N-heterocyclic carbene adducts of cyclopalladated ferrocenylpyridazine: Synthesis, structural characterization, and application in α-arylation of ketones with aryl chlorides

Article abstract of DOI:10.1071/CH12035

A new ferrocene-based ligand 3-chloro-6-pyridazinylferrocene 1 and its N-heterocyclic carbene adducts 2-3 were synthesized and characterized by ¹H NMR and IR spectroscopy, ESI-MS, and elemental analysis. Additionally, detailed structures of complexes 2-3 have been determined by single-crystal X-ray analysis. Complex 3 exhibited high catalytic activity for α-arylation of ketones with aryl chlorides. Typically, using 1mol% catalyst in the presence of 1.5 equivalents of ^tBuOK as base in dioxane at 100°C provided coupled products in good yields.

Full text of DOI:10.1071/CH12035

CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 366–370
Full Paper
http://dx.doi.org/10.1071/CH12035
N-Heterocyclic Carbene Adducts of Cyclopalladated
Ferrocenylpyridazine: Synthesis, Structural
Characterization, and Application in a-Arylation
of Ketones with Aryl Chlorides
A C
,
B
A
A
Chen Xu,
Hong-Mei Li, Zhi-Qiang Wang, Wei-Jun Fu,
B
A
Yu-Qing Zhang, and Bao-Ming Ji
A
College of Chemistry and Chemical Engineering, Luoyang Normal University,
Luoyang, Henan 471022, China.
B
Henan University of Science and Technology, School of Chemical Engineering
and Pharmaceutics, Luoyang, Henan 471003, China.
C
Corresponding author. Email: xubohan@163.com
A new ferrocene-based ligand 3-chloro-6-pyridazinylferrocene 1 and its N-heterocyclic carbene adducts 2–3 were
synthesized and characterized by 1H NMR and IR spectroscopy, ESI-MS, and elemental analysis. Additionally, detailed
structures of complexes 2–3 have been determined by single-crystal X-ray analysis. Complex 3 exhibited high catalytic
activity for a-arylation of ketones with aryl chlorides. Typically, using 1 mol % catalyst in the presence of 1.5 equivalents
t
of BuOK as base in dioxane at 1008C provided coupled products in good yields.
Manuscript received: 21 January 2012.
Manuscript accepted: 9 February 2012.
Published online: 9 March 2012.
Introduction
3-chloro-6-pyridazinylferrocene 1 and its carbene adducts of
palladacycle 2–3 (Scheme 1) and examined their catalytic
activity in the a-arylation of ketones. Here, we report that 3
is an extremely effective catalyst for the a-arylation of ketones
with aryl chlorides.
Since the first report by Buchwald, Hartwig and Miura in
1997,[1–3] palladium-catalyzed a-arylation of ketones with aryl
halides has become an extremely powerful method in organic
synthesis for the formation of Csp2-Csp3 bonds.[4–7] Several Pd
complexes derived from bulky and electron-rich alkylphosphine
ligands are reported to be effective catalysts for the a-arylation
of notoriously unreactive but relatively cheap aryl chlor-
ides.[8–10] As an alternative, N-heterocyclic carbenes (NHCs)
have become a paradigmatically new generation of strong
s-donor ligands and successfully used in palladium-catalyzed
a-arylation.[11–15] Moreover, palladacycles are one of the most
developed and studied classes of catalyst precursors because of
their structural versatility and easy synthetic accessibility.[16,17]
Among them, a wide variety of NHC adducts of palladacycles
have been reported and successfully used in the coupling reac-
tions.[18–23] However, only a few palladacycles have proved to
be efficient for a-arylation of aryl chlorides.[24,25]
We have recently studied the monophosphine adducts of
palladacycles containing the heterocyclic ring ferrocenyl
ligand.[26–28] These adducts combine the stability induced by
the presence of a palladacycle framework with the high activity
commonly associated with phosphine ligands, and were far
more active than the corresponding dimeric palladacycle. In
addition, cyclopalladated ferrocene complexes are quite fasci-
nating since ferrocene is an ideal framework on which planar
chirality can be introduced. In view of these findings and our
continuous interest in applications of chloromercuri-
ferrocene,[26–29] we prepared a new ferrocene-based ligand
Results and Discussion
Synthesis and Characterization of Compounds 1–3
The coupling reaction of chloromercuriferrocene and 3,6-
dichloropyridazine readily afforded 1 according to the published
procedures.[26–28] The following cyclopalladation reaction was
carried out with 1 and 1.1 equivalents of Li2PdCl4 and NaOAc in
methanol at room temperature for 24 h. The resulting red solids
of 1a were collected by filtration, washed several times with
methanol, and were assigned as a chloride-bridged palladacyclic
dimer.[26–28] Because of its poor solubility in all common
organic solvents, 1a was not characterized and was instead
directly subjected to the following reaction. Two new carbene
adducts of palladacycle 2–3 have been easily prepared in situ
from the reaction of 1a and 1,3-di-4-methoxyphenylimidazolium
chloride or 1,3-bis(2,6-diisopropylphenyl) imidazolium chloride
(IPrHCl) in THF at room temperature under a N2 atmosphere.
