Diverting β-Hydride Elimination of a ?-Allyl PdIICarbene Complex for the Assembly of Disubstituted Indolines via a Highly Diastereoselective (4 + 1)-Cycloaddition

Article abstract of DOI:10.1021/acs.orglett.0c02374

A Pd0-catalyzed formal (4 + 1)-cycloaddition approach to 2,3-disubstituted dihydroindoles is described. The diastereoselective formation of dihydroindoles that is highlighted by a carbene migratory insertion/reductive elimination sequence proceeding via a ?-allyl PdII-species compliments existing methods of indoline assembly.

Full text of DOI:10.1021/acs.orglett.0c02374

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Letter
Diverting β‑Hydride Elimination of a π‑Allyl PdII Carbene Complex
for the Assembly of Disubstituted Indolines via a Highly
Diastereoselective (4 + 1)-Cycloaddition
Zachary D. Tucker, Harrison M. Hill, Andrew L. Smith, and Brandon L. Ashfeld*
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ABSTRACT: A Pd0-catalyzed formal (4 + 1)-cycloaddition
approach to 2,3-disubstituted dihydroindoles is described. The
diastereoselective formation of dihydroindoles that is highlighted
by a carbene migratory insertion/reductive elimination sequence
proceeding via a π-allyl PdII-species compliments existing methods
of indoline assembly.
lthough less developed than the (3 + 2) analog,1 (4 + 1)-
cycloadditions offer a powerful, complementary retro-
rearrangement) to a migratory insertion/reductive elimination
sequence reticent of Pd-catalyzed enyne and diene cyclo-
isomerizations (Scheme 1b).5 Herein, we disclose the
successful execution of a Pd-catalyzed (4 + 1)-cycloaddition
via the migratory insertion of a PdII−carbene complex with
high levels of diastereoselectivity.
A
synthetic disconnect for the generation of highly substituted
five-membered carbo- and heterocyclic systems.2 A major
challenge in the design and implementation of (4 + 1)-
cycloadditions is the identification of compatible C4- and C1-
atom synthons that provide the desired cycloadduct in a
stereoselective fashion with a high degree of functional group
compatibility.3 Recently, we reported on the development of
(4 + 1)-cycloaddition strategies that employ diazo compounds
as C1-synthons in the presence of a RhII−carboxylate catalyst
(Scheme 1a).4 The net cycloaddition occurs via the initial
Seminal work by the Wang and Van Vranken groups have
revealed the utility of palladium carbenes in a variety of
different cross-couplings employing aryl/alkyl halides,6a−e
boronic acids,7a−c and nitrogen/carbon nucleophiles8a−d
wherein migratory insertion serves as the critical C−C bond
forming event. While initial reports focused on migratory
couplings of metallocarbenes to η1-ligands, as demonstrated by
Van Vranken’s approach to allyl amines (Scheme 2a), inner
sphere migrations involving η3-allyl metal complexes have
likewise proven effective. For example, Wang demonstrated
that the Pd-catalyzed oxidative addition to allyl acetates and
allyl halides in the presence of a diazo ester led to a migratory
insertion/β-hydride elimination sequence resulting in 1,3-diene
formation (Scheme 2b). We sought to divert β-hydride
elimination through the incorporation of an internal
nucleophile resulting in a net cycloaddition. To achieve this
aim, we speculated that treatment of vinyl benzoxazinone 1
and the diazo compound generated in situ from N-sulfonyl
hydrazone 2 and base in the presence of a Pd0 catalyst would
provide dihydroindole 3 via sequential, intramolecular C−C
and C−N bond formations from π-allyl Pd complex 4 (Scheme
2c).9 An important consideration in our proposed (4 + 1)-
cycloaddition was the coordination capability of the pendant
Scheme 1. Mechanistic Divergence in (4 + 1)-
Cycloadditions
cyclopropanation of either a vinyl ketene or vinyl isocyanate
followed by a 1,3-carbon migration to provide the correspond-
ing cyclopentenone or lactam, respectively.4b−d Enantioselec-
tive formation of the cyclopentenone α-quaternary carbon was
achieved employing a Davies chiral RhII −carboxylate complex.
