Synthesis of conjugated dienes via a biomimetic aerobic oxidative coupling of Two Cvinyl-H bonds

Article abstract of DOI:10.1002/chem.201301771

A time to dienes: A highly efficient and general method has been developed to prepare conjugated dienes through a biomimetic approach. This aerobic oxidative coupling, involving two C_vinyl-H bonds, proceeds under low Pd catalyst loading and emp

Full text of DOI:10.1002/chem.201301771

COMMUNICATION
DOI: 10.1002/chem.201301771
Synthesis of Conjugated Dienes via a Biomimetic Aerobic Oxidative
À
Coupling of Two Cvinyl H Bonds
Nicolas Gigant and Jan-E. Bꢀckvall*[a]
The development of green, mild and efficient methods for
the synthesis of small but complex molecular frameworks is
at the heart of modern organic chemistry.[1] During the past
be accomplished only under less favorable reaction condi-
tions. These reaction conditions require high catalyst loading
(10–20 mol%), stoichiometric or almost often over-stoichio-
metric amounts of various non-environmentally friendly ad-
ditives (copper(II), silver(I), peroxides, etc.) and sacrificial
oxidant other than simple and ideal molecular oxygen.[5] In
addition, the reactions are frequently limited to a restricted
substrate scope. Therefore, a more general and compatible
protocol to achieve these transformations would be desira-
ble.
Driven by the wish to elaborate either new environmen-
tally friendly or general synthetic routes to synthesize these
derivatives and guided by our previous results,[10,11] we were
particularly interested in developing an oxidative olefin–
olefin coupling to conjugated dienes through a biomimetic
approach (Scheme 1). On the basis of this concept, biomi-
À
decade, metal-catalyzed C C bond formation via activation
À
of two C H bonds has appeared as a powerful tool for the
functionalization of various organic molecules, not only for
the development of new methodologies, but also for the
total synthesis of natural products.[2] This strategy, common-
ly named “dehydrogenative coupling”, does not require the
traditional pre-functionalization step, reducing the number
of side-products and avoiding the amount of waste by
losing, in theory, simple hydrogen gas. The direct Pd-cata-
lyzed coupling of arenes and alkenes, known as the Fuji-
wara–Moritani reaction,[3] has been the topic of numerous
reports during the past decade.[4] On the other hand the
direct dehydrogenative coupling of two alkenes involving
À
cleavage of two Cvinyl H bonds
has only recently attracted at-
tention.[5,6]
Conjugated dienes and poly-
enes are structural elements
that occur in many natural
products and they are common
in biological chemistry and ma-
terials science.[7] In addition,
conjugated dienes are involved
in a number of important chem-
ical transformations, making
them versatile building blocks
in organic synthesis.[8] Usually,
conjugated dienes are either Scheme 1. Comparison of a) direct reoxidation by O2 and b) reoxidation via low-energy electron transfer with
the use of ETMs.
prepared by metal-catalyzed
cross-coupling reactions starting
from the corresponding pre-functionalized substrates or by
Wittig and similar olefinations.[9] Recently, a number of
metal-catalyzed dehydrogenative methods for olefin–olefin
coupling have been reported.[5,6] However, due to the unfav-
metic processes occur through low-energy electron transfer,
in which a number of catalytic electron-transfer mediators
(ETMs) are engaged.[10] The reduced catalyst can be easily
reoxidized to its active form by the use of air (O2). Com-
pared to “traditional” oxidation, the reoxidation step is
often more complicated due to the high energy barrier gen-
erally associated with the direct reoxidation of the catalyst
by O2 (Scheme 1a). The use of ETMs will lower this barrier
(Scheme 1b). Here, we report our results regarding the
À
orable activation of the C H bond and the complicated re-
generation of the active catalyst, these transformations can
[a] Dr. N. Gigant, Prof. Dr. J.-E. Bꢀckvall
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, 106 91 Stockholm (Sweden)
Fax : (+46)8-154-908
À
aerobic oxidative coupling of two Cvinyl H bonds through a
biomimetic approach.
E-mail: jeb@organ.su.se
We first selected a-methylstyrene 1a (5 equiv) and n-
butyl acrylate 2a (1 equiv) as substrates to test the viability
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301771.
