Palladium-Catalyzed Tandem Oxidative Arylation/Olefination of Aromatic Tethered Alkenes/Alkynes

Article abstract of DOI:10.1002/chem.201605351

We describe herein a palladium-catalyzed tandem oxidative arylation/olefination reaction of aromatic tethered alkenes/alkynes for the synthesis of dihydrobenzofurans and 2 H-chromene derivatives. This reaction features a 1,2-difunctionalization of C?C π-bond with two C?H bonds using O₂as terminal oxidant at room temperature. The products obtained are valuable synthons and important scaffolds in biological agents and natural products.

Full text of DOI:10.1002/chem.201605351

A Journal of
Accepted Article
Title: Palladium-Catalyzed Tandem Oxidative Arylation/Olefination of
Aromatic Tethered Alkenes/Alkynes
Authors: Yang Gao, Yinglan Gao, Wanqing Wu, Huanfeng Jiang,
Wenbo Liu, Xiaobo Yang, and Chao-Jun Li
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To be cited as: Chem. Eur. J. 10.1002/chem.201605351
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Palladium-Catalyzed Tandem Oxidative Arylation/Olefination of
Aromatic Tethered Alkenes/Alkynes
Yang Gao,a, b Yinglan Gao,b Wanqing Wu,b Huanfeng Jiang*,b Xiaobo Yang,a Wenbo Liu,a Chao-Jun
Li*a
Abstract: We herein described
a palladium-catalyzed tandem
oxidative arylation/olefination reaction of aromatic tethered
alkenes/alkynes for the synthesis of dihydrobenzofurans and 2H-
chromene derivatives. This reaction features a 1,2-difunctionalization
of C-C  bond with two C–H bonds using O2 as terminal oxidant at
room temperature. The products obtained are valuable synthons and
important scaffolds in biological agents and natural products.
Carbon−carbon bond formation is a central theme of
chemical syntheses.[1] The cross-dehydrogenative coupling
(CDC) reactions are among the most straightforward strategies
to construct C-C bonds by joining two C−H bonds directly.[2]
Among CDC reactions, the palladium-catalyzed coupling
between arenes and olefins, commonly known as the
Fujiwara−Moritani reaction,[3] has proven to be a highly useful
process for constructing C(sp2)-C(sp2) bonds (scheme 1A). In
the last decades, through the introduction of directing groups or
external ligands,[4] tremendous progress has been made to
achieve such oxidative C-H/C-H couplings regioselectively.
Meanwhile, the intramolecular aromatic C–H alkenylation
reactions have also been developed.[5] Despite the efficiency in
forming a single C-C bond, it would be highly desirable to further
impove the synthetic efficiency, with respect to “step and atom
economy”, by forming multiple C-C bonds related to such C-H/C-
H couplings. As our ongoing interest in CDC reactions, we
envisaged a tandem oxidative arylation/alkenylation of aromatic
tethered C-C bond with two C–H bonds, a Fujiwara−Moritani-
type difunctionalization reaction of C-C mutiple bond with
aromatic C–H bond and vinylic C–H bond (Scheme 1B). Such a
reaction will lead to the simultaneous formation of two new
vicinal C-C bonds, and thus could rapidly increase molecular
complexity. [6]
Scheme 1. The concept of Fujiwara-Moritani type difunctionalization reaction.
either by oxidative addition of C(sp2)-X bonds or transmetalation,
have been reported to produce benzoheterocycles.[8] In addition,
intramolecular hydroarylation of alkenes/alkynes,[9] radical
addition/cyclization reactions[10] and electrophilic cyclization
reactions[11] have been employed as well. Recently, Liu, Zhu and
co-workers reported the PdII/PdIV oxidative arylacetoxylation,
arylalkylation and aryltrifluoromethylation methods for the
synthesis of oxindoles.[12] While these reactions are only limited
to the N-arylacrylamides substrates and stoichiometric stronger
oxidants such as PhI(OAc)2 are needed. None of these
precedents allows the tandem arylation/olefination of C-C
multiple bonds via the functionalization of two C–H bonds, with
green oxdiant such as O2 under mild conditions. Herein, we
presented such a strategy.
On the other hand, benzo-fused heterocycles such as
dihydrobenzofurans and 2H-chromene are prominent in a variety
of natural products and biologically active compounds [e.g.
Parviflorene J (I), Morphine (II and III) and Tephrowatsin B (IV),
Figure 1], which exhibit a wide range of important biological
activities.[7] Consequently, tremendous effort has been made in
developing new synthetic methods towards these scaffolds.
Previously, the intramolecular Heck-type reactions, initiated
Figure 1. Representative bioactive compounds with dihydrobenzofurans and
2H-chromene structural motifs.
