Direct C-H aminocarbonylation ofN-heteroarenes with isocyanides under transition metal-free conditions

Article abstract of DOI:10.1039/d1ob00245g

A C-C bond forming amide synthesis through direct C-H aminocarbonylation ofN-heteroarenes with isocyanides was developed. The reaction was mediated by an inorganic persulfate salt under transition metal-free conditions. Mechanistic studies suggested a radical pathway for this reaction without the participation of H₂O and O₂. This method also showed merits of substrate availability, easy operation and atom economy. It provided an efficient route for straightforward synthesis ofN-heteroaryl amides.

Full text of DOI:10.1039/d1ob00245g

Organic &
Biomolecular Chemistry
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COMMUNICATION
Direct C–H aminocarbonylation of N-heteroarenes
with isocyanides under transition metal-free
conditions†
Cite this: Org. Biomol. Chem., 2021,
19, 2917
Received 8th February 2021,
Accepted 15th March 2021
a,b
Zhong Zhou,a Huihui Ji,a Qing Li,a Qian Zhang*a and Dong Li
*
DOI: 10.1039/d1ob00245g
rsc.li/obc
A C–C bond forming amide synthesis through direct C–H amino- and are widely involved in organometallic chemistry.5 In
carbonylation of N-heteroarenes with isocyanides was developed. recent years, the employment of isocyanates as C1 synthons
The reaction was mediated by an inorganic persulfate salt under has emerged as an efficient strategy for the amide synthesis.
transition metal-free conditions. Mechanistic studies suggested a These methods are generally performed through transition
radical pathway for this reaction without the participation of H2O metal-catalyzed aminocarbonylation with isocyanides and stoi-
and O2. This method also showed merits of substrate availability, chiometric H2O and/or terminal oxidants (Scheme 1a).6
easy operation and atom economy. It provided an efficient route Considering the cost of transition-metal catalysts and the
for straightforward synthesis of N-heteroaryl amides.
requirement for removal of trace amounts of metal residues in
the products especially in pharmaceutical manufacturing,
development of metal-free methodologies is also highly
desired.7 Several groups successfully achieved the transition
metal-free aminocarbonylation with isocyanides through base
or visible light-promoted reactions of aryl diazonium salts or
azo sulfones (Scheme 1b).8,9 However, the application of these
methods is limited by the pre-functionalized substrates. The
development of a direct C–H aminocarbonylation method with
isocyanides under transition metal-free conditions will be
intriguing and attractive.
Introduction
Amides are of great importance because of their ubiquitous
existence and essential role in natural products, biologically
active molecules, functional materials and organic synthesis
intermediates.1 Great efforts have been devoted to the develop-
ment of facile and efficient methods for amide synthesis all
the time. The amide bond is typically constructed via C–N
bond formation reactions between carboxylic acid surrogates
and appropriate amine sources in both laboratories and indus-
tries.2 In recent decades, C–C bond forming aminocarbonyla-
tion has been developed as an efficient route to this end,
which significantly broadened the avenue of amide synthesis
methodologies.3 Among these methods, isocyanides have been
revealed as efficient building blocks.
Isocyanides have attracted increasing attention in organic
synthesis because they feature high synthetic efficiency and
molecular diversity in many heterocycle syntheses and multi-
component reactions.4 As isoelectronic species of carbon mon-
oxide, isocyanides exhibit a strong metal coordination ability
aHubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei
University of Technology, Wuhan 430068, China.
E-mail: zhangqian620@hotmail.com, dongli@mail.hbut.edu.cn
bHubei Key Laboratory of Drug Synthesis and Optimization, Jingchu University of
Technology, Jingmen 448000, China
†Electronic supplementary information (ESI) available: For detailed experi-
mental procedures, analytical data and the copies of NMR spectra. See DOI:
10.1039/d1ob00245g
Scheme 1 Amides synthesis through aminocarbonylation with
isocyanides.
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Table 1 Optimization of the reaction conditionsa
Entry
Deviations from the standard conditions
Yieldb (%)
1
2
3
4
5
6
7
8
None
75(15)c
24
Trace
Trace
0
K2S2O8 instead of Na2S2O8
(NH4)2S2O8 instead of Na2S2O8
Oxone instead of Na2S2O8
TBHP instead of Na2S2O8
DTBP instead of Na2S2O8
mCPBA instead of Na2S2O8
DCE instead of CH3CN
DMF instead of CH3CN
HFIP instead of CH3CN
H2O instead of CH3CN
CH3CN/H2O (1 : 1) instead of CH3CN
1.5 equiv. of Na2S2O8
2 equiv. of tBuNC
0
Fig. 1 Representative drugs containing N-heteroaryl amide moieties.
