Nickel-Catalyzed C-CN Bond Formation via Decarbonylative Cyanation of Esters, Amides, and Intramolecular Recombination Fragment Coupling of Acyl Cyanides

Article abstract of DOI:10.1021/acs.orglett.7b01905

An efficient nickel-catalyzed decarbonylative cyanation reaction which allows the direct functional-group interconversion of readily available esters into the corresponding nitriles was developed. This reaction successfully offers access to structurally diverse nitriles with high efficiency and excellent functional-group tolerance and provides a good alternative to classical synthetic pathways from diazonium salts or organic halide compounds.

Full text of DOI:10.1021/acs.orglett.7b01905

Letter
pubs.acs.org/OrgLett
Nickel-Catalyzed C−CN Bond Formation via Decarbonylative
Cyanation of Esters, Amides, and Intramolecular Recombination
Fragment Coupling of Acyl Cyanides
Adisak Chatupheeraphat,†,§ Hsuan-Hung Liao,†,§ Shao-Chi Lee,† and Magnus Rueping*,†,‡
†Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
‡King Abdullah University of Science and Technology (KAUST), KAUST Catalysis Center (KCC), Thuwal 23955-6900, Saudi
Arabia
S
* Supporting Information
ABSTRACT: An efficient nickel-catalyzed decarbonylative cyanation
reaction which allows the direct functional-group interconversion of
readily available esters into the corresponding nitriles was developed.
This reaction successfully offers access to structurally diverse nitriles
with high efficiency and excellent functional-group tolerance and
provides a good alternative to classical synthetic pathways from
diazonium salts or organic halide compounds.
he development of transition-metal-catalyzed cross-cou-
pling reactions has emerged as a powerful method for
to explore alternative ways involving metal-catalyzed conversions
of aryl (pseudo) halides into more diversely substituted aromatic
nitriles.5 In order to accomplish our goal and develop a first ester
to cyanide functional group interconversion, we needed to
overcome several challenges: (1) the strongly binding cyanide
anions may deactivate and poison the catalyst; (2) the propensity
of acyl cyanide formation instead of the desired nitrile product;6
(3) nitrile product decomposition due to the lack of chemo-
selectivity7 (Scheme S2).
We herein describe a significant advance in the realm of nickel-
catalyzed decarbonylative reactions which successfully sup-
pressed the deleterious side reactions mentioned above and
provides a new method for the formation of C(sp2)−CN bonds.
Guided by our previous reaction development in the area of
functional group interconversions,8 survey coupling experiments
with phenyl 2-naphthoate (1a) under Ni(cod)2 catalysis were
carried out. From the different cyanide reagents available, use of
zinc cyanide (2a) provided a successful decarbonylative
introduction of the CN group. Further experiments were
conducted with a series of trialkyl-monodentate phosphine
ligands, including PnBu3 and PCy3 which resulted only in a low
yield of the desired nitrile product 3a (Table 1, entries 1−2).
Carbene ligands such as 1,3-bis(2,6-diisopropylphenyl) imida-
zolidin-2-ylidene (SIPr) did not provide the product (Table 1,
entry 3). Further evaluation of bidentate phosphine complexes
revealed a beneficial effect in terms of reactivity (Table 1, entries
4 and 5). The best ligand was identified as dcype [1,2-bis
(dicyclohexylphosphino) ethane] providing the product in 72%
yield (Table 1, entry 4). Next, further bases were tested in order
to reach higher yields (Table 1, entries 6 and 7), and the best
result was obtained with HCO2Na (Table 1, entry 7). A decrease
T
carbon−carbon bond formations. Recently, an increasing
emphasis on environmentally friendly approaches has stimulated
research aimed at finding lower toxicity and economical cross-
coupling partners, which avoid the corrosive halide-containing
waste production in transition-metal-catalyzed reactions. Partic-
ular attention has been drawn to the use of carboxylic acid
derivatives, such as esters and amides in decarbonylative
transformations,1,2 to further complement organic halides as
coupling partners. Commonly, aromatic and unsaturated as well
as saturated aliphatic rests have been introduced on (hetero)-
aromatic substrates, leading to a wide range of new C−C bond
forming reactions (Scheme 1, left). In contrast, less is known
Scheme 1. Decarbonylative Transformations: C−C Bond
Formations
regarding the direct introduction of C-based functional groups
on (hetero)aromatic substrates. With these considerations in
mind we decided to target the C−CN bond formation via a
decarbonylative transformation (Scheme 1, right).
