Enantioselective Hydrogenation of Endocyclic Enones: the Solution to a Historical Problem?

Article abstract of DOI:10.1002/cjoc.202000617

The enantioselective hydrogenation of endocyclic enones has been a historical problem for homogeneous catalysis. We herein report an efficient method to reduce endocyclic enones with molecular hydrogen. Catalyzed by a rhodium/Zhaophos complex, a variety of enones with five-, six- or seven-member ring were hydrogenated with high enantioselectivity (92%—99% ee). Excellent chemo- and enantioselectivity demonstrated this method was successfully applied in the enantioselective hydrogenation of citral to produce enantio-enriched citronellal.

Full text of DOI:10.1002/cjoc.202000617

Chinese Journal
of Chemistry
CJC
中 国 化 学 - An International Journal
Accepted Article
Title: Enantioselective Hydrogenation of Endocyclic Enones: the Solution to a
Historical Problem
Authors: Qiwei Lang, Huaxin Yang, Guoxian Gu, Qiang Feng, Jialin Wen* and Xumu
Zhang*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2020, 38, 10.1002/cjoc.202000617.
The final Version of Record (VoR) of it with formal page numbers will soon be
published online in Early View: http://dx.doi.org/10.1002/cjoc.202000617.
ISSN 1001-604X • CN 31-1547/O6
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Zhang Xumu (Orcid ID: 0000-0001-5700-0608)
Cite this paper: Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
Enantioselective Hydrogenation of Endocyclic Enones: the Solu-
tion to a Historical Problem
Qiwei Langa, †, Huaxin Yanga, †, Guoxian Gua, Qiang Fenga, Jialin Wen*a, b and Xumu Zhang*a
a Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology. 1088
Xueyuan Road, Shenzhen, 515055, China. E-mail: wenjl@sustech.edu.cn, zhangxm@sustech.edu.cn
b Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology. 1088 Xueyuan Road, Shenzhen, 515055,
China.
† Qiwei Lang and Huaxin Yang contributed equally.
Keywords
asymmetric catalysis | hydrogenation | cyclic enone | rhodium | citral reduction
Main observation and conclusion
The enantioselective hydrogenation of endocyclic enones has been a historical problem for homogeneous catalysis. We herein report
an efficient method to reduce endocyclic enones with molecular hydrogen. Catalyzed by a rhodium/Zhaophos complex, a variety of
enones with five-, six- or seven-member ring were hydrogenated with high enantioselectivity (92% ~ 99% ee). Excellent chemo- and
enantioselectivity demonstrated This method was successfully applied in the enantioselective hydrogenation of citral to produce en-
antio-enriched citronellal.
Comprehensive Graphic Content
O
O
*
n
n
R
R
CF3
=
n =
examples
R
alkyl
3
23
aryl,
1, 2,
~
ee
92% 97%
BF4/Zhaophos
S
Rh[NBD]2
H
atm), toluene 35 °C
2 (30
F3C
N
N
H
H
*
Fe
Ph2P
O
O
S,R
(
)-ZhaoPhos
Ph2P
citronellal
ee
citral
E/Z 98:2
)
90%
=
(
*E-mail: wenjl@sustech.edu.cn, zhangxm@sustech.edu.cn
View HTML Article
Supporting Information
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
This article has been accepted for publication and undergone full peer review but has not been
through the copyediting, typesetting, pagination and proofreading process which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1002/cjoc.202000617
This article is protected by copyright. All rights reserved.

