Electrochemical dehydrogenative cross-coupling of xanthenes with ketones

Article abstract of DOI:10.1039/d0cc02580a

A new external oxidant-free electrochemical dehydrogenative cross-coupling of xanthenes and ketones for the preparation of functionalized 9-alkyl-9H-xanthenes was developed. This method enables the formation of a new C(sp3)-C(sp3) bond through release of

Full text of DOI:10.1039/d0cc02580a

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Electrochemical dehydrogenative cross-coupling of xanthenes
with ketones
Yong-Zheng Yang,a‡ Yan-Chen Wu,a‡ Ren-Jie Song,*a and Jin-Heng Li*abc
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
possible hydroperoxide intermediate in this transformation
(Scheme 1a). Although these methods are efficient, the
requirements for high pressure O2, longer reaction time, substrate
scope and lower yields are drawbacks, which limited the application
of this transformation. Therefore, it is attracting and promising to
find a mild, efficient, eco-friendly, and safe synthetic strategy to
construct this valuable chemical structure.4
A new external oxidant-free electrochemical dehydrogenative
cross-coupling of xanthenes and ketones for the preparation of
functionalized 9-alkyl-9H-xanthenes was developed. This method
enables the fromation of a new C(sp3)-C(sp3) bond through release
of H2 as the major byproduct at room temperature, and features
mild conditions, high atom economy, excellent functional-group
tolerance, scalability and facile applications in pharmaceutical
chemistry.
Functionalized xanthenes not only are valuable skeleton present in
a broad range of pharmaceuticals and bioactive natural products
but also are used as versatile fluorescent materials in catalytic
chemistry (Figure 1).1 For example, 1H-Indene-1,3(2H)-dione is
known as important antitumor agents.1a As a result, much efforts
over the last few decades have been devoted to construct or
modify the active compounds.2
Figure 1. Examples of important functionalized xanthenes.
Scheme 1. Electrochemical dehydrogenative cross-coupling
reactions of xanthenes with nucleophiles.
Direct C-C coupling reaction which using oxygen as the oxidant
has been regarded as a fascinating strategy to formation C-C
bonds.3a-b For instance, the group of Klussmann3c-e reported a series
of aerobic organocatalytic oxidative C-C bond formation reaction of
xanthene derivatives with various C-nucleophiles and showed the
In the past several years, electrochemical anodic oxidation has
witnessed rapid development and attracted a great deal of interest
in organic synthetic chemistry due to its highly efficient and
environmentally benign features.5 In particular, electro-oxidative
dehydrogenative cross-coupling reactions have been proven to be a
promising and feasible alternative to construct new C-C bonds or C-
X (X = N, O, etc…) bonds.6 For instance, in 2017, zeng and co-
a.Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources
Recycle, Nanchang Hangkong University, Nanchang 330063, China
b.State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,
Changsha 410082, China
c.State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou
730000, China
workers realized
a
ferrocene catalyzed intermolecular
dehydrogenative reactions of xanthenes with N-alkoxyamides
under electrochemical conditions for C-N bond construction
(Scheme 1b).7a To the best of our knowledge, the direct
electrochemical for the construction C(sp3)-C(sp3) bond under
oxidant-free conditions is rare and still a challenge in organic
E-mail: srj0731@hnu.edu.cn, and jhli@hnu.edu.cn
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
‡ These authors contributed equally to this work
This journal is © The Royal Society of Chemistry 20xx
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chemistry.8 With our continuing interest in green organic synthesis,
Journal Name
the reaction proceeded smoothly and gave the desirVeiepwrAordticulectOn3lianae
DOI: 10.1039/D0CC02580A
with lower yield (66%) (entry 5). Additionally, other electrolytes,
here, we report clean and practical electro-oxidative
a
dehydrogenative cross-coupling of xanthenes with ketones to
synthesize 9-alkyl substituted 9H-xanthenes under mild conditions
without an external oxidant (Scheme 1c). This electrooxidative
reaction enables the formation of C(sp3)-C(sp3) bond via a C-H
functionalization process, which is characterized by its mild
conditions, high atom economy, excellent functional-group
tolerance and scalability.
such as LiClO4, TBAI and TBAB, were also investigated. As compared
with ammonium salts electrolyte, LiClO4 has showed decreased
reaction efficiency due to its low conductivity in the reaction (entry
6). Notably, TBAI as the electrolyte failed to deliver the desired
product 3aa (entry 7), while 50% yield of product 3aa was isolated
when TBAB as the electrolyte (entry 8). Unfortunately, the desired
product could not be detected under no MsOH condition, and gave
75% yield oxidation by-product 9H-xanthen-9-one (4) (entry 9). In
addition, other additive acids, such as p-TsOH and BF3·C2H5O were
examined for this transformation, but they were shown to be less
effective (entries 10 and 11). In this transformation, MsOH may
promote the enol isomerization of ketones, which in turn facilitates
the nucleophilic reaction.3c-e Other solvents, namely CH3OH, DMSO
and DCM, were also investigated and MeCN was found to be the
optimal one (entry 1 vs entries 12-14). Almost the same yield can be
retained when the reaction worked at argon atmosphere (entry 15).
