Enantioselective synthesis of 3-aryl-phthalides through a nickel-catalyzed stereoconvergent cross-coupling reaction

Article abstract of DOI:10.1039/d1ob00487e

A nickel-catalyzed asymmetric Suzuki-Miyaura cross-coupling of racemic 3-bromo-phthalides and arylboronic acids was realized for the synthesis of diverse chiral 3-aryl-phthalides in moderate to excellent reaction yields. The reaction proceeded in a stereoconvergent manner and high enantioselectivities were observed for most examined examples. A number of functional groups like aldehyde, ester and bromide were well tolerated. Heteroaromatic boronic acids were also competent coupling partners in this reaction.

Full text of DOI:10.1039/d1ob00487e

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Enantioselective synthesis of 3-aryl-phthalides
through a nickel-catalyzed stereoconvergent
cross-coupling reaction†
Cite this: DOI: 10.1039/d1ob00487e
Received 12th March 2021,
Accepted 26th April 2021
Si-Yu Xu,a Rui Zhang,a Shu-Sheng Zhang *a and Chen-Guo Feng
*
a,b,c
DOI: 10.1039/d1ob00487e
rsc.li/obc
A nickel-catalyzed asymmetric Suzuki–Miyaura cross-coupling of noboron reagents as coupling partners has been employed
racemic 3-bromo-phthalides and arylboronic acids was realized for universally in both industry and academia.15 Due to several
the synthesis of diverse chiral 3-aryl-phthalides in moderate to serious problems like slow oxidative addition with alkyl elec-
excellent reaction yields. The reaction proceeded in a stereocon- trophiles and the decomposition of alkyl organometallics via
vergent manner and high enantioselectivities were observed for β-elimination, the traditional palladium-catalyzed Suzuki–
most examined examples. A number of functional groups like alde- Miyaura cross-coupling with alkyl (pseudo)halides was rather
hyde, ester and bromide were well tolerated. Heteroaromatic challenging.16 In the meantime, nickel has received much
boronic acids were also competent coupling partners in this attention as a superior catalyst for this kind of transformation.
reaction.
Furthermore, the nickel-catalyzed stereoconvergent coupling
reaction of alkyl electrophiles could convert a racemic mixture
of enantiomers to an enantioenriched product, representing a
powerful method to construct chiral saturated hydrocarbon
frameworks.17–19 As the development of convenient prepa-
ration of diverse chiral 3-aryl-phthalides is still in demand,6a
Phthalides, namely 3H-isobenzofuran-1-ones, are important
structural motifs embedded in many natural products,1 which
often show a wide range of biological activities such as anti-
oxidant,2 antiplatelet,3 antihyperglycemic,4 and analgesic
activities.5 Among these, chiral 3-substituted phthalides are
important members, and their enantioselective synthesis has
received considerable research interest.1,6 Although asym-
metric reactions using chiral auxiliaries, reagents or precursors
have proved to be applicable,7 much effort has been devoted to
the development of asymmetric transition metal-catalyzed pro-
cesses such as hydrogenation/transfer hydrogenation,8
dynamic resolution,9 aldol/Michael reaction,10 allylation/alkyl-
ation-lactonization,11
addition-lactonization12
sequential
process, and cyclization reactions (Scheme 1).13
Transition metal-catalyzed cross-coupling reactions com-
prise efficient methods for the construction of C–C bonds.14
Among them, the Suzuki–Miyaura reaction that utilizes orga-
aThe Research Center of Chiral Drugs, Innovation Research Institute of Traditional
Chinese Medicine, Shanghai University of Traditional Chinese Medicine, Shanghai
201203, China. E-mail: zhangss@shutcm.edu.cn, fengcg@shutcm.edu.cn
bCAS Key Laboratory of Synthetic Chemistry of Natural Substances, Center for
Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry,
University of Chinese Academy of Sciences, 345 Lingling Road, Shanghai, 200032,
China
cShanghai Key Laboratory for Molecular Engineering of Chiral Drugs, Shanghai Jiao
Tong University, Shanghai, 200240, China
†Electronic supplementary information (ESI) available. CCDC 2061494. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1ob00487e
Scheme 1 Transition metal-catalyzed synthesis of chiral 3-substituted
phthalides.
