Synthesis and catalytic activity of chiral dicarbene dipalladium complexes incorporating the S-binaphthol unit

Article abstract of DOI:10.3184/174751918X15168768395864

A series of chiral di-N-heterocyclic carbene (NHC) dipalladium complexes, [{PdPyCl₂}₂(di-NHC)], in which di-NHC represents a diimidazolylidene, featuring an (S)-3,3'-dimethyl-2,2'-dimethoxy-1,1'-binaphthalene spacer between the carbene units, have been prepared. The influence of ligand size on the catalytic activity of these complexes in the Suzuki reaction of phenylboronic acid with p-bromotoluene has been investigated. The most sterically hindered complex, bearing the di-isopropylphenyl group, showed the greatest catalytic activity, and it is active for various aryl halides with different electronic and steric properties.

Full text of DOI:10.3184/174751918X15168768395864

54 RESEARCH PAPER
VOL. 42 JANUARY, 54–56
JOURNAL OF CHEMICAL RESEARCH 2018
Synthesis and catalytic activity of chiral dicarbene dipalladium complexes
incorporating the S-binaphthol unit
Guowen Zhanga, Man Chaoa, Shuting Wanga, Mengxia Zhua, Dou Wanga, Guangsheng Pangb and
Yanhui Shia*
aSchool of Chemistry and Material Science and Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University,
Xuzhou, Jiangsu 221116, P.R. China
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012, P.R. China
A series of chiral di-N-heterocyclic carbene (NHC) dipalladium complexes, [{PdPyCl } (di-NHC)], in which di-NHC represents a di-
imidazolylidene, featuring an (S)-3,3′-dimethyl-2,2′-dimethoxy-1,1′-binaphthalene spacer2b2etween the carbene units, have been prepared.
The influence of ligand size on the catalytic activity of these complexes in the Suzuki reaction of phenylboronic acid with p-bromotoluene
has been investigated. The most sterically hindered complex, bearing the di-isopropylphenyl group, showed the greatest catalytic activity,
and it is active for various aryl halides with different electronic and steric properties.
Keywords: palladium, N-heterocyclic carbene, (S)-binaphthol, Suzuki reaction, catalysis
Over the last few decades, the chemistry of palladium
N-heterocyclic carbene (NHC) complexes has been extensively
studied and widely applied in various C–C, C–N and other
C–heteroatom bond-coupling reactions.1–3 Generally, the many
unique properties of Pd-NHC complexes, including facile
synthesis, stability to oxygen, moisture and heat, and tunability
of their steric and electronic properties, make their synthesis
and application attractive. A great number of chiral NHCs have
been developed and successfully applied as chiral ligands in
various metal-catalysed asymmetric reactions.4–10 Moreover,
many chiral NHC transition metal complexes have been
synthesised and used in asymmetric catalysis.11–12 Chiral
binaphthol (BINOL) is one of the most studied chiral ligands.
Several chiral NHC ligands have been prepared by introducing
NHCs onto the binaphthol unit and have been successfully
applied in some metal-catalysed asymmetric reactions.13–16
It has been our interest to study the preparation and properties
of binuclear palladium-NHC complexes carrying various
linking groups, bearing in mind that introducing two catalytic
sites into a catalyst may induce cooperative effects and result in
better catalytic activities.17–21 Herein, we report the preparation
of a series of chiral binuclear Pd-NHC complexes incorporating
the (S)-binaphthol (S-BINOL) unit. In order to elucidate the
influence of the steric and electronic effect of the ligands, we
changed one of the wingtip substituents on the nitrogen of the
imidazole ring and investigated the catalytic activity of these
complexes in Suzuki–Miyaura reactions.
The synthesis of chiral di-NHC dipalladium complexes with
a dimethoxy binaphthalene spacer is shown in Scheme 1.
(S) -3,3′-Bis (chloromethyl) -2,2′-dimethoxy-1,1′-
binaphthalene was synthesised from commercially available
(S)-1,1′-binaphthalene-2,2′-diol via a four-step reaction with
45% yield according to the following procedure.22 First,
(S)-2,2′-dimethoxy-1,1′-binaphthalene was obtained by the
methylation of (S)-1,1′-binaphthalene-2,2′-diol with 95%
yield. Introduction of an aldehyde group in the 3,3′-position
O
H
OH
1) n-BuLi/TMEDA,
Et
K
CH
2CO3
3I,
2O
OCH3
OCH3
OH
OH
OCH3
OCH3
NaBH4
OCH3
OCH3
actone, 60 °C
2) DMF
3) H3O+
H
O
OH
N
Cl
Pd
Cl
R
N
N
Cl
N
N
N
R
R
N
R
N
PdCl2 K CO
,
OCH3
OCH3
K
SOCl2 2CO3
,
OCH3
OCH3
2
3
OCH3
OCH3
80 °C
CH3CN
Py., 80 °C
N
Cl
N
N
R
Cl
Pd
Cl
2
Cl
N
=
2
1
3
( )
4
R
( )
( )
( )
Scheme 1 Synthesis of dipalladium-NHC complexes 1–4.