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Table 1. Optimization of the biomimetic aerobic oxidative coupling between a-meth-
ylstyrene 1a and n-butyl acrylate 2a.[a]
The applicability of this methodology to a wide
range of substrates is shown in Table 2. We specu-
lated that the biomimetic coupling could be easily
performed with more electron-rich substrates,
giving us the opportunity to decrease the loading of
PdACTHNUTRGNEUNG
(OAc)2 and ETMs.[12] We subsequently per-
Entry 1a
[equiv]
2a
[equiv]
Cat. [Pd]
[mol%]
Oxidant
[mol%]
Fe(Pc) Solvent
ACHTUNGTRNE[NUNG mol%]
Yield
[%][b]
G
G
G
U
formed additional experiments with protected
glycal 1b and found a second set of optimal condi-
tions. Thus, 3b was isolated in a good 76% yield
1
5
5
5
5
5
2
2
1
2
2
2
2
2
1
1
1
1
1
1
1
3
1
1
1
1
1
Pd(OAc)2
(5)
Pd(OAc)2
(5)
Pd(OAc)2
(5)
Pd(TFA)2
(5)
Pd(OAc)2
(2.5)
Pd(OAc)2
(6)
Pd(OAc)2
(5)
Pd(OAc)2
(5)
Pd(OAc)2
(5)
Pd(OAc)2
(5)
Pd(OAc)2
(5)
Pd(OAc)2
(5)
Pd(OAc)2
(5)
N
BQ (20)
BQ (20)
BQ (20)
BQ (20)
BQ (20)
BQ (20)
BQ (20)
BQ (20)
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
AcOH
EtCOOH
PivOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
56
2
60
(messy)
0
with only 2.5 mol% of PdACHTNUTRGNENG(U OAc)2, 5 mol% of p-ben-
zoquinone (BQ) and 1 mol% of iron phthalocya-
nine (Fe(Pc)) in acetic acid/dimethylacetamide (1:1)
at 708C under O2 for 24 h (Method B). Also, dihy-
dropyrane derivative 3c was obtained in 66% yield
under these reaction conditions (Method B). Func-
tionalized naphthalene 1d as well as indene 1e
were suitable coupling partners with Method A,
giving the desired conjugated dienes in 52 and 67%
yield, respectively. Unfortunately, methylenecyclo-
hexane 1 f, only gave a low yield of coupling prod-
uct with 2b.
To enlarge the molecular diversity, the reactivity
of five nitrogen-containing substrates was investi-
gated (Table 2, entries 7–11). To our delight, enam-
ides 1g, 1h, and protected uracil 1i afforded good
yields of the corresponding coupling products 3g,
3h, and 3l under standard optimized conditions
(Method B). On the other hand, 1-methyl-2-pyri-
done 1j afforded 3j only in 16% yield due to the
slow conversion of the starting material. However,
the use of pivalic acid instead of acetic acid, provid-
ed 3j in a 53% yield. Pivalic acid has been recently
employed in many reactions as a proton shuttle,
and its better reactivity in comparison to acetic acid
3
4
23
17
53
56
27
35
28
32
49
5
6
7
8
9
2,6-DMBQ 2.5
(20)
chloranil
(20)
10
11
12
13
2.5
2.5
5
BQ (10)
BQ (20)
BQ (20)
2.5
AcOH/DMA 19
(1:1)
[a] Reactions were performed at 908C for 24 h. [b] Isolated yields. BQ=p-benzoqui-
none, 2,6-DMBQ=2,6-dimethoxybenzoquinone, Fe(Pc)=iron phthalocyanine,
DMA=dimethylacetamide, Piv=pivalyl.