Seeking to exploit this synthetic disconnect beyond vinyl
ketenes/isocyanates, we speculated that greater architectural
diversity could be achieved by altering the mechanistic profile
from a ring expansion event (i.e., vinyl cyclopropane
Received: July 16, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02374
© XXXX American Chemical Society
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dihydroindole in 77% yield and >20:1 dr. Employing
Pd(PPh3)4 led to only trace amounts of 3a, while the addition
of P(furyl)3, a common phosphine employed in Pd-carbene
cross-couplings, to [Pd(allyl)Cl]2 or Pd(OAc)2 provided 3a in
29% and 19% yield, respectively (entries 6−8).6b,10,11a−c The
bidentate phosphine dppe also failed to improved the yield of
3a (entries 9; dppp: 3a = 28%).
Scheme 2. Selected Pd−Carbene Migratory Insertions
With optimized conditions in hand, we turned our attention
toward evaluating the architectural limitations of vinyl
benzoxazinone 1 in a Pd-catalyzed (4 + 1)-cycloaddition
with N-tosyl phenylhydrazone 2a (Scheme 3). Olefin R2-alkyl
Scheme 3. Structural Variations on Vinyl Benzoxazinone 3
sulfonamide upon oxidative addition to the vinyl benzox-
azinone, and whether this secondary ligation would inhibit
diazo decomposition. Additionally, the presence of a
phosphine ligand to facilitate π-allyl PdII formation could
diminish the capability of accommodating a metallocarbene,
thereby leading to catalyst inhibition.
In an effort to assuage these initial concerns, we began our
study by examining the Pd-catalyzed (4 + 1)-cycloaddition of
vinyl benzoxazinone 1a with hydrazone 2a (Table 1). Our
a
Table 1. Optimization of Reaction Conditions
Entry
Catalyst
Ligand
Base
Solvent Yield (%)
1
2
3
4
5
6
7
8
9
Pd2dba3·CHCl3
Pd2dba3·CHCl3
Pd2dba3·CHCl3
Pd2dba3
−
−
−
−
−
−
tBuOK
tBuOK
tBuOK
tBuOK
Cs2CO3
tBuOK
tBuOK
tBuOK
tBuOK
THF
DMF
PhMe
THF
THF
THF
THF
THF
THF
69
5
32
74
b
Pd2dba3
Pd(PPh3)4
87(77)
<5
29
[Pd(allyl)Cl]2
Pd(OAc)2
P(furyl)3
P(furyl)3
dppe
and -aryl substitution at C2 provided dihydroindoles 3b and 3c
in 91% and 90% yield, respectively. However, olefin 1,1-
dimethyl substitution failed to yield the anticipated dihy-
droindole 3d, but favored C−N reductive elimination to give
dihydroquinoline 5a in quantitative yield. However, sub-
stitution of the benzylic position and 1,2-substitution of the
alkene were not tolerated under the optimized reaction
conditions. Interestingly, the (4 + 1)-cycloaddition proved
sensitive to the N-substituent on 1. For example, the N-nosyl
benzoxazinone gave 3e in 52% yield, while the corresponding
N-Boc benzoxazinone led to selective deacylation of the
carbonate to give 1a′ (R1 = H) in 92% yield. Resubjection of
1a′ to the optimized reaction conditions failed to undergo
cycloaddition to 3g; rather an intractable mixture of
byproducts was observed. In contrast, substitution of the
arene was well tolerated, but product distribution proved
dependent on the electronic impact of the substituent. For
example, 5-chloro vinyl benzoxazinone yielded dihydroindole
3h in 82%, but a CF3-group at the 5-position led to a 1.5:1
mixture of the (4 + 1)-cycloadduct 3i and S- allylation product
6a in 96% combined yield. Similarly, 4-methoxy substitution
19
Pd(OAc)2
40
a
Conditions: 3a (0.05 mmol), 4a (0.06 mmol), base (0.06 mmol), Pd
catalyst (5 mol %), solvent (0.5 mL). Performed on 3 mmol scale.
b
initial attempts at facilitating the (4 + 1)-cycloaddition of
benzoxazinone 1a with N-tosylhydrazone 2a focused on
assessing the feasibility of conducting the reaction under
essentially ligandless conditions by employing Pd2dba3·CHCl3
(5 mol %) and tBuOK in THF. While conducting the reaction
at room temperature resulted only in recovered benzoxazinone
1a, conducting the reaction at 60 °C provided the 2,3-anti
dihydroindole 3a in 69% yield after 4 h as a single diastereomer
(entry 1). The relative stereochemistry was assigned by X-ray
crystallography. Variation of the solvent provided the formal
cycloadduct in diminished yields (entries 2 and 3). While
t
employing Pd2dba3 with BuOK increased the yield of 3a to
74%, using Cs2CO3 to generate the desired diazo compound
improved the yield to 87% (entries 4 and 5). These conditions
also proved tolerable on a 3 mmol scale, providing the desired
B
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gave cycloadduct 3j and S-allyl sulfone 6b in a ratio of 1.9:1
and 80% yield.
