Palladium-catalyzed denitrative α-arylation of ketones with nitroarenes

Article abstract of DOI:10.1021/acs.orglett.0c04104

The palladium-catalyzed α-arylation of ketones with readily available nitroarenes and nitroheteroarenes provides access to useful α-aryl and α-heteroaryl ketones. The use of the Pd/ BrettPhos catalysts was critical to achieve high efficiency for these transformations, whereas other catalysts led to decreased yields or no conversions. The intramolecular type substrate was also applied in this methodology and gave a chromone derivative. Polyaromatic carbonyl compounds can be easily obtained by multicomponent tandem reactions, via nucleophilic aromatic substitution (S_NAr) or cross-coupling reaction followed by this denitrative arylation. Kinetic experiments show that the electronic effect of nitrobenzenes has a greater effect on the reaction rate than the electronic effect of ketones.

Full text of DOI:10.1021/acs.orglett.0c04104

pubs.acs.org/OrgLett
Letter
Palladium-Catalyzed Denitrative α‑Arylation of Ketones with
Nitroarenes
Zhirong Li, Yonggang Peng, and Tao Wu*
Cite This: Org. Lett. 2021, 23, 881−885
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ABSTRACT: The palladium-catalyzed α-arylation of ketones with
readily available nitroarenes and nitroheteroarenes provides access
to useful α-aryl and α-heteroaryl ketones. The use of the Pd/
BrettPhos catalysts was critical to achieve high efficiency for these
transformations, whereas other catalysts led to decreased yields or
no conversions. The intramolecular type substrate was also applied
in this methodology and gave a chromone derivative. Polyaromatic carbonyl compounds can be easily obtained by multicomponent
tandem reactions, via nucleophilic aromatic substitution (SNAr) or cross-coupling reaction followed by this denitrative arylation.
Kinetic experiments show that the electronic effect of nitrobenzenes has a greater effect on the reaction rate than the electronic effect
of ketones.
α-Arylated carbonyl compounds are widely present in
numerous natural products, pharmaceuticals, agrochemicals,
efficient ligands, organic materials, etc.1 Consequently, the
development of highly efficient synthetic methodologies for α-
arylation of carbonyl compound is of great significance to
organic and medicinal chemists. To accompany the pioneering
work by Miura, Buchwald, and Hartwig,2 aryl iodides,3a aryl
bromides,3b aryl chlorides,3c aryl triflates,3d,e and aryl
arenesulfonates3f have been extensively used in palladium-
catalyzed α-arylation of carbonyl compounds (Figure 1).
However, aryl bromides and iodides tend to be more
expensive, aryl chlorides are less active, and aryl triflates and
aryl arenesulfonates require prior synthesis and are less stable.
The search for new aromatic sources is still important for
modern arylation processes.4
In recent years, activation of inert bonds has increasingly
attracted the attention of chemists.5 These inexpensive and
readily available aryl compounds have become new sources of
aromatic bases through associated sp2 C−H bond,6 C−O
bond,7 C−F bond,8 C−N bond,9 or C−C bond10 cleavage.
Nitroaromatic compounds, as one of the cheapest and most
readily available aryl-containing species, merely act as aryl
precursors via SNAr type reaction due to the electron-deficient
nitro group.11 Until recently, Nakao and Sakaki developed
denitrative Suzuki−Miyaura coupling between nitrobenzenes
and aryl boronic acids, in which C−NO2 bond activation was
involved.12 Besides, denitrative arylation strategies have been
implemented in other transformations, such as Buchwald−
Hartwig amination,13 Sonogashira coupling,14 Mizoroki−Heck
reaction,15 and reductive hydrogenation.16 This novel strategy
of nitroaromatics as aryl precursors has not been employed in
enolate chemistry.17 Herein, we report the first case of
denitrative α-arylation reactions of ketones with nitrobenzenes.
Our study began by optimizing the reaction between p-
nitrotoluene 1a and phenyl benzyl ketone 2a (Table 1). The
conditions for the Suzuki−Miyaura coupling reaction initially
were utilized in our model reaction, and the desired product,
α-aryl ketone 3a, was obtained with a 45% GC yield (entry 1).
