Macrocyclic palladium(II) complexes in C-C coupling reactions: Efficient catalysis by controlled temporary release of active species

Article abstract of DOI:10.1002/adsc.201000458

Stabilization of palladium species against agglomeration is essential for reasonable catalytic activity in C-C coupling reactions. In contrast to common methods of palladium(O) complex or particle stabilization, a new concept is introduced here: it is demonstrated that a controlled release of palladium from an inactive precatalyst provides stability, too, and leads to high catalytic activity. This paper presents surprising catalytic results for Heck and Suzuki reactions with aryl chlorides and bromides, using three highly stable macrocyclic palladium complexes as catalyst precursors. Three different behaviour patterns for the macrocyclic complexes can be deduced from the evaluation of catalytic activities, UV-Vis spectroscopy, recycling studies of immobilized complexes, and ligand addition experiments, (i) Palladium tetraphenylporphyrin reversibly releases only extremely low amounts of palladium during the reactions, and low coupling activities are observed, (ii) Release of palladium from its phthalocyanine complex is irreversible; cumulative release of palladium into the reaction mixtures leads to high catalytic activity, (iii) Extraordinary results were obtained with a Robson-type complex of palladium, which reversibly releases effectual amounts of palladium into solution under reaction conditions. This controlled release prevents the formation of inactive palladium agglomerates under harsh conditions and leads to high catalytic performances. Even strongly deactivated electron-rich aryl chlorides (4-chloroanisole) can be completely and selectively converted by the in situ formed anionic palladium halide complexes; the addition of typical stabilizing additives (TBAB) was found to be unnecessary. The bimetallic palladium complex is regenerated at the end of the reaction. These results contribute to the current understanding of the active species in C-C coupling reactions of Heck and Suzuki types.

Full text of DOI:10.1002/adsc.201000458

FULL PAPERS
DOI: 10.1002/adsc.201000458
À
Macrocyclic Palladium(II) Complexes in C C Coupling
Reactions: Efficient Catalysis by Controlled Temporary
Release of Active Species
Christoph Rçhlicha and Klaus Kçhlera,*
a
Department of Chemistry, Catalysis Research Center, Technische Universitꢀt Mꢁnchen, Lichtenbergstr. 4, 85747
Garching, Germany
Fax : (+49)-89-289-13183; e-mail: klaus.koehler@ch.tum.de
Received: June 11, 2010; Published online: September 8, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000458.
Abstract: Stabilization of palladium species against dium into the reaction mixtures leads to high catalyt-
agglomeration is essential for reasonable catalytic ac- ic activity. (iii) Extraordinary results were obtained
À
tivity in C C coupling reactions. In contrast to with a Robson-type complex of palladium, which re-
common methods of palladium(0) complex or parti- versibly releases effectual amounts of palladium into
cle stabilization, a new concept is introduced here: it solution under reaction conditions. This controlled
is demonstrated that a controlled release of palladi- release prevents the formation of inactive palladium
um from an inactive precatalyst provides stability, agglomerates under harsh conditions and leads to
too, and leads to high catalytic activity. This paper high catalytic performances. Even strongly deactivat-
presents surprising catalytic results for Heck and ed electron-rich aryl chlorides (4-chloroanisole) can
Suzuki reactions with aryl chlorides and bromides, be completely and selectively converted by the in
using three highly stable macrocyclic palladium com- situ formed anionic palladium halide complexes; the
plexes as catalyst precursors. Three different behav- addition of typical stabilizing additives (TBAB) was
iour patterns for the macrocyclic complexes can be found to be unnecessary. The bimetallic palladium
deduced from the evaluation of catalytic activities, complex is regenerated at the end of the reaction.
UV-Vis spectroscopy, recycling studies of immobi- These results contribute to the current understanding
À
lized complexes, and ligand addition experiments. (i) of the active species in C C coupling reactions of
Palladium tetraphenylporphyrin reversibly releases Heck and Suzuki types.
only extremely low amounts of palladium during the
reactions, and low coupling activities are observed.
(ii) Release of palladium from its phthalocyanine Keywords: C C coupling; homogeneous catalysis;
complex is irreversible; cumulative release of palla- macrocyclic ligands; palladium
À
Introduction
action, and are the actual catalytically active spe-
cies.[9,10,14] The same has been observed repeatedly for
Since their development in the early 1970s, Pd-cata- supported Pd sources.[9,11–14] Although not profiting
À
lyzed C C coupling reactions have enriched organic from stabilizing and electron-donating ligands, such li-
chemistry and fine chemical synthesis.[1–4] In the last gandless Pd species are demonstrated to be highly
few decades, several thousand papers have been pub- active, but also prone to agglomerate and form inac-
lished on the development of new homogeneous and tive Pd metal particles (Pd black) during the reaction.
heterogeneous catalysts for these reactions. Most
Figure 1 presents a simplified schematic view of the
common are phosphine/phosphine oxide and carbene processes concerning palladium transformation during
complexes of Pd,[5,6] pincer ligand complexes,[7] and C C coupling reactions. The reaction usually starts
À
palladacycles.[8] It has been shown, however, that, for with Pd(II) bound in a precursor. This can be a Pd
example, even highly stable pincer ligand complexes complex or simple Pd salts, also immobilized on a
of palladium are only reservoirs for “ligandless” Pd support. Part of the Pd is released into solution (equi-
species, which are released in the first stages of the re- librium a), depending on the complex stability, and
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Figure 1. Simplified model for the equilibria concerning Pd species during C C coupling catalysis.
reduced under reaction conditions to give a coordina- in excess) for Pd(0), such as phosphane and carbene
tively unsaturated Pd(0) species. Reduction of Pd in ligands. Other additives or solvents have been used to
the precursor and subsequent release is also imagina- stabilize the Pd particles. Tetraalkylammonium salts,
ble. The free Pd(0) species can be considered to be a especially tetra-n-butylammonium bromide (TBAB,
mono- or dinuclear Pd complex with solvent mole- so called Jeffery conditions[19]), are among the most
cules, halide ions, and additives (including parts of the prominent. Stabilizers are, in general, unavoidable for
primary precursor complex, like phosphanes) as coor- efficient conversion of non-/deactivated aryl chlorides
dinating ligands. It is most likely anionic.[15] The Pd in at high temperatures.
