced online publication: 25.10.2006

DOI: 10.1055/s-2006-951539; Art ID: S13306ST

LETTER

Precursors for Chelating Carbene Complexes of Palladium in the Mizoroki–Heck Reaction

2895

Table 1 Chelating Imidazolium Salts as Carbene (NHC) Precursors

zoles. rac-1-(Phenylethyl)imidazole (1a, Figure 2), (R)-1-

(phenylethyl)imidazole (1b),^30aand the new modified

rac-1-(1¢-naphthylethyl)imidazole (2), could be synthe-

sized via this procedure.²⁹In the case of aromatic groups,

the synthesis according to Liu et al. can be applied

(Scheme 2).^30b

R²

N

H

N

H

N

H

N

H

+

N

R

R¹

1

2 X^–

10, 15

3–9, 11–14, 19–25

N

*

N

+

N

+

1a rac

1b R

2

N

R¹

H

2 X^–

Figure 2

16–18

In order to synthesize a library of bridged imidazolium

salts, we used a general synthetic procedure. Two equiva-

lents of the R-imidazole (R = alkyl, aryl) were dissolved

in THF and brought into an ACE pressure tube. One

equivalent of the corresponding dibromo compound was

added and the resulting mixture was heated. The work-up

as well as the purification of the air-stable hygroscopic

colorless-to-pale-brown products¹⁷were achieved by fil-

tration of the obtained solids by washing with THF, and

were afterwards dried in high vacuo.³¹To the best of our

knowledge these are the first bridged diimidazolium salts

with the chirality next to the N-atoms reported to date.

Table 1 gives an overview of the prepared 1,3-bisazolium

salts.

Salt

R¹

R²

X^–

Yield

(%)

3²⁶

4

Me

CH₂

Br

I

83

96

79

98

67

80

72

68

86

90

85

71

90

71

75

49

83

76

97

93

80

75

i-Pr

5²²

6³²

7

t-Bu

Cy

Ad

Br

Cl

Br

8

2,6-(i-Pr)₂C₆H₃

9²³

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

Mes

Me

R¹

CH₂(p-C₆H₄)CH=CH₂CH₂

EtOH

N

r.t.

O

i-Pr

CH₂CH₂

–

R³

NH₂

+

20 h

R³

R²

t-Bu

R²

Ad

R¹

NH₄Cl

Me

H₂CO

R³

N

reflux

i-Pr

R²

t-Bu

–

Scheme 2 Synthesis of aryl-substituted imidazoles

Ad

–

Free carbenes have become available on a broad basis

through the ‘ammonia method’ (Scheme 3); thereby the

deprotonation takes place at low temperatures in liquid

ammonia with NaH as a strong base.¹⁸The deprotonation

of the corresponding imidazolium salts is carried out in an

argon atmosphere using a specially designed apparatus.¹⁸

The corresponding 1,3-bisimidazolium salt is suspended

in THF and ammonia is condensed via this special appa-

ratus into the reaction vessel at –78 °C. After removing

the cooling bath, NaH is added; an excess of NaH being

recommendable, especially if the ammonia has not been

dried before with potassium. After two hours a light yel-

low solution is formed.

rac-CH(C₆H₅)CH₃

rac-CH(C₁₀H₇)CH₃

rac-CH(C₆H₅)CH₃

rac-CH(C₁₀H₇)CH₃

CH₂

o-xylene

m-xylene

p-xylene

2,6-(CH₂)₂(C₅H₃N) Br

hexane. The product can be obtained in good yields at low

temperatures (–30 °C or –78 °C). This method is applica-

ble for the synthesis of sterically hindered cyclic free car-

benes, for saturated and unsaturated imidazolium salts or

After removal of ammonia, the free carbenes can be iso-

lated and crystallized. At the end the solvent (THF) is re-

moved in vacuo and the free carbene is extracted with n-

Synlett 2006, No. 18, 2894–2907 © Thieme Stuttgart · New York

2896

T. Scherg et al.

LETTER

⁺X^–

Å] (48), respectively, and the related non-chelating com-

plex (49) [cis-{(CH₃)NC(H)=C(H)N(CH₃)C}₂PdI₂] with

1.990(3) and 1.997(3) Å.³⁷The C–C and C–N bond dis-

tances within the imidazolin-2-ylidene based ring systems

and the Pd–C bond distances in complex 28 are consistent

with both contributions from s- and p-donation to the

metal center and p-stabilization of the carbene onto the

adjacent nitrogen centers. Other bond angles within the

molecule are unexceptional and do not require further

comments.

NaH

N

R

N

R

N

R

THF/NH₃, –78 °C

H

– NaX

– H₂

Scheme 3 Deprotonation of imidazolium salts via the ‘ammonia

method’

acyclic diamino carbenes, such as the bis(N,N-diiso-

propylamino)carbene.³¹However, as shown by Alder³³

and Herrmann,³⁴the route does not work for cyclic sub-

stituted acyclic bis(N,N-dialkylamino) carbenes. The

‘ammonia method’ can be used to deprotonate ethylene-

or higher bridged imidazolium salts, but as mentioned

previously not methylene-bridged imidazolium salts.

The neutral di(carbene)palladium(II) halide complexes 27

and 28 were prepared via an adaptation of an in situ car-

bene-generation procedure¹⁵which utilized longer reac-

tion times and milder temperatures to increase yields of

the complexes, particularly when bulky alkyl substituents

are present in the methylene-bridged diimidazolium salt

precursors²²(Scheme 4). The easiest way to prepare di-

iodide complexes 29 and 30 is via halide exchange of the

dibromo complexes 32,³⁵35³⁶with NaI (Table 2), rather

than direct reaction of the diimidazolium diiodide salt pre-

cursor with Pd(OAc)₂(31). The dihalide complexes crys-

tallize as very fine needles and have excellent solubility in

DMSO, good solubility in hot acetonitrile, are sparingly

soluble in CH₂Cl₂and THF and have no solubility in di-

ethyl ether and hydrocarbon solvents. Complex 28 crys-

tallizes as colorless rods suitable for X-ray structure

determination (Figure 3).

Figure 3 ORTEP-style plot of compound 28 in the solid state. Ther-

mal ellipsoids are drawn at the 50% probability level. Hydrogen

atoms are omitted for clarity. Selected bond lengths [Å] and bond an-

gles [deg]: Pd–Br1 2.4969(4), Pd–Br2 2.4858(4), Pd–C1 1.991(3),

Pd–C2 1.973(3), N1–C1 1.366(4), N1–C3 1.459(4), N1–C4 1.373(4),

N3–C2 1.357(4), N3–C3 1.456(4), N3–C7 1.379(4); Br1–Pd–Br2

93.33(1), Br1–Pd–C1 170.43(8), Br1–Pd–C2 89.90(8), Br2–Pd–C1

92.08(7), Br2–Pd–C2 175.62(8), C1–Pd–C2 84.3(1).

¹H NMR and ¹³C NMR spectra of the complexes 27–31

and 34–40 are in agreement with their assigned structures.

The appearance of inequivalent methylene proton reso-

nances for 29, 30 and 31 indicate the retention of a confor-

mationally restrained boat-shaped six-membered chelate

ring for the complexes, as was determined by X-ray crys-

tal structure analysis of the dibromide complex 28 and the

neutral complex 46. Such conformationally rigid ring sys-

tems have been noted by us and other groups in related

complexes as examples which are fluxional at room tem-

perature.^20j,32,40A detailed discussion about the coales-

cence temperature of different biscarbene ligands and the

coordinated anion of this palladium-complex-type is de-

picted in Table 2.

The crystal structure of complex 28 has shown the com-

pound to be monomeric with the dicarbene ligand chelat-

ing the palladium(II) center in a cis-fashion with a boat

conformation being observed for the six-membered

C₃N₂Pd ring. The remaining two coordination sites of

the distorted square planar coordinated palladium center

are occupied by bromine anions. A summary of important

bond distances and angles for complex 28 is depicted

in Figure 3. The Pd–C distances [1.991(3), 1.973(3)

Å] of the complex can be respectively compared with

those found in the dicationic²³and neutral chelating¹⁵

dicarbene

palladium

complexes

[cis-

CH₂{NC(H)=C(H)N(CH₃)C}₂Pd(NCCH₃)₂]²⁺[BF₄]^–

2

In the ¹H NMR an AB set for the protons of the NCH₂N-

bridged complexes were found at room temperature

[1.966(2), 1.972(3) Å] (47, see Figure 5) and [cis-

CH₂{NC(H)=C(H)N(CH₃)C}₂PdI₂] [1.988(7), 1.989(8)

N

R

N

Pd(OAc)₂

NaI

+

X

Pd

X

I

Pd

I

N

R

1. DMSO, 50 °C, 4 h

2. DMSO, 190 °C, 20 min

MeCN, H₂O

2 X^–

R

27: X = Cl, R = CH₂(p-C₆H₄)CH=CH₂

28: X = Br, R = (R)-CH(C₆H₅)CH₃

29: R = Et

30: R = Bz

31: R = Cy

Scheme 4

Synlett 2006, No. 18, 2894–2907 © Thieme Stuttgart · New York

LETTER

Precursors for Chelating Carbene Complexes of Palladium in the Mizoroki–Heck Reaction

2897

Table 2 ¹H NMR Measurements for the [cis-CH₂{N(H)C=C(H)N(R)C}₂PdX₂] Complexes, in Dependence of the N-Substituents and the

Anions of the Protons of the NCH₂N-Bridge

Complex

R

X

NCH₂N (25 °C) [ppm]

NCH₂N [ppm]

Coalescence

temperature (°C)

33²³

34^15,37

32³⁵

29

Me

Et

Br

I

6.28 (s)

–

6.15, 6.54 (AB, ²J_HH= 10.9 Hz)

6.09, 6.39 (AB, ²J_HH= 11.1 Hz)

6.35, 6.62 (AB, ²J_HH= 13.5 Hz)

6.21, 6.57 (AB, ²J_HH= 11.9 Hz)

6.13, 6.46 (AB, ²J_HH= 12.2 Hz)

6.28, 6.61 (AB, ²J_HH= 12.5 Hz)

6.19, 6.46 (AB, ²J_HH= 12.1 Hz)

6.23, 6.58 (AB, ²J_HH= 11.4 Hz)

6.11, 6.39 (AB, ²J_HH= 11.8 Hz)

6.00, 6.27 (AB, ²J_HH= 12.9 Hz)

6.30, 6.60 (AB, ²J_HH= 12.8 Hz)

6.41 (s)

6.35 (s)

6.31 (s)

6.44 (s)

6.41 (s)

6.49 (s)

6.33 (s)

6.39 (s)

–

56

Br

I

60

Et

79

35³⁶

30

Bz

Br

I

69

Bz

84

36²¹

37²¹

38³⁵

31

i-Pr

Cy

Br

I

128

154

141

>180

Br

I

Cy

39²²

40²²

t-Bu

Br

I

–

(Table 2). This occurs from retention of the bend-boat

conformation in these complexes (see Figure 4). There-

fore, fluctuation effects in solution were studied and for a

choice of complex the coalescence temperature was deter-

mined. The coalescence temperature is influenced by the

bulkiness of the N-substituents and the coordinated anion

(Br, I). We found that a ring inversion occurs already at

room temperature for R = CH₃and bromide as the coordi-

nated anion. In general the coalescence temperature for

secondary N-substituents is between 128 °C and 154 °C,

for tertiary N-substituents a temperature higher than

180 °C is necessary.

