Iron-Catalyzed α-Arylation of Deoxybenzoins with Arenes through an Oxidative Dehydrogenative Approach

Article abstract of DOI:10.1021/acs.orglett.6b02516

A novel α-arylation of deoxybenzoins with non-prefunctionalized arenes is developed through an iron-catalyzed oxidative dehydrogenative approach. The reaction shows broad substrate scope and functional group tolerance and thus provides efficient access to synthetically useful 1,2,2-triarylethanones. A reasonable mechanism is also proposed.

Full text of DOI:10.1021/acs.orglett.6b02516

Letter
pubs.acs.org/OrgLett
Iron-Catalyzed α‑Arylation of Deoxybenzoins with Arenes through
an Oxidative Dehydrogenative Approach
Tao Chen, Yi-Fan Li, Yang An, and Fu-Min Zhang*
State Key Laboratory of Applied Organic Chemistry and Department of Chemistry, Lanzhou University, Lanzhou 730000, P. R.
China
S
* Supporting Information
ABSTRACT: A novel α-arylation of deoxybenzoins with non-prefunctionalized arenes is developed through an iron-catalyzed
oxidative dehydrogenative approach. The reaction shows broad substrate scope and functional group tolerance and thus provides
efficient access to synthetically useful 1,2,2-triarylethanones. A reasonable mechanism is also proposed.
ligands, general performance under basic conditions, as well as
poor compatibility of halogenated substrates. Moreover,
prefunctionalization to prepare the coupling precursor contain-
ing the C−heteroatom bond is always required, which
dramatically limited their wide practicability. In this respect,
the more direct transition-metal-catalyzed arylation of deoxy-
benzoins with arenes through cleavage of a C−H bond on the
aryl ring is particularly attractive but remains unexplored. Herein,
we report a FeCl3-catalyzed α-arylation of deoxybenzoins with
arenes through an oxidative dehydrogenative strategy9−11
(Scheme 1b).
We started to test the oxidative dehydrogenative reaction by
using deoxybenzoin 1a and p-xylene 2a as model substrates
(Table 1). However, such an oxidative transformation would
require solutions to several problems: (1) the benzylic C−H
bond of deoxybenzoin can be easily oxidized to form benzil
derivatives12 or cleaved into other species13 under oxidative
conditions; (2) intermolecular self-coupling of deoxybenzoin
produces unexpected 1,4-dicarboyl byproducts;14 (3) undesired
bis-arylation may compete with monoarylation;9c (4) the
benzylic C−H bond of p-xylene is easily oxidized to different
reaction intermediates, which could further participate in
undesired transformations;15 (5) in particular, oxidative
dehydrogenative arylations involving the C(sp3)−H bonds that
are not adjacent to a nitrogen or an oxygen atom have not been
well explored.9,16 The desired product 3a was obtained when the
reaction was conducted in the presence of FeCl3 under an O2
atmosphere at 80 °C, even though the yield was only 9% (entry
1). Screening of oxidants showed 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) to be the best choice, giving the product
in 77% yield (entries 2−5). The temperature is also crucial for the
reaction, which affects the yields of 3a dramatically: lower
temperature leads to low conversion, while higher temperature
α-Arylation of ketones has been a central topic in organic
synthetic chemistry because the resulting products exist widely in
numerous bioactive natural products, clinical drugs, and
functional molecules.1,2 Inspired by the seminal works of
Miura,3a Buchwald,3b and Hartwig,3c transition-metal-catalyzed
α-arylation of carbonyl compounds were intensively investigated
because this process features a few advantages, such as the use of
simple aryl halides or pseudohalides instead of organometallic
reagents as aryl sources, good chemo- and regioselectivity,
various possible catalytic systems, and ideal reaction yields.
