Copper-catalyzed transformation of alkyl nitriles to N -arylacetamide using diaryliodonium salts

Article abstract of DOI:10.1039/d1ra02305e

This work reports a simple and efficient method for the copper-catalyzed redox-neutral transformation of alkyl nitriles using eco-friendly diaryliodonium salts and leading to N-arylacetamides. The method features high efficiency, broad substrate scope and good functional group tolerance.

Full text of DOI:10.1039/d1ra02305e

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Copper-catalyzed transformation of alkyl nitriles to
N-arylacetamide using diaryliodonium salts†
Cite this: RSC Adv., 2021, 11, 15885
a
a
Romain Sallio,a Pierre-Adrien Payard,b Paweł Pakulski,
Iryna Diachenko,
Ilaria Ciofini,
a
a
c
Indira Fabre,b Sabine Berteina-Raboin,
Cyril Colas,
b
a
*
*
Laurence Grimaud and Isabelle Gillaizeau
Received 23rd March 2021
Accepted 15th April 2021
This work reports a simple and efficient method for the copper-catalyzed redox-neutral transformation of
alkyl nitriles using eco-friendly diaryliodonium salts and leading to N-arylacetamides. The method features
high efficiency, broad substrate scope and good functional group tolerance.
DOI: 10.1039/d1ra02305e
rsc.li/rsc-advances
Buchwald7 described an enhanced version by using chelating
nitrogen ligands and only 1 mol% of air-stable CuI. More
Introduction
recently, Xiang and Wang studied a copper-catalyzed amination
of aryl halides with nitriles, which are easily available, in pres-
ence of N,N0-dimethyl-1,2-ethanediamine as the ligand.8a Aryl-
boronic acids have also been investigated as coupling partner.8b
Major breakthroughs by Wang9 or Cui10 demonstrated an
elegant N-arylation approach to respectively (aryl)methylamines
or N-aryl-cyanamide using diaryliodonium salts. The arylation
of secondary acyclic amides has also been achieved by Olofsson
with diaryliodonium salts under mild and metal-free condi-
tions.11 Chen12 has also described the copper-catalyzed selective
arylation of arylnitriles, however restricted to the use of six-
membered cyclic diaryl iodonium salts only.
Nonetheless, the scope of the aforementioned trans-
formations is mainly limited to (hetero)aryl nitriles and the
development of effective general protocols for the arylation of
alkyl nitriles with diaryliodonium salts is still highly desirable;
only the singular case of acetonitrile having being studied by
several groups. In this context and relying on our earlier work in
copper-catalyzed reactions,13 we envisioned performing N-ary-
lamide synthesis by the copper-catalyzed oxidative trans-
formation of readily available alkyl nitriles in presence of
diaryliodonium salts. The later has recently aroused consider-
able interest as they advantageously replace iodoarenes due to
their excellent reactivity and environmentally friendly nature.14
Herein, we report the rst catalytic, single-step, and redox-
neutral transformation of alkyl nitriles acting as amine surro-
gate into N-arylacetamides (Scheme 1). This method also
N-Arylacetamides constitute the core of many structural motifs
found in biologically active compounds, drugs (e.g., lidocaine,
atorvastatin), and agrochemicals (e.g., boscalid).1 Amide bond
formation is a key step that has proved to be one of the most
important processes in organic and bioorganic chemistry. For
illustration, up to 25% of all current pharmaceuticals contain
amide bonds, and polyamides are one of the most widely rep-
resented categories of synthetic polymers.2 However, in spite of
the considerable efforts that have been made for their prepa-
ration, and taking into account their synthetic signicance, the
implementation of atom economical and environmentally-
friendly processes is still highly desirable.3 Common
approaches for the synthesis of N-arylacetamides involve the
aminolysis of activated carboxylic acid derivatives, such as
halides, anhydrides, azides, or activated esters, which are
mostly generated in an extra step using hazardous, expensive or
waste-intensive reagents.4 Alternatively, it may involve peptide
coupling reagents, such as carbodiimides or phosphonium
salts. However, such processes usually demonstrate a poor atom
economy and generate toxic by-products. As a result, catalytic
methods have recently been developed to access N-arylaceta-
mides.4b,5 Pioneering work in this eld was reported by Gold-
berg6 in the copper catalyzed synthesis of N-arylamide from
amide through a C–N coupling process, albeit under harsh
conditions limiting its broad application in organic synthesis.
a
´
Institute of Organic and Analytical Chemistry, ICOA UMR 7311 CNRS, Universite
´
´
d'Orleans, rue de Chartres, 45100 Orleans, France. E-mail: Isabelle.gillaizeau@
univ-orleans.fr
bLaboratoire des Biomolecules, LBM, Departement de chimie, Ecole normale
´
´
´
´
´
superieure, PSL University, Sorbonne Universite, CNRS, 75005 Paris, France. E-mail:
laurence.grimaud@ens.psl.eu
cInstitute of Chemistry for Health and Life Sciences, I-CLeHS, Chimie ParisTech, PSL
University, CNRS, 75005 Paris, France
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra02305e
Scheme 1 Present work.
