Cross-Coupling Catalysis by an Anionic Palladium Complex

Article abstract of DOI:10.1021/acscatal.6b03655

Recent studies have shown that anionic palladium complexes are viable catalysts for a range of catalytic cross-coupling reactions. We present a one-step synthesis of the anionic "ligandless" palladium complex [NBu₄][Pd(DMSO)Cl₃] together with its crystal structure. This compound has been shown to be an active precatalyst in the Mizoroki-Heck reaction. Under Jeffery conditions, activated aryl chlorides can be coupled in yields of up to 94% without the need of an additional ligand. The presence of a small amount of water was necessary for product formation. An Amatore-Jutand-type catalytic cycle is consistent with the results presented herein. For comparison with the known mixed complex [(pym-Im-Me)₂PdCl][Pd(DMSO)Cl₃], the cationic complex [(pym-Im-Me)₂PdCl]PF₆ (pym = pyrimidyl, Im = imidazolin-2-ylidene) has been synthesized and characterized using standard techniques.

Full text of DOI:10.1021/acscatal.6b03655

Letter
pubs.acs.org/acscatalysis
Cross-Coupling Catalysis by an Anionic Palladium Complex
Felix Schroeter, Johannes Soellner, and Thomas Strassner*
Physikalische Organische Chemie, TU Dresden, Bergstrasse 66, 01062 Dresden, Germany
*
S Supporting Information
ABSTRACT: Recent studies have shown that anionic
palladium complexes are viable catalysts for a range of catalytic
cross-coupling reactions. We present a one-step synthesis of
the anionic “ligandless” palladium complex [NBu ][Pd-
4
(
DMSO)Cl ] together with its crystal structure. This
3
compound has been shown to be an active precatalyst in the
Mizoroki−Heck reaction. Under Jeffery conditions, activated
aryl chlorides can be coupled in yields of up to 94% without
the need of an additional ligand. The presence of a small
amount of water was necessary for product formation. An
Amatore−Jutand-type catalytic cycle is consistent with the
results presented herein. For comparison with the known mixed complex [(pym-Im-Me) PdCl][Pd(DMSO)Cl ], the cationic
2
3
complex [(pym-Im-Me) PdCl]PF (pym = pyrimidyl, Im = imidazolin-2-ylidene) has been synthesized and characterized using
2
6
standard techniques.
KEYWORDS: palladium, Mizoroki−Heck reaction, C−C coupling, NHC, anionic complex
1
. INTRODUCTION
Of the many palladium-catalyzed reactions developed in the
past 50 years, catalytic carbon−carbon cross-couplings are
among the most important and thoroughly studied trans-
1
formations. Consequently, the 2010 Nobel Prize in chemistry
was awarded jointly to Richard F. Heck, Ei-ichi Negishi, and
2
Akira Suzuki. Particularly interesting, in terms of cost and
3
atom efficiency, is the concept of “ligandless” catalytic systems,
e.g., the absence of stabilizing phosphine or N-heterocyclic
4
carbene (NHC) ligands, or the use of nanoparticles in ionic
5
liquids.
Although anionic species are frequently proposed to
6
participate in cross-coupling catalysis, the direct use of anionic
palladium complexes remains scarce and is mostly limited to
compounds that contain imidazolium cations. However, these
7
Figure 1. Selected palladium precatalysts for the Mizoroki−Heck and
related reactions.
cannot be considered as truly ligandless systems, since, under
the basic reaction conditions, the formation of NHC complexes
7
a
is possible. On the other hand, several neutral coordination
8
at the NHC ligands (e.g., those substituted by a methyl group),
two ligands are bound at a cationic palladium center,
compounds of palladium salts and DMSO are known, and
these compounds show an interesting catalytic behavior in
oxidation and cycloisomerization reactions, as demonstrated by
accompanied by a [Pd(DMSO)Cl ] counterion (1, Figure 1).
3
9
In the Mizoroki−Heck reaction, complex 1 showed superior
activity compared to the other neutral complexes. We reasoned
that the higher catalytic activity might result from the anionic
palladium species. Indeed, recent examples of catalytically active
anionic palladium species were reported by Navarro for the
Stahl and others. The trichlorido dimethylsulfoxido
palladate(II) ([Pd(DMSO)Cl ]) anion combines these types
3
of catalysts. It has been described in a limited number of
1
0
11
complexes a2nd salts, yet only few accounts on its catalytic
1
activity exist.
