Synergistic Copper-Catalyzed Reductive Aminocarbonylation of Alkyl Iodides with Nitroarenes

Article abstract of DOI:10.1021/acs.orglett.9b04092

We have developed a Cu-catalyzed reductive aminocarbonylation of alkyl iodides using nitroarenes as the nitrogen source. The reaction proceeds with a single copper catalyst playing dual roles of synergistically mediating both carbonylation of alkyl iodide

Full text of DOI:10.1021/acs.orglett.9b04092

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synergistic Copper-Catalyzed Reductive Aminocarbonylation of
Alkyl Iodides with Nitroarenes
Siling Zhao and Neal P. Mankad*
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States
S
* Supporting Information
ABSTRACT: We have developed a Cu-catalyzed reductive
aminocarbonylation of alkyl iodides using nitroarenes as the
nitrogen source. The reaction proceeds with a single copper
catalyst playing dual roles of synergistically mediating both
carbonylation of alkyl iodides and reduction of nitroarenes,
providing acyl iodides and anilines as possible reactive
intermediates that then do amide coupling spontaneously. A diverse range of secondary N-aryl alkylamides are accessible
from a variety of primary, secondary, and tertiary alkyl iodides using this method.
mides are among of the most important chemical units in
all of organic and biological chemistry,1 and so the
Scheme 1. Reductive Aminocarbonylation with Nitroarenes
A
synthetic community has maintained an interest in developing
different, complementary synthetic methods to target diverse
amide compounds.2 For example, the classic Ritter reaction
involves protonation of an alcohol or alkene and in situ
trapping of the resulting carbocation with a nitrile
nucleophile;3 thus, only N-alkyl amides are accessible.
Similarly, amidation of esters or transamidation between
amides and amines typically only produce N-alkyl amides
except in rare cases.4 An efficient method for N-aryl amide
synthesis is aminocarbonylation of aryl halides with anilines
under CO atmosphere;5 here use of alkyl electrophiles has
proven challenging and limits the scope of accessible products
to arylamides.
Nitroarenes can act as surrogates for anilines under reductive
conditions, in some cases exhibiting orthogonal functional
group tolerance to classical amine nucleophiles in various
coupling reactions.6 However, there are only a few reports of
successful reductive aminocarbonylation using nitroarenes as
the nitrogen source, none of which involve C(sp3)-hybridized
electrophiles. Driver reported a Pd-catalyzed reductive amino-
carbonylation of aromatic C−H bonds with Mo(CO)6 serving
as both reducing agent and CO source, producing N-aryl
arylamides as long as the substrate contained an o-pyridyl
directing group (Scheme 1a).7 Additionally, recent advances
by Hu, Wu, and others have enabled Ni-catalyzed reductive
aminocarbonylation of aryl halides (Scheme 1b), esters, and
aryl boronic acids.8 The only example of alkylamide synthesis
by reductive aminocarbonylation was reported by Beller, who
disclosed a Pd-catalyzed coupling of terminal alkenes with
nitroarenes under syngas pressure to provide N-aryl
alkylamides (Scheme 1c).9 Here, the range of accessible
products was limited by the fact that only monosubstituted
alkenes were amenable to coupling. Recently, Hu also
furnished alkylamides through reductive transamidation
between nitroarenes and amides.10 Although C(sp3)−X
electrophiles have not been investigated in reductive amino-
carbonylation, they have been used in aminocarbonylation
with amine nucleophiles,11 including a recent report by
Alexanian of stereospecific aminocarbonylation of alkyl
tosylates with primary and secondary amines.12
Our group has reported several examples of carbonylative
coupling reactions involving single-electron transfer activation
