Suzuki cross coupling followed by cross dehydrogenative coupling: An efficient one pot synthesis of Phenanthrenequinones and analogues

Article abstract of DOI:10.1016/j.tetlet.2020.151701

An efficient one pot protocol has been developed towards the synthesis of substituted phenanthrenequinone and analogous derivatives via Suzuki cross coupling followed by copper catalyzed cross dehydrogenative coupling.

Full text of DOI:10.1016/j.tetlet.2020.151701

Journal Pre-proofs
Suzuki Cross Coupling followed by Cross Dehydrogenative Coupling: An Ef‐
ficient One Pot Synthesis of Phenanthrenequinones and Analogues
Pompy Sarkar, Atiur Ahmed, Jayanta K. Ray
PII:
S0040-4039(20)30124-6
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.151701
TETL 151701
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
3 December 2019
28 January 2020
31 January 2020
Please cite this article as: Sarkar, P., Ahmed, A., Ray, J.K., Suzuki Cross Coupling followed by Cross
Dehydrogenative Coupling: An Efficient One Pot Synthesis of Phenanthrenequinones and Analogues,
Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.151701
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Suzuki Cross Coupling followed by Cross
Dehydrogenative Coupling: An Efficient One Pot Synthesis of
Phenanthrenequinones and Analogues
Pompy Sarkara, Atiur Ahmedb, Jayanta K. Raya,*
aDepartment of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India
bDepartment of Chemistry, Memari College, Purba Burdwan, West Bengal 713146, India
E-mail of corresponding author: jkray@chem.iitkgp.ernet.in
Abstract: An efficient one pot protocol has been developed towards the synthesis of substituted
phenanthrenequinone and analogous derivatives via Suzuki cross coupling followed by copper
catalyzed cross dehydrogenative coupling.
Keywords: Phenanthrenequinone, One pot reaction, Suzuki coupling, Cross dehydrogenative
coupling.
9,10-Phenanthrenequinones are important structural confirmations found in a plethora of natural,
metabolic, or synthetic products and other biologically active molecules. For example
Biruloquinone,1a,b isolated from Cladonia macilenta, a lichen forming fungus, has been found to
display inhibition capacity towards acetylcholinase. Bulbophyllanthrone1c has been analyzed to
exhibit antiproliferative activity towards P388 murine leukemia as well as BSC cell lines.
Phenanthrenequinones have also been explored as elementary unit in many research works such
as in electron accepting polymers,2a polytriphenylene dendrimers emitting blue light,2b phenazine
derivatives in liquid crystalline form,2c 2,2’ -diacyl-1,1’ –biaryls with high functionalizations2d
and fluorenone derivatives.2e They have also been studied for sensing urea and amides2f and also
for the inhibition for tyrosin phosphatase CD45.2g The metal complexes of 9,10-
phenanthrenequinone diimine displays sequence-specific recognition of DNA2h and have been
studied for the development of artificial nucleases for shape-selective DNA photo cleavage.2i
Phenanthrenequinone thiosemicarbazone itself and its metal complexes of have been investigated
as potential anticancer agents.2j
O
O
OH
O
OH
O
HO
O
MeO
O
O
MeO
O
O
OH
H3C
OH
MeO
H3C
OMe
OMe
OMe
OMe
O
O
OMe
Biruloquinone
Sch 68631
Bulbophyllanthrone
Cypripediquinone A
Figure 1: Some promising bioactive molecules containing phenanthrenequinone

