Iridium-Catalyzed Aryl C-H Sulfonamidation and Amide Formation Using a Bifunctional Nitrogen Source

Article abstract of DOI:10.1021/acs.orglett.8b01977

A new strategy for the sequential formation of aryl and amidyl C-N bonds is reported. Using trichloroethoxysulfonyl azide as a bifunctional nitrogen source, Ir-catalyzed aryl C-H sulfonamidation and subsequent desulfonative amide formation proceed effectively without any need of oxidants or coupling reagents. This protocol is suitable for readily available benzamides and stable carboxylates including primary, secondary, and tertiary alkyl, alkenyl, and phenyl carboxylates, thereby providing a direct and efficient method for the synthesis of biologically and chemically useful N-arylamides.

Full text of DOI:10.1021/acs.orglett.8b01977

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iridium-Catalyzed Aryl C−H Sulfonamidation and Amide Formation
Using a Bifunctional Nitrogen Source
Meng Yu,†,⊥ Tao Zhang,†,⊥ Hitesh B. Jalani,‡ Xunqing Dong,† Hongjian Lu,*,† and Guigen Li†,§
†Institute of Chemistry & BioMedical Sciences, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, 210023, China
‡Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115,
United States
§Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States
S
* Supporting Information
ABSTRACT: A new strategy for the sequential formation of
aryl and amidyl C−N bonds is reported. Using trichlor-
oethoxysulfonyl azide as a bifunctional nitrogen source, Ir-
catalyzed aryl C−H sulfonamidation and subsequent
desulfonative amide formation proceed effectively without
any need of oxidants or coupling reagents. This protocol is suitable for readily available benzamides and stable carboxylates
including primary, secondary, and tertiary alkyl, alkenyl, and phenyl carboxylates, thereby providing a direct and efficient
method for the synthesis of biologically and chemically useful N-arylamides.
Scheme 1. N-Activation Strategies Used in the Synthesis of
N-Arylamides
N-Phenylamides, containing aryl and amidyl C−N bonds, are a
key structural fragment in agrochemicals and pharmaceutical
substances such as Lidocaine, Tocainide, Mepivacaine,
Prilocaine, Bentiromide, Boscalid, and Fluxapyroxad.1 Metal-
catalyzed C−H amination of nonfunctionalized arenes is one
of the most simplified and convergent methods for
constructing aryl C−N bonds, and has been gaining attention
and emerging as a rapidly evolving research field.2,3 However,
this strategy for direct synthesis of N-arylamides has challenges.
The main reason is that while the metal-nitrene species were
commonly used in catalytic C−H amination chemistry,3,4 the
analogous metal-carbonylnitrenes generated from amides are
prone to convert to isocyanates via Curtius rearrangement
instead of being inserted into C−H bonds.5,6 In this context,
Chang7,8 and others9,10 developed elegant ways to conduct
functional-group-directed metal-catalyzed aryl C−H amination
under mild reaction conditions (Path a, Scheme 1A). Organic
azides7,9 and 1,4,2-dioxazol-5-ones8,10 were used as efficient
preactivated aminating reagents, most of which are derived
from the benzoic acids and linear alkyl carboxylic acids. Very
recently, Chang and co-workers reported the design of new
iridium catalysts which can successfully catalyze intramolecular
aryl and alkyl C−H amination of different alkyl substituted
1,4,2-dioxazol-5-ones to form various γ-lactams.11 Construc-
tion of an aryl C−N bond, followed by coupling of the
resulting anilines with acid to generate an amidyl C−N bond is
an efficient alternative way to form N-arylamides (Path b,
Scheme 1A). Direct installation of an NH2 group in inert
arenes with high efficiency is, however, challenging.12−14
Nicewicz and co-workers recently developed an organic
photoredox-based catalyst system to promote site-selective
amination of a variety of aromatic structures using ammonium
Morandi et al. reported an iron-catalyzed C−H amination of
arenes with MsONH3OTf to form anilines.12b However, the
substrate scope in these systems was limited to electron-rich
arenes and, in most cases, the yields or regioselectivities were
moderate.
