Synthesis of Axially Chiral Olefin-Oxazoline Ligands via Pd-Catalyzed Multiple C-H Functionalization

Article abstract of DOI:10.1021/acs.orglett.0c03093

We report herein the Pd-catalyzed oxazoline-directed C-H olefination of the N-arylindole skeleton, affording two diastereomers of axially chiral olefin-oxazoline ligands in a one-step procedure. Modifications at the 3- and 3′-positions were facilely achieved via electrophilic substitution of the indole fragment and subsequent oxazoline-directed C-H amidation or olefination of the arene fragment.

Full text of DOI:10.1021/acs.orglett.0c03093

pubs.acs.org/OrgLett
Letter
Synthesis of Axially Chiral Olefin−Oxazoline Ligands via Pd-
Catalyzed Multiple C−H Functionalization
Ling-Jun Li, Jun-Jie Chen, Chen-Fei Feng, Han-Yuan Li, Xing Wang, Hui Xu, and Hui-Xiong Dai*
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sı Supporting Information
ABSTRACT: We report herein the Pd-catalyzed oxazoline-directed C−H olefination of the N-arylindole skeleton, affording two
diastereomers of axially chiral olefin−oxazoline ligands in a one-step procedure. Modifications at the 3- and 3′-positions were facilely
achieved via electrophilic substitution of the indole fragment and subsequent oxazoline-directed C−H amidation or olefination of
the arene fragment.
4
hiral ligands possessing axial chirality have been widely
asymmetric hydrogenation and C−C bond formation. Despite
the significant achievements, there has been ongoing interest in
ligand modification to improve the catalytic efficiency and
enantioselectivity in some asymmetric syntheses. Modifications
at the ortho positions of the coordinating group are highly
appealing because of the significant influence of the electron
density and steric hindrance. Zhou demonstrated that 6,6′-
diaryl substitution of SIPHOS resulted in remarkable improve-
ment in the Ni-catalyzed asymmetric reductive coupling of 1,3-
C
employed in transition-metal-catalyzed enantioselective
1
synthesis (Scheme 1A). Many types of axially chiral ligands
Scheme 1. 3,3′-Modification of the Axially Chiral Ligand
5
dienes with benzaldehyde. Very recently, Shi investigated a
series of BINOLs and found that 3,3′-disubstituted BINOLs
can significantly enhance the reactivity and enantioselectivity
6
in Pd(II)-catalyzed methylene C−H alkynylation. However,
ligand modifications often require excess organolithium
reagents, prefunctionalization of the ligand, and multistep
1
synthesis. Direct C−H functionalization at the 3- and 3′-
positions to diversify the axially chiral ligands is the most
efficient and straightforward way. However, only a few
7
examples have been reported in the literature. To develop a
practical methodology for ligand modifications, we turned our
attention to N-arylindole, which is an important structure
8
motif in organic chemistry. We envisioned that direct C2−H
amination, phosphination, oxidation, or olefination assisted by
a coordination group could be used to construct axially chiral
bidentate ligands. The 3- and 3′-positions of the ligand could
be facilely modified via electrophilic substitution at the 3-
position of the indole fragment and directed C−H
have been developed in the past decades. For example, the P,P-
ligand BINAP, discovered by Noyori, is a powerful ligand in
Ru-catalyzed asymmetric hydrogenation. The P,N-ligand
Received: September 15, 2020
2
QUINAP shows broad catalytic utility in Rh-catalyzed
asymmetric hydroboration of alkenes and asymmetric cyclo-
3
addition. The monodentate spiro phosphorus ligand SIPHOS,
developed by Zhou, has proved to be highly efficient in
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c03093
A
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functionalization at the 3′-position of the arene fragment
palladium salts were also effective, including PdCl , Pd(acac) ,
2
2
(Scheme 1B).
