Dihalogen-bridged NHC-palladium(i) dimers: Synthesis, characterisation and applications in cross-coupling reactions

Article abstract of DOI:10.1039/c9cc02239b

A di-iodo-bridged Pd^I dimer bearing an N-heterocyclic carbene ligand was made accessible via the reduction of [(IPr)PdI₂]₂ in basic methanol solution. The structural features of [(IPr)PdI]₂ were explored by single-crystal X-ray analysis. It was found to be a convenient one-component precatalyst with high activity in Suzuki-Miyaura, Buchwald-Hartwig, and Sonogashira reactions.

Full text of DOI:10.1039/c9cc02239b

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DOI: 10.1039/C9CC02239B
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Dihalogen-bridged NHC-palladium(I) dimers: synthesis,
characterisation and applications in cross-coupling reactions
Nico Pirkl,§a Alessandro Del Grosso,§a Bert Mallick,b Angelino Doppiuc and Lukas J. Gooßen*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A di-iodo-bridged PdI dimer bearing an N-heterocyclic carbene Colacot)6 which quickly dissociate in solution, or amine-
ligand was made accessible via the reduction of [(IPr)PdI2]2 in basic containing palladacycles (Buchwald),7 which irreversibly
methanol solution. The structural features of [(IPr)PdI]2 were eliminate indolines and carbazoles.
explored by single-crystal X-ray analysis. It was found to be a The use of such additional components is avoided in dimeric
convenient one-component precatalyst with high activity in Suzuki- [LPdHal]2 precatalysts featuring dihalogen-bridged Pd in the
Miyaura, Buchwald-Hartwig, and Sonogashira reactions.
unusual +1 oxidation state (Scheme 1).8,9
Palladium catalysts are widely employed in synthetically useful
organic transformations, particularly in C–C and C–heteroatom
bond-forming reactions. Numerous review articles illustrate the
versatility and scope of Pd catalysis in research and industry.1 In
most of the validated catalytic cycles, the oxidation state of
palladium alternates between Pd0 and PdII, but other cycles, e.g.
of PdII/PdIV, have also been shown to be functional.2
Traditionally, the key steps of catalytic cross-couplings are the
oxidative addition of a substrate, e.g. an aryl halide, to a
coordinatively unsaturated Pd0 species, transmetalation or salt
exchange at the resulting PdII complex, and liberation of the
product with regeneration of the initial Pd0 species.
The actual catalyst is thus a highly reactive, coordinatively
unsaturated Pd0 species, such as a 12-electron complex bearing
only a single electron-rich and bulky phosphine or NHC ligand.
Since such species cannot be stored or handled, they must be
generated in situ. The usual entry point to any catalytic reaction
is thus a precatalyst, ideally one that is air- and water-stable,
can be synthesized in consistent quality, and rapidly forms the
desired Pd0 species under the reaction conditions.
I
Previous work: halogen-bridged, phosphine-ligated Pd dimers
t
t
Mingos/Vilar: L = P( Bu) ,P( Bu) Ph; X = Br, I
X
X
3
2
t
Hartwig: L = P(1-Ad)( Bu) ; X = Br
L
Pd
Pd
L
2
t
t
Schoenebeck: L = P( Bu) iPr; X = I; L = P( Bu) ; X = SCF
2
3
3
I
This work: iodine-bridged, NHC-ligated Pd dimer
N
N
N
N
I
I
Pd
Pd
Pd-1
Figure 1. Known phosphine–PdI and new NHC–PdI complexes.
Such PdI dimers were first described by Mingos and Vilar, and
used in cross-couplings by Hartwig,8,10 Garg,11 Faul,12 and
others.13 In this reaction type, their activity was found to
surpass that of structurally related Pd0 and PdII precursors.8 This
was attributed to energetically favourable activation routes.