Therefore, many efforts have been made to expand this
pioneering work for α-arylation of deoxybenzoins to synthesize
valuable 1,2,2-triarylethanones,4,5 including the Pd-catalyzed
transformation by Miura, Hartwig and others,6 a Cu-catalyzed
approach by Taillefer,7 and a Ni-catalyzed approach by Itami
(Scheme 1a).8 These coupling reactions generally rely on the
formation of the corresponding enolate intermediates from
ketones under basic conditions, which act as ideal nucleophiles
for the electrophilic species generated through oxidative addition
of transition metals to aryl halides or pseudohalides. However,
some shortcomings still existed, such as employment of high cost
Scheme 1. Transition-Metal Catalyzed α-Arylation of
Deoxybenzoins
Received: August 22, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02516
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Letter
a
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Scope of Arenes
b
entry
cat. (%)
solvent
oxid (equiv)
temp (°C) yield (%)
1
FeCl3 (20)
FeCl3 (20)
FeCl3 (20)
FeCl3 (20)
FeCl3 (20)
FeCl3 (20)
FeCl3 (20)
FeCl3 (20)
FeCl3 (20)
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
O2
80
80
80
80
80
70
60
50
90
80
80
80
80
80
80
80
9
15
8
2
K2S2O8 (1.2)
c
3
BQ (1.2)
d
4
DTBP (1.2)
11
77
70
41
29
49
0
e
5
DDQ (1.2)
6
DDQ (1.2)
DDQ (1.2)
DDQ (1.2)
DDQ (1.2)
DDQ (1.2)
7
8
9
15
16
17
18
19
FeCl3 (20)
FeCl3 (10)
FeCl3 (5)
10
64
5
DDQ (1.2)
DDQ (1.2)
DDQ (1.2)
DDQ (1.2)
DDQ (1.2)
AlCl3 (20)
FeCl3 (20)
FeCl3 (20)
18
99
73
f
22
g
23
a
Reaction was performed using deoxybenzoin (0.2 mmol) and p-
xylene (0.6 mmol) in 1.0 mL of solvent at 80 °C under argon.
a
Reaction conditions: 20 mol % of FeCl3, 120 mol % of DDQ,
deoxybenzoin (0.2 mmol), and arenes (0.6 mmol) in 1.0 mL of DCE
b
c
d
Isolated yield. BQ = p-benzoquinone. DTBP = di-tert-butyl
e
f
b
peroxide. DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. 20
at 80 °C under argon. Isolated yield of product. Performed on a 10
mmol scale, and 1.386 g product 3a was isolated. 50 mol % of FeCl3
g
c
equiv of p-xylene was applied. 99.99% FeCl3 was used.
was applied.
results in the decomposition of deoxybenzoin 1a (entries 6−9).
Screening of solvents indicated no superior results was
obtained.17 Reducing DDQ equivalents slightly decreased the
product yield.17 In the absence of FeCl3 or DDQ, no or a small
amount of the product 3a was isolated, indicating the
indispensability of both the iron catalyst and the oxidant (entries
10 and 11). FeCl3 loading dramatically affected the yields (entries
12 and 13). Other iron salts did not give better results.17
Screening metals other than iron demonstrated that they failed to
promote the current reaction,17 except AlCl3, which gave 18%
yield of product 3a (entry 14). Superior to 5 or 10 equiv, the use
of 20 equiv of p-xylene led to 99% yield (entry 15). To make the
reaction as cost-effective as possible, we selected 3 equiv of p-
xylene as the optimal conditions in the subsequent experiments.
To ascertain the current reaction through the iron catalysis,17 an
ultrapure 99.99% FeCl3 was used, resulting in the formation of 3a
in 73% yield (entry 16).
oxygen-based substituents on the arylated partner because
mono-, tri-, tetra-, and pentasubstituted aromatic products (3b,
3a, 3l, 3u, and 3v) were obtained. Our reaction also proceeded
with a MeS-substituted substrate, although the product 3w was
obtained in slightly lower yield and 50% FeCl3 was required.
As shown in Scheme 3, a variety of deoxybenzoins were also
tested under the optimal reaction conditions. For the benzoyl
a
Scheme 3. Scope of Deoxybenzoins
Under the optimized conditions (entry 5, Table 1), various
arenes gave the expected products in moderate to good yields
(Scheme 2). 1,2,2-Triphenylethanone 3b5b was obtained in 40%
yield from benzene. The reaction performed very well with
anisoles containing F, Cl, Br, I, or OCF3 substituents, providing
the corresponding products (3c-j) in satisfactory yields.
Dibenzo-p-dioxine also afforded the product 3k in 56% yield.
To examine the flexibility of the reaction toward aryl substrates,
we subjected tri- or tetra-substituted aromatic substrates to the
current reactions, and the desired products (3l-3v) were
obtained in good yields. The structures of 3m and 3o were
further confirmed by X-ray crystallography, respectively.18 The
reactions worked well even with the sterically hindered arenes,
producing products 3l, 3r, 3u, and 3v in moderate to good yields.
