Monodentate Transient Directing Group Assisted Ruthenium(II)-Catalyzed Direct ortho-C-H Imidation of Benzaldehydes for Diverse Synthesis of Quinazoline and Fused Isoindolinone

Article abstract of DOI:10.1021/acs.orglett.1c01083

2-Fluoro-5-(trifluoromethyl)aniline was found to be a suitable monodentate transient directing group (MonoTDG) to enable Ru(II)-catalyzed intermolecular direct ortho-C(sp2)-H imidation of benzaldehydes. N-Tosyloxyphthalimide was used as an alternative azide-free amidation reagent to achieve high efficiency and good functional group tolerance. Moreover, the reaction could be enlarged to gram scale, and the amidated product could be readily converted into useful quinazoline and fused isoindolinone scaffolds by one-step derivatization.

Full text of DOI:10.1021/acs.orglett.1c01083

pubs.acs.org/OrgLett
Letter
Monodentate Transient Directing Group Assisted Ruthenium(II)-
Catalyzed Direct ortho-C−H Imidation of Benzaldehydes for Diverse
Synthesis of Quinazoline and Fused Isoindolinone
Yongdi Wu,# Na Liu,# Meifang Qi, Huihao Qiao, Xuelian Lu, Li Ma, Yirong Zhou,*
and Fang-Lin Zhang*
Cite This: Org. Lett. 2021, 23, 3923−3927
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ABSTRACT: 2-Fluoro-5-(trifluoromethyl)aniline was found to be a
suitable monodentate transient directing group (MonoTDG) to
enable Ru(II)-catalyzed intermolecular direct ortho-C(sp2)−H
imidation of benzaldehydes. N-Tosyloxyphthalimide was used as an
alternative azide-free amidation reagent to achieve high efficiency and
good functional group tolerance. Moreover, the reaction could be
enlarged to gram scale, and the amidated product could be readily
converted into useful quinazoline and fused isoindolinone scaffolds
by one-step derivatization.
available. In most cases, stoichiometric amounts of strong
oxidants were required, and accompanying waste was
generated. Therefore, it is urgent to develop novel environ-
mentally benign atom-economic construction methods for
quinazoline and isoindolinone compounds from stable and
easily accessible substrates.
uinazoline and fused isoindolinone skeletons stand as
Q two kinds of important nitrogen-containing hetero-
cycles. They have wide existence in natural products and
pharmaceutical molecules which exert a remarkably wide
variety of biological and pharmacological activities, such as
antimalarial, antimicrobial, antiviral, antihypertensive, antitu-
berculosis, antipsychotic, and anticancer activities, etc. (Figure
1).1,2
For the last two decades, the transition-metal-catalyzed inert
C−H bond activation has become an essential tool for the
direct and efficient formation of a C−C bond and C−hetero
bond.5 Moreover, a vast number of directing groups have been
successfully utilized to enable various transition metal catalysts
for the direct C(sp2)−H amination and amidation of arenes
(Scheme 1a).6 Taking advantage of the assistance of the
transient directing group (TDG),7 the aldehyde compounds
have also rendered step-economic C−H amidation via an in
situ formed imine intermediate.8,9 Shi, Yu, and other groups
independently reported iridium-catalyzed ortho-C−H amida-
tion of benzaldehydes by using organic azide (TsN3) as
amidating reagents (Scheme 1b).8 Later, dioxazolone as
another suitable amino source was employed to fulfill similar
transformations by using Rh and Co catalysts (Scheme 1b).9
Besides, Ackermann and Zhang’s group demonstrated that
aldehyde and ketone could act as a weakly coordinating
directing group for C−H imidation of ketone and amidation of
Figure 1. Selected examples of 2-aryl-substituted quinazolines and
fused isoindolinones.
