Efficient Heterogeneous Palladium-Catalyzed Oxidative Cascade Reactions of Enallenols to Furan and Oxaborole Derivatives

Article abstract of DOI:10.1002/anie.201911462

A heterogeneous palladium-catalyzed oxidative cyclization of enallenols has been developed for the construction of highly substituted furan and oxaborole derivatives. The heterogeneous catalyst (Pd-AmP-MCF) exhibits high activity, high site- and stereoselectivity, and efficient palladium recyclability in the transformations.

Full text of DOI:10.1002/anie.201911462

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Accepted Article
Title: Efficient Heterogeneous Palladium-Catalyzed Oxidative Cascade
Reactions of Enallenols to Furan and Oxaborole Derivatives
Authors: Jan-E. Bäckvall, Man-Bo Li, Daniels Posevins, Antoine
Geoffroy, and Can Can
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Efficient Heterogeneous Palladium-Catalyzed Oxidative Cascade
Reactions of Enallenols to Furan and Oxaborole Derivatives
Man-Bo Li, Daniels Posevins, Antoine Geoffroy, Can Zhu* and Jan-E. Bäckvall*
Abstract:
A
heterogeneous palladium-catalyzed oxidative
cyclizations of enallenols has been developed for the construction of
highly substituted furan and oxaborole derivatives. The
heterogeneous catalyst (Pd-AmP-MCF) exhibits high activity, high
site- and stereoselectivity, and efficient palladium recyclability in the
transformations.
[
1]
[2]
Pd-catalyzed oxidations such as the Wacker process,
[
3]
[4]
alkene functionalizations, and C-H activations constitute an
important research field and these reactions have become
increasingly important in organic synthesis, as they enable the
diverse functionalizations of unsaturated C‒C bonds and
unreactive C‒H bonds. In spite of the significant progress in
homogeneous Pd-catalyzed oxidations, the achievement of high
[
5]
efficiency is still challenging in these reactions due to the
generation of palladium black from active palladium species
during the catalytic cycle (Scheme 1a). To circumvent the Pd
deactivation problem in homogeneus oxidation reactions,
considerable efforts have been made by different groups to
develop ligands that are stable towards the oxidizing conditions
Scheme 1. Pd deactivation problem in oxidations and the solutions.
[
6]
and that can keep Pd(0) in solution (Scheme 1b). Apart from
solving the Pd deactivation problem shown in Scheme 1a,
control of selectivity and efficient recyclability of the precious
palladium catalyst are other challenges in Pd-catalyzed
the cis-palladium intermediate (Int-B) with the Pd(II) and the allyl
alcohol group on the same side of the C=C bond (Scheme 2a).
In the presence of the terminal alkyne 2a, ligand exchange and
reductive elimination would produce the (Z)-tetrasubstituted
[
7]
oxidations.
Our group has a long-standing interest in Pd-catalyzed oxida-
olefin 3a. Interestingly, treatment of 1a and 2a with Pd(OAc)
and p-benzoquinone (BQ) in CDCl at room temperature did not
2
3
[
1a,b,8]
tions.
Recently, we reported on a heterogeneous nanopalla-
give 3a. Instead a tetrasubstituted furan 5a was obtained as the
exclusive product, which might be generated from the
isomerization of 4a (Scheme 2b). In a previous report, we found
a similar transformation of allenic amides without the pending
olefin by using homogeneous palladium. However, in that case
the reactivity of the corresponding allenic alcohol did not give the
corresponding tetrasubstituted furan (cf. 5a) under homo-
dium catalyst, where palladium is immobilized into the cavities of
aminopropyl-functionalized siliceous mesocellular foam (Pd-
[
9a]
AmP-MCF).
The performance of this catalyst has been
preliminary tested in organic transformations, such as dynamic
[
9]
kinetic resolution, hydrogenation, and oxidation. This catalyst
has shown high activity as well as excellent recyclability. Inspir-
ed by these results,we present herein the work on the use of Pd-
AmP-MCF as a general and efficient heterogeneous catalyst in
[
11]
geneous conditions.
3
Pd-catalyzed oxidative cyclizations of enallenols via C(sp )‒H
oxidations. This strategy provides
a new and satisfactory
solution of the Pd deactivation problem (Scheme 1c), leading to
high Pd efficiency, high site- and stereoselectivity and palladium
recyclability. As far as we known, there are only a few reports on
3
[9c,d,10]
the heterogeneous Pd-catalyzed C(sp )‒H oxidations.
We initiated our study by using enallenol 1a and alkyne 2a as
the model substrates. We envisioned that the simultaneous
coordination (Int-A) of the allene and olefin unit of 1a would
[
8c]
3
trigger the allene attack via C(sp )‒H bond oxidation to form
[*]
Dr. M.-B. Li, D. Posevins, A. Geoffroy, Dr. C. Zhu, Prof. Dr. J.-E.
