Elimination of an alkyl group from imidazolium salts: imidazole-coordinated dinuclear monodentate nhc-palladium complexes driven by self-assembly and their application in the heck reaction

Article abstract of DOI:10.1002/ejic.200801207

A new family of imidazole-coordinated monodentate NHC-metal complexes 9 was obtained as well as cis-chelatedbidentate NHC-metal complexes 10 from novel bis(imid-azolium) salts 4 with a biphenyl backbone. The structuresof complexes 9a and 10b were determin

Full text of DOI:10.1002/ejic.200801207

SHORT COMMUNICATION
DOI: 10.1002/ejic.200801207
Elimination of an Alkyl Group from Imidazolium Salts: Imidazole-Coordinated
Dinuclear Monodentate NHC–Palladium Complexes Driven by Self-Assembly
and Their Application in the Heck Reaction
Lian-jun Liu,[a] Feijun Wang,*[a] and Min Shi*[a,b]
Keywords: Heck reaction / Carbene ligands / Palladium / Self-assembly / NHCs (N-Heterocyclic carbenes)
A new family of imidazole-coordinated monodentate NHC–
metal complexes 9 was obtained as well as cis-chelated
bidentate NHC–metal complexes 10 from novel bis(imid-
azolium) salts 4 with a biphenyl backbone. The structures
of complexes 9a and 10b were determined by X-ray crystal-
structure diffraction, and their application in the Heck reac-
tion showed good to excellent catalytic activities under very
mild conditions.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
NHC 1 (Figure 1). However, up till now, complexes 3 have
yet to be reported, except those generated from the coordi-
nation of an extra imidazole with NHC metal complexes.[7]
Introduction
N-Heterocyclic carbenes (NHCs), first reported by Öfele
and Wanzlick,[1] have become a very important class of li-
gands due to their stability to air and moisture and their
strong σ-donor but poor π-acceptor abilities. The design
and synthesis of novel and effective NHCs have attracted a
great deal of attention from both academia and industry.[2]
Unlike phosphanes, the coordination of NHCs to metal
centers requires the activation or deprotonation of an imid-
azolium salt precursor. Thus far, many activation strategies
have been used to prepare NHC–metal complexes.[3]
Among these methods, the most important one is the de-
protonation of the corresponding imidazolium salt with a
base (for example, tBuOK, nBuLi, or Cs2CO3) to generate
the NHC ligand.[4] Under basic conditions, imidazolium
salts can also be converted into the corresponding imid-
azoles, but this process only takes place for imidazolium
salts bearing an electron-withdrawing β substituent.[5] Re-
cently, Whittlesey and coworkers reported ruthenium-
induced elimination of an isopropyl group from an imid-
azolium salt to provide the corresponding imidazole-coor-
dinated ruthenium complex through C–N bond activation
and tautomerization.[6] It is conceivable that novel NHC–
metal complexes 3 can be designed on the basis of such an
elimination of an alkyl group R4 to give imidazole 2 and
deprotonation from the imidazolium salt precursor to give
Figure 1. The transformation of imidazolium salts.
Encouraged by the development of monodentate
NHCs[8] and combined with other coordination groups
such as oxazolines and phosphanes and the successful ap-
plications of their palladium catalysts in bond–bond acti-
vations, such as C(sp3)–H bond activation,[9] we envisaged
that it might be feasible to use C2-symmetric bis(imidazol-
ium) salts as precursors for the preparation of monodentate
NHC and imidazole-coordinated metal complex 3 in the
presence of palladium salts. Herein, we wish to report the
elimination of an alkyl group from imidazolium salts and a
novel self-assembly strategy for the preparation of imid-
azole-coordinated dinuclear monodentate NHC palladium
complexes from C2-symmetric bis(imidazolium) salts.