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Table 2 Ligand screeninga,b,c
we envisioned that nickel-catalyzed asymmetric Suzuki–
Miyaura cross-coupling may provide a flexible and modular
strategy toward this goal.20
3-Halo-isobenzofuranones are widely used as electrophiles
in the Friedel–Crafts reaction for the racemic synthesis of
3-aryl-phthalides.21 We decided to use these as coupling part-
ners for the nickel-catalyzed, stereoconvergent synthesis of
chiral 3-substituted phthalides. Herein, we describe our
success in achieving this goal (Scheme 1).
We started our investigation with 3-bromo-isobenzofuranone
1a and phenylboronic acid 2a as model substrates. Preliminary
studies indicated that pyridyloxazoline ligand L1 exhibited high
activity and the reaction was accomplished in the presence of a
nickel catalyst in situ generated by mixing 10 mol%
NiCl2·diglyme and 12 mol% L1. After extensive screening, the
desired phthalide product 3a was generated in an excellent yield
with a very good ee value (Table 1, entry 1). A screening of bases
showed that they had a crucial effect on the reaction. While the
replacement of K2CO3 resulted in a lower reaction yield and
enantioselectivity (Table 1, entry 2), the reaction was totally
inhibited when Cs2CO3, K3PO4 or EtONa was used as the base
(Table 1, entries 3–5), which was ascribed to a faster degra-
dation of 3-bromo-isobenzofuranone with these bases. The reac-
a Reactions were carried out at 70 °C for 12 h by using 3-bromo-isoben-
zofuranone 1a (0.2 mmol), 2a (0.4 mmol), Ni catalyst (0.02 mmol),
ligand (0.024 mmol), and K2CO3 (0.4 mmol) in THF (2.0 mL). b Yield
was determined by 1H NMR using dibromomethane as the internal
standard. c The ee value was determined by HPLC analysis on a chiral
stationary phase.
tion also proceeded smoothly in other ether solvents like ation. Thus, ligand screening was then performed to further
dioxane, diglyme and dimethyl ether, but showed reduced improve the efficiency and enantioselectivity (Table 2).
efficiency (Table 1, entries 6–8). Attempts to further improve the Although excellent reaction yields were also observed, the re-
enantioselectivity by performing the reaction at a lower temp- placement of the t-Bu group of L1 by less sterically hindered
erature proved to be unsuccessful (Table 1, entry 9). Both reac- groups resulted in a significant loss in enantioselectivity. The
tion yield and enantioselectivity were decreased when a lower introduction of CF3 substitution at the 5- or 4-position of the
catalyst loading of 5 mol% was used (Table 1, entry 10).
pyridyl ring could slightly increase the enantioselectivities (L5
On the basis of the previous literature, the bidentate and L6). While the steric hindrance around the nitrogen atom
N-ligands often played a crucial role in this kind of transform- of the pyridyl ring was increased by switching the phenyl ring
to the quinolinyl group, the reaction yield decreased signifi-
cantly albeit with comparable enantioselectivity. The bis(oxazo-
line) ligands often are competent ligands in similar cross-
couplings. However, all the tested bis(oxazoline) ligands
were not effective at promoting the current transformation
(L8–L10).16
Table 1 Optimization of reaction parametersa
To explore the scope of this nickel-catalyzed process, a
variety of boronic acids with diverse steric and electronic pro-
perties were firstly examined in the reaction with 3-bromo-iso-
benzofuranone 1a (Table 3). Under the optimized reaction con-
ditions, a number of arylboronic acids with different substitu-
ents on the phenyl ring were coupled smoothly with 3-bromo-
isobenzofuranone with moderate to very good levels of
enantioselectivity. The reaction yield was affected by both the
electronic and steric properties of the arylboronic acids.
