Tricyclohexylphosphine-cyclopalladated ferrocenylimine complexes: Synthesis, crystal structures and application in Suzuki and Heck reactions

Article abstract of DOI:10.1039/b608825b

A series of novel tricyclohexylphosphine (PCy₃)-cyclopalladated ferrocenylimine complexes 2c-2g have been easily synthesized. These new palladacycles are thermally stable and are not sensitive to air and moisture. Their detailed structures have been determined by single-crystal X-ray analysis and six different types of intermolecular hydrogen bonds are found to be existed in the crystals of these complexes. The use of 2c-2g as catalysts for Suzuki and Heck reactions was examined. They were found to be very efficient for the Suzuki reaction of aryl chlorides with phenylboronic acid. Typically, using 0.1 mol% of catalyst in the presence of 1.5 equivalent of Cs₂CO ₃ as base in dioxane at 100 °C provided coupled products in excellent yields. These complexes also displayed good activity in the Heck reaction of a range of aryl bromides with acrylic acid ethyl ester although they were not particularly useful for the activation of aryl chlorides. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608825b

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Tricyclohexylphosphine-cyclopalladated ferrocenylimine complexes: synthesis,
crystal structures and application in Suzuki and Heck reactions†
Chen Xu, Jun-Fang Gong,* Su-Fang Yue, Yu Zhu and Yang-Jie Wu*
Received 21st June 2006, Accepted 24th July 2006
First published as an Advance Article on the web 9th August 2006
DOI: 10.1039/b608825b
A series of novel tricyclohexylphosphine (PCy3)-cyclopalladated ferrocenylimine complexes 2c–2g have
been easily synthesized. These new palladacycles are thermally stable and are not sensitive to air and
moisture. Their detailed structures have been determined by single-crystal X-ray analysis and six
different types of intermolecular hydrogen bonds are found to be existed in the crystals of these
complexes. The use of 2c–2g as catalysts for Suzuki and Heck reactions was examined. They were found
to be very efficient for the Suzuki reaction of aryl chlorides with phenylboronic acid. Typically, using
0.1 mol% of catalyst in the presence of 1.5 equivalent of Cs2CO3 as base in dioxane at 100 ◦C provided
coupled products in excellent yields. These complexes also displayed good activity in the Heck reaction
of a range of aryl bromides with acrylic acid ethyl ester although they were not particularly useful for
the activation of aryl chlorides.
formed secondary or tertiary phosphine (HP(tBu)2, HPCy2 or
Introduction
PCy3, PtBu3, e.g.) adducts of dimeric phosphorus- or nitrogen-
Palladium-catalyzed coupling reactions such as the Suzuki cou-
containing palladacycles are far more active than the dimeric
pling, the Heck reaction and the Buchwald–Hartwig amination
palladacycles and exhibit excellent activity at very low catalyst
have become an extremely powerful method in organic synthe-
loadings when aryl chlorides, both activated and unactivated are
sis for the formation of carbon–carbon or carbon–heteroatom
bonds.1 Of the many commonly used coupling reactions, the
used as substrates in the Suzuki reaction. We have also found that
the activity of cyclopalladated ferrocenylimine dimers 1a and 1b
Suzuki and Heck reactions are perhaps the two most important
in the Suzuki reaction of aryl chlorides has been greatly enhanced
ones.2,3 A plethora of palladium catalyst precursors, usually
simple palladium salts or complexes associated or not with
with the corresponding tricyclohexylphosphine (PCy3)-containing
monomers 2a and 2b (1.1 vs. 76.4% GC yield with 1b and 2b
as catalyst, respectively, under the same reaction conditions).11
In view of these findings and our continuous interest in the
cyclopalladation of ferrocenylimines as well as applications of
these systems,12 we prepared a series of novel PCy3-cyclopalladated
ferrocenylimine complexes 2c–2g and examined their catalytic
activity in the Suzuki reaction. Their application to Heck reaction
was also investigated since we were interested to see whether a
great enhancement in activity was still observed in Heck reaction.
The results are presented below.