16
Mizoroki−Heck reaction and by Trzeciak for the related
Originating from the use of (C^C)-chelating biscarbene
17
1
3
14
oxidative Heck reaction (Figure 1).
palladium complexes in the Mizoroki−Heck reaction, we
investigated a related series of (C^N)-chelating NHC
palladium complexes (Figure 1).
10b
Several other palladium-
Received: December 23, 2016
Revised: March 13, 2017
containing cations of the latter type comprising the pyrimidyl
15
moiety were reported. We found that, for small substituents
©
XXXX American Chemical Society
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DOI: 10.1021/acscatal.6b03655
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Letter
2
1c
To investigate the origin of the catalytic activity of 1, we
decided to separately synthesize the palladium-containing
species in complex 1 to identify the palladium species
responsible for the superior catalytic activity. During the course
of the investigation, we found that water plays an important
role in the Mizoroki−Heck reaction. Finally, the scope of this
reaction using different aryl bromides and chlorides has been
explored.
leads to a decrease. The characteristic S−O vibration mode
−1
in liquid DMSO has a wavenumber of 1019 cm , while this
−1
value increases to 1123 cm for complexes 1, 5, and 7 and
1119 cm− for complex 6, respectively. In contrast, no S−O
vibration mode with a lower wavenumber, compared to free
DMSO, has been observed. These results indicate that, in
complexes 1 and 5−7, the DMSO ligand is bound to the
palladium center via the S atom.
1
Solid-state structures of compounds 4 and 5 have been
obtained via slow diffusion of diethyl ether into a saturated
solution of the corresponding complex in DCM (see Figures 2a
2
. RESULTS AND DISCUSSION
Synthesis and Analytical Data. The synthesis of complex
from the imidazolium salt 2 and dichloro(1,5-
1
cyclooctadiene)palladium(II) (Pd(cod)Cl ) has been described
2
1
0b
previously. Complex 4 was synthesized following a modified
literature procedure by Chen (Scheme 1, i−iii). In the first
15c
a
Scheme 1. Synthesis of Compounds 3−7
Figure 2. ORTEP representations of the synthesized compounds.
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn
at the 50% probability level. (a) Compound 4 (excluding one molecule
of co-crystallized DCM and the disordered hexafluorophosphate
counterion). [Selected bond lengths (Å), angles (deg), and torsions
(
deg): Pd1−Cl1, 2.3350(8); Pd1−C1, 1.969(3); Pd1−C9, 1.976(3);
Pd1−N3, 2.094(2); C1−Pd1−C9, 97.73(11); C1−Pd1−C9−N6, 73.9
3).] (b) Compound 5. [Selected bond lengths (Å), angles (deg) and
(
torsions (deg): Pd1−Cl1, 2.295(2); Pd1−Cl2, 2.298(2); Pd1−S1,
2.230(2); S1−O1, 1.460(5); Cl1−Pd1−S1−O1, 51.6(2).]
a
Conditions: (i) KPF , H O/MeOH, rt, 2 d, 94%; (ii) Ag O, MeCN,
6
2
2
rt, overnight; (iii) Pd(cod)Cl , MeCN, rt, overnight, 70% (over two
2
steps); (iv) NBu X, DMSO, 60 °C, 15 min, 5: 95%, 6: 93%; and (v)
4
NaCl, DMSO, DCM, reflux, 5 d, 94%.
and 2b). Selected bond lengths, angles, and torsions are given
in the caption of Figure 2. In both cases, palladium adopts a
slightly distorted square planar geometry. In compound 4, the
Pd−C1 and the Pd−C9 distances are similar, having values of
1.969(3) Å and 1.976(3) Å, respectively. This shows that the
nature of the metal−carbene bond is only slightly affected by
1
8
step, the imidazolium hexafluorophosphate salt 3, which was
1
9
generated by salt metathesis from the known compound 2, is
deprotonated by silver oxide, yielding an intermediate silver
carbene complex. The crude product is used in the following
transmetalation reaction to palladium, yielding compound 4.