Recently, activation of amines, a reversal of the widely used
carboxylic acid activation strategy, has emerged as a potentially
important pathway for the construction of an amide bond since
this procedure employs commonly available carboxylic acids
and preactivated amines (Scheme 1B).15−19 Recognizing that
N-activation strategies can greatly promote both catalytic C−H
amination and amide bond formation, we hypothesized that a
Received: June 24, 2018
12a
−
carbamate (H4N+H2NCO2 ) as a nitrogen source,
and
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01977
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Letter
single nitrogen atom could be associated with two different
activating groups, one to promote the aryl C−H amination and
the other to facilitate the transformation to an amide
functionality (Scheme 1C). If such a method is successful, it
will construct both aryl and amidyl C−N bonds without
oxidants or coupling reagents in a straightforward manner. The
reactions would likely encounter chemoselectivity issues due to
the competitive reactions between the two activating groups.
The selection of the activation groups is difficult in view of the
fact that only a few activated amines such as N-(imidazolyl-
carbonyl)amine,16 isocyanide,17 iminophosphorane (in situ),18
and isothiourea19 were used in amide formation, and the
sulfonyl group often used as a protecting group in catalytic C−
H amination was removed generally under harsh reaction
conditions.3,4
having a strong electron-withdrawing group such as 2,2,2-
trichloroethoxyl sulfonyl azide24,25 (TcesN3, 1e) was em-
ployed. To our delight, we found that the intramolecular
substitution reaction forming 5a′ was totally blocked (Path b,
Scheme 2) and a C−H sulfonamidation/desulfonylative amide
formation cascade reaction was accomplished (Path a, Scheme
2), affording the desired product (4a) in 87% isolated yield in
an [IrCp*Cl2]2/AgOTf (5 mol %/30 mol %) catalytic system.
Next, the scope of the benzamides in this reaction was
explored (Scheme 3). Benzamides with various N-alkyl
Scheme 3. Scope of Benzamides
Organic azides have been recognized as an efficient,
convenient, and activated amino source in catalytic C−H
amination reactions.20,21 Green and mild conditions can be
used with azides because N2 gas is the sole byproduct and
oxidative or basic conditions are unnecessary. Chemicals used
in such reactions are maximally tolerant of other reaction
conditions, leading potentially to their application in
sequential/cascade reactions. Campagne et al. reported the
synthesis of amides using imidazolylcarbonyl as the activation
group of amines, in which imidazole acted as the leaving group
(LG).16 Led by these and our own C−H amination work with
different azides,22,23 we became interested in finding a suitable
azide to test our hypothesis (see the details in Table S1 in the
Supporting Information (SI)). Our investigation began using
2,2,2-trichloroethoxyl carbonyl azide (TrocN3, 1a) as a
nitrogen source, whose azido group may serve as an activated
amino source in a metal-catalyzed C−H amination reaction.20
The Troc group can serve as an activating group for the
subsequent amide formation since 2,2,2-trichloroethoxyl is a
good leaving group (Scheme 2).16 In practice however, when
a
Conditions A: 2 (0.20 mmol), 1e (0.24 mmol), 3a (0.40 mmol),
[IrCp*Cl2]2 (5 mol %), AgOTf (30 mol %), PhCl (2 mL), 120 °C, 24
h; isolated yield. 2 mmol scale. Only one regioisomer was obtained.
b
c
substituents possessing different electronic and steric proper-
ties reacted smoothly (4a−f). Both electron-donating and
-withdrawing groups present at the para-position of the phenyl
ring were compatible (4g−n), and many functional groups,
such as methoxyl (4h), amino (4i, 4j), halogen (4k), ester
(4m), and amide (4n), were tolerated. Irrespective of the
electronic and steric properties of a meta-substituent on the
phenyl ring, the C−H amination is facilitated prominently at
the less hindered para-position with exclusive regioselectivity
(4o−r). In addition to the phenyl ring, the C−H bond in
naphthalene could be involved in the reaction, forming the
desired product (4s) with high regioselectivity and good yield.
Unfortunately, ortho-substituted benzamides, such as N-(tert-
butyl)-2-iodobenzamide and N-(tert-butyl)-2-methylbenza-
mide, and other substrates, such as N-(tert-butyl)-3-bromo-
benzamide, N-(tert-butyl)-3-(trifluoromethyl)benzamide, and
N-(tert-butyl)-4-bromobenzamide, do not work well; only a
few desired products could be observed. When a tertiary
benzamide such as N-(tert-butyl)-N-methylbenzamide was
used, no desired product was formed.
Furthermore, different types of carboxylates were screened
(Scheme 4). Linear alkyl carboxylate (EtCOOK), electron-
deficient carboxylate (CF3COOK), and aryl carboxylate
(PhCOOK) were tolerated well (7b, 7e−f). Some potassium
carboxylates, however, failed to furnish the desired products,
possibly due to the difficulty in initiating the C−H amination.
