Asymmetric Counteranion Directed Catalytic Heck/Tsuji-Trost Annulation of Aryl Iodides and 1,3-Dienes

Article abstract of DOI:10.1021/acs.orglett.1c00910

A chiral anion-mediated asymmetric Heck/Tsuji-Trost reaction of aryl iodides and 1,3-dienes is presented. Chiral indoline derivatives could be afforded with remarkably higher yields and enantioselectivities than our previous chiral ligand-based method. Silver carbonate is employed as both base and halide scavenger to ensure fast and recyclable exchange of the catalytic amount of chiral anions. Fast salt metathesis, as well as the acceleration effect of the chiral anion, could both benefit the stereocontrol of the reaction.

Full text of DOI:10.1021/acs.orglett.1c00910

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Letter
Asymmetric Counteranion Directed Catalytic Heck/Tsuji−Trost
Annulation of Aryl Iodides and 1,3-Dienes
Jia-Cheng Xu, Yi-Zhuo Yin, and Zhi-Yong Han*
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ABSTRACT: A chiral anion-mediated asymmetric Heck/Tsuji−Trost reaction of aryl iodides and 1,3-dienes is presented. Chiral
indoline derivatives could be afforded with remarkably higher yields and enantioselectivities than our previous chiral ligand-based
method. Silver carbonate is employed as both base and halide scavenger to ensure fast and recyclable exchange of the catalytic
amount of chiral anions. Fast salt metathesis, as well as the acceleration effect of the chiral anion, could both benefit the stereocontrol
of the reaction.
interactions between the metal center and the chiral anion,
rather than the strong coordination between the metal and the
ligand, achieving high enantioselectivity with chiral anions is
rather difficult. Nevertheless, the field of chiral anion-mediated
asymmetric transition-metal catalysis has witnessed a con-
tinuous development in the past decades, and has become a
powerful platform in asymmetric synthesis.3,5
Palladium-catalyzed asymmetric Heck-type reactions are of
great importance for the synthesis of synthetically valuable
chiral compounds.4 While most of the asymmetric Heck-type
transformations rely on the use of various chiral ligands,
Toste’s group developed an alternative chiral anion phase
transfer strategy that employed chiral phosphate anions as the
sole chiral source (Scheme 1b).6 A series of asymmetric Heck-
type reactions of aryl diazonium salts and various alkenes have
been reported applying this strategy.7 While these successes
have demonstrated the value of the chiral anion strategy in
asymmetric Heck-type reactions, the great utilities of the
methodologies are, however, somehow tempered by the
potential instability of the irreplaceable aryl diazonium salts.8
ransition-metal catalyzed asymmetric transformations
Thave served as a powerful means to access chiral
compounds.1 Typically, two alternative approaches exist for
the introduction of chirality in these reactions: using chiral
ligands that bind directly to the metal center2 or introducing
chiral anions to form chiral ion pairs linked by electrostatic
interactions,3 as exemplified by the asymmetric Heck-type
reactions4 (Scheme 1a). Considering the weak electrostatic
Scheme 1. Strategies for Asymmetric Heck-Type Reactions
Received: March 16, 2021
Published: May 7, 2021
https://doi.org/10.1021/acs.orglett.1c00910
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3834−3838
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Albeit that frequently used diazonium tetrafluoroborates are
often significantly more stable, some of them, such as 3-pyridyl
diazonium tetrafluoroborate, can still undergo spontaneous
decomposition.9 Further, high electrophilicity of diazonium
salts may also cause side reactions and poor compatibility with
certain functional groups. On the other hand, aryl halides, as a
standard aryl source in Heck-type reactions, feature high
stability, compatibility, as well as feasible accessibility and
satisfying reactivity.4c Thus, developing alternative methods
employing aryl halides as the substrates instead of aryl
diazoniums is of practical importance in chiral anion-mediated
chemistry.7 Previously, our group reported an enantioselective
multicomponent carbonyl allylation of aldehydes with dienes
and alkynyl bromides.10 Inspired by this work, as well as
previously mentioned references using aryl diazoniums, we
envisioned that aryl halides might be ideal substrate in the
presence of a proper halide scavenger and a chiral anion
source. As shown in Scheme 1c, with basic silver salts as the
halide scavenger11 in the presence of a chiral Brønsted acid,12
the oxidative addition complex can be readily converted to a
chiral anion pair, and renders the following Heck-type
transformations enantioselective.