* Correspondent. E-mail: yhshi@jsnu.edu.cn

JOURNAL OF CHEMICAL RESEARCH 2018 55
of (S)-2,2′-dimethoxy-1,1′-binaphthalene was achieved by
ortho-lithiation followed by formylation, with an 85% yield
overall. (S)- (2,2′-Dimethoxy-[1,1′-binaphthalene]-3,3′-diyl)
dimethanol was prepared by reduction of (S)-2,2′-dimethoxy-
[1,1′-binaphthalene]-3,3′-dicarbaldehyde with NaBH4. Finally,
(S)-3,3′-bis(chloromethyl)-2,2′-dimethoxy-1,1′-binaphthalene
was prepared by chlorination of (S)-(2,2′-dimethoxy-[1,1′-
binaphthalene]-3,3′-diyl)dimethanol with thionyl chloride.
The chiral bis(imidazolium) chlorides were prepared by direct
nucleophilic substitution reaction of (S)-3,3′-bis(chloromethyl)-
2,2′-dimethoxy-1,1′-binaphthalene with the corresponding aryl
imidazoles following the literature procedure.23 The synthesis
of chiral Pd-NHC complexes (1–4) was achieved by the reaction
of the corresponding bis(imidazolium) chlorides with PdCl2 in
the presence of K2CO3 in pyridine at 80 °C for 18 h.
The elemental analyses for the C, H and N contents of these
complexes were in the expected regions.
The palladium-catalysed C–C coupling reaction has found
wide application in organic synthesis. The catalytic activity
of complexes 1–4 in the Suzuki reaction was tested in order
to elucidate the influence of the ligands. The reactions were
conducted in a vial in the presence of catalytic amounts of
palladium catalysts 1–4 and KOH in a mixture of 1-bromo-
4-methylbenzene with phenylboronic acid at 65 °C for 24 h
(Table 1, entries 1–4). The results show that all of these
complexes 1–4 could catalyse the Suzuki reaction with good
yields. However, complex 4, with the most sterically hindered
substituent (di-isopropyl) on the phenyl group, had the best
catalytic activity, affording 93% of 4-methyl-1,1′-biphenyl with
99% conversion of phenyl bromide. In contrast, complexes 1–3,
with the less sterically hindered substituents (dimethyl, diethyl)
on the phenyl group, gave lower yields of the product. This
implies that the steric effect in the reaction plays an important
role in the activity of these catalysts.
In addition, a number of aryl halides with electron-donating
or electron-withdrawing substituents were investigated in the
Suzuki reaction with phenylboronic acid catalysed by complex
4 (Table 1, entries 5–10). The results show that an aryl iodide,
bromide or even chloride would react smoothly and generate
the product in good yield. The reaction went well with
substrates bearing electron-donating (Table 1, entries 4 and 10)
or electron-withdrawing substituents (Table 1, entries 5 and 8),
and even with substrates having little steric hindrance (Table 1,
entries 7 and 9). Unfortunately, these complexes gave very poor
results in a more hindered asymmetric reaction. The reaction
of 2-methyl-1-naphthalenylboronic acid with 2-methyl-1-
naphthalenylbromide catalysed by 4 afforded 2,2′-dimethyl-1,1-
binaphthalene in 32% yield with only 1% enantiomer excess.
The chiral di-NHC dipalladium complexes 1–4 were fully
1
characterised by H and 13C NMR, IR and elemental analysis.
Unfortunately, attempts to grow single crystals for an X-ray
structural study did not succeed. The structures of these
complexes are proposed by comparison with other related
palladium species, whose structures have been determined
in our previous studies,17–20 and their spectral and analytical
data are consistent with the proposed structures. In the NMR
spectra of these complexes, all signals corresponding to
pyridine, imidazole, phenyl and alkyl moieties are observed
1
in their expected regions. For example, the H NMR spectrum
of 1 displayed pyridine protons including Hα-Py as a doublet at
8.67 ppm, Hγ-Py as a triplet at 7.66 ppm and Hβ-Py as a doublet at
7.39 ppm. A doublet at 7.87 ppm corresponds to the imidazole
protons, and one multiplet around 7.34–7.19 ppm corresponds
to the phenyl and imidazole protons. A singlet at 6.92 ppm
corresponds to the 4,4′-proton of binaphthalene. The methene
protons appeared as two doublets at 6.30 and 6.11 ppm,
respectively, due to the chirality of the molecule. The protons
of the methoxy groups appeared as a singlet at 3.25 ppm,
whereas the protons of the methyl groups appeared as two
singlets at 2.37 and 2.28 ppm, respectively. In the 13C NMR
spectrum, carbon resonances of the three alkyl groups were
observed at 61.0, 51.1 and 19.2 ppm, along with other aromatic
carbons. In addition, the signal at 154.9 ppm was assigned to the
Ccarbene-Pd resonance, providing direct evidence of metalation.