The one-pot synthesis avoids multi-step reactions employing
free carbenes.[30] These new compounds were fully character-
ized by elemental analysis, ESI-MS, and IR and 1H NMR
spectroscopy. The 1H NMR spectra of compounds 1 and 2–3 are
consistent with the proposed structures; 1 exhibits peaks for the
Cp ring with a proton ratio of 2 : 2 : 5, while 2–3 exhibit those
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

N-Heterocyclic Carbene Adducts of Palladacycle
367
HgCl
Cl
N N
[Pd]
ꢁ
Li2PdCl4
Fe
Fe
Cl
Cl
N N
1
Cl
tBuOK, THF
Cl
N N
N
N
R1
R1
Fe
ꢂ
Pd Cl
Fe
Pd
Cl
R2
Cl
R1
Nꢁ
N
R2
R2
2
N
R1
R1
R1
R1
N
2
3
R1 ꢀ H; R2 ꢀ OCH3
R1 ꢀ CH(CH3)2; R2 ꢀ H
1a
R1
R2
2–3
Scheme 1. Synthesis of compounds 1–3.
C8
C6
C24
C18
C9
C10
C7
C7
O1
C19
C20
C17
C16
C8
C10
Fe1
C21
C6
C23
C3
C2
C9
C24
C22
C4
C5
C26
C25
C15
C22
C12
Fe1
C13
C11
N2
C4
C5
C21
C20
C23
C18
C1
N3
C3
C2
N1
C14
C11
C19
C11
N2
Pd1
C12
C12
C28
C27
C1
N3
C13
C11
N4
C41
C15
C29
C39
C35
C16
C17
N1
Pd1
C14
C40
C30
C36
N4
C37
C12
C31
C38
C34
C33
C25
C26
C32
C30
C27
Fig. 2. Molecular structure of complex 3ꢀCH2Cl2 (representation of one of
the two independent crystal structures). Displacement parameters are drawn
at the 50 % level. CH2Cl2 and H atoms are omitted for clarity. Selected bond
C29
C28
˚
lengths (A) and angles (8): Pd(1)–C(1) 1.974(5), Pd(1)–N(1) 2.148(4),
Pd(1)–C(27) 1.991(11), Pd(1)–Cl(2) 2.3853(15), and C(1)–Pd(1)–N(1)
80.48(19), N(1)–Pd(1)–Cl(2) 93.01(13), Cl(2)–Pd(1)–C(27) 94.82(13),
C(1)–Pd(1)–C(27) 91.82(19).
O2
C31
attributed to [M–Cl]þ. In order to further investigate the struc-
tures of these complexes, their detailed structures have been
determined by X-ray single-crystal diffraction.
Fig. 1. Molecular structure of complex 2. Displacement parameters are
drawn at the 50 % level. H atoms are omitted for clarity. Selected bond
˚
lengths (A) and angles (8): Pd(1)–C(1) 1.973(5), Pd(1)–N(1) 2.121(4),
Pd(1)–C(15) 1.979(18), Pd(1)–Cl(2) 2.4037(13), and C(1)–Pd(1)–N(1)
80.24(18), N(1)–Pd(1)–Cl(2) 96.41(11), Cl(2)–Pd(1)–C(15) 94.38(14),
C(1)–Pd(1)–C(15) 89.3(2).
