Catalytic Enantioselective Aminopalladation–Heck Cascade

Article abstract of DOI:10.1002/anie.202016001

Domino processes initiated by intramolecular nucleopalladation of alkynes have been developed into powerful synthetic tools for the synthesis of functionalized heterocycles. However, a catalytic enantioselective version of this class of reactions remains

Full text of DOI:10.1002/anie.202016001

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 7093–7097
International Edition: doi.org/10.1002/anie.202016001
German Edition: doi.org/10.1002/ange.202016001
Homogeneous Catalysis
Catalytic Enantioselective Aminopalladation–Heck Cascade
Yu-Ping He+, Jian Cao+, Hua Wu, Qian Wang, and Jieping Zhu*
Abstract: Domino processes initiated by intramolecular
nucleopalladation of alkynes have been developed into power-
ful synthetic tools for the synthesis of functionalized hetero-
cycles. However, a catalytic enantioselective version of this
class of reactions remains scarce. We report herein that reaction
of 2-alkynylanilines with prochiral cyclopentenes in the
presence of a catalytic amount of Pd(OAc)2, a chiral bidentate
pyrox ligand and O2 as terminal oxidant affords the structurally
diverse indole-cyclopentene conjugates bearing two stereocen-
ters in a highly diastereo- and enantio-selective manner. One of
the products is converted to a heavily functionalized tetracyclic
indolinone derivative.
T
he nucleopalladation of alkenes is an important elementary
step in the transitional metal catalyzed transformations. As
the reaction generates at least one stereocenter, the develop-
ment of catalytic enantioselective versions has attracted
a great deal of recent attention (Scheme 1a).[1] In parallel,
the nucleopalladation of alkynes followed by trapping of the
resulting achiral vinylpalladium intermediates has also been
developed into a powerful synthetic tool (Scheme 1b-i).[2] For
example, the PdII-catalyzed 5-endo cyclization of 2-alkynyl-
anilines[3] has been successfully combined with Heck reac-
tion,[4] Sonogashira reaction,[5] carbocyclization,[6] Suzuki
reaction,[7] amination,[8] carboesterification,[9] alkoxycarbony-
lation,[10] imidoylation[11] and nucleophilic addition to alde-
hydes[12] for the synthesis of 2,3-difunctionalized indoles
(Scheme 1b-i). However, applying this strategy to the syn-
thesis of chiral molecules remains scarce (Scheme 1b-ii).
Indeed, except for recent achievement on the synthesis of
axially chiral biaryls,[13] enantioselective domino reaction
initiated by nucleopalladation of alkynes for the construction
of enantioenriched centrally chiral molecules remains, to the
best of our knowledge, unknown.
Scheme 1. Nucleopalladation and oxidative asymmetric Heck reaction:
The state of the art and our reaction design.
Heck reaction between arylboronic acids and alkenes.[14–20]
However, the heteroarene-derived boronic acids, with few
exceptions, are notably absent in the scope exploration of
these reports. In this regard, the successful use of 1-tosyl-1H-
indol-3-yl boronic acid as a reaction partner in the asymmetric
redox-relay oxidative arylation of alkenols is noteworthy.[16a]
While the enantioselectivity was high, the yield is moderate
due to the instability of these boronic acids under the reaction
conditions. To address this issue, Sigman developed an
enantioselective dehydrogenative Heck reaction of indoles
with trisubstituted alkenols (Scheme 1c).[21] While the reac-
tion works well with 2,3-unsubstituted indoles, the yield of the
Heck adduct drops significantly when the 2-substituted indole
is used as a reaction partner. The presence of strong electron-
withdrawing group in the phenyl ring of the indole is also not
tolerated in line with the proposed reaction mechanism.
Therefore, a new approach is still needed in order to
incorporate the diversely substituted indole moiety to the
chiral molecules.
Recent years have witnessed the significant progress in
the development of PdII-catalyzed enantioselective oxidative
[*] Dr. Y.-P. He,[+] Dr. J. Cao,[+] Dr. H. Wu, Dr. Q. Wang, Prof. Dr. J. Zhu
Laboratory of Synthesis and Natural Products
Institute of Chemical Sciences and Engineering
Ecole Polytechnique Fꢀdꢀrale de Lausanne
EPFL-SB-ISIC-LSPN
BCH 5304, 1015 Lausanne (Switzerland)
E-mail: jieping.zhu@epfl.ch
The in situ generated indolylPdX species A (Scheme 1c)
is proposed to be the intermediate in the reaction developed
by Sigman. Since the PdX2-catalyzed aminopalladation of 2-
alkynylaniline 1 generated the same intermediate A, we
reasoned that it could be interesting to combine this reaction
with the enantioselective Heck reaction for the synthesis of
indole-containing chiral molecules. We report herein the
realization of this endeavour by developing a domino amino-
palladation-oxidative Heck reaction of 2-alkynylanilines
Homepage: http://lspn.epfl.ch
Dr. J. Cao[+]
Key Laboratory of Organosilicon Chemistry and Material Technology
of Ministry of Education, Hangzhou Normal University
Hangzhou 311121 (P. R. China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016001.
