Naphtho[2,3-: B] furan-4,9-dione synthesis via palladium-catalyzed reverse hydrogenolysis

Article abstract of DOI:10.1039/c8cc09369e

A reverse hydrogenolysis process has been developed for two-site coupling of 2-hydroxy-1,4-naphthoquinones with olefins to produce naphtha[2,3-b]furan-4,9-diones and hydrogen (H₂). The reaction is catalyzed by commercially available Pd/C without oxidants and hydrogen acceptors, thereby providing an intrinsically waste-free approach for the synthesis of functionalized and potentially biologically relevant naphtha[2,3-b]furan-4,9-diones.

Full text of DOI:10.1039/c8cc09369e

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DOI: 10.1039/C8CC09369E
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Naphtho[2,3-b]furan-4,9-dione synthesis via palladium-catalyzed
reverse hydrogenolysis
Jimei Li,a Jie Zhang,a Mingfei Li,a Chenyang Zhang,a Yongkun Yuan,b and Renhua Liu*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A reverse hydrogenolysis process has been developed for two-site
coupling of 2-hydroxy-1,4- naphthoquinones with olefins to
produce naphtha[2,3-b]furan4,9-diones and hydrogen(H2). The
reaction proceeds catalyzed by commercially available Pd/C
without oxidants and hydrogen acceptors, thereby providing an
intrinsically waste-free approach for the synthesis of
functionalized and potentially biologically relevant naphtha[2,3-
b]furan-4,9-diones.
Previous work
O
O
O
O
O
OH
m-CPBA
CAN
SPh
SPh
acetonitrile
dichloromehane
rt, 12h
O
O
0
6h
℃,
O
O
O
O
R'
O
I
OH
or
O
R'
PdCl2(PPh3)2-CuI
or Cu2O,base
Ph
R
O
R
I
O
O
O
O
R1
R2
O
The C–H bonds and X (N, O, S, and P)–H bonds are the most
commonly encountered structural motifs in organic molecules.
Consequently, the development of selective, direct and
efficient methods for converting C–H and X–H bonds into C–X
bonds may greatly simplify the synthesis of complex organic
molecules and represent an attractive long-standing goal in
chemistry. However, the conversion of C–H and X–H bonds
into the C–X bonds is extremely difficult to achieve so that only
a few catalytic systems have been developed for this
conversion by resorting to stoichiometric hydrogen acceptors
or oxidants. These hydrogen acceptors or oxidants are often
expensive or highly hazardous or toxic compounds, and may
generate a large amount of environmental effluents in
industrial production.1 The use of stoichiometric expensive
metallic or highly toxic oxidants or hydrogen acceptors is
problematic in industrial settings, which limits the applications
of the reactions in large-scale synthesis. In this context, there
is a great need for development of an atom- and step-efficient,
and intrinsically waste-free catalytic system available for
performing the conversion. The catalytic reverse
hydrogenolysis, the reverse process of hydrogen cracking, can
convert C–H bonds and X–H bonds into the C-X bonds with the
evolution of hydrogen without the need for a stoichiometric
oxidant or a hydrogen acceptor, and thereby have the
O
O
Cl
Cl
base
R1
R2
O
O
O
O
O
OH
NaOMe (1.5eq)
Pd/C
R2
R1
R1
R2
O
℃
O
DMSO, 140
11min
W，
32%
CN
75
NH2
this work
O
O
O
H
H
O
H
R2
R3
10 mol % Pd/C
R1
R2
R1
+
2H2
+
H
20 mol % Cs2CO3
130 oC, DMA, N2
R3
O
O
Scheme 1 Methods for synthesis of 2-furanonaphthoquinones.
potential to reshape retrosynthetic analysis strategies, and can
streamline the synthesis of natural products, medicines and
materials. Adding to the advantages, reverse hydrogenolysis
processes are able to harness native structural units (C–H and
X–H bonds) of starting material to construct chemical bonds,
thereby dispensing with the need for substrate
prefunctionalization. However, despite the advantages and
potentials, synthetic methods utilizing reverse hydrogenolysis
procedures remain extremely rare to date.
