Pd-Catalyzed Cyanation of (Hetero)Aryl Halides by Using Biphosphine Ligands

Article abstract of DOI:10.1002/chem.201704178

Tetraadamantylbiphosphine (TABP; L1), which showed superior activity in the palladium-catalyzed cyanation of 4-chloroanisole compared to standard phosphines, was synthesized as a new ligand. The generality of the new catalytic system was shown by the cyanation reaction of approximately 30 (hetero)aryl halides including hindered, electron-rich, and electron-poor aryl chlorides. These reactions constitute the first examples of using biphosphine ligands in Pd-catalyzed coupling reactions.

Full text of DOI:10.1002/chem.201704178

A Journal of
Accepted Article
Title: Pd-Catalyzed Cyanation of (Hetero)Aryl Halides Using
Biphosphine Ligands
Authors: Shaoke Zhang, Helfried Neumann, and Matthias Beller
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To be cited as: Chem. Eur. J. 10.1002/chem.201704178
Link to VoR: http://dx.doi.org/10.1002/chem.201704178
Supported by

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COMMUNICATION
Pd-Catalyzed Cyanation of (Hetero)Aryl Halides Using Biphos-
phine Ligands
[a]
[a]
[a]
Shaoke Zhang, Helfried Neumann, Matthias Beller*
Abstract: Tetraadamantylbiphosphine (TABP) L1 has been
synthesized as a new ligand, which shows superior activity in the
palladium-catalyzed cyanation of 4-chloroanisole compared to
standard phosphines. The generality of the new catalytic system is
shown in the cyanation of around 30 (hetero)aryl halides including
hindered, electron-rich and electron-poor aryl chlorides. These
reactions constitute the first examples of using biphosphine ligands
in Pd-catalyzed coupling reactions.
Notably, TABP is a highly moisture-stable compound, even
in water at 80°C.[8] Due to the extraordinary stability and
convenient access we believe that this and related sterically
hindered biphosphines have a significant potential for catalysis,
organic synthesis, and other fields. However, up to date, the
utilization of this kind of phosphorous compounds is scarcely
explored.[
9]
Based on our renewed interest in metal-catalyzed cyanation
reactions,[10] we got attracted to the influence of such potential
ligands for the synthesis of aromatic nitriles. The nitrile group is
an integral part of several natural products, but also of industrial
Ancillary ligands, especially phosphorous-based derivatives
have emerged as the most powerful tool to control the activity
and selectivity of molecularly-defined catalysts.[1] Among the
plethora of phosphine ligands nowadays routinely applied in
organometallic catalysis, tertiary mono-[2] and diphosphine
ligands[3] dominate. In comparison, the use of secondary
phosphines is rare and concentrated so far on secondary
phosphine oxides.[4] Moreover, the use of phosphorous
compounds with P-P bonds such as biphosphines is one of the
first examples in catalysis.[5]
dyes,
herbicides,
agrochemicals,
and
especially
pharmaceuticals.[ Traditionally, aromatic nitriles are prepared
11]
by the Rosenmund–von Braun reaction of aryl halides[12] or the
diazotization of anilines with subsequent Sandmeyer reaction
On a larger scale, heterogeneously-catalyzed ammoxidation at
high temperature (300-550 °C) prevails.[14] All these processes
suffer from lack of functional group tolerance or harsh reaction
conditions.
[13]
.
For some time we are interested in the use of phosphines
with adamantyl substituents in metal-catalyzed coupling and
related reactions.[6] Compared to other bulky phosphines, the
introduction of an adamantyl substituent leads to improved
stability and the resulting ligands are convenient to handle. As
an example for this unusual stability diadamantylphosphine is a
stable non-pyrophoric secondary phosphine.[7]
Since Takagi et al. reported in 1973 the first example of a
Pd-catalyzed cyanation of aryl halides,[15] this method attracted
substantial attention.[16] Numerous cyanide sources such as
MCN (M = Na, K, etc),[ Zn(CN)
17]
,
[18]
acetone cyanohydrin,
[19]
2
Toluenesulfonyl Cyanide (TsCN)[20] and trimethylsilyl cyanide
(TMSCN)[21] have been exploited for this catalytic
[22]
4 6
transformation. In 2004, we introduced K [Fe(CN) ] as a more
benign cyanation source.[ Since then, we as well as other
groups demonstrated the usefulness of this low toxicity reagent
in several cyanation reactions.[24] Meanwhile, phosphine ligands
23]
In order to synthesize novel adamantly-based PNP pincer
ligands,
diadamantylchlorophosphine L9, which is conveniently available
in two steps from adamantane and PCl . Mixing L9 with lithium
recently
we
investigated
the
lithiation
of
play an essential role for the activation of aryl bromide and
3
[25]
chloride substrates.