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1 with prochiral cyclopentenes 2 for the synthesis of
(c = 0.17 M, oxygen atmosphere, 608C) in the presence of
catalytic amount of Pd(OAc)2 (10 mol%) and L5
enantioenriched
(Scheme 1d).
indole-cyclopentene
conjugates
3
a
(12 mol%). Under these conditions, 3a was isolated in 75%
yield with 93% ee (d.r. > 19:1, entry 13). We note that in the
absence of oxygen under otherwise identical conditions,
reaction of 1a with 2a afforded compound 3a in only 7%
yield corresponding roughly to the catalyst loading.
With the optimized conditions in hands, we proceed to
explore the scope of this domino indolization/enantioselec-
tive oxidative Heck reaction starting with substituted cyclo-
pentenes 2 (Scheme 2). The C1 aromatic substituent bearing
an electron-donating (MeO, Me) and an electron-withdraw-
ing group (F, Cl, Br, NO2) at different positions (para, meta
We began our studies by examining the reaction of N-(2-
(phenylethynyl)phenyl)methanesulfonamide (1a) with N-
benzyl-1-phenylcyclopent-3-ene-1-carboxamide (2a). After
initial survey of the reaction parameters, the structure of
chiral ligands was fine-tuned by performing the reaction in the
presence of PdII complex (0.1 equiv),
a chiral ligand
(0.12 equiv) at 508C under oxygen atmosphere. Heating
a 1,2-dichloroethane solution of 1a and 2a in the presence of
palladium(II) acetate, Box ligand L1 (Figure 1) afforded the
desired product 3a in 25% yield with 70% ee (entry 1,
Table 1). Using pyrox ligand L2 under otherwise identical
Figure 1. Structure of the chiral ligands.
conditions, the product 3a was isolated in a slightly increased
yields and enantiopurity. Dramatically improved enantiose-
lectivities were observed when CF3-pyrox L3–L5 were
employed as ligands and L5 stood out in terms of enantio-
selectivity (entry 5). Other palladium(II) salts, such as Pd-
(OPiv)2 and Pd(TFA)2 gave the product 3a in lower yields
(entries 6–7). After further optimization of the reaction
parameters varying the palladium loading, Pd/L5 ratio,
reaction temperature and solvents, the optimum conditions
consisted of performing the reaction of 1a with 2a in DCE
Table 1: Optimization of reaction conditions.[a]
Scheme 2. Scope of cyclopenetenes. [a] General conditions:
1 (0.12 mmol), 2 (0.1 mmol), Pd(OAc)2 (10 mol%), L5 (12 mol%),
DCE (0.17 M), 608C, O2 balloon. The ee was determined by SFC
analysis on a chiral stationary phase. [b] d.r.=15:1.
Entry
L*
PdX2
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
L1
L2
L3
L4
L5
L5
L5
L5
L5
L5
L5
L5
L5
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OPiv)2
Pd(TFA)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Toluene
THF
25
35
39
43
38
15
18
72
67
50
63
38
75
70
À72
À88
88
and ortho) is compatible with the reaction conditions (3b–
3g), so is the a-naphthyl unit (3h). The C1 aliphatic
substituent including the functionalized one (Me, Bn,
CH2CO2tBu) in compound 2 is well tolerated affording the
desired products 3i–3k without event. Finally, the C1
unsubstituted cyclopentene participates in the reaction to
afford 3l in a moderate yield. A cyclopentene containing both
an indole and an oxindole moiety 3m is similarly prepared
starting from the corresponding spirooxindole. The amide
part can also be varied as showcased by the preparation of
compound 3n bearing an isobutyl carboxyamide. The
(1S,4R)-absolute configuration of 3c is determined by X-ray
crystallographic analysis and the configuration of the other
products are assigned in analogy.[22]
93
ND
ND
93
85
85
91
93
93
8[d]
9[d]
10[d]
11[d]
12[d]
13[e]
DMF
CHCl3
DCE
[a] General conditions: 1a (0.1 mmol), 2a (0.1 mmol), Pd(OAc)2
(0.1 equiv), chiral Ligand (0.12 equiv), DCE (2.0 mL), 508C, O2 balloon,
24 h. [b] Isolated yield, d.r. >19:1. [c] Determined by SFC analysis on
a chiral stationary phase. [d] Pd(OAc)2 (0.15 equiv), chiral Ligand
(0.18 equiv) were used, the reaction time was 72 h. [e] 1a (0.12 mmol),
2a (0.1 mmol), DCE (0.6 mL) at 608C for 72 h.
A series of structurally diverse 2-alkynylanilines are next
evaluated to further probe the reaction scope (Scheme 3).
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Scheme 4. Possible reaction pathway.
Scheme 3. Scope of 2-alkynylanilines. [a] General conditions:
1 (0.12 mmol), 2 (0.1 mmol), Pd(OAc)2 (10 mol%), L5 (12 mol%),
DCE (0.17 M), 608C, O2 balloon. The ee was determined by SFC
analysis on a chiral stationary phase. [b] d.r.=13:1. [c] d.r.=15:1.
the desired aminopalladation of 2-alkynylaniline 1a. On the
other hand, the reaction of compound H (Scheme 4, inset)
with 1a failed to produce the desired product indicating the
importance of the amide function for the present trans-
formation.