Results and Discussion
A novel family of bis(imidazolium) salts with a 1,1Ј-bi-
naphthalenyl-2,2Ј-diamine (binam) framework (R = Me)
was developed in our research group.[10] Their cis-chelated
bidentate NHC–PdII complexes could be obtained by treat-
ment with Pd(OAc)2 in the presence of a base such as
tBuOK. However, an imidazole-coordinated monodentate
NHC–Pd complex was not obtained, presumably as a result
of the fact that elimination of an alkyl group from the benz-
imidazole ring in the sterically bulky binaphthalene scaffold
[a] Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science and Technology
130 Mei Long Road, Shanghai, 200237, China
Fax: +86-21-64166128
E-mail: Feijunwang@ecust.edu.cn
mshi@mail.sioc.ac.cn
[b] State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry
354 Fenglin Road, Shanghai, 200032, China
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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L.-j. Liu, F. Wang, M. Shi
SHORT COMMUNICATION
is difficult, which was ascertained on the basis of the X- complex suitable for X-ray crystal structure analysis were
ray crystal structure of the cis-chelated bidentate NHC–PdII grown from dichloromethane/hexane (1:4), and its ORTEP
complex.[10c] Relative to the rigid binaphthalene skeleton, drawing is shown in Figure 2. From the crystal structure of
sterically more flexible bis(imidazolium) salts 4 were de- 9a, it can be seen that benzimidazole was formed with the
signed and synthesized. These substrates posses a biphenyl elimination of an ethyl group from 4a, which coordinated
scaffold bearing a variety of alkyl groups and it was be- with Pd(OAc)2 through the nitrogen atom, combined with
lieved that elimination of one of the alkyl groups from the the NHC ligand to give the C2-symmetric dinuclear palla-
benzimidazole ring would be possible.
dium complex driven by self-assembly. However, by using
According to our previously reported synthetic pro- 4b as the precursor, cis-chelated bidentate NHC–PdII com-
cedure,[10] the preparation of bis(imidazolium) salts 4 is plex 10b was obtained in 70% yield without the formation
shown in Scheme 1. Racemic 6,6Ј-dimethoxybiphenyl-2,2Ј- of complex 9b under the standard conditions.[10b] Single
diamine 5 was treated with 2-bromonitrobenzene in toluene crystals of complex 10b suitable for X-ray crystal structure
at 80 °C to produce desired compound 6 in 98% yield after analysis were grown from dichloromethane/hexane (1:3),
48 h. Reduction of 6 with 10% Pd–C/H2 at 60 °C afforded and its ORTEP drawing is shown in Figure 3.
compound 7 in 95% yield. Subsequent cyclization with tri-
ethyl orthoformate catalyzed by toluenesulfonic acid
(TsOH) at 100 °C gave benzimidazole 8 in 88% yield after
24 h. Quaternization of the benzimidazole ring of 8 with a
variety of alkyl iodides upon heating in acetonitrile or diox-
ane provided the corresponding dibenzimidazolium salts
4a–d in quantitative yields.
Figure 2. ORTEP drawing of bis(NHC)–PdII complex 9a with ther-
mal ellipsoids at the 30% probability level. Selected bond lengths
[Å] and angles [°]: Pd1–N1 2.089(7), N2–C1 1.373(12), N1–C1
1.297(11), N2–C1 1.408(12), Pd1–C50 2.014(9), N7–C50 1.347(11),
N8–C50 1.330(11), Pd1–I1 2.6164(10), Pd1–I2 2.6071(10), Pd2–
C20 1.961(9), Pd2–I3 2.5962(10), Pd2–I4 2.6074(10); C1–N1–C2
106.6(8), N1–C1–N2 112.2(8), I1–Pd1–N1 90.4(2), I1–Pd1–I2
171.75(4), N1–Pd1–N2 90.6(2), N1–Pd1–C50 175.3(4), N8–C50–
Pd1 122.8(7), N7–C50–Pd1 128.4(7), C50–Pd1–I1 C50–Pd1–I1
91.2(2).
Scheme 1. The synthesis of bis(imidazolium) salts 4.
