Selective Palladium-Catalyzed Allenic C?H Bond Oxidation for the Synthesis of [3]Dendralenes

Article abstract of DOI:10.1002/anie.201706211

A highly selective palladium-catalyzed allenic C?H bond oxidation was developed, and it provides a novel and straightforward synthesis of [3]dendralene derivatives. A variety of [3]dendralenes with diverse substitution patterns are accessible with good efficiency and high stereoselectivity. The reaction tolerates a broad substrate scope containing various functional groups on the allene moiety, including ketone, aldehyde, ester, and phenyl groups. Also, a wide range of olefins with both electron-donating and electron-withdrawing aryls, acrylate, sulfone, and phosphonate groups are tolerated.

Full text of DOI:10.1002/anie.201706211

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Accepted Article
Title: Highly Selective Palladium-Catalyzed Allenic C-H Bond Oxidation
for Synthesis of [3]Dendralenes
Authors: Jan-E. Bäckvall, Youai Qiu, and Daniels Posevins
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201706211
Angew. Chem. 10.1002/ange.201706211
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10.1002/anie.201706211
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Highly Selective Palladium-Catalyzed Allenic C–H Bond Oxidation
for Synthesis of [3]Dendralenes
Youai Qiu+,* Daniels Posevins+, and Jan-E. Bäckvall*
Abstract: A highly selective palladium-catalyzed allenic C–H bond
oxidation was developed, which provides
a
novel and
straightforward synthesis of [3]dendralene derivatives. A variety of
[3]dendralenes with diverse substitution patterns are accessible with
good efficiency and high stereoselectivity. The reaction tolerates a
broad substrate scope containing various functional groups on the
allene moiety, including ketone, aldehyde, ester and phenyl groups.
Also, a wide range of olefins with both electron-donating and
electron-withdrawing aryls, acrylate, sulfone and phosphonate
groups are tolerated.
alladium-catalyzed direct C–H functionalization with
various directing groups has provided a straightforward
P
approach for selective formation of carbon-carbon and
carbon-heteroatom bonds in recent years.[1] Compared to the
widely developed transition-metal catalyzed C–H functiona-
lization with the assistance of specialized directing groups, in the
absence of assisting/directing groups, the reaction outcome is
mainly determined by intrinsic reactivity of the substrate. In the
palladium-catalyzed allylic C–H acetoxylation reaction, early
studies on cyclic alkenes by Åkermark[2] and our group[3] showed
that the reaction proceeds via an π-allylpalladium(II) inter-
mediate generated through allylic C–H bond cleavage. White et
al. has more recently reported on Pd(II)-catalyzed allylic C–H
oxidation of acyclic olefins.[4] The addition of sulfoxide ligands
promoted the allylic C–H acetoxylation via the π-allyl-
palladium(II) intermediate Int-B (Scheme 1a).
On the basis of these observations and our group’s long-
term interest in palladium-catalyzed oxidation of allenes bearing
assisting π-bond moieties[5-8] to construct carbocyclic
skeletons,[9,10] we were particularly interested in palladium-
catalyzed selective allenic C–H oxidation of allene 1a in the
absence of conventional directing groups (Scheme 1b). We
envisioned that π-allylpalladium(II) intermediate Int-D may be
generated through selective allenic C–H bond cleavage,
followed by rearrangement to vinylpalladium intermediate Int-E
that would undergo subsequent intermolecular olefin insertion[11]
to afford [3]dendralene derivatives 3.[12] The challenge would be
the control of the involved regio- and stereoselectivity in the
absence of directing groups. [3]Dendralenes or cross-
conjugated trienes have attracted the attention of organic
chemists, due to their unique role in polymer and synthetic
Scheme 1. Palladium-catalyzed selective allylic and allenic C–H oxidation.
chemistry, as well as their occurrence in various natural
products.[12,13] A noticeable breakthrough in practical synthesis of
dendranlenes was reported by the Sherburn group in 2009.[14]
.