Scheme 5. Synthesis of Optically Enriched Indolines
In general, varying the structural and electronic properties of
N-tosyl hydrazone 2 in the (4 + 1)-cycloaddition of vinyl
benzoxazinone 1a resulted in moderate to good yields of the
corresponding cycloadducts 3 (Scheme 4). Alkyl hydrazones
Scheme 4. Structural Variations on the N-Tosylhydrazone 2
Thus, exposure of benzoxazinone 1a and the sodium sulfonate
Na-2a to Pd2dba3 (5 mol %), stilbene diamine ligand L2 (10
mol %), Cs2CO3, and BnNEt3Cl provided dihydroindole 3a in
53% yield and 31% ee. Employing ligands L3 and quinox
derivative L4 failed to improve the yield or optical enrichment
of 3a.14 These results demonstrate the feasibility of inducing
asymmetry in 2,3-disubstituted indoline assembly wherein the
enantio-determining step is presumably a stereoselective
migratory insertion of a PdII-carbene into a pendant π-allyl
ligand. Whether the optical enrichment of dihydroindole 3a
occurs via a DyKAT mechanism or kinetic resolution of the
starting racemic benzoxazinone is unclear at the present time.
In an effort to elucidate the role of the catalyst and base in
the key C−C and C−N bond forming events, we evaluated the
conversion of benzoxazinone 1a to indoline 3a while omitting
either Pd0 or base (Scheme 6). Treatment of 1a and hydrazone
were not tolerated under the reaction conditions; however, aryl
hydrazones bearing the resonance electron donating groups 4-
N,N-dimethylamino and 4-silyloxy gave cycloadducts 3k and 3l
in 70% and 51% yields, respectively. Alkyl substitution of the
aryl hydrazone at the para, meta, and ortho positions led to
cycloadducts 3m−o in 32−61% yields, where the more
sterically demanding 2-methyl aryl hydrazone was the least
efficient substrate of this series, giving the corresponding
dihydroindole 3o in 32% yield. Similarly, the 2-bromo aryl
hydrazone led to cycloadduct 3p in 32%, while the 4-bromo
analog yielded adduct 3q in 61% yield. Notably, no side
products resulting from oxidative addition to the Ar−Br bond
were detected. Additionally, the 4-CF3, 4-CO2Me, and 4-NO2
substituted aryl hydrazones underwent cycloaddition in 39−
55% yield providing 3r−t. In contrast, the 3-pyridyl hydrazone
was not tolerated under the reaction conditions, resulting in
complete recovery of 1a. Whether this lack of reactivity is a
result of catalyst inhibition by the pyridine ring or inefficient
diazo decomposition is unclear at this time. The cinnamyl
derived hydrazone also underwent (4 + 1)-cycloaddition with
1a to provide the anticipated dihydroindole 3v in 72% yield.
Using the conversion of racemic benzoxazinone 1a to
indoline 3a, we sought to evaluate the potential of the Pd-
catalyzed (4 + 1)-cycloaddition to provide the formal
cycloadducts in optically enriched form via a dynamic kinetic
asymmetric transformation (DyKAT). Employing a series of
mono- and bidentate chiral ligands L1−L4 known for inducing
asymmetry in Pd-catalyzed processes, we surveyed chiral
ligands, well-known for their capability to induce asymmetry
in Pd-catalyzed transformations. However, the addition of (R)-
MonoPhos (L1) under our optimized reaction conditions
resulted in exclusive N-allylation of hydrazone 2a (Scheme 5).
In an effort to suppress N-allyl hydrazone formation, we
modified those conditions shown by Liang and co-workers to
minimize hydrazone addition to propagyl-PdII intermediates.13
Scheme 6. Elucidating the Role of Pd0 and Base
2a in the absence of Cs2CO3 led to quantitative formation of
dihydroquinoline 5b, presumably arising via reductive
elimination of the initial π-allyl PdII complex formed upon
oxidative addition of Pd0 to 1a. Not surprisingly, the absence of
Pd2dba3 resulted in complete recovery of benzoxazinone 1a.
