Synthesis of amides by palladium-catalyzed decarbonylative coupling of carboxylic acids with isocyanides

Article abstract of DOI:10.1002/ejoc.201300982

Amides were synthesized from carboxylic acids and isocyanides under Pd ^II/Ag^I/base catalytic conditions. The reaction conditions were optimized, and the scope was studied. Various carboxylic acids and aryl isocyanides are compatible with this reaction, but alkyl isocyanides are not. A plausible mechanism, which features distinctive Pd^II-mediated decarbonylative coupling and unprecedented nucleophilic attack of carboxylates to Pd^II-ligated isocyanides, was proposed to account for this reaction. N-Arylamides are assembled in high yields from carboxylic acids and isocyanides under Pd^II/Ag^I/base catalytic conditions. The decarbonylative coupling features unprecedented nucleophilic attack of the carboxylate on the isocyanide, which acts as an electrophile as a result of its coordination to Pd^II, and this inverted reactivity is proposed to account for this transformation.

Full text of DOI:10.1002/ejoc.201300982

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300982
Synthesis of Amides by Palladium-Catalyzed Decarbonylative Coupling of
Carboxylic Acids with Isocyanides
Lin Huang,[a] Haili Guo,[a] Lingxia Pan,[a] and Chunsong Xie*[a]
Keywords: Amides / Carboxylic acids / Palladium / Decarbonylation / Isocyanides
Amides were synthesized from carboxylic acids and iso-
cyanides under PdII/AgI/base catalytic conditions. The reac-
tion conditions were optimized, and the scope was studied.
Various carboxylic acids and aryl isocyanides are compatible
with this reaction, but alkyl isocyanides are not. A plausible
mechanism, which features distinctive PdII-mediated decarb-
onylative coupling and unprecedented nucleophilic attack of
carboxylates to PdII-ligated isocyanides, was proposed to ac-
count for this reaction.
harsh reaction conditions and restricted functional group
tolerance, as well as the need for an extra removing step if
the amides are the desired products. Thus, the search for
straightforward access to amides, especially under catalytic
conditions, from carboxylic acids and isocyanides is of im-
portance. During our investigations on transition-metal-
catalyzed isocyanide reactions,[8] we occasionally found that
under palladium catalysis a formal decarbonylative cou-
pling of isocyanides with carboxylic acids could occur to
produce amide products in high efficiency. Given that the
amides can be formed directly and that the reaction can be
performed with a catalytic amount of PdII/AgI/base, we
think that this reaction will serve as a foundation for the
further development of a simpler and economic avenue to
amide synthesis.
Introduction
Owing to its prevalence in functional organic molecules,
the amide structure represents a continual synthetic target
in organic synthesis.[1] Among the various strategies that
have been developed for amide assembly,[2] direct amidation
of carboxylic acids with amines by using an activation rea-
gent, such as acyl chlorides, carbonyldiimidazole, carbodi-
imides, onium salts, and trialkylaluminum,[3] is the most
widely used. However, in these processes, a stoichiometric
or over-stoichiometric amount of the activation reagent is
always needed. The use of these activators has several prac-
tical drawbacks, as most of these reagents are toxic and
expensive. Wasteful byproducts are often generated, and
therefore, extra purification is needed. Hence, the develop-
ment of novel and simpler methods for the direct amidation
of carboxylic acids is of significance.
Towards this end, there are currently two main strategies
that can be considered. The first is the amidation of carbox- Results and Discussion
ylic acids by using amines under more economic conditions.
We commenced our investigation by choosing 2-meth-
Typical examples include the direct amidation of carboxylic
acids by using amines under thermal[4] and catalytic condi-
tions.[4b,5] The second is the use of alternative amidation
components by which the carboxylic acids can be amidated
through different mechanisms. One successful reaction of
this kind developed in recent years is the formimidation of
carboxylic acids by isocyanides under microwave (MW) ir-
radiation conditions.[6] Although this reaction, pioneered
mainly by Danishefsky and his co-workers, has been used
successfully in the synthesis of a variety of biologically
active peptides,[7] it still suffers from limitations such as
ylbenzoic acid (1a) and 1-ethyl-4-isocyanobenzene (2a) as
model substrates (Scheme 1). By using Pd(OAc)2 as the cat-
alyst and KOAc as the base, N-(4-ethylphenyl)-2-methyl-
benzamide (3aa) and N-(4-ethylphenyl)acetamide (3la) were
produced in 59 and 113% yield (based on 1a), respectively.