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provides a singular way of synthesizing these compounds salt countꢁerion demonstrated that weakly coordinating anions
directly from alkyl nitriles, compared to other well-known BF4ꢁ, PF6 gave poor results.17c To prove the scalability of this
methods using the carbon of nitriles as a carbonyl source as transformation, the reaction was performed using 14 mmol of
in the Ritter reaction15 or in the hydration of nitriles.16
1a (scaling factor: 28) yielding 3aa in good yield (95%) (entry 11).
Our initial investigation started with the study of the copper- Furthermore, by applying Chen's conditions,12 the reaction of
mediated arylation of the a-methylphenylacetonitrile 1a, with benzyl cyanide 1a and diphenyliodonium triate 2a failed (entry
diphenyliodonium triate 2a, chosen as model substrates, to 12).
identify the optimal reaction conditions (Table 1). At the outset,
Having identied the optimal reaction conditions (Table 1,
the reaction was carried out using 2.0 equiv. of Cu(OTf)2 in entry 11), we next investigated the generality of this protocol.
presence of Cs2CO3 (2 equiv.) as a base (entry 1). The desired The reaction proceeded smoothly with a wide functional group
target, N-arylacetamide 3aa, was formed in a yield of 85% within tolerance. As shown in Scheme 2(A), various substituted benzyl
ꢀ
2 h in CH2Cl2 at 80 C (TLC monitoring). Encouraged by these nitriles 1b–j or alkyloxynitriles 1k-z participated in the reaction
results, a series of copper salts (e.g. Cu(OAc)2, Cu2O, Cu(CH3- leading respectively to N-arylacetamides 3ba–ja or 4a–p in good
CN)4$PF6, CuI) were screened under similar conditions (entries to excellent yields. Benzylnitriles bearing either electron-
2–5). Thus, it was found that Cu(OTf)2 presented the highest donating groups (1c–e) or electron-withdrawing groups (1f–i),
activity and efficiency, and that the reaction could be mediated with different aryl substitution patterns were compatible with
by a copper(I) salt, albeit with a lower yield (entry 6).17a The effect standard conditions, affording the desired products 3ca–ia in
of the solvents was investigated and toluene was found to be the good yields. Halide substituents such as Br were particularly
most suitable solvent (entries 7–8). It is worth mentioning that well tolerated, forming 3fa–3ha, which can be further func-
a
satisfying yield of 78% was obtained by using tionalized via cross-coupling reactions. It is worth noting that
environmentally-benign dimethylcarbonate (DMC) (entry 8). the inuence of the aryl substitution pattern was not evidenced.
Lowering the temperature of the reaction was unsatisfactory The reaction turned out to be compatible with tertiary alkyl
(entry 9). Finally, the best conditions were found using 0.3 nitriles such as diphenylacetonitrile 1j, but quaternary alkyl
equiv. of the (CuOTf)2.toluene complex leading to the quanti- nitriles, such as the commercially available 2-methyl-2-
tative formation of the N-arylacetamide 3aa within only 20 min phenylpropanenitrile,
(entries 10–11).17b Control experiments showed no reactivity in
the absence of catalyst or base. Moreover, in comparison with
other inorganic bases (i.e. K2CO3, NaOH.), Cs2CO3 was the
best base for the present transformation. Altering the iodonium
proved
to
be
unsuccessful.
Table 1 Optimization studies.a,e
Entry Catalyst (equiv.)
Solvent
T
ꢀC Time
Yield (%)
1
2
3
4
5
6
7
8
Cu(OTf)2 (2)
Cu(OAc)2 (2)
Cu2O (2)
Cu(CH3CN)4$PF6 (2)
CuI (2)
Cu(OTf)2 (2)
Cu(OTf)2 (2)
Cu(OTf)2 (2)
Cu(OTf)2 (2)
(CuOTf)2$toluene (0.3) Toluene 80
(CuOTf)2$toluene (0.3) Toluene 80
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene 80
DCE
DMC
CH2Cl2
80
80
80
80
60
2 h
2 h
2 h
2 h
2 h
2 h
2 h
2 h
24 h
2 h
85
0
0
9
30
99
61
78
11
100
80
80
60
9
10
11c,d
12f
20 min 100 (95)b
17 h
CuCl (0.1)
DCE
70
0
a
Reaction conditions: in a sealed tube, 1a (0.5 mmol), Ph2I+, ꢁOTf (2
b
equiv.), Cs2CO3 (2 equiv.) in solvent (3 mL).