First author et al.
Report
large-scale application.
Background and Originality Content
Transition metal-catalyzed homogenous hydrogenation of C=C
bonds, with no doubts, has played an important role in the con-
struction of chiral compounds.1 Over the last half century, several
successful catalytic systems with numerous chiral ligands have
been developed for the conversion of prochiral C=C bonds to chi-
ral carbons.2 A key strategy for high enantioselectivity is the coor-
dination of the functional group of the substrates.3 The secondary
interaction assists to form two diastereomeric substrate-catalyst
complexes which lead to the formation of two product enantio-
mers. Mechanistic studies have confirmed this secondary interac-
tion in many successful catalytic systems such as Wil-
Results and Discussion
We began the initial investigations with the hydrogenation of
model substrate 3-phenylcyclopent-2-en-1-one (1a) to evaluate
several chiral bisphosphine ligands. A variety of bisphosphine lig-
ands with different backbones were surveyed under relatively
harsh conditions (60 atm H2, 35°C, 48 hours). To our disappoint-
ment, commercially available chiral bisphosphine ligands, no
matter electron-rich or not, failed in the rhodium catalyzed hy-
drogenation of this endocyclic enone (Table S1 in the supporting
information). The rhodium complexes with these ligands showed
insufficient reactivities and poor enantioselectivities. Gratefully, a
thiourea-containing bisphosphine, Zhaophos that was developed
by our group, succeeded in this chemical transformation. Cata-
lyzed by a Rh/Zhaophos complex, 3-phenylcyclopent-2-en-1-one
(1a) was hydrogenated to the cyclopentanone bearing a stereo-
center at the β-position with a full conversion and 95% ee. After
screening the reaction parameters, the optimized condition was
obtained (97% ee in toluene, 30 atm H2, 35 oC). To our delight, no
ketone reduction was observed. Lowering the hydrogen pressure
or the reaction temperature, however, would lead to a decrease
in conversion. Explanations for the reason of the superiority of
Zhaophos to other bisphosphine ligands include: (1) the hydrogen
bond between thiourea and the carbonyl might activate the sub-
strate by lowering the HOMO, (2) the secondary interaction be-
tween the substrate and the catalyst would enhance the enan-
tiodiscrimination (the interaction model could be found in figure
1).
kinson/Osborn’s cationic Rh complex for dehydroamino acids3, 4
,
Ru/bisphosphine complexes for conjugate carboxylic acids and
Noyori’s RuH-NH systems for ketone reduction.5
In the history of asymmetric hydrogenation, endocyclic
enones has always been a challenging task.6 In our perspective,
the difficulties lie in such areas: (1) as functionalized olefins, the
carbonyl in endocyclic enones could not form efficient coordina-
tion to the metal due to the ring strain and therefore the enanti-
omeric control fails in the aforementioned catalytic systems. (2)
the electron density of the C=C bonds are diluted by the conjugate
carbonyl; together with steric hindrance caused by the ring, the
coordination of the C=C bonds to the metal are weak.
secondary
ensure
enones
interaction to
H
enantioselecivity
challenges for
O
of
high
hydrogenation cyclic
R1
O
R1
R2
P
H
•
*
stable
super
Rh
conjugation
coordination to metal
steric hindrance
•
•
no
P
H
X
O
n
R2
Table 1. Screening of reaction conditions for the asymmetric hy-
NH
M
N
drogenation of 3-phenylcyclopent-2-en-1-one. [a]
R
=
=
or
Ru Ir
classical mechanism for Rh-
enamide
M
X
or
alkali metal
catalyzed
of
H
O
O
hydrogenation
BF4
/
mol%)
Zhaophos (1
Rh(NBD)2
work
This
CF3
H2 solvent, 35 °C
,
F3C
Ph
Ph
O
n
O
1a
2a
S
BF4/Zhaophos
Rh(NBD)2
N
CH3
*
H
H
atm), toluene 35 °C
2 (30
N
Entry
1
2
3
4
5
6
7
Solvent
CH2Cl2
DCE
H2 / atm
60
60
60
60
60
60
60
30
Time
48 h
Conversion
>99%
12%
ee
n
H
R
R
O
95%
86%
95%
86%
94%
96%
96%
97%
97%
97%
96%
H
=
R
n =
examples
Ph2
P
alkoxyl
23
92% 97%
aryl,
alkyl,
3
~
ee
1, 2,
48 h
48 h
48 h
48 h
48 h
48 h
48 h
24 h
24 h
24 h
Rh
S
Fe
R
H
THF
65%
10%
19%
92%
P
Ph2
MeOH
iPrOH
Figure 1. Challenges for asymmetric hydrogenation of cyclic enones.
EtOAc
toluene
toluene
toluene
toluene
toluene
>99%
>99%
>99%
97%
Reports of the maneuvers on AH of this class of substrates are
limited. Although unfunctionalized olefins are hydrogenated with
chiral analogues of Crabtree’s complex, only sporadic reports on
AH of cyclic has been documented.7 Hydrogenation catalyzed by a
cationic ruthenium complex could give (+)-cis-methyl dihydro-
jasmonate with a dr value of 88:12.8 A rhodium/Segphos complex
was applied in the asymmetric hydrogenation of 6-member-ring
endocyclic enones, but the moderate regioselectivity or ee was
still an issue.9 In 2008, asymmetric hydrogenation of cyclic enones
was achieved by Jaekel and coworkers using a hydroformylation
catalyst and this catalytic system could be applied in the synthesis
of enantioenriched citronellal.10 Efforts by Börner and co-workers
were reported to develop a series of P‑chirogenic Xantphos and
their application of asymmetric hydrogenation of isophorone with
high enantioselectivity11, although broad reaction scope was not
disclosed. In comparison, functionalized cyclic enones such as
N-acyl enamides, with higher reactivity and easier enantiomeric
control, could also be hydrogenated smoothly with a rhodium
catalyst.12 An alternative strategy, transfer hydrogenation, was
reported by Macmillan13 and List14 respectively using organocata-
lysts. But the relatively low catalytic efficiency slowed down its
8
9
30
10
30
10
11[b]
45%
[a] reaction conditions: Rh(NBD)2BF4/ligand/1a (0.1 mmol) ratio of
1/1.1/100, 1.0 mL solvent. The conversion was determined by 1H NMR
and the ee value was determined by HPLC analysis using a chiral sta-
tionary phase. [b] The reaction was carried out at 25 °C.
We applied the optimized conditions to explore the reaction
scope. Various aryl cyclic enones underwent hydrogenation
smoothly to yield chiral β-aryl chiral cyclic ketones (figure 2). The
substitution groups on the benzene ring, no matter elec-
tron-donating or electron-withdrawing, showed limited impact on
both yields and enantioselectivities. Heteroaromatic substituents,
such as thiophene, did not make this reaction sluggish (2p). Alkyl
cyclic enones (2q and 2r) could also be reduced with high enanti-
oselectivities. If the ring size expand from five-member ring to six-
or seven-member ring, no significant influence on the reactivity or
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Chin. J. Chem. 2021, 39, XXX－XXX

