Rh-catalyzed Transient Directing Group Promoted C—H Amidation of Benzaldehydes Utilizing Dioxazolones

Article abstract of DOI:10.1002/cjoc.201700726

Transition-metal catalyzed C—H functionalization of benzaldehydes is of great interest in organic synthesis. Herein, we developed a transient directing group assisted amidation of benzaldehydes catalyzed by rhodium catalyst. With the employment of 10 mol% of 4-trifluoromethyl aniline, the in situ generated imine groups as the directing group efficiently enable this transformation. By using this protocol, a wide range of benzaldehydes were efficiently converted into the corresponding N-(2-formylphenyl)benzamides utilizing dioxazolones as the nitrogen source.

Full text of DOI:10.1002/cjoc.201700726

Rh-catalyzed Transient Directing Group Promoted C–H Amidation of Benzalde-
hydes Utilizing Dioxazolones
Xiaoyang Wang,a Song Song,a and Ning Jiaoa, b,*
a State Key Laboratory of Natural and Biomimetic Drugs, Peking University, Xue Yuan
Rd. 38, Beijing 100191, China.
b State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China.
* E-mail: Email address; Tel.: telephone number ((optional)); Fax: fax number ((optional))
Received ((will be filled in by the editorial staff)); accepted ((will be filled in by the editorial staff));
published online XXXX, 2013.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cjoc.2013xxxxx or from the author. ((Please delete if not appropriate.)).
Dedicated to … on the Occasion of … ((optional))
Transition-metal catalyzed C–H functionalization of benzaldehydes is of great inter-
est in organic synthesis. Herein, we developed a transient directing group assisted ami-
dation of benzaldehydes catalyzed by rhodium catalyst. With the employment of 10 mol%
of 4-trifluoromethyl aniline, the in situ generated imine groups as the directing group
efficiently enable this transformation. By using this protocol, a wide range of benzalde-
hydes were efficiently converted into the corresponding N-(2-formylphenyl)benzamides
utilizing dioxazolones as the nitrogen source.
Keywords amidation, benzaldehydes, rhodium, C-H functionalization, transient di-
recting group
Introduction
moval of such auxiliary groups was often chal-
lenging, which limit their application in organic
synthesis.
Benzaldehydes are important building blocks
in natural products, materials and pharmaceuticals.
Recently, transition-metal catalyzed C-H func-
tionalization becomes an intriguing research topic
in methodology.1 The abundance and synthetic
versatility of benzaldehydes make aldehyde-di-
rected C−H functionalization an alluring area.2.3
However, there were only limited reports in this
field owning to the weak coordinating ability of
an aldehyde group and its instability toward oxi-
dation.4
To overcome the above limitations, an optional
and promising approach is to introduce transient
directing groups which could bind reversibly with
substrates. In the traditional directing group as-
sisted C-H bond functionalization, the directing
groups could efficiently control the regioselectiv-
ity of C–H activation thus enabling regiospecific
introduction of other groups. However, the instal-
lation of directing groups adds steps and the re-
Alternatively, a transient directing group strat-
egy might enable the C–H bond functionalization
without changing the functional groups of the sub-
strates (Scheme 1, a). Lewis’ group represented
the pioneering example of selective ortho C-H
bond alkylation utilizing ortho-metalated ruthe-
nium complexes.5 Jun and coworkers complete an
elegant example by using 2-aminopyridine as ex-
ternal directing group for Rh-catalyzed C–H bond
functionalization of aldehydes.6 In the assistance
of catalytic amount of phosphinite liga-
Scheme 1 Transient directing group promoted
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C-H functionalization of benzaldehydes.
(Scheme 1, c). With the employment of 10 mol%
of aniline, the in situ generated imine groups as
the directing group efficiently enable this transfor-
