Synthesis of Stable Diarylpalladium(II) Complexes: Detailed Study of the Aryl–Aryl Bond-Forming Reductive Elimination

Article abstract of DOI:10.1002/chem.201602849

The synthesis of diarylpalladium(II) complexes by twofold aryl C?H bond activation was developed. These intermediates of oxidative cyclization reactions are stabilized by chelation with acetyl groups while still maintaining sufficient reactivity to study

Full text of DOI:10.1002/chem.201602849

A Journal of
Accepted Article
Title: Synthesis of Stable Diarylpalladium(II) Complexes — Detailed Study of the
Aryl-Aryl Bond Forming Reductive Elimination
Authors: Tobias Gensch; Nils Richter; Gabriele Theumer; Olga Kataeva; Hans-
¨
Joachim Knolker
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To be cited as: Chem. Eur. J. 10.1002/chem.201602849
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Synthesis of Stable Diarylpalladium(II) Complexes ― Detailed
Study of the Aryl–Aryl Bond Forming Reductive Elimination**
Tobias Gensch+,[a] Nils Richter+,[a] Gabriele Theumer,[a] Olga Kataeva,[a,b] and Hans-Joachim Knölker*[a]
Abstract: We have developed the synthesis of diarylpalladium(II)
complexes by twofold aryl C–H bond activation. These intermediates
of oxidative cyclization reactions are stabilized by chelation with
acetyl groups while still maintaining sufficient reactivity to study their
reductive elimination. We found four distinct triggers for the reductive
elimination of these complexes to dibenzofurans and carbazoles.
Thermal elimination occurs at very high temperatures, whereas
ligand-promoted and oxidatively induced reductive eliminations
proceed readily at room temperature. Under these conditions, no
isomerization occurs. In contrast, weak Brønsted acids, such as
acetic acid, lead to a sequence of proto-demetalation, isomerization
to a κ3-diarylpalladium(II) complex and reductive elimination to non-
symmetrical cyclization products.
converted to carbazoles in a palladium(II)-catalyzed oxidative
cyclization, an intramolecular variant of cross-dehydrogenative
coupling.[8] As part of this ongoing program, we have recently
achieved the isolation of a diarylpalladium(II) complex, the key
intermediate of this oxidative cyclization.[9] Chelation by
appended weakly coordinating ligands stabilized this
intermediate sufficiently for isolation (2% yield), thus confirming
the mechanistic hypothesis for the oxidative cyclization.
However, detailed investigations were not possible due to the
only low amounts of diarylpalladium(II) complex obtained at that
time.[9] Herein, we describe the design and synthesis of more
stable diarylpalladium(II) complexes still having sufficient
reactivity to allow the study of the reductive elimination. With
these compounds in hand, we have discovered four
mechanistically distinct triggers for reductive elimination from
diarylpalladium(II) complexes, resulting in selective formation of
different isomers of the cyclized products.
Palladium-catalyzed arene cross coupling is one of the most