Conclusion
We have synthesised a series of chiral dipalladium di-NHC
complexes 1–4 carrying the BINOL unit with different wingtip
substituents on one of the N positions of the NHC backbone.
The results of the Suzuki reaction of phenyl bromide with
phenylboronic acid show that the different substituents on the
NHC ligand have an obvious effect on the catalytic activity of the
Pd-NHC complexes. Complex 4, with the most sterically hindered
substituent (di-isopropyl) on the phenyl group, performed best,
giving the highest yield of product. It also showed good catalytic
activity for a variety of phenyl halides (iodide, bromide, chloride)
with different electronic and steric properties.
Table 1 Suzuki reaction of phenyl bromide with phenylboronic acid
catalysed by 1–4
B(OH)2
0.5 mol% Pd-NHC
Experimental
+
X
KOH, dioxane
Anhydrous diethyl ether and dioxane were distilled over Na, and
anhydrous pyridine was distilled over calcium hydride under an argon
atmosphere prior to use. The synthesis of (S)-3,3′-bis(chloromethyl)-
2,2′-dimethoxy-1,1′-binaphthalene22 and the corresponding bis-
imidazolium chlorides23 followed literature procedures. All other
chemicals were obtained from commercial suppliers and were used
65o 24 h
C,
R
R
Entrya Catalyst
R
X
Yield (%)
Conversionb
92
1
2
3
4
5
6
7
8
9
10
1
2
3
4
4
4
4
4
4
4
p-Me
p-Me
p-Me
p-Me
p-CN
m-CN
Br
Br
Br
Br
Br
Br
Br
83
85
86
93
>99
>99
83
95
95
without further purification. H and 13C spectra were recorded on a
1
Bruker AV 400 MHz spectrometer at room temperature and referenced
in parts per million (ppm) to the residual signals of the solvent.
Elemental analyses were performed on a EuroVektor Euro EA-300
elemental analyser. IR spectra were recorded in KBr pellets using an
FTIR-Tensor 27 spectrometer. GC-MS was performed on an Agilent
6890-5973N system with electron ionisation (EI) mass spectrometry.
>99
>99
>99
90
>99
95
o-OMe
p-NHCOCH3 Br
93
o-OMe
p-Me
I
88
Synthesis of di-NHC dipalladium complexes (1–4); general procedure
Cl
72
82
In a 25 mL flask containing a stirrer bar under argon, anhydrous
pyridine (5 mL) was added to a mixture of bis(imidazolium) chloride,
PdCl2 (0.18 g, 1 mmol) and K2CO3 (1.38 g, 10 mmol), which was
aReaction conditions: phenyl halide (0.5 mmol), phenylboronic acid (0.6 mmol, 73.2 mg),
Pd-NHC complex (0.0025 mmol) and KOH (1 mmol, 56 mg) in dioxane (2 mL).
bDetermined by GC using dodecane as internal standard.

56 JOURNAL OF CHEMICAL RESEARCH 2018
ground to a fine powder prior to use. The reaction mixture was heated
at 80 °C for 16 h, after which time the mixture was filtered through
Celite and washed with CH2Cl2. The filtrate was concentrated under
vacuum, and the residue was recrystallised with CH2Cl2/ether to give
the Pd-NHC complex as a yellow solid.
J = 6.8 Hz, 6H, CH3), 1.05 (d, J = 6.8 Hz, 6H, CH3), 0.98 (d, J = 7.2 Hz,
6H, CH3); 13C NMR (100 MHz, CDCl3): δ 154.9, 152.0, 151.4, 146.9,
146.8, 137.6, 134.9, 134.7, 132.5, 130.3, 129.3, 128.3, 127.1, 126.3, 125.3,
125.1, 124.3, 124.2, 123.9, 122.2, 60.7, 51.4, 28.4, 26.3, 23.3, 23.0; IR
(KBr) (νmax/cm−1): 2963, 2867, 1604, 1448, 1414, 1359, 1242, 1218, 1105,
1006, 757, 694; Anal. calcd for C64H68Cl4N6O2Pd2 (1307.93 g mol−1): C,
58.77; H, 5.24; N, 6.43; found: C, 59.02; H, 5.31; N, 6.52%.
