Chiral cobalt(II) complex catalyzed Friedel-Crafts aromatization for the synthesis of axially chiral biaryldiols

Article abstract of DOI:10.1039/c7cc05266a

An efficient atroposelective synthesis of axially chiral biaryldiols via asymmetric Friedel-Crafts aromatization between p-quinones and 2-naphthols was developed. A chiral cobalt(ii) complex of N,N′-dioxide enabled the process to generate axially chiral biaryldiols in up to 98% yield and 95% ee. A large range of substituents at different positions of p-quinones and 2-naphthols was tolerable. The configuration of the product and the chiral N,N′-dioxide-Co(ClO₄)₂ catalyst was identified by X-ray crystal diffraction analysis and a possible catalytic model was suggested.

Full text of DOI:10.1039/c7cc05266a

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Chiral Cobalt(II) Complex Catalyzed Friedel-Crafts-Aromatization
for Synthesis of Axially Chiral Biaryldiols
Received 00th January 20xx,
Accepted 00th January 20xx
Chaoran Xu, Haifeng Zheng, Bowen Hu, Xiaohua Liu*, Lili Lin and Xiaoming Feng*
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient atroposelective synthesis of axially chiral biaryldiols
via asymmetric Friedel-Crafts-aromatization between p-quinones
and 2-nathphols was developed. A chiral cobalt(II) complex of
N,N’-dioxide enabled the process to generate axially chiral
biaryldiols in up to 98% yield and 95% ee. A large range of
substituents at different positions of p-quinones and 2-nathphols
were tolerable. The configuration of the product and the chiral
N,N’-dioxide-Co(ClO4)2 catalyst was identified by X-ray crystal
diffraction analysis and a possible catalytic model was suggested.
Scheme 1. Access to Axially Chiral Biaryldiols.
Salvio and Bella8c (Scheme 1; a, b). However, the phosphoric acid
could give excellent enantioselectivity but limiting to 2-
substituted 1,4-quinone, and chiral quinine derivative could
tolerate several substituents on 2,6-dichloro-1,4-quinones but
lacking of high enantioselectivity. Interestingly, none of chiral
Lewis acid catalysts has been reported efficiency in these
cases. The coordination ability of the formed biarylols to the
metal ions, and Lewis-acid initiated polymerization of quinone
substrate11 might be big trouble. Additionally, the previous
usage of BINOL derivatives in asymmetric catalysis exhibits
that substituents on the biaryl backbone have significantly
influence the stereocontrol of the reaction. Therefore, to
develop an alternative chiral Lewis acid catalytic system to
synthesize various substituted biaryltriols in high
enantioselectivity is interesting.
Biaryls are commonly encountered in natural products with
biological activity1, and their derivatives, such as biaryldiols
also serve as useful backbone of chiral ligands and catalysts2
for asymmetric transformations. Over the past decade, several
strategies have been reported for the atroposelective
synthesis of biaryl derivatives3-8, which can be summarized as
five methods: transition metal catalyzed aryl-aryl coupling3,
classical resolution4 and dynamic kinetic resolution5,
desymmetrization of prochiral biaryls6, atroposelective
construction of an aromatic ring7 and central-to-axial chirality
exchange8. Recently, chiral organocatalysts have shown
efficiency for the construction of axially chiral biaryl
derivatives. For example, chiral phosphoric acids are used for
the
synthesis
of
biaryl
derivatives6a,
2,2’-
binaphthyldiamines8e,f, and naphthyl-indole skeletons8h. In
addition, pyrrolidinyl-tetrazole was used to catalyze aldol
We envisioned the possibility of chiral Lewis acid-promoted
Friedel-Crafts reaction/aromatization process to establish
biaryltriols. The introduction of an ester group into 1,4-
quinone enhances both the coordination preferences and the
reactivity (Scheme 1, c). Asymmetric Friedel-Crafts reactions of
arylols were suitable for using chiral N,N’-dioxide complex
catalytic systems10. Herein, we demonstrated the application
of a chiral N,N’-dioxide-cobalt(II) catalyst in the development
of a Friedel-Crafts-Aromatization reaction with 2-nathphols
and 1,4-quinones. A series of biaryltriol derivatives were
obtained in moderate to excellent yields with good to
excellent enantioselectivities.
condensation
to
generate
1,1’-binaphthalene-2-
carbaldehydes.7a For the atroposelective synthesis biaryldiol or
biaryltriol derivatives, the method of central-to-axial chirality
exchange makes a breakthrough. BINOL/NOBIN biaryls by [3,3]-
rearrangement was achieved by Kürti, Xu and coworkers8b.