The preparation of the cationic species 42, 43, 44 and 46²³

was achieved by exchange reactions involving Na[PF₆],

K[PF₆] or Ag[BF₄] in cases where alkali metal salts were

effective in only replacing one halide ion, and isolated as

air-stable acetonitrile adducts (Scheme 5).

Figure 4 Solution-phase fluctuation effects in [cis-

CH₂{N(H)C=C(H)N(R)C}₂PdX₂] complexes

plex. The crystal structure determination has shown the

compound to be monomeric with the dicarbene ligand

chelating the palladium(II) center, the remaining two co-

ordination sites of the distorted square planar coordinated

palladium center being occupied by molecules of

Crystals of complex 46 suitable for X-ray crystal structure

determination were grown by vapor diffusion of diethyl

ether into a concentrated acetonitrile solution of the com-

⁺[PF₆]^–

Ag[BF₄]

or

Na[PF₆]

²⁺2X^–

N

R

N

K[PF₆]

MeCN Pd

Br

Br Pd

Br

MeCN Pd

MeCN

MeCN, H₂O

R

–

43: R = (R)-CH(C₆H₅)CH₃; X = BF₄

28: R = (R)-CH(C₆H₅)CH₃

39: R = t-Bu

45: R = Mes

42: R = t-Bu

–

44: R = t-Bu; X = BF₄

–

46: R = Mes, X = PF₆

Scheme 5

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2898

T. Scherg et al.

LETTER

acetonitrile (Figure 6). The Pd–C distances are, with

1.9653(8) and 1.9893(5) Å, nearly the same as those

in

the

analogous

methyl

complex

[cis-

CH₂{NC(H)=C(H)N(CH₃)C}₂Pd(NCCH₃)₂]²⁺[BF₄]^–

2

(47) [1.966(2), 1.972(3) Å]²³or the related non-chelating

[cis-{(CH₃)NC(H)=C(H)N(CH₃)C}₂PdI₂] complex (49)

[1.990(3), 1.997(3) Å].³⁷All complexes are, how-

ever, shortened significantly relative to the

dicationic

[{CH₂[NC(H)=C(H)N(CH₃)C]₂}₂Pd]²⁺2I^–(50) and the

chiral dicarbene complex [trans-{[syn-

tetracarbene

complex

Ph(Me)CH{NC(H)=C(H)N(Ph)C}₂]}₂PdI₂] (51, see

Figure 5) in which the Pd–C distances measure 2.137(5),

2.049(4) Å^18,21and 2.018(7), 2.042(7) Å,³⁸respectively, in

the absence of strong trans-effects of iodide and acetoni-

trile substituents.

Figure 6 HyperChem⁴¹model of complex 46 based on the data of a

single-crystal X-ray experiment. Hydrogen atoms are omitted for cla-

rity. Selected bond lengths [Å] and bond angles [deg] are taken from

ref.⁴²: Pd–N5 2.0664(2), Pd–N6 2.0589(4), Pd–C10 1.9893(5), Pd–

C14 1.9653(8), N5–C26 1.1394(3), C26–C27 1.4557(3); N5–Pd–N6

87.41(3), N5–Pd–C10 95.95(7), N5–Pd–C14 175.04(3), N6–Pd–C10

173.87(4), C10–Pd–C14 85.05(3).

²⁺2[BF₄]^–

N

MeCN Pd

MeCN

I

Pd

I

Pd

I

²⁺2[BF₄]^–

N

47

48

49

N

Me

N

Me

N

MeCN Pd

MeCN

MeCN Pd

MeCN

N

²⁺2I^–

Me

N

52

53

N

Pd

I

Pd

I

Figure 7

N

All described dicarbene-substituted complexes 27–53 are

air- and moisture-stable for periods of months in the solid

state and in solution, and the yields of the reactions are not

reduced by using wet solvents/reactants at any point. The

complexes display high thermal stability, decomposing

only much higher than 215 °C in the solid state, and are

50

51

Figure 5 Known palladium complexes.

1

stable in refluxing DMSO for varying periods. The H

The formation of the six-membered chelate ring of dicar-

bene complex 46 distorts the coordination geometry of the

palladium only slightly, with the C–Pd–C bite angle being

reduced to 85.05(3)°. The cis- and trans-N–Pd–C angles

are also effected by the chelate ring formation with 91.17,

95.95(7), 173.87(4) and 175.04(3)°, respectively, whereas

the N–Pd–N angle, being free from such distortion, is

close to ideal with 87.41(3)°. Summaries of selected bond

distances and angles appear in Figure 6.

NMR and ¹³C NMR spectra of the complexes 42–46, 52,

53 are in agreement with their assigned structures. The ap-

pearance of inequivalent methylene proton resonances for

the complexes 42–44 indicate the retention of the same

conformationally restrained boat-shaped six-membered

chelate ring structure discussed for complexes 29–40

(Table 2). Unfortunately, we could not find an AB set for

the complexes 46,²³52, 53 in DMSO-d₆for the bridged

methylene protons.

We have demonstrated the general synthetic applicability

of this procedure by preparing analogous examples based

on triazolin-2-ylidene (52) and benzimidazolin-2-ylidene

(53) heterocyclic ring systems in addition to the examples

based on the imidazolin-2-ylidene heterocyclic ring sys-

tem (Figure 7), using the dibromo precursors.^39,40All

complexes have excellent solubility in DMSO, acetoni-

trile and nitromethane, have good solubility in methanol

and are insoluble in diethyl ether, CH₂Cl₂, THF and hy-

drocarbons.

The palladium-catalyzed arylation of olefins has found

wide application in organic synthesis. In this reaction, ho-

mogeneous catalytic systems are highly efficient. The N-

heterocyclic dicarbene complexes of type 27–40 have

been shown excellent catalysts for the arylation of olefins,

such as styrene or n-butyl acrylate, with aryl bromides. In

the past, the catalytic activity of these complexes was in-

vestigated in detail with activated, non- and deactivated

aryl halides.

Synlett 2006, No. 18, 2894–2907 © Thieme Stuttgart · New York

LETTER

Precursors for Chelating Carbene Complexes of Palladium in the Mizoroki–Heck Reaction

2899

room temperature and referenced to the residual ¹H and ¹³C signals

of the solvents. Coupling constants J are given in Hz. GC-MS spec-

tra were measured on a Hewlett Packard gas chromatograph GC

5890 A equipped with a mass selective detector MS 5970 B. Quan-

titative analyses were performed on a Hewlett Packard GC 5890 A

equipped with a flame ionization detector (GC/FID). Elemental

analyses were carried out by the Microanalytical Laboratory at the

TU München. Mass spectra were performed at the TU München

Mass Spectrometry Laboratory on a Finnigan MAT 90 Spectrome-

ter using the CI or FAB technique. ESI mass spectra were per-

formed on a Thermo Electron LCQ Classic spectrometer. IR spectra

were recorded using a Jasco FT-IR 460, FTS 575C BIO-RAD or a

Perkin-Elmer 1650 spectrometer. Optical rotations were measured

with a Perkin Elmer 341 digital polarimeter (c, 1.00). Melting points

were measured with a Büchi melting point apparatus system (Dr.

The catalytic performance of complex 27 was tested in the

Heck coupling reaction of a variety of aryl halides with

styrene or n-butyl acrylate. Full conversion was obtained

for the coupling of p-bromoacetophenone with styrene or

n-butyl acrylate after four hours with 0.15 mol% of the

catalyst (entry 1 and 2, Table 3). The reaction of bro-

mobenzene with both olefins gave still yields over 90% of

the desired products (entry 3, 4; Table 3). Even the less re-

active deactivated p-methoxybromobenzene gave yields

around 90% (entry 5, 6; Table 3). A significant decrease

in yield was observed by changing the bromoaryl com-

pound to the chloro derivatives. In these cases, yields be-

tween 29–50% for activated chloro precursors (p-

nitrochlorobenzene and p-chloroacetophenone) were ob- Tottoli). The achiral imidazoles were already described in the liter-

ature and were synthesized according to literature procedures.^29,30,43

served, by using 0.15 mol% of catalyst 27 (entry 7–10;

Table 3).

Spectroscopic data for complex 46 are presented in the literature.²³

Under Mizoroki–Heck conditions complex 27 shows the

possibility to form a C–C coupling product at the vinyl

group (Scheme 6). We obtained after a 12-hour reaction

period of catalyst 27 with p-bromoacetophenone a mix-

ture of complex 41 and the analogous complexes, where

the chloro ligand is substituted by a bromo or an acetate

ligand. To obtain only one defined complex, anion ex-

change was performed on the mixture of products giving

41 in 78% yield.

rac-1-(Phenylethyl)imidazole (1a)

12.1 g (0.1 mol) of rac-1-phenylethylamine were dissolved in 100

mL H₂O and H³PO₄(85%) was added until a pH of 2 was reached.

After addition of 3.0 g of paraformaldehyde (0.10 mol) and 11.5 mL

of glyoxal (40% in H₂O, 0.1 mol), the reaction mixture was heated

to 100 °C and a sat. NH₄Cl solution (5.35 g, 0.10 mol, 20 mL H₂O)

was added dropwise over a period of 1 h. After stirring for another

1 h at 100 °C, the reaction mixture was cooled to 0 °C and NaOH

was added until a pH higher than 12 was observed. The product was

extracted three times with CH₂Cl₂(100 mL), dried over MgSO₄and

afterwards the solvent was removed in vacuo. The crude product

was purified by vacuum distillation to obtain a colorless oil as a ra-

cemic product mixture.^30c

In this manuscript a large variety of bridged imidazolium

salts and their easy synthetic access has been presented.

These salts are useful chelating ligands for a wide variety

of complexes. The catalyst 27 has shown activity in the

Mizoroki–Heck reaction. It showed self-coupling charac-

teristics at the vinyl group to form a C–C coupling product

(41). Furthermore fluctuation effects and the coalescence

temperature in solution were studied at palladium com-

plexes. Herein the influence of bulky N-substituents and

the coordinated anion have been shown.

Yield: 5.81 g (34%); bp 102 °C/1 mbar.

¹H NMR (270 MHz, CDCl₃): d = 7.52 (s, 1 H, NCHN), 7.28–7.22

(m, 3 H, CH_arom), 7.13 (m, 2 H, CH_arom), 7.09 (d, 1 H, ³J_HH= 2.0 Hz,

NCH=), 7.02 (d, 1 H, ³J_HH= 2.0 Hz, NCH), 5.27 [q, 1 H, ³J_HH= 6.9

Hz, CH(C₆H₅)CH₃], 1.78 (d, 3 H, ³J_HH= 7.2 Hz, CH₃).

