Efficient Palladium-Catalyzed Aerobic Arylative Carbocyclization of Enallenynes

Article abstract of DOI:10.1002/anie.201810501

Herein, we communicate a selective and efficient protocol for oxidative arylating carbocyclization of enallenynes using O₂ as the oxidant. The key to success for this aerobic transformation is the application of a specific electron transfer mediator (ETM), a bifunctional catalyst consisting of a metal-macrocycle and quinone moieties. This catalyst significantly facilitates the reoxidation of Pd⁰ to Pd^II under atmospheric pressure of O₂. Diverse functionalized enallenynes react with aryl boronic acids to afford the corresponding cyclic tetraenes in moderate to good yields.

Full text of DOI:10.1002/anie.201810501

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Accepted Article
Title: Efficient Palladium-Catalyzed Aerobic Arylative Carbocyclization
of Enallenynes
Authors: Jan-E. Bäckvall, Jie Liu, Alexander Ricke, and Bin Yang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810501
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10.1002/anie.201810501
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Efficient Palladium-Catalyzed Aerobic Arylative Carbocyclization
of Enallenynes
Jie Liu, Alexander Ricke, Bin Yang,* and Jan-E. Bäckvall*
Dedicated to Prof. Elias J. Corey and Prof. Xiyan Lu on the occasion of their 90th birthday
Abstract: Herein, we communicate a selective and efficient protocol
for oxidative arylating carbocyclization of enallenynes using O2 as
the oxidant. The key to success for this aerobic transformation is the
application of a specific electron transfer mediator (ETM) – a
bifunctional catalyst consisting of a metal-macrocycle and quinone
moieties. This catalyst significantly facilitates the reoxidation of Pd(0)
to Pd(II) under atmospheric pressure of O2. Diverse functionalized
enallenynes react with aryl boronic acids to afford the corresponding
cyclic tetraenes in moderate to good yields.
tems in which a metal-macrocycle and quinones are merged into
one molecule have been reported by our group.[13] These bifunc-
tional catalysts led to an increased efficiency of the electron
transfer compared to that observed for the system with the qui-
none and metal-macrocycle as separate molecules.[13] Based on
these state-of-the-art works, we present work on the use of such
a bifunctional catalyst (Co(salophen)-HQ; HQ = hydroquinone,
Scheme 1b) as an efficient electron transfer mediator in Pd-
catalyzed aerobic oxidative carbocyclizations of enallenynes.
The use of molecular oxygen (O2) in enzyme-catalyzed oxidation
reactions is highly widespread in nature.[1] In the perspective of
synthetic organic chemistry, molecular oxygen is an inexpensive,
abundant and highly atom-efficient oxidant that generates no
toxic byproducts, thus fulfilling the requirements of “green
chemistry”.[2] Over the past decades, palladium-catalyzed
aerobic oxidations have provided the basis for streamlined
conversion of various feedstocks into valuable products.[3]
Illustrative examples include Wacker oxidations,[4] alcohol
oxidations,[5] alkene functionalizations[6] and oxidative C-H
activations.[7] In spite of the significant synthetic progress offered
by these reactions, the relative low catalyst efficiency due to
palladium deactivation still remains challenging in most cases.[8]
This “Oxidation Problem” can be explained by the slow electron
transfer directly between Pd(0) and O2, compared to the rapid
precipitation of palladium black from active palladium species
(Scheme 1a).
Scheme 1. a) The “Oxidation Problem” in palladium-catalyzed aerobic
reactions and the solutions. b) The cobalt-based bifunctional catalyst
(Co(salophen)-HQ) as an electron transfer mediator in this study.