34
48
0
0
0
9
10
11
12
13
14
15
16
17
18
On the other hand, N-heteroarenes such as pyridines, qui-
nolines and isoquinolines are important structure motifs
which pervasively exist in natural products, pharmaceuticals,
functional materials and organic synthesis ligands.10 The
development of new methodologies for synthesis of
N-heteroarene derivatives has attracted great attention.11
0
66
36
58
14
61
50
100 °C
80 °C
12 h
4 h
Represented by the Minisci-type reaction,
a number of
a Reaction conditions: Isoquinoline (1a) (0.2 mmol), t-butyl isocyanide
(2a) (0.6 mmol), and Na2S2O8 (0.4 mmol) in CH3CN (2.0 mL) under
stirring at 120 °C for 24 h. b Isolated yield. c Recovery of the starting
materials.
methods have been developed for the direct C–H functionali-
zation of N-heteroarenes.12 The amide bond attached to
N-heteroarenes is also an important class of moieties which is
present in many drugs and biologically active molecules
(Fig. 1).13 Previously, N-heteroaryl amides were mainly pre-
pared through the reaction of N-heteroarenes or heteroarene
N-oxides with amide counterparts such as formamides, hydra-
zine carboxamides or oxamic acids.14 However, this transform-
ation still required transition metal catalysts, or pre-functiona-
lized substrates. To the best of our knowledge, the preparation
of N-heteroaryl amides through the C–H aminocarbonylation
reaction between N-heteroarenes and isocyanides has been
undeveloped. Recently, we paid attention to both the develop-
ment of amide synthesis methods and functionalizations of
N-heteroarenes.15,16 Herein, we reported the first transition
metal-free direct C–H aminocarbonylation of N-heteroarenes
with isocyanides (Scheme 1c). The reaction was promoted by a
cheap, stable and readily available inorganic persulfate salt
under transition metal-free conditions.17 It provided an
efficient route for straightforward synthesis of N-heteroaryl
amides in an atom economical manner.
or anaerobic conditions. Using K2S2O8 instead of Na2S2O8 led
to a drastic decrease of the product yield (entry 2). Other in-
organic persulfate salts such as (NH4)2S2O8 and oxone were
not effective for this reaction (entries 3 and 4). Organic per-
oxide oxidants such as TBHP, DTBP and mCPBA were also
tested. Except for mCPBA which afforded 34% yield, the others
resulted in no reaction (entries 5–7). Other solvents such as
DCE, DMF, HFIP and H2O were not suitable (entries 8–11).
Unlike most previous studies of aminocarbonylation with iso-
cyanides, a mixed solvent of CH3CN and H2O was unfavorable
for this reaction (entry 12). On reducing the amount of
Na2S2O8 to 1.5 equiv., the yield decreased to 66% (entry 13). A
much lower yield was obtained after reducing the amount of
2a to 2 equiv. (entry 14). The temperature was also essential for
this reaction. A lower yield was obtained when the temperature
was reduced to 100 °C (entry 15). And the reaction hardly pro-
ceeded at a much lower temperature (entry 16). Finally we also
investigated the effect of reaction time. Shortening the reaction
time to 12 hours only generated 61% product yield (entry 17).
Results and discussion
We carried out our initial study with the reaction between iso- A much lower yield was obtained within 4 hours (entry 18).
quinoline (1a) and a commercially available t-butyl isocyanide
After the optimization of the reaction conditions, we then
as the model reaction (2a). After examining a series of reaction investigated the substrate scope of this method. At first, a
parameters, we were pleased to find out that 1a reacted with 3 series of N-heteroarenes (1) were examined for reactions with
equiv. of 2a in the presence of 2 equiv. of Na2S2O8 in CH3CN at t-butyl isocyanide (2a) (Scheme 2). Different substituents such
120 °C for 24 hours to provide the aminocarbonylation as methyl, chlorine and bromine were tolerated on the C3-, 4-,
product 3aa in 75% yield with 15% recovery of the starting 5-, 6- or 7-position of isoquinoline. 2-Aminocarbonylation pro-
materials (Table 1, entry 1). Notably, the reaction can be ducts were obtained selectively in similar yields without a sig-
handled under air without requirements of anhydrous solvents nificant electronic or steric hindrance effect (3b–3h). Generally
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benzo[d]oxazole and benzo[d]imidazole were not suitable for
this reaction (3u and 3v).
The scope of isocyanides was then investigated with the
reactions of isoquinoline and 4-cyanoquinoline (Scheme 3).