¹³C NMR (67.5 MHz, CDCl₃): d = 142.4 (NCHN), 136.5, 130.2,

129.4, 128.9, 126.1 (Ar), 118.1 (NCH=), 56.6 [CH(C₆H₅)CH₃], 21.9

(CH₃).

GC-MS: m/z (%) = 173 (73) [MH⁺].

All experiments were carried out under dry argon using standard

Schlenk or dry-box techniques. Solvents were dried by standard

methods and distilled under nitrogen. ¹H NMR and ¹³C NMR spec-

tra were recorded on a JEOL JMX-GX 400 MHz spectrometer at

By using (R)-1-phenylethylamine, (R)-1-(phenylethyl)imidazole

(1b) was obtained according to the literature.^30a

N

Pd

Cl Cl

27

1. NaOAc, DMAc, 120 °C, 12 h

– NaX, – HOAc

MeCO

Br

2. NaCl, DMSO, 50 °C, 2 h

N

Pd

N

Cl Cl

MeCO

COMe

41

Scheme 6

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2900

T. Scherg et al.

LETTER

Table 3 Homogeneous Heck Coupling Catalysis with Complex 27 as the Catalyst^a

R²

X

cat. [Pd], NaOAc

+

R²

– NaX, – HOAc

R¹

R¹= OMe, H, COMe, NO₂

R²= Ph, CO₂-n-Bu

X = Cl, Br

Entry

Aryl halide

Alkene

Time (h)

Temp (°C)

150

Conv.^b(%)

Yield (%)^c

TON^d

640

667

613

600

587

667

333

193

220

200

1

2

4-MeCOC₆H₄Br

C₆H₅Br

CH₂=C(H)Ph

4

100

92

96

100

92

CH₂=C(H)COOn-Bu

CH₂=C(H)Ph

150

3

20

44

20

44

42

43

150

4

C₆H₅Br

CH₂=C(H)COOn-Bu

CH₂=C(H)Ph

150

90

5

4-MeOC₆H₄Br

4-O₂NC₆H₄Cl

4-MeCOC₆H₄Cl

150

88

81

6

CH₂=C(H)COOn-Bu

CH₂=C(H)Ph

150

100

50

100

50

7

150

8

CH₂=C(H)COOn-Bu

CH₂=C(H)Ph

150

29

9

150

33

10

CH₂=C(H)COOn-Bu

150

30

^aReactions were performed in sealed pressure tubes without the exclusion of oxygen/moisture and with non-dried solvents. Yields and product

identification were determined by GC-MS. Typical reaction conditions: a molar ration of 1:1.25:1.5 was used for the aryl halide (10 mmol)/

alkene/base (NaOAc anhyd). In each reaction 0.15 mol% of catalyst 27 and DMAc (10 mL) as solvent was used.

^bConversion of the aryl halide with diethylene glycol-n-butyl ether as internal standard.

^cGC yield of the trans-isomer based on the aryl halide.

^dTON = [moles of coupling product (all isomers)]/(moles of Pd).

rac-1-(1¢-Naphthylethyl)imidazole (2)

IR (KBr): 3149, 3130, 3043, 1785, 1698, 1670, 1583, 1561, 1546,

1454, 1440, 1395, 1331, 1164, 1085, 900, 865, 781, 765, 731, 682,

625, 614, 432 cm^–1.

¹H NMR (400 MHz, D₂O): d = 9.28 (s, 2 H, NCHN), 7.83 (s, 2 H,

NCHCHN), 7.63 (s, 2 H, NCHCHN), 6.76 (s, 2 H, NCH₂N), 4.01

(s, 6 H, NCH₃).

rac-1-(1-Naphthylethyl)imidazole was synthesized similar to 1a

from rac-1-naphthylethylamine (5.00 g, 29.0 mmol). For purifica-

tion, the crude racemic product was extracted with Et₂O to obtain a

yellow oil.

Yield: 2.10 g (33%).

¹H NMR (270 MHz, CDCl₃): d = 7.90–7.79 (m, 2 H, CH_arom), 7.61

(s, 1 H, NCHN), 7.55–7.20 (m, 5 H, CH_arom), 7.08 (s, 1 H, NCH),

6.96 (s, 1 H, NCH), 6.24 [q, 1 H, J = 6.8 Hz, CH (naphthylethyl)],

2.00 [d, 3 H, J = 7.1 Hz, CH₃(naphthylethyl)].

¹³C NMR (67.5 MHz, CDCl₃): d = 136.6 (NCN), 129.1, 118.6

(NC), 136.3, 134.0, 130.4, 129.3, 127.0, 126.1, 125.5, 123.4, 122.2,

120.0 (Ar), 53.2 [CH (naphthylethyl)], 22.0 [(CH₃(naphthylethyl)].

¹³C NMR (100.5 MHz, D₂O): d = 136.8 (NCHN), 124.8

(NCHCHN), 122.6 (NCHCHN), 58.7 (NCH₂N), 35.8 (NCH₃).

MS-FAB: m/z = 256.9 [M⁺], 177.0 [M²⁺].

Anal. Calcd for C₉H₁₄N₄Br₂(338.04): C, 31.98; H, 4.17; N, 16.57.

Found: C, 31.92; H, 3.77; N, 16.25.

1,1¢-Diisopropyl-3,3¢-methylenediimidazolium Dibromide (4)

Yield: 2.3 g (96%); mp >280 °C (dec.).

GC-MS: m/z = 222 [M⁺], 155 [M⁺– imidazole], 128 [naphthyl⁺].

IR (KBr): 3134, 3120, 3066, 2957, 2848, 2242, 1758, 1711, 1638,

1573, 1546, 1474, 1434, 1397, 1376, 1334, 1298, 1267, 1181, 1159,

1135, 1111, 941, 858, 759, 728, 654, 630, 614, 598, 466 cm^–1.

General Synthesis of Methylene-Bridged Imidazolium Salts

To a solution of the R-imidazole (12.2 mmol) in 5 mL THF in an

ACE pressure tube 1.18 g (0.94 mL, 6.1 mmol) of CH₂Br₂was add-

ed. A colorless precipitate formed after the solution was heated for

72 h at 130 °C. The precipitate was filtered and washed three times

with THF (5 mL). A colorless powder was obtained, after the prod-

uct was dried in vacuo.

¹H NMR (400 MHz, D₂O): d = 8.96 (s, 2 H, NCHN), 7.71 (d, 2 H,

3

³J_HH= 2.4 Hz, NCHCH), 7.66 (d, 2 H, J_HH= 2.4 Hz, CHCHN),

4.70 (s, 2 H, NCH₂N), 4.63 [m, 2 H, CH(CH₃)₂], 1.49 [s, 12 H,

CH(CH₃)₂].

¹³C NMR (100.5 MHz, D₂O): d = 152.3 (NCHN), 122.7 (NCHCH),

120.6 (CHCHN), 58.9 (NCH₂N), 53.7 [NCH(CH₃)₂], 21.8

[NCH(CH₃)₂].

1,1¢-Dimethyl-3,3¢-methylenediimidazolium Dibromide (3)

Yield: 1.77 g (83%); mp >280 °C (dec.).

Synlett 2006, No. 18, 2894–2907 © Thieme Stuttgart · New York

LETTER

Precursors for Chelating Carbene Complexes of Palladium in the Mizoroki–Heck Reaction

2901

MS-FAB: m/z (%) = 313 (26) [M⁺], 233 (100) [M²⁺].

¹³C NMR (100.5 MHz, DMSO-d₆): d = 144.2 [CCH(CH₃)₂], 138.4

(NCHN), 131.1 (NC), 129.5 (CH, Ar), 125.1 (CH, Ar), 123.9

(CHCHN), 122.6 (NCHCH), 58.2 (NCH₂N), 27.3 [CH(CH₃)₂], 23.1

[CH(CH₃)₂].

Anal. Calcd for C₁₃H₂₂N₄Br₂(394.15): C, 39.61; H, 5.63; N, 14.21.

Found: C, 39.38; H, 5.39; N, 14.00.

1,1¢-Di-tert-butyl-3,3¢-methylenediimidazolium Dibromide (5)

Yield: 2.03 g (79%); mp >300 °C.

MS-FAB: m/z (%) = 585 (6) [MH⁺– CH(CH₃)₂], 551 (4) [M⁺], 469

(100) [M²⁺; and MH⁺– C₆H₃CH(CH₃)₂].

IR (KBr): 3051, 2952, 2881, 2851, 2781, 2459, 2422, 2237, 1754,

1711, 1628, 1565, 1541, 1481, 1464, 1433, 1408, 1377, 1301, 1240,

1207, 1132, 1051, 996, 944, 858, 820, 752, 724, 660, 623, 614, 572,

486, 404 cm^–1.

¹H NMR (400 MHz, D₂O): d = 8.72 (s, 2 H, NCHN), 7.62 (s, 2 H,

NCHCH), 7.41 (s, 2 H, CHCHN), 6.57 (s, 2 H, NCH₂N), 1.43 [s, 18

H, NC(CH₃)₃].

¹³C NMR (100.5 MHz, D₂O): d = 135.3 (NCHN), 122.4 (NCHCH),

120.1 (CHCHN), 58.1 (NCH₂N), 48.7 [NC(CH₃)₃], 28.8

[NC(CH₃)₃].

Anal. Calcd for C₃₁H₄₂N₄Br₂(630.50): C, 59.05; H, 6.71; N, 8.89.

Found: C, 59.93; H, 6.78; N, 8.92.

1,1¢-Dimesityl-3,3¢-methylenediimidazolium Dibromide (9)

Yield: 2.38 g (72%); mp >318 °C (dec.).

IR (KBr): 3142, 3040, 2961, 2765, 1630, 1608, 1547, 1486, 1442,

1419, 1379, 1319, 1298, 1265, 1211, 1158, 1110, 1066, 1028, 969,

936, 857, 846, 822, 808, 763, 734, 658, 610, 580, 551, 438 cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 10.56 (s, 2 H, NCHN), 8.96 (d,

2 H, NCHCHN), 8.54 (d, 2 H, NCHCHN), 7.61 (s, 4 H, CH_mesityl.),

7.41 (s, 2 H, NCH₂N), 3.79 [d, 6 H, C(CH₃)], 2.78 [s, 6 H, C(CH₃)],

2.49 [s, 6 H, C(CH₃)].

¹³C{¹H}-NMR (100.5 MHz, DMSO-d₆): d = 140.6 [s,

CC(CH₃)_arom], 134.6 (ipso-C_mesityl), 134.2 (o-C_mesityl), 129.7

(m-C_mesityl), 129.4 (p-C_mesityl), 122.8 (NCHCHN), 58.9 (NCH₂N),

17.4 (p-CH₃), 17.0 (o-CH₃).

MS-FAB: m/z (%) = 341 (47) [M⁺], 261 (100) [M²⁺].

Anal. Calcd for C₁₅H₂₆N₄Br₂(422.20): C, 42.67; H, 6.21; N, 13.27.