Hartwig et al. suggested that the PdI dimers are converted into
the 12-electron species Pd0–PtBu3 via disproportionation or
reduction.8,14 Others propose homolytic bond scission followed
by reduction of the radical species.13a
There are also a few examples in which PdI species seem to
enable novel catalytic pathways, e.g. Schoenebeck’s catalytic
exchange of halides for SCF3 and SeCF3 groups13b,c and some
isomerization processes Table2developed by our group.15
Since the donor strength of N-heterocyclic carbenes (TEP(IPr) =
2052 cm-1)16a,b is even higher than that of the above phosphines
(TEP(PtBu3) = 2056 cm-1),16b-d it is highly desirable to gain access
to PdI complexes bearing NHC ligands, and to evaluate their
Many users prefer simple one-component precatalysts to multi-
component concoctions.3 In such precatalysts, the metal
centers may be protected by pyridines (Organ’s PEPPSITM),4
dienes (Beller)5 and allyl groups (Nolan, Shaughnessy, and
aEvonik Chair of Organic Chemistry. Faculty of Chemistry and Biochemistry, Ruhr
Universität Bochum, ZEMOS 2.27, Universitätsstr. 150, 44801 Bochum, Germany
E-mail: lukas.goossen@rub.de
bChair of Inorganic Chemistry II. Faculty of Chemistry and Biochemistry, Ruhr
Universität Bochum, NC2/73 Universitätsstr. 150, 44801 Bochum, Germany.
E-mail: bert.mallick@rub.de
cUmicore AG&Co. KG, Rodenbacher Chaussee 4, 63457 Hanau.
§ Nico Pirkl and Alessandro Del Grosso contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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catalytic activity.17 En route to the desired NHC–PdI dimers, we
investigated several potential synthetic approaches previously
used for phosphine–PdI complexes. These include Mingos and
Vilar’s synthesis of [(tBu3P)PdX]2 via oxidation of the
corresponding Pd0 complexes in the presence of CHX3,9a the
comproportionation of various Pd0 and PdII complexes by Vilar,
Goossen, and Schoenebeck,9b,18 and Colacot’s reduction of
Pd(COD)Br2 / phosphine mixtures in basic methanol.19
View Article Online
DOI: 10.1039/C9CC02239B
Figure 3. Crystal structure of the dimeric PdI iodide complex Pd-1 (left) and the H–PdII
bromide species Pd-4 (right). The thermal ellipsoids are depicted at 50% probability and
C–H hydrogen atoms are omitted for clarity.
a) [Pd (dba)
]
+
PR
c) PR
e) [Pd (dba) ] + PR
2
3
3
3
2
3
3
+ CHX
18/23%
+
Pd (COD)X 90%
+
PdX
2
48%
3
2
NaOH
X = Br
Mingos/ X = Br, I
X = I
Colacot
Schoenebeck
Vilar
t
t
L = Bu
L = Bu
t
L = Bu
X
X
X-ray analysis of the PdI iodo complex revealed that Pd-1 had
crystallised in two conformers, which differ from each other in
the orientation of the imidazole ring. Thus, the asymmetric unit
consists of two slightly different half-molecules. Both Pd-1
conformers are C2-symmetric. The dihedral angle between the
plane of the imidazole ring and that defined by the I—Pd—I
atoms is 27.75° in one molecule, and 82.40° in the other. The
average Pd–I bond length of 2.601(2) Å is similar to that in
[(tBu3P)PdI]2 (2.598(1) Å).9a The Pd–Pd bond distances in the
two molecules of 2.629(1) and 2.654(1) Å, respectively, are in
the expected range for Pd–Pd bonds in PdI dimers.9a
Having thus found an expedient synthetic entry to Pd-1, we next
investigated its performance in Suzuki-Miyaura cross-couplings
(Table 1). The reaction of p-chlorotoluene and phenylboronic
acid was chosen as benchmark, because this synthetically
meaningful coupling is possible only with state-of-the art,
electron-rich Pd catalysts.20
L
Pd
Pd L
1996
2000
2010
2011
2014
Mingos/ X = Br
Vilar
X = Br
Goossen
t
t
L = Bu
L = Bu
b) [Pd (dba)
]
+
4 PR
d) PdX
2
3
3
2
+
2 Pd (COD)X 60%
+ Pd(PR )
2 87%
3
2
Figure 2. Established entries to phosphine-bridged PdI dimers.