Our investigation began with the coupling of 1-methoxy-4-
(2-methylallyloxy)benzene (1a) and methyl acrylate (2a) using
Pd(OAc)2 (10 mol %) as catalyst in HOAc (0.5 mL) under 1 atm
O2, and the desired product 3a was detected in 10% NMR yield
(Table 1, entry 1). The addition of trifluoroacetic acid (TFA) was
found to be critical to improve the yields and 3a was obtained as
a mixture of isomers (3a:3a' = 7:1) in 41% NMR yield with TFA
(3.0 equiv) as additive (Table 1, entry 2). During the reaction,
some decomposition of the catalyst, as palladium black or
mirror, was observed. Adding ligands such as pyridine and
DMSO which have been proven to be useful in oxidative
palladium processes,[13] only afforded 3a in 12% and 40% NMR
yields, respectively (Table 1, entries 3 and 4). No improvement
in yield was observed by addding some common bases such as
[*]
Y. Gao, X. Yang, W. Liu, Prof. Dr. C.-J. Li
Department of Chemistry and FQRNT Centre for Green Chemistry
and Catalysis, McGill University
801 Sherbrooke St. W., Montreal, Quebec H3A 0B8 (Canada)
E-mail: cj.li@mcgill.ca
Y. Gao, Y. Gao, Dr. W. Wu, Prof. Dr. H. Jiang
Key Laboratory of Functional Molecular Engineering of Guangdong
Province, School of Chemistry and Chemical Engineering, South
China University of Technology, Guangzhou 510640 (PR China)
E-mail: jianghf@scut.edu.cn.
Supporting information for this article is given via a link at the end of
the document.
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Reaction conditions A: all reactions were performed with 1 (0.1 mmol), 2a
(0.15 mmol), Pd(OAc)2 (10 mol%), Cu(OAc)2 (20 mol%), BQ (10 mol%), TFA
(3.0 equiv) in HOAc (0.5 mL) under 1 atm O2 at room temperature for 10 h.
Isolated yields are given in all cases.
NaOAc and CsOPiv, which are known to assist the C–H
activation via the CMD process[14] (Table 1, entries 5 and 6).
Various oxidants such as 1,4-benzoquinone (BQ), TBHP and
AgOAc were then examined, which could only increase the
yields slightly (Table 1, entries 7-9). Next, inspired by the co-
oxidant system,[15] i.e. the BQ(0.1eq)/FePC/O2 system (Table 1,
entriy 10) and the classical Cu2+/O2 system (Table 1, entry 11),
we were pleased to find the yield could be increased significantly
to 76% when 0.2 equiv Cu(OAc)2 was added as co-oxidant.
Since BQ could stabilize the Pd(0) species by in situ generating
the Pd(0)-BQ complexes,[16] the yield was further improved in
presence of BQ (10 mol%) and no palladium black was
observed (Table 1, entry 12). Finally, the selectivity of 3a to 3a'
was improved to 15:1 when the reaction temperature was
decreased to room temperature (Table 1, entry 13).
With the optimized reaction conditions in hand, we next
investigated the substrate scope of this transformation. As
summarized in Table 2, apart from acrylates, other electron
deficient olefins, such as acrylaldehyde (3c), but-3-en-2-one
(3d), methylsulfonylethene (3e), N-isopropylacrylamide (3f) and
even acrylic acid (3g) all showed effectiveness in this tandem
CDC reaction to give the desired dihydrobenzofurans in
moderate to good yields (54%-80%). Gratifyingly, styrenes also
delivered comparable yields under the optimized conditions and
various styrenyl derivatives (3h−3j) were obtained in 59%-74%
isolated yields. Notably, this reaction showed an excellent
stereoselectivity, with E-configuration product obtained
exclusively in most cases. Interestingly, the reaction of 1a with
α-methyl substituted acrylates proceeded to form the terminal
alkene 3k rather than the more stable internal isomer.
Table 1. Optimization studies for the formation of 3a.[a]
Entry
Oxidant
Ligand
Additive
3a:3a'
Yield%[b]
1[c]
2
3[d]
4[d]
5[e]
6[f]
7
O2
O2
-
-
-
10
41
Table 3. Substrate scope of allyl ethers 1
-
-
7:1
-
O2
Py
-
12
O2
DMSO
-
9:1
6:1
7:1
5:1
6:1
10:1
5:1
8:1
9:1
15:1
40
O2
-
-
-
-
-
-
-
-
-
NaOAc
36
O2
CsOPiv
30
BQ (2.0 equiv)
TBHP (2.0 equiv)
AgOAc (2.0 equiv)
BQ/FePC/O2
Cu(OAc)2/O2
Cu(OAc)2/BQ/O2
Cu(OAc)2/BQ/O2
-
-
-
-
-
-
-
43
8
55
9
46
10[g]
11[h]
12
40
76
85
13[i]
83 (76)
[a] Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), Pd(OAc)2 (10 mol%),
[b]
TFA (3.0 equiv) in HOAc (0.5 mL) under 1 atm O2 at 60 °C for 10 h.