It should be noted that complex 9a was a self-assembled
cyclodimer possessing a chiral “parallelogram” grid[11] by
To evaluate the coordinating behavior of bis(imidazol- ligand and metal coordination. The chirality of the biphenyl
ium) salts 4 to the metal center, the reactions of 4 with axis was found to have the (S,S)-configuration, as deter-
Pd(OAc)2 in thf were carried out under reflux in the pres- mined by the crystal structure (Figure 2). The imidazole ni-
ence of tBuOK for 24 h. It was surprisingly found that the trogen atom has a weaker trans effect than that of the iodine
use of 4a as the precursor led to isolation of new NHC– anion, and the NHC has a stronger trans effect than that
PdII complex 9a as the major product after silica-gel col- of the iodine anion.[12] That is the reason why complex 9a is
umn chromatography, but only 15% yield of cis-chelated a trans-chelated NHC–PdII complex with a bite angle (C50–
bidentate NHC–PdII complex 10a was produced on the ba- Pd1–N1) of 175.3°, whereas complex 10b is a cis-chelated
1
1
sis of H NMR spectroscopy.[10b] The H NMR spectrum bidentate NHC–PdII complex with a bite angle (C1–Pd–C8)
of complex 9a revealed that the corresponding two methoxy of 93.2°. The benzimidazole ring in 4a is almost perpendic-
groups of the biphenyl framework located at 2.59 and ular to the benzene ring, and relative to the repulsion ob-
2.83 ppm, respectively, and one ethyl group was eliminated served when (S)-4a coordinates with (S)-4a, this can pro-
on the basis of proton integration. Single crystals of this duce higher steric repulsion between the imidazole and the
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Elimination of an Alkyl Group from Imidazolium Salts
When the alkyl (R) group in bis(imidazolium) salt 4c is
an isopropyl group, PdII complex 9c was obtained in only
22% yield along with trace amounts of cis-chelated biden-
tate NHC–PdII complex 10c, presumably as a result of the
steric hindrance of the isopropyl group under the identical
conditions (Table 1, Entry 3). Moreover, as for bis(imidaz-
olium) salt 4d in which the R group is a normal propyl
group, PdII complex 9d was obtained in 46% yield along
with trace amounts of NHC–PdII complex 10d (Table 1,
Entry 4).
The influence of a base in the preparation of the palla-
dium complexes was also investigated, and the results of
these experiments are summarized in Table 1 (Entries 5–7).
It was found that the use of Cs2CO3 or NaOCH3 as the
base could also give dinuclear NHC–Pd complex 9a in 48
and 42% yield, respectively, along with 10a in 23 and 16%
yield, respectively (Table 1, Entries 5 and 6). Only 5% yield
of 9a was obtained without the formation of complex 10a
in the absence of a base, suggesting that a base is required
in this interesting coordination process (Table 1, Entry 7).
It was interestingly found that imidazolium salt 4 bearing
a β-H on the alkyl group was converted into the corre-
sponding imidazole as well as the carbene intermediate.
This β-H may be activated by palladium to release an ethyl-
ene molecule in a mechanism similar to that observed for
dealkylation induced by ruthenium complexes.[6] To deter-
mine whether Pd was involved in the dealkylation process,
a control experiment was carried out by using bis(imidazol-
ium) salt 4a and tBuOK as the substrates in thf (10 mL)
Figure 3. ORTEP drawing of bis(NHC)–PdII complex 10b with
thermal ellipsoids at the 30% probability level. Selected bond
lengths [Å] and angles [°]: Pd–C1 1.996(12), Pd–C8 1.959(11), Pd–
I1 2.6691(11), Pd–I2 2.6715(13), C1–Pd–I1 88.4(3), C1–Pd–C8
93.2(5), C1–Pd–I1 167.3(4), I1–Pd–I2 92.54(4), C8–Pd–I1 167.8(3),
C8–Pd–I2 88.5(3).