of the reaction (Table 1). After some initial attempts, Pd-
ACHTUNGTRENNUNG(OAc)2 (5 mol%), p-benzoquinone (BQ; 20 mol%) and
can be explained by its ability to both lower transition-state
energies and increase the basicity of its conjugated base.[13]
4-Enaminone 1k did not furnish a satisfactory result with
any of the methods employed. We next probed the reactivity
of chromone 1l toward AcOH or PivOH as solvent with a
slight difference of reactivity, 45 and 60%, respectively. a-
Oxoketene dithioacetal 1m was easily functionalized into
iron phthalocyanine (Fe(Pc); 2.5 mol%) in the presence of
acetic acid at 908C under O2 (1 atm) for 24 h gave the de-
sired conjugated diene 3a in 56% yield and good E/Z selec-
tivity (84:16; Method A). A similar yield was obtained in
propionic acid, whereas no reaction was observed in pivalic
acid (Table 1, entries 2 and 3). We next focused our atten-
tion on the palladium salt (Table 1, entries 4–6). PdACTHNUTRGNEUNG(OAc)2
was found to be the best catalyst in a satisfying 5 mol%
loading. Decreasing the amount of 1a from 5 to 2 equiv did
not affect the yield of the coupling (Table 1, entry 7). At the
same time, performing the reaction with an excess of acryl-
ate 2a was not efficient (Table 1, entry 8). Two other benzo-
quinone derivatives, chloranil and 2,6-dimethoxybenzoqui-
none, were also investigated (Table 1, entries 9 and 10);
however, only a lower reactivity was observed. A decrease
of the BQ loading or increase of the Fe(Pc) loading had a
negative effect on the yield of the reaction (Table 1, en-
tries 11 and 12). A final attempt was made by decreasing
the acidity of the medium by running the reaction in a 1:1
mixture of AcOH and dimethylacetamide (DMA), but this
resulted in a disappointing yield (Table 1, entry 13).
Figure 1. Scale-up of the biomimetic aerobic oxidative coupling between
substrate 1b and methyl acrylate 2b.
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Synthesis of Conjugated Dienes
COMMUNICATION
Table 2. Scope of the biomimetic aerobic oxidative coupling with ester acrylates.
skeleton 3m. Starting from spi-
rodihydropyran 1n, a low yield
was obtained for the double
coupling due to the low stability
of the starting material in the
reaction medium. Finally, cou-
marin 1o was unreactive under
these biomimetic conditions,
and only trace amounts of the
desired alkynated product 3o
were obtained with all three
methods.
Entry
1
Substrate
Product
Method[a]
A
Yield [%][d]
56 (84:16)[e]
2
B
76
We next probed the scope of
our methodology towards the
reactivity of a broad range of
external alkenes (Table 3). The
reaction between acrylate esters
2a–2c and styrene derivatives
1a or 1q furnished the conju-
gated dienes 3a, 3p–3r in mod-
erate yields and good to high
selectivity. Notably, also acro-
lein, stryrene and acrylonitrile
were good alkenylating partners
with protected glycal 1b. As ex-
pected, allyl acetate 2g was less
reactive and gave an insepara-
ble mixture of conjugated and
unconjugated 3-funtionnalized
pyrane 3v, due to the fact that
two syn-eliminations are possi-
ble during the process. Finally,
both diethyl vinylphosphonate
2h and phenyl vinyl sulfone 2i
were screened with success,
giving the corresponding 5-al-
kynated-2-pyridones 3w and
3x.
We also demonstrated that
the coupling reaction can be
run on a slightly larger prepara-
tive scale. Thus, Method B
(with less solvent) was success-
fully applied to the synthesis of
3b on a 2.0 g scale to give 81%
yield of 3b (Figure 1).
In light of previous studies
regarding the Fujiwara–Morita-
ni reaction, an electrophilic pal-
ladation pathway is envisage-
d.[4a,14] After formation of the
palladated substrate, alkene in-
sertion and subsequent b-H
elimination, the desired coupled
product is generated. In our
case, the key step is the reoxi-
dation of Pd0 to PdII through
3
4
B
66
A
52 (97:03)[e]
5
6
7
A
A
B
67
8
66
8
B
B
63
74
9
B
19
53
B[b]
10
11
B
16
B
45
60
B[b]
12
13
B
C
13
63
14
15
B[c]
16
0
A, B or C
[a] Reaction conditions: Method A: 1 (2 equiv), 2 (1 equiv), Pd
(2.5 mol%) in AcOH (0.5 mLmmolÀ1) for 24 h at 908C. Method B: 1 (1 equiv), 2 (2 equiv), Pd
(2.5 mol%), BQ (5 mol%), Fe(Pc) (1 mol%) in AcOH/DMA (1:1; 1.2–1.5 mLmmolÀ1) for 24 h at 708C.