None of these approaches led to the formation of bromo-
bridged NHC–PdI dimers when using (IPr)2Pd in place of
phosphines. However, [(IPr)PdIII2]2, which was obtained by
reaction of [(IPr)PdIICl2]2 with KI, was successfully reduced in
7
basic methanol solution to [(IPr)PdII]2 (Pd-1, Scheme 1). In the
course of the reaction, the colour changed to green, indicating
the formation of PdI. This was confirmed by 13C NMR, in which
a substantial shift of the carbene signal from 165.4 in Pd-2 to
183.7 ppm was observed. Pd-1 was isolated in 60% yield and
high purity as a green solid by simple toluene extraction. Its
identity was established by X-ray analysis (Figure 3A).
Table 1. Optimisation of reaction conditions for Suzuki-Miyaura coupling. a
a) Sy nthesis of Pd-1 by reduction of Pd-2
Cl
0.5 mol% Pd-1
Ph
Ph
HO
Base
B
I
+
I
KOH/MeOH
I
I
IPr
OH
2a
Pd
Pd IPr
I
IPr Pd
Pd IPr
I
toluene, 6 h, 25 °C
1a
3aa
Pd-2
Pd-1
Entry
Catalyst
T [°C]
2a/eq.
Base/eq.
3aa [%]b
b) Reduction attempt of Pd-4
1
2
3
4
5
Pd-1
"
"
60
40
"
"
"
1
"
1.2
"
Cs2CO3/1.2
86
86
98
16
0
Br
H
IPr Pd IPr
Br
KOH/MeOH
toluene, 6 h, 25 °C
Br
Br
"
IPr Pd
Pd IPr
Br
Cs2CO3/1.4
[(tBu3P)PdIBr]2
[(tBu3P)PdII]2
"
"
"
Pd-3
Pd-4
a 0.5 mmol 1a, 1.0 eq. 2a, 1.2 eq. base, 0.5 mol% Pd-1, 1 mL EtOH, 4 h. b GC yield
using n-tetradecane as an internal standard. Average of two runs.
Scheme 1. Synthesis of Pd-1 and attempted synthesis of its bromo-analogue.
When attempting to synthesize the analogous bromo-bridged
complex from Pd-3 under identical reaction conditions, the solution
turned brown, and a different set of signals was seen in the NMR
spectra. The main product was identified as the PdII–hydride
complex Pd-4, and its structure confirmed by X-ray analysis
(Figure 3B). The attempted comproportionation of (IPr)2Pd with
PdBr2 led to the formation of Pd black along with trans-
(IPr)2PdBr2. Its structural identity was confirmed by NMR and X-
ray in combination with independent syntheses from different
precursors (see the supporting information). These results
indicate that bromo-bridged NHC–PdI complexes seem to have
a lower tendency to form than either the iodo-bridged NHC- or
the bromo-bridged phosphine–PdI analogues.
Some deviations from standard reaction protocols were
necessary to adapt the conditions to catalyst Pd-1.8
A
combination of caesium carbonate as the base and ethanol as
the solvent was found to be most effective. Using a slight excess
of phenylboronic acid 2a and base along with 0.5 mol% catalyst,
3a was obtained in near-quantitative yield within 4 h at 40 °C.
This catalyst activity is very good for an easy-to-use, one-
component precatalyst. Under identical conditions,
[(tBu3P)PdIBr]2, which displays record-setting activity for aryl
bromides, gave unsatisfactory yields, and [(tBu3P)PdII]2 was
inactive (entries 4 and 5).
