Palladium-Catalyzed Decarboxylative γ-Olefination of 2,5-Cyclohexadiene-1-carboxylic Acid Derivatives with Vinyl Halides

Article abstract of DOI:10.1021/acs.orglett.8b00486

This study explores a Pd-catalyzed decarboxylative Heck-type Csp³-Csp² coupling reaction of 2,5-cyclohexadiene-1-carboxylic acid derivatives with vinyl halides to provide γ-olefination products. The olefinated 1,3-cyclohexadienes can be further oxidized to produce meta-alkylated stilbene derivatives. Additionally, the conjugated diene products can also undergo a Diels-Alder reaction to produce a bicyclo[2.2.2]octadiene framework.

Full text of DOI:10.1021/acs.orglett.8b00486

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium-Catalyzed Decarboxylative γ‑Olefination of 2,5-
Cyclohexadiene-1-carboxylic Acid Derivatives with Vinyl Halides
Chi-Hao Chang and Chih-Ming Chou*
Department of Applied Chemistry, National University of Kaohsiung, 700 Kaohsiung University Road, Nanzih District, Kaohsiung
81148, Taiwan
S
* Supporting Information
ABSTRACT: This study explores a Pd-catalyzed decarboxylative Heck-type Csp3−Csp2 coupling reaction of 2,5-
cyclohexadiene-1-carboxylic acid derivatives with vinyl halides to provide γ-olefination products. The olefinated 1,3-
cyclohexadienes can be further oxidized to produce meta-alkylated stilbene derivatives. Additionally, the conjugated diene
products can also undergo a Diels−Alder reaction to produce a bicyclo[2.2.2]octadiene framework.
he alkene moiety is an important building block in organic
synthesis and widely exists in pharmaceuticals, natural
Moreover, MacMillan reported a photocatalytic decarboxylative
olefination of alkyl, α-amino, and α-oxy carboxylic acids with
vinyl sulfones or vinyl halides using a photoredox nickel dual
catalysis activation, realizing Csp3−Csp2 bond formation.13
In 2011, Studer and co-workers reported a Pd-catalyzed
stereospecific decarboxylative γ-arylation using 2,5-cyclohexa-
diene-1-carboxylic acids as substrates coupled with aryl iodides to
afford arylated 1,3-cyclohexadienes (Scheme 1b),14a,b which was
applied to the total synthesis of resveratrols, macheriols, and
THC.14c,d Recently, we and Studer’s group have independently
T
products, and functional organic π-materials.1 Thus, the
development of synthetic protocols toward these scaffolds has
attracted considerable attention. In general, conventional
methods for the construction of alkene moieties with good
stereoselectivity include Wittig−type reactions,2 Mizoroki−
Heck reactions,3 and transition-metal catalyzed cross-coupling
reactions.4 However, the shortcomings of these methods include
being step-noneconomic and waste-exhaustive and requiring
expensive or air-sensitive organometallic reagents (organozinc,
-boron, and -tin compounds).
Scheme 1. Decarboxylative Olefinations or Arylation of
During the past decade, transition-metal catalyzed decarbox-
ylative cross-couplings have become powerful tools for the
formation of carbon−carbon bonds.5 Decarboxylative cross-
couplings take advantage of the fact that the nucleophilic
coupling partners are generated in situ from readily available
carboxylic acids by extrusion of carbon dioxide instead of toxic
metal halides. Decarboxylative couplings can provide a greener,
more practical, and step-economic synthetic alternative to the use
of common organometallic reagents.
Various Carboxylic Acid Derivatives
Since Myers and co-workers reported the first decarboxylative
Heck-type olefination of aromatic carboxylic acids with alkenes in
2002,6 extensive research on decarboxylative olefinations using a
variety of carboxylic acids coupled with a broad range of alkene
sources has been developed. For example, Su,7 Gooβen,8 and
others9 showed that successful decarboxylative couplings of
aromatic carboxylic acids with alkenes or vinyl halides can be
achieved using various metal catalysts or oxidants. In addition,
decarboxylative olefinations using alkynyl,10 cinnamyl,11 or α-
keto carboxylic acids12 as substrates have also been disclosed.