The anionic complex 5 was synthesized following a modified
literature procedure by Sharutina (Scheme 1, iv). Equimolar
amounts of tetrabutylammonium chloride and palladium
chloride are dissolved in dimethylsulfoxide (DMSO). The
excess solvent is removed in vacuo and the product is
precipitated by adding diethyl ether to a concentrated solution
of 5 in dichloromethane. This procedure allows for a
convenient one-step access to compound 5 with significantly
2
1
1
the bonding of the ligand in a κ or κ fashion. The κ -bound
ligand is tilted out of the palladium coordination plane by
73.9(3)°.
11c
In compound 5, the Pd−Cl1 and Pd−Cl2 bonds are
2.295(2) and 2.298(2) Å, respectively. Thus, the trans influence
of the DMSO ligand is small. The Pd−S distance of 2.230(2) Å
11c
is similar to analogous reported complexes. All bond lengths,
angles, and torsions of compounds 4 and 5 are similar to those
10b
reported for the mixed compound 1.
1
1b
higher yield, compared to a previously described synthesis.
Compound 6 has been synthesized analogously for comparison
Scheme 1, iv). Complexes 5 and 6, with melting points of 76
In the packing diagram of compound 5, chains of DMSO
ligands are visible, with an average intermolecular C−O
distance of 3.18 Å, slightly shorter than the sum of the van
(
20
22
and 51 °C, respectively, are ionic liquids. To investigate the
influence of the cation, compound 7 was synthesized by
refluxing a suspension of 1 equiv each of sodium chloride,
palladium chloride, and DMSO (Scheme 1, v).
To understand the coordination of the DMSO ligand toward
the anionic palladium center, infrared (IR) spectroscopy has
been employed (see Figure S1 in the Supporting Information
der Waals radii of 3.22 Å (Figure 3). This indicates that
significant dipole−dipole interactions of the highly polar
DMSO molecules among themselves are still present in the
solid-state structure of complex 5. The Pd atom and the three
Cl atoms are attached perpendicular to these DMSO chains.
Between two palladium ligand spheres, the noncoordinating
tetrabutylammonium counterion is enclosed. The unusual
folding of one butyl group has also been reported in some
(
SI)). The coordination modes of DMSO to transition metals
21
23
have been investigated thoroughly. Generally, S-coordination
of the DMSO ligand leads to an increase of the wavenumbers of
the characteristic S−O vibration mode while O-coordination
structurally related palladium complexes.
Catalysis. Following the observation that complex 1
10b
performs exceptionally well in the Mizoroki−Heck reaction,
3
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Table 1. Results of the Mizoroki−Heck Reaction with Aryl
a
Chlorides and Different Stabilizers
b
b
c
entry
catalyst
5 (0.5)
stabilizer
base [equiv]
yield [%]
1
2
3
4
5
6
7
8
9
TBACl (20)
TBACl (20)
TBACl (20)
TBACl (20)
TBACl (20)
TBACl (20)
TBACl (20)
LiCl (20)
1.1
1.1
1.1
2.2
2.2
2.2
2.2
2.2
2.2
2.2
50
58
66
74
44
5 (0.1)
5 (0.05)
5 (0.05)
5 (5)
d
5 (0.05)
5 (0.05)
5 (0.05)
5 (0.05)
5 (0.05)
11
e
54
4
1
f
TBAPF6 (20)
g
1
0
TBABr (20)
21
TBACl (15)
TBABr (5)
TBACl (20)
TBACl (20)
TBACl (20)
NaCl (400)
NaCl (400)
TBACl (50)
1
1
5 (0.05)
2.2
67
0
h
h
1
2
3
5 (0.05)
5 (0.05)
2.2
2.2
,
i
j
1
55 (48)
Figure 3. Packing of compound 5 as an ORTEP drawing. Hydrogen
atoms are omitted, and only one tetrabutylammonium cation is shown
for clarity. Thermal ellipsoids are drawn at the 50% probability level.