Compared with electron-withdrawing groups (3d–3i), electron-
donating substituents had beneficial effects on the reaction
yields (3a–3c). The sterically more hindered ortho-methyl sub-
stituted phenyl boronic acid gave a relatively lower yield (3k).
Notably, several useful functional groups like halides (Cl and
Br), ester and aldehyde were well tolerated, providing handles
for further derivatizations (3e, 3f, 3h and 3i). The replacement
of the phenyl ring by the naphthyl group was compatible. A
range of substrates containing heterocyclic fragments, such as
Entry
Deviation from standard conditions
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
None
98
84
Trace
Trace
Trace
79
90
74
40
50
82
76
—
—
—
64
70
78
72
67
Na2CO3 instead of K2CO3
Cs2CO3 instead of K2CO3
K3PO4 instead of K2CO3
EtONa instead of K2CO3
Dioxane instead of THF
Diglyme instead of THF
Dimethyl ether instead of THF
50 °C instead of 70 °C
5 mol% NiCl2·glyme and 6 mol% L1
10
a Reactions were carried out at 70 °C for 12 h by using 3-bromo-isoben-
zofuranone 1a (0.2 mmol), 2a (0.4 mmol), Ni catalyst (0.02 mmol), L1
(0.024 mmol), and K2CO3 (0.4 mmol) in THF (2.0 mL) unless otherwise
noted. b Yield was determined by 1H NMR using dibromomethane as
the internal standard. c The ee value was determined by HPLC analysis
on a chiral stationary phase.
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Table 3 Cross-coupling of 3-bromo-isobenzofuranone with various
aryl boronic acidsa,b,c
Table 4 Cross-coupling of various 3-bromo-phthalides with phenyl-
boronic acida,b
a Isolated yield. b The ee value was determined by HPLC analysis on a
chiral stationary phase.
Fig. 1 X-ray crystallographic structure of 3f.
a Reactions were carried out at 70 °C for 12 h by using 3-bromo-isoben-
zofuranone 1a (0.3 mmol), aryl boronic acid (0.6 mmol), Ni catalyst
(0.03 mmol), ligand (0.036 mmol), and K2CO3 (0.6 mmol) in THF
(3.0 mL). b Isolated yield. c The ee value was determined by HPLC ana-
lysis on a chiral stationary phase.
product 3f was obtained by recrystallization and its absolute
configuration was also confirmed by X-ray crystallography ana-
lysis (Fig. 1).22 For other products, their stereochemistry was
assigned as indicated based on the assumption of an analo-
gous reaction pattern.
thiophene (3m), pyridine (3n) and benzofuran (3o), were also
successfully employed, albeit with reduced reaction yields.
Next, we assessed the substituents on the phenyl ring of
3-bromo-phthalides (Table 4). Although the enantioselectivi-
ties fluctuated with different substituents at different positions
of the phenyl ring, they were at very good levels for most tested
Conclusions
cases. However, methyl substitution at the 4-position, which is In summary, we have developed a nickel-catalyzed asymmetric
close to the reaction center, had a significant effect on enantio- Suzuki–Miyaura coupling reaction of racemic 3-bromo-phtha-
selective control, giving a low ee value of 7% (3w). In contrast lides and arylboronic acids for the efficient preparation of
to the substituent effect on arylboronic acid, electron-with- diverse chiral 3-aryl-phthalides. This protocol proceeds in a
drawing substituents at the 5-position of the phenyl ring gave stereoconvergent manner, showing very good enantioselectivi-
better reaction yields (3p–3s).
ties for most tested cases. Both aryl and heteroaromatic
The newly formed stereogenic center of known structures boronic acids were competent coupling partners, and a
shown in Tables 3 and 4 was assigned by comparison with the number of active functional groups like halides (Cl and Br),
reported optical rotation data. In addition, the optically pure ester and aldehyde were well tolerated.
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Conflicts of interest
The authors declare there is no conflict of interest regarding
the publication of this paper.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21772216 and 91956113), the Shanghai
Municipal Committee of Science and Technology
(18401933500 and 20XD1423400), and the Shanghai Municipal
Education Commission (2019-01-07-00-10-E00072).
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