Crystal Structures of Complexes 2–3ꢀCH2Cl2
The crystals were obtained by recrystallization from CH2Cl2/
petroleum ether solution at room temperature. The molecules
are shown in Figs 1–2. The bicyclic system formed by the pal-
ladacycle and the C5H3 moiety is approximately coplanar
(dihedral angles of 6.88 and 6.78 for complexes 2–3ꢀCH2Cl2,
respectively). The Pd atom in each complex is in a slightly
distorted square-planar environment bonded to the C atom of
corresponding peaks with a proton ratio of 1 : 1 : 1 : 5, clearly
showing that they were ortho-cyclopalladated products. The
1H NMR spectra of 2–3 showed only one set of signals in a
symmetrical environment indicating the exclusive formation of
the isomer. In the mass spectra of 2–3, the most intense peak was

368
C. Xu et al.
Table 1. Optimization of reaction conditions for the a-arylation
Table 2. a-Arylation of ketones with aryl chlorides catalyzed by
of acetophenone with phenyl chlorideA
complex 3A
O
O
R3
O
O
Cat 3
tBuOK, dioxane
R2
Cl
ꢁ
Cat
Cl
R1
R1
ꢁ
Base, solvent
R3
R2
Yield [%]B
Entry
R1
R2
R3
YieldB [%]
Entry
Catalyst [mol %]
Base
Solvent
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
p-Me
92
91
1
3 (1)
K2CO3
NaH
Dioxane
Dioxane
Dioxane
Dioxane
Toluene
THF
16
56
2
H
m-OMe
p-OMe
p-CF3
p-NO2
o-Me
2
3 (1)
3
3 (1)
tBuONa
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
78
3
H
93
4
H
68
4
3 (1)
90
5
H
Trace
93
5
3 (1)
86
6
H
6
3 (1)
60
7
H
o-OMe
95
7
3 (1)
DMF
46
8
H
1,3-dimethyl
p-OMe
96
8
2 (1)
Dioxane
Dioxane
Dioxane
Trace
Trace
52
9
CH3
CH3
CH3
CH3
H
90
9
10C
1a (0.5)
1a/IPrHCl (0.5/2)
10
11
12
13
14
15
16
o-OMe
92
1,3-dimethyl
p-CF3
94
AReaction conditions: phenyl chloride (1.0 mmol), acetophenone
(1.2 mmol), base (1.5 mmol), solvent (3 mL), reflux temperature, 3h.
BIsolated yields (average of two experiments).
70
4-OMe-Ph
4-OMe-Ph
o-Me-Ph
o-Me-Ph
o-Me
89
H
p-OMe
87
CtBuOK (3.0 mmol).
H
o-Me
85
H
p-OMe
82
AReaction conditions: aryl chlorides (1.0 mmol), ketones (1.2 mmol),
tBuOK (1.5 mmol), dioxane (3 mL), 1008C, 3 h.
BIsolated yields (average of two experiments).
NHC, the chlorine atom, the nitrogen atom and the C atom of the
˚
ferrocenyl moiety. The Pd–Ccarb [1.991(11) A] bond length of
complex 3ꢀCH2Cl2 is similar to those of related carbene adducts
[1.992–1.998 A],[20,22,24] while it is longer than those of related
˚
˚
complex 2 [1.979(18) A] possibly owing to the steric bulk of the
˚
IPr ligand. In addition, The Pd–N [2.148(4) A] bond length of
t
Under the optimized reaction conditions (3, BuOK, diox-
complex 3ꢀCH2Cl2 is also longer than that of complex 2 [2.121
ane), a-arylation of ketones with a variety of electronically and
structurally diverse aryl chlorides were carried out to explore the
scope of this system (Table 2). Reactions of electron-rich aryl
chlorides such as 4-chlorotoluene, 3- and 4-chloroanisoles also
gave very high yields (92, 91 and 93 %, respectively, entries
1–3). However, for electron-deficient aryl chlorides such as
4-trifluroromethyl chlorobenzene, the yield decreased notice-
ably (68 %, entry 4). In the case of 4-nitrochlorobenzene, 3 was
almost inactive under the similar reaction conditions (entry 5).