hi
4 ,
1
5 (0.05)
2.2
82
7
15 ,
hi
5 (0.05)
5 (0.05)
2.2
2.2
6 ,
hi
94 (88)
k
l
1
we investigated to what extent each of the palladium species in
the complex contributes to the catalytic activity. The reaction of
bromoacetophenone and styrene in the presence of sodium
acetate as a base in dimethylacetamide was chosen as a model
system (see Scheme 2). A first direct comparison revealed that
h
h
h
h
h
h
,
,
,
,
,
,
i
i
i
i
i
i
17
18
19
20
1 (0.05)
4 (0.05)
6 (0.05)
7 (0.05)
TBACl (50)
TBACl (50)
TBACl (50)
TBACl (50)
TBACl (50)
TBACl (50)
2.2
2.2
2.2
2.2
2.2
2.2
55
7
77
48
l
2
1
2
PdCl (0.05)
69 (35)
2
2
PdBr (0.05)
70
Scheme 2. The Mizoroki−Heck Reaction Using Aryl
2
a
a
Bromides
General reaction conditions: 1 mmol 4-chloroacetophenone, 1.4
b
mmol styrene, 5 mL DMAc, sodium acetate, 140 °C, 6 h. Catalyst
and stabilizer amounts (shown in parentheses) are given in units of
c
mol %. Yield given for the major product; the 1,1-regioisomer was not
d
observed. The yield after 24 h was identical in every case. At 120 °C.
e
f
g
At 160 °C. Tetrabutylammonium hexafluorophosphate. Tetrabutyl-
h
i
ammonium bromide. Using rigorously dried reagents. 50 μL water
added. 100 μL water added. Corresponding to a TON of 1880 and a
TOF of 313 h , After 2 h.
j
k
−1
l
a
General reaction conditions: 1 mmol 4-bromoacetophenone, 1.4
that catalyst decomposition occurs. Doubling the amount of
sodium acetate further increased the yield to 74% (Table 1,
entries 3 and 4). An increase of catalyst load to 5 mol % was
less efficient, as well as varying the reaction temperature (Table
mmol styrene, 0.014 mol % [Pd], 1.1 equiv NaOAc, 5 mL DMAc, 140
°
C, 24 h. All yields were determined by GC using dodecane as internal
standard and are given as an average of two runs. The 1,1-regioisomer
was not detected.
1
, entries 5−7).
Subsequently, different stabilizers were tested. The use of
the cationic precatalyst 4 itself does not lead to full conversion,
even after 24 h. However, with any of the anionic complexes
lithium chloride or tetrabutylammonium hexafluorophosphate
led to an almost-complete loss of catalytic activity, indicating
that the simultaneous presence of chloride and tetrabutylam-
monium ions is necessary for the reaction (Table 1, entries 8
and 9). However, if tetrabutylammonium bromide (TBABr) is
added instead of TBACl, the yield is decreased to 21%,
compared to 74% yield, respectively (Table 1, entry 10). This
effect persists, to a lesser extent, if a mixture of TBACl and
TBABr is employed (Table 1, entry 11). Other stabilizers also
were less efficient (see Table S1 (entries 7−14) in the SI).
In the course of the optimization, we noticed a complete loss
of catalytic activity, if rigorously dried reagents are employed
(Table 1, entry 12). However, the catalytic activity reappeared
after addition of a defined small amount of water, while larger
amounts of water led to lower catalytic activity (Table 1, entry
13). We reasoned that the presence of trace amounts of water
5−7, full conversion is achieved. In every case, the E/Z
selectivity is ≥98:2, while the 1,1-regioisomer was not observed.
All yields are given for the major product. If the reaction is
performed in air, no product is obtained using complex 4, while
the catalytic activity of complexes 5 and 6 is retained.
We chose complex 5 for further optimization studies, because
it showed high catalytic activity, tolerance toward air, and can
be synthesized in one step, starting from commercially available
reagents.
Coupling of Aryl Chlorides. The Mizoroki−Heck
coupling of aryl chlorides requires different reaction conditions,
since these are less-reactive substrates than aryl bromides (see
Table 1, as well as Table S1 in the SI). The addition of
tetrabutylammonium chloride (TBACl) was crucial for product
formation to occur. The reduction of the catalyst load led to
might be responsible for catalyst activation, as described
24
3a,25
improved yields (Table 1, entries 1−3). We noted that the
previously.