These results would seem to confirm that generation of both
an intermediate π-allyl PdII complex and aryl diazomethane, via
a base-induced desulfonylation of hydrazone 1, are crucial to
dihydroindole 3 formation.
Based on our composite findings, in conjunction with work
by Wang, Van Vranken, and others, a plausible mechanism for
the diastereoselective, (4 + 1)-cycloaddition assembly of 2,3-
disubstituted indolines is illustrated in Scheme 7.6a,7c,15
Oxidative decarboxylation of benzoxazinone 1a by Pd0 results
in formation of π-allyl PdII complex 8,9a,b which upon
decomposition of phenyl diazomethane 9 arising from
desulfonylation of hydrazone 2a leads to Pd-allyl carbene
C
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indicate the viability of a π-allyl PdII carbene intermediate
followed by sequential C−C and C−N bond formation to
construct the indoline core framework. Likewise, the addition
of chiral phosphorus-based ligands demonstrates the potential
of inducing an enantiodiscriminating migratory insertion/
reductive elimination cascade en route to optically enriched
cycloadducts. Current studies are focused on the development
of an effective enantioselective (4 + 1)-cycloaddition approach
toward the construction of highly substituted indolines and
elucidating the mechanistic details of the Pd-catalyzed
cycloaddition presented herein.
Scheme 7. Proposed Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02374.
Experimental procedures, spectroscopic data, and
crystallographic data (PDF)
Accession Codes
CCDC 1991894 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
4a.12a,16 A diastereoselective migratory insertion event yields
six-membered palladacycle 10, which upon reductive elimi-
nation affords 3a. While the oxidative addition of Pd0 to vinyl
benzoxinones is relatively well-established,9 the formation of a
π-allyl PdII−carbene complex and subsequent sequential C−C
and C−N bond formations are significantly less precedented.
However, the trend we observed wherein aryl hydrazones
bearing electron-donating substituents provided the corre-
sponding (4 + 1)-cycloadducts in better yields than their
electron-neutral or electron-deficient counterparts is consistent
with a mechanism involving PdII−carbene formation.12a,b In
contrast, a mechanism involving direct outer-sphere attack on
the π-allyl ligand, complementary to the work of Li and Tan,
and that of others, involving sulfur ylides, cannot be ruled
out.9b
AUTHOR INFORMATION
Corresponding Author
■
Brandon L. Ashfeld − Department of Chemistry and
Biochemistry, University of Notre Dame, Notre Dame, Indiana
46556, United States; orcid.org/0000-0002-4552-2705;
Email: bashfeld@nd.edu
Authors
Zachary D. Tucker − Department of Chemistry and
Biochemistry, University of Notre Dame, Notre Dame, Indiana
46556, United States
Harrison M. Hill − Department of Chemistry and Biochemistry,
University of Notre Dame, Notre Dame, Indiana 46556,
United States
While speculative, a rationale for the selective formation of
the anti-dihydroindole products may result from the alleviation
of an unfavorable steric interaction between the aryl
substituent on the palladium-stabilized carbene and the N-
sulfonyl group in intermediate 4a. Assuming the stereo-
determining step is migratory insertion of the carbene center to
the π-allyl ligand in 4a, isomer anti-4a leads to the observed
stereoisomer 10, whereas syn-4a would provide the corre-
sponding syn stereoisomeric intermediate. The favored anti-4a
positions the hydrogen of the carbene and the N-sulfonyl
group syn thereby minimizing the unfavorable steric inter-
actions present in syn-4b. The dynamic equilibrium between
anti-4a and syn-4b may account for the exceptional
diastereoselectivity observed in the formation of the anti-
dihydroindole 3a.
Andrew L. Smith − Department of Chemistry and Biochemistry,
University of Notre Dame, Notre Dame, Indiana 46556,
United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02374
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(CHE-1665440). We thank Dr. Allen G. Oliver at the
University of Notre Dame for assistance in collecting X-ray
crystallographic data.
In conclusion, we were able to demonstrate how the
inherent biphilic reactivity of diazo compounds are harnessed
in (4 + 1)-cycloadditions by exploiting benzoxazinones as
masked 1,4-dipoles in the presence of a Pd0 catalyst. The
resulting 2,3-disubstituted dihydroindoles are obtained in
moderate to high yields and exceptional levels of diaster-
eoselectivity (dr >20:1). A series of control experiments
REFERENCES
■
(1) (a) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1986, 25 (1), 1−20.