Pd(dppf)Cl , and Pd (dba) , albeit in lower yields (entries 3−
2
2
3
In recent years, chiral olefin ligands have emerged as
6). When AgTFA, Ag SO , Ag CO , and AgBF were
2 4 2 3 4
9
powerful ligands in asymmetric catalysis. In 2003, Hayashi
and co-workers reported Rh-catalyzed asymmetric conjugate
employed as the oxidants, 27−53% yields of olefin−oxazoline
ligand 3a were obtained (entries 7−10). Further investigation
of solvents, including DCE, DMF, and THF, showed that
1
0
additions employing bicyclo[2.2.1]heptadiene as the ligand.
11
Since then, chiral diene ligands, chiral phosphine−olefin
CH CN was the optimal solvent (entries 11−13). Although
3
1
2
ligands,
and chiral sulfur−olefin ligands have been
the d.r. value was improved to 4.2:1, the total yield significantly
decreased to 54% when we lowered the reaction temperature
to 60 °C (entry 14). Furthermore, when the reaction was
1
3
developed. By combining olefins with oxazolines, in 2010
Glorius and co-workers reported the bidentate olefin−
oxazoline (OlefOx) ligands, which were successfully applied
carried out under a N atmosphere, the desired product was
1
4
2
in asymmetric 1,4-conjugate addition. Inspired by the
successful applications of axially chiral ligands and the
bidentate olefin−oxazoline skeleton in asymmetric catalysis,
we report herein the synthesis of axially chiral olefin−oxazoline
ligands via Pd-catalyzed oxazoline-directed C−H olefination
formed in only 34% yield (entry 15). The reason for the low
diastereoselectivity may be that the stereogenic center is too far
away or the lower rotational barriers of axially chiral scaffolds
containing five-membered rings cause racemization of products
(
see the Supporting information).
With the optimal reaction conditions in hand, we turned our
(
Scheme 1C). The 3- and 3′-positions of the ligands were
conveniently modified by electrophilic C−H bromination and
attention to examine the substrate scope of indole−phenyl-
oxazoline derivatives 1 and various acrylates 2 (Scheme 2). To
our delight, regardless of their electronic properties, methyl-,
methoxy-, fluoro-, chloro-, bromo-, and trifluoromethyl-
substituted indole−phenyloxazoline derivatives 1 were well-
tolerated in the reaction (Scheme 2A). The corresponding
olefin−oxazoline ligands were obtained in moderate to good
oxazoline-directed C−H amidation and olefination.
Over the past decades, several practical strategies have been
developed for the synthesis of axially chiral biaryls via
transition-metal-catalyzed asymmetric C−H bond functional-
1
5
ization. Considering the wide application of the oxazoline
16
moiety in asymmetric synthesis, we commenced our studies
by choosing indole−phenyloxazoline skeleton 1a as the model
substrate for the synthesis of axially chiral olefin−oxazoline
ligand 3a. The optimization of the reaction conditions is
shown in Table 1. To our delight, in the presence of 10 mol %
Scheme 2. Substrate Scope of Indole−Phenyloxazoline
,
a b
Derivatives
a
Table 1. Optimization of the Reaction Conditions
b
c
entry
variation from the standard conditions
yield (%)
d.r.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
none
no Pd
PdCl2
82
−
1.2:1
−
52
30
46
16
46
27
53
34
40
43
68
54
34
1.3:1
1.6:1
1.2:1
1.3:1
1.3:1
1.2:1
1.2:1
1.1:1
1.1:1
1.4:1
1.5:1
4.2:1
1.2:1
Pd(acac)2
Pd(dppf)Cl2
Pd (dba)
2
3
AgTFA
Ag SO
2
4
Ag CO
2
3
1
1
1
1
1
1
AgBF4
DCE instead of CH CN
DMF instead of CH CN
THF instead of CH CN
3
3
3
at 60 °C
N atmosphere
2
a
Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), Pd(OAc) (10
2
mol %), AgOAc (0.15 mmol), CH CN (1.0 mL), air, 80 °C, 12 h.
3
b
Determined by 1H NMR analysis of the crude products using
c
1
CH Br as an internal standard. Determined by H NMR analysis.
