Decarbonylative Cyanation of Amides by Palladium Catalysis

Article abstract of DOI:10.1021/acs.orglett.7b01199

Transition-metal-catalyzed cyanation of aryl halides is a process of significant importance in the preparation pharmaceuticals, organic materials and agrochemicals. Here, we demonstrate a palladium-catalyzed decarbonylative cyanation of amides by highly selective carbon-nitrogen bond cleavage for the synthesis of a wide range of aryl nitriles. The utility of this technology is demonstrated by the synthesis of isotopically labeled aryl nitriles and orthogonal cross-coupling reactions of bench-stable amides to establish cross-coupling synthons with opposite polarity.

Full text of DOI:10.1021/acs.orglett.7b01199

Letter
pubs.acs.org/OrgLett
Decarbonylative Cyanation of Amides by Palladium Catalysis
Shicheng Shi and Michal Szostak*
Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States
S
* Supporting Information
ABSTRACT: Transition-metal-catalyzed cyanation of aryl halides
is a process of significant importance in the preparation
pharmaceuticals, organic materials and agrochemicals. Here, we
demonstrate a palladium-catalyzed decarbonylative cyanation of
amides by highly selective carbon−nitrogen bond cleavage for the
synthesis of a wide range of aryl nitriles. The utility of this
technology is demonstrated by the synthesis of isotopically labeled
aryl nitriles and orthogonal cross-coupling reactions of bench-stable amides to establish cross-coupling synthons with opposite
polarity.
romatic nitriles represent extremely useful building blocks
Ain organic synthesis that can be exploited to generate an
array of versatile products by standard functional group
transformations, including benzoic acid derivatives, amines,
aldehydes, ketones, imines, and heterocycles.1 Furthermore,
aryl nitriles represent a common structural motif in a large
number of pharmaceuticals, bioactive natural products, agro-
chemicals, dyes, and functional materials (Figure 1).2
Figure 1. Examples of pharmaceutically important benzonitriles.
Figure 2. (a) Cross-coupling of amides. (b) Classical cross-coupling of
aryl halides and equivalents, and palladium-catalyzed redox-neutral
decarbonylative cyanation of amides: a novel strategy for the synthesis
of aryl nitriles.
The synthesis of aryl nitriles has been classically achieved by
the Rosenmund−von Braun3 or Sandmeyer reactions4 of aryl
halides or diazonium salts. In the past decade, significant
progress has been made in the development of methods to
Among many advantages of using amides as precursors to
access aryl nitriles directly by the transition-metal-catalyzed
cross-coupling of aryl halides.5 Furthermore, elegant methods
acylmetal species is the availability of tricoordinate nitrogen
for the chelation-assisted cyanation6 and electrophilic cyanation
geometry that allows controlled access to additional amide
geometries by rational variation of N-substitution.16aa,ab
Methods to replace common aryl halide electrophiles with
stable carboxylic acid derivatives are rare.16k−m,q,17,18
Here, we describe the first palladium-catalyzed decarbon-
ylative cyanation of amides by carbon−nitrogen bond cleavage
for the synthesis of a broad range of aryl nitriles (Figure 2B).
Cross-coupling of an alkenyl amide is also reported. Despite
elevated temperatures, decarbonylative cross-coupling methods
represent an attractive coupling manifold due to (i) the use of
of organometal nucleophiles7 have been developed. Recently,
direct cyanation of arenes by a photoredox mechanism has been
disclosed.8 Noteworthy progress in the synthesis of alkyl nitriles
by transfer hydrocyanation9 and radical relay10 mechanisms has
been made.11,12 However, the synthesis of aryl nitriles from
carboxylic acid derivatives after direct oxidative addition into
the acyl bond13 represents a major challenge. Carboxylic acid
derivatives are cheap, readily accessible, and derived from a
different pool of precursors than halides, phenols, anilines, or
hydrocarbons.14
Of major interest is the development of synthetic methods
Received: April 20, 2017
for the cross-coupling of amide derivatives (Figure 2A).15,16
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01199
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Letter
common carbonyl-containing substrates, (ii) orthogonal
selectivity, (iii) reduction of halide waste.13 The following
features of our protocol should be noted: (1) The process
constitutes the first example of a palladium-catalyzed decarbon-
ylative cross-coupling of bench-stable carboxylic acid derivatives
under ligand-controlled catalysis conditions.13,14 (2) The
reaction proceeds with exclusive decarbonylation/acyl selectiv-
ity via N−C activation. (3) The reaction proceeds in the
absence of external oxidants; (4) the utility has been
highlighted by the synthesis of labeled nitriles. Using amide
derivatives as aryl electrophiles enables tuning of the acyl bond
reactivity for direct oxidative addition.15 Given the pivotal role
of amides19 as key building blocks in peptides, polymers, and
pharmaceuticals,20 methods that allow cross-coupling of the
amide bond are likely to have a broad impact on chemical
synthesis.