this species will, at least temporarily, be coordinative-
In contrast, this work takes on a new concept. As
ly unsaturated, and thus start agglomerating and build Figure 1 shows, stabilization of Pd could also be ach-
larger assemblies of Pd(0) atoms (equilibria b), from ieved by a constant but reluctant release of Pd from
clusters to nanoparticles, depending on a variety of the precursor. If the release equilibrium a is shifted
parameters. All of these species enter (and are inter- towards the initial Pd compound and is reversible,
converted by) the catalytic cycle.[16–18] However, the only low amounts of “free” Pd will be present in solu-
coupling activity is expected to decrease with the size tion. Since the probability of agglomeration is then
of the Pd aggregates. Finally, the irreversible precipi- significantly lower, overall catalytic activity should be
tation of Pd black will occur, shifting the equilibria b increased. This corresponds to the situation of
towards inactive species, and stalling catalytic activity. “homeo
One crucial point for high catalytic activity, as it be very valuable for high C C coupling activity.
seems, is to keep Pd(0) in solution (i) long enough This principle is, however, commonly not consid-
ACHTUNGTRENUNpNG athic” Pd content, which has been found to
[17]
À
and (ii) electron-rich enough, but (iii) unsaturated, so ered or applied. This might be because most publica-
À
that it effectively catalyzes the reaction before it ag- tions on C C coupling reactions do not specifically
glomerates to Pd black and is deactivated.
consider transformations of the Pd precursor during
High initial catalyst amounts, strongly reducing con- the reaction and because release of Pd (or in other
ditions, low reaction rates (TOFs, that is, deactivated words, the destruction of the initial Pd compound) is
substrates for the coupling reaction), and high tem- something intrinsic to many Pd precatalysts that have
peratures will lead to faster agglomeration of Pd and been applied, especially under harsh conditions. This
lower catalytic activity.[16,17] Typical means to prevent includes both homogeneous and heterogeneous preca-
this deactivation are the addition of stabilizers (often talysts.[9,10,13,14] In light of these aspects, it is essential
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Macrocyclic Palladium(II) Complexes in C C Coupling Reactions
to find a precatalyst that remains stable even under plex I, Figure 2) as the precatalyst in the Heck reac-
the harsh conditions needed for the activation and tion of styrene with highly deactivated 4-chloroani-
conversion of demanding substrates (aryl chlorides), sole.
but generates sufficient amounts of coordinatively un-
saturated Pd.
In this paper catalytic results and mechanistic inves-
tigations on the behaviour of macrocyclic Pd com-
À
Macrocyclic Pd complexes (e.g., phthalocyanines, plexes in C C coupling reactions are presented. The
porphyrins, etc.) are characterized by a very high ther- bimetallic complex I and two classical macrocyclic Pd
mal and chemical stability, complete coordinative sat- complexes [palladium(II) phthalocyanine – PdPc; and
uration of the central atom, and a preference for the palladium(II) meso-tetraphenylporphyrin – PdTPP;
Pd(II) oxidation state. Such compounds possess an ex- Figure 2] were applied in Heck and Suzuki reactions.
traordinarily high complex stability, so the release of As it will be shown, in accordance with the new con-
Pd under coupling reaction conditions would be very cept, the interplay between Pd release and recoordi-
limited and its equilibrium largely shifted towards nation is crucial for high catalytic performances with
bound Pd. Since macrocyclic ligands are usually very these precatalysts.
rigid, they cannot dissociate partially. Such complexes
are very applicable model systems for the presented
À
new concept, since they are not able to catalyze C C Results and Discussion
coupling reactions in their undissociated form, but Pd
released by them would be “completely unsaturated” Characteristics of Complex I
and, thus, a highly active catalyst. This concept is spe-
À
cifically applicable for C C coupling reactions, where The bimetallic Pd complex I was prepared via proton-
the species of highest activity (low nuclear halopalla- templated synthesis of the free ligand Ia,[25a] and sub-
date species) is formed in situ from the precursor, re- sequent metallation with two equivalents of palladium
actants and additives; the ligands, therefore, have acetate (Scheme 1). Complex I has been structurally
mainly the task of avoiding the deactivation.
and spectroscopically characterized before.[25b] How-
À
Although Pd-catalyzed C C coupling reactions ever, the stability of the complex has not been ad-
have widely been used to modify macrocyclic ligands dressed, but is an essential factor for the presented
like porphyrins, porphyrazins or phthalocyanines,[20] new concept. Thus, experiments on the stability of I
literature examples of coupling reactions catalyzed by and its capability of releasing low amounts of Pd
Pd macrocycles are seldom. Only in recent years have under conditions similar to those of the reactions
the first examples of such reactions been published. were performed.
For example, Kostas et al. have used a water-soluble
Pd porphyrin as a catalyst for the Suzuki reaction of
aryl bromides with phenylboronic acid in pure Chemical Stability of Complex I
water.[21] Wan et al. reported the first use of a Pd por-
phyrin for the Heck reaction of aryl iodides in ionic Complex I was found to be thermally stable in bulk
liquids as the reaction medium. The intact Pd porphy- until up to 3508C (beyond which it may decompose
rin was detected by UV-Vis spectroscopy after the re- with explosive force). It can be boiled in water for
action.[22] We have recently reported[23] the use of a several hours, or dissolved in trifluoroacetic acid,
macrocyclic Robson-type complex[24,25b] of Pd (com- without decomposition, which was proved by NMR
Figure 2. Macrocyclic complexes of Pd(II) applied in this study: a Robson-type bimetallic complex (I), palladium phthalocya-
nine (PdPc) and palladium meso-tetraphenylporphyrin (PdTPP).