Received: February 9, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00486
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
showed that 2,5-cyclohexadiene-1-carboxylic acids undergo
tandem Pd-catalyzed decarboxylative C−H olefination/rear-
omatization reactions yielding ortho-alkylated vinylarenes.15 We
envisioned that this versatile substrate class, 2,5-cyclohexadiene-
1-carboxylic acids, can also be utilized in the decarboxylative
olefination with vinyl halides to enable a new decarboxylative
Csp3−Csp2 bond-forming reaction. Herein, we disclose a Pd-
catalyzed decarboxylative γ-olefination of 2,5-cyclohexadiene-1-
carboxylic acids with vinyl bromides or vinyl iodides to provide
olefinated 1,3-cyclohexadienes that can be further used in the
preparation of meta-alkylated stilbenes or bicyclo[2.2.2]-
octadiene frameworks.
loading, but only a 66% yield was obtained (entry 13). Therefore,
for further study we used 1.0 equiv of vinyl halides, 1.25 equiv of
acids, 5 mol % of Pd(OAc)2, and 1.4 equiv of Cs2CO3 in toluene
at 110 °C for 3 h as the standard conditions in the following
experiments.
With the optimized reaction conditions in hand, we turned our
attention toward the substrate scope of this Pd-catalyzed
decarboxylative γ-olefination reaction. As shown in Scheme 2, a
Scheme 2. Substrate Scope of the Pd-Catalyzed
a b
,
Decarboxylative γ-Olefination
We began with the optimization of the decarboxylative γ-
olefination reaction by using 1.0 equiv of 1-isopropyl-2,5-
cyclohexadiene-1-carboxylic acid (1b) and 1.1 equiv of (Z)-β-
bromostyrnene (2a) as model substrates, 10 mol % of Pd(OAc)2
catalyst, and Cs2CO3 as a base in toluene at 110 °C for 3 h,
affording γ-olefination product 3b in 45% isolated yield (Table 1,
Table 1. Optimization of the Pd-Catalyzed Decarboxylative γ-
a
Olefination Reaction
d
e
entry
1b:2a
ligand
base
3b (%)
1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.5
1.25:1
1.25:1
1.25:1
−
PPh3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
tBuOK
45
b
f
2
30
c
f
3
Dppe
31
a
b
g
Reaction was performed with 1 (0.125 mmol), and 2 (0.1 mmol) in 2
4
SPhos
22
b
c
c
g
mL of toluene. Isolated yields. Reaction conducted at 1 mmol scale
of 2a. Combined yield of isomers.
5
XantPhos
0
d
g
6
−
−
−
−
−
−
−
−
0
g
7
tBuONa
0
g
8
K2CO3
0
series of 1-alkylated 2,5-cyclohexadiene-1-carboxylic acids 1a−l
were prepared from readily available aromatic carboxylic acids
derivatives using the well-established Birch reduction (see
Supporting Information (SI) for details),16 and coupled with a
variety of (Z)- or (E)-vinyl halides under optimized conditions to
give 3a−3t. Methyl, isopropyl, sec-butyl, n-hexyl, and benzyl
substituents at the 1-position of the 2,5-cyclohexadiene-1-
carboxylic acids coupled with (Z)-β-bromostyrene were
compatible with this procedure, leading to the corresponding
olefinated 1,3-cyclohexadienes 3a−e in good yields.
f
9
Ag2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
0
10
11
12
13
59
87
h
88
i
66
a
Reaction conditions: 0.1 mmol scale at 0.05 M with 10 mol % of
b
Pd(OAc)2 under an argon atmosphere. With 20 mol % of additive.
c
d
e
With 10 mol % of additive. Base/acid = 1.1:1. Isolated yields.
f
g
h
Isopropylbenzene as major product. Recovering of 1b. With 5 mol
i
% of Pd(OAc)2. With 1 mol % of Pd(OAc)2.