of alkyl halides mediated by N-heterocyclic carbene (NHC)
Received: November 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04092
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ClIMes provided almost the same yields (Table 1, entries 4 and
copper catalysts in the presence of reducing agents such as
hydrosilanes.13,14 Thus, we wondered whether this method-
ology could be extended to reductive aminocarbonylation with
nitroarenes as pro-nucleophiles (Scheme 1d), with the copper
catalyst playing dual roles of synergistically mediating nitro-
arene reduction and alkyl halide carbonylation. To our
knowledge, copper catalysts have not been explored previously
for reductive aminocarbonylation.15 Nonetheless, Beller has
successfully reduced nitroarenes to anilines using copper
hydrides,16 and Yin recently showed that copper catalysis is
appropriate for reductive C−N coupling with nitroarenes using
hydrosilane reductants.17 However, to realize our goal that
involves controlling a complex sequence of reduction, radical
carbonylation, and amidation steps, several challenges would
need to be overcome: potential multiple CO insertion,18 high
energy activation of substrates,11d competing reduction of alkyl
halides by the copper hydride catalyst,19 and competing
hydroxymethylation via reduction of carbonylated intermedi-
ates by the copper hydride rather than by the nitrogen
nucleophile.20
Cl
5), BrIMes decreased yield (Table 1, entry 6) and OMeIMes
boosted the yield up to 78% (Table 1, entry 1). The choice of
NaOH as the base and PhSiH3 as the hydrosilane are necessary
for the successful transformation (Table 1, entries 79). It is
worth noting that the use of LiOMe favored the
hydroxymethylation side reactions described above, which is
in agreement with our previous work (Table 1, entry 8).14
Reducing the amount of hydrosilane or raising catalyst loading
both diminished the desired reaction (Table 1, entries 10 and
11). Finally, we observed moderate yields at lower temperature
and pressure (Table 1, entries 12 and 13). Use of alkyl
bromide in place of alkyl iodide resulted in only trace
conversion, but use of alkyl bromide + KI produced the
corresponding amide in 52% yield.
With the optimized conditions in hand, we started to test the
scope of alkyl iodides (Figure 1). Simple primary (3a),
We began our study with nitrobenzene (1a) and 1-iodo-3-
phenylpropane (2a) as substrates, targeting amide compound
3a (Table 1). After screening with different NHC ligands, we
Table 1. Reaction Optimization for Copper-Catalyzed
a b
,
Reductive Aminocarbonylation of Alkyl Iodides
change from optimized yield of 3a yield of 3aa yield of 3aaa
entry
conditions
(%)
(%)
(%)
1
2
none
no catalyst
78
0
40
0
12
0
3
4
5
IPrCuCl catalyst
IMesCuCl catalyst
ClIMesCuCl catalyst
BrIMesCuCl catalyst
LiOH base
38
61
59
38
18
13
0
28
62
51
30
50
34
32
39
22
25
0
18
64
39
31
24
14
0
6
23
18
22
0
Figure 1. Scope of alkyl iodides. Reactions were done on 0.1 mmol
scale. All yields are isolated yields relative to 1 and the average of two
parallel experiments. aReactions were done on 1 mmol scale, reported
as average isolated yield relative to 1 from two parallel experiments.
7
8
9
10
11
12
13
LiOMe base
Ph2SiH2 as silane
2.0 equiv of PhSiH3
10 mol% of catalyst
60 °C
Cl
b ClIMes ligand was used instead of OMeIMes.
3
12
10
6
secondary (3l), and tertiary (3m) alkyl iodides underwent the
transformation smoothly. Substrate containing aryl halides
were efficiently converted to target amides (3b3d), implying
that this methodology is orthogonal to classical Pd-catalyzed
couplings. Synthetically important moieties, such as ether (3e),
cyano (3g), and trifluoromethyl (3h), are all compatible. To
our delight, some base-sensitive groups including benzyl ether
(3f) and methyl ester (3i) survived under these conditions.
