Hexafluoroisopropanol-Enabled Copper-Catalyzed Asymmetric Halogenation of Cyclic Diaryliodoniums for the Synthesis of Axially Chiral 2,2′-Dihalobiaryls

Article abstract of DOI:10.1021/acs.orglett.0c03833

An efficient asymmetric halogenation of cyclic diaryliodonium salts is demonstrated, which gives access to a wide range of axially chiral 2,2′-dihalobiaryls in good to excellent yields and with excellent enantioselectivities. The use of CuX with chiral bisoxazoline ligand and tetrabutylammonium halides in the unique solvent of hexafluoroisopropanol (HFIP) led to the best results in the process. The axially chiral 2,2′-dihalobiaryls can be transformed into a number of enantiopure chiral ligands that could be potentially useful in asymmetric catalysis.

Full text of DOI:10.1021/acs.orglett.0c03833

pubs.acs.org/OrgLett
Letter
Hexafluoroisopropanol-Enabled Copper-Catalyzed Asymmetric
Halogenation of Cyclic Diaryliodoniums for the Synthesis of Axially
Chiral 2,2′-Dihalobiaryls
Jie Ke,† Bing Zu,† Yonghong Guo, Yingzi Li, and Chuan He*
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ABSTRACT: An efficient asymmetric halogenation of cyclic diary-
liodonium salts is demonstrated, which gives access to a wide range of
axially chiral 2,2′-dihalobiaryls in good to excellent yields and with
excellent enantioselectivities. The use of CuX with chiral bisoxazoline
ligand and tetrabutylammonium halides in the unique solvent of
hexafluoroisopropanol (HFIP) led to the best results in the process.
The axially chiral 2,2′-dihalobiaryls can be transformed into a number
of enantiopure chiral ligands that could be potentially useful in asymmetric catalysis.
valuable properties in asymmetric catalysis owing to their
xially chiral compounds are ubiquitous in many important
Aorganic molecules, such as natural products, biologically unique structures and characteristics. Axially chiral 2,2′-
diiodobiaryls are widely used as hypervalent iodine catalyst
precursors, which has led to their broad applications in many
Scheme 1. Significance and Synthesis of 2,2′-Dihalobiaryls
asymmetric transformations, such as the enantioselective α-
functionalization of ketones, dearomatization, the difunction-
alization of alkenes, as well as oxidative rearrangement and
oxidative coupling (Scheme 1a, left).2 In addition, the versatile
aromatic C−I bonds in 2,2′-diiodobiaryls can easily be
elaborated, conveniently delivering a variety of privileged C2-
symmetic chiral ligands or catalysts (Scheme 1a, right).3 Given
the importance of this structural motif, strategies for the
efficient construction of axially chiral 2,2′-diiodobiaryls are
highly desirable. However, methods to access such compounds
have been rather limited to date and have mainly relied on the
Sandmeyer reaction from axially chiral 2,2′-diaminobiaryls
(usually BINAM),4 or the resolution of racemate via a chiral
preparative column.5 As a result, catalytic approaches for the
asymmetric synthesis of axially chiral 2,2′-diiodobiaryls have
remained underdeveloped.
In 2015, the Yoshikai group reported an elegant copper/
diamine-catalyzed iodinative ring-opening reaction of cyclic
diaryliodonium salts toward 2,2′-diiodobiaryls (Scheme 1b).6
Compared with the previous procedures, which underwent
metathesis with KI, followed by pyrolysis at 200−250 °C,7 this
copper-catalyzed process provides a more efficient and
convenient way for the synthesis of 2,2′-diiodobiaryls in
Received: November 19, 2020
Published: December 29, 2020
active compounds, optically pure materials, and privileged
chiral ligands, which have attracted increasing attention over
the past decade.1 Among these elegant and useful axially chiral
structural skeletons, 2,2′-diiodobiaryls display significant and
https://dx.doi.org/10.1021/acs.orglett.0c03833
© 2020 American Chemical Society
Org. Lett. 2021, 23, 329−333
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a
asymmetric iodination of cyclic diaryliodonium salt 1a with the
iodide source nBu4NI in the presence of CuI as the catalyst
and bisoxazoline L1 as the chiral ligand in common solvents,
such as CH3CN, CH2Cl2, toluene, and 1,4-dioxane. To our
surprise, the desired 2,2′-diiodobiaryl product 2a was obtained
in quantitative yield but with very poor enantiomeric excess
(ee) (Table 1, entries 1−4).9 Further screening of the solvent
disclosed that the ee of 2a could be tremendously improved to
97% in quantitative yield in the solvent of HFIP (Table 1,
entry 5). HFIP has recently been shown to display unique
properties as a solvent or cosolvent in synthetic chemistry,
especially in asymmetric catalysis.10 The role of HFIP in this
enantioselective reaction was preliminarily studied by density
functional theory (DFT) calculations. (See Supporting
Information Section 6.) Next, various chiral bisoxazoline-type
ligands were also tested. Replacing the phenyl group of L1 with
a benzyl (L2) or isopropyl (L3) group gave 2a with lower ee
(Table 1, entries 6 and 7). Spirobis(oxazoline) ligands L4, L5,
and L6 did not greatly affect the enantioselectivity, whereas L7
afforded the product 2a with 99% ee and in 93% isolated yield
(Table 1, entries 8−11). PyBox ligand L8 significantly reduced
both the yield and the ee (Table 1, entry 12). Moreover, the
reaction could also proceed smoothly on the gram scale with
only a slight loss of enantioselectivity; even the loadings of CuI
and L7 were reduced by half (Table 1, entry 13).
