One-Step Synthesis of Substituted Benzofurans from ortho- Alkenylphenols via Palladium-Catalyzed C=H Functionalization

Article abstract of DOI:10.1002/adsc.201600082

A dehydrogenative oxygenation of C(sp²)=H bonds with intramolecular phenolic hydroxy groups has been developed, which provides a straightforward and concise access to structurally diversely benzofurans from ortho-alkenylphenols. The reaction is catalyzed by palladium on carbon (Pd/C) without any oxidants and sacrificing hydrogen acceptors.

Full text of DOI:10.1002/adsc.201600082

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DOI: 10.1002/adsc.201600082
One-Step Synthesis of Substituted Benzofurans from ortho-
À
Alkenylphenols via Palladium-Catalyzed C H Functionalization
Dejun Yang,a Yifei Zhu,a Na Yang,a Qiangqiang Jiang,a and Renhua Liua,*
a
School of Pharmacy, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, Reopleꢀs
Republic of China
E-mail: liurh@ecust.edu.cn
Received: January 20, 2016; Revised: March 23, 2016; Published online: May 9, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600082.
Abstract:
A
dehydrogenative oxygenation of
bonds of ortho-alkenylphenols because of the topo-
logical structure, or the overall skeletal structure, of
the starting material obviously resembles that of the
product. Unfortunately, implementing such synthetic
protocols requires sacrificial hydrogen acceptors, for
example, DDQ,[4] or a stepwise procedure in which
the double bond is oxidized to electrophilic epoxy,
carbonyl or iodine functional groups with stoichio-
metric or excess molar amounts of m-CPBA,[5]
2
À
C(sp ) H bonds with intramolecular phenolic hy-
droxy groups has been developed, which provides
a straightforward and concise access to structurally
diversely benzofurans from ortho-alkenylphenols.
The reaction is catalyzed by palladium on carbon
(Pd/C) without any oxidants and sacrificing hydro-
gen acceptors.
[7]
Keywords: acceptorless dehydrogenation; benzofur-
TBHP[6] or I2 followed by the nucleophilic reaction
À
ans; C H functionalization; oxidant-free condi-
of phenolic hydroxyl groups to realize the cyclization
(Scheme 1). Thus the concurrent formation of unde-
sired wastes is inherently unavoidable for these reac-
tions; palladium
À
tions. The C H functionalization represents markedly
different synthetic strategies from traditional ap-
proaches to functionality transformation. Although
The benzofuran ring is a prominent structural motif significant and indicative progress has been made in
found in numerous natural products[1] and pharma- the realm of oxidative C H functionalization with
À
ceutically active compounds.[2] In spite of the impor- stoichiometric oxidants,[8] the C H oxygenation in-
À
tance of this kind of heterocyclic compound, existing volved in the one-step conversion of o-alkenylphenols
methods for benzofuran synthesis are limited in their to benzofurans without oxidants and sacrificing ac-
scope and generality. Current strategies for synthesiz- ceptors is scarcely reported. Herein we reported the
ing substituted benzofurans typically require a chemi-
cal cyclization step for forming the furan ring.[3] How-
ever, these cyclizations rely mainly on traditional
functional group transformations and, consequently,
inherently require substrates with specific functional
groups in the appropriate location. A non-trivial syn-
thetic sequence for installing functional groups prior
to the desired bond construction may be required,
thus increasing the overall cost of synthesis and waste
generation. Moreover, most of these cyclization pro-
cedures with patterns of functional group exchange
are accompanied by loss of small molecules or frag-
ments from the parent compounds. They are therefore
inefficient synthetic protocols with poor atom econo-
my and high waste generation.
A straightforward and concise synthetic approach
to the benzofuran product class is intramolecular cou-
pling of phenolic hydroxy groups and the C(sp ) H
Scheme 1. Synthesis of substituted benzofurans from ortho-
alkenylphenols.
2
À
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Table 1. Optimization of the dehydrogenation conditions.
discovery and development of a novel dehydrogena-
tive coupling of intramolecular phenolic hydroxy
2
À
groups and C(sp ) H bonds through palladium(0)-cat-
À
alyzed C H bond activation, which should in one-step
synthesize substituted furans from ortho-alkenylphe-
nols without any oxidants and sacrificing hydrogen ac-
ceptors.
Entry[a] TEMPO
(mol%)
Base/
Acid
Temp.