After that, a handful of elegant procedures available for the
synthesis of dendralenes were developed by their group and
other groups.[15,16]
Our study began with the palladium-catalyzed reaction of
readily accessible allene 1a[17] with butyl acrylate 2a using BQ
(p-benzoquinone) as the oxidant in toluene at 80 oC for 19 h
(Scheme 2). To our delight, [3]dendralene (2E,5E)-3aa was
obtained in 12% yield with high regio- and stereoselectivity along
with dimer 4a in 10% yield. To the best of our knowledge, to
date there have been no reports on efficient synthesis of
functionalized [3]dendralenes that would involve palladium-
catalyzed allenic C–H bond oxidation.
Scheme 2. Initial attempt for formation of [3]dendralene 3aa.
With these inspiring results in hand, we set out to optimize
the reaction conditions for the selective formation of 3aa (Table
1). Catalyst screening showed that Pd(OAc)2 was still the best
catalyst for this transformation (Table 1, entries 1-5). Solvent
screening revealed that the yield of 3aa could be increased to
23% by the use of CH3CN as solvent (Table 1, entries 6-9). The
favored formation of 3aa in CH3CN is probably due to
[*]
[+]
Dr. Y, Qiu, D. Posevins, Prof. Dr. J.-E. Bäckvall
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, SE-10691, Stockholm, Sweden
E-mail: youai.qiu@organ.su.se; jeb@organ.su.se
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201706211
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coordination of CH3CN to palladium, which could stabilize the
key vinylpalladium intermediate. Based on this observation, we
envisioned that a more strongly coordinating solvent might favor
the formation of product 3aa. The yield of 3aa was improved to
40% by using DMSO as solvent (Table 1, entry 10). The yield
was further increased to 45%, when the reaction was conducted
at room temperature (Table 1, entry 11). The oxidant also plays
a crucial role in the selective Pd-catalyzed olefination (Table 1,
entries 12-15). The use of 2,6-dimethyl-BQ enhanced the
formation of product 3aa (78% yield) and inhibited the formation
of the dimeric byproduct 4a (Table 1, entry 14).
conditions, which shows that electron-donating groups on the
allene moiety are crucial for selective allenic C–H bond oxidation.
Table 1. Optimization of the reaction conditions.[a]
Entry
Catalyst
Solvent
Yield of
Yield of
Recovery of
3aa (%)[b]
4a (%)[b]
1a (%)[b]
Scheme 3. Scope for formation of 3. [a] The reaction was conducted in DMSO
at room temperature using 1 (0.2 mmol), 2 (1.5 equiv), 2,6-dimethyl-BQ (1.1
equiv) in the presence of Pd(OAc)2 (5 mol%).[b] all of the starting material was
consumed.
1
2
3
Pd(OAc)2
Pd(TFA)2
Pd(OPiv)2
toluene
toluene
toluene
12
11
10
10
10
16
5
-
-
We next turned our attention to other substituted substrates
(Scheme 4), it is worth noting that the reactions involving
substrates bearing ketone or aldehyde functionalities all
proceeded well and gave the corresponding [3]dendralenes 6 in
good yields. We were delighted to find that the reaction
conditions were also suitable for the olefination reactions of a
readily accessible 3,4-dienoates 5.[18] Aliphatic acrylate
derivatives, n-butyl, methyl, ethyl, and benzyl, furnished the
corresponding products 7aa-7ad in good yields. Useful olefin
coupling partners, such as the phenyl vinyl sulfone and vinyl
phosphonate also were found to react smoothly to afford the
products 7ae and 7af. Styrenes with both electron-donating and
electron-withdrawing groups on the phenyl ring reacted smoothly.
We also examined the aliphatic olefin, such as 1-octene, which
afforded product 7am in 52% yield. Selective C–C bond
formation in the internal position[19] was also observed with
allylbenzene, ethyl vinyl ether, and 2,3-dihydrofuran which
afforded 7an, 7ao, and 7ap, respectively, in good yields.
