Efficient Palladium-Catalyzed Aerobic Oxidative Carbocyclization to Seven-Membered Heterocycles

Article abstract of DOI:10.1002/chem.202004265

The use of molecular oxygen in palladium-catalyzed oxidation reactions is highly widespread in organic chemistry. However, the direct reoxidation of palladium by O₂ is often kinetically unfavored, thus leading the deactivation of the palladium catalyst during the catalytic cycle. In the present work, we report a highly selective palladium-catalyzed carbocyclization of bisallenes to seven-membered heterocycles under atmospheric pressure of O₂. The use of a homogenous hybrid catalyst (Co(salophen)-HQ, HQ=hydroquinone) significantly promotes efficient electron transfer between the palladium catalyst and O₂ through a low-energy pathway. This aerobic oxidative transformation shows broad substrate scope and functional group compatibility and allowed the preparation of O-containing seven-membered rings in good yields in most cases.

Full text of DOI:10.1002/chem.202004265

Accepted Article
Accepted Article
Title: Efficient Palladium–Catalyzed Aerobic Oxidative Carbocyclization
to Seven–Membered Heterocycles
Authors: Jan-E. Bäckvall and Jie Liu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
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To be cited as: Chem. Eur. J. 10.1002/chem.202004265
Link to VoR: https://doi.org/10.1002/chem.202004265
01/2020

10.1002/chem.202004265
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Efficient Palladium–Catalyzed Aerobic Oxidative Carbocyclization
to Seven–Membered Heterocycles
Jie Liu*, and Jan-E. Bäckvall*
Dedicated to Professor Ei-ichi Negishi on the occasion of his 85th birthday
Abstract: The use of molecular oxygen in palladium–catalyzed
oxidation reactions is highly widespread in organic chemistry.
However, the direct reoxidation of palladium by O2 is often kinetically
unfavored, thus leading the deactivation of the palladium catalyst
during the catalytic cycle. In the present work, we report a highly
selective palladium–catalyzed carbocyclization of bisallenes to
seven–membered heterocycles under atmospheric pressure of O2.
The use of a homogenous hybrid catalyst (Co(salophen)–HQ, HQ =
hydroquinone) significantly promotes efficient electron transfer
between the palladium catalyst and O2 through a low-energy
pathway. This aerobic oxidative transformation shows broad
substrate scope and functional group compatibility and allowed the
preparation of O–containing seven–membered rings in good yields
in most cases.
Jiang and co–workers recently developed an elegant protocol
that utilizes an efficient metal–organic framework (MOF) for the
stabilization
of
the
palladium
catalyst
in
aerobic
functionalizations (Scheme 1b).[6] Although
a
plethora of
strategies to circumvent this oxidation problem have been
reported, the development of highly active catalytic systems that
enable mild Pd-catalyzed aerobic oxidations continues to be an
important topic in this area.
Introduction
Over the past decades, palladium-catalyzed oxidations have
emerged as powerful and valuable tools in modern organic
synthesis.[1] Various protocols have been developed for the
assembly of carbon–carbon and carbon–heteroatom bonds that
provide useful applications in biology, medicine, and materials
science.[2] However, in most cases the use of stoichiometric
oxidants such as Cu(II), Ag(I), and peroxides often leads to poor
selectivity, low atom economy, and considerable amounts of
undesired waste. In the perspective of synthetic organic
chemistry, molecular oxygen is an inexpensive, abundant, and
highly atom-efficient oxidant, which does not generate any toxic
byproducts, thus fulfilling the requirements of “green
chemistry”.[3] Despite significant advances in palladium-
catalyzed aerobic oxidations, a severe problem is that fast
aggregation of palladium black from the active palladium species
(Pd–H or Pd0) slows down and finally stops the homogenous
reaction.[4] Extensive endeavors for solving this problem have
focused on the use of air–stable ligands to restrain Pd0
precipitation during the catalytic cycle (Scheme 1a).[5] Moreover,
Scheme 1. Strategies to improve the efficiency of palladium-catalyzed aerobic
oxidative reactions.
