Recyclable catalysts for palladium-catalyzed C - O coupling reactions, Buchwald-Hartwig aminations, and Sonogashira reactions

Article abstract of DOI:10.1002/anie.201001787

Simply reuse: Homogeneous palladium catalysts are effective and attractive tools for advanced organic synthesis (see example). Compound 1 belongs to a new class of ligands that enable recycling of the precious-metal catalyst directly from the reaction without any heterogenization.

Full text of DOI:10.1002/anie.201001787

Communications
DOI: 10.1002/anie.201001787
Ligand Design
À
Recyclable Catalysts for Palladium-Catalyzed C O Coupling
Reactions, Buchwald–Hartwig Aminations, and Sonogashira
Reactions**
Andreas Dumrath, Xiao-Feng Wu, Helfried Neumann, Anke Spannenberg, Ralf Jackstell, and
Matthias Beller*
Dedicated to Professor Uwe Rosenthal on the occasion of his 60th birthday
Palladium-catalyzed coupling reactions have become an
effective tool for advanced organic synthesis in both academic
and industrial laboratories.[1,2] In particular, homogeneous
palladium complexes allow for the reliable construction of all
nucleophilic phosphane ligands, which allow for easy recy-
À
À
cling and direct reuse in palladium-catalyzed C O, C N, and
À
C C bond-forming reactions.
À
À
kinds of C C and C X bonds (X = O, N, S) from aryl and
heteroaryl halides. Among the multitude of palladium com-
plexes available for modern coupling reactions, bulky elec-
tron-rich phosphanes have become a privileged class of
ligands,[3,4] which in addition to established coupling processes
also enable more challenging reactions with water,[5] alco-
hols,[6] ammonia,[7] fluoride,[8] and synthesis gas.[9] The draw-
backs of these sophisticated ligands are their multistep
preparation and costs, which are often price determining for
the respective catalyst system. Hence, recycling these ligands
still constitutes a major challenge.
Common concepts for the recycling of palladium catalysts
involved in coupling reactions include heterogenization of
ligands on polymeric supports[10] and palladium-supported
heterogeneous catalysts.[11] Recently, Jin and Lee also fixed
homogeneous palladium catalysts onto nanoparticles to
combine the advantages of homogeneous and heterogeneous
catalysis.[12,13] On the basis of the development of liquid–
liquid biphasic catalysis[14,15] and the immobilization of
molecular catalysts on solid surfaces, elegant concepts for
catalyst recycling have been realized in the last decade.[16] For
example, Wasserscheid and co-workers[12] demonstrated the
feasibility of supported ionic liquid phases as a catalyst
support (SILP) in Fischer–Tropsch and hydrogenation reac-
tions.[17]
Scheme 1. Synthesis of imidazolium phosphanes 1–5: a) 1. nBuLi,
THF, À408C, 5 min; 2. MeI, THF, À408C, 20 min. b) 1. nBuLi, THF,
À408C, 20 min; 2. Cl-PR’’2, THF, 30–508C.
Despite all these developments, to date there exist no
examples of the recycling of ligands and palladium catalysts in
difficult coupling reactions. In this respect, we describe a
general and facile generation of novel sterically hindered,
As shown in Scheme 1, our ligand concept is based on 2-
phosphanylmethyl-N,N’-biarylimidazolium salts. Ligands 1–5
are prepared in a straightforward manner in only two or three
steps. The central N,N’-biaryl-1H-imidazolium unit is synthe-
sized either by multicomponent condensation of the corre-
sponding aniline with a-dicarbonyl compounds (glyoxal or
2,3-butadione) and paraformaldehyde or by sequential two-
step reactions.[18] Both strategies allow for the preparation of
imidazolium salts with different substituents in the 1-, 3-, 4-,
and 5-positions. After selective deprotonation at the 2-
position, the resulting carbene was methylated in situ. A
second deprotonation occurred regioselectively at the methyl
group at the 2-position. Final quenching with commercially
available chlorodialkyl- or chlorordiarylphosphanes led, after
work-up and recrystallization from H2O/EtOH, to the desired
phosphane ligands in good to excellent yields. The general
[*] A. Dumrath, X.-F. Wu, Dr. H. Neumann, Dr. A. Spannenberg,
Dr. R. Jackstell, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock e. V.