The reactivity of substrates with different substituents on the
allene moiety was next investigated (Scheme 5). In addition to
two methyl substituents on the allene moiety, two ethyl, and
cyclohexylidene allenes (5b and 5d) also afforded the
corresponding products (7b and 7da) in good yields. It is
noteworthy that the reaction of cyclopentylidene allene 5c
afforded the 7c and 7c’ in 80% and 10% yields, respectively.[20]
7c’ was successfully converted to 7c using copper(II) catalysis
to afford the desired [3]dendralene product 7c in 85% combined
yield (for details, see the Supporting Information). Trisubstituted
allene 5e also underwent the olefination and afforded product 7e
in moderate yield with E/Z value 3/1. The reaction of the
unsymmetrical allene 5f, bearing methyl and phenyl groups, and
5g, bearing methyl and i-Pr groups, afforded 7fa in 82%, 7fb in
80%, and 7g in 91% yield, respectively. To our delight, using
tetrasubstituted allene 5h, the corresponding [3]dendralene
derivative 7h can also be obtained in moderate yield. Styrene,
4
toluene
15
15
-
5
PdCl2
toluene
DCE
-
-
78
18
10
30
17
10
35
81
70
-
6
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
8
15
10
11
25
3
7
i-PrOH
dioxane
MeCN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
8
8
7
9
23
40
45
-
10
11[c]
12[c,d]
13[c,e]
14[c,f]
15[c,g]
3
-
16
78
71
-
2
3
5
[a] The reaction was conducted in the indicated solvent (1 mL) at 80 oC using
1a (0.2 mmol), 2a (1.5 equiv), BQ (1.1 equiv) in the presence of palladium
catalyst (5 mol%). [b] Determined by NMR using anisole as the internal
standard. [c] The reaction was conducted at room temperature. [d] F4-BQ was
used instead of BQ. [e] 2,6-Dimethoxy-BQ was used instead of BQ. [f] 2,6-
Dimethyl-BQ was used instead of BQ. [g] 2,5-Dimethyl-BQ was used instead
of BQ.
Under the optimal reaction conditions, we first studied the
reaction of substrates 1 with different alkene coupling partners
(Scheme 3). Aliphatic acrylate derivatives afforded the
corresponding [3]dendralenes in good yields. In addition to two
methyl substituents on the allene moiety, cyclohexylidene allene
also afforded the corresponding product 3b. Monomethyl-
substituted allene also underwent the olefination and afforded
product 3c in moderate yield with an E/Z value of 3/1. However,
the corresponding [3]dendralenes 3d could not be obtained
using terminal hydrogen-substituted allene (for details, see the
Supporting Information) as a substrate under the standard
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2,6-dimethyl-BQ (1.1 equiv) in the presence of Pd(OAc)2 (5 mol%). [b] 5.0
equivalents of 2 was used.
ethyl vinyl ether, and 2,3-dihydrofuran all reacted smoothly and
afforded 7db, 7dc, and 7dd in good to excellent yields.
A biomimetic oxidation approach with the use of electron-
transfer mediators (ETMs) could decrease the kinetic barrier for
the reoxidation of Pd(0) by O2.[3c,21] Catalytic amounts of oxidant
(2,6-dimethyl-BQ) would be sufficient to realize the desired
transformation. When the reaction of 1a was treated with butyl
acrylate 2a (1.5 equiv), 2,6-dimethyl-BQ (20 mol%), Pd(OAc)2 (5
mol%), and cobalt(salophen) (5 mol%) under an atmosphere of
O2 (balloon), the desired product 3aa was obtained in 76% yield
(eq 1).