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Scheme 1. Synthesis scheme for the bimetallic Pd-complex I.
1
spectroscopy. The H NMR spectra of a saturated so-
In summary, in solutions of I, a release-recatch
lution of I in DMF-d7 are identical at 258C, 60 8C and equilibrium for Pd(II) exists, which is strongly shifted
808C. Complex I also withstands typical reductants towards the intact complex.
for Pd(II): stirring a solution of I with triethylamine
as a reductant did not lead to the formation of Pd
metal, while Pd
ACHTUNGTRENNUNG(OAc)2 is rapidly reduced to the metal Heck Coupling using Complex I
under these conditions. A solution of I could be
À
purged with H2 for several hours at room tempera- As the first C C coupling reaction, we have applied
ture without any visible changes; controlled heating the macrocyclic complexes in the Heck coupling of
resulted in the slow formation of Pd metal at 708C. aryl halides with styrene. The anticipated modus of in
Stirring of a solution of I in DMSO with sodium for- situ release and recatch of active species from the
mate also did not result in the formation of palladium macrocyclic precatalyst is very similar to the findings
metal, but led to the precipitation of an insoluble for heterogeneous catalysts in this reaction. Thus, re-
green compound. It was shown by elemental analysis action conditions which were found to be effective for
that it still contains the Pd from its precursor complex heterogeneously catalyzed Heck reactions[27] were
(30.9 wt% Pd found), while no chlorine content was also applied for these homogeneous precatalysts.
detected. This green precipitate is also generated Sodium acetate was used as the base for reactions
from solutions of I under non-reducing but alkaline with aryl bromides. For aryl chlorides Ca(OH)2 was
conditions (addition of soda, NaOH, amines). Thus, used, and TBAB was initially added as catalyst stabil-
we assume that the green compound is a result of ex- izer. The reactions were performed in NMP. Since it
change of the perchlorate counterions, most probably was found unnecessary and even disadvantageous to
against hydroxide ions. Further characterization of work under an inert atmosphere, all of the reactions
this compound is difficult due to the very low solubili- were performed in air.
ty.
Table 1 lists the results of catalytic experiments of
the Heck reaction with complex I. Good selectivity to
the (substituted) trans-stilbene was obtained, while
the corresponding gem-substituted olefin was found
as a side product in minor quantities (usually about 1/
Scope of Pd Release from Complex I
The chelating effect disfavours a release of Pd from 10 of the trans product). cis-Stilbene or its derivatives
the macrocyclic ligand. As mentioned, from NMR were generally not detected.
studies we know that I does not release significant
Practically quantitative reactions (99% conversion
amounts of Pd when heated; neither should PdPc nor and >70% yield of trans-stilbenes) could be achieved
PdTPP. Very strong chelators such as the thio-Mich- for non- and deactivated aryl bromides (Table 1, en-
ler’s ketone [TMK, 4,4’-bis(dimethylamino)thiobenzo- tries 1 to 4). Comparably low amounts of precatalyst
phenone] show an extremely high affinity to heavy led to successful reactions in several cases (entries 2
metal ions such as Pd(II), with which they form a and 3). In the case of the reaction with bromobenzene
complex with an intense, typical colour. This has been (entry 1), the catalytic activity was even high enough
applied in methods for trace analysis of such to promote the consecutive reaction of stilbene to tri-
metals.[26] When a few drops of a solution of I are phenylethylene (14% yield). In turn, performing the
added to a buffered aqueous solution (pH 3) of TMK, reaction with trans-stilbene as the initial olefin instead
the colour change from yellow to purple indicates of styrene, 67% conversion and a yield of triphenyl-
uptake of Pd(II) by TMK.
ethylene of 61% were obtained.
Aryl chlorides were also effectively coupled using
complex I, with similar selectivities to the trans-stil-
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Macrocyclic Palladium(II) Complexes in C C Coupling Reactions
Table 1. Conditions, conversions and yields for Heck reactions of styrene and different aryl halides catalyzed by complex I.[a]
Entry Cat. [mol%] R/X
TBAB [mmol] T [8C] Reaction Time [h] Conversion of Styrene [%] Yield [%]
1
2
3
4
5
6
7
8
0.5
0.05
0.05
0.5
0.5
0.025
0.5
0.5
0.5
0.0375
0.0375
0.0375
0.0375
0.0375
0.0425
H/Br
–
–
–
–
0.9
0.9
–
0.9
0.9
0.9
0.2
–
140
140
140
140
140
140
140
150
160
160
160
160
160
160
160
8
6
6
8
6
6
6
6
6
6
6
6
6
6
6
98
92
99
99
100
83
26
100
48
81
94
90
94
98
94
70[b]
51
71
92
69
48
3
83
36
63
79
75
64
81
80
Me/Br
MeO/Br
MeO/Br
Ac/Cl
Ac/Cl
Ac/Cl
H/Cl
9
MeO/Cl
MeO/Cl
MeO/Cl
MeO/Cl
Me/Cl
MeO/Cl
F/Cl
10
11
12
13[c]
14[c]
15[c]
–
–
–
[a]
Reaction conditions: aryl halide (3.0 mmol), styrene (1.5 mmol), NaOAc (X=Br, 1.8 mmol) or Ca(OH)2 (X=Cl,
0.9 mmol), NMP (5 mL), air.
An additional yield of triphenylethylene (14%) was obtained.
LiBr (0.6–0.7 mmol) was added.
[b]
[c]
benes (entries 5 to 12). Non- and deactivated aryl which ranks among the best reaction results for these
chlorides were converted effectively. Under non-opti- substrates (styrene+4-chloroanisole) in the current
mized conditions, TBAB is required to enhance con- literature.
versions and yields (compare entries 5 and 7).
