A robust Ru-PNNP catalyst system for the asymmetric hydrogenation of α,β-unsaturated ketones to allylic alcohol

Article abstract of DOI:10.1016/j.tetlet.2013.10.051

A robust and stable Ru-Biphosphinobioxazoline catalyst system is discovered for the highly enantioselective hydrogenation of enones to allylic alcohols. A series of acyclic α,β-unsaturated ketones react well affording the desired products in high yields (

Full text of DOI:10.1016/j.tetlet.2013.10.051

Accepted Manuscript
A robust Ru-PNNP catalyst system for the asymmetric hydrogenation of α,β-
unsaturated ketones to allylic alcohol
Sheng-Mei Lu, Qiang Gao, Jun Li, Yan Liu, Can Li
PII:
S0040-4039(13)01786-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.10.051
TETL 43689
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
29 July 2013
25 September 2013
10 October 2013
Please cite this article as: Lu, S-M., Gao, Q., Li, J., Liu, Y., Li, C., A robust Ru-PNNP catalyst system for the
asymmetric hydrogenation of α,β-unsaturated ketones to allylic alcohol, Tetrahedron Letters (2013), doi: http://
dx.doi.org/10.1016/j.tetlet.2013.10.051
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Graphical abstract:

A robust Ru-PNNP catalyst system for the asymmetric hydrogenation
of α,β-unsaturated ketones to allylic alcohol
Sheng-Mei Lua,b*, Qiang Gaoa, Jun Lia, Yan Liua, Can Lia
a: State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
b: Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College
of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan, Hubei
430079 (China)
* To whom correspondence should be addressed.
Tel: 86-411-84379302; Fax: +86-411-84694447
E-mail: smlu@dicp.ac.cn; http:// www.canli.dicp.ac.cn
Abstract:
A robust and stable Ru-Biphosphinobioxazoline catalyst system is discovered for the
highly enantioselective hydrogenation of enones to allylic alcohols. A series of acyclic
α,β-unsaturated ketones react well affording the desired products in high yields (up to
98%) and enantioselectivities (up to 98% ee). Simple manipulation process and
tolerance of water and air make this catalysis more practical and appealing.
Keywords: ruthenium; biphosphinobioxazoline; hydrogenation; unsaturated ketones,
allylic alcohol

Asymmetric hydrogenation has become a very important transformation to obtain
chiral compounds both in academic and industry. For substrates with only one single
unsaturated C=C, C=O or C=N bond, their asymmetric hydrogenations have been
well developed.1 However, for substrates containing multiple different unsaturated
bonds such as heteroaromatic compounds,2 unsaturated ketones and imines,
asymmetric hydrogenation of them is much more complex and chemoselectivity,
regioselectivity and enantioselectivity are involved in these reactions. For example,
hydrogenation of α,β-unsaturated ketones will produce saturated ketones with C=C
double bond being hydrogenated and allylic alcohols with C=O double bond being
hydrogenated. Controlling the chemoselectivity to obtain one single product with high
enantioselectivity is a challenge. Recently, asymmetric hydrogenation of
α,β-unsaturated ketones to saturated ketones with high enantioselectivity has been
realized using Ir-complex of P,N-ligands as catalyst system.3 For asymmetric
hydrogenation of α,β-unsaturated ketones to allylic alcohol, both Ir-complex and
Ru-complex are effective for this transformation.4 Using in situ prepared
Ir-complexes of chiral spiro aminophosphine ligands, Zhou and Xie’s group reported
highly efficient asymmetric hydrogenation of exo-cyclic enones.5 Noyori’s group6 and
Ohkuma’s group7 reported the asymmetric hydrogenation of acyclic enones by using
prepared complex of trans-RuCl2(phosphine)2(1,2-diamine). Under optimized
conditions, the corresponding allylic alcohols were obtained with high yield and
enantioselectivity. However, the preparation of such complexes and the manipulation
of hydrogenation reactions are usually carried out under the protection of inert
atmosphere in order to remove oxygen and water, which brings some problems in
practical use. Thus, more stable and robust catalyst systems are highly desirable.
Considering that allylic alcohols are important intermediates for the synthesis of a
variety of biologically active pharmaceuticals and natural products and asymmetric
hydrogenation of α,β-unsaturated ketones to allylic alcohols is the most convenient
and efficient approach, herein we describe highly enantioselective hydrogenation of
acyclic α,β-unsaturated ketones to allylic alcohols, catalyzed by a robust and stable

Ru-Biphosphinobioxazoline catalyst system under mild conditions.
From the easily available L-tartaric acid, the
C2-symmetric
bisphosphinobioxazoline [(S,S)-Phos-Biox] L1 can be prepared with good yield.8 It
contains diphosphine structure and rigid dioxazoline ring, but without NH structure.
This is different from diamine ligand in Noyori’s RuII- diphosphine/diamine complex,
in which the NH structure is believed to be very important for the reaction9. L1 has
been applied in hydrosilylation,8,10 allylic alkylation11 and transfer hydrogenation
reactions12. Considering that its special tetradentate PNNP structure may have
different effect on the reaction, we investigated the catalytic performance of Ru-L1
combination for the asymmetric hydrogenation of α,β-unsaturated ketones.
Table 1: Optimization of the reaction conditions.a
Entry
Solvent
Ligand
Base
Convn.b
(%)
>95
>95
>95
>95
>95
>95
>95
46
Eec (%)
1
i-PrOH
i-PrOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
t-BuOK
NaOH
62
85
90
90
90
90
89
90
92
89
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
2
3
NaOH
4
LiOH·H2O
KOH
5
6
t-BuOK
Cs2CO3
K2CO3
7
8
9d
10e
LiOH·H2O
NaOH
>95
>95