Found: C, 42.44; H, 5.70; N, 13.34.

1,1¢-Dicyclohexyl-3,3¢-methylenediimidazolium Diiodide (6)

Yield: 3.38 g (98%); mp >281 °C (dec.).

MS-FAB: m/z (%) = 467 (3) [M⁺], 385 (100) [M²⁺].

IR (KBr): 3126, 3065, 3030, 2927, 2854, 1757, 1713, 1633, 1572,

1547, 1464, 1445, 1434, 1376, 1350, 1296, 1271, 1241, 1189, 1168,

1121, 1051, 987, 897, 853, 815, 758, 707, 626, 610, 509, 462 cm^–1.

Anal. Calcd for C₂₅H₃₀N₄Br₂(546.34): C, 45.96; H, 5.53; N, 10.25.

Found: C, 45.56; H, 5.41; N, 10.57.

¹H NMR (400 MHz, DMSO-d₆): d = 8.79 (s, 2 H, NCHN), 7.27 (s,

2 H, NCHCHN), 7.23 (s, 2 H, NCHCHN), 5.86 (s, 2 H, NCH₂N),

3.58 (sept, 2 H, CH_cy), 1.34, 1.07, 0.91, 0.64, 0.45 (m, 20 H, CH_2,cy).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 136.3 (s, NCHN), 122.2,

121.5 (s, NCHCHN), 59.0 (s, NCH₂N), 32.0 (s, CH_cy), 24.2 (s,

CH_2,cy).

1,1¢-Dimethyl-3,3¢-methylenedibenzimidazolium Dibromide

(10)

Yield: 1.83 g (68%); mp 255 °C (dec.).

IR (KBr): 3098, 3027, 2938, 1613, 1562, 1487, 1454, 1436, 1390,

1334, 1269, 1213, 1136, 1109, 1033, 1019, 830, 794, 784, 766, 697,

639, 590, 568, 549, 522, 435, 421, 396, 388, 379, 362, 357 cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 8.46 (s, 2 H, NCHN), 6.91 (s,

2 H, CH_arom), 6.75 (s, 2 H, CH_arom), 5.53 (s, 2 H, NCH₂N), 4.35 (s,

6 H, NCH₃).

MS-FAB: m/z (%) = 441 (31) [M⁺], 313 (100) [M²⁺].

Anal. Calcd for C₁₉H₃₀N₄I₂(568.28): C, 40.16; H, 5.32; N, 9.86.

Found: C, 39.49; H, 4.83; N, 9.95.

¹³C NMR (100.5 MHz, DMSO-d₆): d = 134.2 (NCHN), 129.4,

128.9, 123.5, 122.8 (s, CH_arom), 58.7 (NCH₂N), 52.7 (NCH₃).

MS-FAB: m/z (%) = 357 (21) [M⁺], 277 (100) [M²⁺].

1,1¢-Diadamantyl-3,3¢-methylenediimidazolium Dibromide (7)

Yield: 2.36 g (67%); mp >320 °C.

IR (KBr): 3186, 3004, 2802, 2688, 2654, 2601, 2570, 2512, 2462,

2417, 2006, 1991, 1949, 1879, 1597, 1576, 1491, 1476, 1452, 1437,

1375, 1364, 1347, 1312, 1287, 1204, 1199, 1196, 1163, 1114, 1083,

1021, 970, 959, 925, 911, 811, 645, 542, 457, 420, 404 cm^–1.

1,1¢-Di(4-vinylbenzyl)-3,3¢-methylenediimidazolium Dichloride

(11)

N,N-Diimidazolylmethane (1.50 g, 10.6 mmol) and 4-vinylbenzyl

chloride (9.70 g, 63.4 mmol) are dissolved in i-PrOH (9 mL) in an

ACE pressure tube and heated for 40 h at 100 °C. The solvent was

removed in vacuo and the precipitate was washed four times with

THF (15 mL) and dried in vacuo.

¹H NMR (400 MHz, D₂O): d = 8.77 (s, 2 H, NCHN), 7.15 (d, 1 H,

3

³J_HH= 2.0 Hz, CHCHN), 6.90 (d, 1 H, J_HH= 2.0 Hz, NCHCH),

4.59 (s, 2 H, NCH₂N), 2.17 (d, 12 H, ³J_HH= 2.5 Hz, NCCH₂), 1.68

(m, 12 H, CH₂), 1.47 (m, 6 H, CH).

¹³C NMR (100.5 MHz, D₂O): d = 134.1 (NCHN), 122.1 (CHCHN),

120.1 (NCHCH), 58.9 (NCH₂N), 29.9 (CH), 55.6 (NC), 43.1

(NCCH₂), 36.1 (CH₂).

Yield: 4.13 g (86%); mp >300 °C.

¹H NMR (400 MHz, DMSO-d₆): d = 10.12 (s, 2 H, NCHN), 8.37 (s,

2 H, NCH), 7.96 (s, 2 H, NCH), 7.54–7.47 (m, 8 H, CH_arom), 6.91 (s,

3

2 H, NCH₂N), 6.75 (dd, 2 H, J_HH= 17.6 Hz, J_HH= 11.0 Hz,

1,1¢-Di(2,6-diisopropylphenyl)-3,3¢-methylenediimidazolium

Dibromide (8)

Yield: 3.07 g (80%); mp 284 °C (dec.).

3

H₂C=CH), 5.88 (d, 2 H, J_HH= 17.6 Hz, H₂C=C), 5.53 (s, 4 H,

NCH₂Ph), 5.31 (d, ³J_HH= 11.0 Hz, H₂C=C).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 138.5 (HC_vinyl), 138.2

(NCHN), 136.4, 134.2, 129.6, 127.1 (Ar), 123.4 (NCH), 115.9

(H₂C_vinyl), 58.5 (NCH₂N), 52.5 (NCH₂Ph).

IR (KBr): 3125, 3069, 2963, 2929, 2870, 1590, 1555, 1536, 1464,

1408, 1388, 1367, 1337, 1308, 1266, 1243, 1187, 1117, 1069, 1016,

956, 936, 866, 808, 760, 742, 699, 675, 619, 555, 460 cm^–1.

Anal. Calcd for C₂₅H₂₆N₄Cl₂(453.41): C, 66.22; H, 5.78; N, 12.36.

Found: C, 66.10; H, 5.94; N, 11.80.

¹H NMR (400 MHz, DMSO-d₆): d = 8.61 (s, 2 H, NCHN), 7.46 (t,

2 H, ³J_HH= 7.7 Hz, CHCHCH), 7.38 (s, 2 H, NCHCH), 7.28 (d, 2

H, ³J_HH= 7.7 Hz, CHCHCH), 6.85 (s, 2 H, NCHCH), 4.54 (s, 2 H,

NCH₂N), 2.27 [sept, 4 H, ³J_HH= 5.9 Hz, CH(CH₃)₂], 1.12 [d, 24 H,

³J_HH= 5.9 Hz, CH(CH₃)₂].

General Synthesis of Ethylene-Bridged Imidazolium Salts

To a solution of the R-imidazole (50 mmol) in THF (5 mL) in an

ACE pressure tube, 4.69 g (3.51 mL, 25 mmol) of CH₂Br₂was add-

ed. A light yellow precipitate was observed after the solution was

Synlett 2006, No. 18, 2894–2907 © Thieme Stuttgart · New York

2902

T. Scherg et al.

LETTER

heated for 48 h at 130 °C. The precipitate was filtered and washed

three times with THF (5 mL). A beige highly hygroscopic powder

was isolated after the product was dried in high vacuum.

Anal. Calcd for C₁₈H₂₀N₄Br₂(452.19): C, 47.81; H, 4.46; N, 12.39.

Found: C, 47.52; H, 4.32; N, 12.21.

General Synthesis of Pyridinyl-Bridged Imidazolium Salts

To a solution of the R-imidazol (25 mmol) in 3 mL THF in an ACE

pressure tube, 2.96 g (12.5 mmol) of 2,6-dibromopyridine was add-

ed. The solution was heated for 48 h at 130 °C and a colorless pre-

cipitate was observed. The precipitate was filtered and washed three

times with THF (3 mL). A colorless powder was isolated after the

product was dried in vacuo.

1,1¢-Diisopropyl-3,3¢-ethylenediimidazolium Dibromide (12)

Yield: 9.18 g (90%).

IR (KBr): 3048, 2975, 2877, 2427, 1762, 1638, 1565, 1542, 1470,

1428, 1406, 1376, 1307, 1293, 1240, 1202, 1165, 1133, 881, 849,

819, 766, 730, 656, 613, 570, 486, 418, 407, 384, 366, 352 cm^–1.

¹H NMR (400 MHz, D₂O): d = 9.08 (s, 2 H, NCHN), 7.83 (m, 2 H,

NCHCH), 7.61 (m, 2 H, CHCHN), 4.93 (s, 4 H, NCH₂CH₂N), 4.81

[m, 2 H, CH(CH₃)₂], 1.68 [s, 12 H, CH(CH₃)₂].

1,1¢-Diisopropyl-3,3¢-pyridinyldiimidazolium Dibromide (16)

Yield: 5.14 g (90%); mp 271 °C (dec.).

¹³C NMR (100.5 MHz, D₂O): d = 156.3 (NCHN), 123.1 (NCHCH),

121.6 (CHCHN), 53.9 (NCH₂CH₂N), 48.8 [NCH(CH₃)₂], 21.9

[NCH(CH₃)₂].

IR (KBr): 3051, 1928, 1741, 1665, 1628, 1613, 1591, 1572, 1532,

1466, 1425, 1392, 1377, 1335, 1308, 1265, 1253, 1223, 1136, 1116,

1092, 1000, 942, 872, 822, 772, 763, 657, 626, 586, 530, 437, 406

cm^–1.

MS-FAB: m/z (%) = 327 (64) [M⁺], 247 (100) [M²⁺].

3

¹H NMR (400 MHz, D₂O): d = 8.42 (t, 1 H, J_HH= 8.4 Hz,

Anal. Calcd for C₁₄H₂₄N₄Br₂(408.18): C, 41.20; H, 5.93; N, 13.73.

Found: C, 40.71; H, 6.10; N, 13.58.

CHCHCH), 8.32 (d, 2 H, ³J_HH= 2.4 Hz, NCHCHN), 7.99 (d, 2 H,

³J_HH= 8.4 Hz, CHCHCH), 7.85 (d, 2 H, ³J_HH= 2.4 Hz, NCHCHN),

4.82 [m, 2 H, CH(CH₃)₂], 1.65 [d, 12 H, ³J_HH= 6.8 Hz, CH(CH₃)₂].

1,1¢-Di-tert-butyl-3,3¢-ethylenediimidazolium Dibromide (13)

Yield: 9.26 g (85%); mp 262 °C (dec.).

¹H NMR (400 MHz, DMSO-d₆): d = 8.51 (s, 2 H, NCHN), 7.12 (s,

2 H, NCHCHN), 6.75 (s, 2 H, NCHCHN), 3.84 (s, 4 H,

NCH₂CH₂N), 1.59 [s, 18 H, NC(CH₃)₃].