To testify the hypothesis, we turned our attention to realize
the palladium-catalyzed enantioselective cascade Heck/Tsuji−
Trost reaction (also known as 1,2-carboamination) of 1,3-
dienes13 with 2-iodoaniline derivatives through a chiral
counteranion strategy. The reaction pioneered by Dieck14
and Larock15 features high efficiency to construct heterocycles
with simple starting materials. However, the enantioselective
version of the intermolecular transformation remained a
challenging problem for decades. Employing chiral phosphor-
amidite ligands, our group disclosed an enantioselective
annulation of 1,3-dienes with 2-iodoanilines, but with only
up to 87% ee (Scheme 2a).16 This reaction might be an ideal
platform to test the chiral anion-mediated Heck-type reactions
with aryl halides. Our detailed design is shown in Scheme 2b
with silver carbonate as the halide scavenger and a catalytic
amount of chiral phosphoric acid (B*-H)12 as the chiral anion
source. Initially, aryl iodide 1 undergoes oxidative addition to
the palladium center to give aryl palladium iodide intermediate
I. Then, salt metathesis17 can occur between palladium iodide
species I and the chiral silver phosphate, which is in situ
generated from silver carbonate and the chiral acid (path a).
1,3-Diene 2 then undergoes migratory insertion with the
resulted chiral ion pair II to give π-allyl−Pd complex III
bearing a chiral counteranion. Subsequent chiral anion-
controlled intramolecular enantioselective Tsuji−Trost proc-
ess18 produces chiral indoline 3 and regenerates both the
Pd(0) catalyst and the chiral phosphoric acid for the next
catalytic cycle. Notably, it is also possible that intermediate I
directly reacts with 1,3-diene 2 to form an achiral π-allyl−Pd
complex III′, which may possibly lead to racemic product. On
the other hand, it is quite likely that III′ can also transform
into III via fast salt metathesis, ensuring the stereocontrol of
the whole process. It is worth mentioning that, if necessary, the
silver source could be recycled from the silver iodide
precipitate through well-established procedures.19 Herein, we
report the chiral anion-mediated asymmetric Heck/Tsuji−
Trost annulation of aryl iodides and 1,3-diene, leading to chiral
indolines with up to 93% ee.
Our investigation started with the reaction of N-tosyl-2-
iodoaniline 1a, (E)-1-Phenyl-1,3-butadiene 2a, Ag2CO3, and 4
Å molecular sieves with catalytic amounts of Pd(PPh3)4 and
BINOL-derived chiral phosphoric acid 4a (Table 1).
Encouragingly, desired chiral indoline 3aa could be obtained
in 59% yield 28% ee (entry 1). Inspired by literature reports of
DMSO effects on Heck-type reactions,7e we were pleased to
find that addition of a small amount of DMSO to the system
could significantly improve the outcome of the reaction to 81%
yield and 46% ee (entry 2). Subsequently, a number of chiral
phosphoric acids were evaluated to further enhance the
enantioselectivity. Enlarging the steric hindrance of substitu-
ents at the 3,3′-positions of the binaphthyl backbone could
clearly benefit the enantioselectivity of the reaction (up to 86%
ee, entries 3−4 vs entry 2). SPINOL-based chiral phosphoric
acids led to 3aa in similar levels of yield, but with lower
enantioselectivity (entries 5−6). Switching to H8-BINOL-
based phosphoric acids resulted in slightly higher enantiose-
lectivities (entries 7−9), and the highest enantioselectivity of
90% ee could be observed with 4h. Switching silver carbonate
to other metal carbonates, such as K2CO3, Na2CO3, and
PbCO3, no enantioselectivity was observed (entries 10−12),
indicating that silver cation might be crucial to ensure fast
anion exchange to form chiral ion pair II. Silver phosphate as
the base and halide scavenger could also render the reaction
enantioselective, but with lower enantioselectivity (entry 13),
while AgBF4, AgF, or organic base DIPEA all led to racemic
product (entries 14−16). These results disclosed that fast
formation of the chiral silver phosphate is also essential for the
stereocontrol of the reaction. The effect of phosphine ligands
was also examined, identifying Ph3P to be the optimal ligand
for the reaction (see Supporting Information for details).