As we expected, an additional substituent (methyl or fluoro) at
the 2- or 3-position of the 2,5-cyclohexadiene-1-carboxylic acid
was well tolerated, and reactions proceeded with excellent
regioselectivity to deliver 3e−3h in 60−83% isolated yields. This
finding is consistent with the decarboxylative γ-arylation of 2,5-
cyclohexadiene-1-carboxylic acids reported by Studer.14a As for
the 4-methyl 2,5-cyclohexadiene-1-carboxylic acid substrate,
compound 3i was obtained and isolated with its isomers in
53% combined yields. However, 4-isopropyl or 4-tert-butyl 2,5-
cyclohexadiene-1-carboxylic acids did not work with this process
due to the large steric effect. It is worth noting that undesired
double bond isomerization products, which are commonly found
as side products in Heck reactions, were not observed using this
procedure.
entry 1). Next, a variety of mono- or bidentate phosphine ligands
were examined. The decarboxylative rearomatization of 1b
occurred in the presence of PPh3 or Dppe formed isopropylben-
zene as the major product, instead of forming 3b (entries 2 and
3). Using Buchwald ligands, Sphos and XantPhos, poor reactivity
was observed and resulted in the recovery of 1b (entries 4 and 5).
Other bases, such as tBuOK, tBuONa, K2CO3, or Ag2CO3 were
also tested and either were found to be ineffective or resulted in
the formation of isopropylbenzene (entries 6−9). Upon
increasing the amount of 2a to 1.5 equiv, the reaction yield
increased to 59% (entry 10). Interestingly, the reaction provided
the γ-olefination product in 87% yield when increasing the
amount of 1b from 1.1 to 1.25 equiv (entry 11), and decreasing
the amount of 2a from 1.1 to 1. Moreover, decreasing the catalyst
loading to 5 mol % still gave a comparable result (entry 12). We
further examined the reaction with a 1 mol % of Pd(OAc)2
Substituted (E)-β-bromostyrenes were also examined using
these reaction conditions. Para-substituents with hydrogen,
benzyloxy, chloro, or nitro groups were well tolerated, delivering
3j−m in high yields. Using para-, meta-, and ortho-methoxy (E)-
B
DOI: 10.1021/acs.orglett.8b00486
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
β-bromostyrenes, the reaction proceeded with good yields (3n−
p). In the case of methylene methyl ester substituted 2,5-
cyclohexadiene-1-carboxylic acid or substituted 1,4-dihydro-1-
naohthoic acids, treatment of para-(E)-β-iodostyrene instead of
para-(E)-β-bromostyrene gave satisfying results (3q−s). The
cyclic substrate, 1-bromocyclohexene, also worked well with this
procedure, affording 3t in 85% yield.
in THF (Scheme 4; see the optimization studies of Lewis acids in
the SI). As a result, the corresponding bicyclo[2.2.2]octadienes
6a and 6b were obtained in 67% and 55% yields, respectively.
Scheme 4. Application of Pd-Catalyzed Decarboxylative γ-
Olefination Products in the Preparation of
ab
,
Bicyclo[2.2.2]octadienes
Meta-substituted stilbenes are an important class of com-
pounds and are widely found in many natural products,
pharmaceuticals, and functional organic π-materials.1f,g,17
Recently, alternative methods in the preparation of meta-
substituted vinylarenes by the strategies of template directing
groups (Yu,18 Maiti,19 Li20) or traceless directing groups
(Miura,21 Gooβen,22 Ackermann,23 Zhao24) have been disclosed
in the area of arene C−H functionalization. However, a large
template directing group is needed to be preformed, and the
expensive catalyst systems are indispensable. Therefore, the
development of an efficient protocol for the synthesis of meta-
substituted stilbenes is highly desired.