These results represent an interesting trend, the oxidative addi-
tion does not affect the overall rate of the reaction and is not the
rate determining step.[31,32] It was noteworthy that the coupling
of hindered aryl chlorides with acetophenone gave the desired
products in excellent yields (entries 6–8), which shows that the
steric hindrances have no deleterious effect on these reactions.
The scope of the a-arylation was further investigated by
varying the aromatic ketones under the same condition. Similar
to the results for acetophenone, good yields were also obtained
in the case of propiophenone (entries 9–12). In cases of
p-methoxyacetophenone and o-methylacetophenone, the elec-
tronic and steric effects have no significant influence on the
reaction (entries 13–16).
˚
(4) A]. The imidazole ring plane of NHC is almost perpendicular
to the square plane formed by the Pd(II) centre (dihedral angles
of 91.78, 72.78 for complexes 2–3ꢀCH2Cl2, respectively). In this
type of arrangement, the N-substituents of NHC reduce the steric
interaction with palladacyclic ligand.
Application in a-Arylation of Ketones
Our initial exploration of reaction conditions focussed on the
coupling of acetophenone with phenyl chloride in the presence
of 1 mol % carbene adduct of palladacycle 3. After screening a
t
t
variety of bases (e.g. K2CO3, NaH, BuONa and BuOK)
(Table 1, entries 1–4), tBuOK was found to give the best result
(90 % yield) and tBuONa displayed moderate efficiency (78 %
yield). Then, a quick survey of solvents indicated that dioxane
was much better than other solvents such as toluene, THF and
DMF (entries 5–7). Toluene also afforded good coupled product
yield (86 %). Furthermore, the activities of several related cat-
alytic systems in the same model reaction were investigated.
Complex 2 was found to be an inactive precatalyst (entry 8). The
dimeric complex 1a was also inactive under the same reaction
conditions (entry 9) and the yield was greatly improved by the
addition of carbene precursor IPrHCl (52 %, entry 10) sug-
gesting that carbene IPr participated in the catalytic cycles.
However, the activity of 1a/IPrHCl system was still obviously
lower than that of the preformed IPr adduct of cyclopalladated
ferrocenylpyridazine. On the basis of these results, we propose
that the carbene ligand may promote the release of the ‘real
catalyst’ species from palladacycle and the coupling reaction
catalyzed by palladacycle proceeded through a Pd0/PdII
cycle.[23,28]
Conclusions
In summary, we have prepared and characterized a new
ferrocene-based ligand 1 and its NHC adducts of pallada-
cycle 2–3. Their detailed structures have been determined by
X-ray single-crystal diffraction. Complex 3 was found to be
an efficient catalyst for a-arylation of ketones with aryl
chlorides.

N-Heterocyclic Carbene Adducts of Palladacycle
369
Experimental
Table 3. Crystallographic data and structure refinement for
2–3ꢀCH2Cl2
General Procedures
All reactions were carried out under a dry nitrogen atmosphere
using standard Schlenk techniques. Solvents were dried and
freshly distilled before use. All other chemicals were commer-
cially available expect for chloromercuriferrocene, which was
prepared according to the published procedures.[33] Elemental
analyses were determined with a Carlo Erba 1160 Elemental
Analyzer. IR spectra were collected on a Bruker VECTOR22
spectrophotometer in KBr pellets. NMR spectra were recorded
on a Bruker DPX-400 spectrometer in CDCl3 with TMS as an
internal standard. Mass spectra were measured on an LC-MSD-
Trap-XCT instrument. Crystallographic data were collected on
a Bruker SMART APEX-II CCD diffractometer.