¹³C NMR (100.5 MHz, D₂O): d = 156.3 (NCHN), 145.7

(CHCHCH), 144.6 (CHCHCN), 121.8 (NCNCH), 119.1

(NCHCH), 114.6 (CHCHN), 54.2 [NCH(CH₃)₂], 21.8

[NCH(CH₃)₂].

MS-FAB: m/z (%) = 376 (21) [M⁺], 296 (100) [M²⁺].

¹³C NMR (100.5 MHz, DMSO-d₆): d = 135.3 (NCHN), 122.8,

120.5 (NCHCHN), 59.8 (NCH₂CH₂N), 48.4 [NC(CH₃)₃], 28.9

[NC(CH₃)₃].

Anal. Calcd for C₁₇H₂₃N₅Br₂(457.21): C, 44.66; H, 5.07; N, 15.32.

Found: C, 44.77; H, 5.12; N, 15.21.

MS-FAB: m/z = 355.3 [M⁺], 275.3 [M²⁺].

1,1¢-Di-tert-butyl-3,3¢-pyridinyldiimidazolium Dibromide (17)

Yield: 4.31 g (71%); mp >320 °C.

Anal. Calcd for C₁₆H₂₈N₄Br₂(436.23): C, 44.05; H, 6.47; N, 12.84.

Found: C, 43.81; H, 6.45; N, 12.69.

IR (KBr): 3069, 2961, 1934, 1861, 1753, 1654, 1615, 1593, 1558,

1522, 1463, 1405, 1377, 1348, 1316, 1273, 1231, 1210, 1127, 1109,

1089, 1048, 1002, 944, 932, 863, 820, 762, 746, 649, 616, 590, 478,

451 cm^–1.

1,1¢-Diadamantyl-3,3¢-ethylenediimidazolium Dibromide (14)

Yield: 10.53 g (71%); mp >330 °C.

IR (KBr): 2912, 2852, 2586, 2523, 2472, 2051, 1956, 1896, 1604,

1499, 1477, 1453, 1438, 1365, 1348, 1312, 1301, 1200, 1113, 1084,

972, 960, 912, 811, 645, 543, 458, 422 cm^–1.

3

¹H NMR (270 MHz, D₂O): d = 8.43 (t, 1 H, J_HH= 5.4 Hz,

3

CHCHCH), 8.36 (s, 2 H, NCHCH), 8.01 (d, 2 H, J_HH= 5.4 Hz,

CHCHCH), 7.96 (m, 2 H, CHCHN), 1.75 [s, 18 H, NC(CH₃)₃].

¹H NMR (400 MHz, D₂O): d = 9.48 (s, 2 H, NCHN), 7.25 (d, 1 H,

¹³C NMR (67.9 MHz, DMSO-d₆): d = 156.4 (NCHN), 145.8

(CHCHCH), 121.3 (CHCHCN), 119.7 (NCNCH), 115.1

(NCHCH), 114.9 (CHCHN), 61.2 [NC(CH₃)₃], 28.4 [NC(CH₃)₃].

3

³J_HH= 2.0 Hz, CHCHN), 7.01 (d, 1 H, J_HH= 2.0 Hz, NCHCH),

4.51 (s, 4 H, NCH₂CH₂N), 2.22 (d, 12 H, ³J_HH= 2.5 Hz, NCCH₂),

1.65 (m, 12 H, CH₂), 1.42 (m, 6 H, CH).

MS-FAB: m/z (%) = 404 (28) [M⁺], 324 (100) [M²⁺].

¹³C NMR (100.5 MHz, D₂O): d = 134.6 (NCHN), 121.7 (CHCHN),

120.4 (NCHCH), 59.2 (NCH₂CH₂N), 55.9 (NC), 42.9 (NCCH₂),

36.6 (CH₂), 29.1 (CH).

MS-FAB: m/z (%) = 380.4 (2) [M⁺– (Br + Ad)], 339.3 (100), 329.4

(22).

Anal. Calcd for C₁₉H₂₇N₅Br₂(485.26): C, 47.03; H, 5.61; N, 14.43.

Found: C, 47.00; H, 5.32; N, 14.57.

1,1¢-Diadamantyl-3,3¢-pyridinyldiimidazolium Dibromide (18)

Yield: 16.1 g (75%); mp >290 °C.

Anal. Calcd for C₂₈H₄₀N₄Br₂(592.45): C, 56.76; H, 6.81; N, 9.46.

Found: C, 57.39; H, 6.49; N, 8.91.

¹H NMR (400 MHz, D₂O): d = 8.91–7.65 (m, 7 H, CHCHCH,

NCHCHN), 2.11 (m, 12 H, NCCH₂), 1.78 (m, 12 H, CH₂), 1.35 (m,

6 H, CH).

¹³C NMR (100.5 MHz, D₂O): d = 158.2 (NCHN), 147.7 (CHCH-

CN), 130.9 (CHCHCN), 123.8 (NCNCH), 122.3 (CHCHN), 111.4

(NCHCH), 63.3 (NC), 41.7 (NCCH₂), 33.7 (CH₂), 31.1 (CH).

1,1¢-Dimethyl-3,3¢-ethylenedibenzimidazolium Dibromide (15)

Yield: 10.2 g (90%).

IR (KBr): 2980, 2914, 2825, 2782, 1611, 1567, 1452, 1385, 1337,

1227, 1184, 851, 824, 610, 437, 395 cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 11.72 (s, 2 H, NCHN), 9.86 (s,

4 H, CH_arom), 7.28 (s, 4 H, CH_arom), 6.25 (s, 2 H, NCH₂CH₂N), 5.84

(s, 2 H, NCH₂CH₂N), 3.28 (s, 6 H, NCH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 155.9 (CH_arom), 139.3

(CH_arom), 135.6 (NCHN), 128.8, 128.5 (s, NCHCHN), 113.8 (s,

NCH₂CH₂N), 19.9 (s, NCH₃).

1,1¢-Di[(R)-1¢¢-phenylethyl]-3,3¢-methylenediimidazolium Di-

bromide (19)

(R)-(1-Phenylethyl)imidazole (860 mg, 5.0 mmol) was dissolved in

10 mL toluene in an ACE pressure tube and 3.0 mmol of CH₂Br₂

was added. The solution was heated for 48 h at 140 °C and a color-

less precipitate was obtained. The precipitate was filtered off and

washed twice with 20 mL THF to obtain a colorless solid.

MS-FAB: m/z (%) = 293 (2) [M²⁺], 147 (100) [C₉H₁₀N₂].

Yield: 635 mg (49%); mp >300 °C; [a]_D²⁰+2.0 (DMSO).

Synlett 2006, No. 18, 2894–2907 © Thieme Stuttgart · New York

LETTER

Precursors for Chelating Carbene Complexes of Palladium in the Mizoroki–Heck Reaction

2903

IR (KBr): 3116, 2917, 1903, 1818, 1667, 1628, 1562, 1544, 1495,

1455, 1385, 1355, 1333, 1300, 1286, 1234, 1202, 1152, 1108, 1081,

1030, 1012, 915, 844, 765, 704, 651, 614, 538 cm^–1.

1,1¢-Di(rac-1¢¢-phenylethyl)-3,3¢-m-xylylenediimidazolium Di-

bromide (22)

Yield: 1.48 g (97%).

¹H NMR (400 MHz, DMSO-d₆): d = 10.11 (s, 2 H, NCHN), 8.80 (s,

2 H, NCH), 8.02 (s, 2 H, NCH), 7.52–7.35 (m, 10 H, CH_arom), 6.82

IR (KBr): 3120, 3073, 3019, 2977, 2055, 1760, 1613, 1554, 1494,

1454, 1382, 1355, 1322, 1311, 1262, 1201, 1149, 1084, 1013, 973,

827, 754, 731, 707, 650, 533, 432 cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 9.67 (s, 2 H, NCHN), 7.87–

7.41 [m, 18 H, CH_arom(10 phenyl + 4 imidazole)], 5.45 (m, 4 H,

CH₂), 5.81 [q, 2 H, ³J_HH= 8.0 Hz, CH (phenylethyl)], 1.88 [d, 6 H,

³J_HH= 7.6 Hz, CH₃(phenylethyl)].

3

(s, 2 H, NCH₂N), 5.93 (d, J_HH= 7.4 Hz, NCH), 1.86 (d, 6 H,

³J_HH= 7.4 Hz, CH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 139.5 (NCHN), 137.7,

129.4, 129.3, 127.3 (CH_arom), 123.1 (NCH), 122.4 (NCH=), 59.6

(NCH), 58.6 (s, NCH₂CH₂N), 21.1 (NCH₃).

MS-FAB: m/z (%) = 437 (13) [M⁺], 357 (57) [M²⁺], 253 (70)

[C₁₅H₁₇N₄], 186 (8) [C₁₂H₁₄N₂], 173 (100) [C₁₁H₁₂N₂].

¹³C NMR (100.5 MHz, DMSO-d₆): d = 139.3 (NCN), 136.0, 135.9,

130.4, 129.7, 129.4, 129.3, 128.5, 127.2 (Ar), 123.4, 122.1 (NC),

59.2 [CH (phenylethyl)], 52.2 (CH₂), 20.9 [CH₃(phenylethyl)].

Anal. Calcd for C₂₃H₂₆N₄Br₂(518.19): C, 53.30; H, 5.06; N, 10.81.

Found: C, 53.73; H, 5.37; N, 10.62.

MS-FAB: m/z (%) = 527 (100) [M⁺].

Anal. Calcd for C₃₀H₃₂Br₂N₄(608.41): C, 59.22; H, 5.30; N, 9.21.

Found: C, 59.34; H, 5.41; N, 9.11.

General Synthesis of o-, m-, p-Xylylene-Bridged Imidazolium

Salts

A rac-imidazole derivative (5 mmol) was dissolved in 5 mL i-PrOH

(20) or THF (21–25) in an ACE pressure tube and 2.5 mmol of a,a¢-

dibromo-o-, -m-, -p-xylylene were added. The solution was heated

for 24 h at 80 °C and a colorless precipitate was obtained. The pre-

cipitate was filtered off and washed three times with 10 mL THF

(22, 25), or recrystallized from CH₂Cl₂–Et₂O (20–24) and dried un-

der high vacuum to obtain a highly hygroscopic colorless powder.

1,1¢-Di(rac-1¢¢-phenylethyl)-3,3¢-p-xylylenediimidazolium Di-

bromide (23)

Yield: 1.41 g (93%); mp >300 °C.

IR (KBr): 3069, 2973, 2847, 2063, 1968, 1756, 1619, 1557, 1518,

1495, 1454, 1430, 1358, 1324, 1310, 1280, 1260, 1201, 1152, 1080,

1015, 972, 864, 768, 728, 708, 652, 629, 521 cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 9.79 (s, 2 H, NCHN), 7.94 (s,

2 H, NCH), 7.90 (s, 2 H, NCH), 7.53–7.30 (m, 14 H, CH_arom), 5.86

[m, 2 H, CH (phenylethyl)], 5.49 (s, 4 H, CH₂), 1.88 [d, 6 H,

³J_HH= 6.0 Hz, CH₃(phenylethyl)].