2
2
Pd(OAc) and 1.5 equiv of AgOAc in CH CN, C−H bond
2
3
olefination occurred smoothly, affording two diastereomeric
ligands 3a with (S,R) and (S,S) configurations in 82% yield
with 1.2:1 d.r. (entry 1). As expected, no desired product was
a
Reaction conditions: 1 (0.1 mmol), 2 (0.15 mmol), Pd(OAc) (10
2
mol %), AgOAc (0.15 mmol), CH CN (1.0 mL), air, 80 °C, 12 h.
3
b
c
d
e
obtained in the absence of Pd(OAc) (entry 2), indicating that
Total yields of two diastereomers are shown. 9 h. 24 h. 100 °C, 36
2
the palladium catalyst was essential in the reaction. Other
h.
B
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Letter
yields (3a−l). The d.r. values of diastereomeric ligands ranged
from 1.1:1 to 1.8:1. Different chiral oxazoline moieties were
compatible with the standard conditions, affording the
diastereomeric ligands 3m−p in 63−80% yield. Next, the
scope of olefins was briefly surveyed (Scheme 2B). Olefin
coupling partners, including α,β-unsaturated esters, amide,
ketone, sulfone, phosphonate, and styrene, afforded olefin−
oxazoline ligands 3q−w in 54−79% yield with d.r. values
ranging from 1.1:1 to 1.8:1.
Scheme 4. Gram-Scale Synthesis
Scheme 5. 3,3′-Modification of Olefin−Oxazoline Ligand 3a
Next, we began to examine the scope of indole−
naphthyloxazoline derivatives (Scheme 3). As shown in
Scheme 3. Substrate Scope of Indole−Naphthyloxazoline
,
a b
Derivatives
a
Reaction conditions: 4 (0.1 mmol), 2 (0.15 mmol), Pd(OAc) (10
compound 7 could be further transformed to 8 via Pd-
catalyzed Suzuki coupling (Scheme 5C).
2
mol %), AgOAc (0.15 mmol), CH CN (2.0 mL), air, 40 °C, 9 h.
3
b
c
d
e
f
Total yields of two diastereomers are shown. 12 h. 60 °C. 5 h. 7
In summary, we have reported herein the synthesis of axially
chiral olefin−oxazoline ligands via palladium-catalyzed C−H
olefination. Two diastereomeric ligands were obtained in a
one-step procedure. Our protocol showed broad substrate
scope and good functional group tolerance. The 3- and 3′-
positions of the axially chiral olefin−oxazoline ligands could be
facilely modified via electrophilic bromination at the 3-position
of the indole fragment and Rh-catalyzed amidation or
olefination at the 3′-position of the arene fragment. Further
applications of these ligands in asymmetric synthesis are
currently ongoing in our laboratory.
g
h. 130 °C, 24 h.
Scheme 3A, indole−naphthyloxazolines 4 bearing methyl,
methoxy, fluoro, and trifluoromethyl substituents were
compatible with the standard conditions, giving the corre-
sponding products 5a−g in moderate to good yields.
Substrates 4h−j with chiral oxazolines bearing i-Pr, t-Bu, and
Bn groups, respectively, were also investigated and substrate 4i
with a t-Bu group on the oxazoline gave the highest d.r. in the
coupling reaction with methyl acrylate. When tert-butyl
acrylate, phenyl acrylate, and ethyl vinyl ketone were employed
as coupling reagents, the desired products 5k−m were
obtained in 60−76% yield (Scheme 3B).
ASSOCIATED CONTENT
sı Supporting Information
■
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03093.
To demonstrate the utility of this protocol, a gram-scale
synthesis using substrate 1a and methyl acrylate 2a was carried
out under the standard conditions, affording the corresponding
product 3a in 78% yield (Scheme 4).
Experimental procedures, characterizations of new
compounds, and NMR spectra (PDF)
The 3- and 3′-positions of axially chiral olefin−oxazoline
ligand 3a could be conveniently modified via electrophilic
bromination at the 3-position and subsequent Rh-catalyzed
olefination or amidation at the 3′-position to give compounds
AUTHOR INFORMATION
Corresponding Author
■
Hui-Xiong Dai − Chinese Academy of Sciences Key Laboratory
of Receptor Research, Shanghai Institute of Materia Medica,
University of Chinese Academy of Sciences, Shanghai 201203,
China; University of Chinese Academy of Sciences, Beijing
6
and 7 in 60% and 78% yield, respectively (Scheme 5A,B).
