Facile access to chiral 4-substituted chromanes through Rh-catalyzed asymmetric hydrogenation

Article abstract of DOI:10.1016/j.cclet.2020.01.001

Rh/ZhaoPhos-catalyzed asymmetric hydrogenation of a series of (E)-2-(chroman-4-ylidene)acetates was successfully developed to prepare various chiral 4-substituted chromanes with high yields and excellent enantioselectivities (up to 99percent yield, 98percent ee). Moreover, the gram-scale hydrogenation could be performed well in the presence of 0.02 molpercent catalyst loading (TON = 5000), the hydrogenation product was easily converted to access other important compounds, which demonstrated the synthetic utility of this asymmetric catalytic methodology.

Full text of DOI:10.1016/j.cclet.2020.01.001

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Facile access to chiral 4-substituted chromanes through Rh-catalyzed
asymmetric hydrogenation
Lin Tao, Qingyang Zhao, Xumu Zhang, Xiu-Qin Dong
PII:
S1001-8417(20)30001-2
https://doi.org/10.1016/j.cclet.2020.01.001
CCLET 5399
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Chinese Chemical Letters
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1 November 2019
17 December 2019
2 January 2020
Please cite this article as: Tao L, Zhao Q, Zhang X, Dong X-Qin, Facile access to chiral
4-substituted chromanes through Rh-catalyzed asymmetric hydrogenation, Chinese Chemical
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Chinese Chemical Letters
journal homepage: www.elsevier.com
Communication
Facile access to chiral 4-substituted chromanes through Rh-catalyzed asymmetric
hydrogenation
Lin Taoa, Qingyang Zhaob,c, Xumu Zhanga,b, Xiu-Qin Dong*a
a Key Laboratory of Biomedical Polymers, Engineering Research Centre of Organosilicon Compounds & Materials, Ministry of Education, College of Chemistry
and Molecular Sciences, Wuhan University, Wuhan 430072, China
b Department of Chemistry and Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055, China
c School of Pharmaceutical Sciences (Shenzhen), Sun Yatsen University, Shenzhen 518107, China
ARTICLE INFO
Article history:
Received 1 November 2019
Received in revised form 17 December 2019
Accepted 20 December 2019
Available online
Graphical Abstract
Rh/ZhaoPhos-catalyzed asymmetric hydrogenation of a series of (E)-2-(chroman-4-ylidene)acetates was successfully developed
to prepare various chiral 4-substituted chromanes with high yields and excellent enantioselectivities (up to 99% yield, 98% ee, 5
000 TON).
ABSTRACT
Rh/ZhaoPhos-catalyzed asymmetric hydrogenation of a series of (E)-2-(chroman-4-ylidene)acetates was successfully developed to prepare various chiral 4-
substituted chromanes with high yields and excellent enantioselectivities (up to 99% yield, 98% ee). Moreover, the gram-scale hydrogenation could be performed
well in the presence of 0.02 mol% catalyst loading (TON = 5 000), the hydrogenation product was easily converted to access other important compounds, which
demonstrated the synthetic utility of this asymmetric catalytic methodology.
Keywords:
Chiral 4-substituted chromanes
Asymmetric hydrogenation
Excellent enantioselectivity
Gram-scale synthesis
Bisphosphine-thiourea ligand
The chromanes have been respresnted an important subclass of benzopyran structural units, which are key core structures and widely
distributed in many natural products, biologically active compounds and drugs [1]. Their great contributions as significant scaffolds
have been exhibited with a broad range of biological activities, such as treatment of stomach, aldose reductase inhibitors, anti-cancer,
anti-bacterial and anti-arrhythmic (Fig. 1) [2].
———
 Corresponding authors.
E-mail addresses: zhangxm@sustc.edu.cn (X. Zhang); xiuqindong@whu.edu.cn (X.Q. Dong)