appropriate ligands, can mediate these reactions under various
reaction conditions. Among them, palladacycles are one of the
most developed and studied classes of catalyst precursors because
of their structural versatility and easy synthetic accessibility.2,3b,4
Since the first report on the use of palladacycles derived from the
cylopalladation of tris(o-tolyl)phosphine for the Suzuki and Heck
reactions by Herrmann et al.,5 a wide variety of known and new
palladacycles with PC, NC, SC or PCN, NCN and SCS pincer-
type coordination modes have been successfully used in these C–C
coupling reactions. However, for the activation of less reactive
aryl halides such as aryl bromides substituted with electron-
donating groups and aryl chlorides under relatively mild reaction
conditions, only a few palladacycles have shown good catalytic
activity.6–10 The best results were obtained with palladacycles
modified with N-heterocyclic carbenes (NHC) or phosphorus-
containing ligands. For example, the NHC-palladacycle complex
developed by Nolan and co-workers8 has been found to be able
to couple unactivated aryl chlorides with arylboronic acids at
room temperature in short reaction times although the catalyst
loadings are relatively high (2 mol%). Indolese and co-workers9
and Bedford et al.10 have demonstrated that isolated or in situ
Results and discussion
Synthesis and characterization of complexes 2c–2g
The chloride-bridged palladacyclic dimers 1c–1g were prepared
according to the published procedures.13 Then each of them was
treated with 1.1 equivalent of PCy3 per palladium in CH2Cl2 at
room temperature for 30 min to afford the corresponding PCy3-
cyclopalladated ferrocenylimine complexes 2c–2g in 73–80% yields
as red solids (Scheme 1). All the new complexes are air- and
moisture-stable both in the solid state and in solution. They are
very soluble in chloroform, dichloromethane and acetone, but
insoluble in petroleum ether and n-hexane.
Department of Chemistry, Henan Key Laboratory of Chemical Biology
and Organic Chemistry, Key Laboratory of Applied Chemistry of Henan
Universities, Zhengzhou University, Zhengzhou, 450052, P R China. E-mail:
wyj@zzu.edu, cngongjf@zzu.edu.cn; Fax: +86 371 67766667; Tel: +86 371
67763207
Complexes 2c–2g were fully characterized by elemental analysis,
1
1
IR, H, 31P{ H} NMR and ESI-MS. The IR spectra of the free
imines show an intense sharp band in the range 1617–1639 cm−1.
For complexes 2c–2g this band shifted to 1559–1609 cm−1,
† The HTML version of this article has been enhanced with colour images.
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N-phenyl ring also showed two close singlets in the 31P NMR
spectra.11 These results suggested the possibility of another kind
of isomerism which is caused by the trans or cis-disposition of the
coordinated PCy3 to the imino nitrogen.15
The crystal structures of complexes 2c–2g were determined to
confirm the details although they could be easily characterized
by means of spectral and elemental analysis. All the crystals
were obtained by recrystallization from CH2Cl2–petroleum ether
solution at room temperature. The single-crystal X-ray analyses
indicates that the palladacycles 2c–2f are trans complexes in the
solid state. For 2g·CH2Cl2 it adopts an anti, trans-geometry in the
solid state. The molecules are shown in Fig. 2–7 (displacement
ellipsoids are drawn at the 30% probability level). Selected bond
◦
˚
lengths (A) and angles ( ) are listed in Table 1.
The Pd atom in each complex is in a slightly distorted square-
planar environment bonded to the phosphorus atom of the PCy3
ligand, the chloride atom, the nitrogen atom and the C6 atom
of the ferrocenyl moiety. The deviations of the Pd atom from
Scheme 1
˚
indicating that the nitrogen atom was coordinated to palladium
through its lone pair. The H NMR spectra of complexes 2c–2f
the plane are 0.1918, 0.1322, 0.0766, 0.0707 and 0.0228 A for
1
complexes 2c–2g·CH2Cl2, respectively. The bicyclic system formed
by the palladacycle and the C5H3 moiety is approximately coplanar
(dihedral angles of 8.7, 7.9, 9.6, 10.3 and 8.6◦ for complexes
2c–2g·CH2Cl2, respectively). In each structure, the two Cp rings
are almost parallel with dihedral angles of 3.4, 1.9, 2.9, 2.4
and 2.4◦, respectively, for 2c–2g·CH2Cl2. The Pd–N (2.150(3)–
were consistent with the proposed structures and the appearance
of only one signal around 46 ppm in 31P NMR spectra suggested
the formation of a single isomer. In contrast to those of 2c–2f,
1
the H NMR spectrum of 2g indicated that it was a mixture of
isomers in solution since the signals in N-phenyl and Cp ring
region appeared in pairs. This type of isomerism might be caused
by the steric hindrance between the ortho-substituted chlorine in
the N-phenyl ring and a-methyl group as well as PCy3 group or
chlorine atom bonded to the palladium.13,14 The free rotation of
the N-phenyl ring about the C–N bond was thus blocked to result
in an anti arrangement of the ortho-substituent and ferrocenyl
moiety and a syn arrangement (Fig. 1). In addition, the 31P NMR
spectrum of 2g was different from those of 2c–2f by the appearance
of two singlets at d 46.72 and 47.45 ppm, respectively. Previously
we found complexes 2a–2b without an ortho-substituent in the
˚
˚
2.183(4) A) and Pd–P (2.2685(10)–2.2838(15) A) bond lengths
of five complexes are similar to those of complexes 2b11 and 316
˚
˚
(2.170(5), 2.173(4) A and 2.2833(16), 2.2815(13) A), while they
are longer than those of the related Ph3P-containing complexes13,17
(2.135(3), 2.130(6), 2.114(4) A and 2.2569(9), 2.247(2), 2.2356(13)
˚
˚
A) possibly due to the steric bulk of the PCy3 ligand.