[oC]
Isolated yield
[%]
Recently, acceptorless dehydrogenation for bond
construction is emerging as a powerful and environ-
mentally benign synthetic strategy.[9] Many highly effi-
cient catalysts capable of acceptorless dehydrogena-
tions of alcohols to the carbonyl compounds and alco-
hol dehydrocoupling reactions have been reported.[10]
To explore the potential of acceptorless dehydrogena-
tion in other types of reactions, we have recently de-
veloped the palladium-catalyzed, oxidant- and accept-
or-free dehydrogenation of amines to imines[11] and of
substituted cyclohexanones to phenols[12] with H2 re-
1
–
–
150
150
150
150
56
16
26
16
12
47
60
68
68
65
72
57
68
37
46
18
50
65
77
28
35
7
2
–
–
–
Cs2CO3
Cs2CO3
TsOH
3[b]
4
5
–
NH2SO3H 150
6
7
8
9
1.5% PBQ
1.5%
5%
10%
15%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
150
150
150
150
150
150
150
150
150
150
150
120
130
140
150
150
140
140
10
11[c]
12[d]
13[b]
14[e]
15[f]
16[g]
17
lease. Key to our findings is the discovery that two ad-
3
À
jacent atoms combined with hydrogen, HC(sp )
3
3
3
À
C(sp )H, HN(sp ) HC(sp ), can be dehydrogenated
to form double bonds, C(sp2)=C(sp2), N(sp2)=C(sp2),
with H2 release by palladium-catalyzed C H bond ac-
À
tivation. As a part of ongoing research on construct-
ing a new bond between two hydrogen-containing
functional groups via the mode of oxidant- and hydro-
gen-acceptor-free dehydrogenation with H2 release,
we anticipated that the palladium(0)-catalyzed system
developed in our laboratory could be adapted to the
dehydrogenation of two unbonded functional groups
to form a new bond without any oxidants and sacrific-
ing hydrogen acceptors. To achieve the goal, we ex-
18
19
20[h]
21[i]
22[j]
23[j]
5%
7
[a]
Reaction conditions: substrate (0.5 mmol), base or acid
(0.1 mmol), Pd/C 26.5 mg (5 mol% Pd), TEMPO, DMA
(2 mL), 1 atm of N2.
Reaction solution was degassed with N2.
7.5 mol% Pd was used.
2.5 mol% Pd was used.
The solvent was DMF.
The solvent was PEG400.
The solvent was xylene.
plored the chemical bond construction with o-alkenyl-
2
À
[b]
[c]
[d]
[e]
[f]
phenol C(sp ) H bond functionalization to form ben-
zofurans as the probe reaction (Scheme 1).
We began the study by investigating the dehydro-
genative coupling reactivity with 2-(2-phenylvinyl)-
phenol (1a) as a representative substrate. At 1508C,
with 5 mol% Pd, DMA as the solvent, 20 mol%
Cs2CO3, 1 atm N2 atmosphere, and 36 h reaction con-
ditions, the reaction afforded an encouraging result:
gas chromatographic (GC) analysis showed that the
starting material was completely converted to a new
[g]
[h]
[i]
Reaction under 1 atm of O2.
Reaction under 1 atm of CO2.
Reaction without Pd/C.
[j]
compound. The compound was purified by silica gel base additives, with significant increase of the yields
column, and its structure was identified as that of de- in the absence of both acids and bases (entries 2–5).
sired compound by 1H NMR and GC-MS analysis. Al- The results of control experiments showed that the
though the reaction provided the desired product and dehydrogenative coupling reaction remains almost un-
the substrate was almost completely converted, it reactive without Pd/C (entry 22 and 23). It is impor-
only reached 16% isolated yields (Table 1, entry 2), tant to note that dioxygen is beneficial to the poly-
and considerable amounts of starting material was merization and oxidation side reactions, giving rise to
converted to polymers that cannot be detected by obvious yield decrease (entry 13 vs. entry 19). This
GC. A variety of reaction conditions was therefore problem arises because phenol substrates are inher-
screened for suppressing the polymerization. Polymer- ently susceptible to oxidation, thus giving rise to by-
ization inhibitors, for example, TEMPO were used, products, especially in alkaline conditions. The best
but they have met with a limited degree of improve- result is highlighted in Table 1 (entry 19).
ment on the reaction (Table 1, entry 1 vs. entry 8). Of
With optimized conditions in hand, we next sought
crucial importance was the non-use of both acid and to define the substrate scope of the dehydrogenative
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coupling reaction. As shown in Table 2, electron-rich a furan ring merely reached a poor yield (2n). From
and electron-poor aromatic nuclei containing sub- an electronic point of view, electron-rich phenol hy-
strates readily undergo intramolecular dehydrogena- droxy groups are prone to couple the electron-poor
tive coupling to furans. A wide range of different coupling partners. To our delight, the substrates with
functional groups was tolerated under the optimized an electron-rich moiety as the phenol hydroxy cou-
coupling conditions. Ethers, aryl fluorides, carboxylic pling partner were also suitable for the intramolecular
esters, trifluoromethylbenzenes, nitriles, and naphtha- dehydrogenative coupling, again affording high isolat-
lene proved compatible. However, the substrate with ed yields (2e, 2f and 2h, 2i). In particular, the 1, 2-
diarylolefin substrates were worth being pursued fur-
ther, because the corresponding 2-arylbenzofuran de-
rivatives exhibit important biological activity.[13] As
Table 2. Scope of the dehydrogenative coupling reaction.