To show the synthetic utility of the γ-olefination products
produced above in Scheme 2, we subjected the olefinated 1,3-
cyclohexadienes 3 to a dichloromethane solution of DDQ (2,3-
dichloro-5,6-dicyano-1,4-benzoquinone, 1.5 equiv) at room
temperature. The corresponding oxidative rearomatization
products 4 were obtained in good to excellent yields (Scheme
3). With this established protocol, meta-alkylated stilbenes with
a
Reaction was performed with 3 (0.2 mmol), 5 (0.6 mmol) and
b
Cu(OTf)2 (0.02 mmol) in 1 mL of tetrahydrofuran. Isolated yields.
On the basis of our experiment results, a plausible mechanism
for this Pd-catalyzed decarboxylative γ-olefination reaction was
proposed and is illustrated in Scheme 5. First, the Pd(OAc)2
Scheme 5. Proposed Mechanism for the Pd-Catalyzed
Decarboxylative γ-Olefination
Scheme 3. Application of Pd-Catalyzed Decarboxylative γ-
Olefination Products in the Preparation of Meta-Alkylated
a b
,
Vinylarenes
reduced to active Pd(0)-species in the presence of 2,5-
cyclohexadiene-1-carboxylic acids 1, followed by oxidative
addition with vinyl halides 2 affording Pd(II)-species A, which
then undergoes ligand exchange with the Cs-carboxylate of 1 to
provide Pd(II)-carboxylate species B. Decarboxylation of B
provides 2,5-cyclohexadienyl Pd(II)-species C. Next, 1,3-Pd
migration to the γ-position affords the thermodynamically stable
2,4-cyclohexadienyl Pd(II)-species D. Interestingly, the 1,3-Pd
migration process to the sterically hindrance position is
prohibited due to the large steric effect of the methyl group at
the 2-position. Finally, the intermediate D undergoes reductive
elimination providing olefinated 1,3-cyclohexadienes 3 and
regenerating the Pd(0)-species to complete the catalytic cycle.
In summary, we have developed a Pd-catalyzed decarboxylative
γ-olefination of 2,5-cyclohexadiene-1-carboxylic acids with vinyl
bromides or vinyl iodides. This method most likely proceeds
through a Pd(0)/Pd(II) catalytic cycle. We have also
demonstrated that the synthesized olefinated 1,3-cyclohexa-
dienes can be rearomatized to meta-alkylated stilbenes or can
undergo the Diels−Alder reaction to construct bicyclo[2.2.2]-
octadiene frameworks. The corresponding γ-olefination products
a
Reaction was performed with 3 (0.1 mmol), and DDQ (0.15 mmol)
b
in 2 mL of dichloromethane. Isolated yields.
highly diverse structures and specific double bond stereo-
chemistry can be rapidly accessed from readily available benzoic
acids derivatives in three steps.
Bicyclo[2.2.2]octadiene frameworks are interesting and
versatile building blocks in organic synthesis.25 In order to
incorporate these frameworks into our γ-olefination system, we
performed Diels−Alder reactions of olefinated 1,3-cyclohex-
adiene 3b or 3j with dienophile 5 (dimethyl acetylene-
dicarboxylate) in the presence of catalytic amounts of Cu(OTf)2
C
DOI: 10.1021/acs.orglett.8b00486
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b00486.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: cmchou@nuk.edu.tw.
ORCID
Chih-Ming Chou: 0000-0002-9894-5365
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully thank the National University of Kaohsiung and the
Ministry of Science and Technology of Taiwan (MOST 104-
2113-M-390-004-MY2 and MOST 106-2113-M-390-001-MY2)
for their financial support.
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DOI: 10.1021/acs.orglett.8b00486
Org. Lett. XXXX, XXX, XXX−XXX