Thus, we designed a two-step sequential reaction (Conditions
B): the first step is the C−H amination with catalytic KOAc to
form the C−H sulfonamidation product (6c). Product 6c can
be easily isolated in high yield (>95%), as N2 is the only
byproduct. The second step, employing the different
a
Scheme 2. Optimization of the Reaction Conditions
a
Isolated yield.
benzamide (2a) was used instead of intermolecular sub-
stitution by acetate to form intermediate B (Path a) which
could lead to the desired product (4a), intramolecular
substitution by the amide group via Path b occurs, providing
the unwanted product (5a).23a Replacement of the 2,2,2-
trichloroethoxyl group with other groups, such as n-heptyl
(1b), 2,2,2-trichloro-1,1-dimethylethoxyl (1c), and 4-methox-
yphenoxyl (1d), failed to produce 4a. Considering the fact that
acyl azides undergo Curtius rearrangement easily, the azide
B
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desired product (7m), and potassium sorbate, which is
primarily used as a food preservative, led to the desired
product (7n) which contains a 1,3-diene functionality. Late
stage C−H amidation of this protocol was successfully
employed to form benzamides containing a natural chemical
structure without racemization. The natural chemical structure
is either in the aryl part, i.e., the estrone-based product (4t) or
in the N-alkyl part, i.e., the cholesterol based product (4u).
This reaction could be useful in medicinal chemistry for SAR
studies of various biological assays. We further explored
benzamide substrate bearing chiral amino acid derivatives such
as alanine (2v) and dipeptide (2w). Under the mild basic
reaction conditions, these provided the desired products (4v−
w) smoothly with complete retention of the original chiral
centers.
Scheme 4. Scope of Carboxylates
a
b
Conditions A; see note a in Scheme 3. Conditions B. First step: 2
(0.20 mmol), TcesN3 1e (0.24 mmol), KOAc (20 mol %),
[IrCp*Cl2]2 (5 mol %), AgOTf (30 mol %), PhCl (2 mL) at 80
°C for 30 min; second step: 3 (0.40 mmol), 6 and PhCl (2 mL) at
120 °C for 10 h, isolated yield was calculated based on two steps.
Several experiments were carried out to examine the
mechanism of the C−H amination step (Scheme 6A, Scheme
c
d
RCOONa instead of RCOOK. 2a instead of 2c.
carboxylates with further heating, leads to the desired products.
The obvious advantage of this procedure is that sodium
carboxylates, which are ineffective in a one-pot reaction
(Conditions A, Table S1) are tolerated very well, giving the
desired products (4a, 7g, or 7j), and sterically hindered
aliphatic carboxylates such as tertiary and even quaternary alkyl
carboxylates to provide good yields of the products (7c−d).
There are difficulties associated with the classical method for
the synthesis of amides from sterically hindered carboxylic
acids and electron-deficient aromatic amines.19b Thiophene-2-
carboxylate provided 7g in good yield. Cinnamic and acrylic
acids are less nucleophilic and are typically employed as
Michael acceptors. Nevertheless, these too participated in the
reaction, furnishing the desired products (7h−j) in acceptable
yields.
Scheme 6. Mechanistic Studies
The applicability of this protocol to naturally occurring
compounds was investigated as shown in Scheme 5. Salts of
unsaturated fatty acids, for example, potassium oleate
participated very well in this reaction, and the cis-configuration
of the double bond in product 7k was retained. When the
reaction was performed with sodium lauroyl sarcosinate, an
ionic surfactant, the desired product (7l) was obtained in good
yield. Sodium isoniconate was tolerated well and furnished the
Scheme 5. Synthesis of Nature Product Derivatives
a
Using 7f instead of 2 in Conditions A (see note a in Scheme 3).
S1 in SI). Replacement of [IrCp*Cl2]2 with the iridiacyclic
complex (8) as the catalyst provided desired product 4c in
62% yield (Scheme 6A). Intermolecular competition reactions
between two electronically distinct substrates 4-methoxy-
benzamide (2h) and 4-(trifluoromethyl)benzamide (2l) was
performed, showing that the predominantly higher C−H
amidation reactivity is associated with the electron-rich arene
(4h/4l = 19/1, Scheme S1A). The parallel kinetic isotope
effect was measured as KIE = 2.6 (Scheme S1B) by an
intermolecular competition experiment between 2c and the
deuterium-labeled C6D5 analog ([D5]-2c). This suggests that
the C−H bond cleavage of benzamide is probably the rate-
limiting step.