Biaryltriols were generated from asymmetric arylative reaction
of 2-naphthols with quinone derivatives as reported by Tan,8a
Key Laboratory of Green Chemistry & Technology, Ministry of Education,
College of Chemistry, Sichuan University, Chengdu 610064, People’s Republic of China.
† E-mail: liuxh@scu.edu.cn, xmfeng@scu.edu.cn; Fax: +86 28 85418249
Electronic Supplementary Information (ESI) available: CCDC 1551259 (3da) and
CCDC 1551260 (L-RaPr2/Co(ClO4)2 complex). For ESI and crystallographic data in CIF
or other elecnic format See DOI: 10.1039/x0xx00000x
Initially, quinone 1a and 2-naphthol 2a were chosen as the
model substrates to optimize the reaction conditions (Table 1).
Using chiral N,N’-dioxide L-RaPr2 as the ligand, several cheap
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metal salts that are capable of bonding bidentate 1,3- easily undergo polymerization and other side reactions at room
dicarbonyl compounds were tested for the model reaction. It temperature11. To avoid impurity contained in benzoquinone
was found that Mg(OTf)2, Cu(OTf)2, and Ni(OTf)2 gave the which affects the ee value remarkably, newly oxidated
desired product 3aa in moderate to good yield as a racemate, quinone 1a from 2-methoxycarbonyl-1,4-phendiol without
although a complete background reaction occurred within 15 further purification was used. It was found that the
minutes at -10 oC (entries 2-4 vs entry 1). The lack of enantioselectivity of the reaction increased to 90% ee with a
enantioselectivity might result from the strong non-asymmetric 91% yield (entry 13). Therefore, the optimized condition
catalytic process and the destroy of chiral N,N’-dioxide-metal involved the use of L-RaPr2/Co(ClO4)2·6H2O as catalyst in DCM
o
complex by the biaryltriol products. To our delight, the L- at -78 C for 60 h, and quenching the reaction with NaBH4 in
RaPr2/Co(ClO4)2·complex was much better at promoting the MeOH (entry 12).
reaction than L-RaPr2/Zn(OTf)2 catalyst, and the product 3aa
With the optimized condition in hand, the reaction scope
was obtained with 90% yield and 64% ee (entry 6 vs entry 5). was next examined (Table 2). Satisfyingly, using 2-naphthol 2a
Next, screening of chiral backbone of the ligand suggested that as the partner, 1,4-quinones 1a-1k with various 2-ester
the L-ramipril derived L-RaPr2 gives better enantioselectivity substituents were smoothly converted into the desired
than L-proline and L-pipecolic acid derived ones (entries 6-8). products 3aa-3ka in good yields and excellent
With the reaction temperature dropped gradually, the enantioselectivities (55-96% yield, and 80-96% ee). Considering
enantioselectivity of the reaction increased due to the fade critical role of the substituents on the 3,3’-positions of BINOL
o
background reaction (entries 9-11). The reaction run at -78 C on the stereocontrol of the reaction, we further explored the
gave a 91% yield and 83% ee. Similar to the work of Salvio and scope of 2-carboxylate substituted 1,4-quinones with
Bella,8c NaBH4 dissolved in MeOH was used to quench the substituents at C5 position (Table 2). To our delight, changing