Herein we reported our results toward achieving this goal. In
the exploration of rapid and green methods for preparation of
naphtho[2,3-b]furan-4,9-dione, we developed
a catalytic
reverse hydrogenolysis reaction where 2-hydroxy-1,4-
naphthoquinones were directly coupled with a variety of
olefins to produce a wide range of naphtha[2,3-b]furan4,9-
diones and H2 without oxidants and hydrogen acceptors. The
palladium-catalyzed reverse hydrogenolysis reaction features
a. School of Pharmacy, East China University of Science and Technology, Meilong
Road 130, Shanghai 200237, China.E-mail: liurh@ecust.edu.cn
Suzhou Yacoo Science Co. Ltd Jinhai Road 17, Suzhou industrial Park, Jiangsu
b.
215125, China
† Electronic Supplementary Information (ESI) available: Experimental procedure
and NMR spectral data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
Chem.Commun., 2013, 00, 1-3 | 1
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the starting material skeleton structures being almost
completely preserved into the products, with only two
hydrogen atoms missing.
a
View Article Online
Table 1 Reaction optimization for the synthesis of naphtho[2,3-b]furan-4,9-diones.
DOI: 10.1039/C8CC09369E
O
O
O
OH
O
conditions
Naphtho[2,3-b]furan-4,9-diones constitute an important class
of structural units commonly present in natural products,
pharmaceutically active compounds, and other functional
synthetics. Many naphtho[2,3-b]furan-4,9-diones are found to
have a broad spectrum of biological activities such as
antimicrobial, antifungal, antiviral, radical scavenging,
antiplatelet and trypanocidal activities;2 in particular,
antitumor activity.3 The selection of naphtho[2,3-b]furan-4,9-
diones synthesis for this study was primarily inspired by the
discovery of the antitumor activity of 2-acetylated
naphtho[2,3-b]furan-4,9-dione.4 This naphtho[2,3-b]furan-4,9-
dione core containing compound, which was first isolated from
Tabebuia cassinoides, is capable of killing many different types
of cancer cells, without causing damage to normal cells under
certain exposure conditions.5 Because of the importance of
naphtho[2,3-b]furan-4,9-dione skeleton toward therapeutic
development and the search for clinically superior analogs,
several synthetic methods have been developed to construct
naphtho[2,3-b]furan-4,9-dione structural ring systems. Despite
their many attractive features; however, these synthetic
methods suffer from several limitations, including (i) the need
for stoichiometric strong oxidants (e.g., ceric ammonium
nitrate) and lead to equimolar amounts of byproducts;6 (ii)
inefficiencies associated with the need for substrate
prefunctionalization and long routes of synthesis;7 and (iii)
harsh reaction conditions or use of expensive or highly toxic
reagents or the starting materials not readily available.8
2H2
O
O
O
1a
2a
3a
Entry
1a:2a
[equiv]
Conditions
Yield b
[%]
1
2
3
4
5
6
7
8
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:1
2:1
1ml DMA/ 10 mol% Pd/ 50 mol% Cs2CO3/120℃/ N2
70
1ml DMF/ 10 mol% Pd/ 50 mol% Cs2CO3/ 120℃/ N2 47
1ml DMSO/ 10 mol% Pd/ 50 mol% Cs2CO3/ 120℃/ N2 45
1ml DMA/ 10 mol% Pd/ 50 mol% TsOH/ 120℃/ N2
1ml DMA/ 10 mol% Pd/ 120℃/ N2
1ml DMA/ 10 mol% Pd/ 50 mol% K2CO3/ 120℃/ N2
1ml DMA/ 10 mol% Pd/ 50 mol% NaOH/ 120℃/ N2
0
10
69
0
1ml DMA/ 10 mol% Pd/ 10 mol% Cs2CO3/ 120℃/ N2 63
1ml DMA/ 10 mol% Pd/ 20 mol% Cs2CO3/ 120℃/ N2 83
1ml DMA/ 10 mol% Pd/ 20 mol% Cs2CO3/ 110℃/ N2 63
1ml DMA/ 10 mol% Pd/ 20 mol% Cs2CO3/ 130℃/ N2 84
9
10
11
12
13
14
15
1ml DMA/ 0 mol% Pd/ 20 mol% Cs2CO3/ 130℃/ N2
0
1ml DMA/ 10 mol% Pd/ 20 mol% Cs2CO3/ 130℃/ O2 57
1ml DMA/ 10 mol% Pd/ 20 mol% Cs2CO3/ 130℃/ N2 67
1ml DMA/ 10 mol% Pd/ 20 mol% Cs2CO3/ 130℃/ N2 80
a Reaction condition:0.2mmol substrates, solvent, Pd/C, b Isolated yields
acceptor, which represents an intriguing but formidable
challenge.