Nitriles are important structural motifs in pharmaceutically
relevant molecules and electronic materials.3 In addition, the
nitrile moiety can be used as an intermediate for a multitude of
transformations4 (Scheme S1). Traditionally, the introduction of
the cyano group into aromatic compounds was achieved by
Sandmeyer and Rosenmund von Braun reactions. However, the
relatively limited substrate scope of these reactions led chemists
Received: June 22, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01905
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Organic Letters
Letter
Table 1. Optimization of the Nickel-Catalyzed
Decarbonylatvie Cyanation Coupling Reaction
Scheme 2. Scope of Nickel-Catalyzed Cyanation Cross-
ab
a
,
Coupling with Aryl and Cinnamoyl Esters
b
entry
ligand
base
solvent
yield (%)
1
2
PnBu3
PCy3
SIPr
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
CsCl
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
iPr2O
33
31
−
3
4
dcype
dcypb
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
−
72
34
82
89
50
72
84
37
5
6
7
HCO2Na
8
−
9
HCO2Na
HCO2Na
HCO2Na
HCO2Na
HCO2Na
HCO2Na
10
dioxane
toluene
toluene
toluene
toluene
c
11
d
e
12
94(90)
df
,
13
14
−
−
d
a
Reaction conditions: 1a (0.3 mmol), 2 (0.6 mmol), solvent (1 mL),
b
150 °C, 48 h. NMR yield, 1,3,5-(OMe)3C6H3 as internal standard.
c
d
e
Using dcype (10 mol %). Using ZnCN2 (0.2 mmol). Isolated yield.
f
Without Ni(cod)2.
in the yield of the product was observed when the reaction was
performed in the absence of base (Table 1, entry 8). Further
optimization focused on other solvents; switching the solvent
from toluene to iPr2O and dioxane did not provide a better yield
(Table 1, entries 9 and 10). Next, the influence of the amount of
ligand was examined, and changing the ratio of catalyst/ligand
from 1:2 to 1:1 reduced the yield of the product to 37% (Table 1,
entry 11). Reducing the amount of zinc cyanide (0.6 equiv) still
provided an excellent yield (Table 1, entry 12). Lastly, control
experiments showed that no desired product was formed in the
absence of nickel catalyst and ligand (Table 1, entries 13 and 14).
With the optimized conditions in hand, we studied the scope
of this newly developed nickel catalyzed decarbonylative
cyanation of esters with zinc cyanide. As revealed in Scheme 2,
a variety of aromatic esters with different electronic properties
and substitution patterns provided the corresponding nitriles
3a−m with moderate to high yields. Notably, the reaction is
chemoselective; use of substrate 1m bearing a methyl and a
phenyl ester group provided 3m in 71% yield. Lastly, esters
incorporating a series of different heterocyclic patterns, including
thiophene, indole, and pyridine, were tested and adducts 3n−p
were isolated with high yields. Importantly, our results
demonstrate that the reaction is not limited to reactive and π-
extended aromatic rings.9 α,β-Unsaturated nitriles, especially
cinnamonitriles, are important synthons for further chemical
transformations.10 Conventionally, the most straightforward
approach toward their synthesis is the transition-metal-catalyzed
cyanation of styrenyl halides. However, some procedures involve
toxic metal cyanides and lack general applicability. More
significantly, none of these reactions are stereoselective and
produce a mixture of cis- and trans-stereoisomers in varying
amounts.11 We, therefore, have probed our developed system for
the construction of versatile alkenyl nitriles. To our delight, a
series of different esters which contain different electronic
properties as well as β-substituted alkene reacted nicely to furnish
the desired products 3q−t with high yields.
a
Reaction conditions: 1 (0.2 mmol), Zn(CN)2 (0.16 mmol), Ni(cod)2
(10 mol %), dcype (20 mol %), HCO2Na (2 equiv), toluene (0.2 M),
150 °C 48 h. At 170 °C. Yields for isolated products 3a−t.
b
To further demonstrate the flexibility of the present approach
for nitrile construction, the C−CN bond formation with amides
was investigated.2,12 We were delighted to see that the
deamidative cyanation proceeded also smoothly and the
products 5a−l were obtained in good to high yields (Scheme 3).
Both being new procedures, the nickel-catalyzed decarbon-
ylative cyanation of esters as well as deamidative cyanation of
amides provided a variety of alkenyl and aryl cyanides in good
yields starting from readily available substrates. However, in both
procedures byproducts in the form of the phenol and
gluatarimide, respectively, are formed. This can be prevented if
a substrate in which the CN nucleophile is already incorporated
could be applied. Thus, we wondered whether the corresponding
acyl cyanides could be applied in a CO extrusion recombination
fragment coupling strategy. A further improvement would be if
nickel(II) salts could be applied in this intramolecular cyanation
process.13 After intensive evaluation of the reaction parameters
and application of a series of nickel(II) catalysts, we found NiBr2·
2-methoxyethyl ether as the optimal catalyst in terms of
reactivity.14 To further demonstrate the generality of this
protocol, a series of simple and activation site-free substrates
have been applied into this extrusion recombination fragment
coupling.15
We were pleased to see that aromatic as well as heteroaromatic
substrates 6a−h were converted into the corresponding products
in good to high yields (Scheme 4). A scale-up reaction with 1 g of
B
DOI: 10.1021/acs.orglett.7b01905
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Organic Letters
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Furthermore, in order to create an easier manipulation of the
intramolecular decarbonylative setting, we have designed a
protocol for the direct preparation of nitriles from acyl chlorides
without purification of acyl cyanide intermediate (Scheme 5).
