Amino-Supported Palladium Catalyst for Chemo- and Stereoselective Domino Reactions

Article abstract of DOI:10.1002/anie.202011708

A solid amino-supported palladium catalyst is used in an oxidative domino reaction for the diastereoselective construction of alkyne-substituted cyclopentenol compounds. This heterogeneous catalyst exhibits high efficiency and excellent chemoselectivity, as well as good recyclability. The chemoselectivity of the domino reactions was readily controlled by switching the solvent and catalyst. Asymmetric syntheses and an oxidative carbocyclization-borylation reaction have also been developed based on the heterogeneous palladium catalyst.

Full text of DOI:10.1002/anie.202011708

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Accepted Article
Title: Amino-Supported Solid Palladium Catalyst for Chemo- and
Stereoselective Domino Reactions
Authors: Jan-E. Bäckvall, Man-Bo Li, Jie Yang, Ying Yang, Guo-Yong
Xu, Gen Luo, and Jianping Yang
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Angewandte Chemie International Edition
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COMMUNICATION
Amino-Supported Solid Palladium Catalyst for Chemo- and
Stereoselective Domino Reactions
+
+
Man-Bo Li, * Jie Yang, Ying Yang, Guo-Yong Xu, Gen Luo, Jianping Yang, and Jan-E. Bäckvall*
Abstract: An amino-supported solid palladium-catalyzed oxidative
domino reaction has been developed for the diastereoselective
construction of alkyne-substituted cyclopentenol compounds. This
heterogeneous catalyst exhibits high efficiency and excellent
chemoselectivity, as well as good recyclability. The chemoselectivity
of the domino reactions was readily controlled by switching the
solvent and catalyst. Asymmetric syntheses and an oxidative
carbocyclization-borylation reaction have also been developed
based on the heterogeneous Pd-catalyzed domino strategy.
A domino reaction constitutes an efficient approach in organic
synthesis involving multiple bond formation, as it produces the
[
1-6]
target molecule in one pot with high atom and step economy.
However, the achievement of high selectivity in a domino
reaction is always challenging, considering the possible side
[
3-6]
reactions during each bond-forming step.
During recent
years, our group has been involved in Pd-catalyzed oxidative
domino processes for the construction of complex molecules
[7-12]
from enallenes (Scheme 1a).
The key feature of these
processes is the generation of a vinyl-palladium intermediate
Scheme 1. Proposed domino process, selectivity challenge, and
representative bioactive compounds.
3
(
Int-B) triggered by an allene attack on palladium via a C(sp )‒H
[
7]
bond cleavage (Int-A). The diverse reactivity of Int-B allows
flexible domino processes, while at the same time significant
selectivity challenges are involved in these reactions.
temperature (Table 1). To our delight, the target product 2a
was obtained in 10% yield as the cis-diasteromer with high
diastereoselectivity
(>20:1
d.r.).
However,
the
In the present work, we have designed a Pd-catalyzed
oxidative domino route to enynes 2 with a cyclopentenol unit
chemoselectivity was not satisfactory, and 5a generated from
β-H elimination of Int-2 was obtained as the major product in
[
13-18]
(
Scheme 1b). This skeleton is a key substructure
and
6
9% yield (Table 1, entry 1). To improve the chemoselectivity
[
19]
synthon
of many bioactive compounds. Representative
of the domino reaction for producing 2a, attempts were made
to optimize the reaction conditions. Replacement of
[
14,15,18]
[13,16,17]
examples include maduropeptin,
kedarcidin
and
their analogues (Scheme 1c). In vivo studies have shown these
molecules to be extremely active against leukemia and
melanoma by binding and cleaving duplex DNA at selected
Pd(OAc)
give any better results, and by using Pd(PPh
oxidized product 3a was obtained, which is generated from
2
by other homogeneous palladium sources did not
3
)
2
2
Cl , only non-
[
13-15]
sites.