To circumvent the problem of getting Pd black in direct
reoxidation of Pd(0) by O2, considerable attention has been
focused on the development of ancillary air-stable ligands such
as amines, pyridines, sulfoxides, and carbene derivatives that
can inhibit precipitation of Pd black during the catalytic cycle.[9]
On the other hand, inspired by nature, a coupled catalyst system
with electron transfer mediators (ETMs) can facilitate the
transport of the electrons from the reduced palladium catalyst to
O2, thereby increasing the efficiency of the reoxidation of Pd(0)
to Pd(II).[10] After the original work by Bäckvall in the late
1980s,[11] several groups have explored this concept by develop-
ing mild and efficient Pd(II)-catalyzed aerobic oxidative reac-
tions.[12] The key to success for these transformations is the use
of macrocyclic metal complexes and quinones as ETMs under
aerobic conditions. Additionally, improved coupled catalyst sys-
Allenes constitute an important class of synthons in organic
synthesis, which can be applied to construct a variety of
valuable molecules.[14] Our group has a long-standing interest in
Pd-catalyzed C−C bond-forming reactions from allene-
substituted unsaturated compounds, such as enallenes,
allenynes, and bisallenes.[15] In the present work, we report for
the first time an example of Pd-catalyzed oxidative carbocycli-
zation of rationally designed enallenynes. These interesting
molecules contain three different C−C π-bond functionalities
(allene, olefin and alkyne groups), all of which can be potential
reaction sites.[16] However, the presence of these three π sys-
tems provides a challenge concerning control of regioselectivity
in the reaction. As shown in Scheme 2, there are three possible
pathways for the transformation of vinylpalladium species Int-A,
which is generated from an initial allenic C-H cleavage[15] of
enallenyne 1 by Pd(II): (1) According to our previous work,[17] the
close-by olefin insertion of Int-A could lead to a highly strained
cyclobutene complex Int-B, followed by arylation to give a four-
membered carbocycle 3’ (Path I). (2) Intermediate Int-A can
also be directly trapped by a nucleophile such as ArB(OH)2 to
give an acyclic arylated product 3’’ (Path II).[18] (3) A ligand
exchange of the olefin for alkyne in Int-A would give Int-C,
which on subsequent alkyne insertion can generate six-
[a]
Dr. J. Liu, A. Ricke, Dr. B. Yang and Prof. Dr. J.-E. Bäckvall
Department of Organic Chemistry, Arrhenius Laboratory,
Stockholm University, SE-10691 Stockholm, Sweden
E-mail: binyang@organ.su.se
jeb@organ.su.se
Homepage: http://www.organ.su.se/jeb/
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Table 1. Evaluation of different electron transfer mediators (ETMs) for
oxidative carbocyclization of enallenyne 1a.[a]
membered ring species Int-D. Trapping of Int-D by ArB(OH)2
would lead to cyclic tetraene 3 (Path III).
Yield of
Recovery
Entry
ETM1 (10 mol%)
ETM2 (20 mol%)
3a [%]
of 1a [%]
1
2
-
-
0
93
70
64
55
56
8
-
BQ
BQ
BQ
BQ
BQ
BQ
HQ
-
15
24
23
21
71
74
63
79
76
3
VO(acac)2
Fe(Pc)
4
5
Co(Pc)
6
Co(salen)
7
Co(salophen)
Co(salophen)
Co(salophen)-HQ
Co(salophen)-HQ
6
8
18
<1
3
Scheme 2. Possible pathways for palladium-catalyzed oxidative
functionalization (arylation) of enallenynes.
9[b]
10[c]
-
With these possibilities in mind, our goal was to develop a
general and efficient catalyst system for oxidative
functionalization of enallenynes in a selective manner. More
specifically, in light of recent growing interest in green and
sustainable chemistry, we have undertaken preliminary
investigations to use molecular oxygen as terminal oxidant.