Compound
2
3ꢀCH2Cl2
Empirical formula
Formula weight
Crystal system
Space group
C31H26Cl2FeN4O2Pd
719.71
C83H94Cl6Fe2N8Pd2
1740.86
Triclinic
Triclinic
P-1
P-1
Crystal size [mm]
˚
a [A]
˚
b [A]
0.26 ꢂ 0.21 ꢂ 0.14
11.0551(15)
11.4501(15)
13.900(3)
111.740(2)
92.674(2)
117.2520(10)
1402.2(4)
1.705
0.39 ꢂ 0.27 ꢂ 0.16
14.019(2)
16.262(3)
20.503(3)
78.153(2)
70.538(2)
81.476(2)
4296.9(11)
1.346
˚
c [A]
a [8]
b [8]
g [8]
3
˚
V [A ]
Dc [g cmꢁ3
]
Synthesis of 3-Chloro-6-pyridazinylferrocene 1
Z
2
2
GOF(on F2)
1.010
0.942
In a flask equipped with reflux condenser and gas inlet, chlor-
omercuriferrocene (1 mmol), 3,6-dichloropyridazine (1.1 mmol),
NaI (2 mmol), Pd(PPh3)4 (0.05 mmol), 18mL absolute THF and
12 mL absolute Me2CO were placed under an N2 atmosphere. The
reaction mixture was then placed in an oil bath and heated at 708C
for 6 h, cooled and quenched with water. The product was sepa-
rated by passing through a short silica gel column with CH2Cl2 as
eluent. The second band was collected, which afforded the red
solid 1, yield 55 %. nmax (KBr)/cmꢁ1 2923, 1574, 1480, 1436,
1389, 1280, 1157, 1110, 1035, 998, 822, 745, 687. dH (400 MHz,
CDCl3) 7.46 (d, J 8.8, 1H, ArH), 7.35 (s, 1H, J 8.8, ArH), 5.01 (s,
2H, C5H4), 4.52 (s, 2H, C5H4), 4.08 (s, 5H, C5H5). m/z (ESI) 299.6
[MþH]þ. Anal. Calc. for C14H11ClFeN2: C 56.3, H 3.7, N 9.4.
Found: C 56.6, H 3.5, N 9.6 %.
Reflections collected
Reflections unique
R1, wR2 [I . 2s(I)]
10451
31135
5171
15531
0.0435, 0.1134
0.0565, 0.1017
1H, ArH), 4.66 (m, 1H, CH), 4.48 (s, 1H, C5H3), 4.35 (s, 1H,
C5H3), 3.98 (s, 1H, C5H3), 3.40 (s, 5H, C5H5), 3.20 (m, 1H,
CH), 2.90–2.97 (m, 2H, CH), 1.61 (s, 6H, CH3), 1.54 (d, J 6.4,
3H, CH3), 1.42 (d, J 6.4, 3H, CH3), 1.17 (d, J 6.4, 3H, CH3),
0.97 (d, J 6.4, 3H, CH3), 0.85 (d, J 6.4, 3H, CH3), 0.58 (d, J 6.4,
3H, CH3). m/z (ESI) 792.5 [Mþ–Cl]. Anal. Calc. for
C41H46Cl2FeN4Pd: C 59.5, H 5.6, N 6.8. Found: C 59.7,
H 5.3, N 7.1 %.
General Procedure for the Synthesis of Carbene Adducts
of Cyclopalladated Ferrocenylpyridazines 2–3
General Procedure for the a-Arylation of Ketones
A mixture of 1 (1 mmol), Li2PdCl4 (1.1 mmol) and NaOAc
(1 mmol) in 20 mL dry methanol was stirred for 24 h at room
temperature. The red solid 1a (yield: 89 %) was collected by
filtration and washed several times with methanol. Without
further purification, it was directly subjected to the following
reaction. A Schlenk tube was charged with 1a (0.5 mmol), the
corresponding imidazolium salt (1.25 mmol) and tBuOK
(2.5 mmol) under nitrogen. Dry THF was added by a cannula
and stirred at room temperature for 2 h. The product was sepa-
rated by passing through a short silica gel column with CH2Cl2
as eluent. The second band was collected, which afforded the
corresponding carbene adduct 2 or 3.