In this respect, we report the first metal
in THF and stirring this mixture at room temperature for two days
gave a white solid, which gradually precipitated from the solution.
After filtration and washing the residue with water
tetraadamantylbiphosphine (TABP) L1 was obtained as pure
product in 77% yield (Scheme 1).
coupling reactions using biphosphine ligands. More specifically,
Pd-catalyzed cyanations of (hetero)aryl halides proceed with
good functional group tolerance and the corresponding aryl
nitriles are obtained in good to excellent yields.
Initial experiments were performed using 4-chloroanisole 1a
as a benchmark substrate in the presence of different ligands.
Based on previous work on cyanation of aryl chlorides,[24f] we
used 0.5 mol% Pd(OAc)
monophosphines), 20 mol% Na
40 °C in NMP as solvent. As shown in Figure 1, surprisingly
tetra-tert-butylbiphosphine (TBBP) and TABP showed better
activity than standard phosphines including dppf, PCy , PPh
BuPAd and well-known electron-rich pyrrole- and imidazole-
2
,
1
mol% ligand (2 mol% for
2
CO , 0.5 mmol K [Fe(CN) ] at
3
4
6
1
3
3
,
Scheme 1. Synthesis of tetraadamantlybiphosphine (TABP) L1
2
based ligands. In order to prove whether secondary phosphides
might be involved as active ligands, di-tert-butylphosphine and
diadamantylphosphine were tested.[
26]
However, both ligands
formation. Similarly,
[
a]
Shaoke Zhang, Dr. Helfried Neumann, and Prof. Dr. Matthias Beller
Leibniz-Institut für Katalyse e. V. an der Universität Rostock,
Albert-Einstein Straße 29a, Rostock, 18059, Germany
Matthias.Beller@catalysis.de
induced
only
minor
product
diadamantylchlorophosphine showed only little activity.
Furthermore, to verify if P-C bond formation took place between
TABP and arenes, Ad PPh and Mor-DalPhos (L7 and L14) were
2
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - A European Journal
10.1002/chem.201704178
COMMUNICATION
tested as ligands. Interestingly, they gave 41% and 57% yield of
the desired product, respectively.
Notably to say, the reaction is quite sensitive to the solvent.
Although comparable results could be obtained with NMP and
DMAc (Table 1, entry 9), in the presence of other tested solvents
very low yield of 2a were achieved (Table 1, entries 10-12). In
agreement with previous findings,[24f] also this novel palladium-
catalyzed cyanation is highly sensitive to the base present in the
reaction medium. Using other organic and inorganic bases
commonly applied in coupling reactions gave less than 20% of
Notably, variation of the substituent of the biphosphine
ligand caused a drastic change of the activity. While the
adamantly- and tert-butyl-based ligands gave 77% and 63%
yield, respectively, the phenyl derivative gave only 1% yield.
2a (Table 1, entries 13-18). Optimal results were observed with
2
0 mol% of inexpensive Na CO (Table 1, entry 19).
2
3
Table 1. Pd-catalyzed cyanation of 4-chloroanisole 1a: Influence of different
reaction conditions[a]
[
Pd]:
Entry
[Pd] /mol% Solvent T / °C Base (mol%) Yield[b] /%
Ligand
1
2
3
4
5
1:2
0.1
0.5
0.5
0.5
0.5
NMP
NMP
NMP
NMP
NMP
120 Na
120 Na
120 Na
120 Na
120 Na
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
(100)
(100)
(100)
(100)
(100)
23
26
41
42
19
1:1
1:1.5
1:2
.