Due to their importance and broad applications, great efforts
have been devoted to the development of efficient preparation
protocols of such useful scaffolds, and several synthetic
strategies have been established.3,4 Although significant
advances have already been achieved, there still existed some
limitations for these reported procedures. For instance, many
prefunctionalized starting precursors were not commercially
Received: March 30, 2021
Published: May 3, 2021
https://doi.org/10.1021/acs.orglett.1c01083
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3923−3927
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Scheme 1. Transition-Metal-Catalyzed C−H Amidation
Scheme 2. TDG Screening for ortho-C−H Imidation of
a
Benzaldehyde
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), [RuCl2(p-
cymene)]2 (5 mol %), AgSbF6 (20 mol %), TDG (30 mol %), and
LiOAc (0.2 mmol) in DCE (2 mL) at 120 °C for 24 h. Isolated yields.
NR means no reaction.
benzaldehyde, respectively (Scheme 1c).10 However, the direct
C−H imidation of benzaldehydes remains an unexplored area.
We envisioned that the newly emerged TDG strategy could
provide a new alternative opportunity for catalytic C−H
imidation of benzaldehyde. Herein, we disclose the first
ruthenium(II)-catalyzed intermolecular direct ortho-C−H
imidation of benzaldehydes. Furthermore, by the treatment
of ammonia and sodium borohydride, 2-aryl quinazoline and a
fused isoindolinone framework would be readily constructed,
respectively (Scheme 1d).
a
Scheme 3. Substrate Scope Exploration
Based on our previous work in TDG-enabled ortho-C−H
functionalization of benzaldehydes,11 it was supposed that the
TDGs play a pivotal role for the successful transformation.
Thus, the effect of a variety of TDGs on the model reaction of
2-methylbenzaldehyde 1a with N-tosyloxyphthalimide 2a was
initially investigated (Scheme 2). Disappointedly, amino acids
T1 and T2, as the mostly widely used bidentate transient
directing groups, failed to facilitate the transformation, and no
desired product was detected. Subsequently, we moved onto
the investigation of a library of substituted anilines as
monoTDGs (T3−T12). Fortunately, this time, expected
product 3a was isolated in moderate to good yields. It was
worthy to note that the electronic property and position of the
substituents on the anilines influenced the reaction signifi-
cantly. After extensive TDG screening, T6 turned out to be the
optimal one for the direct C−H imidation of benzaldehyde,
which generated the highest yield of 81%. Meanwhile, the
control experiment showed that no expected imination product
was noticed in the absence of T6. The base additive, solvent,
and reaction temperature also exhibited evident influence on
the reaction (for detailed optimization information, see SI).
With the optimized reaction conditions in hand, the
substrate scope of the benzaldehydes was explored (Scheme
3). In general, both electron-donating groups and electron-
withdrawing groups at the ortho position of benzaldehydes
a
Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), [RuCl2(p-
cymene)]2 (5 mol %), AgSbF6 (20 mol %), T6 (30 mol %), and
LiOAc (0.2 mmol) in DCE (2 mL) at 120 °C for 24 h. Isolated yields.
were well tolerated to provide the desired products 3a−3g in
good yields. Importantly, a group of halogen atoms, including
F, Cl, Br, and I (3d−3f, 3j−3k, 3m−3v), were all compatible
for this direct C−H imidation, and no side reaction on these
sites was detected. In the case of meta-substituted benzalde-
hydes (1h−1k), it was reasonable to notice the imidation
selectively occurred at the less hindered position due to the
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steric constraint. Furthermore, a series of disubstituted
benzaldehydes were also probed, and all the tested substrates
could deliver imination products in moderate to good yields
(3l−3u). The structure of product 3u was unambiguously
characterized by X-ray crystallographic diffraction. The
heterocyclic substrate, such as 5-bromothiophene-2-carbalde-
hyde (1v), was also examined, and a moderate yield was
obtained. Eventually, another imidating reagent 2b was tested,
and it showed similar reactivity to generate good yield for 3w.