The superiority and necessity of the stabilizing
According to our concept, complex I releases effect of I becomes evident when comparing this
“ligand-free” Pd into the solution, whose concentra- result to the performance of a corresponding amount
tion highly determines the catalytic activity: concen- of simple Pd
ACHTUNGTRENNUNG
trations which are too low result in a slow reaction observed for PdAHCTUNTGRENNUNG
progress, while higher amounts of catalyst lead to a and in the presence of 0.9 mmol TBAB only about
more pronounced agglomeration of Pd.[16,17] There half of the yield using complex I was obtained.
should be an ideal initial amount of I which generates
To further enhance conversions and yields, LiBr
the optimal amount of active Pd. We have exemplari- can be added (entries 13 to 15). The Lewis-acidic Li
À
ly performed an optimization of the precatalyst cation may weaken the C Cl bond and enhance the
amount for the Heck coupling of styrene with highly rate-limiting step of oxidative addition. Bromide ions,
deactivated 4-chloroanisole, and found an initial load- on the other hand, have been stated to stabilize (low
ing of 0.0375 mol% of I to be optimal in this case. nuclear) Pd species in solution, as well as enhance Pd
Conversion and yield were approximately doubled dissolution from nanoclusters.[29] The beneficial effect
compared to the initial results with 0.5 mol% of I of LiBr may also be an indicator that the active spe-
(Table 1, entries 9 and 10). Under these conditions cies is a (most probably anionic) bromo complex of
TBAB was found to inhibit the reaction. Conversion Pd.
and yield increased with decreasing amounts of
TBAB (compare entries 10 and 11). We suggest that,
at the high reaction temperature, TBAB is partially Heck Coupling using PdPc or PdTPP
converted to tributylamine through SN2 substitution
or Hofmann elimination.[28] The amine may block co- Table 2 lists the results for Heck coupling reactions
ordination sites on Pd and reduce Pd(II) species with PdPc or PdTPP as precatalysts. PdTPP was
which facilitates the formation of Pd black. Accord- found to give only low yields with aryl bromides
ingly, the reaction was also performed without TBAB. (entry 1) and to be ineffective for aryl chlorides (en-
Practically quantitative conversion (90%; yield: 75%) tries 4 and 5). In comparison, PdPc gave better con-
was observed in this reaction (Table 1, entry 12), versions and selectivities. Also non- and deactivated
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Table 2. Conditions, conversions and yields for Heck reactions of styrene and different aryl halides catalyzed by PdPc and
PdTPP.[a]
Entry
Cat.
[mol%]
R/X
TBAB
[mmol]
T
[8C]
Reaction
Time [h]
PdPc
Conversion of
PdTPP
Conversion of
G
G
Yield [%]
Yield [%]
11
Styrene [%]
Styrene [%]
1
2
3
4
5
6
7
8
0.5
0.1
0.5
0.5
0.5
0.5
0.5
0.075
H/Br
–
–
–
0.9
–
0.9
0.9
0.9
140
140
140
140
140
150
160
160
8
6
8
6
6
6
6
6
35
44
12
49
5
94
10
54
14
18
1
38
<1
81
0
62
Me/Br
MeO/Br
Ac/Cl
Ac/Cl
H/Cl
9
18
17
0
<1
<1
MeO/Cl
MeO/Cl
45
[a]
Reaction conditions: aryl halide (3.0 mmol), styrene (1.5 mmol), NaOAc (X=Br, 1.8 mmol) or Ca(OH)2 (X=Cl,
0.9 mmol), NMP (5 mL), air.
aryl chlorides could be coupled using PdPc (entries 6
Activated substrates were converted in short reac-
and 8). However, conversions and yields were always tion times and 100% selectivity (Table 3, entries 4 to
significantly lower than with complex I, and addition 6). However, compared to simpler precatalyst systems
of TBAB was required to give reasonable activity in (Pd salts) for example, the reaction is comparatively
reactions with aryl chlorides (compare entries 4 and slow at medium temperatures (Table 3, entries 1 and
5).
5). This may be considered as a sign that the release
The formation of Pd black was observed only for of Pd from I is a temperature-driven process. Interest-
aryl chlorides and at temperatures above 1308C in re- ingly, PdPc gave comparable or higher conversions
actions with complex I. Pd black was not observed in than complex I and similar selectivities in reactions
the experiments with PdTPP. This is a first indicator with deactivated substrates, in contrast to the results
that complex I releases low but effective amounts of in the Heck reaction.
Pd into the solution, while from PdTPP the amounts
Minor quantities of benzene and biphenyl were ob-
of Pd released are too low and low conversions are served as side products in reactions with non- and de-
observed. In the case of PdPc, the intense blue/green activated substrates. These products arise from the de-
colour of the pigment prevents any meaningful judge- boronation and the Pd-driven oxidative coupling of
ment of Pd black formation.
phenylboronic acid.[30] The latter reaction may lead to
a more pronounced reduction of Pd(II) species in the
reaction mixture and, thus, to a higher probability of
Pd metal formation and agglomeration than in the
Heck reaction. Indeed, we observed the formation of
Pd(0) agglomerates even at room temperature when
Suzuki Coupling of Aryl Bromides using Complex I
or PdPc
We also applied I and PdPc in the Suzuki coupling of the simple palladium salt PdACTHNUTRGEN(UNG OAc)2 was added to a
differently substituted aryl halides with phenylboronic Suzuki reaction mixture. Accordingly, we observed
acid. PdTPP was not applied, since it had already the formation of Pd black also in undemanding reac-
shown low activity in the Heck reaction. Two sets of tions (bromobenzene as substrate) with I, when the
reaction conditions (dilute and concentrated) were concentrated (i.e., more reducing) conditions B were
used. Again, ambient atmosphere was found to give applied. Under more dilute conditions A, formation
better results than working under an inert atmos- of Pd black was not observed; however, at high cata-
phere.
lyst loadings (entry 1), the precipitation of a green
Sodium carbonate was used as the base for reac- powder occurred (hydroxide form of complex I, vide
tions with aryl bromides. As Table 3 shows, differently supra).