Notably, a 5.5 mmol scale reaction was conducted and 3aa could be
obtained in 78% yield (entry 16). The same result (80%) was also
obtained by the use of IKA Electrasyn 2.0 equipment (entry 17).
With the optimized reaction conditions in hand, we next
investigated the substrate scope of this electrochemical cross-
dehydrogenative coupling reaction and studied different ketones
(Scheme 2). Different substituted ketones, including cyclic ketones,
heterocyclic ketones, α, β-unsaturated ketones, open-chain ketones,
1,3-diketones and acetophenones, proved compatible with the
reaction conditions, and gave the corresponding products 3ab-az
and 3aA-aC in moderate to good yields. Cyclic ketones
cyclohexanone 2b and cycloheptanone 2c were performed
smoothly with 9H-xanthene (1a) to afford the corresponding 9-
ketone substituted xanthene derivatives 3ab and 3ac in 68% and
83% yields, respectively. To our surprise, 12-membered cyclic
ketone cyclododecanone 2d was also tolerated and afford the 2-
(9H-xanthen-9-yl)cyclododecan-1-one 3ad in 70% yield. Different
substituted cyclic ketones reacted with 9H-xanthenes 1a, affording
the corresponding 9-keto xanthenes 3ae-ag in synthetically useful
yields (40-69%). For heterocyclic ketone substrates 2h and 2i,
corresponding products were isolated with moderate yields
(Products 3ah and 3ai). Benzocyclones 2j-2o were successfully
cross-dehydrogenative coupling with 9H-xanthene leading to 3aj-ao
in 40-71% yields. To our delight, two cyclic α, β-unsaturated
ketones, cyclopent-2-en-1-one 2p and cyclohex-2-en-1-one 2q
participated well. It is noteworthy that acetophenones 2r-t, which
carry electron-rich group (Me) or electron-poor substituents (such
as Cl and NO2 groups) furnished products 3ar-at, albeit in 38-51%
yields. Among these transformations, open-chain ketones 2i-2y
were suitable for the electrochemical oxidative conditions, which
provided the desired products 3ai-ay in 49-80% yields. For example,
Table 1 Screening of optimal reaction conditions.a
Entry Variation from the standard conditions
Yield (%)b
82
none
1
2
Pt(+)/C(-) instead of C(+)/Pt(-)
C(+)/C(-) instead of C(+)/Pt(-)
no electric current
58
60
0
3
4
nBu4NPF6 instead of nBu4NBF4
LiClO4 instead of nBu4NBF4
TBAI instead of nBu4NBF4
TBAB instead of nBu4NBF4
without MsOH
66
55
trace
50
0c
5
6
7
8
9
73
48
60
0
30
80
78
80
p-TsOH instead of MsOH
BF3·Et2O instead of MsOH
CH3OH instead of MeCN
DMSO instead of MeCN
DCM instead of MeCN
Ar
10
11
12
13
14
15
16d
17e
none
none
a Reaction conditions: 1a (0.25 mmol), 2a (0.5 mmol), carbon rod
anode, Pt plate cathode, constant current = 5 mA, MsOH (2
equiv), nBu4NPF6 (0.4 mmol), MeCN (8 mL), undivided cell, at
b
room temperature for 2 h. Isolated yield. c 9H-xanthen-9-one
(4) of 75% yield. d 1a (1g, 5.5 mol) for 18 h. e By IKA Electrasyn.