We then carried out a series of control experiments to
explore the amide formation process (Scheme 6B−C, Scheme
S1C in SI). Compound 9, whose structure was confirmed by
X-ray diffraction analysis, was obtained as a minor product
when PhCOOK was used (Scheme 6B). When using alkyl
carboxylates, such as KOAc, no product similar to 9 was
a
b
Conditions A; see note a in Scheme 3. Conditions B; see note b in
Scheme 4.
C
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observed. Aminolysis of 9 could lead to 7f, but 7f could not be
transformed to 9 under the standard conditions. Reaction of
intermediate 6c with PhCOOK formed 9 and 7f with yields
similar to those obtained in the cascade reaction of 2c. The
crossover reaction between N-isopropyl-4-methylbenzamide
(2x) and N-cyclopentylbenzamide (2y) was completed, but no
N-isopropyl and N-cyclopentyl exchange was observed
(Scheme S1C). These results indicate that product 9 is a
side product, formed possibly as a result of the stabilization by
the phenyl ring and could not be a precursor of the desired
product 7f. To validate the effect of directing and activating
groups on the transformation, a series of experiments were
initiated and are depicted in Scheme 6C. An amide directing
group is necessary to facilitate desulfonylative amide formation
(10a). When other protecting groups such as a carbonyl (10b-
e) or a tosyl (10f) group were used in place of the Tces group,
no desired product (4c) was observed. Finally, the reaction
with 18O labeled KOAc was conducted (Scheme 6D), and the
18O was installed in the directing group and the amide group.
On the basis of our experimental results and the literature on
C−H amination reactions,3 a plausible mechanistic pathway is
proposed in Scheme 7. Combination of [IrCp*Cl2]2, AgOTf,
N-arylamides, known building blocks important in medicine
and biology. The mild reaction conditions allow avoiding the
use of any oxidants and coupling reagents, leading them to
reliably tolerate a variety of functional groups and circumvent
epimerization of stereocenters present in the substrates used.
Mechanistic studies show that the azido group in TcesN3
serves as an activated nitrogen source in the first Ir-catalyzed
C−H amination step and the Tces group functions as an
activating group in the subsequent amide formation step. We
anticipate this work will encourage further the rational design
of novel and efficient reactions for constructing multiple C−N
bonds.
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.8b01977.
Experimental details and characterization data for all
new compounds (PDF)
Accession Codes
CCDC 1822561−1822562 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Scheme 7. Proposed Mechanistic Pathway
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hongjianlu@nju.edu.cn.
ORCID
Hitesh B. Jalani: 0000-0002-2442-9798
Hongjian Lu: 0000-0001-7132-3905
Guigen Li: 0000-0002-9312-412X
Author Contributions
⊥M.Y. and T.Z. contributed equally.
and KOAc forms active Ir species IrCp*(OTf)(OAc). The
acetate component helps induce C−H bond cleavage through
transition state I to form intermediate II, which coordinates
TcesN3 to form intermediate III, subsequently undergoing
migratory insertion and releasing nitrogen to form inter-
mediate IV. Proto-demetalation of IV regenerates the active Ir
species and completes the C−H sulfonamidation cycle,
simultaneously producing the intermediate 6. Possibly due to
the higher steric demand of the sulfonyl group compared to a
carbonyl group, 6 proceeds through an intramolecular
substitution process by oxygen (Path a) instead of nitrogen
(Path b) to form the active cyclic intermediate V which then,
attacked by carboxylate, undergoes the ring opening reaction
to give VI. Intramolecular nucleophilic addition in VI forms
the cyclic intermediate VII, and finally, rearrangement of VII
forms the target product 4 (Path c). If R2 is a phenyl group,
dehydration (Path d) followed by hydrolysis of VII forms side
product 9, possibly as a result of stabilization by the phenyl
group.26
In conclusion, an Ir-catalyzed C−H sulfonamidation of
benzamides and an amide formation sequence was developed
using TcesN3 as a bifunctional nitrogen source. This reaction
provides a direct and efficient method for the construction of
important aryl and amidyl C(sp2)−N bonds resulting through
the breaking of strong C(sp2)−H and N−SO2 bonds
sequentially. Readily available benzamides and functionally
diverse metal carboxylates were employed to provide various
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support by the National Natural
Science Foundation of China (21472085, 21332005) and the
Fundamental Research Funds for the Central Universities (No.
020514380114).
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