mation.
Experimental
[Cp*RhCl2]2 (0.0045 mmol, 2.8 mg), AgSbF6
(0.018 mmol, 6.2 mg), benzaldehydes 1 (0.3
mmol), 1,4,2-dioxazol-5-ones 2 (0.45 mmol) and
4-(trifluoromethyl)aniline (0.03 mmol 4.8 mg)
were added to a 25 mL Schlenk tube with a mag-
netic bar under Ar. PhCl (1.5 mL) was added and
then the mixture was stirred at 120 ℃ under Ar for
20 h. The solution was then washed with HCl (2
M, 10 mL), extracted with ethyl acetate and evap-
orated under vacuum. The crude reaction mixture
was purified by column chromatography on silica
gel to get the desired product.
ands, Bedford’s group reported functionalization
of C–H bonds in phenols and alcohols via reversi-
ble transesterification.7 Dong and co-workers de-
veloped the C–H functionalization through re-
versible formation of enamines.8 Yu and cowork-
ers reported direct C–H arylation with the help of
natural amino acid or 2-hydroxynicotinaldehyde
as transient directing group.9 Ge and co-workers
reported transient directing group assisted site-se-
lective C–H bond functionalization in the assis-
tance of catalytic amount of glyoxylic acid or 3-
aminopropanoic acid.10 Very recently, the groups
of Shi,11 Murakami,12 Bull,13 Xu and Jin,14 Zhang,
Li and Yan,15 Hu16 independently developed tran-
sient directing group assisted Pd-catalyzed C-H
olefination and arylation.
Results and Discussion
Our initial investigation was carried out using
4-ethylbenzaldehyde (1a) and 3-phenyl-1,4,2-di-
oxazol-5-one (2a)26,27g in the presence of 1.5 mol %
[Cp*RhCl2]2 in PhCl for 16 h. Various amines
were chosen to test the catalytic activity (Table 1).
Interestingly, aniline (A1) worked well to produce
the desired product 3a in 62% yield. With 4-meth-
oxyl substituted aniline (A2) as the cocatalyst, the
yield slightly decreased to 59%. Among those ani-
lines examined, 4-trifluoromethyl aniline (A3)
was found to be the most effective one to afford
the desired product 3a in 80% yield (Table 2, entry
1). Moreover, alkyl amines such as cyclohexyla-
mine (A4) and benzylamine (A5) were proved to
be less effective. For transient directing groups
from amides (A6, A7), only trace amount of 3a
was detected. Furthermore, the loading of 4-tri-
fluoromethyl aniline was further screened. Nota-
bly, a yield of 74% could also be obtained with
only 10 mol % of 4-trifluoromethyl aniline (Table
2, entry 3). Increasing the reaction concentration
further improved the efficiency to 81% yield (Ta-
ble 2, entry 4). Exclusion of either metal catalysts
or aniline under standard condition inhibited the
reaction (Table 2, entry 5-7). Those results
demonstrate that imine might be an transient di-
recting group for Rh-catalyzed C–H bond activa-
tion of benzaldehydes.
According to this concept, Csp2–H functional-
ization of benzaldehydes also attracted considera-
ble attentions using imine as transient directing
group. Construction of C-C bond from ortho C-H
bond of benzaldehydes was revealed by the
groups of Seayad,17 Yu,18 Sorensen19 and Wang20
utilizing transient imine as directing group
(Scheme 1, b). The chlorination, and bromination
of benzaldehyde substrates was also realized by
Yu and coworkers (Scheme 1, b).18 Recently,
Sorensen’s group reported Pd(II) catalyzed transi-
ent directing groups promoted ortho C−H hydrox-
ylation of benzaldehyde substrates.21 Moreover,
by using Ir-catalysis, the groups of Yu,18 Shi,22
Chen and He,23 independently reported transient
directing groups promoted C-N bond formation
with azide reagents (Scheme 1, b). Very recently,
C-H amidation utilizing nitrone as a traceless di-
recting group was realized in constructing func-
tionalized arylaldehydes.24 According to our con-
tinued interest in nitrogenation chemistry for C-N
bond formation,25 we investigated the direct C-H
amidation of benzaldehydes. Herein, we report a
transient directing group assisted amidation of
benzaldehydes catalyzed by rhodium catalyst for
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the synthesis of N-(2-formylphenyl)benzamides