We have synthesized [1-(methylsulfonyl)-2-phenyl-3-
yl]boronic acid (4) via BCl3-mediated cyclization of 2-
alkynylaniline (Scheme 5).[25] Reaction of 4 with 2a under
our standard conditions afforded 3a in only 23% yield with
71% ee, together with 1-(methylsulfonyl)-2-phenyl-1H-indole
The electron-rich (3o, R = OMe) and electron-poor phenyl
(3p, R = CO2Me) substituent as well as heterocycle (thio-
phene, 3q) on the alkynyl part are tolerated. Pleasingly,
aliphatic alkynes participate in the reaction leading to the
corresponding products (3r–3u), albeit with diminished
yields. The presence of functional groups such as alkyl
chloride (3s), alkyl benzyl ether (3t) and methoxycarbonyl
group (3u) is compatible with the reaction conditions.
However, the terminal alkyne (R = H) is incompatible with
the present oxidative conditions. The presence of diverse
functional groups such as halide (3aa, R = Cl; 3z R = F), CF3
(3v) and importantly methoxycarbonyl group (3w) in the
aromatic part of the aniline is well tolerated. As it would be
expected, the N-Ts (3q–3z) and the easily removable N-Ns
(3aa) groups are effective nucleophiles for this transforma-
tion. Compound 3aa is prepared in a gram scale with a similar
yield and ee (1.18 g, 50%, 91% ee) indicating the practicality
of the present enantioselective domino process.
Scheme 5. Results of enantioselective oxidative Heck reaction using
pre-synthesized 3-borylated indole as a nucleophile. [a] BCl3, DCM, RT,
then H2O. [b] 2a, Pd(OAc)2 (10 mol%), L5 (12 mol%), O2, DCE, 608C.
resulting from the protodeborination of 4 as a major product
(65%, structure not shown). This result is in line with
Sigmanꢀs earlier observations[16a,21] and illustrates clearly the
advantage of our domino aminopalladation/oxidative Heck
process related to the classical stepwise approach.
A possible reaction pathway is depicted in Scheme 4 using
1a and 2a as reaction partners. An intramolecular amino-
palladation of 1a, catalyzed by (L5)PdX2 B in situ formed by
ligation of pyrox ligand to Pd acetate, would afford the
indolylPdX species C. Coordination of C to the double bond
of 2a directed by the amide function would generate the PdII
complex D which defined the face selectivity of the subse-
Taking advantage of the multiple functionalities in com-
pound 3, a variety of post-transformations could be envisaged.
One of such examples exploiting the selective functionaliza-
tion of the two double bonds in 3aa is shown in Scheme 6.[26]
Removal of the N-Ns group[27] from 3aa afforded 5 which
upon chemoselective epoxidation of indole double bond with
the in situ generated dimethyldioxirane followed by scandium
triflate promoted Wagner–Meerwein rearrangement fur-
nished oxindole 6 in 53% yield.[28] The relative stereochem-
istry of 6 was determined by the X-ray structure analysis of its
racemic form. It is reasonable to assume that epoxidation
occurred preferentially from the a face of the double bond for
the steric reason to afford 7 as a major stereoisomer. Ring
quent syn-carbopalladation to afford the PdII species E.[23]
A
b-hydride elimination from E would afford F which, upon
decomplexation and reductive elimination of HX from the
resulting HPdX would generate the enantioenriched indole
derivative 3a and Pd0. The latter would be oxidized by oxygen
and acetic acid to re-generate the catalytic species B,[24]
completing therefore the catalytic cycle. We note that in
principle compound 2a could also undergo an intramolecular
aminopalladation to afford a 2-azabicyclo[2.2.1]heptanone
skeleton. However, this pathway is apparently dominated by
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Conflict of interest
The authors declare no conflict of interest.
Keywords: asymmetric synthesis · domino reactions ·
homogeneous catalysis · nucleopalladation ·
oxidative Heck reactions
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Scheme 6. Synthetic transformation of product 3aa. Reaction condi-
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steps; [d] OsO4, NMO, acetone/H2O, 51% yield, 92% based on
recovered starting material, d.r.=1.4:1; [e] Pb(OAc)4, MeCN, 08C,
71% yield; [f] CHCl3, 95% yield, d.r.=1.3:1.
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catalytic enantioselective aminopalladation/Heck reaction
cascade for the synthesis of functionalized cyclopentene-
indole conjugates with high diastereo- and enantio-selectiv-
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has a clear-cut advantage in the oxidative Heck reaction
relative to those generated by transmetallation of the
indolylboronic acids or by the CH activation. In view of the
established efficiency of the nucleopalladation process, we
expect that this strategy could be generally applicable to the
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Acknowledgements
We thank EPFL (Switzerland), Swiss National Science
Foundation (SNSF 200021-178846/1) for financial supports.
J.C. thanks China Scholarship Council for visiting scholar
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Accepted manuscript online: December 28, 2020
Version of record online: February 17, 2021
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