[a] College of Material, Chemistry and Chemical Engineering,
Hangzhou Normal University,
Hangzhou 310036, P. R. China
E-mail: chunsongxie@hznu.edu.cn
Homepage: http://ch.hznu.edu.cn/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300982.
Scheme 1. Initial experiment.
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L. Huang, H. Guo, L. Pan, C. Xie
SHORT COMMUNICATION
Under the deduction that 3la might be produced by the lowered (Table 1, entries 16–18). However, the yield was still
reaction of 2a with KOAc, we switched the base to K2CO3 maintained at a synthetically useful level (65%) even after
and pleasingly found that the formation of 3la was com- the reaction temperature was reduced to 70 °C. Upon short-
pletely inhibited (Table 1, entry 1). Control experiments re- ening the reaction time, lower yields were obtained, and this
vealed that palladium and silver were necessary for this re- indicates that 24 h was necessary to keep the reaction at
action, and the reaction was totally halted if either of them high efficiency (Table 1, entries 19–21).
was absent (Table 1, entries 2 and 3). The base did not seem
With the optimized conditions in hand, we next studied
to be so crucial, because 3aa was still able to be produced the scope of the reaction, and the results are summarized
in the absence of a base, albeit in low yield (14%; Table 1, in Table 2. Besides model substrate 1a, other aromatic carb-
entry 4). Further optimization indicated that K3PO4 was oxylic acids such as benzoic acid (1b), 2-chlorobenzoic acid
superior to K2CO3, as it provided the product in higher (1c), and 2-styrylbenzoic acid (1d) were all competent sub-
yield (91%; Table 1, entry 5). Screening of the catalysts re- strates in the reaction, and they all gave the amide products
vealed that Pd(tfa)2 (tfa = trifluoroacetate) was the best in very high yields (Table 2, entries 2–4). It should be em-
choice, as it gave 3aa in quantitative yield (Table 1, entries 6 phasized that 1d, which has a double bond at the ortho posi-
and 7). Coinage metals such as copper and iron were also tion, partook in the reaction even though the double bond
able to catalyze this reaction, but they gave lower yields of has been proven to have a high tendency to undergo intra-
the products (Table 1, entries 8–13). Noteworthy, the reac- molecular nucleophilic attack of the carboxylic group under
tion could be performed by using a catalytic amount of similar conditions.[9] In addition, aromatic carboxylic acids
Ag2CO3 and K3PO4, and the product was obtained in a bearing substituents in the para position were also reliable
comparable yield under these conditions (Table 1, entry 14). reaction partners with 2a, and they afforded the target
Further reduction of the loading of these additives deliv- amides in moderate to good yields (Table 2, entries 5–8). In
ered a slightly lower yield (Table 1, entry 15). Lowering the general, substrates with electron-donating groups gave
reaction temperature proved to be detrimental, and the higher yields than substrates with electron-withdrawing
yield of the product gradually fell as the temperature was groups. The exception was 4-methoxybenzoic acid (1f),
which provided a lower yield of the product than 1e. A pos-
sible reason is that the methoxy group might impede the
Table 1. Optimization of the reaction conditions.[a]
reaction by binding to palladium.[10] Additionally, heterocy-
clic carboxylic acids such as furan-2-carboxylic acid (1i)
also underwent the transformation to provide the heterocy-
clic amide in almost quantitative yield (98%; Table 2, en-
try 9). Likewise, α,β-unsaturated carboxylic acids such as
3-methylbut-2-enoic acid (1j) and cinnamic acid (1k) were
proven to be good substrates in this reaction, and they af-
forded the α,β-unsaturated amides in good to perfect yields
Entry [M]
Base
T
[°C]
t
[h]
Yield[b]
[%]
(Table 2, entries 10 and 11). Moreover, aliphatic carboxylic
acids such as acetic acid (1l) and 2-(4-chlorophenyl)acetic
acid (1m) participated well in this transformation to deliver
the aliphatic amides in high yields (Table 2, entries 12 and
13). The amino-protected natural amino acid (S)-1-acetyl-
pyrrolidine-2-carboxylic acid (1n) was also a competent re-
actant in the reaction, and it produced the α-amino amide
derivative in high yield (Table 2, entry 14).