Due to such tremendous applications synthesis of 9,10-phenanthrenequinone has attracted
attention of synthetic chemists from early decades to modern age. Traditional approach for the
synthesis of 9,10-phenanthrenequinone depends upon oxidation of different phenanthrenes with
various oxidizing agents.3,4 Oxidation of phenanthrene based molecules such as 10-
hydroxyphenanthren-9(10H)- ones,5 9,10- dihydrophenanthrenes,6 phenanthrene-9,10-diol7 and
9-phenanthrenols8 has also been explored as an alternative method. Intramolecular oxidative
coupling of benzils by different oxidants e.g. thallium (III) oxide and vanadium (V) oxyfluoride,9
MoCl5/TiCl4 mixture,10 potassium-graphite intercalation compound,11 C8K12 etc has also been
studied by different groups. Imidazolides were subjected towards intramolecular Friedel−Crafts
reaction by TiCl4 to produce the corresponding phenanthroquinones as reported by Yoshikawa
and co-workers.13 Other nonphenanthrene-based resources such as 2- acetylbiphenyls,14
benzoins,12,15 benzils,11,16 dimethyl biphenyl-2,2′-dicarboxylate17 or 2,2′-dilithiobiphenyl18 have
also been examined to construct the phenanthrenequinone skeleton. One of the most recent
transformation includes Cu(0)-Selectflour catalyzed synthesis from o-aryl chalcones.19
Most of the synthetic strategies include use of toxic reagents, relatively expensive catalysts,
harsh reaction conditions and prolonged reaction time. Other drawbacks are employment of
multistep operations, strain in the preparation of substrates with various functionalities and low
yields of the products.
Now one pot technique has manifested as a robust procedure in terms of resource and energy
efficiency as well as environmental sustainability as several synthetic transformations can be
performed within a single pot. The one pot strategy reduces chemical waste and rationalizes
practical aspects.
Herein we have reported an efficient one pot synthesis of 9,10-Phenanthrenequinones from
easily accessible starting materials within a very fast reaction time. The technique described here
allows the construction of 9,10-Phenanthrenequinones with a diverse range of functionalities in
good to excellent yields.
Initially 2-bromobenzaldehyde (1a) was selected as the model structure for our study. First 1a
was subjected towards Suzuki coupling with 2-formylphenylboronic acid (2) under standard
Pd(0) conditions (Scheme 1). Formation of 3a was observed under standardized Suzuki reaction
condition and confirmed by data in accordance with literature reports.
O
O
O
O
Pd(PPh3)4 (10mol%)
H
H
Br
(HO)2B
Na2CO3, DMF
90 oC, 6 h
1a
3a
2
Scheme 1: Suzuki cross coupling reaction

Next our target was to find a suitable reaction condition for cyclization of the Suzuki coupled
product to form the desired quinone skeleton. After thorough survey of recent literature reports it
was found that cross-dehydrogenative coupling (CDC) is a powerful method to transform two
less reactive C-H bonds into C-C bond in a selective manner in presence of various functional
groups.20 This coupling can be carried out in presence of cheap catalysts e.g. copper and iron
salts as well as different oxidizing agents e.g. tert-butylhydroperoxide, hydrogen peroxide,
dioxygen 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). Moreover this technique does not
require pre-functionalization or preactivation of C-H bonds. So we envisioned that our Suzuki
coupled product (3a) which contains two C-H bonds in close proximity should undergo
cyclization when subjected towards reaction conditions for CDC coupling. For this purpose after
the completion of the Suzuki coupling, monitored by TLC, 3a formed in the reaction mixture
was treated with Cu salt and TBHP. To our much delight we observed the formation of 9,10-
Phenanthrenequinone (4a) along with the consumption of the starting material in a rapid and fast
process within 1 hr.
Next our task was to determine the optimal condition for the one pot operation. A series of
reaction conditions was established and scrutinized to attain the optimized reaction condition for
the one pot analysis.
To study the optimization for the one pot method different set of Pd, Cu catalysts, bases and
solvents were introduced and investigated under the reaction condition. Different palladium
complexes such as Pd(PPh3)4, PdCl2, Pd(OAc)2 and Pd(PPh3)2Cl2 were analyzed. Pd(PPh3)2Cl2
was found to be the most promising one for the one pot technique along with K3PO4 as the base
among different bases such as Na2CO3, K2CO3, Cs2CO3, NaOAc etc. Next for the CDC reaction
an array of various Cu catalysts i. e. CuCl, CuBr, CuI, Cu(OAc)2 and even CuCl2 were employed
one by one and examined to find CuCl as the most effective catalyst. After that different types
and amount of oxidants were also varied for the investigation. Among TBHP, H2O2 and m-
CPBA TBHP in 3 equivalent gave the highest yield of the product. Effect of solvent was also
studied. Under standard Suzuki coupling carried out in DMF solvent when Cu catalyst and the
additive were added the CDC coupling did not take place efficiently. Other solvents like
acetonitrile or dioxane when employed the yield was a little better but also not satisfactory. An
excellent result was furnished when DMSO was incorporated as the solvent and explored. Final
inspection was done to obtain the amount of the Cu catalyst and it was observed that in presence
of CuCl in 5 mol% the reaction produced the product in highest yield. When the reaction was
performed in absence of any catalyst, the reaction did not take place which indicates the
subsistence nature of the Cu catalyst.
Table 1: Optimal condition determination for the one –pot cyclizationa, b