1,1¢-Di(rac-1¢¢-phenylethyl)-3,3¢-o-xylylenediimidazolium Di-

bromide (20)

Yield: 1.26g (83%).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 140.0 (NCN), 136.2, 135.9,

129.7, 129.4, 127.2, 123.5 (Ar), 123.3, 122.2 (NC), 59.3 [CH (phe-

nylethyl)], 51.9 (CH₂), 21.0 [CH₃(phenylethyl)].

IR (KBr): 3117, 3047, 2975, 2869, 1763, 1626, 1603, 1550, 1495,

1455, 1422, 1383, 1356, 1327, 1312, 1281, 1262, 1202, 1184, 1150,

1013, 971, 843, 763, 738, 707, 650, 628, 533, 459 cm^–1.

¹H NMR (400 MHz, CDCl₃): d = 10.39 (s, 2 H, NCHN), 7.69 (s, 2

H, NCH), 7.49–7.20 (m, 14 H, Ar), 7.09 (s, 2 H, NCH), 6.15 (d, 2

H, J_HH= 16.0 Hz, CH₂), 5.93 (d, 2 H, J_HH= 12.0 Hz, CH₂), 5.82 [q,

2 H, J = 8.0 Hz, CH (phenylethyl)], 1.96 [d, 6 H, ³J_HH= 7.6 Hz, CH₃

(phenylethyl)].

¹³C NMR (100.5 MHz, CDCl₃): d = 137.9 (NCN), 136.1, 132.3,

130.4, 130.2, 129.5, 129.5, 127.1 (Ar), 123.1, 121.0 (NC), 60.4 [CH

(phenylethyl)], 50.5 (CH₂), 21.1 [CH₃(phenylethyl)].

MS-FAB: m/z (%) = 527 (100) [M⁺].

Anal. Calcd for C₃₀H₃₂Br₂N₄(608.41): C, 59.22; H, 5.30; N, 9.21.

Found: C, 59.24; H, 5.27; N, 9.17.

General Synthesis of 2,6-Lutidine-Bridged Imidazolium Salts

A rac-imidazol derivative (50 mmol) was dissolved in THF (5 mL)

in an ACE pressure tube and 25 mmol of 2,6-di(bromomethyl)pyri-

dine were added. The solution was heated for 24 h at 80 °C and a

colorless precipitate could be observed. The precipitate was filtered

off, recrystallized from CH₂Cl₂–Et₂O and dried under high vacuum.

A colorless hygroscopic powder was isolated.

MS-FAB: m/z (%) = 527 (100) [M⁺].

Anal. Calcd for C₃₀H₃₂Br₂N₄(608.41): C, 59.22; H, 5.30; N, 9.21.

Found: C, 59.45; H, 5.39; N, 9.13.

1,1¢-Di(rac-1¢¢-phenylethyl)(2,6-lutidine)diimidazolium Dibro-

1,1¢-Di(rac-1¢¢-naphthylethyl)-3,3¢-o-xylylenediimidazolium Di-

bromide (21)

Yield: 1.35 g (76%); mp >300 °C.

mide (24)

Yield: 1.22 g (80%).

¹H NMR (400 MHz, CDCl₃): d = 10.86 (s, 2 H, NCHN), 8.02–7.00

[m, 17 H, CH_arom(10 phenyl + 3 pyridyl + 4 imidazole)], 6.06 [q, 2

IR (KBr): 3120, 3049, 2978, 1623, 1597, 1550, 1510, 1453, 1418,

1398, 1381, 1360, 1318, 1243, 1215, 1147, 1106, 1021, 967, 807,

781, 738, 630, 441, 419 cm^–1.

¹H NMR (270 MHz, CDCl₃): d = 10.50 (s, 2 H, NCHN), 8.20–6.81

[m, 22 H, CH_arom(14 naphthyl, 4 o-Tol, 4 imidazole)], 6.62 [q, 2 H,

³J_HH= 7.5 Hz, CH(naphthylethyl)], 6.21 (d, 2 H, J = 13.5 Hz, CH₂),

5.91 (d, 2 H, ³J_HH= 16.2 Hz, CH₂), 2.13 [d, 6 H, ³J_HH= 7.3 Hz, CH₃

(naphthylethyl)].

¹³C NMR (67.5 MHz, CDCl₃): d = 136.6 (NCN), 134.0, 133.2,

132.3, 132.0, 131.1, 130.3, 129.3, 127.9, 126.6, 125.6, 124.7, 122.4,

122.3 (Ar), 122.9, 121.1 (NC), 56.5 [CH (naphthylethyl)], 50.8

(CH₂), 21.4 [CH₃(naphthylethyl)].

3

H, J_HH= 5.6 Hz, CH (phenylethyl)], 5.79 (d, 2 H, J = 14.8 Hz,

CH₂), 5.67 (d, 2 H, J = 14.0 Hz, CH₂), 1.99 [d, 6 H, ³J_HH= 6.0 Hz,

CH₃(phenylethyl)].

¹³C NMR (100.5 MHz, CD₃CN): d = 153.3 (lutidineNC), 138.9

(NCN), 138.9, 136.3, 129.2, 129.1, 127.0, 123.0 (Ar), 123.6, 122.3

(NC), 59.5 [CH (phenylethyl)], 53.4 (CH₂), 20.3 [CH₃(phenyleth-

yl)].

MS-FAB: m/z (%) = 528 (100) [M⁺].

Anal. Calcd for C₂₉H₃₁Br₂N₅(609.40): C, 57.16; H, 5.13; N, 11.49.

Found: C, 57.25; H, 5.23; N, 11.53.

MS-FAB: m/z (%) = 627 (100) [M⁺].

1,1¢-Di(rac-1¢¢-naphthylethyl)(2,6-lutidine)diimidazolium Di-

Anal. Calcd for C₃₈H₃₆Br₂N₄(708.53): C, 64.42; H, 5.12; N, 7.91.

Found: C, 64.53; H, 5.17; N, 7.85.

bromide (25)

Yield: 1.33 g (75%).

2904

T. Scherg et al.

LETTER

¹H NMR (400 MHz, CDCl₃): d = 10.85 (s, 2 H, NCHN), 8.15–7.35

[m, 17 H, CH_arom(14 phenyl + 3 lutidine)], 7.24 (s, 2 H, NCH), 6.92

(s, 2 H, NCH), 6.87 [q, 2 H, J = 7.6 Hz, CH (naphthylethyl)], 5.74

(d, 2 H, J = 14.8 Hz, CH₂), 5.56 (d, 2 H, J = 14.4 Hz, CH₂), 2.08 [d,

6 H, J = 7.2 Hz, CH₃(naphthylethyl)].

¹³C NMR (100.5 MHz, CDCl₃): d = 153.0 (lutidineNC), 139.0

(NCN), 137.2, 134.0, 133.0, 130.4, 129.2, 127.7, 126.6, 125.5,

124.6, 123.8, 123.6 (C_arom), 122.5, 120.2 (NC), 55.9 [CH (naphthyl-

ethyl)], 53.6 (CH₂), 20.8 [CH₃(naphthylethyl)].

¹H NMR (400 MHz, DMSO-d₆): d = 7.63–7.33 (m, 10 H, Ar), 6.92

(s, 2 H, NCH), 6.79 (s, 2 H, NCH), 6.35 (br, 2 H, NCH₂N), 5.91 (br,

2 H, NCH), 2.06 (br, 6 H, CH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 172.2 (NCN), 140.2, 137.3,

129.5, 129.1, 128.8, 128.2, 127.3, 126.7 (Ar), 123.6, 123.2, 122.8,

122.6 (NCH), 63.2 (NCH₂), 58.9, 57.0 (NCH), 21.3, 21.0 (CH₃).

MS-FAB: m/z (%) = 623 (36) [MH⁺], 543 (100) [M⁺], 461 (45)

[M²⁺].

Anal. Calcd for C₂₃H₂₄N₄Br₂Pd (622.68): C, 44.36; H, 3.88; N, 9.00.

Found: C, 44.36; H, 3.96; N, 8.95.

MS-FAB: m/z (%) = 628 (100) [M⁺].

Anal. Calcd for C₃₇H₃₅Br₂N₅(709.52): C, 62.63; H, 4.97; N, 9.87.

Found: C, 62.84; H, 5.13; N, 9.72.

Crystal structure analysis of compound 28: C₂₃H₂₄Br₂N₄Pd,

M_r= 622.68, colorless fragment (0.25 × 0.41 × 0.48 mm³), ortho-

rhombic, P2₁2₁2₁(No.: 19), a = 11.1056(5), b = 14.1269(7),

c = 14.6360(8) Å, V = 2296.2(2) Å³, Z = 4, d_calc= 1.801 g cm^–3,

F₀₀₀= 1224, m = 4.306 mm^–1. Preliminary examination and data

collection were carried out on a IPDS device (Stoe & Cie) at the

window of a rotating anode (NONIUS FR591) with graphite mono-

chromated Mo-K_aradiation (l = 0.71073 Å). Data collection was

performed at 293 K within the k range of 2.72° < k < 30.16°. A total

of 26891 intensities were integrated. Raw data were corrected for

Lorentz, polarization, and, arising from the scaling procedure, for

latent decay and absorption effects. After merging (R_int= 0.081),

6751 [5915: I_o> 2s(I_o)] independent reflections remained and all

were used to refine 273 parameters. The structure was solved by a

combination of direct methods and difference-Fourier syntheses.

All non-hydrogen atoms were refined with anisotropic displace-

ment parameters. All hydrogen atoms were placed in calculated

positions and refined using a riding model. Full-matrix least-

squares refinements were carried out by minimizing Sw(F_o²– F_c²)²

and converged with R₁= 0.0269 [I_o> 2s(I_o)], wR₂= 0.0585 [all

data], GOF = 0.991, and shift/error < 0.001. The choice of the

correct enantiomer is proved by Flack’s parameter × = 0.004(5).

The final difference-Fourier map shows no striking features

(De_min/max= +0.66/–0.66 eÅ^–3).⁴⁴

Deprotonation of the Imidazolium Salts

The deprotonation of the corresponding imidazolium salts was per-

formed under argon in a specially designed apparatus.²⁷Then, 10.0

mmol of the corresponding bridged 1,3-di-(R)-imidazolium salt was

suspended in 20 mL THF. NH₃(100 mL) was condensed into the

vessel at –78 °C. After removing the cold bath, NaH (520 mg, 21.6

mmol) was added; an excess of NaH is advantageous. After 2 h a

light yellow solution was formed. After the NH₃was removed, the

free carbene was isolated and crystallized. Therefore, THF was re-

moved completely under high vacuum, and the free carbene was ex-

tracted three times with n-hexane.

1,1¢-Dibutyl-3,3¢-ethylenediimidazolin-2,2¢-diylidene (26)

Yield: 1.27 g (47%).