Running title
Chin. J. Chem.
enantioselectivity was observed (2s and 2t).
aldehyde or the other C=C bond) was indeed a tough challenge
during the condition optimization. Low hydrogen pressure and
shorter reaction time was applied in order to achieve desired
chemo- and regioselectivity. With a minor-modified Zhaophos17,
geranial could be hydrogenated to give citronellal with high enan-
tioselectivity (90%) and high yield (70%). The over-reduction to
alcohol was the main reason for the loss in yield.
Cyclic enones were normally selected as model substrates to
study chemo- and enantioselective hydrogenation of important
α,β-unsaturated ketones or aldehydes in industry.15 We envi-
sioned that this catalytic system could be applied in the produc-
tion of value-added chiral aldehydes such as enantio-enriched
citronellal. Asymmetric hydrogenation of citral to produce enan-
tiopure citronellal was regarded as commercially practical. In ad-
dition, this transformation from citral to citronellal, which has
been a historical problem in fragrance industry, was also a key
step for menthol production. We envisioned that our catalytic
system could be applied in the preparation of enantio-enriched
citronellal.
CF3
BF
Rh(NBD)2 4 (1
mol%)
mol%)
S
O
O
ZhaoPhos
(1.1
F3C
N
N
H
atm), toluene
2 (30
35 °C, 6 h
H
H
Ar2P Fe
Ar2P
3,5-dimethoxyphenyl
3
4
=
citral, E/Z 98:2
mmol
70% isolated
O
O
yield,
=
Ar
ee
BF
Rh(NBD)2 4 (1
mol%)
ZhaoPhos mol%)
0.3
90%
(1.1
Scheme 1. Challenge in the AH of citral.
or
H
atm), PhMe, 35 °C, 24 48 h
2 (30
R
R
1
2
O
O
O
O
Conclusions
2a
2d
2c
97%yield
ee
97%
2b
96%
yield
97%
98%
98%
95%
yield
ee
yield
ee
97%
ee
In summary, we report a synthetic method of AH of endocyclic
enones, which has been a challenge for a long time. Catalyzed by a
rhodium/Zhaophos complex, a series of cyclic enones, no matter
has a 5-, 6- or 7-member ring, could be hydrogenated with high
enantioselectivities. This catalytic system could be successfully
applied in the enantioselective conversion of citral to enan-
tio-enriched citronellal.
Me
Me
Me
O
O
O
O
2f
2g
90%
96%
2h
2e
98%
97%
95%
96%
99%
95%
yield
yield
ee
yield
yield
ee
ee
ee
Me
tBu
OMe
F
Me
Experimental
O
O
O
O
O
O
2i
2j
2l
2k
In an argon-filled glovebox, a solution of Zhaophos (9.6 mg,
0.011 mmol) and Rh(NBD)2BF4 (3.7 mg, 0.01 mmol) in 1.0 mL an-
hydrous toluene was stirred at room temperature for 40 min. 100
uL of the resulting solution was transferred via syringe into a vial
that was charged with a solution of 1 (0.1 mmol) in 0.9 mL anhy-
drous toluene. This vial was placed into an autoclave, which was
then purged with H2 (3 times) and pressurized with 30 atm of H2.
The mixture was stirred at 35 oC (oil bath) for the indicated period
of time. After the indicated period of time, the hydrogen gas was
carefully released in a fume hood. The products were purified by
flash chromatography and the enantiomeric excess was deter-
mined by HPLC analysis.
96%
96%
yield
97%
96%
97%
95%
yield
yield
99%
96%
yield
ee
ee
ee
ee
O
Cl
CF3
CO2Me
Me
O
O
O
2p
95%
96%
2o
2n
99%
95%
2m
yield
98%
97%
yield
ee
98%
97%
yield
ee
yield
ee
ee
S
NO2
O
O
2t
O
2q
2r
2s
95%
98%
yield
96%
96%
96%
99%
yield
yield
97%
92%
yield
ee
ee
ee
ee
Supporting Information
Ph
Ph
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2021xxxxx.
O
O
O
2v
2w
2u
97%
95%
95%
92%
yield
yield
95%
95%
yield
ee
ee
ee
Me
Acknowledgement
Me
OMe
This project was financially supported by the Science, Tech-
nology
and
Innovation
Commission
of
Shenzhen
Figure 2. Substrate scope for asymmetric hydrogenation of cyclic enones.
(KQTD20150717103157174, JCYJ20170817104350391), the Na-
tional Natural Science Foundation of China (21801118) and
Guangdong Provincial Key Laboratory of Catalysis (No.
2020B121201002). This paper is dedicated to the 10th founding
anniversary of Department of Chemistry, SUSTech.
In our perspective, AH of citral to produce enantio-pure cit-
ronellal has many challenges. First, compared to their acyclic
counterparts, cyclic enones are configurationally stabilized due to
the ring strain. The enantiomeric induction of acyclic
α,β-unsaturated carbonyl compounds was therefore less effective.
Second, chemoselectivity for C=C bonds over aldehyde C=O bond
and regioselectivity for the conjugate C=C bond over the other
one has to be achieved in a single chemical transformation.
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Noyori, R., Pioneering Perspectives on Asymmetric Hydrogenation. Acc.
Since AH of C=C bonds without secondary interaction was
normally an enantiodivergent process16, we therefore selected
geranial as the targeting substrate. Over-reduction (either the
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de