Fig. 2 shows that complex 2c exists as a dimer in the crystal
due to intermolecular hydrogen bonds between chlorine atom and
the adjacent C–H group of PCy3 (Cl1 · · · H33A = Cl1A · · · H33 =
˚
2.75 A). This is different from that of 3 which has an additional
Fig. 1 Possible isomers of complex 2g.
Table 1 Selected bond lengths (A) and angles ( ) for 2c–2g·CH2Cl2
◦
˚
Compound
2c
2d·CH2Cl2
2e·CH2Cl2
2f·CH2Cl2
2g·CH2Cl2
Pd(1)–C(6)
Pd(1)–N(1)
Pd(1)–P(1)
Pd(1)–Cl(1)
N(1)–C(11)
1.998(3)
2.150(3)
2.2685(10)
2.3868(9)
1.274(4)
1.999(4)
2.151(4)
2.2692(14)
2.3859(15)
1.288(6)
2.015(5)
2.183(4)
2.2838(15)
2.3948(13)
1.284(6)
2.014(4)
2.172(3)
2.2813(14)
2.3987(11)
1.288(5)
2.004(7)
2.162(5)
2.2797(17)
2.4037(18)
1.297(8)
C(6)–Pd(1)–N(1)
C(6)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
C(6)–Pd(1)–Cl(1)
N(1)–Pd(1)–Cl(1)
P(1)–Pd(1)–Cl(1)
C(11)–N(1)–Pd(1)
80.87(11)
96.58(9)
164.54(7)
167.00(8)
91.38(7)
93.62(3)
112.9(2)
80.81(16)
95.99(13)
170.75(11)
166.73(13)
90.26(11)
94.29(6)
80.49(18)
94.86(15)
171.59(12)
170.86(15)
90.78(11)
94.14(6)
80.35(15)
95.38(12)
172.04(9)
170.74(12)
90.63(9)
93.83(5)
113.9(3)
79.7(2)
96.34(19)
174.37(15)
168.80(19)
89.45(16)
94.26(7)
114.1(3)
113.2(4)
114.4(4)
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structure of complex 2e·CH2Cl2 (Fig. 5). The molecular structure
of 2f·CH2Cl2 is shown in Fig. 6. Basically, the crystal structure of
2f·CH2Cl2 is quite similar to that of 2e·CH2Cl2 by the existence of
similar types of hydrogen bonds in the crystal. The hydrogen bond
˚
lengths in 2f·CH2Cl2 are 2.82 A [(Cp)CH · · · Cl(benzene ring)] and
˚
2.78 A [(benzene ring)CH · · · Cl], respectively.
Complex 2g·CH2Cl2 like 2c exists as a dimer. In this dimeric
structure, there are two types of intermolecular hydrogen bonds
(Fig. 7). One hydrogen bond is between the chlorine atom
and the adjacent C–H group of benzene ring (Cl1 · · · H18A =
˚
Cl1A · · · H18 = 2.87 A), while the chlorine atom in benzene ring
does not participate in hydrogen bonding. The other hydrogen
˚
bond is C–H · · · M type (Pd1–H17A = Pd1A–H17 = 2.83 A). The
Pd–H bond length in complex 2g·CH2Cl2 is longer than those
in the complexes containing a single phosphine ligand (1.61–
18
˚
2.73 A), while it is shorter than those in the other complexes
reported in the literature (3.10 and 3.20 A).19 In addition, the Pd–
˚
˚
Pd distance (12.101 A) in complex 2g·CH2Cl2 is longer than that
˚
in complex 2c (10.161 A).
Catalytic studies
Suzuki coupling reaction. We have previously shown that
complexes 2a and 2b are efficient catalysts for the coupling of
a range of aryl chlorides with phenylboronic acid. In a typical
experiment, aryl chloride, phenylboronic acid and Cs2CO3 were
used in a ratio of 1 : 1.5 : 2.11 In the present study, the ratio of aryl
chloride, phenylboronic acid and base was 1 : 1.2 : 1.5 and all the
novel palladacycles 2c–2g still showed high activity in the Suzuki
reaction of aryl chlorides (Scheme 2).
Fig. 2 The dimeric structure of complex 2c showing the hydrogen
bonds. Non-hydrogen bonding H atoms are omitted for clarity. Symmetry
transformations used to generate equivalent atoms: A, x + 1, y + 1, z.
CHO group in the unpalladated Cp ring. In complex 3, chlorine
atom not only forms hydrogen bond with the adjacent C–H group
of PCy3, but also forms hydrogen bond with the adjacent C–H
group of the Cp ring.16 In the crystal of 2d·CH2Cl2 there also exist
CH · · · Cl hydrogen bonds (Cl1A · · · H3 = Cl1B · · · H3A = 2.74
˚
A). In this case, the hydrogen bonds between the chlorine atom and
Scheme 2
the adjacent C–H group of Cp ring instead of PCy3 construct the
one-dimensional chain structure of complex 2d·CH2Cl2 (Fig. 3).