Isolated yields.
c
d
Monitored by GC-MS. Gram-scale conditions: 1a (14 mmol, 2.07 g),
2a (30.8 mmol, 13.12 g), Cs2CO3 (28 mmol, 9.12 g) in toluene (40 mL).
e
f
Isolated yield aer purication by column chromatography. 1a (1.2
Scheme 2 (A) Scope of the Cu-catalyzed N-arylation reaction using
substituted alkyl- or benzyl nitriles 1b–z. (B) Smiles rearrangement
from 4b.
mmol), 2a (1.0 mmol), CuCl (10 mol%), H2O (1.15 mmol), DCE (10.0
mL) under argon at 70 ꢀC for 17 h in a sealed tube.12 DMC:
dimethylcarbonate.
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Furthermore, it should be noted that this copper-catalyzed N-
arylation reaction did not allow the transformation of 2-cyano-
phenylacetonitrile. Similarly, phenoxyacetonitriles 1k–z affor-
ded the corresponding N-arylacetamides 4a–p with moderate to
very good yields. The benets of our approach lie both in the
diversity offered by the initial choice of the functionalized
phenol derivative, precursor of 1k–z or aryliodonium salt, and
mild reaction conditions. Substrates bearing a pharmaceuti-
cally important uorine atom (4b) and a naphthyl moiety (4o)
were amenable in this reaction. In addition, conventional
palladium cross-coupling reactions may be performed from the
bromoaryl moiety in 4m. The hindered ([1,10-biphenyl]-2-yloxy)
acetonitrile 1z was also tolerated, yielding 4p albeit in a low
yield.18 In order to demonstrate the value of the methodology
developed, with the 2-uoroaryloxymethylamide 4b in hand, the
synthesis of 2H-1,4-benzoxazin-3-(4H)-ones, a privileged struc-
ture in the arena of pharmaceutical and agrochemical products
was investigated (Scheme 2(B)).19 We sought to take advantage
of the Smiles rearrangement20 in presence of Cs2CO3 as a base
in DMF at 120 ꢀC for 2 h. Consequently, the targeted 2H-1,4-
benzoxazin-3-(4H)-one 5 was isolated in 63% yield. It is worth
noting that diversity can be introduced on aryl substituents of 5.
Thereaer, the reactivity of various symmetrical (2f–g) and
unsymmetrical diaryliodonium salts13,21 (2b–e with Ar2 ¼ 2,4,6-
trimethylphenyl, 2h with Ar2 ¼ phenyl) was studied from 1a
leading to new N-arylacetamides 3ab–ah with good yields
(Scheme 3 and ESI†). As previously reported with unsymmet-
rical diaryliodonium salts in metal-catalyzed conditions, the
less bulky aryl group was transferred more readily than the
bulky one. Moderate yield was also obtained with a thienyl
group allowing access to the heterocyclic product 3ah.
Scheme 4 Time course experiments of Cu-catalyzed N-arylation of
1a (LC-HRMS). HPLC yield accounting for the response factor of 1a
(black curve), 3aa (blue curve) and 6 (orange curve).
To investigate the competition between the substrate and the
product with respect to copper, we resorted to cyclic voltam-
metry (CV) of a nitromethane solution of CuII(OTf)2. The inter-
action of both CuII and electrogenerated CuI with a ligand could
be assessed by this method, while the low coordinating ability
of nitromethane avoided any binding competition issues.22
MeCN selected as a model substrate23 displayed only a weak
affinity for CuII but proved to stabilize CuI due to the formation
of a mixture of [CuI(NCMe)2]+ c2 and [CuI(NCMe)3]+ c3 – the
latter being favoured at high MeCN concentration (see the ESI,
§IV†).24 This result was conrmed by DFT calculations:25 while
the formation of c3 is predicted to be the most exergonic
process, both the formation of c2 (formation energy of
4.8 kcal molꢁ1 higher) and c4 (formation energy of only
1.1 kcal molꢁ1 higher) may be accessible as a function of the
In order to better understand this reaction, we started
mechanistic studies. HRMS analyses performed during the
reaction of 1a with 2a under standard conditions revealed the
formation of the ketenimine 6 within a few minutes (Scheme 4
and ESI, §VI†). Conversely, the N-arylacetamide 3aa probably
formed due to a slow hydrolysis of the ketenimine intermediate
6 under basic conditions. When adding a stoichiometric
amount of nal acetamide at the beginning of the reaction, no
evolution could be detected consistently with an inhibition of
the catalyst by the product (see ESI, §VI†) (Scheme 5).
