Ruthenium-catalysed synthesis of chiral exocyclic allylic alcoholsviachemoselective transfer hydrogenation of 2-arylidene cycloalkanones

Article abstract of DOI:10.1039/d1gc00026h

An exclusive asymmetric reduction of C=O bonds of 2-arylidene four-, five-, six-, and seven-membered cycloalkanones has been studied systematically. The asymmetric transfer hydrogenation was performed using a robust and commercially available chiral diamine-derived ruthenium complex as a catalyst and HCOOH/Et₃N as a hydrogen source under mild conditions, giving 51 examples of chiral exocyclic allylic alcohols in up to 96% yield and 99% ee. This method was also applicable to the gram-scale synthesis of the active intermediates of the anti-inflammatory loxoprofen and natural product (?)-goniomitine.

Full text of DOI:10.1039/d1gc00026h

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Ruthenium-catalysed synthesis of chiral exocyclic
allylic alcohols via chemoselective transfer
hydrogenation of 2-arylidene cycloalkanones†
Cite this: Green Chem., 2021, 23,
1628
Received 4th January 2021,
Accepted 27th January 2021
Kaili Zhang,‡ Qixing Liu,‡ Renke He,‡ Danyi Chen, Zhangshuang Deng,
DOI: 10.1039/d1gc00026h
Nianyu Huang and Haifeng Zhou
*
rsc.li/greenchem
An exclusive asymmetric reduction of CvO bonds of 2-arylidene still a challenging problem due to the twisting force of the
four-, five-, six-, and seven-membered cycloalkanones has been ring. In 2010, Zhou developed an efficient asymmetric hydro-
studied systematically. The asymmetric transfer hydrogenation was genation of 2-arylidene cycloalkanones to prepare chiral exo-
performed using
a robust and commercially available chiral cyclic allylic alcohols using an iridium/chiral spiro amino
diamine-derived ruthenium complex as a catalyst and HCOOH/ phosphine complex as a catalyst.6 Since then, three analogues
Et3N as a hydrogen source under mild conditions, giving 51 have been reported. One is catalysed by an iridium complex of
examples of chiral exocyclic allylic alcohols in up to 96% yield and the ferrocene-based chiral phosphine/oxazoline ligand.7 The
99% ee. This method was also applicable to the gram-scale syn- other two are catalysed by ruthenium complexes of chiral
thesis of the active intermediates of the anti-inflammatory loxo- diphosphine/diamine8
and
diphosphine/oxazoline
profen and natural product (−)-goniomitine.
(Scheme 1a).9 However, asymmetric hydrogenation requires
air-sensitive phosphine ligands and high-pressure vessels.
Thus, the development of mild and green methods for the
asymmetric synthesis of chiral exocyclic allylic alcohols is yet
to be explored.
Introduction
Asymmetric transfer hydrogenation, featuring convenient
operation and avoidance of dangerous hydrogen gas, is
regarded as a green alternative to hydrogenation.10 Since the
distinguished chiral monosulfonyl diamine ruthenium com-
plexes were developed by Noyori for the asymmetric transfer
hydrogenation of ketones,11 various analogous ruthenium,
rhodium, and iridium complexes have been developed for the
asymmetric transfer hydrogenation of ketones.12 Very interest-
ingly, this type of catalyst is exclusive for the reduction of CvO
bonds and is tolerant to CvC bonds.13 In recent years, our
group has made some progress in the synthesis of various
functionalized chiral alcohols through asymmetric transfer
hydrogenation.14 For example, a variety of 1-cycloalkyl chiral
Chiral allylic alcohols, particularly cyclic allylic alcohols, are
important and versatile intermediates which can be readily
converted into a variety of pharmaceuticals, agrochemicals
and other biologically active molecules (Fig. 1).1 As a result,
much attention has been paid to developing efficient methods
to access these compounds.2 Although the asymmetric
reduction of enones is one of the most straightforward
methods for the preparation of chiral allylic alcohols, the com-
petition between 1,2- and 1,4-reduction makes it challenging.3
In 1995, the [RuCl2(diphosphine)(diamine)] complex catalysed
asymmetric hydrogenation of enones was pioneered by Noyori,
which represents the most reliable approach to prepare chiral
allylic alcohols.4 Over the past two decades, a variety of acyclic
and endocyclic enones have been hydrogenated with this type
of catalyst, giving the corresponding chiral allylic alcohols with
good chemoselectivities and enantioselectivities.5 However,
the asymmetric synthesis of chiral exocyclic allylic alcohols is
Hubei Key Laboratory of Natural Products Research and Development, Key
Laboratory of Functional Yeast, China National Light Industry, College of Biological
and Pharmaceutical Sciences, China Three Gorges University, Yichang 443002,
China. E-mail: zhouhf@ctgu.edu.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00026h
Fig. 1 Representative bioactive compounds bearing chiral cyclic allylic
‡These authors contributed equally to this work.
alcohols and their derivatives.