Scheme 3. Scope of Nickel Catalyzed Deamidative Cyanation
of Amides
a
Scheme 5. Nickel-Catalyzed Acyl Cyanide Intramolecular
a
Decarbonylative Reaction in Sequential Reactions
a
Reaction conditions: (1) 8 (0.4 mmol), CuCN (2 equiv), MeCN (0.4
M). 2) NiBr2·2-methoxyethyl ether (10 mol %), dcype (20 mol %),
Mn act as reducing agent of Ni(II) salt (2 equiv), toluene (0.2 M), 150
°C, 16 h.
For this purpose, 2-naphthoyl chloride (8) was reacted with a
stoichiometric amount of copper cyanide and then the crude
product was filtrated over Celite prior to the decarbonylative
step. The desired 2-naphthoyl cyanide 7j was obtained in 72%
overall yield.
Finaly, the power of this methodology is exemplified through
its application to the synthesis of cyanostilbenes (Scheme 6),
which due to their π-conjugated luminophores can be applied to
optical device materials.
a
Reaction conditions: 4 (0.2 mmol), Zn(CN)2 (0.4 mmol), Ni(cod)2
(10 mol %), dcype (20 mol %), KF (2 equiv), toluene (0.2 M), 150
b
°C, 20 h. Reaction conditions: 4a (3.74 mmol, 1g), Zn(CN)2 (7.48
Scheme 6. Site-Selective Sequential Decarboxylation−
a
mmol), Ni(acac)2 (10 mol %), dcype (20 mol %), KF (2 equiv),
toluene (0.2 M), under MW 180 °C, 6 h. At 170 °C. 40 h.
Decarbonylation Reaction
c
d
Scheme 4. Scope of Nickel-Catalyzed Acyl Cyanide
Intramolecular Decarbonylative Cyanation Reaction
a
Reaction conditions: (1) 9 (1.2 mmol), cinnamic acid 10 (1.0 mmol),
PdBr2 (0.03 mmol), CuBr (0.1 mmol) 1,10-phenanthroline (0.1
mmol), K2CO3 (1 mmol), 3 Å MS (250 mg), NMP (1.5 mL),
quinoline (0.5 mL), 150 °C. (2) 11 (0.2 mmol), Zn(CN)2 (0.16
mmol), HCO2Na (0.4 mmol), Ni(cod)2 (10 mol %), dcype (20 mol
%), toluene (0.2 M), 170 °C, 48 h.
In the first step we have exploited the molecular recognition
properties and chemoselectivity of the palladium-catalyzed
decarboxylative coupling17 of phenyl 4-iodobenzodate 9 with
cinnamic acid 10 as the coupling partner to provide the
corresponding stilbene ester 11 in satisfactory yield (62%). In the
second step, the phenyl ester moiety underwent decarbonylative
cyanation reaction leading to cyanostilbene 12 in 89% yield. We
herein paved the way for a new synthetic route for the synthesis
of a demanding compound under a sequential decarboxylation−
decarbonylation manipulation.
In summary, we have developed the first nickel-catalyzed
decarbonylative cyanation protocol which allows the transfer of a
series of readily available aryl and heteroaryl esters as well as
amides to the corresponding nitriles.2o This protocol provides a
new possibility to cleave carboxylic esters and amides while
forming a C−CN bond. The newly developed protocols
successfully suppress the undesired acyl cyanide formation,
nitrile product decomposition, and catalyst posioning processes.
In order to prevent byproduct formation, a nickel(II)-catalyzed
extrusion recombination fragment coupling strategy allows direct
a
Reaction conditions: 6 (0.2 mmol), NiBr2·2-methoxyethyl ether (10
mol %), dcype (20 mol %), Mn act as reducing agent of Ni(II) salt (2
equiv), toluene (0.2 M), 150 °C, 16 h. Reaction on 1 g scale: 6c (6.2
mmol).
b
acyl cyanide 6c was carried out in order to demonstrate the
scalability of our newly developed methodology and provided
the nitrile product 7c in good yield (83%). Notably, para-ketone
substituted acyl cyanide 6d was successfully converted into nitrile
7d without decarbonylation of ketone as recently reported,16
which preserves the ketone group for further transformations.
Lastly, the conjugate acyl cyanide system 6i gives cinnamonitrile
7i in high yield.
C
DOI: 10.1021/acs.orglett.7b01905
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Organic Letters
Letter
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3095.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01905.
■
S
Detailed experimental procedures, spectral data for all
compounds, and copies of 1H and 13C spectra (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: magnus.rueping@kaust.edu.sa.
ORCID
Magnus Rueping: 0000-0003-4580-5227
Author Contributions
§A.C. and H.-H.L. contributed equally.
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.orglett.7b01905
Org. Lett. XXXX, XXX, XXX−XXX