1:3
6
7
8
9
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
NMP
NMP
140 Na
160 Na
140 Na
140 Na
140 Na
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
(100)
(100)
(100)
(100)
(100)
(100)
(100)
83
90
0
DMSO
DMAc
DMF
63
18
7
10
11
12
13
14
15
dioxane 140 Na
CH CN 140 Na
3
22
15
0
NMP
NMP
140
K
2
CO
CO
PO
3
(100)
140 Cs
2
3
(100)
1:2
1:2
1:2
1:2
1:2
0.5
0.5
0.5
0.5
0.5
NMP
NMP
NMP
NMP
NMP
140
140
K
3
4
(100)
4
Figure 1. Pd-catalyzed cyanation of 4-chloroanisole in the presence of
16
KF (100)
19
10
0
selected phosphine ligands. Reaction conditions: 4-Chloroanisole (2 mmol),
1
7
8
140 TMEDA (100)
K
4
[Fe(CN)
diphosphines, 2 mol% for monophosphines), NMP (2 mL), Na
40 °C, 16 h; GC yields with hexadecane as an internal standard.
6
]
(0.5 mmol), Pd(OAc)
2
(0.5 mol%), ligand (1 mol% for
1
140 NaOtBu (100)
2
CO (20 mol%),
3
1
19
140
Na
2
CO
3
(20)
77
With these exciting results in hand, we studied the influence
[
4 6 2
a] 1a (2 mmol), K [Fe(CN) ] (0.5 mmol), Pd(OAc) (0.1-0.5 mol%), TABP (0.2-
of key reaction parameters on this transformation. Selected
results are shown in Table 1. Firstly, [Pd] loading and [Pd] ligand
ratio were optimized. A two-fold excess of ligand (with respect to
1.5 mol%), solvent (2 mL), base (20-100 mol%), 16 h, 120-160 °C. [b] Yields
were determined by GC analysis using hexadecane as an internal standard.
In order to elucidate the structure of the active catalyst in situ
31P NMR spectroscopic measurements were made. After mixing
[
Pd]) and higher loading of [Pd] are favorable for the generation
of 2a (Table 1, entries 1-4). Higher ligand loading decreased the
activity, which might be explained by blocking free coordination
sites on the central metal (Table 1, entry 5). Increasing the
temperature from 120 °C to 140 °C, improved the yield of 2a
from 42% to 83% (Table 1, entry 6). At 160 °C another slight
improvement is observed (Table 1, entry 7).
TABP and Pd(OAc) in toluene, the parent ligand ( = 33.70
2
ppm) was fully converted and mainly two other phosphorous
species were detected ( = 60.01 and 121.70 ppm, ratio of 1:14),
which unfortunately could not be assigned. Meanwhile, several
attempts to crystallize Pd/TABP complexes were undertaken.
However, no suitable crystals for X-ray crystallography were
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Chemistry - A European Journal
10.1002/chem.201704178
COMMUNICATION
obtained. Stirring TBBP instead of TABP in the presence of
(Figure 3). Using the new TABP ligand, aryl chlorides containing
Pd(OAc)
2
in toluene resulted mainly in the formation of a new
2
electron-donating groups such as –OMe, –Me, –OH, and –NH ,
phosphorous species at 125.82 in the 31P NMR. Crystallization
of this species showed the formation of a phosphido-bridged
were successfully cyanated to the corresponding nitriles with
good to excellent yields (4a - 4e). Substrates having another
halide substituent in para- or meta- position are also converted
to the corresponding nitriles (4g - 4h). As a special case, 1,3-
dichlorobenzene gave 30% of the single cyanated product and
24% of the double cyanated product (4h - 4l). Furthermore, aryl
chlorides containing electron-withdrawing groups such as –CN,
dimer
((di-t-butylphosphinato)-(di-t-butylphosphinato)-(di-t-
known
butylphosphinous acid)-palladium), which is
a
compound.[27] Based on this result, we guess that the P-P bond
is cleaved at some point during catalysis. However, the structure
of the active complex is yet unknown. Nevertheless, due to the
interesting catalytic results in the benchmark system, the
substrate scope was further evaluated.