At the outset of our investigations, the palladium-catalyzed
aerobic oxidation of a readily accessible enallenyne 1a and
phenylboronic acid 2a in H2O/acetone was chosen as the
benchmark reaction (Table 1).[19] In the absence of ETMs, we
did not observe any formation of the arylated product under
aerobic conditions and the starting material 1a could be
recovered in 93% yield (entry 1). When catalytic amounts of BQ
[a] Unless otherwise noted, the following reaction condition were employed
with 1a (0.1 mmol, 1.0 equiv.), 2a (0.15 mmol, 1.5 equiv.), Pd(OAc)2 (5
mol%), ETM1 (10 mol%), ETM2 (20 mol%), H2O (1.0 equiv.) in 0.1 M
acetone, O2 (1 atm) at room temperature (23 oC) for 48 h. Yield and
conversion are determined by 1H NMR using anisole as internal standard.
[b] 10 mol% Co(salophen)-HQ was added. [c] 5 mol% Co(salophen)-HQ with
2 mol% Pd(OAc)2 was added.
(p-benzoquinone) was added,
a highly unsaturated six-
membered carbocycle 3a was selectively formed albeit in low
yield (15%, entry 2).[20] It is noteworthy that this unexpected
cyclization takes place between the allene moiety and distal
triple bond of enallenyne 1a, while the olefin group remains
intact.[21] This interesting result motivated us to further explore
various established metal-based electron transfer mediators to
improve the reaction efficiency. The application of VO(acac)2,
Fe(Pc), and Co(Pc) (Pc = phthalocyanine) as ETMs did not lead
to any significant improvement of the transformation of 1a to 3a.
(21%−24% yields, entries 3−5). To our delight, the use of cobalt-
based salen and salophen complexes led to good yields of 3a
(71 and 74% yields, entries 6 and 7). As to the reduced state of
BQ, HQ (hydroquinone) also furnished the cyclization product 3a
in 63% yield (entry 8). Interestingly, a bifunctional cobalt catalyst
(Co(salophen)-HQ), combining
a
metal-macrocycle and
quinones moieties, was found to be the best performing catalyst
and afforded the desired product 3a in 79% yield (entry 9).
Notably, even under lower catalyst loading (2 mol% Pd(OAc)2
and 5 mol% Co(salophen)-HQ), no significant decrease in yield
was observed (76% yield, entry 10 vs. 79% yield, entry 9).
Figure 1. Reaction progress with different ETMs.
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phen)-HQ (10 mol%), H2O (1.0 equiv.) in 0.1 M acetone, O2 (1 atm) at room
temperature (23 oC) for 48 h. Isolated yield. [a] Conditions employing 1.1 equiv.
BQ instead of 10 mol% Co(salophen)-HQ under Ar. [b] Reaction time of 72 h.
Next, the catalytic reaction progress with different electron
transfer mediators (ETMs) was examined using 1 mol% of
Pd(OAc)2, 2.5 mol% Co(salophen)-HQ or 2.5 mol%
o
Co(salophen) and 5 mol% quinone, at 30 C under 1 atm of air.
As shown in Figure 1, the use of Co(salophen)-HQ resulted in a
faster reaction rate than that with Co(salophen) and quinone as
separate ETMs. This result indicates that the intramolecular
electron transfer between the hydroquinone unit and the
oxidized metal-macrocycle of this bifunctional catalyst leads to a
more efficient overall reaction under aerobic conditions.[22]
We next investigated the reactivity of structurally diverse
enallenynes 1 in this aerobic arylating carbocyclization (Scheme
4). Various substitutions on the allene moiety of 1, such as
phenyl (3j), cyclopentyl (3k) and cyclohexyl (3l) groups were
well tolerated, furnishing products in 61%−81% yields. Moreover,
functional groups on the alkyne moiety, bearing −OAc (3m),
−OBn (3n), and −CN (3o) were compatible with the aerobic
conditions and gave the corresponding carbocycles in moderate
to good yields (65%−79%). Under the optimized aerobic
condition, cyclohexylacetylene-substituted enallenyne substrate
1p afforded 3p in only 36% yield. We attribute this diminished
reactivity to the increased steric bulk of the alkyne moiety. In
addition to an ester group, enallenynes containing butyl (3q),
benzyl (3r), hydroxyl (3s), silyl (3t), sulfonamide (3u), and imide