Table 1 Optimization of transient directing
groups. a
not work well (3n, 22%). With the increasing of
4-trifluoromethyl aniline catalyst to 20 mol %, the
yield of 3n was improved to 61%. Besides, 1-
naphthaldehyde and 2-naphthaldehyde showed
good reactivity giving the products 3o and 3p in
good yields with excellent regioselectivity, re-
spectively (72% and 81%).
Table 3 The reactions with substituted benzal-
dehyde a,b
Table 2 Optimization of Reaction Parameters.
a
a
Standard conditions: 1a (0.3 mmol), 2a (0.45
mmol), [Cp*RhCl2]2 (1.5 mol %), AgSbF6 (6 mol
%), 4-trifluoromethyl aniline (10 mol %), PhCl
(1.5 mL), stirred at 120 ℃ under Ar (1 atm) for 20
h. b Isolated yields. c 4-trifluoromethyl aniline (20
mol %) was used.
a
Reaction conditions: 1a (0.3 mmol), 2a (0.45
mmol), [Cp*RhCl2]2 (1.5 mol %), AgSbF6 (6 mol
%), at 120 ℃ under Ar (1 atm). b Isolated yields.
With the optional conditions in hand, we ex-
plored the reaction scope by using various substi-
tuted benzaldehydes together with 3-phenyl-
1,4,2-dioxazol-5-one as the reaction partner (Ta-
ble 3). All the substrates worked well to produce
the desired products in moderate to excellent
yields. The o-, m- and p-substituted methyl ben-
zaldehydes were then investigated under these
conditions and gave the corresponding products
3c-3e in 83-96% yields. It was found that the effi-
ciency was not affected by the steric effect. Fur-
thermore, the halogenated benzaldehydes pro-
duced the halo-substituted products 3f-3k in 60-
79% yields, which could be used for further cou-
pling transformations as a coupling partner. With
electron-donating substituent at the para-position,
4-methoxybenzaldehyde gave the desired product
3l in 71% yields. For substrates with electron-
withdrawing group such as CO2Me, the desired
product 3m was formed in 81% yield. However,
the reaction of benzaldehyde with CF3 group did
For the scope of dioxazolones, the applicability
was further investigated using benzaldehyde 1a as
the partner reagent (Scheme 3). For alkyl substi-
tuted 3-Phenyl-1,4,2-dioxazol-5-one reagents,
these reactions worked well to afford the amide
products (4b-4d) in good yields. It is noteworthy
that halogenated dioxazolones performed well to
afford the corresponding amide products (4e-4g)
in 76% - 85% yield. Furthermore, electron-donat-
ing and electron-withdrawing substituents worked
well to afford 4h and 4i in 64% and 61% yields,
respectively. Meanwhile, except for phenyl sub-
stituted 1,4,2-dioxazol-5-one reagent, the reaction
with 3-(thiophen-2-yl)- 1,4,2-dioxazol-5-one as
nitrogen source could also afford the correspond-
ing thiophene ring product 4j in 23% yield.
Table 4 Scope with respect to dioxazolones a,b
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[Cp*RhIII] is generated with the assistance of
AgSbF6. The cationic catalyst would coordinate
with the imine intermediate I and undergoes C−H
bond activation to generate the rhodacyclic inter-
mediate II. Coordination of 3-phenyl-1,4,2-diox-
azol-5-one 2a to rhodium(III) affords intermedi-
ate III. With the release of CO2 and migratory in-
sertion, Rh(III) amido species (IV) is then formed.
Subsequently, the protonation of intermediate IV
occurs to regenerate cationic Cp*Rh(III) catalyst
and furnish the amidation process with V as prod-
uct. Finally the hydrolysis of amide intermediate
V occurs to form target molecule and release the
aniline catalyst for a new catalytic cycle.
Conclusions
In conclusion, we have achieved an efficient
Rh-catalyzed
approach
diverse
N-(2-
a
Standard conditions: 1a (0.3 mmol), 2a (0.45
formylphenyl)- benzamides through the transient
directing group assisted C-H activation. 4-Tri-
fluoromethyl aniline was approved as a good co-
catalyst in this case. This reaction complements β-
C–H functionalization of benzaldehydes with a
broad substrate scope and moderate to excellent
yields. Further mechanistic studies and other ap-
plications are now under investigation.
mmol), [Cp*RhCl2]2 (1.5 mol %), AgSbF6 (6
mol %), 4-trifluoromethyl aniline (10 mol %),
PhCl (1.5 mL), stirred at 120 ℃ under Ar (1 atm)
for 20 h. b Isolated yields.
To probe the reaction mechanism, imine sub-
strate 5a was synthesized in advance and was em-
ployed under the standard reaction conditions
without addition of 4-trifluoromethyl aniline (Eq.
1). The amide product 3b was obtained in 86%
yield in this case, which indicates that imines
could serve as the key intermediates in this trans-
formation.
Acknowledgement
We thank National Basic Research Program of
China (973 Program) (No. 2015CB856600), the
National Natural Science Foundation of China
(No. 21632001, 21772002), National Young Top-
notch Talent Support Program, and Peking Uni-
versity
Health
Science
Center
(No.
BMU20160541) for financial support of this work.
We thank Bencong Zhu in this group for reproduc-
ing the results of 3b and 4f.
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Entry for the Table of Contents
Page No.
Rh-catalyzed Transient Direct-
ing Group Promoted C–H Ami-
dation of Benzaldehydes Utiliz-
ing Dioxazolones
Transition-metal catalyzed C–H functionalization of benzalde-
hydes is of great interest in organic synthesis. Herein, we developed
a transient directing group assisted amidation of benzaldehydes cat-
alyzed by rhodium catalyst. With the employment of 10 mol% of
4-trifluoromethyl aniline, the in situ generated imine groups as the
directing group efficiently enable this transformation. By using this
protocol, a wide range of benzaldehydes were efficiently converted
Xiaoyang Wang; Song Song; Ning into the corresponding N-(2-formylphenyl)benzamides utilizing di-
Jiao* oxazolones as the nitrogen source.
This article is protected by copyright. All rights reserved.