Bäckvall
Department of Organic Chemistry, Arrhenius Laboratory,
Stockholm University, SE-10691, Stockholm, Sweden
Dr. M.-B. Li
Institute of Physical Science and Information Technology,
Anhui University, Hefei, Anhui, 230601, P.R. China
E-mail: jeb@organ.su.se, can.zhu@su.se
3
Scheme 2. a) Envisioned Pd-catalyzed C(sp )‒H oxidative alkynylation of
Supporting information for this article is given via a link at the end
of the document.
enallenol. b) Observed results. c) The catalyst efficiency comparison.
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Catalyst screening showed distinct differences between the
homogeneous palladium and the heterogeneous Pd-AmP-MCF
catalysts concerning catalyst efficiency (for more details on
catalyst screening, see Supporting Information, P. S13). As
shown in Scheme 2c, 1 mol% of Pd(OAc)
yield at room temperature in 12 h. The addition of 1,10-
phenanthroline to Pd(OAc) completely shut down the reaction,
2
only gave 5a in 36%
2
probably due to the strong coordination of 1,10-phenanthroline
to Pd(II) disfavoring the formation of Int-A and the subsequent
2
allene attack. Replacement of Pd(OAc) by the heterogeneous
Pd-AmP-MCF (1 mol%) gave 5a in 82% yield.
Scheme 3. Solvent-controlled chemoselectivity.
2
In the homogeneous reaction with Pd(OAc) , Pd black was
enallenols 1 worked well with terminal alkynes 2 to give (Z)-
tetrasubstituted olefins or dihydrofurans 4, respectively.
Enallenol 1ab with one more carbon in the pending olefin (see
ref. 13) also reacted with an alkyne in CH OH to give the
corresponding dihydrofuran 4ab in 80% yield.
chemoselectivity of the Pd-AmP-MCF-catalyzed oxidative
cyclization of enallenols 1 is simply controlled by using CHCl as
the solvent to give tetrasubstituted furan 5. To be noted, 4h‘ and
h‘ were isolated as the final products in Scheme 4, which are
generated from olefin isomerization of the initially formed 4h and
observed in the reaction mixture after completion, while there
was no observed generation of Pd black in the reaction using
the heterogeneous Pd-AmP-MCF (Scheme 2c). These results
demonstrate the high efficiency of the Pd-AmP-MCF catalyst in
3
3
[13]
The
3
Pd-catalyzed C(sp )-H oxidations, which is credited to the
protection of Pd species from aggregating to Pd black.
Further solvent screening showed interesting results: by using
CH CN, CH OH or CHCl as the solvent, (Z)-tetrasubstituted
3 3 3
olefin 3a, 2,5-dihydrofuran 4a, or tetrasubstituted furan 5a was
obtained as the dominant product, respectively (Scheme 3, for
more details on the solvent effect, see Supporting Information, P.
S13).
3
5
[14]
4i, respectively (for the formation of 4h‘ and 5h‘, see ref. 14)
.
Pd leaching was not observed in the heterogeneous catalysis
based on a hot filtration test, which rules out a reaction via a
homogeneous pathway. Kinetics studies (Fig. 1) revealed that
the rate and yield of the reaction are essentially maintained
With the optimized reaction conditions in hand, we explored
3
the substrate scope of Pd-AmP-MCF-catalyzed C(sp )‒H
oxidations of enallenols 1 for the divergent syntheses of 3, 4,
and 5 (Scheme 4), which are important building blocks and core
st
th
between the 1 and the 6 runs, which demonstrates that Pd-
Amp-MCF is robust and has maintained its activity in 6 runs.
However, we cannot exclude the possible loss of activity in the
structures of
a
large variety of natural and bioactive
[
12]
compounds. By using CH CN or CH OH as the solvent,
3
3
[15]
long run of the heterogeneous catalyst.
3
Scheme 4. Substrate scope and solvent-controlled selectivity of the Pd-AmP-MCF-catalyzed C(sp )‒H oxidation of enallenols 1 for the divergent construction of
(Z)-tetrasubstituted olefin 3, dihydrofuran 4, and tetrasubstituted furan 5
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Figure 1. Recycling of Pd-AmP-MCF in the transformation from 1a to 5a.
We compared the X-ray Photoelectron Spectroscopy (XPS)
and Transmission Electron Microscopy (TEM) images of Pd-
AmP-MCF before reaction and after six cycles. The
deconvoluted Pd3d XPS spectrum of Pd-AmP-MCF before
reaction showed intensive Pd(II) signals at 338 eV and 343 eV
Scheme 5. Control experiments.
was transformed into 5a in CHCl
3
in good yield (Scheme 5b).