(3v) functionalities underwent this transformation smoothly
highlighting the broad substrate scope of this protocol. Late-
After optimizing the reaction conditions, we continued to
explore the scope of this transformation with various arylboronic
acids (Scheme 3). First, when applying PhB(OH)2 as the aryla-
ting partner, 77% isolated yield of cyclization product 3a was
achieved. Importantly, this aerobic catalytic system appeared to
be equally effective in comparison to the application of
stoichiometric BQ as oxidant (76% yield in Scheme 3), therefore
highlighting the advancement of this efficient and sustainable
method. In addition to PhB(OH)2, arylboronic acids bearing an
electron neutral group (3−Me), donating group (3−OMe) and
withdrawing group (4−Acyl) all reacted well and produced the
corresponding cyclic tetraenes (3b−3d) in good yields (61−77%,
respectively). Various halogen-containing substrates (3−F, 3−Cl,
and 4−Br) were well compatible with this methodology and gave
the desired products (3e−3g) in 54−74% yields. 2-Naphthyl-
boronic acid also reacted smoothly and a moderate yield (53%
of 3h) could be achieved. Moreover, when 4-vinylphenylboronic
acid was used as the substrate, the olefin bond remained intact
and 52% yield of the desired product 3i was obtained. However,
2-thienylboronic acid, 4-pyridinylboronic acid and trans-2-
phenylvinylboronic acid did not give the corresponding tetraenes
under the optimal reaction conditions.
stage oxidative reaction is
a powerful approach for the
streamlining and diversification of complex natural products and
medicinal compounds. Here, an estrone-derived substrate,
participated in this reaction efficiently to afford a useful yield
(75%) of a functionalized complex molecule 3w.
Scheme 3. Substrate scope of different arylboronic acids 2. Reaction condi-
tions: 1a (1.0 equiv.), ArB(OH)2 2 (1.5 equiv.), Pd(OAc)2 (5 mol%), Co(salo-
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Scheme 4. Substrate scope of different enallenyne 1. Reaction conditions:
enallenyne 1 (1.0 equiv.), PhB(OH)2 2a (1.5 equiv.) Pd(OAc)2 (5 mol%),
Co(salophen)-HQ (10 mol%), H2O (1.0 equiv.) in 0.1 M acetone, O2 (1 atm) at
room temperature (23 oC) for 48 h. Isolated yield.
of Int-IV by alkyne insertion gives a cyclic vinylpalladium Int-V.
Transmetallation of Int-V with arylboronic acid 2,[26] followed by
reductive elimination provides the target product 3 and Pd(0)
species Int-VI, respectively. Finally, with the assistance of the
cobalt hybrid catalyst as electron transfer mediator, aerobic
oxidation of Pd(0) regenerates Pd(II) to close the catalytic
cycle.[27]
Furthermore, to demonstrate the necessity of the olefin
group (X = C=CH2) in substrate 1, control experiments were
performed with substrates lacking the olefin group (Table 2). At
first, allenyne 1m’, without the olefin group in the β-position (X =
CH2) of the allene moiety, was applied under the standard
reaction conditions. Notably, the six-membered carbocycle 3m’
was not observed (entry 2 vs. entry 1). Allenynes containing
heteroatom linkers such as O (1x) and NTs (1y) were evaluated
in this aerobic arylative carbocyclization; however, neither of
them was found to be effective (entries 3 and 4). In addition to
an olefin group, we recently showed that a hydroxyl functionality,
can trigger allene attack by weak coordination to the Pd(II)
center, enabling efficient oxidative carbocyclizations of
enallenols.[23] We therefore examined the reactivity of an
allenynol 1z, with a hydroxyl group at the β-position of the allene
moiety. However, the desired oxidative carbocyclization did not
take place in this case (entry 5). These experiments show that
the initial step of allenic C(sp3)−H cleavage by Pd(II) requires the
coordination of a pending olefin bond.[18] A recent computational
study by the Liu group supports that the assisting olefin group
plays an indispensable role in the formation of the vinyl-
palladium intermediate.[24]
Scheme 5. Proposed mechanism.