reaction before flash chromatography, and a slight enhanced the substituted group on the 5-position of quinone moiety had
enantioselectivity was obtained (87% ee; entry 12). Quinones
limited influence on the reactivity. Good to excellent yields
Table 2. Substrate Scope of Qinonesa
Table 1. Optimization of the Reaction Conditionsa
HO
R2
O
O
Co(ClO4)2•6H2O/L-RaPr2
OH
OR1
(1:1, 10 mol%)
R1O2C
OH
OH
R3
DCM, -78 oC, 60 h
R2
R3
O
2
1
3
HO
MeO2C
HO
EtO2C
HO
nPrO2C
HO
OH
OH
OH
OH
OH
OH
nBuO2C
OH
OH
3aa
3ba
3ca
3da
91% yield
90% ee
82% yield
92% ee
80% yield
90% ee
83% yield
90% ee
HO
HO
HO
O
OH
OH
iPrO2C
OH
OH
Entry
Matel salt
Ligand
T (oC)
t (h)
Yield
Ee
iBuO2C
OH
OH
O
(%)b
21
(%)c
0
(aR)-3da
1
2
-
-
-10
-10
-10
-10
-10
-10
-10
-10
-30
-50
-78
-78
-78
0.25
12
12
12
12
12
12
12
12
24
60
60
60
3ea
60% yield
94% ee
3fa
80% yield
90% ee
3ga
55% yield
96% ee
Mg(OTf)2
Cu(OTf)2
Ni(OTf)2
Zn(OTf)2
Co(ClO4)2
Co(ClO4)2
Co(ClO4)2
Co(ClO4)2
Co(ClO4)2
Co(ClO4)2
Co(ClO4)2
Co(ClO4)2
L-RaPr2
L-RaPr2
L-RaPr2
L-RaPr2
L-RaPr2
L-PrPr2
L-PiPr2
L-RaPr2
L-RaPr2
L-RaPr2
L-RaPr2
L-RaPr2
62
87
58
91
90
92
92
86
90
91
89
91
0
HO
O
Br
HO
HO
HO
3
0
O
O
Br
OH
OH
OH
OH
BnO2C
OH
OH Br
OH
OH
4
0
O
O
O
5
42
64
44
20
66
71
83
87
90
3ha
96% yield
80% ee
3jab
3ka
93% yield
84% ee
3iab
6
92% yield
85% ee
70% yield
86% ee
7
HO
OMe
HO
Et
HO
HO
MeO2C
8
MeO2C
OH
OH
MeO2C
OH
OH
MeO2C
OH
OH
OH
OH
9
MeO
10
11
12d
13d,e
3lbc
3mad
3nad
3oad
84% yield
85% ee
71% yield
80% ee
47% yield
75% ee
92% yield
83% ee
a
Unless noted otherwise, all reactions were carried out with
1
(0.1
(0.1 mmol), and L-RaPr2/Co(ClO4)2·6H2O (1:1, 10
without
mmol), 2-naphthol
2
mol%) in DCM (2.5 mL) under N2 for 60 h. Newly oxidated
1
a
Unless noted otherwise, reactions were carried out with 1a (0.1 mmol),
2a (0.1 mmol), and ligand/metal salt (1:1, 10 mol%) in DCM (2.5 mL) under
further purification was used. After 60 h, NaBH4 (0.8 mg) in MeOH (0.2
mL) was added at -78 oC. Isolated yield, and ee was determined by
b
c
N2. Isolated yield. Determined by HPLC analysis on a chiral stationary
phase. d After 60 h, NaBH4 (0.8 mg) in MeOH (0.2 mL) was added at -78 oC.
b
HPLC analysis on a chiral stationary phase. 1,4-benzoquinone
1 (2.0
e
Use newly oxidated 1a without further purification. Co(ClO4)2·6H2O was
equiv) for 36 h. c Reaction at -60 oC, 60 h. d Reaction at -50 oC, 36 h.
used. DCM = dichloromethane
2 | J. Name., 2012, 00, 1-3
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(47-92% yield) and good ee values (75-85% ee) for 3lb
COMMUNICATION
-3oa
To show the synthetic utility of the catalyst system, the
could be achieved after raising the temperature to -60 oC or -50 oC. reaction of quinone 1a and 2-naphthol 2a was performed on a
2-Naphthol derivatives with electron-withdrawing or gram scale (eq 1). The desired product 3aa was obtained in
electron-donating substituents at 6- or 7-position (Table 3) can 97% yield with 86% ee, and can be recrystallized to give an
react smoothly to afford the corresponding products 3ab-3ae excellent enantiomeric excess (96% ee).
