A convenient synthesis of N-aryl benzamides by rhodium-catalyzed ortho-amidation and decarboxylation of benzoic acids

Article abstract of DOI:10.1002/chem.201406031

The rhodium-catalyzed amidation of substituted benzoic acids with isocyanates by directed C-H functionalization followed by decarboxylation to afford the corresponding N-aryl benzamides is demonstrated, in which the carboxylate serves as a unique, removable directing group. Notably, less common meta-substituted N-aryl benzamides are generated readily from more accessible paraor ortho-substituted groups by employing this strategy.

Full text of DOI:10.1002/chem.201406031

DOI: 10.1002/chem.201406031
Communication
&
CÀH Activation
A Convenient Synthesis of N-Aryl Benzamides by Rhodium-
Catalyzed ortho-Amidation and Decarboxylation of Benzoic Acids
Xian-Ying Shi,*[a] Ke-Yan Liu,[a] Juan Fan,[a] Xue-Fen Dong,[a] Jun-Fa Wei,[a] and Chao-Jun Li*[b]
alkenyl CÀH bond to isocyanates with subsequent cyclization,
Abstract: The rhodium-catalyzed amidation of substituted
respectively.[10] Very recently, Ackermann and co-workers devel-
benzoic acids with isocyanates by directed CÀH function-
oped a versatile route to phthalimides by RuII-catalyzed CÀH
alization followed by decarboxylation to afford the corre-
activation of amides with isocyanates.[11] In all of these reports,
sponding N-aryl benzamides is demonstrated, in which
the amide group was introduced ortho to the directing group,
the carboxylate serves as a unique, removable directing
while the directing group remains in place “permanently”.
group. Notably, less common meta-substituted N-aryl ben-
Transition metal-catalyzed decarboxylative cross-coupling re-
zamides are generated readily from more accessible para-
actions have recently emerged as a new and important cate-
or ortho-substituted groups by employing this strategy.
gory of organic transformation that finds versatile applications
in the construction of CÀC and CÀheteroatom bonds.[12] How-
ever, the new CÀC or CÀheteroatom bond is usually formed at
In recent decades, transition metal-catalyzed CÀH functionali-
the original position of the carboxylate group (Scheme 1a) and
zation reactions have emerged as a powerful method in organ-
ic synthesis.[1] Chelation-assisted CÀH bond activation and sub-
sequent addition to unsaturated molecules have offered many
efficient syntheses in an atom-economic fashion.[2] The direct
insertion of CÀH to unsaturated alkene and alkyne derivatives
has been extensively investigated;[3] whereas analogous addi-
tions across polarized CÀN multiple bonds have seen consider-
ably less progress.
However, the installment of nitrogen-based functional
groups into molecules through the direct addition of CÀH
bonds to unsaturated CÀN multiple bonds represents a worth-
while pursuit with profound synthetic potentials.[4] In particular,
the direct insertion of CÀH bonds into isocyanates is highly de-
sirable since it can effectively provide synthetically valuable
amide moieties,[5] which not only represent an important struc-
tural motif present in many natural products, polymers, phar-
maceuticals, and biological systems,[6] but also are of great im-
portance as intermediates for the preparation of various useful
compounds.[7] In this context, Cheng and co-workers illustrated
an efficient RuII-catalyzed amidation of 2-arylpyridines with iso-
cyanates by CÀH bond activation.[8] A RhIII-catalyzed protocol
Scheme 1. Decarboxylative CÀC cross-coupling reactions.
for amidation of anilide and enamide CÀH bonds with isocya-
nates was developed by Ellman and co-workers.[9] The groups
of Takai and Li reported ReII and RhIII-catalyzed addition of an
there are only few reports using carboxylic acids as a traceless
directing group (Scheme 1b).[12b,13,14] In the course of our study
on the cascade cyclization of o-toluic acid with phenyl isocya-
nate,[15] we unexpectedly observed that the acid-free 3-methyl-
N-phenylbenzamide was generated under rhodium catalysis
(Scheme 1c). This finding indicates that the reaction involved
a carboxyl-directed ortho-amidation of o-toluic acid via CÀH
functionalization followed by decarboxylation. Namely, the car-
boxy group acted as a removable ortho-directing group. We
reasoned that, by employing this strategy, N-aryl benzamides
with more difficult substitutions could be obtained readily
[a] Dr. X.-Y. Shi, K.-Y. Liu, J. Fan, X.-F. Dong, Prof. Dr. J.-F. Wei
Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Edu-
cation), Key Laboratory for Macromolecular Science of Shaanxi Province,
School of Chemistry and Chemical Engineering, Shaanxi Normal University,
Xi’an 710062 (P. R. China)
E-mail: shixy@snnu.edu.cn
[b] Prof. Dr. C.-J. Li
Department of Chemistry, McGill University
Montreal, QC, H3A 0B8 (Canada)
E-mail: cj.li@mcgill.ca
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406031.