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Homepage: www.catalysis.de
[**] This work was funded by the State of Mecklenburg-Western
Pomerania, the BMBF, and the DFG (Leibniz Prize). We thank Dr. W.
Baumann, Dr. C. Fischer, and S. Buchholz (all LIKAT) for their
analytical support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001787.
8988
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Angewandte
Chemie
Table 1: Hydroxylation of 2-bromomesitylene in the presence of different
ligands.[a]
synthesis sequence is easily carried out on a multigram scale
and no chromatographic purification is involved. Notably,
despite their cationic character, all of the imidazolium salts
behaved as nucleophilic donor ligands. Single-crystal X-ray
measurements on 4 demonstrate the steric bulk of the
imidazolium part of the ligand (Figure 1).
Entry
Ligand
R
Yield[b]
[%]
1
2
3
4
5
1
2
3
4
5
96
57
31
66
79
6
7
8
Cy
Ad
Ph
0
95
0
9
10
11
Cy
Ph
o-tolyl
0
0
0
PR3
12
tBu
7
[a] Reaction conditions: 2-Bromomesitylene (1.0 mmol) [{Pd-
(cinnamyl)Cl}2] (1.0 mol%), ligand (4.0 mol%), CsOH·H2O
(3.0 equiv), hexadecane as internal standard (0.2 mmol), 1,4-dioxane
Figure 1. Structure of ligand 4. Included solvent (one molecule of
ethanol), the IÀ counterion, and H atoms are omitted for clarity.
Selected bond lengths [pm] and angles [8]: C1–N1 134, C1–N2 134.4,
C30–P1 187.5, C31–P1 183.7; C1-C30-P1 118.8, C31-P1-C37 102.5.
(1.2 mL), 1008C, 20 h. [b] Determined by gas chromatography.
To compare the catalytic performance of the new ligands
1–5 with standard electron-rich phosphanes, the palladium-
catalyzed hydroxylation of aryl bromides with 1,4-dioxane
was studied as a benchmark reaction. Apart from recent
examples where copper catalysts were used,[19] only highly
electron-rich, biaryl ligands, and to a lesser extent PtBu3, in
combination with palladium allow for this transformation.[5]
Exploratory studies demonstrated that the reaction pro-
ceeded with CsOH in 1,4-dioxane (Table 1). Catalysts pre-
pared with all the new imidazolium-based ligands 1–5
revealed significant activity in the hydroxylation of 2-bromo-
mesitylene, and gave moderate to excellent yield (31–96%) of
the corresponding phenol (Table 1, entries 1–5). The previ-
ously published “state-of-the-art” ligand bisadamantylphos-
phanylimidazol gave 95% of the corresponding product
(Table 1, entry 7). To our delight, the combination of [{Pd-
(cinnamyl)Cl}2] and 1 gave a similar yield (96%). Cyclohexyl-
substituted ligands, for example, 5, performed better than the
tert-butyl- and phenyl-substituted phosphanes. Nevertheless,
the performance of the phenyl-substituted imidazolium salt 4
is remarkable. This is the first time that a non-alkyl phosphane
has shown significant activity in hydroxylation reactions. In
contrast, the cyclohexyl- and phenyl-substituted imidazol-
based ligands gave no product at all (Table 1, entries 6 and 8).
Moreover, commercially available monodentate and biden-
date ligands led to no product or a low yield, which
demonstrates the challenging nature of this coupling reaction
(Table 1, entries 9–12). It is noteworthy that the hydroxyl-
ation reaction can be easily scaled up to 20 mmol of substrate
and the reaction time can be decreased to 30 minutes by using
a microwave.