Since water might play a dual role, i.e., also
26
yield was identical after reaction times of 6 and 24 h, indicating
leading to catalyst deactivation, we chose to increase the
3
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amount of stabilizer. Indeed, the addition of a large amount of
sodium chloride, in addition to TBACl, led to a yield of 82%
bromides can be coupled in excellent yields using a smaller
amount of stabilizer and base, since these are more reactive
substrates (Figure 4, condition B). Generally, both inductively
and mesomerically electron-withdrawing substituents in the
meta- or para-position of the aryl halide are tolerated.
Substrates with larger ortho-substituents were unreactive
under the employed conditions. Different alkenes were
tolerated in the reaction. The use of electron-rich styrenes
still led to high yields and high selectivity for the (E)-product,
while only small amounts of the 1,1-regioisomer were observed.
The 1,1-regioisomer was not observed if styrene was used as
the alkene.
(Table 1, entry 14), while adding only sodium chloride led to
low yields (Table 1, entry 15). A yield of 94% was achieved if
the amount of TBACl was increased to 50 mol % (Table 1,
entry 16). Comparing different precatalyst systems under the
optimized reaction conditions, we found that catalyst 5 is
superior not only to the other complexes presented, but also to
commercially available palladium sources (see Table 1, entries
1
7−22, as well as Table S1 (entries 18 and 19)).
Reaction Scope. Various aryl bromides and chlorides were
coupled under the optimized reaction conditions. Activated aryl
chlorides with varying meta- or para-substituents were coupled
with styrene in very good yields (Figure 4, condition A). For
example, 1-chloro-4-nitrobenzene was coupled to styrene in an
excellent yield of 91%. Coordinating functional groups are able
to poison the catalyst, leading to yields of 36% and 15% for
Mechanistic Considerations. Based on the results in this
16
paper and the available literature, a catalytic cycle involving
anionic palladium species is reasonable (Scheme 3). This
a
Scheme 3. Proposed Mechanism
4‑chlorobenzonitrile and 3-chloropyridine, respectively, while
deactivated aryl chlorides were unreactive. However, deacti-
vated aryl bromides were coupled efficiently. Activated aryl
a
(A) Reduction of the palladium(II) precursor, (B) oxidative addition,
(C) coordination of the alkene, (D) migratory insertion, (E) β-hydride
elimination and simultaneous decoordination, (F) reductive elimi-
nation, (G) reversible formation of stabilized palladium nanoparticles
by aggregation, (H) irreversible formation of palladium black. L =
DMSO, X = Cl, Br.
important mechanistic proposal has been made by Amatore and
Jutand, who studied the influence of quaternary ammonium
6
a,27
salts on the Mizoroki−Heck reaction in detail.
In this
mechanism, a palladium(II) precursor (I) is transformed to the
active anionic, dicoordinate palladium(0) species (II) by
3a
reduction (Scheme 3, step A). The presence of TBACl
ensures a high chloride ion concentration and thus might
enhance the stability of a highly active anionic palladium(0)
species II, leading to improved catalytic activity of complex 5.
The anionic species II undergoes oxidative addition more easily
6
e
than their neutral counterparts (Scheme 3, step B), leading to
an anionic palladium(II) species (III). After coordination of the
alkene by displacement of an anionic ligand, migratory insertion
occurs, followed by β-hydride elimination and subsequent base-
Figure 4. Results in the Mizoroki−Heck reaction using different
substrates. Reaction conditions: 1 mmol haloarene, 1.4 mmol styrene,
5
mL DMAc, 140 °C, 6 h ((A) 2.2 mmol NaOAc, 50 mol % TBACl,
16
assisted reductive elimination (Scheme 3, steps C−F). The
active species II could be in equilibrium with palladium
nanoparticles (VII) as a resting state, serving as a catalyst
5
0 μL water; (B) 1.1 mmol NaOAc, 20 mol % TBACl); isolated yield,
average of two runs. [Footnotes shown in figure: (a) At full
conversion. In a side reaction, the methyl ether was cleaved, leading
to lowered yields. (b) For X = Cl, hydrolysis of butyl acrylate occurred,
instead of product formation. (c) 3% of the 1,1-regioisomer were
observed. (d) 1% of the 1,1-regioisomer was observed.]
24,28
reservoir (Scheme 3, step G).