To explore the feasibility of this two-site reverse
hydrogenolysis reaction, 2-hydroxy-1,4-naphthoquinone and a
chalcone were first selected as the representative substrates
and Pd/C as the catalyst. Because catalysts are known to can
promote both the forward and reverse reactions
simultaneously, we speculated that Pd/C, a widely accepted
hydrogenation or hydrogenolysis catalyst, can also be used to
catalyze reverse hydrogenolysis. The initial experiment was
performed with 10 mol % of Pd, 1 mL of DMA, 50 mol%
Cs2CO3, 120°C, N2 atmosphere, and reaction for 30 hour, the
reaction provided us with an exciting result of 70% isolated
yields, which clearly indicated the assumption of synthesis of
naphtho[2,3-b]furan-4,9-diones via reverse hydrogenolysis is
feasible. A systematic study was undertaken to optimize the
reaction conditions, including reaction temperature, solvent,
base, and use amount of the catalyst and the results are
partially exhibited in Table 1 (total results see electronic
supplementary information). As can be seen, strong polar
aprotic solvents are beneficial to the reaction. The effects of
various acidic and basic additives on the reaction was
evaluated. Although most acidic and basic additives screened
were unable to promote the desired reaction, alkali
carbonates were found effective on the reaction, Cs2CO3 is
among the best of alkali carbonates. However, overuse of
Cs2CO3 can lead to selectivity decrease. The reaction offered
high selectivity but relatively low conversion in the absence of
both acids and bases. It was worth noting that molecular
oxygen did not provide expected high isolated yields. In fact,
despite high conversion, molecular oxygen dramatically
reduces the selectivity of the reaction, which arises maybe
because the reaction is inherently prone to side-reactions
under oxidative conditions. To our delight, we obtained 84%
The development of an innovative approach capable of
overcoming these problems for synthesis of naphtho[2,3-
b]furan-4,9-diones is therefore highly desirable. In light of
retrosynthetic analysis, we imagine a hydrogenolysis process
whereby naphtho[2,3-b]furan-4,9-diones are cleaved into
olefins and 2-hydroxy-1,4-naphthoquinones, two readily
available fragments, by hydrogen. Catalytic coupling of these
two fragments produce naphtho[2,3-b]furan-4,9-diones with
hydrogen release via a reverse hydrogenolysis process, which
would be a very attractive synthetic strategy of naphtho[2,3-
b]furan-4,9-diones. Recently, we reported
palladium(0)-catalysed oxidant- and
a
class of
acceptor-free
dehydrogenation reactions accompanied by the release of
hydrogen, in which intramolecular C-H and X-H bonds are
converted to C–X bonds, and alkanes are converted to the
alkenes.9 These reactions manifest as
dehydrogenation process or reverse process of
a acceptorless
a
hydrogenolysis. Encouraged by these results and guided by the
concept of reverse hydrogenolysis, we formulated direct
coupling of 2-hydroxy-1,4-naphthoquinones and olefins to
produce naphtho[2,3-b]furan-4,9-diones and hydrogen(H2),
aiming at further exploring and developing intermolecular
reverse hydrogenolysis synthetic methodologies. However,
this synthetic method designed requires selective scission of
three C(sp2)–H bonds and a O−H bond and metatheses of them
into a C(sp2)–C(sp2) bond, a C(sp2)–O bond, and two H–H
bonds without use of a stoichiometric oxidant or a hydrogen
2 | Chem Commun., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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COMMUNICATION
moderate isolated yields.10 To further explore the substrate