substituted aryl bromides were converted to the cor-
Since I was found unchanged after the reaction (as
responding biphenyls with very good selectivities (90– shown by UV-Vis spectroscopy, see Section – Spectro-
100%) under these conditions. In general, reactions scopic Analysis of Reaction Mixtures), it could princi-
with higher concentrations of the reactants (“condi- pally be reused as a precatalyst for a repeated cou-
tions B”) gave better results than dilute reactions pling reaction. Thus, complex I and PdPc were both
(“conditions A”). This difference became more pro- immobilized to ensure facile separation from the reac-
nounced with decreasing activation of the aryl bro- tion mixtures, and used in repeated experiments in a
mide.
mild Suzuki reaction. In these studies PdPc showed
high initial activity, which dropped severely after the
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Macrocyclic Palladium(II) Complexes in C C Coupling Reactions
Table 3. Conditions, conversions and yields for Suzuki coupling reactions of phenylboronic acid and different aryl bromides
catalyzed by complex I and PdPc.[a]
Entry
R
T
[8C]
Reaction
Time [h]
Conditions
Complex I
PdPc
Conversion of
Conversion of
Yield [%]
Yield [%]
ArBr [%]
ArBr [%]
1
2
3
4
5
6
7
8
9
H
H
H
NO2
NO2
CN
Me
Me
MeO
MeO
60
16.5
6
6
1
1
2
6
6
6
A[c]
A
B
A
A
A
A
B
64
95
98
100
69
100
88
99
53
89
78[b]
112[b]
94[b]
100
69
100
88
99
100
100
100
80
100
100
100
100
100
85
92
100
74
100
84
99
73
92
80[b]
84[b]
100
74
93
82
88
66
87
A
B
46
83
10
6
[a]
Conditions A: PhB(OH)2 (1.2 mmol), aryl bromide (1.0 mmol), Na2CO3 (1.2 mmol), precatalyst (0.5 mol%), NMP
(5 mL), H2O (1.5 mL), air. Conditions B: PhB(OH)2 (3.2 mmol), aryl bromide (3.0 mmol), Na2CO3 (3.2 mmol), precata-
lyst (0.1 mol%), NMP (5 mL), H2O (2 mL), air.
Yields may contain contributions from oxidative coupling of PhB(OH)2.
Reaction was performed with 2.5 mol% of precatalyst.
[b]
[c]
first run. Complex I revealed a lower initial activity is also possible in the Suzuki reaction. When complex
but a higher durability over several reuses in this re- I was used in the absence of TBAB, the coupling re-
action. This is in line with our observations (see Sec- action of 4-chloroacetophenone was only slightly af-
tion – Ligand Addition Experiments) that the release fected (compare entries 3 and 4). On the other hand,
of Pd from PdPc is irreversible, so that most Pd is lost significantly lower conversions were obtained for
after the first run, while complex I can recoordinate PdPc in reactions without TBAB (compare entries 1
Pd and, thus, remain active over further reuses. A and 2).
more detailed description and discussion of these ex-
Non- or deactivated aryl chlorides (namely chloro-
periments is given in the supporting information of benzene and 4-chloroanisole) could also be converted
this article.
using I and PdPc, but with lower conversions and
yields. Increased temperatures had a positive effect
on the conversion, but lower selectivities to the cou-
pling product were obtained (Table 4, entries 10, 11,
13 and 16). For chlorobenzene, TBAB was also found
to be necessary in reactions with complex I (entry 12).
In all of the experiments with complex I Pd black
Suzuki Coupling of Aryl Chlorides using Complex I
or PdPc
We were soon interested whether I and PdPc were
also able to achieve the Suzuki coupling of aryl chlor- was observed, with the exception of experiments at
ides. The base was changed to NaOH in these reac- 808C under dilute conditions. This implies again that
tions. As shown in Table 4, the Suzuki coupling of the release of Pd from I may be temperature-driven.
aryl chlorides can also be catalyzed by complex I and
PdPc. The reaction of an activated substrate (4-chlor-
oacetophenone) was possible even at 808C (Table 4, Ligand Addition Experiments
entries 5 to 8); however, a higher temperature of
1208C led to better results in shorter reaction times It is clear that the C C coupling reactions can hardly
(entries 1 to 4). In the presence of TBAB, both mac- occur at the bound and saturated Pd in the intact
rocyclic complexes gave similar conversions and complexes. As mentioned, we have observed the for-
yields in this reaction, regardless of whether condi- mation of Pd black in several cases during C C cou-
À
À
tions A or B were applied (entries 1, 3 and 5).
pling reactions. Accordingly, Pd must have left the
Since we had found that the Heck coupling of aryl macrocyclic complex in an overall significant amount
chlorides could also be performed in the absence of under the reaction conditions.
TBAB as stabilizer, we were interested whether this
Adv. Synth. Catal. 2010, 352, 2263 – 2274
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Christoph Rçhlich and Klaus Kçhler
FULL PAPERS
Table 4. Conditions, conversions and yields for Suzuki coupling reactions of phenylboronic acid and different aryl chlorides
catalyzed by complex I and PdPc.[a]
Entry
R
T
[8C]
Reaction
Time [h]
TBAB
[mmol]
Conditions
Complex I
PdPc
Conversion of
G
Conversion of
Yield [%]
Yield [%]
ArCl [%]
ArCl [%]
1
2
3
4
5
6
7
8
Ac
Ac
Ac
Ac
Ac
Ac
Ac
Ac
H
H
H
H
MeO
MeO
MeO
MeO
120
120
120
120
80
80
80
80
120
120
140
140
140
140
120
150
6
6
6
6
17
17
17
17
6
6
6
6
6
6
6
6
0.6
–
0.9
–
0.6
–
–
0.9
0.6
0.9
0.9
–
0.6
0.9
1.9
A
A
B
B
A
A
B
B
A
B
B
B
A
B
B
95
78
70
61
53
30
61
91
75
70
61
47
20
57
91
3
64
91
0
64
41
26
12
53
57
26
53
32
13
7
53
9
47
35
66
64
34
11
23
41
32[b]
17[b]
28[b]
8[b]
2
<1
12
11
42[b]
14[b]
1[b]
10
11
12
13
14
15
16
39
23
17
3
38
6
[a]
Conditions A: PhB(OH)2 (1.2 mmol), aryl chloride (1.0 mmol), NaOH (1.2 mmol), precatalyst (0.5 mol%), NMP (5 mL),
H2O (1.5 mL), air. Conditions B: PhB(OH)2 (3.2 mmol), aryl chloride (3.0 mmol), NaOH (3.2 mmol), precatalyst
(0.1 mol%), NMP (5 mL), H2O (2 mL), air.