Like 2d·CH2Cl2, 2e·CH2Cl2 also has a one-dimensional chain
structure formed by CH · · · Br (H atom in Cp ring, Br1 · · · H9A =
Our initial exploration of reaction conditions focused on
the coupling of phenylboronic acid with chlorobenzene. After
screening a variety of bases (e.g., Cs2CO3, KOH, NaOH, K2CO3,
t
˚
Br1A · · · H9B = 2.91 A) hydrogen bonds (Fig. 4). In addition,
Na2CO3, CsF, KF·2H2O, KOAc, NaH, K3PO4, and BuONa),
hydrogen bonds between chlorine atom and the adjacent C–H
group of benzene ring are present in the crystal (the distance is
Cs2CO3 was found to give the best result (some of data are shown
in Table 2, entries 1–6). Then, a quick survey of solvents indicated
that dioxane was much better than toluene and DMF (entries
˚
2.77 A), which are attributed to construct the 2D lamellar
Fig. 3 One-dimensional chain structure of complex 2d·CH2Cl2 showing the hydrogen bonds. CH2Cl2 and non-hydrogen bonding H atoms are omitted
for clarity. Symmetry transformations used to generate equivalent atoms: A, x + 1, y, z.
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Fig. 4 One-dimensional chain structure of complex 2e·CH2Cl2 showing the hydrogen bonds. CH2Cl2 and non-hydrogen bonding H atoms are omitted
for clarity. Symmetry transformations used to generate equivalent atoms: A, x − 1, y, z.
Fig. 5 2-D lamellar structure of complex 2e·CH2Cl2 formed by hydrogen bonds. CH2Cl2 and non-hydrogen bonding H atoms are omitted for clarity.
Symmetry transformations used to generate equivalent atoms: A, x − 1, y, z; B, x + 2, y + 1, z + 1.
1, 7 and 8). Other solvents such as THF (at 70 ◦C) and
polar protic solvents cyclohexanol, 1-butanol as well as isopropyl
alcohol also afforded very low coupled product yields (29.9,
27.2, 15.3 and 10.8%, respectively. data not shown). With the
appropriate solvent (dioxane) and base (Cs2CO3) in hand, the
relative activities of several palladacycles 2c–2g and 3 for the
same model reaction were then studied. All the complexes
exhibited high activity with catalyst loadings as low as 0.1 mol%.
Among them, complex 3 which had an additional CHO group
in the unpalladated Cp ring in comparison with 2c showed the
lowest activity (81.8% yield, entry 9). The N-aryl-substituted
complexes 2d–2g displayed comparable activity producing the
coupled products in yields around 90% (entries 11–14) and
the N-alkyl-substituted complex 2c was slightly more active
than 2d–2g (97.4% yield, entry 10). The possible reason for the
result was that alkyl substituents at nitrogen promoted oxidative
addition of aryl chloride to the Pd intermediate by making
palladium more electron rich and increased the reaction rate
consequently. The dimeric complex 1c was almost inactive under
Fig. 6 Molecular structure of complex 2f·CH2Cl2. CH2Cl2 and hydrogen
atoms are omitted for clarity.
the same reaction conditions (entry 15) and the yield was greatly
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Fig. 7 The dimeric structure of complex 2g·CH2Cl2 showing the hydrogen bonds. CH2Cl2 and non-hydrogen bonding H atoms are omitted for clarity.
Symmetry transformations used to generate equivalent atoms: A, x + 1, y + 2, z + 1.
Table 2 Influence of base, solvent and catalyst on the Suzuki coupling of
(Table 3). For example, ortho-substituents were tolerated and even
the very sterically hindered 2-chloro-m-xylene provided the biaryl
product in 90.1% isolated yield (entries 2–3, 5). 2-chloropyridine
was found to be an efficient coupling partner in this system giving
96.8% isolated yield (entry 8). However, for 2-chlorothiophene
only 22.5% isolated yield was obtained under the same reaction
conditions (entry 10).
phenyl chloride with phenyl boronic acida
Entry
Catalyst (mol%)
Base
Solvent
t/h Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
3 (0.1)
Cs2CO3
KOH
K2CO3
CsF
Dioxane 10
Dioxane 10
Dioxane 10
Dioxane 10
Dioxane 10
Dioxane 10
84.8
55.9
48.1
43.9
36.2
36.1
53.3
34.2
81.8
97.4
91.2
90.9
87.0
89.6
trace
46.8
Heck coupling reaction. The high activity of palladacycles 2c–
2g in the Suzuki coupling of aryl chlorides encouraged us to
examine their activity toward the Heck reaction (Scheme 3).