With the optimal reaction conditions in hand, the generality
of 2-iodoanilines was first investigated for the reactions with
1,3-diene 2a (Scheme 3). The substrates with either an
electron-donating or electron-withdrawing substituent (1b−
1i) were amenable to this reaction, delivering the correspond-
ing indolines 3ba−3ia in high yields of up to 89% and with
high enantioselectivities ranging from 86% to 93% ee.
Meanwhile, for 4-substituted substrates, electron-donating
Scheme 2. Two Strategies for the Asymmetric Heck/Tsuji−
Trost Annulation Reaction
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a
a
Table 1. Reaction Optimization
Scheme 3. Substrate Scope
b
c
entry
CPA
base
yield (%)
ee (%)
d
1
2
3
4
5
6
7
8
4a
4a
4b
4c
4d
4e
4f
4g
4h
4h
4h
4h
4h
4h
4h
4h
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
K2CO3
Na2CO3
PbCO3
Ag3PO4
AgBF4
59
81
78
78
76
79
78
80
80
72
75
64
78
68
76
51
28
46
82
84
74
78
83
86
90
0
0
0
76
0
0
9
10
11
12
13
14
e
f
15
16
AgF
DIPEA
0
a
Unless otherwise indicated, reaction conditions were as follows: 1a
(0.10 mmol), 2a (0.20 mmol), Pd(PPh3)4 (0.010 mmol), CPA (0.012
mmol), base (0.15 mmol), 4 Å molecular sieves (40 mg), DMSO (30
μL), and solvent (1 mL) at N2 atmosphere 70 °C for 12 h.
b
1
Determined by H NMR analysis using 1,3,5-triacetylbenzene as an
c
d
internal standard. Determined by HPLC analysis. Without DMSO.
e
f
0.1 mmol Ag3PO4. 0.3 mmol AgBF4.
a
Reactions of 1b−1k (0.10 mmol), 2a (0.20 mmol), Pd(PPh3)4
(0.010 mmol), Ag2CO3 (0.15 mmol), 4h (0.012 mmol), 4 Å
molecular sieves (40 mg), and DMSO (30 μL) were carried out in
toluene (1 mL) at 70 °C for 12 h.
groups are slightly more favored regarding the yield of the
reaction (3ba, 3ca vs 3ea, 3fa). In contrast, electron-
withdrawing groups benefit the yield when the substitution is
at the 5-position (3ga vs 3ha, 3ia). It is worth mentioning that
in our previous reports using chiral phosphoramidite ligand,
only 58−77% ee were observed for 5-substituted 2-iodoani-
lines. Moreover, the variation of N-protecting groups was also
amenable to the reaction, leading to good yields and high
enantioselectivities (3ja, 3ka). Subsequently, the generality of
1,3-dienes was explored (3ab−3am). The reactions of phenyl-
substituted 1,3-dienes bearing electron-donating or electron-
withdrawing groups all proceeded smoothly to afford the
corresponding chiral indoline in 72−86% yield and 85−90%
ee, regardless of the substitution pattern of the benzene ring
(3ab−3ah). 2-Furanyl and 2-naphthyl substituted 1,3-dienes
also turned out to be good substrates for the reaction. Notably,
1-alkyl-1,3-dienes could also undergo the cascade reaction to
afford the desired indolines with good yields and enantiose-
lectivities (3ak, 3al). In the case of (E)-penta-1,3-diene, the
indoline product was obtained as a 10:1 mixture of regioisomer
in 72% yield and 75% ee of the major isomer (3am).
an hour resulted in complete consumption of 1a, affording aryl
palladium iodide 5 bearing two triphenylphosphine ligands.