Further investigations showed that Pd-1 can be widely
employed as a precatalyst in Suzuki couplings of electron-rich
and electron-poor aryl chlorides in combination with various
2 | J. Name., 2012, 00, 1-3
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Table 4. Scope of the Buchwald-Hartwig amination.a
boronic acids (Table 2). The yields are high throughout, and
common functionalities are well tolerated. Interestingly, the
reaction is more sensitive to steric hindrance on the side of the
aryl halide than on that of the boronic acid: Whereas the ortho-
methyl group in o-tolylboronic acid barely affected the reaction,
the coupling of o-chlorotoluene took 16 hours to go to
completion.
View Article Online
DOI: 10.1039/C9CC02239B
R1
0.5 mol% Pd-1
Cl
R1
HN
N
R2
t
1.5 equiv. BuOK
+
R
R
R2
THF, 2 h, 40 °C
1a-h
4a-c
5aa-5hc
5aa R = p-Me
5ba R = p-H
5ca R = p-CO Et
97%
80%
32%
Table 2. Scope of the Suzuki-Miyaura coupling.a
O
N
N
2
0.5 mol% Pd-1
1.4 equiv. base
Cl
+
Ar
5ab 97%
Ar
HO
5da R = p-OMe
5ea R = p-CN
97%
82%
81%
R
H
N
B
R
R
Ph
OH
5fa R = p-NO
2
1a-i
2a-e
3aa-3ie
5ga R = o-Me
5ac 61%
97%
3aa R = Me
3ba R = H
3ca R = CO Et
92%
94%
93%
a 1 mmol 1, 1.05 eq. 4, 1.5 eq. tBuONa, 0.5 mol% Pd-1, 1 mL THF, 2 h, 40 °C.
Ph
Ph
Ph
2
To further explore the applicability of Pd-1 as a convenient, one-
component precatalyst in coupling reactions, we moved on to
Sonogashira couplings, which have a different set of catalyst
requirements to the above reactions. Traditionally, a copper co-
3da R = OMe
3ea R = CN
80%
96%
93%
R
N
3ha 96%
3ia 96%
3fa R = NO
2
3ga R = CF
96%
3
3ab Ar = p-OMeC H 81%
Ar
6
4
catalyst is employed as
a -acid that facilitates the
3ac Ar = o-tol
3ad Ar = 2-naphty l
3ae Ar = o-CNC H
84%
97%
53%
deprotonation of the terminal alkyne. In modern systems, Pd
needs to take on this role in addition to its function as a cross-
coupling catalyst. We started our investigations using the
conditions described by Cheng et al.,22 which entail using 3
equiv. of K2CO3 in THF at 65 °C, and applied them to the model
reaction of iodobenzene with phenylacetylene. Decisive step-
ups in the yield were achieved by replacing the carbonate base
by triethylamine and the solvent by acetonitrile, as well as by
reducing the reaction temperature. Near-quantitative
conversion was reached within 16 h at 40°C. We benchmarked
Pd-1 against [(tBu3P)PdII]2 and [(tBu3P)PdIBr]2 under identical
conditions. In this reaction, all precursors worked equally well
(see the supporting information).
b
3hc 68%
6
4
a 0.55 mmol 1, 1.2 eq. 2a, 1.4 eq. Cs2CO3, 0.5 mol% Pd-1, 1mL EtOH, 4 h, 40 °C.
b 16 h. Aryl groups originating from ArB(OH)2 on the right-hand side.
Next, we investigated the performance of Pd-1 in Buchwald-
Hartwig aminations using the reaction of 4-chlorotoluene with
morpholine as model. When the reaction was conducted under
Hartwig’s conditions,8 full conversion was achieved within 16 h
at 25 °C or within 2 h at 40 °C (Table 3). When monitoring the
reaction progress over time, the induction period was too brief
to be detected (see the supporting information).
Table 3. Optimisation of the conditions for the Buchwald-Hartwig amination.a
With Pd-1, the reaction gave high yields for various
functionalized aryl iodides, ranging from electron-rich to
electron-deficient derivatives and bearing various functional
groups (Table 5).