Concerning the construction of this important
[20,21]
Pd-catalyzed intramolecular oxypalladation
(Table 1,
skeleton, the synthetic route in Scheme 1b would be a highly
efficient approach. However, considering the possible side
reactions of each palladium intermediate and the newly formed
chiral centers during the domino process, chemo- and
stereoselective formation of 2 would be challenging.
entry 3). Compound 5a was the dominant product in most of
the solvents (Table 1, entries 4-8), and was obtained as the
sole product in 90% yield in CHCl
CH CN as the solvent, 4a was obtained as the major product
in 85% yield (Table 1, entry 8). This result could be explained
by the interaction between CH CN and Pd(II), which inhibits
the Pd-catalyzed olefin insertion step.
3
. Intriguingly, by using
3
Our initial attempt began with the reaction of 1a and
3
phenylacetylene in the presence of 5 mol% of Pd(OAc)
and1.1 equiv. of benzoquinone (BQ) in DCE at room
2
[22]
Under the
homogeneous reaction conditions, 2a was always the minor
product. We then turned our attention to a solid catalyst
where palladium is immobilized on amino-functionalized
[*]
Prof. Dr. M.-B. Li, Y. Yang, Dr. G.-Y. Xu, Prof. Dr. G. Luo
Institute of Physical Science and Information Technology,
Anhui University, Hefei, Anhui, 230601, P.R. China
E-mail: mbli@ahu.edu.cn
[23,24]
siliceous mesocellular foam
(Pd-AmP-MCF). By using
this catalyst, our group has successfully realized the
oxidative transformations of allenes with high activity and
[
25-28]
Prof. Dr. J.-E. Bäckvall, J. Yang
selectivity.
Surprisingly, highly improved selectivity for
Department of Organic Chemistry, Arrhenius Laboratory,
Stockholm University, SE-10691, Stockholm, Sweden
E-mail: jeb@organ.su.se
2
a was observed when the catalyst was switched to Pd-
AmP-MCF, and 2a was now obtained in 44% yield (Table 1,
entry 9). In our previous work the addition of catalytic amount
of AgOTf dramatically improved the activity of this
Dr. J. Yang
School of Materials Science and Engineering,
Jiangsu University of Science and Technology, Zhenjiang, 212003,
P.R. China
heterogeneous palladium catalyst in oxidative carbonylation
[28]
+
of allene amides.
However, addition of AgOTf in the
These two authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
present reaction resulted in the sole production of 3a (Table
[29]
1, entry 10).
Finally, after screening other additives, we
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COMMUNICATION
were delighted to find that addition of Et N is favorable for
3
switching the selectivity of the reaction towards 2a. Thus, the
use of 0.1 equiv. of Et N affored 2a in 84% yield (Table 1,
3
entry 11). Notably, the solid palladium catalyst loading can
be reduced to 1 mol%, producing 2a in 83% yield (Table 1,
entry 12). A similar solid palladium catalyst immobilized on
renewable amino-functionalized crystalline nanocllulose
[28]
foam
(Pd-AmP-CNC) also showed high activity and
selectivity for the formation of 2a (Table 1, entry 13).
b
Yield (%)
Table 2. Amine effect under homogeneous and heterogeneous conditions.
Entry
Cat. (5 mol%)
Sol.
2
a
3a
4a
5a
can be explained by the deactivation of active Pd(II) to Pd black.
As a result, Pd-AmP-MCF exhibited much higher activity and
chemoselectivity than the homogeneous Pd catalyst, which is
credited to the interaction between its porous amino support and
Pd(II) (Figure 1e).
1
2
3
4
5
6
7
8
Pd(OAc)
Pd(TFA)
Pd(PPh Cl
2
DCE
DCE
10
-
-
22
89
-
12
10
-
69
60
-
2
)
3 2
2
DCE
-
Pd(OAc)
2
THF
5
20
20
-
71
74
90
54
3
Pd(OAc)
2
Toluene
-
-
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
CHCl
Acetone
CH CN
3
-
-
2
-
10
-
25
85
5
2
3
-
9
Pd-AmP-MCF
Pd-AmP-MCF
Pd-AmP-MCF
Pd-AmP-MCF
Pd-AmP-CNC
DCE
DCE
DCE
DCE
DCE
44
-
-
35
-
c
10
94
-
-
d
11
84
83
80
-
4
de
12
-
-
4
de
13
-
-
6
a
Unless otherwise noted, the reaction was carried out by using 0.2 mmol
of 1a, 0.25 mmol of phenylacetylene, 5 mol% of catalyst, 1.1 equiv. of
b
BQ and 1.0 mL of solvent at room temperature for 12 h. Determined by
c
NMR using anisole as the internal standard. 5 mol% of AgOTf was
d
e
added. 0.1 equiv. of Et
3
N was added. 1 mol% of catalyst was used, 8 h.
a
Table 1. Optimization of the reaction conditions.