NHC moiety containing the benzene rings and methoxy under reflux for 12 h without the addition of Pd(OAc)2
groups if (S)-4a coordinates with (R)-4a. Therefore, the co- (Scheme 2). Surprisingly, new compound 11 was isolated in
ordinative self-assembly gave the imidazole-coordinated di- 90% yield. Although imidazolin-2-one is well known to be
nuclear monodentate NHC–Pd complexes as (R,R)-9a and prepared by deprotonation and oxidation of the corre-
(S,S)-9a from racemate 4a exclusively.
sponding benzimidazolium salt under an ambient atmo-
Table 1. The coordination of bis(imidazolium) salt with Pd(OAc)2.[a]
Entry
Imidazolium salt
Base
Yield of 9
Yield of 10
[%][b]
[%][c]
1
2
3
4
5
6
7
4a
4b
4c
4d
4a
4a
4a
tBuOK
tBuOK
tBuOK
tBuOK
Cs2CO3
NaOCH3
none
9a, 63
9b, none
9c, 22
9d, 46
9a, 48
9a, 42
9a, 5
10a, 15
10b, 70
10c, trace
10d, trace
10a, 23
10a, 16
none
[a] Reactions of imidazolium salts 4 with Pd(OAc)2 in thf were carried out under reflux in the presence of a base for 24 h. [b] Isolated
yield by silica-gel column chromatography. [c] Isolated yield by silica-gel column chromatography.
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L.-j. Liu, F. Wang, M. Shi
SHORT COMMUNICATION
sphere,[13] to the best of our knowledge, this is the first ex- in good to high yields in most cases (Table 2, Entries 1–5).
ample to indicate that imidazole and imidazolin-2-one The electronic properties of the R and RЈ groups signifi-
could be both obtained from one molecule in the presence cantly affected the outcome of the Heck reactions catalyzed
of tBuOK. On the basis of this control experiment, it is by 9a. For electron-rich 4-bromoanisole, the reaction was
clear that Pd was not involved in the dealkylation process, somewhat sluggish and product 12a was obtained in 80%
although the exact mechanism for the transformation of the yield under identical conditions (Table 2, Entry 1). For dif-
imidazolium salt to the corresponding imidazole and benz- ferent halide atoms, iodobenzene gave product 12d in 99%
imidazolin-2-one to afford compound 11 is still obscure at yield, but only trace amounts of cross-coupling product was
the present stage.
obtained for chlorobenzene. NHC–Pd complex 10b was
also used as the catalyst to catalyze the Heck reaction, and
it showed similar catalytic activity to that of 9a (Table 2,
entries 8–14).
Conclusions
In summary, we reported the first example of the imid-
azole-coordinated dinuclear monodentate NHC–Pd com-
plexes 9a, 9c, and 9d as well as cis-chelated bidentate NHC–
metal complexes 10a and 10b from the novel bis(imidazol-
ium) salts 4a, 4c, and 4d and their application in the Heck
reaction. These dinuclear monodentate NHC–Pd com-
plexes possess a chiral “parallelogram” grid by coordinative
self-assembly. This work may allow the further development
of NHC chemistry in two important directions: (1) Novel
NHC–metal catalysts can be designed on the basis of such
elimination of an alkyl group from an imidazolium salt pre-
cursor to generate the in situ coordinative imidazole moi-
ety; (2) Molecular architectures such as triangles, squares,
rectangles, and helical polymer by self-assembly from
NHC–metal coordination chemistry can be realized, as the
design and synthesis of novel ligands have been widely used
in coordinative self-assembly for the construction of molec-
ular subunits (tectons).[16] At the same time, a novel syn-
thetic method for such compounds bearing imidazole and
imidazolin-2-one groups was developed. The application of
these novel NHC–PdII complexes in asymmetric catalysis as
well as the design and synthesis of new bis(imidazolium)
salts are currently under investigation in our laboratory.
Scheme 2. The synthesis of bis(imidazolium) salts 4.