The structure of 5 was unambiguously confirmed via single
crystal X-ray diffraction analysis. On the basis of literature
reports,17 silver metathesis is not guaranteed to take place
between labile halide complexes and silver salts. Typical
exceptions would result in the formation of metal−metal-
bonded adducts, along with the lack of silver halide
precipitation in the system. To confirm the silver metathesis
in our reaction, while direct characterization was not
successful, a clear solution of aryl palladium iodide 5 was
poured into the solution of chiral silver phosphate Ag-(R)-
TRIP obtained from chiral phosphoric 4b. A light yellow
precipitate, most like the silver iodide, was observed instantly,
indicating the presence of a fast anion exchange process (see
Supporting Information for photographs). The reaction of
palladium complex 5, 1.0 equiv of Ag-(R)-TRIP and diene 2a
proceeded well to give indoline 3aa in 75% yield and 84% ee
(Scheme 4b), a similar level of enantioselectivity compared to
the model reaction of 1a and 2a (Table 1, entry 3). On the
other hand, leveling up the catalyst loading of 4b to 100 mol %
To verify the hypothesized mechanism of the chiral anion-
mediated Heck/Tsuji−Trost reaction, control experiments and
kinetic studies were performed. As shown in Scheme 4a,
treatment of 1a with Pd2dba3 and PPh3 at 70 °C in toluene for
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Accession Codes
Scheme 4. Isolation of Reaction Intermediate, Control
Experiments, and Kinetic Studies
CCDC 2051745−2051746 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Zhi-Yong Han − Hefei National Laboratory for Physical
Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei
230026, China; orcid.org/0000-0002-9225-5064;
Email: hanzy2014@ustc.edu.cn
Authors
Jia-Cheng Xu − Hefei National Laboratory for Physical
Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei
230026, China
Yi-Zhuo Yin − Hefei National Laboratory for Physical
Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei
230026, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00910
Notes
The authors declare no competing financial interest.
for the reaction of iodide 1a and 1,3-diene 2a could indeed
increase the enantioselectivity to 90% ee. The acceleration of
enantioselectivity is probably due to a more complete anion
exchange process brought by in situ generation of complex 5
through oxidative addition. All these observations are
consistent with the hypothesized chiral anion mediated
reaction pathway in Scheme 2. Another factor that could
facilitate the stereocontrol of the reaction is the acceleration
effect of the chiral counteranion. As shown in Scheme 4d,
kinetic studies revealed that the presence of chiral phosphoric
acid 4b as the anion source led to a remarkable boost of the
reaction rate, thus further reducing the impact of the undesired
racemic process (see Supporting Information for details and
discussions). Notably, product 3aa could be converted to (S)-
methyl indoline-2-carboxylate without erosion of the optical
purity in three steps (see Supporting Information for details).
In summary, we have developed a chiral anion-mediated
enantioselective Heck/Tsuji−Trost reaction of 2-iodoanilines
and 1,3-dienes. In general, the current method affords chiral
indolines with higher yields and enantioselectivities compared
to our previous chiral ligand-based methodology. By employ-
ing aryl iodides as the substrate in chiral anion-mediated Heck-
type reaction, the protocol has also demonstrated the potential
of readily available halides in chiral anion-mediated transition
metal catalysis.
ACKNOWLEDGMENTS
We are grateful for the financial support from NSFC (Grants
21971231, 21772184, 21831007).
■
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00910.
Experimental details and characterization data (PDF)
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