In summary, we have developed an efficient Pd(II)-catalyzed
oxidative carbocyclization of enallenynes using molecular
oxygen as the oxidant. By applying a bifunctional catalyst
(Co(salophen)-HQ) as an efficient electron transfer mediator, a
wide range of enallenynes and arylboronic acids were
transformed into the corresponding six-membered carbocycles
in moderate to good yields. In view of the reaction efficiency and
good selectivity, this protocol is expected to complement the
current approach for oxidative functionalization in homogeneous
catalysis and organic synthesis.
Table 2. Investigation of different linkers in allenynes for Pd-catalyzed
aerobic arylating carbocyclization.[a]
Entry
Substrate 1
Product 3
Yield of 3 [%]
1m, X = C=CH2
1m’, X = CH2,
1x, X = O,
1
2
3
4
3m
3m’
3x
65
0
Acknowledgments
0
We are grateful for the financial support from European
Research Council (ERC AdG 247014), Swedish Research
Council (2016-03897, 2018-00830), and Carl Tryggers
Foundation. We thank Dr. Zoltán Bacsik for the characterization
of catalyst.
1y, X = NTs,
3y
0
5
1z, X = CH-OH
3z
0
[a] Reaction conditions: allene 1 (1.0 equiv.), PhB(OH)2 2a (1.5 equiv.),
Pd(OAc)2 (5 mol%), Co(salophen)-HQ (10 mol%), H2O (1.0 equiv.) in 0.1 M
acetone, O2 (1 atm) at room temperature (23 oC) for 48 h.
Keywords: palladium • aerobic oxidation • carbocyclization •
enallenyne • electron transfer mediator •
On the basis of these experimental findings, the mechanism
given in Scheme 5 is proposed for this oxidative arylating
carbocyclization. Initially, the coordination of allene and olefin
units to the Pd(II) center leads to a chelate palladium complex
Int-II. This special coordination of the close-by olefin to Pd(II) is
essential for triggering the allenic C(sp3)−H cleavage and
generating a vinylpalladium intermediate Int-III.[18] Next, instead
of direct arylation to give 3’’ or olefin insertion to form a
cyclobutene complex Int-B (vide supra, Scheme 2), the
envisioned ligand exchange of olefin[25] by the distant alkyne
moiety takes place to give Int-IV. Subsequent carbocyclization
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COMMUNICATION
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equilibrium toward the formation of PhB(OH)2. Please see the
Supporting Information for details.
[8]
[9]
[20] The side product 3a’ and 3a’’ in Scheme 2 were not obtained. For a
discussion on the selectivity, please see the Supporting Information.
[21] The isomer of arene from aromatization of tetraene 3a was not
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[26] In the absence of ArB(OH)2, no product was obtained. The Int-V could
not be detected because a fast trapping of Int-V by ArB(OH)2 occurs
and generates the corresponding cyclic tetraene.
[27] In addition to facilitate electron transfer from the reduced palladium
catalyst to O2, we speculate the quinone moiety of the oxidized
Co(salophen)-HQ can act as a ligand that coordinates to the Pd(II)
intermediate during the catalysis. Quinone coordination to Pd(II) could
withdraw electron density from the Pd(II) center, therefore promoting
the subsequent reductive elimination step.
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10.1002/anie.201810501
Angewandte Chemie International Edition
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Jie Liu, Alexander Ricke, Bin Yang,* and
Jan-E. Bäckvall*
Page No. – Page No.
Efficient Palladium-Catalyzed Aerobic
Arylative Carbocyclization of
Enallenynes
Co catalyst as co-catalyst: We report the first example of a Pd(II)-catalyzed
oxidative carbocyclization of the rational designed enallenynes using molecular
oxygen. By applying a bifunctional catalyst (Co(salophen)-HQ) as an efficient
electron transfer mediator, enallenynes are effectively arylated to the corresponding
six-membered carbocycles in good yields.
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