Our research group has
a long–standing interest in
palladium–catalyzed oxidations where molecular oxygen is used
as a green oxidant.[7] However, as stated above, the direct
reoxidation of palladium by molecular oxygen is often kinetically
unfavored.[4] To solve this problem, we and others have
employed coupled catalytic systems with electron transfer
mediators (metal–macrocycle and quinone) to facilitate the relay
of electrons between the palladium catalyst and O2 (Scheme
1c).[8] These coupled catalytic systems have been demonstrated
to be highly efficient in Wacker oxidations[8b],[9] alcohol
oxidations[10], oxidative olefin functionalizations[11], and oxidative
C–H activations[12] where molecular oxygen is the oxidant. In
1993, a more efficient hybrid catalyst, involving a cobalt–
porphyrin with pendant hydroquinone groups in one molecule,
was reported by our group as well.[13] Later on, an improved
hybrid Schiff base–hydroquinone [Co(salophen)-HQ] as a redox
relay catalyst for aerobic oxidation was reported.[14] The latter
bifunctional catalyst led to a faster reaction rate compared to the
system with the quinone and metal–macrocycle as separate
molecules in Pd-catalyzed aerobic carbocyclization of
enallenynes and dienallenes.[8h],[15]
[a]
Dr. J. Liu, and Prof. Dr. J.-E. Bäckvall
Department of Organic Chemistry, Arrhenius Laboratory,
Stockholm University, SE-10691 Stockholm, Sweden
E-mail: jieliu@hnu.edu.cn, jeb@organ.su.se
Homepage: http://www.organ.su.se/jeb/
Prof. Dr. J.-E. Bäckvall
Department of Natural Sciences, Mid Sweden University, Holmgatan
10, SE-85170, Sundsvall, Sweden
Dr. J. Liu
[b]
[c]
College of Chemistry and Chemical Engineering, Hunan University,
410082, Changsha, China.
During the past decade, our research group has been
involved in the development of palladium-catalyzed oxidative
carbocyclizations of allenes bearing an additional unsaturated
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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moiety, such as an olefin, alkyne, or allene.[16] Interestingly, we
found that an assisting group (AG), [17] such as an olefin, alkyne
or hydroxyl group, is required to make 6-membered
rings[8e,h],[15],[18] from enallenes and allenynes and to make 7-
membered rings from bisallenes due to the initial allenic C–H
cleavage by Pd(II) (Scheme 2a). Afterward, cascade carbocycli-
zation and transmetallation with a nucleophile can occur and
give the cyclic product. However, without an activating group, no
reaction takes place with the distal π–bond to give the larger
rings. This interesting neighboring group effect allows for
palladium–catalyzed carbocyclization in
a highly selective
manner. We recently reported an efficient approach for the
synthesis of seven–membered rings via olefin–assisted
palladium–catalyzed
carbonylative
carbocyclization
of
bisallenes.[19] However, O–containing seven–membered rings
were not explored in the previous study. Also, there is no
example on the use of oxidative carbocyclization of bisallenes
for the synthesis of useful organoboron molecules.[20] Herein we
report a general and efficient catalytic system for the synthesis
of O–containing seven–membered heterocycles via cascade
borylative carbocyclizations (Scheme 2b). This catalytic system
involves the use of a Pd(II) catalyst with a hybrid ETM catalyst
for the biomimetic aerobic oxidations.
Scheme 3. Synthesis of Schiff base-hydroquinone hybrid catalysts.
At the outset of our investigations, the palladium-catalyzed
aerobic oxidation of a readily accessible bisallene 1a with B2pin2
2a was chosen as the benchmark reaction (Table 1). In the
absence of electron transfer mediators (ETMs), we did not
observe any formation of the desired borylation product under
aerobic conditions and the starting material 1a was recovered in
93% yield (entry 1). When catalytic amounts of p-benzoquinone
(BQ) was added, the seven-membered carbocycle 3a was
selectively formed in low yield (15%, entry 2). Notably, the use of
cobalt-based salophen complex with p-benzoquinone (BQ) or
hydroquinone (HQ) as separate ETMs led to increased but
moderate yields of 3a (48 and 35% yields, entries 3 and 4). To
our delight, the bifunctional cobalt catalyst (Co(salophen)-HQ),
combining a metal-macrocycle and a quinone moieties, was
found to be a highly active catalyst and afforded the desired
product 3a in 78% yield (entry 5). Importantly, this aerobic
catalytic system appeared to be equally effective in comparison
to the application of stoichiometric amounts of BQ as oxidant
(78%, entry 6), therefore highlighting the advancement of this
efficient and sustainable method.[21]
Scheme 2. (a) Palladium-catalyzed oxidative carbocyclization with an
assisting group; (b) Current work.