2,4,4-Trimethyl-2-pentanyl isocyanide reacted well with both
isoquinoline and 4-cyanoquinoline, affording the desired
amide products in moderate to good yields (4a and 5a). The
reactions of cyclohexyl isocyanide also proceeded with isoqui-
noline and 4-cyanoquinoline in moderate yields (4b and 5b).
Benzyl and phenyl isocyanides reacted with isoquinoline to
give the desired products; however the yields were low (4c and
4d). A reaction of 4-cyanoquinoline with an ester-containing
isocyanide occurred to generate the corresponding products in
moderate yield (5c).
Subsequent studies were carried out to investigate the reac-
tion mechanism (Scheme 4). At first, the reaction was con-
ducted under an atmosphere of N2 and O2 respectively
(Scheme 4a). Both reactions proceeded smoothly without a sig-
nificant change in the product yields. It excluded the involve-
ment of oxygen in this reaction. Then the addition of the
radical scavenger TEMPO or BHT was examined. Both the
reagents inhibited the reaction entirely. It indicated the exist-
ence of radical intermediates in the reaction pathway.
However, the reaction was not significantly influenced by the
t
addition of a base (K2CO3 or BuOK), which suggested that the
proton was not essential for this reaction. Isotopic labeling
experiment with H218O was also conducted. With 3 equiv. of
H218O, the product was formed in 66% yield in which the 16O
product still predominated (Scheme 4b). This suggested that
H2O didn’t participate in this reaction as the oxygen source
either. As heteroarene N-oxides and isocyanates might be
formed from the oxidation of heteroarenes and isocyanides
respectively,18 we tested these two possible intermediates inde-
pendently. The reaction between isoquinoline N-oxide (1a′)
and t-butyl isocyanide (2a) didn’t proceed with or without an
Scheme 2 Substrate scope of N-heteroarenes (1). Reaction conditions:
heteroarene (1) (0.2 mmol), t-butyl isocyanide (2a) (0.6 mmol), and
Na2S2O8 (0.4 mmol) in CH3CN (2.0 mL) under stirring at 120 °C for 24 h.
Isolated yield.
unreacted starting materials remained after reaction without
obvious by-product observed. A polycyclic phenanthridine also
reacted well to provide the corresponding product in good
yield (3i). Pyridines with various substituents were examined
subsequently. Functional groups such as methyl, t-butyl, meth-
oxyl, CN and Br were tolerated on the pyridine ring (3j–3o).
Generally, 2-aminocarbonylation occurred on 4-substituted
pyridines and 4-aminocarbonylation on 2-substituted pyri-
dines (3j–3l). Di-substituted pyridines also undergo this trans-
formation to provide the desired products in moderate to
excellent yields (3m–3o). Quinolines were also applicable in
this method to give the desired products in moderate to good
yields (3p–3s). Notably, the reaction of 2-phenylquinoline
occurred on the C4-position of quinoline without the influence
of the phenyl ring (3r). Finally, an N,S-containing heteroarene
benzo[d]thiazole was tested which also reacted to give the ami-
Scheme 3 Substrate scope of isocyanide (2). Reaction conditions: het-
eroarene (1) (0.2 mmol), isocyanide (2) (0.6 mmol), and Na2S2O8
(0.4 mmol) in CH3CN (2.0 mL) under stirring at 120 °C for 24 h. Isolated
nocarbonylation product in moderate yield (3t). However, yield.
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Conclusions
In summary, we have developed a novel protocol for persulfate
salt-mediated direct C–H aminocarbonylation of N-heteroarenes
with isocyanides under transition metal-free conditions.
Isocyanates act as C1 synthons which make the amide synthesis
atom economical. Mechanistic studies suggested a radical
pathway for this reaction. It also indicated that neither H2O nor
O2 participate as the oxygen source, unlike most previous
reports on isocyanide-involving aminocarbonylation. But the
reaction was moisture and air tolerable which can be easily
handled under air. It also showed broad N-heteroarene sub-
strate scope and good functional group compatibility. It pro-
vided an efficient route for straightforward synthesis of
N-heteroaryl amides.
Conflicts of interest
There are no conflicts to declare.
Scheme 4 Control experiments.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (No. 21702038), the open project of Hubei Provincial
Key Laboratory of Green Materials for Light Industry (No.
202007A04) and the Scientific Research Project of Hubei
Education Department (T2020023) for financial support.
oxidant (Scheme 4c). The same outcomes were obtained with
the reaction between isoquinoline (1a) and t-butyl isocyanate
(2a′) (Scheme 4d). These results ruled out the involvement of
N-oxides and isocyanates as reaction intermediates.
On the basis of the above results and previous literature, a
plausible mechanism for this reaction was proposed
(Scheme 5). At first, homolytic cleavage of the persulfate anion Notes and references
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Scheme 5 Proposed mechanism.
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