in moderate to good yields (53-91% yield) and excellent ee
values (81-95% ee). Moreover, a wide range of 2-naphthols
(2f-2o) with different substituted 6-aryl groups reacted quite well
with quinone 1a to form the products 3af-3ao in satisfied
results (33-95% yield, 84-94% ee). Additionally, the
heteroaromatic substituted 2p, 2q and [1,2'-binaphthalen]-6'-
To gain insight into the reaction process, several control
experiments were carried out. Firstly, 1,3-dimethyl-2-nathphol
2w was used to react with quinone 1a, and the corresponding
dearomatization product 3aw was obtained in 75% yield albeit
with 10% ee (eq 2). This result confirmed that 2-nathphol
could initially undergo a Friedel-Crafts arylation process. It was
found that the biaryltriol product 3aa could be oxidized into 3-
β-nathphol-substituted 1,4-quinone 4aa by the substrate 1a
even at low reaction temperature (eq 3). It was in consistence
with the result that the yield of the biaryl product 3aa dropped
when excessive amount of 1,4-quinone 1a was used (eq. 4).
Furthermore, HRMS spectra confirmed that the product 3aa
can coordinate with chiral L-RaPr2/Co(ClO4)2 complex (eq. 5),
which implied that the catalyst probably suffered poisoning
from the products which hampered the asymmetric catalytic
process.
ol 2r were also suitable for the reaction, and the
corresponding biaryltriols (3ap-3ar) were afforded in 84-98%
yield and 88-92% ee. Notably, the use of alkenyl or alkyl
substituted naphthols also afforded the desired products (3as
-
3av) in good to excellent stereocontrol (86-91% ee) and
excellent isolated yields. X-ray crystal diffraction analysis
allowed to attribute aR absolute configuration of the product
3da
.
Table 3. Substrate Scope of 2-naphthols a
The X-ray crystal diffraction analysis of the catalyst12 showed
that N,N’-dioxide L-RaPr2 coordinated with cobalt(II) via tetra-
oxygen bonding, forming
a polycyclic octahedron metal
complex. The coordination manner of N,N’-dioxide to cobalt(II)
is similar to our previous reports9, confirming the coordination
characteristic of this type of novel ligand. In light of the
structures of the catalyst and the product 3da, a catalytic
model was proposed for the reaction (Scheme 2). The
a The same as the footnote a in Table 2.
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HO
Re-Re face attack
O
4
5
1d
2a
O
RO
HN
O
O
N
Co
O
O
NH
O
N
L-RaPr2/Co(ClO4)2/THF
HO
HO
RO2C
OH
OH
RO2C
O
H
H
6
7
O
(aR)-3da R = nBu
Scheme 2. Proposed catalytic model.
tetradentate N,N’-dioxides L-RaPr2 and the bidentate quinone
ester 1d coordinate to the CoII center. The Si face of the
quinone ester is shielded by the neighboring amide group of
the ligand and the 2-nathphol attack takes from the Re face to
form the intermediate 4da with good diastereo- and
enantioselectivity. Central-to-axial chirality exchange occurring in
the rearomatization of 4da affords the desired product (aR)-
8
a) Y. H. Chen, D. J. Cheng, J. Zhang, Y. Wang, X. Y. Liu, B. Tan,
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Zhou, C. Xu, H. Sun, L. Kurti, Q. L. Xu, J. Am. Chem. Soc., 2016
,
3da
In summary, an efficient Lewis-acid-catalyzed asymmetric
Friedel-Crafts-aromatization between p-benzoquinone
.
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,
derivatives and 2-naphthols has been developed by using a
chiral cobalt(II) complex of N,N’-dioxide. The corresponding
axially chiral products were obtained in up to 98% yield and
95% ee under mild reaction conditions. This highly convergent
and functional group tolerable approach allows for the rapid
construction of axially chiral biaryldiols from simple starting
materials. An X-ray crystal diffraction analysis was used to
identify the configurations of the product and the chiral N,N’-
dioxide-Co(ClO4)2 catalyst, and a possible catalytic model was
suggested. Further application of the catalyst to develop other
novel asymmetric reactions is ongoing in our laboratories.
We appreciate the National Nature Science Foundation of
China (No. 21290182, 21432006, 21625205) for financial
support.
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12 CCDC 1551259 (3da) and CCDC 1551260 (L-RaPr2/Co(ClO4)2
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from The Cambridge Crystallographic Data Centre.
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An efficient Lewis-acid-catalyzed asymmetric Friedel-Crafts-aromatization
between p-benzoquinone derivatives and 2-naphthols has been developed by using a
chiral cobalt(II) complex of N,N’-dioxide.