range of olefins, we carried out the reaction using non-Michael
View Article Online
DOI: 10.1039/C8CC09369E
Table 2 Synthesis of naphtho[2,3-b]furan-4,9-diones catalyzed by Pd/C a, b
O
O
O
H
H
acceptor olefins, e.g., styrene and stilbene as the partners of 2-
hydroxy-1,4-naphthoquinones. To our delight, the reverse
hydrogenolysis reaction could also provide the corresponding
products (Table 2, 3z, 3α). The result clearly showed that the
substrate scope of olefins was not limited to Michael acceptor
olefins. For 2-hydroxy-1,4-naphthoquinone substrates, the
naphthoquinones bearing functional groups seldom interfered
with the reverse hydrogenolysis reaction. For example, the
reaction obtained 81% isolated yields when substrate 2x was
coupled with chalcone under the optimized coupling
conditions. Furthermore, 2-hydroxy-1,4-naphthoquinones
could be replaced with 3-hydroxy-4H-chromen-4-ones. When
the later was used as the coupling partner of a chalcone, the
reverse hydrogenolysis reaction could provide 49% yields of
the corresponding product (3y). The result clearly indicated
that the coupling partners of chalcone were not limited to
naphthoquinones; a range of 3-hydroxy-4H-chromen-4-ones
would be viable in the reverse hydrogenolysis reaction.
Subsequently, we undertook preliminary mechanistic studies
to manage to have an insight into the formation of the C-C and
the C-O bonds. Because α, β-unsaturated enones are a class of
typical Michael acceptors, we originally thought that the C-C
bond formation involved a Michael addition. However, the
hypothesis that the C-C bond is formed through a Michael
addition is questioned by a disproof. As can be seen in Table 2,
the reverse hydrogenolysis reaction could also occur when α,
β-unsaturated enones were replaced by non-Michael acceptor
olefins. For example, 2-hydroxy-1,4-naphthoquinone could be
coupled with styrene and stilbene to produce the
corresponding products (3z, 3α). The results showed that the
C-C bond formation did not necessarily need to undergo a
Michael addition reaction. By contrast, the C-C bond formation
via a formal Heck reaction is relatively rational. The Heck
reaction-like process starts with a keto–enol tautomerism
equilibrium. Subsequently, the palladation of the carbonyl α-C-
H bond of the intermediate B forms the organopalladium C
(see scheme 2). The process of the palladium (0) inserting into
the electron-poor C-H bond to form the intermediate of
palladium (II) complex C is electronically matched, and similar
to that of palladium (0) inserting into the C-X bonds of Heck
H
H
R2
R3
O
10 mol % Pd/C
R2
R1
R1
+
+
2H2
20 mol % Cs2CO3
130 oC, DMA, N2
R3
O
1
O
3
2
Products (reaction time, yield)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OCH3
3a (31 h, 84%)
3b(32 h, 80%)
3c (35 h, 71%)
3d (30 h, 79%)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
F
Cl
3e (48 h, 61%)
3f (36 h, 70.5%)
3g (32 h, 59%)
3h (33 h, 72%)
OCH3
Cl
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
3j (31 h, 80%)
3l (41 h, 71%)
3i (40 h, 69%)
3k (48 h, 68%)
OCH3
Cl
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
F
C2H5
Cl
3m (38 h, 69%)
3n (40h, 74%)
3o (30h, 70%)
3p (48 h, 52%)
Cl
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
F
3q (46 h, 63%)
3r (35h, 77%)
3s (48 h, 47%)
3t (48 h, 61%)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Cl
3u (46 h, 63%)
3v (47 h, 50%)
3w (49 h, 59%)
3x (40 h, 81%)
O
O
O
O
O
O
O
O
O
O
3y (48 h, 49%)
3z (18h, 55%)
3
(16h, 40%)
α
a Reaction conditions: 0.2 mmol substrates, 10 mol% Pd/C (10%), 20 mol% Cs2CO3,
1ml DMA, 130 °C, N2 atmosphere, the mole ratio of substrate 1: substrate 2 is 1:2.