The palladium-catalyzed Heck reaction has turned out
to be one of the most powerful methods for the formation
of carbon–carbon bonds in organic synthesis.[14] A number
of palladium/N-heterocyclic carbene catalyzed systems have
been employed with great success.[15] Herein, we turned our
interest to investigate the catalytic abilities of NHC–Pd
complexes 9a and 10b in the Heck reaction. Typically, n-
butyl acrylate was used as a model substrate with aryl halo-
genides for the Heck reaction with the use of NHC–Pd
complexes (0.1 mol-% Pd) as the catalyst in the presence of
Na2CO3 at 140 °C. The results are summarized in Table 2.
It was found that Heck reaction products 12 were obtained
Table 2. The coordination of bis(imidazolium) salt with
Pd(OAc)2.[a]
Entry
R
RЈ
X
Complex Time [h] Yield [%][b]
Experimental Section
1
2
3
4
5
6
7
8
OCH3
CH3
CHO
H
H
H
H
H
H
H
CH3
H
H
H
H
H
Br
Br
Br
Br
Br
I
Cl
Br
Br
Br
Br
Br
I
9a
9a
18
18
18
18
18
18
18
18
18
18
18
18
18
18
12a, 80
12b, 90
12c, 95
12d, 93
12e, 82
12d, 99
trace
General Remarks: Dichloromethane was freshly distilled from cal-
cium hydride; thf and toluene were distilled from sodium (Na) un-
der an argon (Ar) atmosphere. Melting points were determined
with a digital melting point apparatus and temperatures are uncor-
rected. 1H and 13C NMR spectra were recorded with a Bruker AM-
300 or AM-400 spectrophotometer. Infrared spectra were recorded
9a
9a
9a
9a
H
9a
OCH3
CH3
CHO
H
H
H
10b
10b
10b
10b
10b
10b
10b
12a, 83
12b, 85
12c, 94
12d, 88
12e, 76
12d, 99
trace
with
a Perkin–Elmer PE-983 spectrometer. Flash column
9
10
11
12
13
14
chromatography was performed by using 300–400 mesh silica gel.
For thin-layer chromatography (TLC), silica-gel plates (Huanghai
GF254) were used. Elementary analysis was taken with a Carlo–
Erba 1106 analyzer. Mass spectra were recorded by EI and ESI
and were measured with a HP-5989 instrument.
H
CH3
H
H
H
Cl
[a] Reactions of NHC–Pd complex 9a or 10b (0.1 mol-% Pd), so-
dium carbonate (2.0 mmol), tetrabutylammonium bromide (TBAB,
0.1 mmol), aryl halogenides (1.0 mmol), and n-butyl acrylate
(1.5 mmol) in N,N-dimethylacetamide (DMAc, 3.0 mL) were car-
ried out at 140 °C for 18 h. [b] Isolated yield by silica-gel column
chromatography.
NHC–PdII Complex 9a and 10a: Compound 4a (455.1 mg,
0.60 mmol), Pd(OAc)2 (134.7 mg, 0.60 mmol), and tBuOK
(168.3 mg, 1.50 mmol) were heated at reflux in thf (10 mL) for 24 h.