After completion of substrate scope exploration, a gram-scale
enlarged reaction and the chemo divergent derivatization were
performed to reveal the synthetic application of this new
strategy. As illustrated in Scheme 4, a 10 mmol scale model
Scheme 6. Proposed Reaction Mechanism
Scheme 4. Synthetic Applicability
cleavage delivers the complex E, which subsequently
regenerates the active catalyst A, thereby liberating the ortho-
imidated imine F. The eventual hydrolysis step provided the
desired product 3 and T6 for the next catalytic cycle.
In conclusion, the first direct ortho-C−H imidization of
benzaldehydes with N-tosyloxyphthalimide was successfully
rendered. A novel monotransient directing group assisted
Ru(II) catalytic strategy was developed to achieve high efficacy
and broad functional group tolerance. Synthetically useful yield
was obtained for a gram-scale enlarged model reaction, and
important 2-aryl quinazoline and fused isoindolinone skeletons
were facilely constructed by simple one-step late-stage
derivatization.
reaction of 1a afforded 1.99 g of product 3a in 75% yield. It
was noteworthy to indicate that the efficacy did not diminish
evidently for the enlarged reaction under standard conditions.
Further treatment with ammonia in methanol 3a was readily
converted into quinazoline 4 in 65% yield. On the other hand,
partial reduction of 3a using NaBH4 delivered fused
isoindolinones 5 in 89% yield.
To gain more information about the reaction mechanism, a
series of intermolecular competition experiments between
differently substituted benzaldehydes were performed. The
results in Scheme 5 indicated that electron-rich benzaldehydes
reacted preferentially compared with electron-poor ones.
ASSOCIATED CONTENT
* Supporting Information
■
sı
Scheme 5. Intermolecular Competition Experiments
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01083.
Experimental details and spectra for important com-
pounds including NMR spectra and X-ray structure of
3u (PDF)
Accession Codes
CCDC 2055086 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Based on previous reports8−12 and the above mechanistic
findings, a plausible catalytic cycle was proposed in Scheme 6.
With silver salt taking the counter chloride anion, and
following anion exchange with acetate, the ruthenium dimer
catalyst precursor releases the mono active species A. In situ
condensation between benzaldehyde 1 and TDG T6 yields
imine B. Then the imine group directed ortho-C−H activation
leads to the formation of the five-membered cycloruthenium
intermediate C. Subsequent coordination with imidating
reagent 2a gives complex D. The following N−O bond
AUTHOR INFORMATION
Corresponding Authors
■
Fang-Lin Zhang − School of Chemistry, Chemical Engineering
and Life Sciences, Wuhan University of Technology, Wuhan
430070, China; Shenzhen Research Institute, Wuhan
University of Technology, Shenzhen, Guangdong 518057,
China; orcid.org/0000-0002-7472-9713;
Email: fanglinzhang@whut.edu.cn
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Tongji Medical College, Huazhong University of Science and
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Authors
Yongdi Wu − School of Chemistry, Chemical Engineering and
Life Sciences, Wuhan University of Technology, Wuhan
430070, China
Na Liu − School of Chemistry, Chemical Engineering and Life
Sciences, Wuhan University of Technology, Wuhan 430070,
China
Meifang Qi − School of Chemistry, Chemical Engineering and
Life Sciences, Wuhan University of Technology, Wuhan
430070, China
Huihao Qiao − School of Chemistry, Chemical Engineering
and Life Sciences, Wuhan University of Technology, Wuhan
430070, China
Xuelian Lu − Shenzhen Research Institute, Wuhan University
of Technology, Shenzhen, Guangdong 518057, China
Li Ma − School of Chemistry, Chemical Engineering and Life
Sciences, Wuhan University of Technology, Wuhan 430070,
China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01083
Author Contributions
#Y.W. and N.L. contributed equally to this work.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (No.
21971200) and the Science and Technology Program of
Shenzhen (JCYJ20190809180001759).
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