¹H NMR (400 MHz, C₆D₆): d = 7.21 (s, 4 H, NCHCHN), 4.24 (s, 4

H, NCH₂CH₂N), 2.14 (s, 6 H, NCH₂), 1.31 (6 H, CH₃), 1.25 (6 H,

CH₂).

¹³C NMR (100.5 MHz, C₆D₆): d = 218.0 (NCN), 123.4, 120.0

(NCHCHN), 49.8 (NCH₂CH₂N), 35.2 (NCH₂), 19.2 (CH₂), 12.2

(CH₃).

General Procedure for Dicarbene Complexes of Type [cis-

CH₂{NC(H)=C(H)N(R)C}₂PdX₂]

Diiodo(1,1¢-diethyl-3,3¢-methylenediimidazolin-2,2¢-

diylidene)palladium(II) (29)

Yield: 502 mg (89%); mp 252 °C.

¹H NMR (400 MHz, DMSO-d₆): d = 7.25 (s, 2 H, NCH), 7.16 (s, 2

H, NCH), 6.62, 6.28 (AB, 2 H, ²J_HH= 13.5 Hz, NCH₂N), 4.18 (br,

4 H, CH₂CH₃), 1.19 (br, 6 H, CH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 169.7 (NCN), 123.4 (NCH),

122.9 (NCH), 62.4 (CH₂), 46.7 (CH₂CH₃), 19.8 (CH₃).

1,1¢-Di-(R)-3,3¢-methylenediimidazoliumdihalogen salt (1.00

mmol) and Pd(OAc)₂(225 mg, 1.00 mmol) were dissolved in

DMSO (10 mL) and heated for 4 h at 50 °C. Afterwards the solution

was refluxed for 20 min at 190 °C, were a clear yellow solution was

formed. The volatile compounds were removed in vacuo and the

precipitate was washed twice with THF (5 mL). The complex was

recrystallized by slow condensation of Et₂O, EtOH or MeOH into a

sat. DMSO solution of the desired complex.

MS-FAB: m/z (%) = 565 (9) [MH⁺], 434 (100) [M⁺].

Dichloro[1,1¢-di(4-vinylbenzyl)-3,3¢-methylenediimidazolin-

2,2¢-diylidene]palladium(II) (27)

Yield: 491 mg (88%); mp: 273 °C.

Anal. Calcd for C₁₁H₁₆N₄I₂Pd (564.50): C, 23.40; H, 2.86; N, 9.93.

Found: C, 23.37; H, 2.78; N, 9.62.

¹H NMR (400 MHz, DMSO-d₆): d = 7.60 (s, 2 H, NCH), 7.30 (s, 2

H, NCH), 7.39–7.28 (m, 8 H, aryl), 6.69 (dd, 2 H, ³J_HH,trans= 17.6

Hz, ³J_HH,cis= 11.0 Hz, H₂CH), 6.34 (br, 2 H, NCH₂N), 6.17 (d, 2 H,

Diiodo(1,1¢-dibenzyl-3,3¢-methylenediimidazolin-2,2¢-

diylidene)palladium(II) (30)

Yield: 544 mg (79%); mp 283 °C.

¹H NMR (400 MHz, DMSO-d₆): d = 7.35 (s, 2 H, NCH), 7.02 (s, 2

H, NCH), 7.78–7.32 (m, 10 H, Ar), 6.52, 6.26 (AB, 2 H, ²J_HH= 11.2

Hz, NCH₂N), 6.46, 6.13 (AB, 4 H, ²J_HH= 12.2 Hz, NCH₂Ph).

3

²J_HH= 14.6 Hz, NCH₂Ph), 5.81 (d, 2 H, J_HH,trans= 17.6 Hz,

CH₂=CH), 5.29 (d, 2 H, ²J_HH,trans= 14.6 Hz, NCH₂Ph), 5.27 (d, 2 H,

³J_HH,cis= 11.0 Hz, CH₂=CH).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 156.4 (NCN), 136.7, 136.3,

136.0, 128.2, 126.4, 114.6 (2 C_vinyl, 4 C_aryl), 122.3 (NCH), 121.9

(NCH), 67.9 (CH₂), 52.5 (NCH₂Ph).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 165.4 (NCN), 136.8, 129.3,

128.7, 128.1 (Ar), 122.0 (NCH), 121.9 (NCH), 63.7 (CH₂), 54.9

(NCH₂Ph).

MS-FAB: m/z (%) = 522 (44) [M⁺], 487 (100) [M²⁺].

MS-FAB: m/z (%) = 689 (21) [MH⁺], 562 (100) [M⁺], 435 (71)

[M²⁺].

Anal. Calcd for C₂₅H₂₄N₄Cl₂Pd (557.81): C, 53.83; H, 4.34; N,

10.04. Found: C, 53.93; H, 4.79; N, 8.97.

Anal. Calcd for C₂₁H₂₀N₄I₂Pd (688.64): C, 36.63; H, 2.93; N, 8.14.

Found: C, 36.93; H, 2.49; N, 8.61.

Dibromo{1,1¢-di[(R)-1¢¢-phenylethyl]-3,3¢-methylenediimidazo-

lin-2,2¢-diylidene}palladium(II) (28)

Yield: 529 mg (85%); mp 299 °C; [a]_D²⁰+182 (DMSO).

LETTER

Precursors for Chelating Carbene Complexes of Palladium in the Mizoroki–Heck Reaction

2905

Diiodo{1,1¢-dicyclohexyl-3,3¢-methylenediimidazolin-2,2¢-

diylidene}palladium(II) (31)

Yield: 525 mg (78%); mp 268 °C.

¹H NMR (400 MHz, DMSO-d₆): d = 7.62 (s, 2 H, NCH), 7.52 (s, 2

H, NCH), 6.39, 6.11 (AB, 2 H, ²J_HH= 11.8 Hz, NCH₂N), 5.09 (br,

2 H, CH,_cy), 2.15–1.50 (m, 20 H, CH_2,cy).

¹³C NMR (100.5 MHz, DMSO-d₆): impossible because of its solu-

bility.

MS-FAB: m/z (%) = 673 (23) [MH⁺], 545 (100) [M⁺].

and Ag[BF₄] (187 mg, 0.96 mmol) in MeCN (15 mL) was heated

for 12 h at 60 °C. Afterwards the formed AgBr was removed by fil-

tration and the solvent was removed in vacuo. The colorless product

was recrystallized from an MeCN–Et₂O solution.

Yield: 317 mg (92%); mp 231 °C.

IR (KBr): 2321.7 (C≡N), 2304.1 (C≡N) cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 7.82–7.29 (m, 14 H), 6.62,

6.34 (AB, 2 H, ²J_HH= 12.9 Hz, CH₂), 5.94 (br, 2 H, NCHPh), 2.06

(br, 6 H, CH₃), 1.74 (s, 6 H, CH₃).

Anal. Calcd for C₁₉H₂₈N₄I₂Pd (672.69): C, 33.92; H, 4.20; N, 8.33.

Found: C, 33.94; H, 4.45; N, 8.49.

¹³C NMR (100.5 MHz, DMSO-d₆): d = 147.0 (NCN), 142.0, 140.2,

129.4, 128.9, 128.4, 127.0, 126.7, 126.0 (aryl), 123.9, 123.7, 120.6,

120.3 (NCH), 118.6 (NCCH₃), 62.9 (CH₂), 58.8, 57.9 (NCHPh),

22.6 (CH₃), 20.0 (CH₃), 1.5 (NCCH₃).

Dichloro{1,1¢-di[p-acetophenone(4-trans-vinylbenzyl)]-3,3¢-

methylenediimidazolin-2,2¢-diylidene}palladium(II) (41)

A solution of dichloro[1,1¢-di(4-vinylbenzyl)-3,3¢-methylenediimi-

dazolin-2,2’-diylidene]palladium(II) (27, 279 mg, 0.50 mmol), p-

bromoacetophenone (398 mg, 2.00 mmol) and NaOAc (246 mg,

3.00 mmol) in DMAc was heated for 2 h at 120 °C. The volatile

compounds were removed in vacuo and the precipitate was washed

twice with MeOH (10 mL) and THF (10 mL). Afterwards the pre-

cipitate was dissolved in DMSO (10 mL) and NaCl (120 mg, 2.05

mmol) was added and heated for 1 h at 50 °C. The solvent was re-

moved in vacuo and the desired complex was recrystallized from an

MeCN–MeOH solution.

Anal. Calcd for C₂₇H₃₀N₆B₂F₈Pd (718.60): C, 45.13; H, 4.21; N,

11.70. Found: C, 44.83; H, 3.99; N, 11.39.

Bisacetonitrile(1,1¢-di-tert-butyl-3,3¢-methylenediimidazolin-

2,2¢-diylidene)palladium(II) Ditetrafluoroborate (44)

A solution of dibromo[1,1¢-di(tert-butyl)-3,3¢-methylenediimidazo-

lin-2,2¢-diylidene]palladium(II) (39, 193 mg, 0.47 mmol) and

Ag[BF₄] (143 mg, 0.95 mmol) in MeCN (15 mL) was heated for 12

h at 60 °C. Afterwards the formed AgBr was removed by filtration

and the solvent was removed in vacuo. The colorless product was

recrystallized from an MeCN–Et₂O solution.

Yield: 310 mg (78%); mp 236 °C.

IR (KBr): 1716.1 (CO) cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 7.89–7.28 (br, 20 H, 4 NCH +

16 C_aryl), 6.56 (br, 4 H, CH), 6.21 (br, 2 H, NCH₂N), 5.93 (br, 4 H,

NCH₂Ph), 2.62 (s, 6 H, CH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 196.3 (CO), 162.4 (NCN),

136.3, 136.1, 135.3, 135.0, 134.1, 133.8, 132.5, 129.5, 128.5, 127.5

(8 C_aryl+ 2 C=C), 122.4 (NCH), 121.3 (NCH), 67.7 (NCH₂N), 52.5

(CH₂), 22.1 (CH₃).

Yield: 246 mg (84%); mp 262 °C.

IR (KBr): 2323.2 (C≡N), 2304.7 (C≡N) cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 7.81 (br, 2 H, NCH), 7.72 (br,

2 H, NCH), 6.90, 6.45 (AB, 2 H, ²J_HH= 12.9 Hz, CH₂), 2.06 (s, 6 H,

NCCH₃), 1.62 (s, 18 H, CH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 143.8 (NCN), 123.3 (NCH),

122.0 (NCH), 118.0 (NCCH₃), 63.8 (CH₂), 60.1 [C(CH₃)₃], 30.9

(CH₃), 1.6 (NCCH₃).