Moreover, the arylations proceeded smoothly even for relatively
electron-deficient anisoles bearing a NO2 substituent and
afforded the corresponding products (3s, 3t) in acceptable
yields. Notably, the current approach does not appear to require
a
Performed under the optimal conditions. Isolated yield of product 3.
moiety in deoxybenzoin, arenes containing various substitutents,
such as a strong electron-donating group (OMe), weak electron-
withdrawing groups (F, Cl, Br) at the para-position, and a methyl
group at the ortho-, meta-, or para-position, were well tolerated
and gave the expected products (3x−ad) in 52−79% yields.
Furthermore, replacing the monosubstituted phenyl moiety with
disubstituted p-xylene or a tert-butyl group did not affect the
reaction, affording the ketones 3ae and 3af in yields of 66% and
68%, respectively. For the benzyl moiety in deoxybenzoin,
various substituents on the aromatic ring were also tolerated. For
B
DOI: 10.1021/acs.orglett.6b02516
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Letter
example, either a Me group at various positions or F, Cl, Br, or Ph
at the para-position provided the corresponding products (3ag−
am). Notably, the expected products (3an−ap) with different
substituents on the two aryl rings were also obtained.
Scheme 5. Some Control Experiments
Finally, we investigated the α-arylation of deoxybenzoin
derivatives with various arenes (Scheme 4). Di-, tri-, and
a
Scheme 4. Scope between Deoxybenzoins and Arenes
bond cleavage at the benzylic position in 1a is a rate-determining
step. Additionally, arylation products 3b and 3b-D were obtained
in a ratio of 1:1 when a mixture of benzene-d6 and benzene (in
1:1 ratio) was employed under the optimal conditions (eq 4),
excluding the possibility of C−C bond formation as the rate-
determining step. Finally, regioisomeric products 3ba and 3bb
were formed by using m-bromoanisole as the arylation partner,
and the structure of 3bb was further confirmed by X-ray
crystallography18 (eq 5); replacing FeCl3 with AlCl3 led to
product 3a in 18% yield (entry 14, Table 1). Combination of
these results and the remarkable substituent effect in the aryl ring
(Scheme 2) suggest that our transformation involves a Friedel−
Crafts-type process.
a
Performed under the optimal conditions. Isolated yield of product 3.
tetrasubstituted arene derivatives reacted smoothly with a
number of deoxybenzoins bearing different substituents on the
aryl ring, affording 1,2,2-triarylethanone derivatives (3aq−az)
with three different substituted aryl rings in good yield, which is
difficult to synthesize by known methods.6−8 It is noteworthy
that synthetically valuable groups, such as F, Cl, Br, I, and OMe,
were tolerated in many cases.
A reasonable mechanism was proposed on the basis of the
experimental results above (Scheme 6).11,19 As suggested by the
Clearly, the results above suggest that this novel α-arylation
approach is rapid and efficient and tolerates a broad range of both
arenes and deoxybenzoins. The fact that our approach preserves
the halo atoms (F, Cl, Br, and I) on the aryl ring means that the
resulting products can be readily coupled with other reaction
partners through C−C or C−X bond formation reactions, which
allows the construction of more complex and diverse functional
molecules. Furthermore, the nitro group on the aryl ring can be
converted to NH2 or other synthetically useful groups.
Therefore, the current approach could be a useful and alternative
procedure for preparing 1,2,2-triarylethanone derivatives5
bearing various halo atoms (even iodine atom) and a NO2
group on the aryl ring. Notably, the product 3a was isolated in
70% yield on gram scale (Scheme 2).
To elucidate the reaction mechanism, we performed some
preliminary experiments (Scheme 5). First, in the presence of
radical-trapping agents, such as 2,2,6,6-tetramethylpiperdin-1-
yl)oxyl (TEMPO) or 2,6-di-tert-butyl-4-methylphenol (BHT),
no product 3a can be obtained, suggesting that a radical process
can be involved in the arylation reaction (eq 1). Second, α-
chlorodeoxybenzoin 4 was also detected and isolated as a side
product in 15% yield in the process of the synthesis of 3b (eq 2),
which was actually also generated in some other cases. The
formation of 4 indicates that a carbocation intermediate may be
generated, which subsequently was trapped by a chloride ion.