NaH
K3PO4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Toluene
DMF
10
10
Dioxane 12
Dioxane 12
Dioxane 12
Dioxane 12
Dioxane 12
Dioxane 12
Dioxane 12
Dioxane 12
2c (0.1)
2d (0.1)
2e (0.1)
2f (0.1)
2g (0.1)
1c (0.05)
Scheme 3
Initially, the Heck reaction of 4-bromoacetophenone with
acrylic acid ethyl ester was carried out with various bases and
solvents in the presence of 0.1 mol% of 2c. The results from
this study are summarized in Table 4. Et3N was found to be
the most effective base (90.8% yield, entry 1) and CsF as well
as KF showed comparable results (entries 2 and 3). KOAc
and K3PO4·3H2O displayed moderate efficiency, producing the
coupled products in 80.7 and 72.5% yield, respectively (entries 4
and 5). Interestingly, Cs2CO3 proved to be ineffective in the present
system (entry 8) although it gave the best result in the above-
mentioned Suzuki reaction. Among the tested solvents, DMF was
the most productive and non-polar solvents toluene as well as
xylene also gave high yields (entries 1, 13 and 14). The relative
catalytic activity of 2c–2g and 3 were examined in the presence
of Et3N as base in DMF. All the tested palladacycles showed
good activity for this specific reaction giving the coupled product
in 75–91% yields. Similarly to the results of the Suzuki reaction,
c
1c/PCy3
a Reaction conditions: PhCl (0.5 mmol), PhB(OH)2 (0.6 mmol), base
(0.75 mmol), solvent (3 mL), 100 ◦C. b Isolated yields (average of two
experiments). c 0.1 mol% 1c, 0.3 mol% PCy3.
improved by the addition of PCy3 (46.8%, entry 16) suggesting
that PCy3 participated in the catalytic cycles. However, the activity
of 1c/PCy3 system was still obviously lower than that of the
preformed PCy3-cyclopalladated ferrocenylimine complex 2c.
A variety of electronically and structurally diverse aryl chlorides
including 3-chloropyridine could be coupled efficiently with
phenylboronic acid by using 0.1 mol% of complex 2c under
the optimized reaction conditions (Cs2CO3, dioxane, 100 ◦C)
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Table 3 Suzuki coupling of aryl chlorides with phenyl boronic acid
catalyzed by 2ca
(entries 6–9). For activated aryl bromides, it was not surprising
that excellent yields were obtained (entries 10 and 11). In contrast
to their high activity in the Suzuki coupling of aryl chlorides, this
series of palladacycles showed little activity for the coupling of aryl
chlorides with acrylic acid ethyl ester (entries 12–14). In the case of
chlorobenzene, 2c was almost inactive under the similar reaction
conditions (entry 12). Increasing catalyst loading to 1 mol% only
gave 13.6% yield (entry 13). For activated chlorides such as 3-
chloronitrobenzene, the yield of the coupled product could be
reached 47.1% by using 1 mol% of 2c (entry 15). Furthermore,
the addition of NBu4Br cocatalyst, which was reported to be
able to increase the activity of most catalytic systems,12b,d,20 did
not give better results for the present catalytic system. Finally, it
was noteworthy that all of these reactions studied showed high
regioselectivity for trans-coupling and cis-product was not found.
Entry
1
Aryl halide
Product
Yieldb (%)
94.0
2
3
92.4
90.1
4
5
90.3
74.1
Conclusions
A series of novel PCy3-cyclopalladated ferrocenylimine complexes
2c–2g have been easily synthesized from the reaction of chloride-
bridged palladacyclic dimers with PCy3. These complexes are
thermally stable and are not sensitive to air and moisture. Single-
crystal X-ray analysis confirms that there are altogether six
types of intermolecular hydrogen bonds in the crystals of 2c–
2g·CH2Cl2. Although these complexes are very efficient catalysts
for the Suzuki coupling of aryl chlorides with phenylboronic acid,
they are not particularly useful for the Heck reaction of aryl
chlorides. Applications involving this type of complexes in other
palladium-catalyzed reactions are currently under investigation in
our laboratory.
6
98.5
7
98.9
96.8
84.3
22.5
8
9
Experimental
10
General
All reactions were carried out under a dry nitrogen atmosphere
using standard Schlenk techniques. Solvents were dried and freshly
distilled prior to use. All other chemicals were used as purchased.
Melting points were measured using a WC-1 microscopic appara-
tus and were uncorrected. Elemental analysis were determined
with a Carlo Erba 1160 Elemental Analyzer. IR spectra were
collected on a Bruker VECTOR22 spectrophotometer in KBr
a Reaction conditions: catalyst 2c (0.1 mol%), aryl chloride (0.5 mmol),
PhB(OH)2 (0.6 mmol), Cs2CO3 (0.75 mmol), dioxane (3 mL), 100 ◦C, 12 h
(time not optimized). b Isolated yields (average of two experiments).
the N-aryl-substituted complexes 2d–2g also showed comparable
activity and the N-alkyl-substituted complex 2c was the most
efficient among these palladacycles in Heck reaction (entries 1
and 15–19). In contrast, complex 3 displayed the activity between
that of 2c and 2d–2g. Additionally, the dimeric complex 1c
generated the coupled product in low yield under the same reaction
conditions (53.2%, entry 20) and the yield was improved to 66.1%
by the addition of PCy3 (entry 21).