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Table 1 Optimization of the reaction conditionsa
Entry
Cat.
[H] sourceb
Solvent (v/v)
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
(S,S)-1a
(S,S)-1b
(S,S)-1c
(S,S)-1d
(S,S)-1e
(S,S)-1f
(S,S)-1g
(S,S)-1h
(S,S)-1i
(S,S)-1j
(S,S)-1k
(S,S)-1f
(S,S)-1f
(S,S)-1f
(S,S)-1f
(S,S)-1f
(S,S)-1f
(S,S)-1f
(S,S)-1f
(S,S)-1f
(S,S)-1f
(S,S)-1f
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (5 : 2)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
H2O
Toluene
MeOH
EtOH
CF3CH2OH
DMF
CHCl3
CHCl3
CHCl3
CHCl3
69
77
60
57
66
75
68
62
66
63
66
64
51
66
70
61
71
62
86
71
65
89
98
93
83
94
77
98
89
91
90
96
65
98
96
93
98
96
91
97
99
99
99
93
9
10
11
12
13
14
15
16
17
18
19
20c
21d
22e
Scheme 1 Asymmetric synthesis of exocyclic allylic alcohols via hydro-
genation and transfer hydrogenation.
HCOONa
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
F : T (1.1 : 1)
allylic alcohols were prepared by a selective asymmetric trans-
fer hydrogenation of cycloalkyl vinyl ketones.15 In order to
study the chemoselective asymmetric transfer hydrogenation
of CvO and CvC bonds deeply, we report an exclusive asym-
metric transfer hydrogenation of CvO bonds of 2-arylidene
four-, five-, six-, and seven-membered cycloalkanones to
prepare chiral exocyclic allylic alcohols systematically, using an
air- and moisture-stable and commercially available catalyst
under very mild conditions (Scheme 1b).
Results and discussion
Initially, the asymmetric transfer hydrogenation of (E)-2-benzyl-
idenecyclopentanone (2a) was conducted with an oxo-tethered
chiral diamine ruthenium complex (S,S)-1a as a catalyst and
HCOOH–NEt3 (molar ratio = 1.1/1) as a hydrogen source at
35 °C for 6 hours in dichloromethane. To our delight, it was
found that the CvO bond was reduced exclusively, affording
exocyclic allylic alcohol 3a in 69% yield and 98% ee (Table 1,
entry 1). Under the identical reaction conditions, a survey of
different chiral monosulfonyl diamine ruthenium complexes
1b–1f, rhodium complex 1j, and iridium complex 1k indicated
that (S,S)-1f was the best one with respect to yields and
a Reaction
conditions:
(E)-2-Benzylidenecyclopentanone
(2a;
0.2 mmol), 2 mol% catalyst, 1 mL of solvent, 35 °C, 6 h; isolated yield.
The ee values were determined by HPLC analysis. b F : T
=
HCOOH : NEt3; the data in the parentheses are the molar ratios.
c 1 mol% catalyst was used. d 25 °C. e 60 °C.
enantioselectivities (Table 1, entries 2–6, 10, and 11). It was entry 20). The yield decreased to 65% at 25 °C (Table 1,
reported that the counter anion effect plays an important role entry 21). When the reaction temperature was increased to
in the reactivities and enantioselectivities.16 Therefore, the 60 °C, a slightly increased yield of 89% was obtained with a
anion effect of ruthenium complexes was also evaluated with lower ee value of 93% (Table 1, entry 22). Finally, the opti-
(S,S)-1g, (S,S)-1h and (S,S)-1i, but no positive effect was mized reaction conditions were determined as follows:
observed (Table 1, entries 7–9). Next, other hydrogen sources 2 mol% of (S,S)-1f as a catalyst, HCOOH–NEt3 (molar ratio =
like HCOOH–NEt3 (molar ratio = 5/2) and HCOONa were 1.1/1) as a hydrogen source, 1 mL of CHCl3 as a solvent, 35 °C,
attempted, providing comparable or inferior results (Table 1, 6 h (Table 1, entry 19).