[b]
Yields may contain contributions from oxidative coupling of PhB(OH)2.
A useful test for this is the addition of the free rene and bromobenzene in the presence of I and
ligand to the reaction. This shifts the equilibrium of 1 equivalent of the free ligand Ia, only a low amount
Pd release in favour of the coordinated Pd, which of product was formed (Table 5, entry 2). At a I/Ia
should result in a drop in activity if the reaction ratio of 1/3, product formation was no longer ob-
occurs at released Pd and not at the intact complex. served (not shown). This demonstrates the strong che-
In fact, when performing the Heck reaction with sty- lating ability of Ia, since even ultra-trace amounts of
Pd have been shown to effectively catalyze the Heck
reaction of bromobenzene and styrene.[31]
The practically quantitative Heck reaction with 4-
chloroanisole (as shown in Table 1, entry 14) also did
not occur anymore, when 1 equivalent of Ia was pres-
ent in the reaction mixture, thus changing the amount
of released Pd away from the found optimum value
(Table 8, entry 2).
Table 5. Effect of free ligand addition on the yield in the
Heck coupling of styrene and bromobenzene. Pd recatching
ligand Ia hinders the reaction while insoluble H2Pc has no
effect. H2TPP may still leave enough Pd in solution for the
Heck reaction to occur unaffected.
In Suzuki reactions of 4-chloroacetophenone con-
taining a 1:1 ratio of complex I and Ia, only trace
amounts (1–2%) of the coupling products were ob-
served (Table 7, entry 2). All of these results prove
that the reaction is catalyzed by Pd that is released
from complex I and indicate a release-recatch equilib-
rium that is largely in favour of coordinated Pd.
The effect of ligand addition was also tested for
H2Pc and H2TPP in the Heck coupling with bromo-
benzene, and the Suzuki reaction of 4-bromotoluene.
As Table 5 and Table 6 show, addition of H2Pc to the
Entry Precatalyst Ligand
added
Ligand Amount
[equiv. of cat.]
Yield[a]
[%]
1
2
3
4
I
I
I
I
None
Ia
H2Pc
70 (14)
13 (0)
64 (20)
66 (19)
1
1
1
H2TPP
[a]
Reaction conditions: as described in Table 1, entry 1;
yields in parentheses are yields of triphenylethylene.
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À
Macrocyclic Palladium(II) Complexes in C C Coupling Reactions
Table 6. Effect of free ligand addition on the yield in the
Suzuki coupling of 4-bromotoluene and phenylboronic acid.
Pd recatching ligands Ia and H2TPP show a negative effect
on the reaction, while insoluble H2Pc has no effect.
reaction mixture did not have a negative effect in
these reactions, regardless of whether I or PdPc was
used as precatalyst (Table 5 entry 3, Table 6 entries 2
and 6). H2Pc was not dissolved in the reaction mix-
ture. This implies that the release of Pd from PdPc is
an irreversible process, since the recapture of Pd is
not possible for insoluble H2Pc.
When H2TPP was added to the reaction mixture,
significantly lower yields were obtained in the Suzuki
coupling with 4-bromotoluene (Table 6, entries 3, 4
and 7). H2TPP is soluble in the reaction mixture, and
may thus coordinate Pd that was released from I or
PdPc, making it unavailable for the catalytic reaction.
This indicates that the release of Pd from PdTPP may
be reversible, but the bound Pd is too strongly fav-
oured so that low catalytic activities are observed.
The presence of H2TPP did, however, hardly affect
the yield in the Heck reaction of styrene and bromo-
benzene (Table 5, entry 4). As mentioned before, this
reaction can be performed with trace amounts of Pd.
We have, in fact, observed that in a solution of
Entry Precatalyst Ligand
added
Ligand Amount
[equiv. of cat.]
Yield[a]
[%]
1
2
3
4
5
6
7
I
I
I
I
None
H2Pc
H2TPP
H2TPP
None
H2Pc
88
86
40
29
82
80
11
1.4
1
2
PdPc
PdPc
PdPc
1
1
H2TPP
[a]
Reaction conditions: as described in Table 3, entry 7.
H2TPP and PdACHTNUGTRNEUNG(OAc)2 in NMP, only about 17%
Table 7. Influence of the order of addition of Ia and
Pd(OAc)2 on the yield of Suzuki coupling product.
(NaOAc as base) or 2% (soda as base) of H2TPP are
converted to PdTPP after stirring for 3 h at 1408C.
The uptake of Pd by this ligand may, thus, be depen-
dent on the availability and type of base, the intricate
kinetics, and several other factors. In turn, H2TPP
may still leave enough Pd in solution for the Heck re-
action to occur undisturbed, but affects the Suzuki re-
action.
The stabilizing effect of the release-recatch equilib-
rium can be nicely demonstrated with the following
experiments. When Suzuki reactions with 4-chloroace-
tophenone were performed by directly using corre-
ACHTUNGTRENNUNG
Entry
Catalyst
Yield [%]
A[a]
B[b]
1
2
3
4
5
Complex I
75
<1
65
68
32
61
2
64
61
25
I+1 equiv. Ia
Ia+2 equiv. Pd
Ia+2 equiv. PdACHTUNGTRENNUNG
G
sponding amounts of Ia and PdACTHUNGTERNNU(G OAc)2, seemingly fast
in situ formation of complex I occurred in the cold re-
action mixture. Yields of the coupling product (65
and 64%, respectively) were then observed, similar to
when complex I had been used (Table 7, entry 3).