We commenced the study by taking 9H-xanthene (1a) as the
representative substrate and cyclopentanone (2a) as the partner,
employing our previously optimized conditions for cross-coupling
reactions under electrooxidative condition. As indicated in Table 1,
the electrolyses was carried out in a single compartment cell under
constant-current condition (5 mA) using carbon rod as working
electrode and platinum plate as cathode electrode. As expected,
82% yield of 2-(9H-xanthen-9-yl)cyclopentan-1-one (3aa) was
obtained when the reaction was happen in the presence of
nBu4NBF4 as the electrolyte, CH3CN as the solvent and MsOH as
additive at room temperature (entry 1). Two other combinations of
the electrodes were attempted and when carbon rod anode was
replaced by Pt plate or the Pt plate cathode was replaced by carbon
rod, the efficiency of this transformation was decreased (entries 2-
3). However, no desired product could be obtained without electric
current (entry 4). The choice of electrolyte has also affected the
reaction efficiency. When we switched from nBu4NBF4 to nBu4NPF6,
phosphate
phenylethyl)phosphonate 2w was translated to the corresponding
product diethyl (2-oxo-2-phenyl-1-(9H-xanthen-9-
ester
substrate
diethyl
(2-oxo-2-
yl)ethyl)phosphonate 3aw in 80% yield. In addition, 1,3-diones (2z
and 2A) and keto esters (2B and 2C) were also studied, providing
the corresponding xanthene derivatives 3az and 3aA-aC in 51-86%
yields. Notably, three drug molecules 5α-Dihydroprogesterone 2D,
Progesterone 2E and Canrenone 2F were compatible with the
2 | J. Name., 2012, 00, 1-3
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electrochemical oxidative conditions, and led to the complex dione 2A was also worked well, furnished the 2-(2-fluoro-9H-
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DOI: 10.1039/D0CC02580A
xanthen-9-yl)-1,3-diphenylpropane-1,3-dione 3ia in 65% yield.
compounds 3aD-aF in 60%, 67% and 62% yields, respectively.
Table 2. Substrate scope of ketones (2).a
Table 3. Substrate scope of xanthenes (1).a
a
Reaction conditions: carbon rod anode, Pt plate cathode,
constant current = 5 mA, 1 (0.25 mmol), 2a (0.5 mmol), MsOH
n
(2 equiv), Bu4NBF4 (0.4 mmol), MeCN (8 mL), undivided cell, at
room temperature for 2 h.
Based on the previous reports, CV results and our experimental
results, a plausible reaction mechanism for the electrochemical
synthesis of xanthenes was proposed in Scheme 3.3, 9-10 The initial
step involves losing one electron from the xanthene forming a
short-lived radical cation A as the first intermediate. Since two
oxidation peak is observed in the cyclic voltammetric curve (See
Figure S2). Intermediate C is formed from A via twice SET process.
The stabilized carbocation C will react with the nucleophilic enol D
which formation from 2a in the presence of MsOH to yield product
3aa. At the cathode, H+ are reduced to generate H2, obviating the
need for electron and proton acceptors for the electrochemical
dehydrogenative cross-coupling reaction.
a
Reaction conditions: carbon rod anode, Pt plate cathode,
constant current = 5 mA, 1a (0.25 mmol), 2 (0.5 mmol), MsOH
n
(2 equiv), Bu4NBF4 (0.4 mmol), MeCN (8 mL), undivided cell, at
room temperature for 2 h.
We also studied the scope for electrochemical cross-
dehydrogenative coupling reactions of with xanthenes 1 using
cyclopentanone 2a as the coupling partner and the results was
shown in Table 3. In general, both electron-donating and electron-
withdrawing substituents, such as Me, Cl, OH, OMe and F groups,
on the benzene ring of xanthenes were well-tolerated in this
reaction. For example, xanthene 1d bearing an unprotected
hydroxyl group (was also tolerated leading to 2-(2-hydroxy-9H-
xanthen-9-yl)cyclopentan-1-one 3da in 42% yield. Similarly,
substrates with naphthalene ring showed tolerance for the
optimized conditions (Products 3fa-ha). We also found the reaction
between 2-fluoro-9H-xanthene 1i and 1,3-diphenylpropane-1,3-
Scheme 3. Possible reaction mechanism.
In summary, we have established an electro-oxidative
dehydrogenative cross-coupling of xanthenes with ketones to
produce the important valuable molecules under green, simple, and
mild conditions. This reaction avoided the use of external oxidants
and H2 was the only byproduct. The developed methodology is able
to accommodate a wide variety of ketones, including cyclic ketones,
heterocyclic ketones, α, β-unsaturated ketones, open-chain ketones,
1,3-diketones, β-keto esters, acetophenones even drug ketone
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molecules with a α-C(sp3)-H bond. We have also proposed a
possible mechanism based on experimental evidence. Further
studies involving the application of the developed electro-oxidative
methodology to the synthesis of heterocyclic molecules are ongoing
in our laboratory.
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6
We thank the National Natural Science Foundation of China (Nos.
51878326, 21762030, 21625203 and 21871126) for financial
support. Ren-Jie Song was also supported by the China Scholarship
Council (contract 201908360088).
Conflicts of interest
The authors declare no conflict of interest.
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