Next, we tested the potential recycling of our cationic
catalysts. For this purpose, we took advantage of the solubility
behavior of ligand 1 in 1,4-dioxane.[20] While the ligand itself is
not soluble at room temperature, under the reaction con-
ditions it forms a soluble palladium complex, which can easily
be separated from the corresponding precipitated phenolate.
As shown in Table 2 the hydroxylation of 2-bromomesitylene
was performed eight times without adding any new ligand or
palladium. This procedure led to a catalyst productivity of
nearly 700 being realized, which is the highest turnover
number achieved so far in catalytic hydroxylations of aryl
halides. Phosphorus NMR studies of the reaction solution of
the recycling experiments showed that the ligand endured the
reaction without significant oxidation (< 5%). However, the
concentration of the base decreased slightly with each run.
Indeed, a strong influence of additional water on the catalyst
productivity was observed. As shown in Table 3, the yield
rapidly dropped when water was added. Interestingly, the
ligand itself is stable in water and can be stored for several
months under water without significant decomposition
(< 5%, see the Supporting Information). The viability of
our catalyst recycling was further demonstrated in four
consecutive hydroxylations of 1-bromo-3,5-dimethylbenzene
on a 10 mmol scale. Again, no significant decrease in the
product yield occurred (90–95% yield).
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Table 2: Recycling of catalysts in the hydroxylation of 2-bromomesityl-
ene.[a]
Table 4: Palladium-catalyzed hydroxylation of aryl bromides.[a]
Entry
Aryl halide
Product
Yield[b]
[%]
Run
Yield [%][b]
TON
1
2
3
4
5
6
7
8
95
96
94
92
90
87
79
61
95
1
2
3
4
96
95
191
285
377
467
554
633
694
90[e]
97
[a] Reaction conditions: 2-Bromomesitylene (1.0 mmol) [{Pd-
(cinnamyl)Cl}2] (1.0 mol%), ligand (4.0 mol%), CsOH·H2O
(3.0 equiv), hexadecane as internal standard (0.2 mmol), 1,4-dioxane
(1.2 mL), 1008C, 20 h. [b] Determined by gas chromatography, all
reactions were carried out twice.
1
5
6
7
76[d]
92
Table 3: Influence of water on the hydroxylation of 2-bromomesitylene.[a]
Entry
Water [mL]
Yield [%][b]
70[c]
1
2
3
0.0
0.5
1.0
96
17
3
[a] Reaction conditions: Aryl bromide (1.0 mmol), [{Pd(cinnamyl)Cl}2]
(1.0 mol%), ligand 1 (4.0 mol%), CsOH·H2O (3.0 equiv), hexadecane as
internal standard (0.2 mmol), 1,4-dioxane (1.2 mL), 1008C, 20 h.
[b] Determined by gas chromatography. [c] 1208C, 30 h. [d] 1008C,
30 h. [e] 1208C, 15 h.
[a] Reaction conditions: 2-Bromomesitylene (1.0 mmol), [{Pd-
(cinnamyl)Cl}2] (1.0 mol%), ligand (4.0 mol%), CsOH·H2O
(3.0 equiv), hexadecane as internal standard (0.2 mmol), 1,4-dioxane
(1.2 mL), 1008C, 20 h. [b] Determined by gas chromatography, all
reactions were carried out twice.
1
heteroaryl bromides proceeded smoothly to 3-(oct-1-ynyl)-
thiophene, 2-(oct-1-ynyl)-mesitylene, 2,4,6-trimethylbiphe-
nyl, and 4-methoxybiphenyl in good to excellent yields (60–
99%). Finally, we demonstrated the generality of the
recycling concept in a copper-free Sonogashira coupling. As
Next, we applied the optimized reaction conditions to the
hydroxylation of various aryl bromides (Table 4). Both steri-
cally hindered and unhindered substrates reacted well
(Table 4, entries 1 and 2). Activated 4-bromonitrobenzene
and also deactivated 2-bromoanisole were successfully
hydroxylated in 92% and 70% yield, respectively (Table 4,
entries 6 and 7). In particular, the hydroxylation of different
bromochlorobenzenes proceeded with high chemoselectivity
to the corresponding chlorophenols in 76–97% yield.