b Isolated yields.
isolated yield with the optimized reaction conditions of 10
mol% Pd, 20 mol% Cs2CO3, 1atm of N2, and 130 °C.
O
O
After having established the optimized conditions, we
proceeded to investigate the scope of 2-hydroxy-1,4-
naphthoquinones and olefins for this reverse hydrogenolysis
process. The results showed that this synthetic method was
suitable for large numbers of substituted α, β-unsaturated
enones. As shown in table 2, the enones with a variety of
groups such as methyl, ethyl, isobutyl, methoxyl could be
directly coupled with 2-hydroxy-1,4-naphthoquinone to give
moderate to high isolated yields. Fluorine- and chlorine-
containing enones were unaffected. Non-aromatic enones
such as 4-phenyl-3-buten-2-one (2s) and substituted 1,5-
diphenylpenta-1,4-dien-3-ones (2t, 2u, 2v, 2w) could also be
smoothly coupled with the 2-hydroxy-1,4-naphthoquinones to
afford the corresponding naphtho[2,3-b]furan-4,9-diones with
O
O
O
PdH
O
R2
R1
O
I
O
R1 R2
O
O
O
D
O
O
O
O
HPdH
R2
R2
HPdH
R2
H2
O
O
R1
O
PdH
R1
E
R1
O
H2
O
H
O
Pd
PdH
OH
R1
Pd
PdH
R2
O
O
O
O
O
O
C
R2
F
O
O
R1 O
OH
G
O
O
B
A
Scheme 2 Proposed catalytic cycle.
This journal is © The Royal Society of Chemistry 20xx
Chem.Commu., 2013, 00, 1-3 | 3
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5
6
(a) M. Ogawa, J. Koyanagi and A. Sugaya, Biosci. Biotechnol.
reactions. The C-O bond formation also underwent a formal
Heck reaction process, in which involves oxidative addition,
syn addition and beta-hydride elimination steps. In our
previous studies, we reported similar processes of the C-O
bond formation where phenolic or carboxylic acidic hydroxyl
groups could be coupled with olefins to form C(sp2)-O bonds
with release of H2.9c, 9d, 9f When the olefins were styrene and
stilbene, the formation of C-O bonds (from F to I) is very
similar to a process of C-O formation we have reported
previously. 9c
In conclusion, we have developed a general approach to the
synthesis of complex naphtha[2,3-b]furan-4,9-diones via the
direct palladium catalyzed reverse hydrogenolysis coupling of
2-hydroxy-1,4-naphthoquinones and a variety of olefins. The
reaction does not need for oxidants and hydrogen acceptors,
with H2 as the only byproduct. The high isolated yields and
atom economy, broad substrate scope, and the use of a simple
and recycled catalyst of the catalytic system highlight that the
reaction is intrinsically easy to industrializate. This reverse
hydrogenolysis process also provides a useful method for two-
site functionalization of the adjacent C(sp2)-H bonds of olefins.
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8
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Conflicts of interest
There are no conflicts to declare.
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10 The products 3u and 3v contain a small amount of 3u' and
3v' products formed by the reaction of 2-hydroxy-1,4-
naphthoquinone with another double bond of 2u and 2v.
2
3
4
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4 | Chem Commun., 2012, 00, 1-3
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