Table 1. Optimization of Pd-Catalyzed Decarbonylative
Cyanation of Amides
a
b
entry
catalyst
ligand
M(CN)x
yield (%)
1
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
Pd(dba)2
−
−
−
−
CuCN
25
46
51
75
21
81
27
59
<2
53
23
70
12
85
2
KCN
3
NaCN
4
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
Zn(CN)2
5
PPh3
6
P(4-MeO-C6H4)3
P(4-CF3-C6H4)3
dppb
7
8
Our development of Pd-,16k,q Ni-,16l and Rh-catalyzed16m
decarbonylative reactions by N−C cleavage prompted us to
question whether this bond activation platform might be
exploited for the development of redox-neutral (cf. oxidative)
decarbonylative cyanation of carboxylic acid derivatives.13
Notably, the use of a versatile palladium catalysis platform in
the coupling makes this method synthetically appealing.21
After a very extensive survey of various reaction parameters
we discovered a catalyst system that promotes the desired
cross-coupling [Table 1, Pd(OAc)2, 5 mol %; PCyPh2, 20 mol
%; Zn(CN)2, 2 equiv; dioxane, 150 °C]. Under the optimized
conditions the cross-coupling of 1 afforded the cyanation
product in 91% yield. Key optimization results are shown in
Table 1. Various catalysts were tested, and Pd(OAc)2 showed
the best activity. The key optimization result involved
identification of Zn(CN)2 as the cyanide source. We
hypothesize that low concentration of cyanide ions and faster
transmetalation of cyanide to the acylpalladium contribute to its
high reactivity. Note that zinc cyanide is one of the preferred
cyanide sources for industrial development.5a Importantly, a
71% yield of the coupling product was obtained using only 0.5
equiv of Zn(CN)2 (entry 25), providing an entry point for
future studies. Control experiments in the absence of catalyst
resulted in recovery of 1. Using the less distorted anilides,16a N-
Boc-carbamates, and N-Ts-sufonamides,15,16 as well as acyl
chlorides, benzoates,18a−d and thiobenzoates,18e only a trace or
none of the cross-coupled product was formed.16ab Note that
N-glutarimide amides are easy to handle, bench-stable,
crystalline solids readily prepared from carboxylic acids.15a,b
The insertion occurred selectively at the N−C bond, with
cleavage of the alternative σ N−C bond and amide deamidation
not observed.16y To our knowledge, this transformation
represents the first example of decarbonylative cyanation of
amides. The successful decarbonylative cross-coupling of 1
highlights the advantageous use of amide derivatives as
coupling partners in decarbonylative manifolds.15−17
9
dppp
10
11
12
13
14
15
Sphos
Xphos
Xanthphos
PCy3HBF4
PCy2Ph
PCyPh2
PCyPh2
PCyPh2
PCyPh2
PCyPh2
PCyPh2
PCyPh2
PCyPh2
PCyPh2
PCyPh2
PCyPh2
c
91
d
16
80
60
3
e
17
f
18
g
19
27
70
83
69
89
86
71
h
20
i
21
j
22
23
24
k
25
Pd(OAc)2
a
Conditions: amide (0.2 mmol), M(CN)x (2.0 equiv), catalyst (5 mol
b
%), ligand (20 mol %), dioxane (0.25 M), 150 °C, 16 h. GC/1H
NMR yields. Isolated yield. Pd(OAc)2 (3 mol %), ligand (12 mol
c
d
e
f
g
h
i
j
%). PCyPh2 (10 mol %). 80 °C. 100 °C. 130 °C. Toluene. THP.
k
Zn(CN)2 (0.50 equiv). See Supporting Information (SI) for full
details.
amides have not been tested. Of particular note, amides
containing sensitive aldehyde (2q), ketone (2r), ester (2s), and
nitrile (2t) groups were tolerated. Aldehydes, ketones, and
nitriles are not tolerated in related C−O couplings,18a−d clearly
showing the advantage of amide electrophiles. At this stage, a
nitro group has not been tested. In addition, extended
naphthalene units (2u−2v) and conjugated polyarenes (2w−
2x) were well accommodated.