The
yield was determined by NMR analysis with mesitylene as the internal
[c]
[d]
standard (isolated yield is in parentheses).
Without TFA.
Ligand (20
[e]
[f]
[g]
mol%).
NaOAc (1.0 equiv). CsOPiv (1.0 equiv).
BQ (0.1 equiv)/FePC
[h]
[i]
(0.05 equiv)/O2.
Cu(OAc)2 (0.2 mmol)/BQ (0.1 mmol)/O2. Cu(OAc)2 (0.2
mmol)/BQ (0.1 mmol)/O2 at room temperature.
Table 2. Substrate scope of olefins 2
All reactions performed under condition A. Isolated yields are given in all
cases. a In toluene 0.5 mL. b TFA 0.1 mL was used as co-solvent, at 40 oC for
12h. c Ratio determined by crude 1H NMR. d TFA (5.0 equiv), 2a (2.0 equiv) at
80 oC for 12h.
Next, a range of allyl ethers 1 derived from commercially
available phenols were investigated, leading to highly
substituted dihydrobenzofurans products. As illustrated in Table
3, various functional groups such as methyl, methoxyl, fluoro,
chloro, bromo, ester, alkenyl, free hydroxyl and amide, were all
well tolerated; and the desired products were isolated in
moderate to good yields (45%-79%). The halogenated
substrates (3t, 3v) highlight the potential for further product
elaboration. The substituent position has little influence on the
product yields (e.g., 3m, 3n and 3o). Notably, with meta-
substituted substrates, the reaction occurred selectively with the
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10.1002/chem.201605351
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less-hindered C−H bond, as was confirmed by NOE (3n, 3w and
3x). Electronic effect also played an important role in this
reaction, since electron-rich substrates (3m-3q) performed much
better than those bearing electron-withdrawing groups such as
CO2Me (3r), Ac (3s) and F (3w). 2-(Allyloxy)naphthalene also
proceeded well under the optimized conditions and the more
electron-rich C1 position was favoured to form the product 3y
with good selectivity (a:a' = 20:3). Moreover, substrate with β-
OMe could also be converted to the corresponding product 3z in
55% yield. Notably, as important pharmaceutically relevant
compound, the 4-(2-methylallyloxy)-2H-chromen-2-one could
also afford the corresponding product 3z'.
carried out (0.84 g, 64% yield). Moreover, given the importance
of 2a,3,4,5-tetrahydro-2H-naphtho[1,8-bc]furan in natural
products as illustrated in Figure 1, the present method provides
an efficient and practical synthesis of this motif by the following
sequence: the corresponding dihydrobenzofuran derivatives
were obtained by the present method in two steps from the
commercially available phenols; then, treatment of the
derivatives by m-CPBA afforded the epoxide 6; finally, Au3+
catalyzed epoxide-ring-opening delivered the desired 7 in good
yields.[18]
Subsequently, such a tandem oxidative arylation/vinylation
was also found to be effective for difunctionalization of aromatic
tethered alkynes, by slightly modifying the reaction conditions.
Various 2H-chromenes derivatives, which are important
structural motifs in pharmaceuticals and functional materials,[17]
were obtained readily. Acrylate, styrene, and acrylaldehyde were
all good reaction partners in this reaction (5a-5e). Different aryl
propargyl ethers
4
were smoothly converted into the
corresponding 2H-chromene products in moderate to good
yields (46%-80%). Functional groups such as OMe (5d), Br (5f)
and CO2Me (5i) were all tolerated, with the electron-rich
substrates produced higher yields. With meta-methyl substituted,
a mixture of isomers (5g, a:a' = 1:1) was obtained. It should be
noted that some of the products are fluorescent, which may find
applications in optoelectronic materials and the fluorescence
properties of 5d and 5e were preliminary tested (see SI for
detail).
Scheme 2. Gram-scale synthesis of 3aa and synthetic utilities. (i) 3-bromo-2-
methylprop-1-ene (1.2 equiv), K2CO3 (2.0 equiv), DMF, rt. (ii) Condition A. (iii)
m-CPBA = meta-chloroperbenzoic acid. (iv) AuCl3 2.5 mol %, AgOTf 7.5
mol %.
Table 4. Substrate scope of 2H-chromene products
Scheme 3. A tentative mechanism for this Fujiwara−Moritani-type C-C  bond
difunctionalization reaction.