Third, an intermolecular kinetic isotope experiment using a
mixture of 1a and deuterated 1a-D19 (in 1:1 ratio) gave the
corresponding products 3a and 3a-D in a 3:1 ratio, indicating a
KH/KD of approximately 3 (eq 3). This result indicates that C−H
Scheme 6. Proposed Reaction Mechanism
kinetic isotope experiments, the substrate (1a) is initially
enolized in a process in which FeCl3 acts as a Lewis acid.19c
Single-electron oxidation of the enolized intermediate I-1 with
DDQ19e generates the radical I-2,13a along with the activated
oxidative species II (path a1), in which FeCl3 only serves as a
Lewis acid, assisting this oxidation.19c The resulting radical I-2 is
further oxidized by the species II and provided the corresponding
cation I-4 (path b1),13c which subsequently undergoes Friedel−
Crafts alkylation with the aromatic compound (2a) to afford the
arylated product 3a. Considering the fact that product 3a can be
also isolated in the absence of DDQ (entry 16, Table 1), an
alternative approach to the carbocation I-4 is also possible: the
enolized intermediate I-1 carries out a single electron trans-
formation19b to form the radical I-2 (path a2), which could be
further oxidized by Fe3+ and generates the cation I-4 (path b2).
The catalytic cycle of Fe3+ is achieved by oxidation of Fe2+ with
DDQ.
C
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Chem. Soc. 1990, 112, 3068. (d) Smith, A. G.; Johnson, J. S. Org. Lett.
2010, 12, 1784.
In summary, we have developed the first iron-catalyzed
arylation of deoxybenzoins through an oxidative dehydrogen-
ative approach. This transformation features broad substrate
scope, uses inexpensive and environmentally friendly iron
chloride as a catalyst under mild reaction conditions, and
generates the synthetically and pharmaceutically valuable 1,2,2-
triarylethanones in moderate to good yields. Compared with
previously reported transformations catalyzed by Pd, Cu, or Ni,
this reaction did not require any extra ligand and was conducted
under the oxidative conditions. We have proposed a reaction
mechanism based on preliminary studies. We are currently
attempting to apply this strategy to other oxidative dehydrogen-
ative reactions involving the C(sp3)−H bond as well as explore
its synthetic applications.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02516.
Experimental procedures and spectroscopic data of
compounds data (PDF)
X-ray data for compound 3m (CIF)
X-ray data for compound 3o (CIF)
X-ray data for compound 3bb (CIF)
(11) For selected reviews for iron-catalyzed transformation, see:
(a) Bolm, C.; Legros, J.; Le Paih, J. L.; Zani, L. Chem. Rev. 2004, 104,
AUTHOR INFORMATION
Corresponding Author
6217. (b) Bauer, I.; Knolker, H.-J. Chem. Rev. 2015, 115, 3170.
̈
■
(12) Yu, J. W.; Mao, S.; Wang, Y. Q. Tetrahedron Lett. 2015, 56, 1575.
(13) For selective examples, see: (a) Su, Y.; Sun, X.; Wu, G.; Jiao, N.
Angew. Chem., Int. Ed. 2013, 52, 9808. (b) Maji, A.; Rana, S.; Akanksha;
Maiti, D. Angew. Chem., Int. Ed. 2014, 53, 2428. (c) More, N. Y.;
Jeganmohan, M. Org. Lett. 2014, 16, 804.
*E-mail: zhangfm@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
(14) Mao, S.; Zhu, X. Q.; Gao, Y. R.; Guo, D. D.; Wang, Y. Q. Chem. -
Eur. J. 2015, 21, 11335.
ACKNOWLEDGMENTS
■
(15) For selected examples, see; (a) Li, Z.; Cao, L.; Li, C.-J. Angew.
Chem., Int. Ed. 2007, 46, 6505. (b) Xie, P.; Xie, Y.; Qian, B.; Zhou, H.;
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Li, Y. M.; Li, Z. Chin. Sci. Bull. 2012, 57, 2382. (d) Yang, K.; Song, Q.
Org. Lett. 2015, 17, 548. (e) Curto, J. M.; Kozlowski, M. C. J. Am. Chem.
Soc. 2015, 137, 18. (f) Li, C.; Jin, T.; Zhang, X.; Li, C.; Jia, X.; Li, J. Org.
Lett. 2016, 18, 1916.
We gratefully acknowledge financial support from the NSFC
(Nos. 21272097 and 21290181) and PCSIRT of MOE (No.
IRT-15R28).
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D
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