entries 12 and 13). The screening of solvents showed that
Under the optimized reaction conditions, the scope of
chloroform (CHCl3) was the best one among dichloromethane 2-arylidene cyclopentanones was investigated (Scheme 2). In
(CH2Cl2), chloroform (CHCl3), toluene, water, methanol, general, the electron-donating or electron-withdrawing substi-
ethanol, trifluoroethanol, and N,N-dimethylformamide (DMF) tuent on the phenyl ring of the substrates had no effect on the
(Table 1, entries 6 and 13–19). A lower yield was obtained enantioselectivities but a slight effect on the reactivities. For
when the catalyst loading was decreased to 1 mol% (Table 1, example, irrespective of the substituents like F, Cl, Br, Me,
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Scheme 2 Scope of 2-arylidene cyclopentanones. Reaction conditions:
2-Arylidene cyclopentanone 2 (0.2 mmol), (S,S)-1f (2 mol%), HCOOH/
NEt3 (molar ratio = 1.1 : 1, 5 equiv.), 35 °C, 6 h; isolated yield. The ee
values were determined by HPLC analysis.
Scheme 3 Scope of 4-, 6-, and 7-membered cycloalkanones. Reaction
conditions: 2-Arylidene cycloalkanone 4 (0.2 mmol), (S,S)-1f (2 mol%),
OMe, or tBu at the ortho-, meta-, or para-position of the
benzene ring, the corresponding chiral exoallylic alcohols HCOOH/NEt3 (molar ratio = 1.1 : 1, 5 equiv.), 35 °C, 6 h; isolated yield.
The ee values were determined by HPLC analysis.
3a–3p were obtained in above 97% ee and 71–86% yields. The
trimethyl substituted cyclopentanone 2q gave 67% yield and
98% ee. Additionally, 2-naphthylidene cyclopentanones 2r and
2s also provided the corresponding products 3r and 3s in 98% were investigated. The substrates with electron-donating
ee. Pleasingly, excellent catalytic behaviours were also observed groups provided higher enantioselectivities than those with
for heterocyclic substrates like 2-pyridylidene cyclopentanone electron-withdrawing ones. For example, the electron-rich 5n
(2t) and 2-furylidene cyclopentanone (2u), affording chiral (OMe) was obtained in 95% ee. In contrast, the electron-
2-heteroarylidene cyclopentanols 3t and 3u in more than 98% deficient 5s (CF3) was obtained in 71% ee. The substituents at
ee. Furthermore, the aryl substituent can be replaced by alkyl different positions of the phenyl ring had no apparent effect
groups, such as benzyl and cyclohexyl group, to afford the on the reactivities and enantioselectivities. For example, the
desired products 2-benzylidene cyclopentanol (3v) and 2-cyclo- ortho-substituted 4m and the meta-substituted 4p provided the
hexylidene cyclopentanol (3w) in 94% and 93% ee, same ee value of 83%. 2-Naphthylidene cyclohexanones 4v and
respectively.
4w also gave the desired products 5v and 5w with 83% ee.
Encouraged by the good results of the chemoselective asym- Moreover, this asymmetric transfer hydrogenation was also
metric transfer hydrogenation of 2-arylidene cyclopentanones, amenable for seven-membered 2-arylidene cycloheptanones
we explored the reaction of four-, six- and seven-membered 4x–4a′. The desired chiral exoallylic alcohols 5x–5a′ were
cycloalkanone substrates, as shown in Scheme 3. It was found obtained in 79–87% yields and 84–90% ee.
that the size of the cycloalkyl group had an effect on the
To examine the utility of this asymmetric transfer hydrogen-
enantioselectivities of the products. First, the four-membered ation, a gram-scale reaction of 2m (1.25 g) was carried out with
2-arylidene cyclobutanones 4a–4h with different substituents 1 mol% of (S,S)-1f at 35 °C for 24 h. To our delight, product
(Me, Cl, Br, and CF3) at the ortho-, meta-, or para-position of 3m, a key intermediate in the preparation of the active form of
the phenyl rings were examined. Electronic properties had no the anti-inflammatory loxoprofen, was isolated in 81% yield
apparent effect on the reaction, and 85–96% yields and (1.1 g) and 90% ee. After recrystallization, the ee value was
94–99% ee were observed. When the phenyl ring was replaced increased to 99% (Fig. 2). Additionally, a gram-scale asym-
by naphthalene, 91% yield and 93% ee were also obtained for metric transfer hydrogenation of compound 6 (1.61 g) was con-
5i. Next, the six-membered 2-arylidene cyclohexanones 4j–4u ducted with 1 mol% of (S,S)-1f at 40 °C for 24 h. The cis
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Experimental section
General information
Unless otherwise noted, all reagents, catalysts and solvents
were purchased from commercial suppliers and used without
further purification. Column chromatography was performed
with silica gel. NMR spectra were recorded on a Bruker
AVANCE III (400 MHz) spectrometer. CDCl3 was used for NMR
analysis with tetramethyl silane as the internal standard.