With the optimal conditions in hand, we next examined the
scope for the substituted cyclic diaryliodonium salts (Scheme
2). A regioselective problem always existed in the ring-opening
reactions of the cyclic diaryliodonium salts with N, S, and O
nucleophiles. This problem limited the scope of the cyclic
diaryliodonium salts to be symmetric structures or to contain a
substituent group at one ortho position of the iodonium ions,
increasing the steric hindrance. In this iodination procedure,
no regioselective problem arose, which certainly enlarged the
diversity of the scope of cyclic diaryliodonium salts. We found
that 6,6′-dimethyl cyclic diaryliodonium salts with substituents
at the para, meta, and ortho positions of the aromatic ring all
provided the desired products in high yields with excellent
enantioselectivities (2b−2k). Various substituted groups on
the aromatic ring were well tolerated, such as fluoro, chloro,
cyano, tosylate, ester, difluoromethyl, −CH2OAc, and
−CH2Nphth groups (2b−2i). The substituted groups at the
6,6′-positions of cyclic diaryliodonium salts for restraining the
rotation were investigated next. Substrates displaying not only
alkyl substituents (such as ethyl and isopropyl) but also
electron-withdrawing substituted groups (such as trifluoro-
methyl, −OBz, tosylate, fluoro, chloro, and bromo) could also
be introduced, smoothly giving the corresponding iodination
product (2l−2s). It is worth mentioning that axially chiral
2,2′,6,6′-tetrahalobiaryls (2q−2s) could be obtained in 90−
93% yield with 99% ee and could be used as chiral synthons for
the construction of numerous interesting and useful biaryl
atropoisomers via the transformation of carbon−halogen
bonds. Furthermore, the reaction was not limited to 1,1′-
biphenyl substrates; compounds bearing 1,1′-binaphthalene,
naphthyl-phenyl, and phenyl-heterocyclic skeletons could all be
isolated in 84−92% yields with 99% ee (2t−2v). In addition,
this asymmetric ring-opening reaction was also applied to the
other halogen anions, such as nBu4NBr and nBu4NCl, which
gave access to the corresponding 2,2′-iodobromobiaryl and
2,2′-iodochlorobiaryl products without any difficulties (2w−
2y).
Table 1. Reaction Development
b
c
entry
chiral ligand
solvent
yield (%)
ee (%)
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L8
L7
CH3CN
CH2Cl2
toluene
1,4-dioxane
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
98
98
99
98
99
99
99
99
99
−18
−8
−13
−11
−97
−84
−86
95
97
−96
99
9
10
11
12
99
99 (93)
31
24
94 (99)
d
13
98 (1.27 g)
a
Standard conditions: 1a (0.1 mmol), CuI (5 mol %), ligand (10 mol
%), nBu4NI (1.0 equiv), solvent (1.0 mL), room temperature, 24 h,
Ar. GC yield with dodecane as a standard. Isolated yield in
parentheses. HPLC conditions: Chiralpak OD-3, isopropanol/
b
c
d
hexanes 0.1:99.9, flow: 0.5 mL/min. Gram-scale conditions: 1a
(3.0 mmol), CuI (2.5 mol %), L7 (5 mol %), nBu4NI (1.0 equiv),
HFIP (30 mL), room temperature, 24 h, Ar. ee of 2a after a single
recrystallization in parentheses.
good yields under mild conditions. Despite the fact that this
facile transformation generates 2,2′-diiodobiaryls with high
efficiency, no axially chiral example is shown there. Because the
overarching goal is to access axially chiral 2,2′-diiodobiaryls, we
questioned whether we could expand the toolbox and develop
an asymmetric halogenation reaction of cyclic diaryliodonium
salts via chiral transition-metal catalysis. The feasibility of the
hypothesis can be supported by the recent enantioselective
ring-opening of cyclic diaryliodonium salts with N, S, and O
nucleophiles reported by the Gu and the Zhang groups.8 The
key to the success of these reactions is the use of appropriate
chiral copper−oxazoline type complexes that discriminates the
two conformers of cyclic diaryliodonium salts and affords the
ring-opening products with high enantiomeric excess. Herein
we present the development of an asymmetric halogenation of
cyclic diaryliodonium salts, which gives access to a wide range
of axially chiral 2,2′-dihalobiaryls in good to excellent yields
and with excellent enantioselectivities under mild conditions
(Scheme 1c). Crucial to the successful outcome of this
reaction is the use of chiral copper−bisoxazoline catalysts in
the unique solvent of HFIP (hexafluoroisopropanol).