The volatiles were then removed under reduced pressure, and the
residue was purified by a silica-gel flash column chromatography
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Elimination of an Alkyl Group from Imidazolium Salts
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(petroleum ether/ethyl acetate, 8:1–2:1 and 1:1–0:1, for 9a and 10a,
respectively) to give 9a as a yellow solid (315.5 mg, 63%) and 10a
as a yellow solid (77.7 mg, 15%). A single crystal of 9a suitable
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NHC–PdII Complex 9a: Yellow solid. M.p. Ͼ250 °C. 1H NMR
(300 MHz, CDCl3, TMS): δ = 1.88 (t, J = 7.2 Hz, 6 H, CH3), 2.60
(s, 6 H, OCH3), 2.83 (s, 6 H, OCH3), 4.83–4.95 (m, 2 H, CH2),
5.51–5.63 (m, 2 H, CH2), 6.72 (d, J = 8.1 Hz, 2 H, ArH), 6.83 (d,
J = 8.4 Hz, 2 H, ArH), 7.00 (d, J = 7.8 Hz, 2 H, ArH), 7.09 (d, J =
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[4]
H, ArH), 9.67 (s, 2 H, CH) ppm. IR (KBr, neat): ν = 3075, 2925,
˜
2853, 1732, 1680, 1587, 1503, 1465, 1435, 1398, 1344, 1313, 1260,
1161, 1084, 1014, 847, 792, 742, 722, 699, 656 cm–1. MS (ESI): m/z
(%) = 1694.8 (18.3) [M]+, 1670.8 (43.5) [M]+, 1543.0 (13.3) [M]+,
1317.2 (12.2) [M]+, 803.6 (100) [M]+, 721.1 (58.6) [M]+, 519.2 (73.1)
[M]+. HRMS (Micromass LCT): calcd. for C60H53I4N8O4Pd2
1668.8438; found 1668.8477.
NHC–PdII Complex 10a: Yellow solid. M.p. Ͼ250 °C. 1H NMR
(300 MHz, CDCl3, TMS): δ = 1.22 (t, J = 7.2 Hz, 6 H, CH3), 2.86
(s, 6 H, OCH3), 4.04–4.12 (m, 2 H, CH2), 5.43–5.50 (m, 2 H, CH2),
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7.16–7.27 (m, 5 H, ArH), 7.28–7.45 (m, 5 H, ArH) ppm. IR (KBr,
[5]
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[7]
[8]
neat): ν = 3032, 2926, 2853, 1678, 1579, 1468, 1437, 1343, 1265,
˜
1118, 1083, 1060, 1013, 801, 779, 742 cm–1. MS (ESI): m/z (%) =
739.0 (23.5) [M]+, 735.0 (100) [M]+. HRMS (Micromass LCT):
calcd. for C32H30IN4O2Pd 735.0448; found 735.0472.
Typical Procedure for the Heck Reaction: NHC–Pd complex 9a
(8.3 mg, 0.005 mmol), sodium carbonate (212.0 mg, 2.0 mmol), tet-
rabutylammonium bromide (TBAB; 32.2 mg, 0.10 mmol), 1-bro-
mobenzene (157.0 mg, 1.0 mmol), n-butyl acrylate (192.3 mg,
1.50 mmol), and N,N-dimethylacetamide (DMAc; 3.0 mL) were in-
troduced into an Schlenk tube. The mixture was stirred at 140 °C
for 18 h. The reaction mixture was diluted with H2O (15 mL) and
ethyl acetate (15 mL), followed by extraction with ethyl acetate
(2ϫ). The combined organic layer was dried with MgSO4, filtered,
and evaporated under reduced pressure to give the crude product.
The pure product was isolated by column chromatography (hexane/
ethyl acetate, 15:1) on silica gel to give 4-acetylbiphenyl 12d
[9]
[10]
1
(190.0 mg, 93%) as a yellow oil, which was analyzed by H NMR
spectroscopy.
X-ray Crystal Structure Determinations: CCDC-675585 (9a) and
-686012 (10b) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
datarequest/cif.
[11]
[12]
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and characterization data for 4, 6, 7,
8, 9, 10, 11, and 12.
[13]
[14]
Acknowledgments
Financial support from the Shanghai Municipal Committee of Sci-
ence and Technology (06XD14005 and 08dj1400100–2), National
Basic Research Program of China (973)-2009CB825300, and the
National Natural Science Foundation of China (20872162,
20672127, 20732008, and 20702013) are greatly acknowledged.
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Eur. J. Inorg. Chem. 2009, 1723–1728
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1727

L.-j. Liu, F. Wang, M. Shi
SHORT COMMUNICATION
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Received: December 15, 2008
Published Online: January 28, 2009
1728
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1723–1728