1
1
pellets. H, 31P{ H} NMR spectra were recorded on a Bruker
DPX-400 spectrometer in CDCl3 with TMS as an internal
1
standard for H NMR and 85% H3PO4 as external standard for
1
31P{ H} NMR. Mass spectra were measured on a LC-MSD-Trap-
XCT instrument.
Synthesis
The Heck coupling of a variety of aryl bromides with acrylic acid
ethyl ester catalyzed by complex 2c were carried out under the op-
timized reaction conditions (Et3N, DMF, 140 ◦C). The results are
shown in Table 5. The coupled products were isolated in moderate
yields for non-activated and deactivated aryl bromide substrates
with this system (entries 1 and 3–5). Prolonging the reaction time
from 16 to 24 h resulted in good yield (entry 2). The catalyst
system was very effective for the coupling of bromopyridines and
bromonaphthalenes giving the coupled products in 84–97% yields
General method for the synthesis of the tricyclohexylphosphine-
cyclopalladated ferrocenylimine complexes 2c–2g. A solution of
chloride-bridged palladacyclic dimer 1c–1g (0.1 mmol) and PCy3
(0.22 mmol) in dichloromethane (10 mL) was stirred at room
temperature for 30 min. The product was separated by passing
through a short silica gel column with CH2Cl2 as eluent. The
first band was collected and afforded complex 2c–2g after the
evaporation of the solvent.
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Table 4 Influence of base, solvent and catalyst on the Heck coupling of 4-bromoacetophenone with acrylic acid ethyl estera
Entry
Catalyst (mol%)
Base
T/◦C
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21c
2c (0.1)
2c (0.1)
2c(0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
2c (0.1)
3 (0.1)
2d (0.1)
2e (0.1)
2f (0.1)
2g (0.1)
1c (0.05)
1c/PCy3
Et3N
CsF
KF
KOAc
K3PO4·3H2O
Na2CO3
K2CO3
Cs2CO3
NaH
140
140
140
140
140
140
140
140
140
140
140
110
110
140
140
140
140
140
140
140
140
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Dioxane
Toluene
Xylene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
90.8
89.9
87.2
80.7
72.5
55.1
52.3
23.9
34.2
18.3
20.2
75.2
88.9
89.9
85.3
75.2
81.6
80.7
79.8
53.2
66.1
KtOBu
NatOBu
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
a Reaction conditions: 4-bromoacetophenone (0.5 mmol), acrylic acid ethyl ester (0.75 mmol), base (0.75 mmol), solvent (3 mL), 11 h. b Isolated yields
(average of two experiments). c 0.05 mol% 1c, 0.3 mol% PCy3.
5
5
1
=
[PdCl{[(g -C5H5)]Fe[(g -C5H3)CH NCy]}(PCy3)], 2c. Red
solid, yield 80.4%. Found: C, 58.56; H, 7.61; N 1.87. Calc.
for C35H53ClFeNPPd: C, 58.67; H, 7.46; N, 1.95%. Mp: 196–
(m, 15H, PCy3), 1.27 (m, 9H, PCy3). 31P{ H} NMR (162 MHz,
CDCl3): d 46.89. MS-ESI+: m/z 766.3 (M+ − Cl).
◦
−1
=
197 C. IR (KBr, cm ): 2928, 2852, 1609 (C N), 1445, 1362,
1224, 1107, 1002. 1H NMR (400 MHz, CDCl3): d 8.01 (br s, 1H,
–CH N), 4.52 (s, 1H, C5H3), 4.39 (br s, 2H, C5H3), 4.25 (m,
5
5
=
[PdCl{[(g -C5H5)]Fe[(g -C5H3)C(CH3) NC6H4-4-Cl]}(PCy3)],
2f. Red solid, yield 76.3%. Found: C, 52.40; H, 6.09; N 1.54.
Calc. for C36H48Cl2FeNPPd·CH2Cl2: C, 52.66; H, 5.97; N, 1.66%.
=
overlapped with C5H5, 1H, NCHC5H10), 4.20 (s, 5H, C5H5), 2.54
(m, 3H, P(CHC5H10)3), 2.34 (m, 1H, NCy), 2.02 (m, 7H, 6 PCy3
+ 1 NCy), 1.78 (m, 17H, 15 PCy3 + 2 NCy), 1.28 (m, 15H, 9 PCy3
◦
−1
=
Mp: 257 C (decomp.). IR (KBr, cm ): 2928, 2851, 1579 (C N),
1
1446, 1364, 1224, 1086, 1006. H NMR (400 MHz, CDCl3): d
+ 6 NCy). 31P{ H} NMR (162 MHz, CDCl3): d 45.33. MS-ESI+:
1
7.31 (d, J = 8.2 Hz, 2H, C6H4), 6.89 (d, J = 8.2 Hz, 2H, C6H4),
4.54 (s, 1H, C5H3), 4.45 (s, 1H, C5H3), 4.42 (s, 1H, C5H3), 4.22 (s,
5H, C5H5), 2.51 (m, 3H, P(CHC5H10)3), 2.06 (m, 6H, PCy3), 2.03
m/z 680.3 (M+ − Cl).