Given the superior performance of the chiral Cu/Box
catalytic system in the enantioselective ring-opening of cyclic
diaryliodonium salts,8 we commenced our studies of the
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a
Scheme 2. Substrate Scope
a
Standard conditions: 1 (0.1 mmol), CuI (5 mol %), L7 (10 mol %), nBu4NX (1.0 equiv), HFIP (1.0 mL), room temperature, 24 h, Ar; isolated
b
c
d
yields. L1 was used as ligand. CuBr (5 mol %) was used as catalyst. CuCl (5 mol %) was used as catalyst.
To further demonstrate the utility of these axially chiral 2,2′-
diiodobiaryls, compound 2a was then converted into various
privileged C2-symmetic chiral ligands or catalysts by diversity-
oriented transformations (Scheme 3). First, the consecutive
treatment of 2a with nBuLi and nitrobenzene successfully
delivered the corresponding 6,6′-dimethyl biphenol 3 in 63%
yield with 96% ee. Compared with the widely used BINOL
scaffold, the 6,6′-dimethyl groups made the biphenol more
configurationally stable,11 which found unique applications in
the asymmetric Mannich-type reactions.12 It is noteworthy that
previously, the preparation of chiral 6,6′-dimethyl biphenol
relied on a four-step resolution synthesis, which obviously
limited its further applications in asymmetric catalysis.13 The
lithiation of 2a with tBuLi and trapped with benzophenone
afforded BAMOL (1,1′-biaryl-2,2′-dimethanol) derivatives 4 in
68% yield with 97% ee, which have displayed excellent
performance in the hetero-Diels−Alder reaction as a hydrogen
bonding catalyst.14 Moreover, axially chiral bisphenylsulfane 5
and BIPHEMP 6 could be easily obtained by simple
nucleophilic substitution between the lithium intermediate of
2a and phenyl disulfide and chlorodiphenylphosphine,
respectively. Further consecutive treatment of 2a with nBuLi
and CO2 smoothly produced the chiral dicarboxylic acid 7 in
80% yield with 97% ee, which has been exploited in a wide
range of enantioselective transformations.15 Finally, the
replacement of CO2 into N,N-dimethylformamide (DMF)
for the synthesis of the axially chiral dialdehyde 8 also
proceeded without any problems. This dialdehyde 8 could be
further transformed into the axially chiral dibenz[c,e]azepine 9
by simple reductive amination.
In summary, we have developed a highly efficient copper-
catalyzed asymmetric halogenation of cyclic diaryliodonium
salts with simple tetrabutylammonium halides, which gives
access to a wide range of axially chiral 2,2′-dihalobiaryls in
good to excellent yields with excellent enantioselectivities.
Crucial to the successful outcome of this reaction is the use of
CuI with the chiral bisoxazoline ligand in the solvent of HFIP.
We also demonstrated that the axially chiral 2,2′-dihalobiaryl
products can be further smoothly transformed into a number
of enantiopure chiral ligands, which could be potentially useful
in asymmetric catalysis.
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Letter
Yonghong Guo − Shenzhen Grubbs Institute and Department
of Chemistry, Guangdong Provincial Key Laboratory of
Catalysis, Southern University of Science and Technology,
Shenzhen, Guangdong 518055, China
Scheme 3. Application of Axially Chiral 2,2′-Diiodobiaryls
Yingzi Li − Shenzhen Grubbs Institute and Department of
Chemistry, Guangdong Provincial Key Laboratory of
Catalysis, Southern University of Science and Technology,
Shenzhen, Guangdong 518055, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03833
Author Contributions
†J.K. and B.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the Thousand
Talents Program for Young Scholars, Shenzhen Science and
T e c h n o l o g y I n n o v a t i o n C o m m i t t e e
(JCYJ20190809142809370) and Guangdong Provincial Key
Laboratory of Catalysis (no. 2020B121201002). We dedicate
this paper to the 10th anniversary of the Department of
Chemistry, SUSTech.
a
b
i. nBuLi, Et2O, PhNO2, −78 °C; ii. MeOH, rt. tBuLi, THF,
c
d
PhCOPh, −78 °C. nBuLi, Et2O, PhSSPh, −78 °C. nBuLi, Et2O,
e
f
Ph2PCl, −78 °C. nBuLi, Et2O, CO2, −78 °C. nBuLi, THF, DMF,
g
−78 °C. BnNH2, NaBH3CN, CH3CN, rt.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03833.
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Materials and methods, experimental procedures,
characterization data, computational studies, H, 13C,
1
and 19F NMR spectral data, HPLC spectra, and mass
spectrometry data of new compounds (PDF)
Accession Codes
CCDC 2002852 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
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bridge Crystallographic Data Centre, 12 Union Road,
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AUTHOR INFORMATION
Corresponding Author
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Chuan He − Shenzhen Grubbs Institute and Department of
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Authors
Jie Ke − Shenzhen Grubbs Institute and Department of
Chemistry, Guangdong Provincial Key Laboratory of
Catalysis, Southern University of Science and Technology,
Shenzhen, Guangdong 518055, China
Bing Zu − Shenzhen Grubbs Institute and Department of
Chemistry, Guangdong Provincial Key Laboratory of
Catalysis, Southern University of Science and Technology,
Shenzhen, Guangdong 518055, China
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