(b) Gothelf, K. V.; Jørgensen, K. A. Chem. Rev. 1998, 98 (2), 863−
910. (c) Trost, B. M.; Silverman, S. M.; Stambuli, J. P. J. J. Am. Chem.
Soc. 2007, 129 (41), 12398−12399. (d) Trost, B. M.; Bringley, D. A.
Angew. Chem., Int. Ed. 2013, 52 (16), 4466−4469. (e) Trost, B. M.;
D
https://dx.doi.org/10.1021/acs.orglett.0c02374
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Bringley, D. A.; Zhang, T.; Cramer, N. J. J. Am. Chem. Soc. 2013, 135
(44), 16720−16735. (f) Hashimoto, T.; Maruoka, K. Chem. Rev.
2015, 115 (11), 5366−5412. (g) Trost, B. M.; Huang, Z.; Murhade,
G. M. Science 2018, 362, 564−568.
(14) We are grateful to a reviewer for suggesting the low
temperature diazo generation from the corresponding (2,4,6-
triisopropylphenyl)sulfonyl hydrazone of 2a, in order improve the
enantioselectivity, as reported in: Wang, Y.; Wen, X.; Cui, X.; Wojtas,
L.; Zhang, X. P. J. Am. Chem. Soc. 2017, 139, 1049−1052.
Unfortunately, this modification failed to increase the % ee of 3a at
lower temperatures.
(15) Xia, Y.; Wang, J. Chem. Soc. Rev. 2017, 46, 2306−2362.
(16) Maxwell, J. L.; Brown, K. C.; Bartley, D. W.; Kodadek, T.
Science 1992, 256, 1544−1547.
́
(2) (a) Spino, C.; Rezaei, H.; Dupont-Gadet, K.; Belanger, F. J. J.
Am. Chem. Soc. 2004, 126 (32), 9926−9927. (b) Coscia, R. W.;
Lambert, T. H. J. J. Am. Chem. Soc. 2009, 131 (7), 2496−2498.
̈
(c) Chen, J.-R.; Dong, W.-R.; Candy, M.; Pan, F.-F.; Jorres, M.; Bolm,
C. J. J. Am. Chem. Soc. 2012, 134 (16), 6924−6927. (d) Roiser, L.;
Zielke, K.; Waser, M. Synthesis 2018, 50 (20), 4047−4054.
(3) (a) Chen, J.-R.; Hu, X.-Q.; Lu, L.-Q.; Xiao, W.-J. Chem. Rev.
2015, 115, 5301−5365. (b) Gao, Z.; Wang, C.; Yuan, C.; Zhou, L.;
Xiao, Y.; Guo, H. Chem. Commun. 2015, 51 (63), 12653−12656.
(c) Kaur, T.; Wadhwa, P.; Bagchi, S.; Sharma, A. Chem. Commun.
2016, 52 (43), 6958−6976. (d) Eckert, K. E.; Ashfeld, B. L. Org. Lett.
2018, 20, 2315−2319. (e) Roiser, L.; Zielke, K.; Waser, M. Synthesis
2018, 50 (20), 4047−4054. (f) Zhou, Y.-Y.; Uyeda, C. Science 2019,
363 (6429), 857−862. (g) Eckert, K. E.; Lepore, A. J.; Ashfeld, B. L.
Helv. Chim. Acta 2019, 102 (12), No. e1900192.
(4) (a) Rodriguez, K. X.; Vail, J. D.; Ashfeld, B. L. Org. Lett. 2016,
18, 4514−4517. (b) Rodriguez, K. X.; Kaltwasser, N.; Toni, T. A.;
Ashfeld, B. L. Org. Lett. 2017, 19, 2482−2485. (c) Meloche, J. L.;
Ashfeld, B. L. Angew. Chem., Int. Ed. 2017, 56, 6604−6608.
(d) Rodriguez, K. X.; Pilato, T. C.; Ashfeld, B. L. Chem. Sci. 2018,
9, 3221−3226.
(5) (a) Trost, B. M. Acc. Chem. Res. 1990, 23, 34−42. (b) Trost, B.
M. Chem. - Eur. J. 1998, 4, 2405−2412.
(6) (a) Greenman, K. L.; Carter, D. S.; Van Vranken, D. L.
Tetrahedron 2001, 57, 5219−5225. (b) Barluenga, J.; Moriel, P.;
́
Valdes, C.; Aznar, F. Angew. Chem., Int. Ed. 2007, 46, 5587−5590.