Chemical shifts were observed up field to TMS (0.00 ppm) for
1H NMR spectra and relative to CDCl3 (77.0 ppm) for 13C NMR
spectra. HPLC analyses were conducted on a Waters 2489
Series instrument with chiral columns OJ-H and OD-H. Optical
rotations were measured on an MCP-500 polarimeter. Melting
points were determined using X-4 made by Peking Taike
Apparatus Co., Ltd. HRMS spectra were recorded using an
Agilent 6540 ESI/TOF mass spectrometer.
General procedure for the asymmetric transfer hydrogen-
ation of 2-arylidene cycloalkanones. In a 10 mL Schlenk tube,
formic acid (1.1 mmol, 42 μL), triethylamine (1.0 mmol,
139 μL), and 1.0 mL of CHCl3 were added. After stirring the
mixture for 1 h, (S,S)-1f (0.004 mmol, 2.49 mg) and 2-arylidene
cycloalkanone (0.2 mmol) were added. Then, the mixture was
stirred at 35 °C for 6 h. The solvent was removed under
reduced pressure. Saturated sodium bicarbonate (5 mL) was
added and extracted 3 times with ethyl acetate. The combined
organic layer was dried over Na2SO4 and concentrated under
reduced pressure. The residue was further purified using a
silica gel column to give the desired 2-arylidene cycloalkanol.
Fig. 2 Gram scale synthesis.
Fig. 3
A mode of ATH of 2-arylidene cyclopentanone: the CH/π
interaction.
Gram-scale synthesis of compound 3m
In a 30 mL Schlenk tube, formic acid (5.5 mmol, 208 μL), tri-
ethylamine (5.0 mmol, 695 μL), and 10 mL of CHCl3 were
added. After stirring the mixture for 1 h, (S,S)-1f (0.025 mmol,
6.2 mg) and compound 2m (1.25 g, 5 mmol) were added.
Then, the mixture was stirred at 35 °C for 24 h. The solvent
was removed under reduced pressure. Then, saturated sodium
bicarbonate (5 mL) was added and extracted 3 times with ethyl
acetate. The combined organic layer was dried over Na2SO4
and concentrated under reduced pressure. The residue was
further purified using a silica gel column to give compound
3m (1.01 g, 81% yield, and 90% ee). The ee value was improved
to 99% by recrystallization from ethyl acetate/petroleum ether.
product 7 was obtained in 76% yield (1.23 g), 94% ee and
>99 : 1 dr, which is a key intermediate for the synthesis of the
natural product goniomitine.17
Based on the experimental results and reported putative
outer sphere mechanism for the reduction of aryl ketones,18
we propose that the enantioselectivity originates from the CH/
π interaction between the C–H bond of the η6-arene ligand and
the π bond of the benzyl ring or the CvC bond (2v, 3w),
affording the chiral exocyclic alcohol with an S configuration19
(Fig. 3).
Gram-scale synthesis of compound 7
Conclusions
In a 30 mL Schlenk tube, formic acid (5.5 mmol, 208 μL), tri-
In summary, we have developed a chemoselective asymmetric ethylamine (5.0 mmol, 695 μL), and 10 mL of CHCl3 were
transfer hydrogenation of 2-arylidene four-, five-, six-, and added. After stirring the mixture for 1 h, (S,S)-1f (0.025 mmol,
seven-membered cycloalkanones with a commercially available 6.2 mg) and compound 6 (1.61 g, 5 mmol) were added. Then,
catalyst under mild conditions. The desired chiral exocyclic the mixture was stirred at 40 °C for 24 h. The solvent was
allylic alcohols were obtained in up to 99% ee and moderate to removed under reduced pressure. Saturated sodium bicarbon-
good yields. This reaction could also be performed on a gram- ate (5 mL) was added and extracted 3 times with ethyl acetate.
scale, and the resulting products can be used as intermediates The combined organic layer was dried over Na2SO4 and con-
for drugs and natural products. This asymmetric protocol pro- centrated under reduced pressure. The residue was further
vides an efficient and mild methodology for the synthesis of purified using a silica gel column to give compound 7 (1.23 g,
various chiral exocyclic allylic alcohols.
76% yield, and 94% ee).
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the 111 Project (D20015) and a
research fund for excellent dissertation of China Three Gorges
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