Fig. 1. Some examples of biologically active compounds bearing chiral chromane motifs.
Owing to the great importance of these privileged chromanes motifs, much attraction has been obtained to develop efficient synthetic
methods. Therefore, some asymmetric catalytic methodologies for the constrution of chiral chromanes promoted by transition metal
catalysis and organocatalysis have been well established in the past decades [1i,2b,3-13], such as asymmetric cascade reactions
involving ortho-hydroxycinnamaldehydes [1i,3] or ortho-nitrovinylphenols [2b, 4] substrates, asymmetric intramolecular [4+2]
cyclization of ortho-quinone methides with olefins [5] or aldehydes [6], asymmtric conjugate addition of alkynes to 3-
alkoxycarbonylcoumarines [7], dynamic kinetic asymmetric acylation of 2-chromanols [8], intramolecular desymmetric aryl C-O
coupling reaction of 2-(2-haloaryl)-1,5-diols [9], asymmetric 6-exo-trig Michael addition-lactonization of enone-acid [10],
intramolecular ylide annulation [11], allenylidene-ene reactions [12]. Asymmetric catalytic reduction is a direct and powerful synthetic
methodology to construct chiral molecules [14]. However, there are limited asymmetric reduction examples concerning the synthesis of
chiral chromanes [15,16]. In 2017, Zhang and coworkers described a highly efficient Ir-catalyzed asymmetric hydrogenation of
substituted 2H-chromenes and substituted benzo[e][1,2]oxathiine 2,2-dioxides in high yields with excellent enantioselectivities [15a].
Zhou and coworkers realized Ni-catalyzed asymmetric (transfer) hydrogenation of α,β-unsaturated esters with excellent results, which
involved the example of synthesis of chiral 4-substituted chromane [16]. Encouraged by these great achievements and in the
continuation of our efforts in the field of asymmetric catalytic hydrogenation, we herein developed Rh-catalyzed asymmetric
hydrogenation of (E)-2-(chroman-4-ylidene)acetates to afford a series of chiral 4-substituted chromanes with high yields and excellent
enantioselectivities (up to 99% yield, 98% ee), the gram-scale hydrogenation could be proceeded efficiently in the presence of 0.02
mol% catalyst loading (TON = 5 000) (Scheme 1).
Scheme 1. Rh-catalyzed asymmetric hydrogenation of (E)-2-(chroman-4-ylidene)acetates.
We began our initial studies of Rh-catalyzed asymmetric hydrogenation of model substrate ethyl (E)-2-(chroman-4-ylidene)acetate
1a, which was investigated in the presence of various chiral diphosphine ligands, 50 atm H2 in CH2Cl2. As shown in Table 1, the
bisphosphine-thiourea ligands L1 and L2 were applied (Fig. 2), which were developed by our group [17], could promote this Rh-
catalyzed asymmetric hydrogenation to provide the desired product 2a in full conversion and excellent enantioselectivities (>99%
conversion, 95%-97% ee, Table 1, entries 1 and 2). The axially chiral bisphosphine ligands (S)-SegPhos and (S)-Binap displayed poor
catalytic activity and asymmetric induction (5%-9% conversions, 18%-20% ee, Table 1, entries 3, 5). Although moderate
enatioselectivities could be obtained employing (Rc, Sp)-DuanPhos and (S, S)-Me-DuPhos as the ligands, very poor conversions were
given (6%-13% conversions, 56%-60% ee, Table 1, entries 4, 6). Among the ligands probed, the ligand ZhaoPhos L1 was proved to be
the best with regard to both conversion and enantioselectivity for this Rh-catalyzed hydrogenation.

