Cobalt-Catalyzed Asymmetric Hydrogenation of 1,1-Diarylethenes

Article abstract of DOI:10.1021/acs.orglett.6b00453

Highly enantioselective cobalt-catalyzed hydrogenation of 1,1-diarylethenes was developed by using bench-stable chiral oxazoline iminopyridine-cobalt complexes as precatalysts. A unique o-chloride effect was observed to achieve high enantioselectivity. Easy removal as well as further transformations of the chloro group make this protocol a potentially useful alternative to synthesize various chiral 1,1-diarylethanes. This process can be successfully performed under 1 atm of hydrogen at room temperature on gram scale.

Full text of DOI:10.1021/acs.orglett.6b00453

Letter
pubs.acs.org/OrgLett
Cobalt-Catalyzed Asymmetric Hydrogenation of 1,1-Diarylethenes
Jianhui Chen, Chenhui Chen, Chonglei Ji, and Zhan Lu*
Department of Chemistry, Zhejiang University, 866 Yuhangtang Road, Hangzhou 310058, China
S
* Supporting Information
ABSTRACT: Highly enantioselective cobalt-catalyzed hydro-
genation of 1,1-diarylethenes was developed by using bench-
stable chiral oxazoline iminopyridine−cobalt complexes as
precatalysts. A unique o-chloride effect was observed to achieve
high enantioselectivity. Easy removal as well as further
transformations of the chloro group make this protocol a
potentially useful alternative to synthesize various chiral
1,1-diarylethanes. This process can be successfully performed
under 1 atm of hydrogen at room temperature on gram scale.
Ohgo et al have investigated the asymmetric hydrogenation
of alkenes using dimethylglyoximatocobalt(II) complexes to
afford the products in 7−49% ee.8a Pfaltz employed chiral
semicorrin cobalt complex-catalyzed asymmetric hydrogenation
of α,β-unsaturated esters to raise the ee value to 96%.8b Very
recently, Chirik and co-workers successfully utilized rapid
evaluation of libraries of chiral diphosphine ligands for asym-
metric hydrogenation of alkenes under 34 atm of hydrogen gas,
but only limited substrates have been investigated.9 By using
moisture- and air-sensitive C1-asymmetric bisiminopridine
cobalt complexes, the same group studied the asymmetric
hydrogenation of α-2°-alkyl styrene with high levels of
enantioselectivity.10 After our revised manuscipt was submitted,
Co-catalyzed enantioselective hydrogenation of 1,1-diary-
lalkenes was reported by Chirik group, however, the enantio-
selectivities were less than idea (<5−77% ee). To the best of
our knowledge, chiral base metal-catalyzed highly enantiose-
lective hydrogenation of 1,1-diarylalkenes has not previously
been described (Scheme 1).
he 1,1-diarylalkane scaffold is an important motif in many
1
Tbiologically active molecules,
among which 1,1-diaryl-
ethanes show potential treatment of various diseases, such as
inflammation, insomnia, cancer, and obesity (Figure 1).2 On
Figure 1. Bioactive 1,1-diarylethanes.
account of their potential applications in pharmaceuticals, the
development of efficient methods for asymmetric synthesis of
1,1-diarylethanes has received much attention.3−5 Among a
variety of catalytic processes,3,4 asymmetric hydrogenation
of 1,1-diarylalkenes is one of the most efficient and useful
methods.5 Transition-metal-catalyzed highly enantioselective
hydrogenation of 1,1-diarylethenes is challenging due to the
difficulty in differentiating two enantiotopic faces in prochiral
substrates.
Although precious metal catalysts including rhodium,
iridium, and ruthenium have been used for hydrogenation of
1,1-disubstituted alkenes to achieve high enantioselectivities,6
the high cost of removing trace amounts of precious metals for
pharmaceutical utility and potential depletion of precious
metals make the development of green, sustainable base-metal
catalysts for hydrogenation highly desirable. Furthermore, the
distinct electronic structures and unique redox ability of base-
metals provide numerous opportunities for exploring new
reactivity.