can be seen from the Table 2, most of this kind of
substrates is reactive, allowing for the synthesis of 2-
arylbenzofurans with high isolated yields. We also ex-
amined 1-alkyl-2-arylalkene substrates (1r, 1s, 1t).
Substrate 1r provided a high isolated yield while 1s
and 1t remained elusive recalcitrant substrates. More-
over, control experiments without TEMPO were car-
ried out. The results shown in Table 2 (in parentheses)
displayed that most of yields were reduced in the ab-
sence of TEMPO.
To further probe the substrate scope of the dehy-
drogenative coupling system, we synthesized 1,1’-di-
arylolefin substrates (1w–1aa). At first we thought
that these compounds may be recalcitrant substrates
2
À
because the C(sp ) H bonds (the phenol hydroxy
coupling partners) are believed to be less reactive due
to the C(sp ) H being without a direct connection to
2
À
any aromatic groups. Interestingly, this kind of sub-
strates seldom interfered with the catalytic coupling
reaction, being also smoothly converted to the corre-
sponding furans with high isolated yields (as can be
seen in Table 3). Furthermore, these 1,1’-diarylolefin
substrates seldom allowed for polymerization side re-
actions to occur, thereby dispensing with the need for
the TEMPO polymerization inhibitor in these cou-
pling reactions.
Coupled with our previously reported palladi-
um(0)-catalyzed acceptorless dehydrogenation with
H2 release, we speculate an overall mechanism for
this catalytic dehydrogenative coupling reaction,
which can be seen in Scheme 2. The reaction mecha-
nism is formally similar to a Heck reaction mecha-
nism. The reaction begins with an oxidative addition
À
in which palladium inserts itself in the O H bond to
form the organopalladium intermediate 3. A similar
À
process in which palladium(0) inserts itself into O H
bonds is observed in an oxidant- and acceptor-free de-
hydrogenation of alcohols.[14] Then the double bond
inserts itself in the palladium-oxygen bond in a syn
addition step to generate 4. The intermediate 4 is con-
verted into the target compound by a beta-hydride
[a]
Reaction conditions: 1a–1v (0.5 mmol), Pd/C 26.5 mg elimination step with the formation of HPdH. The
(5 mol% Pd), TEMPO (5 mol%), DMA (2 mL), 1408C,
1 atm of N2.
Reaction conditions: 1a–1v (0.5 mmol), Pd/C 26.5 mg
(5 mol% Pd), DMA (2 mL), 1408C, 1 atm of N2.
10 mol% Pd was used.
palladium(0) compound is regenerated by reductive
elimination of the HPd(II)H with H2 release. The H2
is detected in the reaction by GC (as can be seen in
the Supporting Information). On the basis of the reac-
[b]
[c]
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Table 3. Further scope of the dehydrogenative coupling re-
action.
reactions achieve high isolated yields without any oxi-
dants and sacrificing hydrogen acceptors. With the de-
velopment of improved methods for highly efficient
and scalable reactions, oxidant- and hydrogen-accept-
or-free dehydrogenation bond formation methods of
this type could increasingly displace elaborate con-
ventional laboratory and industrial synthetic methods.
Experimental Section
General Procedure
To a two-neck flask loaded with a stir bar was added the o-
alkenylphenol (0.5 mmol), Pd/C (26.5 mg, 0.025 mmol Pd),
TEMPO (5.6 mg, 0.025 mmol, only for 2a–2s) and DMA
(2 mL). The flask was first degassed and then sealed with an
N2 balloon. The reaction mixture was heated in an oil bath
to 1408C with vigorous stirring for an appropriate time, as
detected by GC and TLC. After the reaction was completed,
the solvent was evaporated under reduced pressure. The res-
idue was loaded onto a silica gel column and purified by
flash chromatography (hexanes/EtOAc mixture, gradient
from 0–30%) to afford the pure product.
[a]
Reaction conditions: 1w–1aa (0.5 mmol), Pd/C 26.5 mg
(5 mol% Pd), DMA (2 mL), 1408C, 1 atm of N2.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (grants 21072055).
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