In order to understand the mechanism of this transformation,
we performed the reaction with the deuterium-labeled substrate
1a-d6. The expected [3]dendralene product 3aa-d6 was obtained
in 75% yield (Scheme 6a). We also performed the same reaction
in the presence of acetic acid (1.0 equivalent) as an additive (for
details, see the Supporting Information). The product 3aa-d6 was
obtained in 65% yield without any scrambling of the deuterium
content. We also examined the reaction of a malonate-tethered
substrate 1d, but formation of the envisioned product 8 was not
observed (Scheme 6b).These comparative experiments indicate
that the C–H bond cleavage selectively occurs at the benzylic
position.
Scheme 4. Scope for formation of 6 or 7. [a] The reaction was conducted in
DMSO at room temperature using 5 (0.2 mmol), 2 (3.0 equiv), 2,6-dimethyl-BQ
(1.1 equiv) in the presence of Pd(OAc)2 (5 mol%). [b] 5.0 equivalents of 2 was
used. [c] Selectivity of Heck reaction is 30:1 (ratio refers to the amount of the
desired isomer versus all other isomers). [d] Selectivity of Heck reaction is
10:1. [e] Selectivity of Heck reaction is 5:1.
Scheme 6. Comparative experiments.
The observation that there is no deuterium loss in the CD3
groups in the transformation of 1a-d6 to 3aa-d6 rules out a
reaction mechanism that involves allenic C–H bond cleavage of
a methyl group to form a vinyl-Pd intermediate followed by C–C
coupling and rearrangement to give [3]dendralene 3aa. The
ruling out of the latter mechanism was also supported by
deuterium kinetic isotope effect studies.[22] An intermolecular
competition experiment was conducted at room temperature
using a 1:1 mixture of 1a and 1a-d6, which afforded a
competitive KIE of kH/kD = 1.02. Furthermore, parallel kinetic
experiments using 1a and 1a-d6, gave a KIE (kH/kD from initial
rate) value of kH/kD = 1.02.
Scheme 5. Scope of allenes for formation of 7. [a] The reaction was
conducted in DMSO at room temperature using 5 (0.2 mmol), 2 (3.0 equiv),
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Based on the reaction outcome, a possible mechanism for
the palladium-catalyzed oxidative allenic C–H bond oxidation to
afford [3]dendralenes is proposed in Scheme 7. The reaction of
Pd(OAc)2 with allene could give intermediate Int-1. Then π-
allylpalladium(II) intermediate Int-2 may be generated through
selective allenic C–H bond cleavage. Intermediate Int-2 may
transfer to vinylpalladium intermediate Int-3, which could
undergo an intermolecular olefin insertion to form the
intermediate Int-4. Subsequent β-H elimination would produce
the [3]dendralene derivatives.
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Scheme 7. Proposed mechanism.
In conclusion, we have described
a highly selective
approach for formation of [3]dendralene derivatives via
palladium-catalyzed allenic C–H bond oxidation, that involves an
intermolecular C–C bond formation. The reaction is proven to be
highly regio- and stereoselective and proceeds under very mild
conditions tolerating various functional groups. Another
attractive feature of this study is the diverse functionality of
obtained [3]dendralene derivatives, that offer new opportunities
in organic synthesis and materials science. Further studies on
the scope and synthetic application of this newly developed
reaction are currently carried out in our laboratory.
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Acknowledgements
Financial support from the European Research Council (ERC
AdG 247014), the Swedish Research Council (2016-03897), the
Berzelii Center EXSELENT, and the Knut and Alice Wallenberg
Foundation is gratefully acknowledged.
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Keywords: allenic C–H oxidation • [3]dendralene • palladium •
olefination • regio- and stereoselectivity
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10.1002/anie.201706211
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Youai Qiu,* Daniels, Posevins, and Jan-
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Highly Selective Palladium-Catalyzed
Allenic C–H Bond Oxidation for
Synthesis of [3]Dendralenes
A highly selective and straightforward formation of [3]dendralene derivatives via
palladium-catalyzed allenic C–H bond oxidation was developed. The reaction is
highly stereoselective and compatible with numerous functional groups on the
allene substrate and the olefin.
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