On the basis of the above results, a tentative mechanism for
this Fujiwara−Moritani-type C-C
 bond difunctionalization
reaction is proposed (Scheme 3). The reaction could be initiated
by the electrophilic metalation of the cationic species [Pd(TFA)]+
to the substrate 1 to produce the aryl-Pd species VI. In this
process, the selective ortho C-H bond functionalization could be
controlled by the coordination of cationic species [Pd(TFA)]+ to
the C=C bond.[19] Then, intramolecular insertion of olefin into the
C−Pd bond formed the alkyl-Pd intermediate VII, which was
followed by insertion of activated olefin and β-hydride elimination,
afforded the corresponding product 3 and regenerated the active
All reactions were performed with 4 (0.1 mmol), 2 (0.15 mmol), Pd(OAc)2 (10
mol%), Cu(OAc)2 (10 mol%), BQ (10 mol%) in HOAc/toluene = 5:1 (0.6 mL)
a
under 1 atm O2 at 60 oC for 8 h. Isolated yields are given in all cases. TFA
(1.0 equiv) was added. b Ratio determined by crude 1H NMR.
To further demonstrate the synthetic utility of this
transformation, a gram-scale synthesis of product 3a was
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Pd(II) species by the oxidation of Cu2+/O2. Alternatively, the
alkyl-Pd intermediate VII could be formed via the intramolecular
trans-nucleopalladation of aryl to C=C bond.[20] A kinetic isotope
effect experiment (KH/KD = 1.9, see SI for detail) did not provide
concluding evidence for either mechanism.
Asian J. 2014, 9, 26; h) C. J. Scheuermann, Chem. Asian J. 2010, 5,
436.
For seminal work, see: a) Y. Fujiwara, I. Moritani, S. Danno, R. Asano,
S. Teranishi, J. Am. Chem. Soc. 1969, 91, 7166; b) I. Moritani, Y.
Fujiwara, Tetrahedron Lett. 1967, 8, 1119. For recent review, see: c) J.
Le Bras, J. Muzart, Chem. Rev. 2011, 111, 1170.
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In summary, a tandem oxidative Fujiwara−Moritani-type 1,2-
difunctionalization of C-C multiple bonds with two C–H bonds
has been developed.
A
wide range of important
dihydrobenzofurans and 2H-chromene derivatives bearing
various function groups can be synthesized readily via this
process. As an application, an efficient route from the
commercially available phenols to the important 2a,3,4,5-
tetrahydro-2H-naphtho[1,8-bc]furans was accomplished. This
reaction extends the concept and applicability of the present
CDC reaction. Ongoing studies in our laboratory are dedicated
to the asymmetric catalysis and synthetic applications of this
transformation.
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To a 10 mL U-shape tube were added into allyl ethers 1 (0.1 mmol),
acrylates 2 (0.15 mmol), Pd(OAc)2 (2.24 mg, 10 mol%), Cu(OAc)2 (3.60
mg, 20 mol%), BQ (1.08 mg, 10 mol%), TFA (24 uL, 3.0 equiv) in 0.5 mL
HOAc. Then the tube was sealed and the mixture was stirred at room
temperature under 1 atm O2 (oxygen balloon) for 10 hours. After
completion, the reaction was quenched with aqueous NaHCO3 and the
crude product was extracted with ethyl acetate. The organic extracts
were concentrated in vacuum, and the resulting residue was purified by
silica gel column chromatography using light petroleum ether/ethyl
acetate as eluent to afford the desired products 3.
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We are grateful to the Canada Research Chair Foundation (to
C.-J. Li), the CFI, FQRNT Center for Green Chemistry and
Catalysis, NSERC, McGill University and the International
Cooperation Projects (21420102003) for support of our research.
Y. Gao is grateful for the SCUT Doctoral Student Short-Term
Overseas Visiting Study Funding Project. Thanks Z. Qiu, N.
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Keywords: Pd-catalyzed • CDC • Fujiwara−Moritani reaction •
dihydrobenzofurans • 2H-chromene
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COMMUNICATION
A Fujiwara−Moritani-type
1,2-difunctionalization of
C-C multiple bonds with
two C–H bonds has been
developed. This protocol
provides an efficient and
economic rout for the
synthesis of
dihydrobenzofurans and
2H-chromene derivatives
with broad functional group
compatibility.
Yang Gao, Yinglan gao,
Wanqing Wu, Huanfeng Jiang
Xiaobo Yang, Wenbo Liu and
Chao-Jun Li*
Page No. – Page No.
Palladium-Catalyzed
Tandem Oxidative
Arylation/Olefination of
Aromatic Tethered
Alkenes/Alkynes
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