important reactions for the construction of complex molecules in
organic chemistry, owing to its broad scope, reliable applicability,
and choice of protocols.[1] Fundamental mechanistic
understanding has helped developing this class of reactions
even further.[2] In traditional cross coupling reactions, oxidative
addition of an aryl halide to a palladium(0) species leads to an
arylpalladium(II) complex, which then undergoes ligand
exchange via transmetalation to diarylpalladium(II) intermediates.
These readily undergo reductive elimination, releasing the biaryl
product. Recently, many methods have been developed in which
either one or both aryl ligands are introduced to palladium via C–
H bond activation.[3,4] This approach can shorten syntheses and
reduce waste generation, as well as open up new possible
retrosynthetic disconnections.[5] Despite the importance of
diarylpalladium(II) complexes as intermediates in traditional and
direct cross coupling catalysis, their study and characterization
has been limited by their inherent instability and propensity to
undergo reductive elimination.[6]
We have developed a general strategy of consecutive
palladium-catalyzed coupling reactions for the synthesis of
biologically active carbazole derivatives.[7] Thus, diarylamines
are obtained by Buchwald–Hartwig amination and subsequently
Scheme 1. General mechanism of the oxidative cyclization of diaryl
compounds by palladium(II).
In the palladium(II)-catalyzed oxidative cyclization of diaryl
compounds I the biaryl bond formation proceeds via two
consecutive C–H bond activations (Scheme 1). Reductive
elimination from the resulting diarylpalladium(II) complex III
generates the product IV and a palladium(0) species which can
be re-oxidized to palladium(II). A stabilization of the intermediate
diarylpalladium(II) complexes III can be achieved by rigid ligands
attached to the aryl substituents, thus suppressing the reductive
elimination. In order to study the reductive elimination as the
central step of the catalytic cycle, the stabilization of the
diarylpalladium(II) complex should not be too high. In the
present work, we show that acetyl-containing diaryl compounds,
due to their rigidity and weak coordination, enable the synthesis
of stable diarylpalladium(II) complexes which can be
investigated for their behavior towards reductive elimination.
[a]
[b]
T. Gensch,[+] Dr. N. Richter,[+] G. Theumer, Prof. Dr. H.-J. Knölker
Department of Chemistry, Technische Universität Dresden
Bergstraße 66, 01069 Dresden (Germany)
E-mail: hans-joachim.knoelker@tu-dresden.de
Homepage: http://www.chm.tu-dresden.de/oc2/
Dr. O. Kataeva
A. E. Arbuzov Institute of Organic and Physical Chemistry
Russian Academy of Sciences, Arbuzov Str. 8, Kazan 420088
(Russia)
These authors contributed equally to this work.
Part 128 of “Transition Metals in Organic Synthesis”; for part 127,
see: S. K. Kutz, C. Börger, A. W. Schmidt, H.-J. Knölker, Chem. Eur.
J. 2016, 22, 2487–2500.
[+]
[**]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem

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We obtained the tetracoordinate complex 2a in 17% yield by
reaction of palladium(II) acetate with bis(3-acetylphenyl) ether
(1a) after 4 days at 70 °C in glacial acetic acid (Table 1). Higher
temperatures and longer reaction times led via reductive
elimination to a dibenzofuran with the acetyl substituents in
position 1 and 7 (for systematic numbering of dibenzofuran and
carbazole, see the supporting information). Various solvent
systems, concentrations, and additives (such as acetates)
resulted in decreased yields or gave no product at all. Using the
bis(3-acetylphenyl)amine (1b), the corresponding complex 2b
was obtained in 30% yield after 2 h at 90 °C in a 1:1 mixture of
acetic acid and 1,2-dichloroethane (DCE). Similarly, reductive
elimination of complex 2b to the 2,5-disubstituted carbazole is
the major follow-up reaction, thus limiting the yield of the
palladacycle.
Table 1: Synthesis of the diarylpalladium(II) complexes 2.
Figure 2. Molecular structure of complex 2b in the crystal. Thermal ellipsoids
are shown at 50% probability level. Selected bond distances in Å and angles
in °: Pd–C2 1.935(2), Pd–C2' 1.942(2), Pd–O1 2.1798(18), Pd–O1' 2.1803(18),
C2-Pd-C2' 92.27(10), C2-Pd-O1 81.02(9).
The successful isolation of the diarylpalladium(II) complexes
2 in significant amounts enabled us to study their reductive
elimination behavior (Table 2). First, we tested the thermal
stability of these compounds in indifferent aromatic solvents.
Surprisingly, after 7 days of heating in toluene at reflux, complex
2a could be re-isolated almost quantitatively. An increase of the
temperature to 170 °C (using p-xylene as solvent in a sealed
tube) was necessary in order to achieve a thermal induction of
the reductive elimination of complex 2a. Along with the cyclized
1,9-diacetyldibenzofuran (3a), two products resulting from aldol
addition and condensation could be isolated, which originate
from the immediate elimination product 3a. In contrast, addition
of triphenylphosphane to a solution of complex 2a triggers the
reductive elimination to 3a even at room temperature.[12] We
propose that coordination of the phosphane at the palladium
atom weakens the complex and enables a rapid reductive
elimination. Likewise, treatment of complex 2a with an excess of
hydrogen peroxide (30% aq. soln.) leads to the formation of 3a
at room temperature. This reaction most likely involves the
Entry
X
Reaction conditions[a]
Yield
7%
1
2
3
4
O
1.0 equiv Cu(OAc)2, HOAc, 70 °C, 4 d
HOAc, 70 °C, 4 d
O
17%
12%
30%
NH
NH
HOAc, 80 °C, 3 h
HOAc/DCE (1:1), 90 °C, 2 h
[a] 40–70% of starting material (1) and 10–20% of non-symmetrical cyclization
product (6) but no other side-product were isolated along with complex 2.
The structures of the complexes 2a and 2b could be confirmed
by crystal structure analysis (Figures 1 and 2).[10,11] The
molecular structures are highly planar and very similar to each
other. In comparison with the previously reported pivaloyloxy-
chelated analog featuring
a
six-membered palladacycle,[9]
slightly shorter Pd–C (1.94 vs. 1.95 Å) and longer Pd–O bonds
(2.18 vs. 2.14 and 2.10 Å) are present in the symmetrical
complexes 2a and 2b.
oxidation of 2a to
a Pd(IV) species prior to reductive
elimination.[13]
Table 2: Reaction conditions for the reductive elimination of complex 2a.
Entry
Reaction conditions
2a
3a
4
5
1
2
3
4
toluene, 110 °C, 7 d
98%
27%
21%
29%
p-xylene, 170 °C, 3 d
PPh3 (4 equiv), RT, 8 h
H2O2 (excess), CH2Cl2, RT, 5 h
29%
75%
41%
4%
38%
trace
Figure 1. Molecular structure of complex 2a in the crystal. Thermal ellipsoids
are shown at 50% probability level. Selected bond distances in Å and angles
in °: Pd–C2 1.9321(19), Pd–C2' 1.9341(19), Pd–O2 2.1778(14), Pd–O2'
2.1871(14), C2-Pd-C2' 90.44(8), C2-Pd-O2 80.71(7).