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from other substituted benzoic acids, which are conveniently
more accessible. Herein, we present the convenient synthesis
of N-aryl benzamides by rhodium-catalyzed ortho-amidation
and decarboxylation of benzoic acids.
K2HPO4 and Cu2O was used in the reaction system (Table 1,
entry 20).
Having established the optimized reaction conditions, the
substrate scope was examined with respect to the substituted
benzoic acids (Table 2). No desired product was isolated using
2-nitrobenzoic acid, bearing a strong electron-withdrawing
To understand the nature of this reaction and to optimize
the reaction conditions, different solvents were first examined,
given the importance of solvent effects in catalytic reactions
(Table 1, entries 1–9). Solvent screenings revealed that dioxane
provided a slightly higher yield than THF, chlorobenzene, tolu-
Table 2. Substrate scope for the CÀH functionalization–decarboxylation
Reaction.[a]
Table 1. Selected results for optimizing reaction conditions.[a]
Entry
Additive (equiv)
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
K2HPO4(1.0)
K2HPO4(1.0)
K2HPO4(1.0)
K2HPO4(1.0)
K2HPO4(1.0)
K2HPO4(1.0)
K2HPO4(1.0)
K2HPO4(1.0)
K2HPO4(1.0)
K2HPO4(2.0)
—
THF
C6H5Cl
toluene
o-xylene
dioxane
DCE
13
11
10
11
23
ND[c]
ND
ND
ND
48
ND
ND
trace
ND
12
DMF
CH3CN
9
t-BuOH
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
10
11
12
13
14
15
16
17
18
19
20
Na2HPO4(2.0)
KH2PO4(2.0)
NaH2PO4(2.0)
K3PO4(2.0)
K2CO3(2.0)
25
40
29
14
Na2CO3(2.0)
Li2CO3(21.0)
Ag2CO3(2.0)
K2HPO4 (2.0)/Cu2O (0.15)
65
[a] Reactions were carried out with o-toluic acid (0.1 mmol), phenylisocya-
nate (0.2 mmol), [{Cp*RhCl2}2] (5 mol%), additives, and solvent (0.5 mL) at
1
1508C for 24 h, under argon in pressure tubes; [b] determined by H NMR
spectroscopy of the crude reaction mixture using mesitylene as internal
standard; [c] ND=not detected.
[a] Reactions were carried out with acids (0.2 mmol), isocyanates
(0.4 mmol), [{Cp*RhCl2}2] (5 mol%), dioxane (0.5 mL), K2HPO4 (2.0 equiv),
Cu2O (0.15 equiv), 1508C, 24 h. Yield of isolated product was reported.
ene, or o-xylene, and no target product was detected when di-
chloroethane (DCE), DMF, acetonitrile, or tert-butanol were
used as solvent. The moderate yield of 3a could be enhanced
to 48% by increasing the amount of K2HPO4 to 2.0 equivalents.
No desired product was formed in the absence of K2HPO4
(Table 1, entry 11), which suggested that the choice of salt is
crucial for the success of the present catalytic reaction. There-
fore, a series of salts was also examined for the reaction. Disap-
pointingly, when other alkali phosphates such as Na2HPO4,
KH2PO4, NaH2PO4, and K3PO4 were employed, either no desired
product or only trace amounts thereof was formed (Table 1,
entries 12–15). The use of carbonates also provided the desired
product with decreased yield compared to K2HPO4 (Table 1, en-
tries 16–19). Given that Cu2O is known to promote the decar-
boxylation,[13i,16] different amounts of Cu2O were introduced to
optimize the formation of this product. Finally, the yield of the
product was further improved to 65% when a mixture of
group, which suggests that the parent carboxy group and the
additional electron-withdrawing group render the aryl ring
highly electron-deficient and retard the attacking of electro-
philic rhodium.
The location of substituents on the parent benzoic acid has
a strong influence on the product yield. For example, o-me-
thoxybenzoic acid afforded only 30% yield of the product,
which can be attributed to deactivation resulting from the co-
ordination between an electron-rich heteroatom and rhodium.
Compared with ortho-substituted benzoic acids, those bearing
substituents at the para position were readily converted into
the corresponding products in higher yields (Table 2; 3a vs.