Heterocyclic groups such as furan (3j) and indole (3k) gave
modest yields and required use of ClIMes in place of
3 atm CO
a
b
Reactions were done on 0.1 mmol scale. Yields relative to 1a were
1
determined by H NMR with 1,1,2,2-tetrachloroethane as internal
standard.
found that while direct reduction of alkyl iodides (3aaa) can be
avoided by the choice of ligands (Table 1, entry 5),
hydroxymethylation (3aa) cannot be avoided completely. In
order to compensate the loss of iodide substrate consumed by
hydroxymethylation, we employed 2.0 equiv of 2a in the
reaction (see the SI for more information). Based on further
investigation, we noticed that the electronegativity of ligand is
crucial to improve the yield of 3a. For example, while IMes and
Cl
OMeIMes. This method can also be applied to complex
carbohydrate substrates (3n).
Next, we investigated the substrate scope with respect to
nitroarenes (Figure 2). We found ClIMes performed better
Cl
than OMeIMes in this transformation with substituted
B
DOI: 10.1021/acs.orglett.9b04092
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Mechanistic Studies
Figure 2. Scope of nitroarenes. Reactions were done on 0.1 mmol
scale. All yields are isolated yields relative to 1 and average two
parallel experiments. aProduct was isolated with 10% inseparable
b
c Cl
impurities. IPr ligand was used instead of ClIMes. OMeIMes ligand
was used instead of ClIMes. dPhenolic group was protected as its
pivalic ester and underwent deprotection during the reaction. Yields
e
were NMR yields relative to 1 against 1,1,2,2-tetrachloroethane as
internal standard.
nitroarenes. Aryl halide (3q−3t) and thioether (3w)
substituents posed no problem in the transformation, whereas
protected phenols (3x) and heterocyclic groups (3z) gave
moderate yields. Use of IPr as ligand allowed for smooth
conversion with nitroarenes containing boronic esters (3y) and
Cl
anisole (3u), while OMeIMes was required for trifluorome-
thoxyl substituted nitroarenes (3v) to give good yield.
However, cyanide groups on the nitroarene (4a) inhibited
the reaction, and a polyaromatic nitroarene (4b) performed
poorly in this reaction. We also found this method is highly
sensitive to steric environment on the nitrobenzene partner
(4c and 4d).
a
Reactions were done on 0.1 mmol scale, and yields were determined
by 1H NMR with 1,1,2,2-tetrachloroethane as internal standard.
b
c
Isolated yield. 0.5 equiv intermediate was added instead of 1.0
equiv.
To examine the mechanism of this reductive amino-
carbonylation, a stoichiometric reaction was carried out by
subjecting acyl iodide 5 to the standard reaction in place of 2a
under N2 atmosphere (Scheme 2a). The expected amide 3a
was obtained in 71% yield, implying that acyl iodide might
serve as the carbonylated intermediate during catalysis. Then
the radical nature of this transformation was confirmed by the
addition of radical scavenger TEMPO to the standard reaction.
The target amide 3a was not observed; instead the TEMPO
adduct compound 6 was isolated in 30% yield (Scheme 2b).
To gain insight into the reactive nitrogen nucleophile during
catalysis, several possible intermediates resulting from nitro-
arene reduction were tested under catalytic conditions
(Scheme 2c).21 No amide product was detected with
azobenzene (7c), and nitrosobenzene (7a), N-phenyl hydroxy-
amine (7b), and azoxybenzene (7d) gave low yields. Only
aniline (7e) gave quantitative yield (94%) under the standard
reaction conditions. Thus, we propose that aniline might be
the major reactive intermediate for C−N bond formation.
However, we cannot rule out a potential role of other
intermediates such as hydroxylamine serving as nitrogen
sources by minor pathways (see the SI for more information).
Then the aniline intermediate (7e) was subjected to the
conditions shown in Scheme 2d without the presence of
copper catalyst. Surprisingly, amide 3a or other carbonylated
compounds were not detected. This indicates that the
carbonylation is a copper-mediated radical process rather
than a radical chain-type atom transfer carbonylation.22
Another potential copper amide intermediate 8 was also tested
under optimized conditions shown in Scheme 2e. The absence
of targeted 3a helps to eliminate the possible route of a copper
amide intermediate mediating radical carbonylation.