Anal. Calcd for C₁₉H₃₀N₆B₂F₈Pd (622.51): C, 36.66; H, 4.86; N,

13.50. Found: C, 36.42; H, 4.92; N, 13.28.

Anal. Calcd for C₄₁H₃₆N₄Cl₂O₂Pd (794.08): C, 62.01; H, 4.57; N,

7.06. Found: C, 62.21; H, 4.22; N, 7.57.

Bisacetonitrile(4,4¢-dimethyl-2,2¢-methyleneditriazolin-3,3¢-

diylidene)palladium(II) Ditetrafluoroborate (52)

Acetonitrile[bromo(1,1¢-di-tert-butyl-3,3¢-methylenediimidazo-

lin-2,2¢-diylidene)]palladium(II) Hexafluorophosphate (42)

A solution of dibromo[1,1¢-di(tert-butyl)-3,3¢-methylenediimidazo-

lin-2,2¢-diylidene]palladium(II)²²(39, 303 mg, 0.45 mmol) and

K[PF₆] (1.50 g, 8.15 mmol) in MeCN–H₂O (20:15 mL) was re-

fluxed for 5 h. Afterwards the solvent was removed in vacuo and the

colorless product was washed with H₂O (10 mL). The complex was

recrystallized from an MeCN–Et₂O solution.

A solution of dibromo(4,4¢-dimethyl-2,2¢-methyleneditriazolin-

3,3¢-diylidene)palladium(II)⁴¹(303 mg, 0.68 mmol) and Ag[BF₄]

(265 mg, 1.36 mmol) in MeCN (15 mL) was heated for 12 h at

60 °C. Afterwards the formed AgBr was removed by filtration and

the solvent was removed in vacuo. The colorless product was re-

crystallized from an MeCN–Et₂O solution.

Yield: 314 mg (84%); mp 248 °C.

IR (KBr): 2320.0 (C≡N), 2301.5 (C≡N) cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 8.81 (s, 2 H, NCH), 6.68 (s, 2

H, CH₂), 4.35 (s, 6 H, NCH₃), 2.23 (s, 6 H, CH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 146.8 (NCN), 126.7 (NCH),

118.9 (NCCH₃), 63.0 (CH₂), 30.0 (CH₃), 2.0 (NCCH₃).

Yield: 231 mg (81%); mp 252 °C.

IR (KBr): 2317.4 (C≡N) cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 7.39 (br, 2 H, NCH), 7.15 (br,

2

2 H, NCH), 6.84 (AB, 1 H, J_HH= 12.9 Hz, CH₂), 6.45 (AB, 1 H,

²J_HH= 12.9 Hz, CH₂), 1.99 (s, 3 H, NCCH₃), 1.91 (s, 6 H, CH₃), 1.84

(s, 6 H, CH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 144.6 (NCN), 142.4 (NCN),

123.6 (NCH), 122.9 (NCH), 122.3 (NCH), 119.5 (NCH), 118.5

(NCCH₃), 63.6 (CH₂), 61.0 [C(CH₃)₃], 60.3 [C(CH₃)₃], 39.8 (CH₃),

30.2 (CH₃), 1.0 (NCCH₃).

Anal. Calcd for C₁₁H₁₆N₈B₂F₈Pd (540.33): C, 24.45; H, 2.98; N,

20.74. Found: C, 24.42; H, 2.92; N, 20.28.

Bisacetonitrile(1,1¢-dimethyl-3,3¢-methylenedibenzimidazolin-

2,2¢-diylidene)palladium(II) Ditetrafluoroborate (53)

A solution of dibromo(1,1¢-dimethyl-3,3¢-methylenedibenzimida-

zolin-2,2¢-diylidene)palladium(II)⁴⁰(0.68 mmol) and Ag[BF₄] (265

mg, 1.36 mmol) in MeCN (15 mL) was heated for 12 h at 60 °C. Af-

terwards the formed AgBr was removed by filtration and the solvent

was removed in vacuo. The colorless product was recrystallized

from an MeCN–Et₂O solution.

Anal. Calcd for C₁₇H₂₇N₅BrF₆PPd (632.72): C, 32.27; H, 4.30; N,

11.07. Found: C, 33.62; H, 4.67; N, 11.63.

Bisacetonitrile{1,1¢-di[(R)-1¢¢-phenylethyl]-3,3¢-methylenedi-

imidazolin-2,2¢-diylidene}palladium(II) Ditetrafluoroborate

(43)

A solution of dibromo{1,1¢-di[(R)-1¢¢-phenylethyl]-3,3¢-methylene-

diimidazolin-2,2¢-diylidene}palladium(II) (28, 300 mg, 0.48 mmol)

Yield: 364 mg (84%); mp 221 °C.

2906

T. Scherg et al.

LETTER

IR (KBr): 2322.1 (C≡N), 2304.5 (C≡N) cm^–1.

¹H NMR (400 MHz, DMSO-d₆): d = 7.98–6.99 (m, 8 H, aryl), 6.76

(s, 2 H, CH₂), 4.98 (s, 6 H, NCH₃), 2.23 (s, 6 H, CH₃).

¹³C NMR (100.5 MHz, DMSO-d₆): d = 146.8 (NCN), 137.0, 135.8,

134.8, 129.0, 128.6, 119.9 (NCCH₃), 64.1 (CH₂), 31.0 (CH₃), 1.8

(NCCH₃).

33, 619. (g) Cavell, K. J.; McGuinness, D. S. Coord. Chem.

Rev. 2004, 248, 671. (h) Peris, E.; Crabtree, R. H. Coord.

Chem. Rev. 2004, 248, 2239. (i) Crudden, C. M.; Allen, D.

P. Coord. Chem. Rev. 2004, 248, 2247. (j) Lin, I. J. B.;

Vasan, C. S. Comments Inorg. Chem. 2004, 25, 75. (k) Lin,

I. J. B.; Vasan, C. S. Can. J. Chem. 2005, 83, 812.

(l) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105,

3978. (m) Crabtree, R. H. J. Organomet. Chem. 2005, 690,

5451.

Anal. Calcd for C₂₁H₂₂N₆B₂F₈Pd (638.47): C, 39.50; H, 3.47; N,

13.16. Found: C, 40.43; H, 2.99; N, 13.35.

(11) Herrmann, W. A.; Denk, K.; Gstöttmayr, C. W. K. In

Applied Homogeneous Catalysis with Organometallic

Compounds, 2nd ed., Vol. 2; Cornils, B.; Herrmann, W. A.,

Eds.; Wiley-VCH: Weinheim, 2002, 829ff.

(12) (a) Böhm, V. P. W.; Herrmann, W. A. Chem. Eur. J. 2000,

6, 1017. (b) Herrmann, W. A.; Öfele, K.; von Preysing, D.;

Schneider, S. K. J. Organomet. Chem. 2003, 687, 229.

(13) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.;

Herrmann, W. A. Angew. Chem. Int. Ed. 1998, 37, 2490;

Angew. Chem. 1998, 110, 2631.

General Procedure for the Heck Catalysis

The reaction for Heck coupling studies were typically conducted as

follows: The aryl halide (1.0 equiv, 10 mmol), alkene (1.2 equiv, 12

mmol), base (1.5 equiv, 15 mmol), internal standard (100 mg dieth-

yl glycol-n-butyl ether) and the catalyst were added to a thick-

walled 17 cm ACE pressure tube; the solvent (10 mL) and a mag-

netic stir bar were added. The tube was sealed with an o-ringed

Teflon cap and heated to the appropriate temperature of the experi-

ment. The reaction progress was monitored by the removal of a

small aliquot of the reaction mixture, which was analyzed by GC-

MS. The pressure tubes were then cooled to r.t.; a small aliquot of

the reaction mixture was taken for GC-MS analysis of the reaction.

Products were identified by comparison with authentic samples.

(14) (a) Köcher, C.; Herrmann, W. A. J. Organomet. Chem.

1997, 532, 261. (b) Bortenschlager, M.; Schütz, J.; von

Preysing, D.; Nuyken, O.; Herrmann, W. A.; Weberskirch,

R. J. Organomet. Chem. 2005, 690, 6233.

(15) Herrmann, W. A.; Reisinger, C.-P.; Spiegler, M. J.

Organomet. Chem. 1998, 557, 93.

Acknowledgment

(16) (a) Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. Org.

Chem. 1999, 64, 3804. (b) Böhm, V. P. W.; Gstöttmayr, C.

W. K.; Weskamp, T.; Herrmann, W. A. J. Organomet.

Chem. 2000, 595, 186. (c) Viciu, M. S.; Germaneau, R. F.;

Navarro-Fernandez, O.; Stevens, E. D.; Nolan, S. P.

Organometallics 2002, 21, 5470. (d) Schönfelder, D.;

Nuyken, O.; Weberskirch, R. J. Organomet. Chem. 2005,

690, 4648. (e) Sato, Y.; Yoshino, T.; Mori, M. J.

We are grateful to Dr. Karl Öfele and Dr. Jörg Eppinger for sugge-

stions and fruitful discussions. This work was generously supported

by the Süd-Chemie AG (TS), by the Fonds der Chemischen Indu-

strie (PhD fellowship for SKS and JS), the Bayrische Forschungs-

verbund Katalyse (FORKAT), and by NanoCat, an International

Graduate Program within the Elitenetzwerk Bayern (postdoctoral

fellowship for GDF). We also thank Degussa for a generous gift of

palladium(II) acetate.

Organomet. Chem. 2005, 690, 5753.

(17) (a) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn.

1971, 44, 581. (b) Heck, R. F.; Nolly, J. P. J. Org. Chem.

1972, 37, 2320. (c) Dieck, H. A.; Heck, R. F. J. Am. Chem.

Soc. 1974, 96, 1133. (d) Schoenberg, A.; Heck, R. F. J. Org.

Chem. 1974, 39, 3327. (e) Dieck, H. A.; Heck, R. F. J.

Organomet. Chem. 1975, 93, 259. (f) Dieck, H. A.; Heck, R.

F. J. Org. Chem. 1975, 40, 1083. (g) Melpolder, J.; Heck, R.

F. J. Org. Chem. 1976, 41, 265. (h) Ziegler, C. B.; Heck, R.

F. J. Org. Chem. 1978, 43, 2947. (i) Patel, B. A.; Dickerson,

J. E.; Heck, R. F. J. Org. Chem. 1978, 43, 5018.

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LETTER

Precursors for Chelating Carbene Complexes of Palladium in the Mizoroki–Heck Reaction

2907

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Kubo for measurement of the solid-state structure.

Structural parameters for 46: data collection: Nonius Kappa

CCD area detector; graphite monochromated MoK_a

radiation (l = 0.71073 Å) at 193 K; crystal size:

0.14 × 0.15 × 0.22 mm; crystal system: monoclinic; space

group: P2₁/c (#14); unit cell: a = 13.138 (2) Å, b = 49.664

(5) Å, c = 22.157 (2) Å, b = 91.591 (5)°, U = 14451 (3) Å³,

Z = 4, D_calc.= 0.263 g cm^–1; 4465 unique reflections were

observed; the final residuals were R₁= 0.0311 and

wR₂= 0.0768. Hydrogen atom positions were calculated for

ideal geometry and not refined.

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Article Doi

Bridged imidazolium salts used as precursors for chelating carbene complexes of palladium in the Mizoroki-Heck reaction

DOI: 10.1055/s-2006-951539

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