Fig. 2. The structure of chiral diphosphine ligands in this asymmetric hydrogenation.
Table 1
Screening ligands for Rh-catalyzed asymmetric hydrogenation of ethyl (E)-2-(chroman-4-ylidene)acetate 1a.a
Entry
Ligand
ZhaoPhos L1
L2
(S)-SegPhos
(Rc, Sp)-DuanPhos
(S)-Binap
Conv. (%) b
ee (%) c
97
95
-20
-60
1
2
3
4
5
6
>99
>99
9
13
5
-18
-56
(S, S)-Me-DuPhos
6
a Unless otherwise noted, all reactions were carried out with Rh(NBD)2BF4/ligand/1a (0.2 mmol) ratio of 1:1.1:100 in 1.0 mL of solvent under
50 atm of H2 at 30 °C. The catalyst was precomplexed in CH2Cl2 (0.1 mL for each reaction vial).
b Determined by 1H NMR analysis.
c Determined by chiral HPLC analysis.
Solvents always played an important role in asymmetric catalytic reactions. As shown in Table 2, the Rh/ZhaoPhos L1-catalyzed
asymmetric hydrogenation of model substrate ethyl (E)-2-(chroman-4-ylidene)acetate 1a was then investigated in different solvents.
Interestingly, nearly these solvents provided the same enantioselectivity. We found that toluene, CH2Cl2 and 1,2-dichloroethane (DCE)
led to high conversions and excellent enantioselectivities (98% ~ >99% conversions, 97% ee, Table 2, entries 1, 3, 6). In addition,
i
moderate to good conversions were observed with other polar solvents, such as tetrahydrofuran (THF), 1,4-dioxane and PrOH (60%-
82% conversions, Table 2, entries 8-10). Although MeOH, EtOH and CHCl3 allowed this transformation to proceed with high
enantioselectivities, 28%-40% conversions were detected (Table 2, entries 4, 5, 7). Therefore, CH2Cl2 was the optimal solvent for this
Rh-catalyzed asymmetric hydrogenation. To our delight, when the hydrogen pressure was reduced from 50 atm to 10 atm, this
hydrogenation could be still performed smoothly with the same results (>99% conversion, 97% ee, Table 2, entry 3 vs. entry 11).
Table 2
Screening solvents for Rh-catalyzed asymmetric hydrogenation of ethyl (E)-2-(chroman-4-ylidene)acetate 1a.a
Entry
1
2
3
4
Solvent
Toluene
CF3CH2OH
CH2Cl2
MeOH
EtOH
Conv. (%) b
ee (%) c
97
97
97
97
98
54
>99
40
5
37
97
6
DCE
98
97
7
CHCl3
28
97
8
THF
82
97
9
1,4-Dioxane
81
60
>99
97
97
97
10
iPrOH
11d
CH2Cl2
a Unless otherwise noted, all reactions were carried out with Rh(NBD)2BF4/ZhaoPhos L1/1a (0.2 mmol) ratio of 1:1.1:100 in 1.0 mL of solvent
under 50 atm of H2 at 30 °C. The catalyst was precomplexed in CH2Cl2 (0.1 mL for each reaction vial).
b Determined by 1H NMR analysis.
c Determined by chiral HPLC analysis.