For in-depth understanding of the origin of the high activity
and chemoselectivity in the heterogeneous palladium-catalyzed
domino process for the formation of 2, control experiments were
conducted by using 1 mol% of Pd-AmP-MCF or Pd(OAc)
2
with
[
30]
different amine additives (Table 2).
Some interesting results
were observed and the following conclusions were made: 1)
Under both heterogeneous and homogeneous reaction conditi-
ons, tertiary, secondary, and primary amines improved the
2
Figure 1. XPS Pd3d spectra of the reaction mixture by using Pd(OAc) (a:
before reaction; b: after reaction) or Pd-AmP-MCF (c: before reaction; d: after
reaction) as the catalyst. Inset: Photos of the reaction mixture. e) An
illustration of the heterogeneous catalyst and its three-in-one role.
[
31]
selectivity for 2a,
and tertiary amines dramatically increased
the yield of 2a in the reaction. Diamine completely shut down the
reaction, probably due to the strong coordination of diamine to
Pd(II), which inhibits the catalytic activity of the solid palladium
catalyst. These results indicate that amine coordinates to Pd(II)
With the optimized reaction conditions in hand, we
investigated the substrate scope (Scheme 2). Aromatic and
heteroaromatic terminal alkynes all worked well with enallene 1
to give 2 in high yields (2a-l). Arylalkynes with electron-
withdrawing groups or electron-donating groups at para-, ortho-,
or meta-positions worked equally well with enallene 1, affording
[
32-34]
and affects its catalytic activity and selectivity,
and tertiary,
secondary and primary amines are positive for improving the
selectivity for 2a by suppressing the β-H elimination or promo-
2 in good yields (2b-j). Functional groups such as MeO, O
2
N, F,
[
35,36]
ting the alkyne ligand exchange of Int-2.
2) Under homoge-
Cl, Br, F C and MeO C were well tolerated under the standard
3
2
neous reaction conditions, the starting material was always
partially recovered and considerable amounts of Pd black was
observed after the completion of the reaction (Figure 1a and b,
inset). XPS Pd3d analysis of the reaction mixture showed that
the proportion of Pd(II) was much lower than that of Pd(0)
reaction conditions. Aliphatic terminal alkynes reacted with
enallene 1, giving 5 as the major products, probably due to the
unfavorable alkynylation between aliphatic alkynes and Int-2
(Scheme 1b). Different substituents including cyclohexyl (2a),
cyclopentyl (2p), cyclopropyl (2o), propyl (2m), butyl (2n) and
phenylethyl (2q) at the R position of enallenes 1 worked well to
(
Figure 1a and b). In contrast, under the heterogeneous reaction
[
38]
conditions, Pd(II) was still dominant after the reaction (Figure 1c
give products 2 in >80% yields.
Interestingly, the addition of
[
37]
and d).
These results demonstrate that the porous amino
3
Et N was not necessary when a tertiary amine was introduced in
support (AmP-MCF) protects Pd species from aggregating to Pd
black, and in this way the activity of Pd-AmP-MCF was
maintained during the catalytic cycle. The partial recovery of
starting material under the homogeneous reaction conditions
the substrate (2r), Notably, Pd-AmP-MCF-catalyzed the
oxidative domino process with high efficiency and
diastereoselectivity. For all of the substrates, only 1 mol% of Pd-
AmP-MCF was used and products 2 were obtained in >20:1 d.r.