Several solvents (entries 2−4) were then investigated, and
Received: December 11, 2020
Published: January 25, 2021
Figure 1. (A) Pd-catalyzed α-arylation of carbonyl compound. (B)
Pd-catalyzed denitrative α-arylation of ketones.
https://dx.doi.org/10.1021/acs.orglett.0c04104
© 2021 American Chemical Society
Org. Lett. 2021, 23, 881−885
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a
a
Table 1. Reaction Optimization
Scheme 1. Scope of the Denitrative α-Arylation of Ketones
a
Standard conditions: Pd(acac)2 (5 mol %), Brettphos (20 mol %), 1
(0.2 mmol, 1.0 equiv), 2a (0.24 mmol, 1.2 equiv), K3PO4·3H2O (0.6
mmol, 3.0 equiv) in c-Hexane (0.6 mL), stirred at 130 °C for 24 h,
a
General conditions: [Pd] (5 mol %), Ligand (20 mol %), 1a (0.1
mmol, 1.0 equiv), 2a (0.12 mmol, 1.2 equiv), Base (0.3 mmol, 3
b
b
equiv), in solvent (0.3 mL), stirred at 130 °C for 24 h. GC yields
with 1-tetradecane as internal standard. Isolated yield on a 0.2 mmol
scale (two runs). 3 equiv H2O were added. The reaction was run at
110 °C. The amount of Brettphos was reduced to 10 mol %.
average isolated yield after two runs. 1 mmol scale.
c
d
e
f
affording α-arylated ketones 3a−3f in excellent yields. Other
electron-rich nitroarenes, such as tert-butyl- or dimethyl-
substituted nitrobenzenes worked well to give the correspond-
ing product 3g and 3h in good yields. A phenyl substituent did
not interfere with the reaction and furnished the ketone 3i in
91% yield. To test electron-deficient nitroarenes, 4-trifluoroni-
trobenzene was subjected to the reaction conditions, giving 3j
in a moderate yield. Methyl and ethyl esters were tolerated
under these reaction conditions to furnish the desired products
3k and 3l in moderate to excellent yields. Although free formyl
substituted nitrobenzenes were incompatible with the optimal
reaction conditions, their acetal-protected analogs, 2m,
engaged in the reaction and was transferred to carbonyl-
substituted arylation product 3m in 76% yield. The C−F
bonds in 2n, 2o, and 2p were tolerant and retained in their
corresponding denitrative products. It should be noted that the
reaction of 4-fluoronitrobenzene proceeded in 77% yield,
exclusively via Ar−NO2 activation rather than conventional
SNAr reactions, in which the C−F bond was converted.19
However, other halogen functionalities were much more
reactive than the NO2 group and were not compatible with
the applied reaction conditions. Nitronaphthalenes were also
successfully denitrated, and arylation of 2a furnished the
corresponding ketone 3q in 82% yield. 3-Nitropyridine and 2-
nitrothiophene did not afford the desired products, whereas 1-
ethyl-6-nitroindole 2r and 1-ethyl-6-nitro-1H-indazole 2s
smoothly furnished the corresponding arylated product in
62% and 60% yield, respectively.
cyclohexane (c-Hex) gave the best yield, up to 60%. In
previously reported palladium-catalyzed denitritative reactions,
ligands often played crucial roles. The screening of ligand in
our reaction also indicated that Brettphos was the best choice,
whereas other Buchwald ligands, such as Xphos and Cphos,
afforded undetectable or trace conversion (entries 5−6). Other
phosphine ligands conventionally employed in the Buchwald−
Hartwig α-arylation of ketones did not generate 3a (see
Supporting Information). Moreover, NHC L was not effective
for this arylation (entry 7).18 When PdCl2 or Pd(PPh3)4 was
employed instead of Pd(acac)2, the reaction was shut down
(entries 8−9). Subsequently, the impact of bases on the
progress of the reaction was examined (entries 10−14). The
yield of 3a was improved when CsF was replaced with Cs2CO3,
whereas K3PO4 trihydrate was chosen and gave an unparalleled
yield of 3a, up to 92% GC yield with an 84% isolated yield.
The water contamination in K3PO4 seemed important, as its
presence (3 equiv) with predried K3PO4 resulted in a poorer
yield; the rationale for the effect was unclear (entry 11 vs 12 vs
14). Lower temperature or lower loading of ligand would
dramatically decrease the yield of 3a (entries 14−15). Finally,
the optimized reaction conditions of the denitrative α-arylation
of ketones included those listed in entry 12 in Table 1.