Finally, the reaction scope could be readily extended to the
synthesis of valuable vinyl nitriles (2y).5a Full selectivity in the
amide cross-coupling in the presence of an activated ester that
has been reported in Suzuki cross-coupling using Pd-NHC
catalysis16r,23 is observed (2z). This result highlights the
potential to achieve high selectivity using amide electrophiles
under orthogonal cross-coupling conditions. Importantly,
amides containing protected alcohols (2aa) and amines (2ab)
were also competent substrates. NaCN gave a higher yield of
2ab. The presence of these groups is crucial for further
synthetic transformations. For example, benzonitriles such as
2aa react smoothly in Fe-catalyzed sp3−sp2 OTs cross-
coupling.24
With the optimized conditions in hand, we explored the
preparative scope of the reaction (Scheme 1). As shown, the
scope of the reaction is very broad and tolerates the coupling of
electron-neutral (2a−2d), electron-rich (2e−2f), and electron-
withdrawing (2g−2h) substrates. Steric hindrance is well-
tolerated (2b). Importantly, fluoro-substituents are readily
accommodated (2i−2l). Furthermore, saturated oxygen hetero-
cycles that are prone to Ni-catalyzed C−O cleavage (2m−
2n),22 as well as quinoline (2o) and thiophene heterocycles
(2p), provided the corresponding coupling products in
moderate to excellent yields. At this stage, other heterocyclic
Scheme 2 highlights two appealing applications of the
decarbonylative cyanation described herein. The reaction utility
B
DOI: 10.1021/acs.orglett.7b01199
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Letter
Scheme 1. Pd-Catalyzed Decarbonylative Cyanation of
Amides
Scheme 2. Applications of Pd-Catalyzed Decarbonylative
Cyanation of Amides
a
a b
,
a
a: 1a′ (1.0 equiv), Pd(OAc)2 (1 mol %), 4-MeO-C6H4-B(OH)2 (1.5
b
equiv), K2CO3 (2.0 equiv), DMA/H2O (5:1), rt, 3 h. b: 1a′ (1.0
equiv), Pd(OAc)2 (2 mol %), CuI (20 mol %), PPh3 (5 mol %), B2pin2
(1.5 equiv), Cs2CO3 (1.5 equiv), CH3CN:H2O (4:1), rt, 12 h.
Scheme 3. Proposed Mechanism
a
Conditions: amide (1.0 equiv), Zn(CN)2 (2.0 equiv), Pd(OAc)2 (5
mol %), PCyPh2 (20 mol %), dioxane (0.25 M), 150 °C, 16 h. Isolated
b
yields. Pd(PPh3)4 (5 mol %), NaCN (2.0 equiv). See SI for full
details.
into the amide N−C bond/transmetalation. Further studies to
elucidate the mechanism are ongoing.
In conclusion, we have developed the first Pd-catalyzed
decarbonylative cyanation of amides by C−N bond cleavage for
the synthesis of aryl nitriles. The method hinges on the use of
amides as selective acyl precursors for the formation of
acylmetal species with high chemoselectivity. Studies directed
at further optimization to achieve improved catalytic efficiency
and broader scope are currently ongoing in our laboratories.
is demonstrated by orthogonal cross-coupling reactions of
bench-stable amides to establish cross-coupling synthons with
opposite polarity for the first time in the context of amide N−C
bond activation (Scheme 2A).15,16 The amide group is not
reactive toward Pd(0)-orthogonal Suzuki−Miyaura and
Miyaura cross-coupling reactions. The utility is further
demonstrated by the synthesis of isotopically labeled aryl
nitriles by formal carbon exchange in carboxylic acid precursors
(Scheme 2B).5,13−15
Several studies were conducted to gain insight into the
reaction mechanism (see SI). A possible mechanism is
presented in Scheme 3. Acyl cyanide was not observed in the
absence of palladium.25 The key step involves metal insertion
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01199.
Procedures and analytical data (PDF)
C
DOI: 10.1021/acs.orglett.7b01199
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: michal.szostak@rutgers.edu.
ORCID
Michal Szostak: 0000-0002-9650-9690
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by Rutgers University. The 500
MHz spectrometer used in this study was supported by the
NSF-MRI grant (CHE-1229030).
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