In addition, we also tested the reaction of 1h with benzyl
isocyanide (2b) and obtained desired amide 3hb in 23%
yield (Scheme 2). Further investigation disclosed that alkyl
isocyanides, such as tert-butyl isocyanide and 1-isocy-
anohexane, were completely incompatible with this reac-
tion. This represents one of the major limitations of this
reaction, which should be improved in further studies. A
possible reason for this incompatibility is that the alkyl iso-
cyanides might reduce PdII to Pd0 in these reactions.[11]
On the basis of these results, we conducted some prelimi-
nary mechanistic experiments on the reaction (Scheme 3).
First, the transition-metal-catalyzed decarbonylative cou-
pling of carboxylic acids and their derivatives has gained
much attention in recent years.[12] From the viewpoint of
the mechanism, these transformations are normally initi-
ated by the formation of acyl–metal species through the oxi-
1
Pd(OAc)2
none
K2CO3
K2CO3
K2CO3
none
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
110
90
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
12
6
55
0
0
2
3[c]
4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
Pd(tfa)2
CuCl2
14
91
87
99
44
39
47
25
26
29
97
84
84
82
65
83
60
47
5
6
7
8
9
10
11
12
CuCl
Cu(OAc)2·H2O K3PO4
FeBr2
FeCl3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
13
Fe(AcO)2
Pd(tfa)2
Pd(tfa)2
Pd(tfa)2
Pd(tfa)2
Pd(tfa)2
Pd(tfa)2
Pd(tfa)2
Pd(tfa)2
14[d]
15[e]
16[d]
17[d]
18[d]
19[d]
20[d]
21[d]
70
130
130
130
3
[a] Reaction conditions: 1a (1 mmol), 2a (2 mmol), [M]
(0.05 mmol), Ag2CO3 (2 mmol), base (2 mmol), dioxane (4 mL).
[b] Yield of isolated product. [c] In the absence of Ag2CO3. [d] The
loading of Ag2CO3 and K3PO4 was 20 mol-%. [e] The loading of
Ag2CO3 and K3PO4 was 10 mol-%.
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Synthesis of Amides by Pd-Catalyzed Decarbonylative Coupling
Table 2. The scope of the reaction.[a]
Scheme 2. Reaction between 1h and 2b.
dative addition of low-valent metals to the C(O)–O bonds
in the carboxylates. The subsequent extrusion of CO from
the as-formed acyl–metal intermediates results in the de-
carbonylative coupling reaction. To test if our reaction pro-
ceeded by a similar pathway, 13C-labled benzoic acid 1b-13C
was treated with isocyanide 2a under the standard condi-
tions. Analysis of the formed product by 13C NMR spec-
troscopy indicated that the labeled carbon atom was re-
tained in amide 3ba-13C [Scheme 3, Eq. (1)]. This implied
that the carbon atom of the extruded CO came from the
isocyanide and that the decarbonylation mechanism of our
reaction was quite different with the above-mentioned tran-
sition-metal-catalyzed decarbonylative couplings.
Scheme 3. Preliminary mechanistic studies.