These results demonstrate that 2,5-dihydrofuran 4a is formed
via Pd-catalyzed intramolecular cyclization of 3a, and that
tetrasubstituted furan 5a is formed via isomerization of 4a.
Based on the experimental results, we propose a mechanism for
the Pd-AmP-MCF-catalyzed oxidative functionalizations of
enallenols, leading to (Z)-tetrasubstituted olefin 3, 2,5-
dihydrofuran 4, or tetrasubstituted furan 5 (Scheme 6).
(
Fig. 2a). After 6 cycles, the catalyst still had a dominant
proportion of Pd(II) (Fig. 2b). TEM images showed that no
palladium black was generated from Pd species of Pd-AmP-
MCF after 6 cycles (Fig. 2c and 2d). Based on these results, we
conclude that the porous support (AmP-MCF) of Pd-AmP-MCF
protect Pd species from aggregating to catalytically inert Pd
black during the catalytic cycle, which leads to the high
efficiency of Pd-AmP-MCF.
Scheme 6. Proposed mechanism.
On the basis of mechanistic understanding of the Pd-AmP-
MCF-catalyzed oxidative cyclizations of enallenols 1, we
extended this strategy for the construction of oxaborole 6, which
[
16]
is an important synthon in organic transformations.
As
key
Figure 2. XPS and TEM analyses of Pd-AmP-MCF before reaction and after
six cycles.
[
16a]
reported by Baran,
this oxaborole skeleton is
a
intermediate permitting rapid and gram-scale access to a variety
of bioactive meroterpenoids. By replacing the alkyne reaction
To gain further insight into the reaction mechanism, we carried
out control experiments (Scheme 5). first, we replaced the vinyl
group of enallenol 1a by an ethyl group to form 1ac. Treatment
of allenol 1ac with terminal alkyne 2a under standard reaction
conditions in different solvents did not afford the corresponding
products, instead 1ac was recovered in >90% yield in these
cases (Scheme 5a). These results show that the olefin group of
enallenols 1 is required for coordinating to palladium to initiate
partner by bis(pinacolato)diboron (B
mol% of Pd-AmP-MCF and 1.1 equiv. of BQ in CHCl
2
pin
2
), in the presence of 1
at room
3
temperature, enallenols 1 were transformed to oxaboroles 6 in
[17]
excellent yields (Scheme 7) . More importantly, by using (S)-
enallenols 1, readily obtained from kinetic resolution of
enallenols 1 with Candida antarctica lipase B (CalB), the
reaction afforded optically pure (>99% ee) oxaboroles (S)-6 in
high yields. During the process, no loss of optical purity was
observed, in spite of the potential racemization of the allylic
alcohol by palladium. This heterogeneous process provides an
3
the C(sp )-H oxidation and control the stereoselectivity,
furnishing a cis-palladium intermediate (Int-B in Scheme 2a).
Second, 3a was treated with 1 mol% of Pd-AmP-MCF in CH
3
OH
or CHCl , which afforded 4a or 5a, respectively. Furthermore, 4a
3
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efficient route towards highly substituted chiral oxaboroles with
high selectivity and step-economy. This method should find use
in medicinal chemistry as a result of the wide application of
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In conclusion, we have developed
a
heterogeneous
palladium-catalyzed oxidative cascade cyclization of enallenols,
constructing highly substituted furan and oxaborole derivatives.
The heterogeneous process circumvents the Pd deactivation
problem in homogeneous Pd-catalyzed oxidations, exhibing high
catalyst efficiency, high site- and stereo-selectivity and efficient
palladium recyclability. It was demonstrated that the support
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Financial support from the Swedish Research Council (2016-
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Knut and Alice Wallenberg Foundation (KAW 2016.0072) is
gratefully acknowledged.
Keywords: heterogeneous palladium • oxidative cyclizations •
efficiency • stereospecific
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3
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[
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Entry for the Table of Contents (Please choose one layout)
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A
highly efficient heterogeneous
Man-Bo Li, Daniels Posevins, Antoine
Geoffroy, Can Zhu* and Jan-E.
Bäckvall*
palladium-catalyzed oxidative
cyclizations of enallenols is developed
here for the construction of highly
substituted furan and oxaborole
derivatives. The support (AmP-MCF)
of the heterogeneous catalyst (Pd-
AmP-MCF) protect Pd species from
aggregating to Pd black during the
catalytic cycle, which leads to the high
Pd efficiency of Pd-AmP-MCF
Page No. – Page No.
Efficient Heterogeneous Palladium-
Catalyzed Oxidative Cascade
Reactions of Enallenols to Furan and
Oxaborole Derivatives
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