C
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Organic Letters
Letter
Y.; Shi, D.-Q. Ruthenium-Catalyzed Carbonylation of Oxalyl Amide-
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carbon monoxide, hydrosilanes, and metal hydrides have all
been shown to reduce nitroarenes,23,4 each of them was
eliminated individually. According to the results shown in
Scheme 2f, we can conclude that only (NHC)CuH
intermediates are competent to reduce nitroarenes under
these conditions.
Collectively, there is definitive evidence that the copper
catalyst plays a dual role of synergistically mediating both alkyl
iodide carbonylation and nitroarene reduction. While the
detailed mechanism of each step requires more study, we
propose that the dual roles of the copper catalyst produce the
two reactive intermediates, acyl iodide and aniline, which then
engage in C−N coupling via a rapid, uncatalyzed step.
In conclusion, we have developed a copper-catalyzed
reductive aminocarbonylation from simple nitroarenes and
alkyl iodides. This methodology has shown good tolerance
with a variety of functional groups and serves as the only
reductive aminocarbonyation method for C(sp3)-hybridized
electrophiles. Mechanistic studies suggested that NHC copper
catalyst serves a dual function in the tandem process: a copper-
catalyzed carbonylation of alkyl iodides followed by amidation
with in situ generated anilines resulting from nitroarenes
reduction catalyzed by the same copper catalyst.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications Web site. Experimental details, supplemen-
tary results, and spectral data (PDF) The Supporting
Information is available free of charge at https://pubs.ac-
s.org/doi/10.1021/acs.orglett.9b04092.
́
(d) Yin, Z.; Zhang, Z.; Soule, J.-F.; Dixneuf, P. H.; Wu, X.-F. Iron-
Experimental details, supplementary results, and spectral
data (PDF)
catalyzed Carbonylative Alkyl-acylation of Heteroarenes. J. Catal.
2019, 372, 272. (e) Shen, N.; Cheung, C. W.; Ma, J.-A. Direct Amide
Synthesis via Ni-mediated Aminocarbonylation of Arylboronic Acids
with CO and Nitroarenes. Chem. Commun. 2019, 55, 13709. (f) Qi,
X.; Zhou, R.; Peng, J.-B.; Ying, J.; Wu, X.-F. Selenium-Catalyzed
Carbonylative Synthesis of 2-Benzimidazolones from 2-Nitroanilines
with TFBen as the CO Source. Eur. J. Org. Chem. 2019, 2019, 5161.
(g) Peng, J.-B.; Geng, H.-Q.; Wu, F.-P.; Li, D.; Wu, X.-F. Selectivity
Controllable Divergent Synthesis of -unsaturated Amides and
Maleimides from Alkynes and Nitroarenes via Palladium-Catalyzed
Carbonylation. J. Catal. 2019, 375, 519. (h) Geng, H.-Q.; Peng, J.-B.;
Wu, X.-F. Palladium-Catalyzed Oxidative Carbonylative Coupling of
Arylallenes, Arylboronic Acids, and Nitroarenes. Org. Lett. 2019, 21,
8215.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: npm@uic.edu.
ORCID
Neal P. Mankad: 0000-0001-6923-5164
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Funding was provided by NSF (CHE-1664632).
■
(9) Fang, X.; Jackstell, R.; Beller, M. Selective Palladium-Catalyzed
Aminocarbonylation of Olefins with Aromatic Amines and Nitro-
arenes. Angew. Chem., Int. Ed. 2013, 52, 14089.
(10) (a) Cheung, C. W.; Ploeger, M. L.; Hu, X. Nickle-Catalyzed
Reductive Transamidation of Secondary Amides with Nitroarenes.
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Iron-Catalyzed Oxidative Coupling Reaction of Isocyanides with
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Carbon Radicals and Pd Catalysts. Acc. Chem. Res. 2014, 47, 1563.
(d) Chow, S. Y.; Odell, L. R.; Eriksson, J. Low-Pressure Radical 11C-
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