O
O
O
O
O
O
H
H
Pd catalyst, ligand
ii. Cu salt
base, solvent
heat, 6 h
Br (HO)2B
TBHP
rt. 1 h
1a
2
3a
4a
Entry
Pd Catalyst
Pd(PPh3)4
Pd(PPh3)4
Ligand
Base
Na2CO3
K2CO3
Cu Catalyst
CuCl (10)
CuCl (10)
Oxidant
TBHP (3)
TBHP (3)
Solvent
DMF
Yield
(%)c
1
2
-
-
<10
DMF
<10
3
Pd(PPh3)4
-
Cs2CO3
CuCl (10)
TBHP (3)
DMF
<10
4
5
6
7
8
Pd(PPh3)4
Pd(PPh3)4
PdCl2
-
-
NaOAc
K3PO4
K3PO4
K3PO4
K3PO4
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
DMSO
DMSO
DMSO
DMSO
DMSO
40
55
58
65
80
PPh3
PPh3
-
Pd(OAC)2
Pd(PPh3)2Cl2
9
Pd(PPh3)2Cl2
-
K3PO4
CuCl (10)
TBHP (3)
CH3CN
55
10
11
12
13
14
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
-
-
-
-
-
K3PO4
K3PO4
K3PO4
CuCl (10)
CuBr (10)
CuI (10)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
Dioxane
DMSO
DMSO
DMSO
DMSO
25
58
<10
24
K3PO4
K3PO4
Cu(OAc)2 (10)
CuCl2 (10)
42
15
16
17
18
19
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
-
-
-
-
-
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
CuCl (10)
CuCl (10)
CuCl (20)
CuCl (30)
CuCl (5)
m-CPBA (3)
H2O2 (3)
DMSO
DMSO
DMSO
DMSO
DMSO
40
24
75
70
85
TBHP (3)
TBHP (3)
TBHP (3)
20
Pd(PPh3)2Cl2
-
K3PO4
-
TBHP (3)
DMSO
N. R.

Entry Pd Catalyst
Ligand
Base
Cu Catalyst
Oxidant
Solvent
Yield
(%)c
21
Pd(PPh3)2Cl2
-
K3PO4
CuCl (5)
TBHP (5)
DMSO
85
22
Pd(PPh3)2Cl2
-
K3PO4
CuCl (10)
TBHP (1)
DMSO
75
a Reagents and conditions: 1a (1 mmol), 2 (1.2 mmol), Pd catalyst (10 mol%), base (1.5 mmol),
in 5 mL DMSO at 90 oC for 6 h. (ii) Cu salt, oxidant, 1 h at rt.
b Reaction was done in a two-necked round-bottomed flask under N2 atmosphere.
c Yields refer to the isolated yields after purification through column chromatography.
N. R. = no reaction
Next to evaluate the scope of this assessment further, various 2-bromobenzaldehydes (1a-i) were
tested under the optimized reaction conditions established above (entry 21 in Table 1).
It was perceived that the protocol prosecuted smoothly to furnish different 9,10-
Phenanthrenequinones with diverse functionalities (4a-i) in moderate to good yields (Table 2).
Table 2 illustrates the extension of the methodology to the formation of different 9,10-
Phenanthrenequinones containing various substituents. Products containing electron donating
groups were furnished in higher yields whereas electron withdrawing group –NO2 drastically
decrease the yield of the product.
The reaction is believed to proceed through radical pathway. As TBHP is well known to produce
t
t-Butyloxy radical and hydroxyl radical in presence of metal catalysts.21 The BuO radical thus
produced can abstract the acyl hydrogen from the substrate to form acyl radical.22 Next the acyl
radical is responsible for the next transformations.
Table 2: Exploration of substrates scope a,b
O
O
O
O
O
O
H
Pd(PPh3)2Cl2
ii. CuCl
H
K3PO4, DMSO
90 0C, 6 h
Br
TBHP
rt. 1 h
(HO)2B
3a-i
4a-i
1a-i
2
Scheme 2: Synthesis of assorted 9,10-Phenanthrenequinones

O
O
O
O
O
O
O
O
F
4a
4b
4c
(85%)
O
(85%)
(80%)
O
O
O2N
O
MeO
MeO
O
MeO
O
OMe
4e
4f
4d
(86%)
(75%)
(90%)
O
O
O
O
O
O
MeO
MeO
4h
4g
4i
(86%)
(85%)
(88%)
aAll the reactions were carried out with (i) 1a-i (1 mmol), 2 (1.2 mmol), Pd(PPh3)2Cl2 (10 mol%),
K3PO4 (1.5 mmol), in 5 mL DMSO at 90 oC for 6 h. (ii) CuCl (5 mol%), TBHP (3 equivalent), 1
h at rt.
bIsolated yields.
However this two step one-pot methodology was found restricted for β-bromo carbaldehydes
containing chromene or thiochromene skeletons as they produced inseparable mixture of various
products under this described reaction condition.
Proposed Mechanism
A proposed mechanism is illustrated in Scheme 3. At first TBHP produces t-Butyloxy radical
and hydroxyl radical in presence of Cu(I). Then the formyl hydrogen of the substrate (3a) is
abstracted by tBuO radical to form the acyl radical 5a. Next this acyl radical will undergo 6-exo