(c) Chen, S.; Wang, J. Chem. Commun. 2008, 4198−4200. (d) Xiao,
Q.; Ma, J.; Yang, Y.; Zhang, Y.; Wang, J. Org. Lett. 2009, 11, 4732−
4735. (e) Xia, Y.; Hu, F.; Xia, Y.; Liu, Z.; Ye, F.; Zhang, Y.; Wang, J.
Synthesis 2017, 49, 1073−1086.
(7) (a) Zhao, X.; Jing, J.; Lu, K.; Zhang, Y.; Wang, J. Chem. Commun.
2010, 46, 1724−1726. (b) Tsoi, Y.-T.; Zhou, Z.; Chan, A. S. C.; Yu,
W.-Y. Org. Lett. 2010, 12, 4506−4509. (c) Kitamura, M.; Sakata, R.;
Okauchi, T. Tetrahedron Lett. 2011, 52, 1931−1933.
(8) (a) Devine, S. K. J.; Van Vranken, D. L. Org. Lett. 2007, 9,
2047−2049. (b) Devine, S. K. J.; Van Vranken, D. L. Org. Lett. 2008,
10, 1909−1911. (c) Xia, Y.; Xia, Y.; Zhang, Y.; Wang, J. Org. Biomol.
Chem. 2014, 12, 9333−9336. (d) Shang, X. S.; Li, N. T.; Siyang, H.
X.; Liu, P. N. J. J. Org. Chem. 2015, 80, 4808−4815.
(9) (a) Wang, C.; Pahadi, N.; Tunge, J. A. Tetrahedron 2009, 65,
5102−5109. (b) Li, T.-R.; Tan, F.; Lu, L.-Q.; Wei, Y.; Wang, Y.-N.;
Liu, Y.-Y.; Yang, Q.-Q.; Chen, J.-R.; Shi, D.-Q.; Xiao, W.-J. Nat.
Commun. 2014, 5, 5500. (c) Guo, C.; Fleige, M.; Janssen-Muller, D.;
̈
Daniliuc, C. G.; Glorius, F. J. J. Am. Chem. Soc. 2016, 138 (25),
7840−7843. (d) Wang, Y.-N.; Wang, B.-C.; Zhang, M.-M.; Gao, X.-
W.; Li, T.-R.; Lu, L.-Q.; Xiao, W.-J. Org. Lett. 2017, 19 (15), 4094−
4097. (e) Li, M.-M.; Wei, Y.; Liu, J.; Chen, H.-W.; Lu, L.-Q.; Xiao,
W.-J. J. J. Am. Chem. Soc. 2017, 139 (41), 14707−14713. (f) Jin, J.-H.;
Wang, H.; Yang, Z.-T.; Yang, W.-L.; Tang, W.; Deng, W.-P. Org. Lett.
2018, 20 (1), 104−107. (g) Das, P.; Gondo, S.; Nagender, P.; Uno,
H.; Tokunaga, E.; Shibata, N. Chem. Sci. 2018, 9 (13), 3276−3281.
(h) Wang, C.; Li, Y.; Wu, Y.; Wang, Q.; Shi, W.; Yuan, C.; Zhou, L.;
Xiao, Y.; Guo, H. Org. Lett. 2018, 20 (10), 2880−2883.
(10) Xia, Y.; Qiu, D.; Wang, J. Chem. Rev. 2017, 117, 13810−13889.
(11) (a) Amatore, C.; Jutand, A.; Meyer, G.; Atmani, H.; Khalil, F.;
Chahdi, F. O. Organometallics 1998, 17, 2958−2964. (b) Zhou, Y.;
Ye, F.; Wang, X.; Xu, S.; Zhang, Y.; Wang, J. J. J. Org. Chem. 2015, 80,
6109−6118. (c) Chen, Z.-S.; Duan, X.-H.; Zhou, P.-X.; Ali, S.; Luo, J.-
Y.; Liang, Y.-M. Angew. Chem., Int. Ed. 2012, 51, 1370−1374.
(12) (a) Qu, Z.; Shi, W.; Wang, J. J. J. Org. Chem. 2001, 66, 8139−
8144. (b) Wong, F. M.; Wang, J.; Hengge, A. C.; Wu, W. Org. Lett.
2007, 9, 1663−1665.
(13) Chen, Z.-S.; Duan, X.-H.; Wu, L.-Y.; Ali, S.; Ji, K.-G.; Zhou, P.-
X.; Liu, X.-Y.; Liang, Y.-M. Chem. - Eur. J. 2011, 17, 6918−6921.
E
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