However, it cannot be
completely ruled out that these nanoparticles also are
catalytically active by themselves. Aggregation of these
3
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palladium nanoparticles might then irreversibly deactivate the
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catalyst (Scheme 3, step H). The stabilization of palladium
29
nanoparticles by TBACl has been discussed, ultimately
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2
(
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tigated. Therefore, the cationic complex 4 has been synthesized
and an improved protocol has been developed as a high-
yielding, one-step access to complex 5. The anionic complex 5
showed higher activity in the Mizoroki−Heck coupling of 4-
bromoacetophenone and styrene, compared to the cationic
complex 4. Complex 5 was also successfully used in the
Mizoroki−Heck reaction of activated aryl chlorides under
Jeffery conditions. The scope of the reaction has been explored,
revealing that complex 5 is well-suited for the Mizoroki−Heck
coupling of aryl bromides and activated aryl chlorides.
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linked to the simultaneous presence of tetrabutylammonium
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that a defined small amount of water is necessary for catalytic
activity might be important for further work on ligandless
catalytic systems. The observations are consistent with an
Amatore−Jutand-type catalytic cycle. Detailed investigations on
the mechanism of the Mizoroki−Heck reaction catalyzed by
complex 5, especially on the activation step and the role of
water, and further cross-couplings will be reported in due
course.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b03655.
■
Corma, A. Angew. Chem., Int. Ed. 2013, 52, 11554−11559. (v) Yin, J.;
Hyland, C. J. T. J. Org. Chem. 2015, 80, 6529−6536. (w) Grosse, S.;
Pillard, C.; Massip, S.; Marchivie, M.; Jarry, C.; Bernard, P.;
Guillaumet, G. J. Org. Chem. 2015, 80, 8539−8551. (x) Jadhav, S.;
Kumbhar, A.; Rode, C. V.; Salunkhe, R. S. Green Chem. 2016, 18,
*
S
Additional figures and tables, experimental procedures,
analytical data and NMR data, and spectra (PDF)
Crystallographic data (CCDC 1540014) for compound 4
1
(
1
898−1911.
5) (a) Kaufmann, D. E.; Nouroozian, M.; Henze, H. Synlett 1996,
996, 1091−1092. (b) Herrmann, W. A.; Bohm, V. P. W. J. Organomet.
Chem. 1999, 572, 141−145. (c) Bohm, V. P. W.; Herrmann, W. A.
̈
(
CIF)
Crystallographic data (CCDC 1540013) for compound 5
CIF)
̈
Chem.Eur. J. 2000, 6, 1017−1025. (d) Zou, G.; Wang, Z.; Zhu, J.;
(
Tang, J.; He, M. Y. J. Mol. Catal. A: Chem. 2003, 206, 193−198.
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AUTHOR INFORMATION
Corresponding Author
E-mail: thomas.strassner@chemie.tu-dresden.de.
■
*
McGlacken, G. P.; Weissburger, F.; De Vries, A. H. M.; Schmieder-van
de Vondervoort, L. Chem.Eur. J. 2006, 12, 8750−8761. (g) Calo,
̀
V.;
Nacci, A.; Monopoli, A.; Cotugno, P. Angew. Chem., Int. Ed. 2009, 48,
6
3
101−6103. (h) Wang, L.; Li, H.; Li, P. Tetrahedron 2009, 65, 364−
ORCID
68. (i) Wang, W.; Yang, Q.; Zhou, R.; Fu, H. Y.; Li, R. X.; Chen, H.;
Thomas Strassner: 0000-0002-7648-457X
Notes
The authors declare no competing financial interest.
Li, X. J. J. Organomet. Chem. 2012, 697, 1−5. (j) Gaikwad, D. S.; Park,
Y.; Pore, D. M. Tetrahedron Lett. 2012, 53, 3077−3081.
(
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ACKNOWLEDGMENTS
Vondervoort, L.; Mommers, J. H. M.; Henderickx, H. J. W.; Walet,
■
M. A. M.; De Vries, J. G. Adv. Synth. Catal. 2002, 344, 996−1002.
We would like to thank Dr. Eberhardt Herdtweck for his help
in solving and refining the solid-state structures of complexes 4
and 5.
(c) Goossen, L. J.; Koley, D.; Hermann, H. L.; Thiel, W. J. Am. Chem.
Soc. 2005, 127, 11102−11114. (d) Carrow, B. P.; Hartwig, J. F. J. Am.
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