Similar yields (68 and 61%) were also obtained when
2 equiv. PdACHTNUTRGNEU(GN OAc)2 +Ia after 5 min
[a]
Reaction conditions: as described in Table 4 entry 2.
Reaction conditions: as described in Table 4 entry 4.
[b]
Pd
ture containing ligand Ia (entry 4). However, if solely
Pd(OAc)2 was first added to the Suzuki reaction mix-
ACHTUNGRTENUN(NG OAc)2 was added to an already hot reaction mix-
Table 8. Influence of the order of addition of Ia and
Pd(OAc)2 on the yield of Heck coupling product.
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
ture, the formation of Pd agglomerates was observed
already at room temperature. Heating this mixture to
the reaction temperature and adding Ia after 5 min re-
sulted in low yields (32 and 25%, entry 5). Part of the
Pd had already been deactivated before it could be
protected by the strongly chelating ligand, and low-
ered activities were observed. Table 8 summarizes the
analogous experiments for the Heck coupling reaction
with 4-chloroanisole. When corresponding amounts of
Entry
Catalyst
Yield[a] [%]
1
2
3
4
5
Complex I
81
0
69
14
4
I+1 equiv. Ia
Ia+2 equiv. Pd
Ia+2 equiv. PdACHTUNGTRENNUNG
the ligand Ia and PdACHTUNTRGNEUNG(OAc)2 were given to the reaction
G
mixture (before heating) instead of the preformed
complex I, 89% conversion of styrene and 69% yield
of 4-methoxystilbene were still observed, comparable
to the results obtained by directly using complex I
2 equiv. PdACHTNUTRGNEU(GN OAc)2 +Ia after 5 min
[a]
Reaction conditions: as described in Table 1 entry 14.
Adv. Synth. Catal. 2010, 352, 2263 – 2274
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Christoph Rçhlich and Klaus Kçhler
FULL PAPERS
(entry 3). Thus, the formation (also in situ) of the
macrocyclic complex I led to highly effective catalytic
coupling.
On the other hand, adding PdACHTNUTRGNEUNG(OAc)2 after 5 min to
an already hot reaction mixture containing ligand Ia
led to significantly lower conversion (39%) and yield
(14%, entry 4). Moreover, when adding the ligand
after 5 min to the hot reaction mixture containing
PdACHTUNGTRENNUNG(OAc)2, the Heck reaction proceeded to an even
less extent (4% yield, entry 5), or did not occur.[23]
Again, direct exposure of free Pd to the warm reac-
tion mixture is found to be disadvantageous for the
Heck coupling, even in the presence of Ia, because
the initially unprotected Pd will enter deactivation
pathways to a significant extent. Only a preformed
macrocyclic complex stabilizes the Pd from the begin-
ning and leads to high catalytic activities.
Spectroscopic Analysis of Reaction Mixtures
UV-Vis spectroscopy turned out to be a viable tool to
observe possible changes of complex I (which has an
absorption band in the visible spectrum) in the reac-
tion mixture. UV-Vis spectra of Suzuki reaction solu-
tions were recorded after the reaction. We only re-
corded spectra for Suzuki reactions with bromo- and
chlorobenzene because substituted biphenyls always
showed an absorption band reaching into the visible
region, superimposed by the absorption band of I. As
Figure 3 shows, no absorption of the free ligand Ia
was observed after the coupling of bromobenzene, in-
stead complex I was observed in its hydroxide form.
This implies, once again, that Pd, which has left the
complex and performed the catalytic coupling, was re-
coordinated afterwards [with re-oxidation to Pd(II)].
In the Suzuki coupling of chlorobenzene, formation
of Pd black had been observed. Accordingly, in the
UV-Vis spectrum of the respective reaction solution,
the free ligand Ia is detected. However, complex I is
present and detectable in the solution after the reac-
tion, once again highlighting the stability of I under
reaction conditions. The same findings could be ob-
tained for the respective Heck reactions of bromo-
and chlorobenzene (not shown), but the spectra are
Figure 3. Comparison of UV-Vis spectra (in NMP). Upper
panel: Filtered reaction solution of entry 3 in Table 3, intact
complex I, and saturated solution of complex I in its hydrox-
ide form. Lower panel: Filtered reaction solution of entry 9
in Table 4, free ligand Ia treated with soda in NMP, and
complex I in its hydroxide form.
superpositioned by the intense peak of the coupling phthalocyanine (H2Pc) is insoluble and precipitates.
product. This may actually be the driving force for the release,
In summary, the coupling reaction is catalyzed by making it thermodynamically favourable, but irrever-
Pd that is released slowly and in low amounts from I. sible. Since the recapture process cannot occur, free
Under the conditions applied for aryl bromides, the Pd accumulates in the reaction mixture, which leads
Pd in solution is stable long enough to be re-coord- to deactivation after some reaction time. Accordingly,
ACHTUNGTRENNUNG
From ligand addition and also recycling experi- is not powered by free ligand precipitation (H2TPP is
ments (see Supporting Information) it can be pre- soluble in the reaction mixtures), so not enough free
sumed that PdPc also releases Pd, but the free ligand Pd is released into the solution, or it is recoordinated
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À
Macrocyclic Palladium(II) Complexes in C C Coupling Reactions
too fast. In other words, the Pd release does not Experimental Section
match the demand for active Pd of the coupling reac-
tion. This results in very low catalytic activities.
Synthesis of Bimetallic Complex I
To a solution of PdACHTNUTRGNE(NUG OAc)2 (90.0 mg, 0.4 mmol) in acetoni-
trile (5 mL), a suspension of Ia (128.1 mg, 0.2 mmol) in ace-
tonitrile (6 mL) was added dropwise. The resulting yellow
suspension was stirred at 808C for 2 h and then allowed to
stir at room temperature for 62 h. The resulting precipitate
was collected by filtration, washed with acetonitrile and di-
ethyl ether, and dried under vacuum. The crude product was
recrystallized by slow addition of diethyl ether to a saturat-
ed DMF solution to furnish a yellow powder; yield:
135.1 mg (83%). Note: Although the complex can be han-
dled without necessary precautions at ambient conditions, it
decomposes with explosive force when heated above 3508C.