The brominated products are versatile stepping stones for
further structure elaboration at the 3-position. For example,
C
https://dx.doi.org/10.1021/acs.orglett.0c03093
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Letter
1
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Email: hxdai@simm.ac.cn
Authors
Ling-Jun Li − Chinese Academy of Sciences Key Laboratory of
Receptor Research, Shanghai Institute of Materia Medica,
University of Chinese Academy of Sciences, Shanghai 201203,
China; University of Chinese Academy of Sciences, Beijing
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Jun-Jie Chen − Chinese Academy of Sciences Key Laboratory
of Receptor Research, Shanghai Institute of Materia Medica,
University of Chinese Academy of Sciences, Shanghai 201203,
China
Chen-Fei Feng − Chinese Academy of Sciences Key
Laboratory of Receptor Research, Shanghai Institute of
Materia Medica, University of Chinese Academy of Sciences,
Shanghai 201203, China
Han-Yuan Li − Chinese Academy of Sciences Key Laboratory
of Receptor Research, Shanghai Institute of Materia Medica,
University of Chinese Academy of Sciences, Shanghai 201203,
China
Xing Wang − Chinese Academy of Sciences Key Laboratory of
Receptor Research, Shanghai Institute of Materia Medica,
University of Chinese Academy of Sciences, Shanghai 201203,
China; University of Chinese Academy of Sciences, Beijing
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(
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Hui Xu − Chinese Academy of Sciences Key Laboratory of
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University of Chinese Academy of Sciences, Shanghai 201203,
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2
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(6) Han, Y.-Q.; Ding, Y.; Zhou, T.; Yan, S.-Y.; Song, H.; Shi, B.-F.
Pd(II)-Catalyzed Enantioselective Alkynylation of Unbiased Methyl-
■
We gratefully acknowledge Shanghai Institute of Materia
Medica, Chinese Academy of Sciences, the National Natural
Science Foundation of China (21772211), the Youth
Innovation Promotion Association of CAS (2014229 and
3
ene C(sp )−H Bonds Using 3,3′-Fluorinated-BINOL as a Chiral
Ligand. J. Am. Chem. Soc. 2019, 141, 4558−4563.
(7) (a) Xiao, B.; Fu, Y.; Xu, J.; Gong, T.-J.; Dai, J.-J.; Yi, J.; Liu, L.
Pd(II)-Catalyzed C−H Activation/Aryl-Aryl Coupling of Phenol
Esters. J. Am. Chem. Soc. 2010, 132, 468−469. (b) Yang, J.; Wang, R.;
Wang, Y.; Yao, W.; Liu, Q.; Ye, M. Ligand-Accelerated Direct C−H
Arylation of BINOL: A Rapid One-Step Synthesis of Racemic 3,3′-
Diaryl BINOLs. Angew. Chem., Int. Ed. 2016, 55, 14116−14120.
2
018293), Institutes for Drug Discovery and Development,
Chinese Academy of Sciences (CASIMM0120163006), the
Science and Technology Commission of Shanghai Municipal-
ity (17JC1405000 and 18431907100), the Program of
Shanghai Academic Research Leader (19XD1424600), the
National Science & Technology Major Project “Key New Drug
Creation and Manufacturing Program”, China
(
8) (a) Hulcoop, D. G.; Lautens, M. Palladium-Catalyzed
Annulation of Aryl Heterocycles with Strained Alkenes. Org. Lett.
ller, M.; Koenigs, R. M.
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007, 9, 1761−1764. (b) Yang, Z.; Mo
̈
Synthesis of gem-Difluoro Olefins through C−H Functionalization
and β-fluoride Elimination Reactions. Angew. Chem., Int. Ed. 2020, 59,
(
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Natural and Biomimetic Drugs for financial support.
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