Table 1. Evaluation of different electron transfer mediators (ETMs) for
borylative carbocyclization of bisallene 1a.[a]
Results and Discussion
Initially,
catalysts were prepared from the ligand precursor
salicylaldehyde–hydroquinone, which was covalently
a class of bifunctional Schiff base–hydroquinone
synthesized according to our previous report.[14] As shown in
Scheme 3, the synthetic route to these oxidation catalysts
commences with condensation of two equivalents of salicyl-
aldehyde–hydroquinone with various diamines such as o–
phenylenediamine, 4–chloro and 4–methoxyl–o–phenylene-
diamine, ethylenediamine and N–(3–aminopropyl)–N–methyl-
propane–1,3–diamine. The resulting salen–hydroquinone
ligands could be directly used for the next step without further
purification. Subsequently, coordination of these ligands with 3d
metal salts (Co, Fe, Ni, and Cu) afforded the corresponding
hybrid catalysts (Cat. 1–9) in general yields of 78–95%.
Entry
ETMs
Yield of 3a [%][b]
0[c]
1
2
3
4
5
6
none
20 mol% BQ
15
10 mol% Co(salophen), 20 mol% BQ
10 mol% Co(salophen), 20 mol% HQ
10 mol% Co(salophen)-HQ (Cat. 1)
100 mol% BQ
48
35
78[d]
78[d]
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isolated yield. With R1 being a benzyl or a cyclohexyl group, the
corresponding products 3b and 3c were obtained in 78% and
60% yields, respectively. Under the optimal aerobic reaction
conditions, cyclohexylidene bisallene 1d afforded 3d in only 30%
yield. We attribute this diminished reactivity to the increased
steric bulk of the cyclohexyl group. Not only can a terminal olefin
act as an assisting group, but also internal olefins were found to
promote the reaction as shown by the formation of products 3e
and 3f in 59% and 65% yield, respectively. In addition to an
ester group on R2, various functional groups at R2 such as alkyl
1g, hydroxyl 1h, silyl 1i, acetate 1j, sulfonamide 1k, and imide 1l
were compatible with the reaction conditions, highlighting the
broad substrate scope of this protocol (52%–75% yields). As an
example of late-stage oxidative functionalization, an estrone–
derived substrate 1m efficiently participated in this reaction to
afford a functionalized complex molecule 3m in a useful yield
(78%). Unfortunately, attempts to synthesize the N-containing
seven-membered heterocycle using the nitrogen analogue of 3g
(where oxygen in the ring had been replaced by NTs) were
unsuccessful (See Supporting Information). Bis(neopentyl
glycolato)diboron and bis(hexylene glycolato)diboron also
worked as the borylating reagent and afforded the
corresponding borylation products 3n and 3o in moderate yields
(40% and 52%, respectively). The seven-membered borylation
product from 1s was not observed due to the steric hindrance of
substrate 1s.
[a] Unless otherwise noted, the following reaction condition were employed
with 1a (0.1 mmol, 1.0 equiv.), B2pin2 2a (0.13 mmol, 1.3 equiv.), Pd(OAc)2 (5
mol%), ETMs (10-20 mol%) in 0.1 M acetone, O2 (1 atm) at room temperature
(25 oC) for 30 h. [b] Yields were determined by 1H NMR using anisole as
internal standard. [c] 93% of starting material 1a was recovered. [d] <1% of
starting material 1a was recovered.