5
5
1
(s, 3H, CH3), 1.72 (m, 15H, PCy3), 1.28 (m, 9H, PCy3). 31P{ H}
=
[PdCl{[(g -C5H5)]Fe[(g -C5H3)C(CH3) NC6H4-4-OCH2CH3]}-
(PCy3)], 2d. Red solid, yield 78.2%. Found: C, 54.68; H, 6.54;
N 1.51. Calc. for C38H53ClFeNOPPd·CH2Cl2: C, 54.88; H, 6.50;
NMR (162 MHz, CDCl3): d 47.56. MS-ESI+: m/z 722.3 (M+ −
Cl).
◦
N, 1.64%. Mp: 262 C (decomp.). IR (KBr, cm−1): 2928, 2850,
1
=
1583 (C N), 1506, 1468, 1447, 1238, 1106, 1050, 1003. H NMR
5
5
=
(400 MHz, CDCl3): d 6.85 (m, 4H, C6H4), 4.58 (s, 1H, C5H3), 4.47
(br s, 2H, C5H3), 4.28 (s, 5H, C5H5), 4.01 (m, 2H, –OCH2CH3),
2.51 (m, 3H, P(CHC5H10)3), 2.04 (m, 6H, PCy3), 2.00 (s, 3H,
CH3), 1.71 (m, 15H, PCy3), 1.40 (t, J = 6.8 Hz, 3H, –OCH2CH3),
[PdCl{[(g -C5H5)]Fe[(g -C5H3)C(CH3) NC6H4-2-Cl]}(PCy3)],
2g. Red solid, yield 72.6%. Found: C, 52.32; H, 5.98; N 1.54.
Calc. for C36H48Cl2FeNPPd·CH2Cl2: C, 52.66; H, 5.97; N, 1.66%.
◦
−1
=
Mp: 259 C (decomp.). IR (KBr, cm ): 2928, 2851, 1579 (C N),
1.27 (m, 9H, PCy3). 31P{ H} NMR (162 MHz, CDCl3): d 46.58.
1
1
1459, 1321, 1226, 1107, 1003. H NMR (400 MHz, CDCl3): the
MS-ESI+: m/z 732.3 (M+ − Cl).
ratio of two isomers is 1.14 : 1. d 7.42 (d, J = 7.8 Hz, 1H, C6H4),
7.39 (d, J = 7.8 Hz, 1H, C6H4), 7.30 (t, J = 7.5 Hz, 1H, C6H4),
7.24 (t, J = 7.5 Hz, 1H, C6H4), 7.12 (t, J = 7.6 Hz, 2H, C6H4),
7.03 (d, J = 7.6 Hz, 1H, C6H4), 6.89 (d, J = 7.6 Hz, 1H, C6H4),
4.56 (s, 2H, C5H3), 4.49 (s, 1H, C5H3), 4.43 (m, 2H, C5H3), 4.39
(s, 1H, C5H3), 4.32 (s, 5H, C5H5), 4.25 (s, 5H, C5H5), 2.49 (m, 6H,
P(CHC5H10)3), 2.06 (m, 12H, PCy3), 2.04 (s, 3H, CH3), 2.01 (s,
5
5
=
[PdCl{[(g -C5H5)]Fe[(g -C5H3)C(CH3) NC6H4-4-Br]}(PCy3)],
2e. Red solid, yield 75.2%. Found: C, 50.26; H, 5.63; N 1.48.
Calc. for C36H48BrClFeNPPd·CH2Cl2: C, 50.03; H, 5.67; N,
1.58%. Mp: 282 ◦C (decomp.). IR (KBr, cm−1): 2925, 2852, 1559
(C N), 1448, 1367, 1276, 1106, 1004. 1H NMR (400 MHz,
CDCl3): d 7.44 (d, J = 7.7 Hz, 2H, C6H4), 6.78 (d, J = 8.2 Hz,
2H, C6H4), 4.70 (m, 3H, C5H3), 4.38 (s, 5H, C5H5), 2.49 (m,
3H, P(CHC5H10)3), 1.99 (m, 6H, PCy3), 1.96 (s, 3H, CH3), 1.68
=
3H, CH3), 1.72 (m, 30H, PCy3), 1.25 (m, 18H, PCy3). 31P{ H}
1
NMR (162 MHz, CDCl3): d 47.45, 46.72. MS-ESI+: m/z 722.3
(M+ − Cl).