Scheme 1. Cobalt-Catalyzed Asymmetric Hydrogenation of
1,1-Disubstituted Alkenes
Because of the lower costs and toxicity of cobalt catalysts,
rapid progress has been made in cobalt-mediated or -catalyzed
transformations during the last two decades.7 However, cobalt-
catalyzed enantioselective hydrogenation of alkenes were rare.8
Received: February 16, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00453
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Organic Letters
Letter
Recently, we have independently synthesized the enantio-
pure oxazoline iminopridine ligands11,12 and their bench-stable
iron and cobalt complexes that could catalyze highly enantio-
selective hydroboration and hydrosilylation of 1,1-disubstituted
alkenes. These results led us to consider that hydrogen as a
mild and clean reagent may also act as a selective hydrogen
donor for asymmetric hydrogenation. Herein, we reported a
highly enantioselective cobalt-catalyzed hydrogenation reaction
of 1,1-diarylethenes.
To benefit from further transformations, we chose 1-(2′-
chlorophenyl)-1-phenylethene (1a) as the substrate in which
the o-chloro group can be easily removed or further transferred
via cross-coupling reactions. Mixing 5 mol % of a cobalt com-
plex L1·CoCl2 with a reductant and 1a in a solution of toluene
at room temperature under 1 atm of hydrogen resulted in
formation of the hydrogenation product 2a in 77% conversion
and 68% ee without any dechlorination product (Table 1).
(1b−d) was tolerated, in which 1-(2,4-dichlorophenyl)-1-
phenylethene (1c) gave the best enantioselectivity and 1-(2,5-
dichlorophenyl)-1-phenylethene (1d) afforded 2d with a
slightly lower reactivity and enantioselectivity. 1,1-Diarylethenes
containing 2-chlorobenzene with both electron-donating and
electron-withdrawing properties all participated to give the
corresponding 1,1-diarylethanes 2e−g in 89−95% ee. Addi-
tionally, the reaction of 1h containing a pyrrolic group on the
5-position of 2-chlorobenzene could also afford 2h smoothly in
92% ee.
Substitutions on the phenyl ring were also investigated. The
p- and m-methoxy groups on the phenyl rings yielded the
hydrogenation products in 90% ee (2i) and 95% ee (2j), respec-
tively. To our delight, the catalytic system distinguished between
the chloro and methoxy group to afford 2k in 85% ee. Although
free alcohols inhibited the reactivity, TBS-protected phenols (2n)
and alcohols (2o) were well compatible under the reaction
conditions. Interestingly, the reaction of electron-deficient 3-CF3
substrate afforded 2p in >99% yield and >99% ee. 3-Phenyl or
4-phenyl substitutions were suitable for the reaction to give
2q or 2r with 89% ee, respectively. A dimethylamino group at
the 4-position (1s) was also tolerated. The substrates with
disubstitutions are good partners to give 2t−v in 89−95% ee. The
chiral indolyl derivatives (2w and 2x) could be obtained in 93%
yield with 96% ee, 97% yield, and 93% ee, respectively.
Table 1. Optimization Studies for Asymmetric
a
Hydrogenation of Alkenes
Various ortho-substitutions were also examined. 1,1-Diary-
lethenes with o-fluoro (3a) and methoxy (3b) substitutions
participated to give the hydrogenation products in >99% yield,
albeit, with a slightly low ee. To our delight, the simple alkyl
groups such as methyl (3c) and ethyl (3d) could give 4c
and 4d with good enantioselectivities. The reaction of 1-(2-
bromophenyl)-1-phenylethene showed poor reactivity (<5%
yield). 1-(1′-Naphthyl)-1-phenylethane can also be accessed
using this method with 77% ee.13 The o- and m-methylphenyl
group could easily be deffiantiated through this method to
afford 4f with 86% ee. The reaction of 3g and 3h gave the
hydrogenation product with 91% ee and 89% ee, respectively.
The reaction of 3h using 1 mol % of L3·CoCl2 could afford 4h
in 80% yield and 88% ee.14 The steric effect on the enan-
tioselectivity is presented in the SI as a function of Charton
steric parameters. There were no solid rules to determine the
high enantioselectivity; however, more steric bulky groups
seemed to promote the enantioselectivity, and the chloro group
showed a unique property.