Second, N-formyl amide 3bbЈ was resubjected to the
standard conditions to see if the final amide was produced
by decarbonylation of 3bbЈ [Scheme 3, Eq. (2)]. However,
no reaction took place, and this implied that 3bbЈ was not
the intermediate. Third, we conducted the reaction with a
stoichiometric amount of KOAc and isocyanide 2a and
found that desired amide 3la was furnished in 48% yield
[Scheme 3, Eq. (3)]. This implied that the direct reaction
with the potassium carboxylate was very slow, and this is
consistent with the fact that the catalytic reaction could not
proceed in the absence of Ag (see above). In contrast, the
[a] Reaction conditions:
(0.05 mmol), Ag2CO3 (0.2 mmol), K3PO4 (0.2 mmol), dioxane
(4 mmol), 130 °C, 24 h. [b] Yield of isolated product.
1 (1 mmol), 2a (2 mmol), Pd(tfa)2
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L. Huang, H. Guo, L. Pan, C. Xie
SHORT COMMUNICATION
direct reaction of a stoichiometric amount of AgOAc with
2a under Pd(tfa)2 catalysis proceeded efficiently to afford
amide 3la in 94% yield [Scheme 3, Eq. (4)]. This indicated
that the silver carboxylate, which is generated in situ, might
be the active intermediate in this reaction. After adding
some molecular sieves (MS) to the reaction, the yield was
reduced to 64%. This showed that the hydrogen in the
amide moiety of the product might come from the water in
the solvent.
Scheme 5. Competing experiment.
Conclusions
On the basis of these results, a plausible mechanism is
proposed, as depicted in Scheme 4. In solution, aryl isocya-
nide 2 coordinates with PdII to form complex I. This coor-
dination would invert the reactivity of the isocyanide from
a nucleophilic species to an electrophilic one. Meanwhile,
carboxylic acid 1 reacts with K3PO4 to afford potassium
carboxylate IЈ, which is further converted into silver carb-
oxylate IIЈ by in situ Ag–K exchange with Ag2CO3. Next,
two possible pathways (A and B) could be involved. Path-
way A involves the direct intermolecular attack of IIЈ to I to
produce imidoyl–PdII intermediate III. Pathway B involves
transmetalation of I with silver carboxylate IIЈ followed by
inner-sphere nucleophilic attack of the coordinated carb-
oxylate to PdII-ligated isocyanide II to give imidoyl–PdII
intermediate III. After the generation of intermediate III,
intramolecular acyl transfer occurs to provide carbamoyl–
PdII IV. Decarbonylation of IV gives amidate–PdII V, which
reacts with the carboxylic acid to produce amide 3 and to
In summary, the PdII-catalyzed decarbonylative coupling
of isocyanides with carboxylic acids to afford amides was
reported. Aryl, heteroaryl, and alkyl carboxylic acids and a
variety of isocyanides, including aryl and benzyl isocya-
nides, were competent substrates in the reaction, but alkyl
isocyanides were incompatible with the reaction conditions.
A possible mechanism was proposed, and distinctive de-
carbonylative coupling involving the unprecedented nucleo-
philic attack of carboxylates to PdII-ligated isocyanides was
proposed to be involved. Modifications to this reaction to
accommodate alkyl isocyanides and further applications of
this transformation are currently ongoing in our laboratory.
Experimental Section
General Methods: All reactions were performed under an Ar atmo-
sphere in oven-dried flasks. Dioxane was distilled from Na by using
benzophenone as an indicator. 1-Ethyl-4-isocyanobenzene (2a) was
prepared according to a literature method.[14] Other materials were
purchased from commercial sources and used without additional
purification. 1H NMR spectra were recorded at 400 MHz by using
tetramethylsilane as internal standard. 13C NMR spectra were re-
corded at 100 MHz. Mass spectrometry data were collected with a
HRMS-EI instrument.
regenerate the PdII catalyst. Given that amidate–PdII
V
could be protonated directly by carboxylic acid 1, the
K3PO4 and Ag2CO3 bases can be used catalytically.