It is noteworthy that the Schiff base-hydroquinone hybrid
catalyst (Co(salophen)-HQ, Cat. 1) leads to significant
improvement of the transformation of 1a to 3a (78% yield). This
interesting result motivated us to further explore the catalytic
performance of various hybrid catalysts synthesized above (vide
supra). As shown in Figure 1, in addition to Co-based catalyst
(Cat. 1), other metal–based catalysts Fe, Ni and Cu (Cat. 2-4)
were less effective than Co in this reaction. This result indicates
that the metal center of the salen catalyst has a significant
influence in the oxygen activation step.[22] Notably, functional
groups such as Cl (Cat. 5) or OMe (Cat. 6) on the diamine
backbone resulted in little change of the reaction yields (77%
and 74%, respectively). The hybrid catalysts with aliphatic
diamine backbones, such as ethylenediamine (Cat. 7), (±)-
trans-1,2-diaminocyclohexane (Cat. 8) and N–(3–aminopropyl)–
N–methylpropane–1,3–diamine (Cat. 9) exhibited slightly lower
reactivity (58%–63%). Therefore, the bifunctional catalyst
Co(salophen)–HQ was found to be the best performing catalyst
in this aerobic carbocyclization to
heterocycle.
a
seven–membered
Figure 1. Evaluation of the bifunctional catalysts for aerobic carbocyclization
of bisallene 1a with B2pin2 to seven membered cycle 3a. Unless otherwise
noted, the following reaction condition were employed with 1a (0.1 mmol, 1.0
equiv.), B2pin2 2a (0.13 mmol, 1.3 equiv.), Pd(OAc)2 (5 mol%), Schiff base–
hydroquinone catalyst (10 mol%) in 0.1 M acetone, O2 (1 atm) at room
temperature (25 oC) for 30 h. Yields were determined by 1H NMR using
anisole as internal standard.
After optimizing of the reaction conditions, we explored the
substrate scope with the optimal oxidation catalyst
Co(salophen)-HQ (Scheme 4). The benchmark borylative
carbocyclization gave the corresponding product 3a in 75%
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product 4 in 48% yield (Scheme 5a). Furthermore, an efficient
cascade reaction of bisallene 1a via an oxidative
carbocyclization–methoxycarbonylation is demonstrated here
(Scheme 5b) and gave the seven-membered carbocycle 5 in
78% yield. In the absence of
a trapping reagent, an
intramolecular oxidative coupling ending with β-H elimination
produced a highly conjugated seven-membered ring 6 in 79%
yield (Scheme 5c).
Scheme 5. Synthesis of seven membered rings via arylative, carbonylative
and beta–H eliminative carbocyclization. Reaction conditions: (a) bisallene 1a
(1.0 equiv.), PhB(OH)2 (1.3 equiv.), Pd(OAc)2 (5 mol%), Cat. 1 (10 mol%), in
0.1 M acetone, O2 (1 atm) at 25 oC for 30 h; (b) bisallene 1a (1.0 equiv.),
Pd(OAc)2 (5 mol%), DMSO (20 mol%), BQ (1.5 equiv.), MeOH (5.0 equiv.),
CO (1 atm) in 0.1 M DCE at 25 oC for 30 h; (c) bisallene 1s (1.0 equiv.),
Pd(OAc)2 (5 mol%), Cat. 1 (10 mol%) in 0.1 M THF, O2 (1 atm) at 25 oC for 30
h.
Scheme 4. Substrate scope for aerobic borylative carbocyclization to seven–
membered rings. Reaction conditions: bisallene 1 (1.0 equiv.), B2X2 2 (1.3
equiv.), Pd(OAc)2 (5 mol%), Cat. 1 (10 mol%) in 0.1 M acetone, O2 (1 atm) at
room temperature (25 oC) for 30 h; all yields are Isolated yields. [a] 15 mol%
Cat. 1 was added.