d The reaction was carried out under 10 atm H2.
After the optimized reaction conditions were established, we focused on the exploration of the substrate scope generality. These
reaction results were summarized in Scheme 2, a variety of substituted (E)-2-(chroman-4-ylidene)acetates were hydrogenated to give
the corresponding desired chiral 4-substituted chromanes. Whether the electron-deficient or electron-rich groups were at C6-position of
the substrates, they were hydrogenated smoothly to prepare products (2b-2d) in high yields and excellent enantioselectivities (>99%
conversion, 95%-97% yields, 96%-98% ee). In addition, other substrates with methyl or isopropyl ester groups were applied into this
Rh-catalyzed asymmetric hydrogenation. The methyl (E)-2-(chroman-4-ylidene)acetate (1e) was hydrogenated to give the product (2e)
with low conversion and moderate enantioselectivity (38% conversion, 30% yield, 82% ee). The isopropyl (E)-2-(chroman-4-
ylidene)acetate (1f) with bulky steric hindrance was difficult to be hydrogenated in this catalytic system, and trace conversion was
detected. The oxygen atom of the model substrate ethyl (E)-2-(chroman-4-ylidene)acetate 1a was switched to carbon atom forming
substrate ethyl (E)-2-(3,4-dihydronaphthalen-1(2H)-ylidene)acetate 1g, which was hydrogenated well to afford product 2g with full
conversion, 96% yield and 96% ee. In addition, the alkyl substrate 1h also gave the corresponding product 2h in high yield and
moderate enantioselectivity (>99% conversion, 99% yield, 73% ee). However, the substrate (1i) with substituted group on the benzene
ring at C7-position almost did not work in this reduction, trace conversion was observed.
Scheme 2. Substrate scope study. Unless otherwise noted, all reactions were carried out with Rh(NBD)2BF4/ZhaoPhos L1/1 (0.2 mmol) ratio
1
of 1:1.1:100 in 1.0 mL CH2Cl2 under 10 atm of H2 at 30 °C. Conversion was determined by H NMR analysis. ee was determined by chiral
HPLC analysis. b Rh(NBD)2BF4/ZhaoPhos L1 (4.0 mol%), 50 atm H2, 50 °C.
Encourged by these promising results, the benzo-seven-membered substrate was further investigated in this catalytic system. And we
found that ethyl (E)-2-(6,7,8,9-tetrahydro-5H-benzo[7]annulen-5-ylidene)acetate 1j was an excellent substrate pattern, which was
hydrogenated efficiently to provide the desired product 2j with >99% conversion, 97% yield and 94% ee (Scheme 3).
Scheme 3. Rh-catalyzed asymmetric hydrogenation of benzo-7-membered substrate ethyl (E)-2-(6,7,8,9-tetrahydro-5H-
benzo[7]annulen-5-ylidene)acetate 1j.
In order to demonstrate the synthetic utility of this asymmetric catalytic methodology, a gram-scale Rh/ZhaoPhos L1-catalyzed
asymmetric hydrogenation of model substrate 1a was performed in the presence of 0.02 mol% catalyst loading (S/C = 5 000) under 50
atm H2, and product 2a was readily obtained in comparable yield nearly without loss of enantioselectivity (>99% conversion, 96%
yield, 95% ee, Scheme 4a). The enantioenriched hydrogenation product 2a can be easily converted into other useful chiral molecules.
For example, the ester group of the product 2a was hydrolyzed smoothly, affording the chiral carboxylic acid 3 in 84% yield and
without loss of ee value (Scheme 4b) [18]. In addition, the reduction of product 2a with LiAlH4 furnished chiral alcohol 4 in nearly
quantitative yield and 98% ee (Scheme 4b) [6b].

Scheme 4. Gram-scale asymmetric hydrogenation and synthetic transformations.
In conclusion, we successfully developed a highly efficient Rh/ZhaoPhos-catalyzed asymmetric hydrogenation of (E)-2-(chroman-4-
ylidene)acetates to construct a series of chiral 4-substituted chromanes with high yields and excellent enantioselectivities (up to 99%
yield, 98% ee). Moreover, the gram-scale hydrogenation could be performed well in the presence of 0.02 mol% catalyst loading (TON
= 5 000), and hydrogenation product transformations were conducted to access other important compounds, such as chiral carboxylic
acid and chiral alcohol.
Declaration of Interest Statement
The authors declare no competing financial interest.
Acknowledgments
We are grateful for the financial support from the National Natural Science Foundation of China (Nos 21432007, 21502145,
21602172), Wuhan Morning Light Plan of Youth Science and Technology (No. 2017050304010307), the Fundamental Research Funds
for and the Central Universities (No. 2042018kf0202), Shenzhen Nobel Prize Scientists Laboratory Project (No. C17783101), Science
and Technology Innovation Committee of Shenzhen (No. KQTD 20150717103157174) and SZDRC Discipline Construction Program.
The Program of Introducing Talents of Discipline to Universities of China (111 Project) is also appreciated.
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