1H NMR (400 MHz, DMSO-d6): d=8.17 (s, 4H, -CHN),
7.79 (s, 4H, Ar-H), 3.69 (s, 8H, -N-CH2-), 2.32 (s, 6H,
Conclusions
À
Stabilization of the catalytic Pd species in C C cou-
pling reactions can be achieved by controlled in situ
release of active species by stable (inactive) precur-
sors. This controlled release prevents the formation of
inactive Pd agglomerates under harsh conditions, and
leads to high catalytic performances even for deacti-
vated substrates. Macrocyclic Pd complexes, combin-
ing thermal and chemical stability (especially complex
stability), were chosen as model precatalysts which
are catalytically inactive unless Pd is released by
them. The applicability of the presented new concept
was verified by using them in Suzuki and Heck cou-
pling reactions. Three different behaviour patterns of
macrocyclic Pd complexes as precatalysts in C-C cou-
pling reactions were identified.
1
-CH3), 2.05 (s, 4H, -CH2-); H NMR (400 MHz, TFA-d): d=
9.25 (s, 4H, -CHN), 9.03 (s, 4H, Ar-H), 5.19 (s, 8H,
-N-CH2-), 3.78 (s, 6H, -CH3), 3.64 (s, 4H, -CH2-) ; elemental
analysis, calcd. % : C 35.40, H 3.22, N 6.88; found: C 35.35,
H 3.50, N 6.95.
General Procedures for Catalytic Tests
Firstly, PdTPP is too stable for catalytic activity. It
releases only very low amounts of Pd into solution.
The free ligand was found to coordinate Pd released
from other Pd sources and to stall its activity. Low ac-
Catalytic reactions were performed in sealed 45-mL glass
tubes in a parallel synthesis unit (Radleys). Samples for re-
action analysis were obtained by extraction with aliquots of
tivities were observed even for demandless substrates. H2O/CH2Cl2, and drying the organic phase over MgSO4.
Substances were identified and quantified by gas chromatog-
raphy (GC) on a Hewlett Packard HP 6890 equipped with
an HP-1 column and a flame ionization detector (FID). Di-
ethylene glycol dibutyl ether (DEGBE) was used as an in-
ternal standard.
Suzuki coupling, conditions A: Phenylboronic acid
(1.2 mmol), base (1.2 mmol; NaOH was mostly added as a
cut pellet), optionally TBAB (0.6 mmol), DEGBE (120 mg
as internal standard for GC analysis), and aryl halide
(1.0 mmol) were introduced into a reaction tube. The cata-
Secondly, PdPc releases Pd into solution. This re-
lease is irreversible due to the insolubility of the free
ligand. Accordingly, high activities in C C coupling
reactions were observed. However, since Pd accumu-
lates in the solution, the presence of stabilizing addi-
tives (TBAB) can still be essential for demanding re-
action conditions. Because of its decomposition, it is
not reusable when immobilized.
À
Thirdly, bimetallic Robson complex I was shown to
be an essentially stable complex. In contrast to PdPc lyst was added either in substance before all other reagents,
or via an aliquot of a stock solution in NMP where applica-
ble. NMP was added to give 5 mL in sum, and water
(1.5 mL) was added shortly before placing the reaction tube
in the heating source. The reaction was performed with vig-
orous stirring (800–1000 rpm) at the desired temperature for
the respective reaction time.
Suzuki coupling, conditions B: Phenylboronic acid
(3.2 mmol), base (3.2 mmol; NaOH was mostly added as a
cut pellet), optionally TBAB (0.9 mmol), DEGBE (180 mg
as internal standard for GC analysis), and aryl halide
(3.0 mmol) were introduced into a reaction tube. Catalyst
was added either in substance before all other reagents, or
via an aliquot of a stock solution in NMP where applicable.
NMP was added to give 5 mL in sum, and water (2 mL) was
added shortly before placing the reaction tube in the heating
source. The reaction was performed with vigorous stirring
(800–1000 rpm) at the desired temperature for the respec-
tive reaction time.
and PdTPP, the release of Pd from I is reversible and
sufficient for high catalytic activity, yet controlled
enough that I can act as a stable Pd reservoir. In ac-
cordance, high catalytic activities were observed, and
further stabilizing agents are not necessary under op-
timized conditions. Quantitative reactions can be per-
formed even with highly unreactive substrates such as
4-chloroanisole without the need for other stabilizers.
I was identified as a durable catalyst for recycling in
mild Suzuki reactions.
The successful verification of our concept is in ac-
cordance with the real (or most) active species being
a mononuclear, anionic Pd halide complex, which is
progressively formed from the release of Pd from the
macrocyclic ligand and the generation of halide ions
during the reaction.
Heck coupling: Base [1.8 mmol; 0.9 mmol for Ca(OH)2],
optionally TBAB (0.9 mmol) or LiBr (0.6 mmol), aryl halide
(3.0 mmol), DEGBE (130 mg as internal standard for GC
analysis), and styrene (1.5 mmol) were introduced into a re-
Adv. Synth. Catal. 2010, 352, 2263 – 2274
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2273

Christoph Rçhlich and Klaus Kçhler
FULL PAPERS
action tube. Catalyst was added either in substance before
all other reagents, or via an aliquot of a stock solution in
NMP where applicable. NMP was added to give 5 mL in
sum, shortly before placing the reaction tube in the heating
source. The reaction was performed with vigorous stirring
(800–1000 rpm) at the desired temperature for the respec-
tive reaction time.
reich, S. S. Soomro, S. S. Proeckl, Adv. Synth. Catal.
2008, 350, 2930.
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