Method C: 1 (1 equiv), 2 (2 equiv), Pd(OAc)2 (5 mol%), BQ (10 mol%), Fe(Pc) (2.5 mol%) in AcOH/DMA
(1:1; 1.35 mLmmolÀ1) for 24 h at 708C. [b] Reactions performed in PivOH (1.3–1.4 gmmolÀ1) for 36 h at
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
1208C. [c] 1 (1 equiv), 2 (4 equiv) PdACTHNUTRGNE(NUG OAc)2 (5 mol%), BQ (10 mol%), Fe(Pc) (2 mol%) in AcOH/DMA
(1:1; 1.5 mLmmolÀ1) for 24 h at 708C. [d] Isolated yields. [e] E/Z ratio determined by NMR of isolated prod-
uct. PMP=p-methoxyphenyl.
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10801

J.-E. Bꢀckvall and N. Gigant
Table 3. Scope of the biomimetic aerobic oxidative coupling with various olefins.
In summary, we have devel-
oped a biomimetic aerobic oxi-
À
dative coupling of two Cvinyl
H
bonds to the synthesis of conju-
gated dienes. Importantly, pro-
ceeding under relatively low
catalyst loading and using mo-
lecular oxygen as the terminal
oxidant, this methodology toler-
ates not only a wide substrate
scope of important heterocycles
but it is also applicable to the
multigram scale. Moreover, the
study confirms the potential of
the biomimetic coupling as an
efficient protocol for the forma-
Entry
1
Olefin 1
Olefin 2
Product
Method[a]
A
Yield [%][d]
56 (84:16)[e]
54 (93:07)[e]
60 (95:05)[e]
2
A
3
4
A
B
41 (93:07)[e]
À
tion of various C C bonds
69
under environmentally friendly
aerobic conditions. Further syn-
thetic studies are underway in
our laboratory.
5
6
7
C
C
B
57
Experimental Section
50
Aerobic oxidative coupling (Meth-
od B): A Schlenk tube, charged with
PdACTHNUTRGENUG(N OAc)2 (2.5 mol%), p-benzoqui-
43 (86:14)[f]
none (5 mol%), iron phthalocyanine
(1 mol%) and substrate 1 (1 equiv),
was degassed three times under re-
duced pressure before introducing
8
9
B[b]
62
49
oxygen gas with
a balloon. Then,
B[b]
olefin (2 equiv) and AcOH/DMA
2
(1:1, 1.2–1.5 mLmmolÀ1) were added
and the resulting mixture was stirred
in an oil bath at 708C for 24 h. The re-
action mixture was cooled to room
temperature, followed by the addition
of saturated aqueous NaHCO3 and
AcOEt. The organic layer was separat-
ed and the aqueous layer was extract-
ed with AcOEt. The combined organic
[a] Reaction conditions: Method A: see legend of Table 2. Method B: see legend of Table 2. Method C: see
legend of Table 2. [b] Reactions performed in PivOH (1.35 gmmolÀ1) for 36 h at 1208C. [c] 1 (1 equiv), 2
(4 equiv), PdACHTUNGTRENNUNG
(OAc)2 (5 mol%), BQ (10 mol%), Fe(Pc) (2 mol%) in AcOH/DMA (1:1; 1.35 mLmmolÀ1) for
24 h at 708C. [d] Isolated yields. [e] E/Z ratio determined by NMR spectroscopy of isolated product. [f] Conju-
gated/unconjugated ratio determined by H NMR spectroscopy of isolated product.
1
layer was washed with brine, dried over MgSO4, filtered and concentrat-
ed. Products were purified by flash chromatography with hexane/ethyl
acetate to yield the desired conjugated dienes.
the biomimetic approach. Catalytic quantity of p-BQ and
Fe(Pc) as electron-transfer mediators and O2 at atmospheric
pressure are able to regenerate the catalyst in its active
form (Scheme 2).
Acknowledgements
Financial support from the European Research Council (ERC AdG
247014) is gratefully acknowledged.
À
Keywords: aerobic oxidation · biomimetic synthesis · C H
activation · conjugated dienes · cross-coupling
Scheme 2. Plausible mechanism for the regeneration of the catalyst under
its active form by the biomimetic approach.
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Synthesis of Conjugated Dienes
COMMUNICATION
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Received: May 8, 2013
Published online: June 28, 2013
Chem. Eur. J. 2013, 19, 10799 – 10803
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10803