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By readily switching the reaction conditons, using AgOTf or
2 3 3
Pd(OAc) (5 mol%) as the catalyst, DCE, CH CN or CHCl as
the solvent, the chemoselectivity of the reaction is simply
controlled to give 3, 4 or 5 as the final products in excellent
yields (Scheme 3). Notably, Pd-AmP-MCF showed much higher
efficiency than Pd(OAc)
partially recovery of the starting materials. The solvent effect
was also extended to the solid catalyst in CH CN, however, in
CHCl , Pd-AmP-MCF catalyzed the reaction, giving the mixure
of 2 and 5 as the products.
2 2
, and 1 mol% of Pd(OAc) resulted in
3
3
[
41]
This catalyst/solvent-controlled
selective domino process provides an efficient pathway towards
the chemodivergent synthesis of 2, 3, 4 and 5.
a
b
a
Scheme 3. Chemodivergent syntheses of 3, 4 and 5. 5 mol% of Pd(OAc)
2
.
1
Scheme 2. Substrate scope for the synthesis of 2. Without Et
3
N
c
mol% of Pd(OAc)
2
, starting materials were partially recovered. 1 mol% of Pd-
AmP-MCF. > 90% conversion, 2 was obtained as the byproducts.
d
Inductively Coupled Plasma Optical Emisson Spectroscopy
ICP-OES) analysis after the reaction indicates that there was no
detectable amount of Pd in the reaction solution (< 0.1 ppm). A
Enantiomerically pure enallene (R)-1, readily obtained from
(
[42]
kinetic resolution of 1 with Candida Antarctica lipase B (CalB),
was selectively transformed to optically pure (>99% ee) products
, 4 or 5 in high yields by simply adjusting the solvent or catalytic
system of the reaction (Scheme 4a). During the domino process,
no loss of optical purity was observed, despite the possible
racemization pathway of the allylic alcohol moiety of allene (R)-1
in the presence of palladium. Under the standard heterogeneous
reaction conditions, by replacing the alkyne reaction partner with
[39]
hot filtration test
showed that no active Pd species were
2
leached out from the solid catalyst during the reaction. These
results rule out a Pd leaching during or after reaction, suggesting
a heterogeneous pathway. Recycling experiments and kinetic
studies (Figure 2) reveal that the solid Pd catalyst is recoverable,
and its activity is essentially maintained between the first and the
seventh cycles, which demonstrates that Pd-AmP-MCF is robust
and highly active for the Pd-catalyzed oxidative domino process.
However, We cannot exclude the possibility that the solid
catalyst might lose its activity slowly after the long run of the
2 2
bis(pinacolato)diboron (B pin ), an oxidative carbocyclization-
borylation domino process was developed to give cyclopentenol
boron compound 6 in high yield and high diastereoselectivity
[
40]
(
Scheme 4b).
reaction.
Figure 2. recycling experiments and kinetic studies.
Scheme 4. Applications of the solid palladium catalyzed domino process.
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Angewandte Chemie International Edition
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COMMUNICATION
Based on the experimental results and our previous work on
Natural Science Foundation of China (21501182, 2065011802)
is gratefully acknowledged.
[
7-
Pd-catalyzed oxidative transformations of allene derivatives,
1
2,25-28]
we propose the mechanism shown in Scheme 5 for the
Pd-catalyzed domino reactions. Initially, simultaneous
Keywords: solid palladium catalyst
•
amino support
•
coordination of the olefin group and the allene unit of enallene 1
to Pd(II) would trigger the allene attack to generate Int-1, which
chemoselective domino reactions • diastereoselectivity
would react with terminal alkyne to give product 4 in CH
3
CN, or
[1] L. F. Tietze, G. Brasche, K. Gericke, Domino Reactions in Organic
undergo a face-selective olefin insertion to generate Int-2 with
Synthesis; Wiley-VCH, Weinheim, 2008.
[11]
hydroxyl group and Pd on the same side.
homogeneous reaction conditions (Pd(OAc) CHCl
subsequent β-H elimilation of Int-2 would produce 5, while under
heterogeneous reaction conditions (Pd-AmP-MCF, Et N, DCE),
Under
[2] L. F. Tietze, Domino Reactions: Concepts for Efficient Organic Synthesis;
2
,
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a
Wiley-VCH, Weinheim, 2014.