3d, 3b vs. 3e, 3c vs. 3 f). Meta-substituted benzoic acids also
reacted smoothly to afford the corresponding products in high
yields and generated two regioisomers (Table 2; 3h and 3h’),
due to the two possible CÀH bond activation sites at C2 and
C6. The amide formation occurred selectively para to the sub-
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stituent, which illustrated that the reaction was mainly under
steric control. This transformation tolerated dual substitution
and 2,4-dimethylbenzoic acid gave the highest yield of the cor-
responding product (Table 2; 3i–l).
rhodium species to the carboxylic oxygen atom and subse-
quent ortho CÀH activation affords a five-membered rhodacy-
clic intermediate B, releasing one equivalent of proton at the
same time. Then, isocyanate is coordinated to B at Rh to form
C, followed by the insertion of the isocyanate into the RhÀC
bond of C to generate the seven-membered rhodium alkoxide
D. Protonation of D by HCl affords the intermediate E and re-
generates the active RhIII species for the next cycle. Finally, de-
carboxylation of E followed by protonation provides the final
product.
To further explore the substrate scope and limitations of this
process, the reactions between 2,4-dimethylbenzoic acid and
various isocyanates were tested (Table 2). 3-methyl, 4-methyl,
and 3,5-dimethyl phenyl isocyanates underwent CÀH activation
and a decarboxylation reaction effectively with 2,4-dimethyl-
benzoic acid, producing the corresponding products in 74%,
80%, and 77% yields, respectively (3m, 3n, 3p). In the pres-
ence of electron-withdrawing groups such as Cl and Br, the de-
sired products were also obtained, although the yields were
somewhat decreased (3o, 3q, 3r). Particularly noteworthy was
that halo groups (Cl, Br) on the aromatic ring of isocyanate re-
mained intact in the product, which provides the possibility for
further useful transformations through common cross-coupling
strategies. Thus, as a proof-of-concept, using 3q as the reac-
tant, a further transformation of the product by employing the
Suzuki reaction afforded the product 4q in 38% yield
(Scheme 2).[17]
In summary, we have succeeded in preparing a series of N-
aryl benzamides from readily available benzoic acids and iso-
cyanates through regiospecific amidation of aromatic ortho-CÀ
H groups with concomitant decarboxylation. In these reactions,
the carboxy function effectively serves as a removable direct-
ing group. Notably, less common meta-substituted N-aryl ben-
zamides can arise readily from more accessible para- or ortho-
substituted groups by employing this strategy.
Experimental Section
Based on known rhodium-catalyzed directing-group-assisted
CÀH bond activation reactions[18] and metal-mediated decar-
boxylation reactions,[19] a tentative mechanism to rationalize
this reaction is depicted in Scheme 3. Dissociation of the dimer
precatalyst [{Cp*RhCl2}2] provides the coordinatively unsaturat-
ed monomer. With the help of K2HPO4, coordination of the
An oven-dried reaction vessel was charged with [{Cp*RhCl2}2]
(6.2 mg, 5 mol%, 0.01 mmol), acid 1 (0.2 mmol), isocyanate 2
(0.4 mmol), K2HPO4 (69.6 mg, 0.4 mmol), Cu2O (4.4 mg, 0.06 mmol).
After the tube was evacuated and purged with argon three times,
the 1,4-dioxane (0.5 mL) was added to the system by syringe. The
mixture was stirred at 1508C for 24 h. When the reaction was com-
plete, the resulting mixture was
cooled to room temperature and
filtered through a short silica-gel
pad. Then, the mixture was con-
centrated in vacuo to give a resi-
due, which was purified by prepa-
rative TLC to afford the corre-
sponding product.
Scheme 2. Suzuki coupling of 3q with phenylboronic acid.
Acknowledgements
The authors are grateful to the
National Natural Science Foundation of China (Grant Nos.
20906059 and 21272145), Shaanxi Innovative Team of Key Sci-
ence and Technology (Grant No. 2013 KCT-17), the Fundamen-
tal Research Funds for the Central Universities (Grant Nos.
GK201102005 and GK261001095), and Canada Research Chair
(to C.J.L.) for providing financial support for this research.
Keywords: amidation · CÀH activation · decarboxylation ·
homogeneous catalysis · synthetic methods
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Received: November 9, 2014
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
4
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
&
CÀH Activation
X.-Y. Shi,* K.-Y. Liu, J. Fan, X.-F. Dong,
J.-F. Wei, C.-J. Li*
Kill the director: The rhodium-catalyzed
amidation of substituted benzoic acids
with isocyanates via directed CÀH func-
tionalization followed by decarboxyla-
tion to afford the corresponding N-aryl
benzamides is demonstrated. The car-
boxylate serves as a unique, removable
directing group. Notably, less common
meta-substituted N-aryl benzamides are
formed readily from more accessible
para- or ortho-substituted groups by
employing this strategy.
&& – &&
A Convenient Synthesis of N-Aryl
Benzamides by Rhodium-Catalyzed
ortho-Amidation and Decarboxylation
of Benzoic Acids
Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