General Procedure for the Decarbonylative Coupling of Isocyanides
with Carboxylic Acids: An oven-dried flask was charged with 2-
methylbenzoic acid (1a; 136 mg, 1.0 mmol), 1-ethyl-4-isocyanoben-
zene (2a; 262 mg, 2.0 mmol), Pd(tfa)2 (17 mg, 0.05 mmol), Ag2CO3
(55 mg, 0.2 mmol), and K3PO4 (42 mg, 0.2 mmol) at room tem-
perature. The system was degassed and backfilled with Ar (3ϫ).
Then, dioxane (4 mL) was injected by syringe. The reaction mixture
was allowed to react at 130 °C for 24 h. Upon completion of the
reaction, the mixture was filtered through a pad of Celite, and the
filtrate was concentrated to dryness. The residue was then separated
on a silica gel column, and the final product was obtained as yellow
crystals (232 mg, 97%).
Scheme 4. Proposed mechanism.
N-(4-Ethylphenyl)-2-methylbenzamide (3aa): 1H NMR (400 MHz,
CDCl3, TMS): δ = 7.66 (s, 1 H), 7.53 (d, J = 7.6 Hz, 2 H), 7.44 (d,
J = 7.4 Hz, 1 H), 7.35 (t, J = 7.4 Hz, 1 H), 7.12–7.30 (m, 4 H),
2.66 (q, J = 7.7 Hz, 2 H), 2.49 (s, 2 H), 1.26 (t, J = 7.4 Hz, 3
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 168.07, 140.70, 136.60,
136.38, 135.64, 131.21, 130.17, 128.41, 126.64, 125.86, 120.10,
28.37, 19.80, 15.72 ppm. HRMS (EI): calcd. for C16H17NO [M]+
239.1310; found 239.1309.
To distinguish if pathway A or B was responsible for the
nucleophilic attack of the carboxylates to the isocyanides,
we performed a stoichiometric reaction by using Pd(OAc)2
and CD3CO2D as competing substrates to react with 2a
with 1 equivalent of Ag2CO3 and K3PO4 (Scheme 5). The
reaction went to completion very quickly and acetamide
product 3la-D was obtained with a deuteration ratio of
80% (see the Supporting Information for details). This indi-
cated that intermolecular attack (pathway A) might be the
likely pathway, because if the reaction proceeded by path-
way B, the deuteration ratio in 3la-D would not exceed
50%.[13]
N-(4-Ethylphenyl)-3-methylbut-2-enamide
(3ja):
1H
NMR
(400 MHz, CDCl3, TMS): δ = 7.44 (d, J = 7.5 Hz, 2 H), 7.26 (s, 1
H), 7.12 (d, J = 7.5 Hz, 2 H), 5.71 (s, 1 H), 2.60 (q, J = 7.6 Hz, 2
H), 2.20 (s, 3 H), 1.87 (s, 3 H), 1.20 (t, J = 7.6 Hz, 3 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 165.10, 152.78, 140.01, 135.89,
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Synthesis of Amides by Pd-Catalyzed Decarbonylative Coupling
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128.23, 119.98, 118.86, 28.30, 27.32, 19.94, 15.66 ppm. HRMS (EI):
calcd. for C13H17NO [M]+ 203.1310; found 203.1311.
N-(4-Ethylphenyl)acetamide (3la): 1H NMR (400 MHz, CDCl3,
TMS): δ = 8.03 (s, 1 H), 7.41 (d, J = 8.4 Hz, 2 H), 7.12 (d, J =
8.4 Hz, 2 H), 2.60 (q, J = 7.6 Hz, 2 H), 2.13 (s, 3 H), 1.21 (t, J =
7.6 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 168.79,
140.35, 135.64, 128.22, 120.34, 28.30, 24.34, 15.63 ppm. HRMS
(EI): calcd. for C10H13NO [M]+ 163.0097; found 163.0098.
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Acknowledgments
The authors gratefully thank the grant support from the Zhejiang
Provincial Research Program (2012C31026), Hangzhou Normal
University, and Program for Changjiang Scholars and Innovative
Research Team in Chinese University (PCSIRT) (IRT 1231).
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Received: July 2, 2013
Published Online: August 15, 2013
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