Characterisation of complexes 1–4
[Bis (dichloropyridyl)]{(s) -3, 3 ′- ((2, 2 ′- dimethox y- [1,1′-
binaphthalene]-3,3′-diyl)bis(methylene))bis(1-(2,6-dimethylphenyl)-
2,3-dihydro-1H-imidazolidene)}dipalladium (1): Yield: 85%;
1H NMR (400 MHz, CDCl3): δ 8.67 (d, J = 4.8 Hz, 4H, H2-Py), 8.54 (s,
2H, ArH), 7.87 (d, J = 8.0 Hz, 2H, Himidazole), 7.66 (t, J = 7.6 Hz, 2H, H4-
Py), 7.39 (d, J = 8.4 Hz, 4H, H3-Py), 7.34–7.19 (m, 14H, ArH, Himidazole),
6.92 (s, 2H, ArH), 6.30 (d, J = 14.4 Hz, 2H, CH2), 6.11 (d, J = 14.4 Hz,
2H, CH2), 3.25 (s, 6H, OCH3), 2.37 (s, 6H, CH3), 2.28 (s, 6H, CH3);
13C NMR (100 MHz, CDCl3): δ = 154.9, 151.4, 151.2, 137.7, 137.5,
136.7, 134.7, 132.4, 130.4, 129.4, 129.1, 128.5, 128.4, 127.1, 126.1, 125.1,
124.3, 124.1, 123.7, 122.8, 61.0, 51.1, 19.2; IR (KBr) (νmax/cm−1): 2944,
1604, 1501, 1481, 1448, 1415, 1357, 1242, 1217, 1106, 1006, 757, 694;
Anal. calcd for C56H52Cl4N6O2Pd2 (1195.71 g mol−1): C, 56.25; H, 4.38;
N, 7.03; found: C, 55.95; H, 4.34; N, 6.97%.
[Bis (dichloropyridyl)]{(s) -3,3 ′- ((2,2′-dimethoxy- [1,1′-
binaphthalene]-3,3′-diyl)bis(methylene))bis(1-mesityl-2,3-dihydro-
1H-imidazolidene)}dipalladium (2): Yield: 76%; 1H NMR (400 MHz,
CDCl3): δ 8.68 (d, J = 5.2 Hz, 4H, H2-Py), 8.53 (s, 2H, ArH), 7.86 (d,
J = 8.0 Hz, 2H, Himidazole), 7.66 (t, J = 7.6 Hz, 2H, H4-Py), 7.41–7.19 (m,
12H, H3-Py, ArH, Himidazole), 7.03 (s, 2H, ArH), 6.99 (s, 2H, ArH), 6.88
(d, J = 1.6 Hz, 2H, ArH), 6.28 (d, J = 14.4 Hz, 2H, CH2), 6.10 (d,
J = 14.8 Hz, 2H, CH2), 3.24 (s, 6H, OCH3), 2.34 (d, J = 10.8 Hz, 12H,
CH3), 2.23 (s, 6H, CH3); 13C NMR (100 MHz, CDCl3): δ 154.9, 151.4,
151.1, 139.1, 137.6, 136.3, 136.2, 135.0, 134.7, 132.4, 130.3, 129.2,
128.4, 127.1, 126.1, 125.1, 124.3, 124.1, 123.8, 122.7, 61.0, 51.1, 21.2,
19.0; IR (KBr) (νmax/cm−1): 2921, 2855, 1605, 1486, 1448, 1413, 1358,
1242, 1217, 1045, 1006, 757, 694; Anal. calcd for C58H56Cl4N6O2Pd2
(1223.77 g mol−1): C, 56.93; H, 4.61; N, 6.87; found: C, 56.78; H, 4.69;
N, 6.92%.
Acknowledgements
We are grateful to the National Natural Science Foundation
of China (NSFC 21571087), the Major Projects of Natural
Science Research in Jiangsu Province (15KJA150004),
the PAPD and TAPP of Jiangsu Higher Education
Institutions, Postgraduate Research & Practice Innovation
Program of Jiangsu Province (KYCX17_1582), the
Research & Practice Innovation Program of Jiangsu
Province for Undergraduate and Postgraduate Students
(KYCX17_1582, 201710320086Y and HXKY2017008)
and the State Key Laboratory of Inorganic Synthesis and
Preparative Chemistry at Jilin University (2016-01) for
financial support.
Received 16 October 2017; accepted 12 January 2018
Paper 1705055
https://doi.org/10.3184/174751918X15168768395864
Published online: 27 January 2018
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132.3, 130.4, 129.8, 129.1, 128.4, 127.1, 126.2, 126.1, 125.1, 124.6,
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1
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