In a Schlenk tube, a mixture of catalyst (0.01 mmol), aryl
chloride (1.0 mmol), ketone (1.2 mmol) and the selected base
(1.5 mmol) in solvent (3 mL) was evacuated and charged with
nitrogen. The reaction mixture was then placed in an oil bath and
heated at reflux temperature for 3 h, then cooled and quenched
with water. The reaction mixture was extracted three times with
CH2Cl2, the combined organic layers were washed with water,
dried (MgSO4), and evaporated to dryness. The pure products
were isolated by flash chromatography on silica gel and iden-
1
tified by comparing melting points or H NMR spectra with
those found in the literature.[2,10,12,31,32]
[PdCl{[(Z5-C5H5)]Fe[(Z5-C5H3)-N2C4H2-Cl]}(C3N2H2)
(C6H4-OCH3)2] (2): Red solid, yield 90 %. nmax (KBr)/cmꢁ1
2933, 1509, 1467, 1375, 1289, 1160, 1148, 1009, 842, 818, 770,
692. dH (400 MHz, CDCl3) 8.39 (d, J 8.8, 2H, ArH), 7.86 (d,
J 8.8, 2H, ArH), 7.52 (d, J 8.8, 1H, ArH), 7.42 (s, 1H,
NCHCHN), 7.35 (s, 1H, NCHCHN), 7.29 (d, J 8.8, 1H, ArH),
7.17 (d, J 8.8, 2H, ArH), 6.86 (d, J 8.8, 2H, ArH), 4.38 (s, 1H,
C5H3), 4.22 (s, 1H, C5H3), 3.91 (s, 1H, C5H3), 3.81 (s, 3H,
OCH3), 3.73 (s, 3H, OCH3), 3.49 (s, 5H, C5H5). m/z (ESI) 684.3
[Mþ–Cl]. Anal. Calc. for C31H26Cl2FeN4O2Pd: C 51.7, H 3.6,
N 7.8. Found: C 51.5, H 3.3, N 8.1 %.
[PdCl{[(Z5-C5H5)]Fe[(Z5-C5H3)-N2C4H2-Cl]}(C3N2H2)
(C6H3-2C3H7)2] (3): Red solid, yield 87 %. nmax (KBr)/cmꢁ1
2960, 1583, 1514, 1469, 1379, 1329, 1253, 1106, 1038, 800,
756, 700. dH (400 MHz, CDCl3) 7.45–7.50 (m, 3H,
NCHCHNþArH), 7.16–7.32 (m, 6H, ArH), 7.04 (d, J 8.8,
Single Crystal X-Ray Structure Determination
Crystallographic data for complexes 2 and 3ꢀCH2Cl2 were
collected on a Bruker SMART APEX-II CCD diffractometer
equipped with a graphite monochromator at 296 K using Mo-Ka
˚
radiation (l ¼ 0.071073 A). The data were corrected for Lorentz
polarization factors as well as for absorption. Structures were
solved by direct methods and refined by full-matrix least-
squares methods on F2 with the SHELX-97 program.[34] All non-
hydrogen atoms were refined anisotropically, while hydrogen
atoms were placed in geometrically calculated positions. Crystal
data and structure refinements are summarized in Table 3.
CCDC reference numbers 857945 and 857946 for 2 and
3ꢀCH2Cl2, respectively. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.

370
C. Xu et al.
Acknowledgements
[18] R. B. Bedford, M. Betham, M. E. Blake, R. M. Frost, P. N. Horton,
M. B. Hursthouse, R. Lo´pez-Nicola´s, Dalton Trans. 2005, 2774.
doi:10.1039/B506286A
We are grateful to the National Natural Science Foundation of China
(No. 20902043) and the Natural Science Foundation of Henan Province
(No. 102300410220) and Luoyang Tackle Key Problem of Science and
Technology (No. 1001060A) for financial support of this work.
[19] G. D. Frey, J. Schu¨tz, E. Herdtweck, W. A. Herrmann, Organome-
tallics 2005, 24, 4416. doi:10.1021/OM049001G
[20] J. Y. Li, M. J. Cui, A. J. Yu, Y. J. Wu, J. Organomet. Chem. 2007, 692,
3732. doi:10.1016/J.JORGANCHEM.2007.05.022
[21] E. A. B. Kantchev, G. R. Peh, C. Zhang, J. Y. Ying, Org. Lett. 2008, 10,
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