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[
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would be reoxidized by BQ to generate active Pd(II) to initiate
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[
[
[
[
[
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In conclusion, we have developed an amino-supported solid
Pd-catalyzed domino reaction for the diastereoselective
construction of cyclopentenols with an alkynyl group, which are
key substructures and synthons of many bioactive compounds.
This heterogeneous catalyst exhibited high activity and
selectivity, as well as good recyclability. It was demonstrated
that the amino support interacts with Pd, on the one hand
improving the chemoselectivity of the heterogeneous palladium
catalyst, and on the other hand contributing to high palladium
activity in the domino reaction by protecting Pd species from
aggregating to Pd black. The chemoselectivity of the domino
reactions was readily controlled by switching the catalyst or
solvent, which allows chemodivergent syntheses of 2, 4 and 5 in
excellent yields. The domino strategy developed was also
extended to the asymmetric syntheses as well as oxidative
carbocyclization-borylation reactions. It is expected that our work
will stimulate more research on the development of
heterogeneous catalytic system for highly active and selective
transformations.
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[29] Based on control experiments, AgOTf catalyzed the cyclization of
enallenol to give without the addition of Pd catalyst. The
1
3
corresponding results are shown in Scheme 3. For AgOTf catalyzed-
cyclization of enallenols, see: a) J. A. Marshall, M. A. Wolf, E. M. Wallace,
J. Org. Chem. 1997, 62, 367-371; b) S. Ma, Z. Shi, J. Org. Chem. 1998,
Acknowledgements
6
3, 6387-6389.
[
30] For more control experiments on the amine effect, see Supporting
Information, p. S2.
Financial support from the Swedish Research Council (2019-
[
31] Under the heterogeneous reaction conditions, there was no significant
difference in the yield of 2a by using secondary, primary, or no amine as
04042), the Olle Engkvist Foundation, the Knut and Alice
Wallenberg Foundation (KAW 2016.0072), and the National
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Angewandte Chemie International Edition
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COMMUNICATION
the additive. This result is reasonable, because the Pd(II) in Pd-AmP-MCF
is already coordinated by primary amine (see ref. 23), and Pd-AmP-MCF
catalyzed the domino reaction to give 2a in moderate yield.
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[
[
6
5
36] T. Bartholomeyzik, J. Mazuela, R. Pendrill, Y. Deng, J.-E. Bäckvall,
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[
[
37] Pd(II) was still dominant after the seventh run, and the XPS spectra of
II
0
Pd-AmP-MCF after run 1 and run 7 showed similar Pd /Pd ratio. For
details, see Supporting Information, p. S 21.
38] The reaction of aromatic R substituted enallenol 1i (R = Ph) with the
1
terminal alkyne resulted in a messy mixture by TLC and H NMR. We
conclude that the electronic effect of aromatic R group might affect the
reactivity of enallenol 1 under the reaction conditins.
[
[
[
39] For details on hot filtration test, see Supporting Information, p. S20.
40] Á. Molnár, A. Papp, Coord. Chem. Rev. 2017, 349, 1-65.
3
41] The interaction between CDCl and the solid palladium catalyst is weak,
and Pd(II) in Pd-AmP-MCF is coordinated by primary amine, which might
partially switch the selectivity of the domino reaction to give 2 as the
byproduct in CDCl
3
. The ratios of product 5 : product 2 in 5a, 5b, 5c and
5d are 59 : 40, 45 : 40, 44 : 42, and 50 : 38, respectively.
[42] For the preparation of enantiomerically pure enallene 1, see Supporting
Information, P. S21.
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Angewandte Chemie International Edition
10.1002/anie.202011708
COMMUNICATION
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COMMUNICATION
An
amino-supported
solid
Pd-
Man-Bo Li*, Jie Yang, Ying Yang, Guo-
Yong Xu, Gen Luo, Jianping Yang and
Jan-E. Bäckvall*
catalyzed oxidative domino reaction is
developed here for the construction of
alkyne-substituted
compounds. The three-in-one property
of the heterogeneous catalyst
contributes to its high efficiency and
excellent chemoselectivity, as well as
good recyclability.
cyclopentenol
Page No. – Page No.
Amino-Supported Solid Palladium
Catalyst for Chemo- and
Stereoselective Domino Reactions
This article is protected by copyright. All rights reserved.