To prove the broad scope of this protocol, the asymmetric
hydrogenation reactions of α-alkylstyrenes were also conducted
smoothly to afford 4i−m with 89−95% ee (Scheme 3). These
transformations could be efficiently completed in 1 h without
alkene-isomerization products.
To propose the initiation step of the reaction, the active
cobalt species L3·CoMe10a,12a was synthesized to catalyze the
reaction of 1a, which afforded 2a with 90% ee (eq 1).15
b
c
entry
[Co]
reductant
yield (%)
ee (%)
1
L1·CoCl2
L2·CoCl2
L3·CoCl2
L4·CoCl2
L3·CoCl2
L3·CoCl2
L3·CoCl2
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
NaBHsBu3
LiBHEt3
77
99
99
16
87
79
0
68
81
90
82
89
89
2
3
4
5
6
7
8
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
NaBHEt3
0
9
CoCl2
0
10
11
12
L3
0
PDI·CoCl2
L5 + CoCl2
37
<5
77
a
Using 1a (0.5 mmol), cat. (5 mol %), reductant (15 mol %), and
toluene (1 mL) with a hydrogen balloon. Yields determined by H
NMR analysis. Ee values were determined by chiral HPLC analysis.
b
1
c
Sterically bulky substituents on oxazoline increased both
reactivity and enantioselectivity (entries 1−3). Although the
less sterically hindered imine precatalyst could also catalyze
the reaction, either the reactivity or enantioselectivity
was compromised. Different reductants, such as NaBHsBu3 or
LiBHEt3, were tested instead of NaBHEt3, affording the
hydrogenation product with good levels of enantioselectivity,
albeit with slightly low yields. Control reactions were con-
ducted and did not occur without ligands, CoCl2, cobalt
precatalysts, or reductants, which implied that all these com-
ponents were necessary for this transformation. The chiral
cobalt catalysts used in the previous literature9,10 were not
efficient for this asymmetric transformation (entries 11 and 12).
The scope of the cobalt-catalyzed asymmetric hydrogenation
of 1,1-diarylethenes is illustrated in Scheme 2. Additional chloro
substitution at the 3-, 4-, or 5-position on 2-chlorobenzene
To showcase the utility of this transformation, a gram-scale
reaction was easily conducted using a hydrogen balloon. The
chloro group could also undergo further palladium-catalyzed
transformations to afford the corresponding dechlorination,16
phenylation, vinylation, and boronation products in excellent
B
DOI: 10.1021/acs.orglett.6b00453
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Letter
a
Scheme 2. Asymmetric Hydrogenation of 1,1-Diarylethenes
a
b
Standard conditions: alkenes (0.5 mmol), L3·CoCl2 (0.025 mmol), NaBHEt3 (0.075 mmol) in 1 mL of toluene at rt with a hydrogen balloon for 3 h. 12 h.
Scheme 3. Asymmetric Hydrogenation of α-Alkylstyrenes
Additionally, the anti-cancer agent 10 could be obtained from
2x via a dechlorination process.
In summary, we have successfully developed a cobalt-
catalyzed highly enantioselective hydrogenation reaction of
1,1-diarylethenes using bench-stable chiral oxazoline iminopyr-
idine ligands. A unique o-chloride effect was observed to
achieve high enantioselectivity. Since the removal and further
transformation of the chloro group are easily realized, this
method provides a potetially useful alternative to synthesize
various chiral 1,1-diarylethanes. Mechanistic studies and further
applications of asymmetric base-metal catalysis are ongoing.
yields (Scheme 4).17 We determined the absolute configura-
tion of hydrogenation products by comparison of optical
rotation after dechlorination with the known compound 9.18
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 4. Further Derivatizations
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00453.
Experimental procedures and characterization data for all
new compounds (PDF)
X-ray crystallographic data for 4h (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: luzhan@zju.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was conducted using funds from the National
Natural Science Foundation of China (21472162), the
“Thousand Youth Talents Plan”, and the Fundamental
Research Funds for the Central Universities (2013QNA3022).
C
DOI: 10.1021/acs.orglett.6b00453
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Organic Letters
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