In analogy with our previous report,[15] there was a rate
enhancement with the hybrid catalyst compared to the use of
Co(salophen) and quinone as separate molecules as shown in
Figure 2. Co(salophen)-HQ hybrid (Cat. 1) resulted in fast
borylative carbocyclization of 1g to 3g (Figure 2a), as well as the
direct carbocyclization of 1s to 6 ending with β-H elimination
(Figure 2b). These results indicate that the intramolecular
electron transfer between the hydroquinone and the oxidized
metal-macrocycle of this bifunctional catalyst leads to a more
efficient palladium reoxidation process under aerobic
conditions.[23]
To further confirm the effect of the pending olefin group in
bisallene 1, we carried out control experiments with 1a’ as the
substrate, in which the vinyl group in 1a had been replaced by
an ethyl group (Eq. 1). Attempted reaction of 1a’ under standard
conditions did not give any product. This result shows that the
pending olefin is an indispensable element for this oxidative
carbocyclization, which is in accordance with our previous
work.[17]
In addition to borylative oxidative carbocyclization, the use
of phenylboronic acid as a transmetallating agent under similar
reaction conditions also afforded the carbocyclization−arylation
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Co(salophen)-HQ (Cat.1)
Co(salophen) and BQ
80
Co(salophen) and HQ
70
60
50
40
30
20
10
0
0
5
10
15
Time (h)
Scheme 6. Proposed catalytic cycle.
50
40
30
20
10
0
Co(salophen)-HQ (Cat. 1)
Co(salophen) and BQ
Co(salophen) and HQ
Conclusion
In summary, we have developed an efficient and selective
palladium-catalyzed carbocyclization of bisallenes under aerobic
oxidative conditions. This reaction avoids overstoichiometric
amounts of non-environmentally friendly oxidants (Cu(II), Ag(I),
peroxide etc.) for the activation of allenic C-H bonds. The use of
molecular oxygen as a green oxidant allows for synthesis of
important seven-membered heterocycles in moderate to good
yields. The key to success of this transformation is the
0
5
10
15
20
25
Time (h)
application of
a special hybrid electron transfer mediator
Figure 2. Reaction profiles with different ETMs: (a) Borylative carbocyclization
of 1g to 3g; (b) Direct intramolecular carbocyclization of 1s to 6. For details,
please see the supporting information.
[Co(salophen)-HQ] – a bifunctional catalyst consisting of a
metal-macrocycle and quinone moieties. This hybrid catalyst
significantly facilitates the reoxidation of Pd0 to PdII using
molecular oxygen as the terminal oxidant. In view of the high
reaction efficiency and selectivity, this protocol is expected to
complement the current approach for oxidative functionalization
in the synthesis of natural products and pharmacologically active
substances.
On the basis of these experimental findings, a possible
mechanism for the oxidative coupling reaction is proposed in
Scheme 6. Initially, the coordination of allene and olefin units to
the PdII center leads to a chelate palladium complex Int-I. This
special coordination of the close-by olefin to PdII is essential for
triggering the allenic C(sp3)−H cleavage and generating a
vinylpalladium intermediate Int-II.[17] Next, the envisioned ligand
exchange of olefin by the distant allene moiety takes place to
give Int-III. Subsequent carbocyclization of Int-III by the second
allene insertion gives a seven-membered (π-allyl)palladium
intermediate Int-IV. Reaction of Int-IV with a trapping reagent
such as B2pin2, PhB(OH)2 or CO/MeOH provides the target
product and a Pd0 species respectively. Finally, with the
assistance of the cobalt hybrid catalyst, aerobic oxidation of Pd0
regenerates PdII to close the catalytic cycle.
Acknowledgements
We are grateful for financial support from the Swedish Research
Council (2019–04042), the Olle Engkvist Foundation, and the
Carl Trygger Foundation.
Keywords: aerobic oxidation • palladium • carbocyclization •
heterocycles • regioseletivity
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10.1002/chem.202004265
Chemistry - A European Journal
COMMUNICATION
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We report a Pd(II)-catalyzed oxidative
carbocyclization of the rational
designed bisallenes using molecular
oxygen. By applying a bifunctional
catalyst (Co(salophen)-HQ) as an
efficient electron transfer mediator,
bisallenes are effectively converted to
the corresponding seven-membered
carbocycles in good yields.
Jie Liu*, and Jan-E. Bäckvall*
Page No. – Page No.
Efficient Palladium–Catalyzed
Aerobic Oxidative Carbocyclization to
Seven–Membered Heterocycles
This article is protected by copyright. All rights reserved.