Scheme
5 CV towards reduction (top) and oxidation (bottom)
potentials of CuII(OTf)2 (1 mM) in the presence of MeCN (158 equiv.)
with increasing amounts of cyclohexylformamide (0, 1, 2, 5, 14, 50, 158
equiv. from red to green), recorded at a steady glassy carbon disk
Scheme
3
Scope of the reaction using symmetrical (2f–g) or electrode (d ¼ 3 mm) in nitromethane containing n-Bu4NBF4 (0.3 M) at
unsymmetrical (2b–e, 2h) diaryliodonium salts.
20 ꢀC with a scan rate of 0.5 V sꢁ1
.
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MeCN concentration (see ESI, §V†). A base is required for the with an energy barrier of only 5.8 kcal molꢁ1 and coordination
reaction to proceed but the best one was Cs2CO3, which is of a new nitrile allowed to regenerate c5 along with the experi-
poorly soluble in toluene. Thus, the concentration of hydroxides mentally observed ketenimine intermediate H2C]C]NPh.
should be kept very low. The impact of hydroxides on the nature
of the catalyst was tedious to evaluate experimentally, as copper
salts tend to precipitate in the presence of hydroxides. To
Conclusions
identify the possible species in the presence of hydroxides, we
resorted to DFT calculations.
In summary, we have developed a new, simple and practical
method for the copper-catalyzed synthesis of N-arylacetamide
For the sake of completeness, we considered the possibility
from easily accessible alkyl or benzyl nitrile. The protocol uses
of forming either monomeric or dimeric CuI complexes with
diaryliodonium salts as the electrophilic coupling partners.
Mechanistic studies proved the formation of an intermediate
ketenimine that is slowly hydrolyzed under the reaction
conditions. CV experiments demonstrated the high affinity of
the product for the catalyst, justifying the catalyst loading
required. Finally, DFT calculations ascertained that a two-
electron activation i.e. oxidative addition is energetically
different ligand stoichiometries (See ESI, §V†). In all the cases,
dimeric copper species were found to be more favourable than
the corresponding hydroxo monometallic complex. The two
hydroxo-bridged complexes [CuI(OH)(MeCN)]2 c6 and
[CuI(OH)(PhNHCOMe)]2 c8 were the most relevant species. c8
turned out to be more stable than c6, due to the stabilizing
effect of two intramolecular hydrogen bonds (see ESI, §V†).
possible. Efforts to expand the utility of the method are in
Consistently with the observed inhibition of the catalyst, ligand
exchange to regenerate the active c6 complex was computed as
not favorable. This nding could, at least partly, explain the
progress in our laboratory.
Conflicts of interest
high catalyst loading required for the reaction to proceed. Two
possible mechanisms for the reaction of diaryliodonium with
There are no conicts to declare.
CuI complexes were next investigated: (i) the SET pathway and
(ii) a two-electron transfer (oxidative addition, OA) from CuI to
+
26
ArI2
.
The addition of 3 equiv. of benzophenone or BHT did
Acknowledgements
not affect the process, which ruled out a SET mechanism. The
OA path was therefore calculated as the reaction can be
promoted by different CuI sources (Scheme 6 and ESI†). The
monomeric [CuI(OH)(NCMe)] c5 can form an adduct (c9) with
Ph2I+ and its formation is exergonic (ꢁ3.9 kcal molꢁ1). Depro-
tonation of c9 to form complex c10 can spontaneously occur
(DG z ꢁ50 kcal molꢁ1). Starting from c10, OA via TS-OA was
accessible with a low barrier (+13.7 kcal molꢁ1) giving complex
c11. The direct OA (TS-OA-bis) of c9 was less favourable, with an
activation free energy of 24.4 kcal molꢁ1. Reductive elimination
through a distorted T-shape transition state TS-RE takes place
R. S. and I. D. thank the Region Centre-Val-de-Loire, the
´
University of Orleans and the Labex Synorg (ANR-11-LABX-0029)
for support. P. A. Payard is grateful to ENS Paris Saclay and
Solvay for a PhD grant.
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© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 15885–15889 | 15889