4736 | Dalton Trans., 2006, 4730–4739
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Table 5 Heck coupling of aryl halide with acrylic acid ethyl ester catalysed by 2ca
Entry
1
Aryl halide
Catalyst (mol%)
0.1
t/h
Product
Yieldb (%)
16
63.6
2
3
4
0.1
0.1
0.1
24
16
16
82.9
58.9
53.7
5
6
0.1
0.1
16
16
53.8
87.0
7
8
0.1
0.1
16
16
97.2
95.6
9
0.1
16
83.5
10
11
12c
0.1
0.1
0.1
16
24
24
97.7
99.5
trace
13c
14c
1
24
24
13.6
13.5
0.1
15c
1
24
47.1
a Reaction conditions: aryl halide (0.5 mmol), acrylic acid ethyl ester (0.75 mmol), Et3N (0.75 mmol), DMF (3 mL), 140 ◦C. b Isolated yields (average of
two experiments). c Base (1.0 mmol).
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Table 6 Crystallographic data and structure refinement for complexes 2c–2g·CH2Cl2
Compound
2c
2d·CH2Cl2
2e·CH2Cl2
2f·CH2Cl2
2g·CH2Cl2
Elemental formula
Mr
C35H53ClFeNPPd
716.45
Monoclinic
C39H55Cl3FeNOPPd
853.41
Triclinic
¯
P1
9.927(2)
12.477(3)
16.594(3)
103.00(3)
90.52(3)
94.15(3)
1996.7(7)
1.419
C37H50BrCl3FeNPPd
888.26
Triclinic
¯
P1
10.118(2)
11.126(2)
18.050(4)
102.98(3)
93.17(3)
105.25(3)
1896.0(7)
1.556
C37H50Cl4FeNPPd
843.80
Triclinic
¯
P1
10.115(2)
11.148(2)
17.938(4)
102.77(3)
92.87(3)
105.73(3)
1885.9(6)
1.486
C37H50Cl4FeNPPd
843.80
Triclinic
¯
P1
10.319(2)
11.270(2)
18.101(4)
78.72(3)
74.37(3)
68.02(3)
1869.2(7)
1.499
Crystal system
Space group
P21/n
˚
a/A
10.668(2)
14.320(3)
22.676(5)
90
102.72(3)
90
3379.1(12)
1.621
4
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Dc/g cm−3
Z
2
2
2
2
Crystal size/mm
F(000)
0.20 × 0.18 × 0.17
0.20 × 0.18 × 0.18
0.20 × 0.17 × 0.17
0.20 × 0.17 × 0.17
0.20 × 0.18 × 0.17
1496
884
904
868
868
l/mm−1
1.112
1.067
5461/0/361
0.0317, 0.0663
1.085
1.037
6145/0/425
0.0447, 0.1181
2.191
1.035
5862/0/407
0.0428, 0.1125
1.214
1.020
5774/0/407
0.0405, 0.0708
1.225
1.036
5754/0/407
0.0579, 0.1297
GOF (on F2)
Data/restraints/parameters
R1, wR2 [I > 2r(I)]a
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢀFo| − |Fcꢀ/ |Fo|, wR2 = [ (Fo − Fc2)2/ w(Fo2)2]1/2
.
2
Suzuki reaction. A Schlenk tube was charged with the appro-
priate aryl chloride (0.5 mmol), phenyl boronic acid (0.6 mmol)
and base (0.75 mmol) under nitrogen. The catalyst was introduced
as a dioxane solution (0.0005 mmol mL−1) via syringe, and
additional dioxane was added to obtain a total volume of
3 mL. The reaction mixture was then placed in an oil bath and
the structures were solved using the SHELX-97 program.22 Details
of crystal structure determination of complexes 2c–2g·CH2Cl2 are
summarized in Table 6.
CCDC reference numbers 284580, 600775, 600773, 602222 and
602221 for complexes 2c–2g·CH2Cl2, respectively.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b608825b
◦
heated at 100 C for 12 h, cooled and quenched with water. The
organic layer was separated and the aqueous layer was extracted
with dichloromethane, then the combined organic layers were
washed with water, dried over MgSO4, filtered, and the solvent was
removed on a rotary evaporator, and the products were isolated
by flash chromatography on silica gel (the purified products were
identified by comparison of melting points with the literature
values21 or by 1 H NMR spectra).
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Project 20472074) and the Innovation Fund for Outstand-
ing Scholar of Henan Province (Project 0621001100) for financial
support of this work.
Heck reaction. A Schlenk tube was charged with the appropri-
ate aryl halide (0.5 mmol), acrylic acid ethyl ester (0.75 mmol) and
base (0.75 or 1 mmol) under nitrogen. The catalyst was introduced
as a DMF solution (0.0005 mmol mL−1) via syringe, and additional
DMF was added to obtain a total volume of 3 mL. The re◦action
mixture was then placed in an oil bath and heated at 140 C for
16 h (24 h). The work-up employed was the same as described
above for the Suzuki reaction.
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