3
–CHO, and –CF gave the desired benzonitriles in good to
excellent yields (4j – 4k, and 4m – 4n), albeit at 160°C. This
catalytic system is also successfully applicable to multi-
substituted aryl chlorides and chloronaphthalene (4o – 4r).
Gratifyingly, sterically hindered 2-chloro-1,3-dimethylbenzene
was converted with an excellent yield of 84% to give 4p. As
expected aryl bromides could be converted to the corresponding
nitriles 4r – 4v with excellent yields at lower temperature (120°C)
and catalyst loading (0.1 mol% [Pd]).
For applications in life sciences (pharmaceuticals,
agrochemicals), it is interesting to note that TABP is effective in
the cyanation of heteroaryl halides. As shown in Figure 4
derivatives of quinazoline 6a, quinoline 6b - 6c, pyridine 6d - 6e,
1,3-benzodioxole 6f - 6g, were all successfully coupled to give
the corresponding nitriles in good to excellent yields. However,
comparing to 6e, 2-chloropyridine was much less active in the
presence of this catalytic system. [28]
Figure 4. Pd-catalyzed cyanation of heteroaryl halides. Reaction conditions for
chlorides: Substrate (2 mmol),
TABP (1 mol%), NMP (2 mL), Na
for bromides: Substrate (2 mmol),
mol%), TABP (0.2 mol%), NMP (2 mL), Na
K
4
[Fe(CN)
CO (20 mol%), 16 h. Reaction conditions
[Fe(CN) ] (0.5 mmol), Pd(OAc) (0.1
CO (1 eq.), 16 h. Isolated yields.
6 2
] (0.5 mmol), Pd(OAc) (0.5 mol%),
2
3
K
4
6
2
2
3
[
a] GC yields using hexadecane as an internal standard.
In conclusion, we have synthesized the new biphosphine
ligand L1, which is stable and can be conveniently handled and
prepared. In one of the first examples, this class of ligands is
being used in Pd-catalyzed coupling reactions. In more detail,
L1 permits the effective cyanation of various aryl (hetero)
chlorides as well as aryl (hetero) bromides. Although the nature
of the exact active species remains unknown, we assume further
catalytic application of bi- or mutiphosphines, specifically in
Figure 3. Pd-catalyzed cyanation of aryl halides. Reaction conditions for
chlorides: Substrate (2 mmol),
TABP (1 mol%), NMP (2 mL), Na
for bromides: Substrate (2 mmol),
mol%), TABP (0.2 mol%), NMP (2 mL), Na
K
4
[Fe(CN)
CO (20 mol%), 16 h. Reaction conditions
[Fe(CN) ] (0.5 mmol), Pd(OAc) (0.1
CO (1 eq.), 16 h. Conversion and
6 2
] (0.5 mmol), Pd(OAc) (0.5 mol%),
2
3
K
4
6
2
2
3
yields were determined by GC using hexadecane as an internal standard.
Next, the substrate scope with respect to aryl chlorides and
bromides was tested under the optimal reaction conditions
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Chemistry - A European Journal
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COMMUNICATION
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ACKNOWLEDGMENTS
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We are grateful for the financial support from the State
Mecklenburg-Western Pomerania and the federal German
Ministry BMBF. Shaoke Zhang thanks the Chinese Scholarship
Council for financial support. We thank the analytical department
of Leibniz-Institute for Catalysis at the University of Rostock for
their excellent service.
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Keywords: biphosphine • cyanation • Pd catalysts • coupling
reaction • aryl halide •
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To demonstrate the stability of TABP, the ligand was suspended in
aqueous solution under air for 24 h. Even after heating this
[
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7]
8]
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Chemistry - A European Journal
10.1002/chem.201704178
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Shaoke Zhang, Helfried Neumann,
Matthias Beller*
Page No. – Page No.
Pd-Catalyzed
Hetero)Aryl
Cyanation
Halides
of
Using
(
Biphosphine Ligands
Give me two phosphorous atoms. A new biphosphine ligand combined with [Pd]
allows for smooth cyanation of all kinds of (hetero)aryl chlorides and bromides.
This article is protected by copyright. All rights reserved.