The optimized conditions for 1 proved to be general for the
palladium-catalyzed denitrative α-arylation of ketones with a
range of nitroarenes (Scheme 1). Various nitroarenes bearing
methyl and methoxy at the o-, m-, and p-positions worked well,
Thereafter, the scope of the denitrative α-arylation
concerning various ketones was investigated. The results for
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4-nitrotoluene 1a were summarized in Scheme 2. Arylketone
derivatives with an electron-donating substituent such as
Scheme 4. Tandem Reactions with Denitrative α-Arylation
of Ketones
a
Scheme 2. Scope of the Denitrative α-Arylation of Ketones
Fluoronitrobenzene 1n, phenol 9, and ketone 2a were utilized
in our standard conditions and resulted in the desired α-
arylation ketone product 10 at 71% yield. Various substituted
α-arylated ketones would be readily synthesized by changing
the nucleophile.
a
Standard conditions: Pd(acac)2 (5 mol %), Brettphos (20 mol %),
The denitrative α-arylation reaction was combined with a
state-of-the-art cross-coupling reaction (Scheme 4B). We
designed the cross-coupling arylation/denitrative arylation of
ketone step by step in one flask with the same palladium/
Brettphos catalytic system. Polyaryl carbonyl compound 14
was afforded in 67% yield. Unexpectedly, an excellent yield of
14 was achieved when all of the starting reagents, ketone 2a,
11, and phenylboronic acid 12, and the catalyst were put
together in one portion (Scheme 4C). These results indicated
that our methodology may be applied to synthesize
polyaromatic carbonyl compounds by simple combination of
ketones, halogenated nitrobenzenes, and arylboronic acids in
one operation.21
1a (0.2 mmol, 1.0 equiv), 4 (0.24 mmol, 1.2 equiv), K3PO4·3H2O
(0.6 mmol, 3.0 equiv) in c-Hexane (0.6 mL), stirred at 130 °C for 24
b
h, average isolated yield after two runs. 1a (0.6 mmol, 3.0 equiv), 4
c
(0.2 mmol, 1.0 equiv). Benzotrifluoride used as solvent.
methoxy or methyl on the C4-position of the phenyl group
gave the corresponding coupling products 5a and 5b in good
yields; electron-withdrawing substituents, such as fluorine or
phenyl, on the phenyl group did not affect the yield and were
converted to their corresponding arylated products 5c and 5d
in good to excellent yield. Free hydroxyl groups were not
compatible with this transformation, but preprotected o-
diphenolic ketones furnished the desired products 5e and 5f
in excellent yield. Benzyl phenyl ketones bearing methyl or
fluorine also worked well and afforded 5g and 5h in moderate
yields. Other alkyl phenyl ketones, such as ethyl, n-propyl, n-
butyl, or 2-phenylethyl, could be applied in this transformation,
whereas an increased amount of nitrotoluene 1a was required
and benzotrifluoride (PhCF3) was used as solvent instead of c-
Hexane.
To investigate the effect of the substituent on the aromatic
ring on the reaction rate, we designed several kinetic
experiments (Scheme 5). Experiments A showed that nitro-
Scheme 5. Kinetic Experiments
More interestingly, our method can also be applied to an
intramolecular type denitrative α-arylation of ketones (Scheme
3). α-Arylation cyclic product 8, a chromone derivative,20
Scheme 3. Trial of Intramolecular Type of Denitrative α-
Arylation of Ketones
which is abundant in natural products, could be gained in
moderate yield from the starting substrate 7, in which ketone
and NO2 groups were in coexistence.
Fluoronitroarenes can be readily substituted by a nucleo-
phile via an SNAr mechanism. Using this feature, we tried
tandem reactions via nucleophilic substitute/denitrative α-
arylation of ketones in one pot (Scheme 4A). para-
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Letter
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04104.
Experimental procedures, characterization data, and
copies of NMR spectra for substrates and products
(PDF)
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AUTHOR INFORMATION
Corresponding Author
■
Tao Wu − The College of Chemistry, Nanchang University,
Nanchang, Jiangxi 330031, P. R. China; orcid.org/0000-
0002-6895-7656; Email: taowu@ncu.edu.cn
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Authors
Zhirong Li − The College of Chemistry, Nanchang University,
Nanchang, Jiangxi 330031, P. R. China
Yonggang Peng − The College of Chemistry, Nanchang
University, Nanchang, Jiangxi 330031, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04104
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Science Foundation of China (21801113, 21961020)
and the start-up Fund of Nanchang University. We gratefully
acknowledge Prof. Tao Xu (Tongji University), Prof. Xiaoyu
Yang (ShanghaiTech University), and Prof. Guoyin Yin
(Wuhan University) for helpful discussions and proofreading
of the manuscript.
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