cyclization to form the intermediate 6a. The final step is the oxidation by Cu(II) to afford the
final phenathrenequinone 4a.
tBuOOH
tBuO + OH
Cu(I)
Cu(II)
OH
O
O
O
O
tBuOH
O
tBuO
-tBuOH
-tBuO
6-exo
6a
3a
5a
Cu(II)
Oxidation
O
O
4a
Scheme 3: Proposed mechanism
Conclusion
Thus we were able to open up an interesting and feasible avenue for the formation of 9,10-
Phenanthrenequinones from easily accessible starting materials. The construction is highly
adaptive and customizable. Also this strategy includes fewer synthetic steps which are simple
and trouble-free to perform. The one pot synthesis of 9,10-Phenanthrenequinone which includes
Suzuki cross coupling followed by cross dehydrogenative coupling has been well established
and it diminishes total number of actions in the overall synthetic procedures along with
maximum transfer of mass from the substrate to the product.
Acknowledgements
Pompy Sarkar thanks IIT Kharagpur for the fellowship and the Department of Science and
Technology (DST), Ministry of Human Resource Development (MHRD), India for providing
funds for the project.

Supplementary data
Supplementary data (experimental procedure and spectral data for the final compounds 4a-i)
associated with this article can be found, in the online version, at
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23. General procedure for the synthesis of phenanthrenequinone: 2-bromobenzaldehyde
(1 mmol), 2-formylphenylboronic acid (1.2 mmol), Pd(PPh3)2Cl2 (10 mol%) and K3PO4
(1.5 mmol) was taken in a two neck round bottomed flask. 5 mL DMSO was added to it
and the reaction mixture was degassed properly with N2. Next the reaction was allowed
to stir at 90oC for 6h. After the completion of the reaction, the reaction mixture was
cooled to room temperature and to it were added CuCl (5 mol %) and TBHP (3
equivalent). It was stirred for another 1 h at rt. Then it was diluted with water and
extracted with ethyl acetate several times. The organic parts were collected together and
concentrated under vacuum after drying over Na2SO4. Products were purified by column
chromatography using 60-120 mesh silica gel and 5:1 pet ether : ethyl acetate as the
eluent.
24. Spectral data of representative compound, phenanthro[3,2-d][1,3]dioxole-5,6-dione
o
(4c): orange solid, yield 85%, m.p. greater than 200 C.1H NMR (400 MHz, CDCl3): δ
8.11 (dd, J = 7.7, 1.3 Hz, 1H), 7.77 (d, J = 8.0 Hz, 1H), 7.67-7.63 (m, 1H), 7.54 (s, 1H),
7.42-7.38 (m, 1H), 7.36 (s, 1H), 6.11 (s, 2H). 13C NMR (100 MHz, CDCl3): δ 180.40,
178.38, 154.88, 149.10, 136.07, 135.74, 134.78, 130.44, 130.37, 129.24, 126.97, 124.80,
109.14, 104.82, 102.75. HRMS (ESI-TOF): [M + Na]+ calculated for C15H8O4Na :
275.0320; found: 275.0316.
 An effective one pot technique has been designed to afford 9,10-phenanthrenequinones.

 Suzuki cross coupling and cross dehydrogenative coupling are the two key steps.
Precursors are simple and readily accessible.

 Reaction conditions are mild, short and very easy to perform.
Graphical Abstract
Suzuki Cross Coupling followed by Cross
Dehydrogenative Coupling: An Efficient One Pot Synthesis of
Phenanthrenequinones and Analogues
Pompy Sarkara, Atiur Ahmedb and Jayanta K Raya,*
aDepartment of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India
bDepartment of Chemistry, Memari College, Purba Burdwan, West Bengal 713146, India
E-mail of corresponding author: jkray@chem.iitkgp.ernet.in
O
O
O
O
O
O
H
Pd(PPh3)2Cl2
ii. CuCl
H
K3PO4, DMSO
90 0C, 6 h
Br
TBHP
rt. 1 h
(HO)2B
Phenanthrenequinone
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.