O
0.5 equiv. Pd-1
Cl
N
O
t
1.5 equiv. BuONa
+
HN
Table 5. Scope of the copper-free Sonogashira coupling.a
1a
4a
5aa
Ph 8aa R = H
8ba R = Me
8ca R = CN
8da R = NO
93%
81%
90%
96%
Entry
Time [h]
Solvent
Temp. [°C]
5aa [%]b
I
1
2
3
4
2
16
0.5
2
THF
25
"
40
"
51
>99
81
Ph
7a
"
"
"
Pd-1
Et N
2
R
R
8ea R = C(O)Me 96%
8fa R = F 93%
6a-f
8aa-8fa
3
>99
a 1 mmol 1a, 1.05 eq. 4a, 1.5 eq. tBuONa, 0.5 mol% Pd-1, 1 mL THF. b GC yield using
n-tetradecane as an internal standard. Average of two runs.
a 1 mmol 6, 1.2 eq. 7a, 2 eq. Et3N, 0.5 mol% Pd-1, 2 mL MeCN, 16 h, 40 °C.
We next probed whether PdI would also allow Sonogashira
couplings with aryl bromide or even chloride substrates. Since
the latter had been converted successfully in both the Suzuki
and Buchwald/Hartwig couplings, there was no doubt that PdI
is capable of inserting into non-activated C–Hal bonds.
However, even after reoptimising the conditions (see the
supporting information), the desired product 10a was obtained
in only 50% yield based on 4-fluorophenyl bromide 9 (Scheme
3). A possible explanation arises from the formation of enyne 11
as a by-product. This was not unexpected, since (E)-selective
head-to-head dimerization of terminal alkynes is known to be
efficiently promoted by (IPr)2Pd.23 According to the proposed
[(tBu3P)PdIBr]2 has been reported to be uniquely effective in
such reactions (88% within 15 min).8 However, under the
optimal conditions for Pd-1, it gave inferior results in the model
reaction (see the supporting information).
Under these conditions, Pd-1 promotes the coupling of a broad
range of aryl chlorides with several amines. The yields were
comparable for electron-rich and electron-poor aryl chlorides
(Table 4). Several functionalities, among them even the base-
sensitive nitro group, were well tolerated.21
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7
8
9
A. Bruneau, M. Roche, M. Alami and S. Messaoudi, ACS Catal.,
catalytic cycle, this reaction is initiated by an oxidative insertion
of Pd0 into the alkyne C–H bond. Apparently, this process is
slower than the oxidative addition of aryl iodides but proceeds
at a comparable rate to the oxidative addition of aryl bromides.
View Article Online
2015, 5, 1386–1396.
DOI: 10.1039/C9CC02239B
J. P. Stambuli, R. Kuwano and J. F. Hartwig, Angew. Chem. Int.
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J. P. White and D. J. Williams, J. Organomet. Chem., 2000, 600,
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2007, 9, 4343–4346.
Ph
0.5 mol% Pd-1
Br
1 equiv. Cs CO
2
3
Ph
+
Ph
+
MeCN, 40 °C
16 h
1.2 equiv.
Ph
F
F
1 equiv.
50%
35%
9
7a
10a
11
Scheme 2. Competition experiment of Sonogashira coupling and alkyne dimerization.
10 a) T. Hama, X. Liu, D. A. Culkin and J. F. Hartwig, J. Am. Chem.
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This example illustrates another interesting facet of the
reactivity of Pd-1, potentially opening up further catalytic
opportunities involving alkyne activation.
In conclusion, [(IPr)PdI]2 is disclosed as the first example of a di-
halogen-bridged PdI–NHC complex. It is an easily stored and
handled catalyst precursor with excellent activity in Suzuki and
Buchwald-Hartwig couplings. The findings nurture the hope
that the still elusive complex [(IPr)PdBr]2 might possess even
higher activity that surpasses the state of the art in this field.
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Acknowledgements
Funded by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) under Germany ́s Excellence
Strategy – EXC-2033 – Projektnummer 390677874 and SFB TRR
88 “3MET”.
Notes and reference
1
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