Table 5: Catalyst recycling in copper-free Sonogashira reactions.[a]
To demonstrate the general synthetic usefulness of the
ionic ligands we also investigated other important palladium-
catalyzed cross-coupling reactions. As shown in Scheme 2, the
new catalyst system successfully promoted the Buchwald–
Hartwig amination of 4-chloro-2-methylquinoline and 2-
bromomesitylene with 1-benzylpiperazine, as well as the
direct amination of 1-chloronaphthalene and 4-chloro-2-
methylquinoline with ammonia. Here, the corresponding
anilines were obtained in 80 and 99% yield, respectively.
Moreover, the catalyst was re-isolated without problems in
the case of the amination of 1-chloronaphthalene. Further-
more, Sonogashira and Suzuki coupling reactions of aryl and
Run
Yield [%][b]
TON
1
2
3
4
96
99
98
95
96
195
293
388
[a] Reaction conditions: Aryl bromide (1.0 mmol), Pd(OAc)2 (1.0 mol%),
ligand 2 (3.0 mol%), Na2CO3 (4.0 equiv), hexadecane as internal
standard (0.2 mmol), 1,4-dioxane (2.0 mL), 1208C, 16 h (reaction time
not optimized). [b] Determined by gas chromatography, all reactions
were carried out twice.
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Scheme 2. Applications of imidazolium phosphane ligands in palla-
dium-catalyzed cross-coupling reactions. Reaction conditions:
1.0 mol% Pd(OAc)2, 2.0 mL 1,4-dioxane, hexadecane as internal stan-
dard. 1a) 3.0 mol% 2, 4.0 equiv Na2CO3, 1.0 mmol 3-bromothiophene,
2.0 mmol 1-octyne. 1b) 1.0 mmol 2-bromomesitylene. 2a) 2.0 equiv
NaOtBu, 2.0 mol% 2, 1.0 mmol 4-chloro-2-methylquinoline, 1.2 mmol
1-benzylpiperazine. 2b) 1.0 mmol 2-bromomesitylene. 3a) 2.0 mol% 2,
1.0 mmol phenylboronic acid, 1.0 mmol 4-chloroanisole. 3b) 1.0 mmol
2-bromomesitylene. 4a) 1.0 mol% Pd(OAc)2, 4.0 mol% 2, 2.0 mL NH3
(0.5m in 1,4-dioxane), 0.2 mmol 1-chloronaphthalene, 10 bar N2.
4b) 0.5 mol% Pd(OAc)2, 2 mol% 2, 0.2 mmol 4-chloro-2-methylquino-
line. Yields were determined by gas chromatography, all reactions were
carried out twice.
shown in Table 5, the reaction was realized four times in very
good yields without the addition of new catalyst.
In summary, a series of new cationic imidazolium-based
phosphanes have been synthesized. These derivatives repre-
sent an interesting class of recyclable ligands because of their
ionic nature. The novel phosphanes are extraordinary stable
towards air as well as water, and have been successfully
applied in various palladium-catalyzed hydroxylations, ami-
nations, and Sonogashira coupling reactions of aryl bromides
and chlorides. Notably, the catalysts can be recycled several
times without significant loss of reactivity. It is clear that the
new ligands offer further potential in modern recycling
technologies such as SILP catalysis.
Received: March 26, 2010
Revised: August 23, 2010
Published online: October 15, 2010
Keywords: catalyst recycling · hydroxylation · palladium ·
.
phosphanes · Sonogashira reaction
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