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Manuscript received: XXXX, 2021
Manuscript revised: XXXX, 2021
Manuscript accepted: XXXX, 2021
Accepted manuscript online: XXXX, 2021
Version of record online: XXXX, 2021
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© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
Chin. J. Chem. 2021, 39, XXX－XXX

Running title
Chin. J. Chem.
Entry for the Table of Contents
Enantioselective Hydrogenation of Cyclic Enones: the Solution to a Historical Problem
Qiwei Lang, Huaxin Yang, Guoxian Gu, Qiang Feng, Jialin Wen* and Xumu Zhang*
Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
O
O
*
n
n
R
R
CF3
=
n =
examples
R
alkyl
3
23
aryl,
1, 2,
~
ee
92% 97%
BF4/Zhaophos
S
Rh[NBD]2
H
atm), toluene 35 °C
2 (30
F3C
N
N
H
H
*
Fe
Ph2P
O
O
S,R
(
)-ZhaoPhos
Ph2P
citronellal
90%
E-citral
ee
The enantioselective hydrogenation of cyclic enones has been a historical problem for homogeneous catalysis. We herein report an efficient method
to reduce cyclic enones with molecular hydrogen. Catalyzed by a rhodium/Zhaophos complex, variety of enones with five-, six- or seven-member ring
were hydrogenated with high enantioselectivity (92% ~ 99% ee). Excellent chemo- and enantioselectivity demonstrated This method was successfully
applied in the enantioselective hydrogenation of citral to produce enantio-enriched citronellal.
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de