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COMMUNICATION
Table 3: Reactions of the diarylpalladium(II) complexes 2 with Brønsted acids.
reductive elimination (Scheme 3). Two factors may dictate the
selectivity of this elimination pathway. Firstly, reductive
elimination from 2 forms a highly strained product and requires
high activation energies, as shown in the thermally induced
reductive elimination (Table 2, entries 1 and 2). Secondly, the κ3-
diarylpalladium(II) complex 8 is less stabilized due to less
chelation, thus enabling elimination at lower temperatures.
Based on our present observations, formation of the isomeric
cyclization products during the synthesis of the complexes 2
most likely occurs directly from the κ3-intermediates, rather than
via intermediate formation of 2 and subsequent isomerization–
X
Reaction conditions
Product, yield
6a, 46%[a]
1a, 85%
a
a
b
b
O
HOAc, reflux, 3 d
elimination.
The
related
pivaloyloxy-substituted
mainly
diarylpalladium(II) complex previously reported
O
HCl/CH2Cl2 (1:4), RT, 1.5 h
HOAc, reflux, 3 d
undergoes reductive elimination without isomerization in HOAc
at 80 °C.[9] We attribute this behaviour to the reduced stability as
a consequence of the less favorable coordination geometry in
the six-membered palladacycle. As a consequence, pathways
leading to reductive elimination prior to isomerization may
become favored. Thus, the preferential formation of the
undesired non-linear isomer in the course of our total synthesis
of antipathine A can be rationalized by the present findings.[15]
NH
NH
6b, 85%
HCl/CH2Cl2 (1:20), RT, 30 min
1b, 100%
[a] By-product: 3-acetyldibenzofuran (8% yield); 24% of complex 2a recovered.
In order to explain the formation of the isomeric non-
symmetrical cyclization products obtained on synthesis of the
diarylpalladium(II) complexes 2, the complexes 2 were subjected
to acidic reaction conditions (Table 3). We observed an
unexpected reactivity of the complexes 2 in glacial acetic acid.
After 3 days of heating in HOAc at reflux, 76% of complex 2a
In conclusion, we have developed the synthesis of stable
diarylpalladium(II) complexes by twofold aryl C–H bond
activation of tethered diaryl compounds. These complexes are
model compounds for the palladium-catalyzed biaryl bond
formation and can be used for studying the reductive elimination
leading to C–C bond formation. We have discovered four distinct
triggers for the reductive elimination from these complexes. The
thermal elimination occurs at very high temperatures (170 °C),
whereas the ligand-promoted reductive elimination using
triphenylphosphane and the oxidatively induced elimination
proceed at room temperature. Each of these variants gives rise
to the cyclization products without isomerization. Weak Brønsted
acids, as a fourth potential trigger, lead to an isomerization–
reductive elimination sequence and thus to the formation of non-
symmetrical cyclization products. The selectivity offered by the
different mechanistic alternatives for the reductive elimination of
diarylpalladium(II) complexes will have an impact on future total
syntheses of biologically active carbazole alkaloids. Moreover,
the present findings are highly useful for interpretation, design,
and application of C–C bond forming reactions using palladium
in a more general context.
were
consumed
and
the
non-symmetrical
1,7-
diacetyldibenzofuran (6a) was formed along with 3-
acetyldibenzofuran. The latter product probably results from 6a
by a proton-initiated retro-Friedel–Crafts acylation. Under the
same reaction conditions, complex 2b was consumed
completely to afford the non-symmetrical 2,5-diacetylcarbazole
(6b) in 85% yield. Refluxing complex 2a in trifluoroacetic acid
leads to an accelerated formation of 6a within a few hours, along
with major decomposition of material, despite the lower reaction
temperature. In neither of these experiments, we could observe
any of the symmetrical 1,9-diacetyldibenzofuran (3a). When the
diarylpalladium(II) complexes 2 were subjected to even stronger
acids like HCl (mixture of conc. HCl and CH2Cl2), demetalation
to the diaryl compounds 1 occurred rapidly at room temperature
(1a: 85% yield; 1b: 100% yield). The reversibility of C–H
palladation under acidic conditions is well-documented.[14] Thus,
the formation of the non-symmetrical cyclization products 6 can
be rationalized by
a
sequence of proto-demetalation,
isomerization to the κ3-diarylpalladium(II) complex 8, and
Scheme 2. Reactivity pathways for the demetalation of diarylpalladium(II) complexes 2.

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Keywords: C–H bond activation • metallacycles • palladium •
reaction mechanisms • reductive elimination
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Chemistry - A European Journal
10.1002/chem.201602849
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Tobias Gensch, Nils Richter, Gabriele
Theumer, Olga Kataeva, and Hans-
Joachim Knölker*
Diarylpalladium(II) intermediates of oxidative cyclization reactions can be
synthesized as stabilized chelates by twofold aryl C–H bond activation. The study of
the reductive elimination of these complexes to dibenzofurans and carbazoles
unveiled four distinct triggers: a) thermal elimination at high temperature, b) ligand-
promoted, and c) oxidatively induced reductive elimination at room temperature,
which all proceed without isomerization. Whereas weak Brønsted acids, such as
acetic acid, induce a sequence of proto-demetalation, isomerization to a κ3-
diarylpalladium(II) complex and reductive elimination to non-symmetrical isomers.
Synthesis of Stable
Diarylpalladium(II) Complexes ―
Detailed Study of the Aryl–Aryl Bond
Forming Reductive Elimination