11f
12
13
EtOH
EtOH
EtOH
EtOH
t-BuOK
>95
<5
88
/
L1
L2
L3
L1
LiOH·H2O
LiOH·H2O
LiOH·H2O
>95
>95
66
90
14g
a
1a: 1.0 mmol; [Ru(cymeme)Cl]2: 1.6 mg (0.0025 mmol); L: 0.0055 mmol;
substrate/catalyst/base: 200/1/10～40. Unless otherwise stated, reactions were conducted at room
temperature under H2 (10 atm) in solvent (3 mL) for 4 hours.
b The conversion was determined by 1H-NMR and all the reactions gave the product 2a with >98%
chemselectivity.
c Enantiomer ratios were determined by HPLC using a Chiralcel OD-H column. The R enantiomer
of product was formed in excess.
d Reaction was carried out at 0 oC;
e Ethanol (2.5 mL) and water (0.5 mL) were used as solvent;
f The reaction was manipulated in glove box using anhydrous ethanol.
g 1a: 5.0 mmol, S/C: 1000/1.
(E)-chalcone (1a) is chosen as a standard substrate. The reaction was performed
without protection and the solvents were reagent pure grade and used as received
without manipulation. Under such conditions, hydrogenation of 1a with the catalyst
prepared in situ from [Ru(cymene)Cl2]2 and L1 in i-PrOH at room temperature under
10 atm of H2 with t-C4H9OK gave complete conversion in 4 hours, affording
(R)-1,3-diphenyl-2-propen-1-ol ((R)-2a) in 62% ee (Table 1, entry 1). No saturated
ketone 1,3-diphenyl-1-propanone (3) and alcohol, 1,3-diphenyl-1-propanol (4) was
1
detected with H-NMR. Without base or hydrogen, no product was detected, which
indicated that base was necessary and transfer hydrogenation did not occur. When
NaOH was used as the base under the same conditions, higher enantioselectivity of 85%
was achieved (entry 2), which indicated that base have great influence on the reaction
in i-PrOH. However, when ethanol was used as the solvent, the reactions with LiOH,
NaOH, KOH, t-C4H9OK and Cs2CO3 as the base all gave full conversion and similar

enantioselectivities of 89-90% (entries 3-7). The reaction only with the weak base
K2CO3 gave 46% conversion, still with 90% ee (entry 8). When the reaction was
o
o
carried out at 0 C, 92% ee was obtained, slightly higher than that at 25 C (entry 9).
Full conversion and 89% ee were obtained when 0.5 mL of water was added to the
reaction, which showed the reaction and the catalyst is not sensitive to water (entry
10). For comparison, a reaction was tried in glove box in anhydrous ethanol and full
conversion and slightly lower enantioselectivity (88% ee) was obtained (entry 11). All
these results indicated that Ru-[(S,S)-Phos-Biox] system is rather stable and robust for
the asymmetric hydrogenation of α,β-unsaturated ketones to allylic alcohols. When
P,N-ligand L2 with the similar half part structure of L1 was used, no product can be
detected (entry 12). Although L3 has the similar PNNP structure as L1, the reaction
with L3 as ligand gave the product with full conversion, but lower enantioselectivity
(entry 13). When the reaction was performed on a 5.0 mmol scale of 1a with a
catalyst loading of 0.1 mol%, conversion (>95%) and enantioselectivity (90% ee)
were maintained under the same conditions (entry 14).
Table 2: Hydrogenation of α,β-unsaturated ketones using Ru-L1 catalyst system.a
Entry Substrate
Yield of 2 ee (%)c
(%)b
Config.d
90 (R)
96 (R)
92 (R)
93 (R)
97 (R)
94 (R)
88 (R)
1
2
3
4
5
6
7
1a, R = H
2a, 98
2b, 94
2c, 96
2d, 95
2e, 89
2f, 94
2g, 92
1b, R = 2-Cl
1c, R = 3-Cl
1d, R = 4-Cl
1e, R = 2-CF3
1f, R = 4-CF3
1g, R = 4-CH3

1h, R = 4-CH3O
1i, Ar = 2-furyl
1j, Ar = 2-thienyl
1k, Ar = 1-naphthyl
1l, R = 4-Cl
2h, 93
2i, 88
2j, 95
2k, 92
2l, 96
2m, 95
2n, 91
90 (R)
82 (R)
84 (R)
91 (R)
93 (R)
89 (R)
86 (R)
8
9
10
11
12
13
14
1m, R = 4-F
1n, R = 4-MeO
15
16
17
18
19
1o
1p
1q
1r
1s
2o, 97
2p, 95
2q, 97
2r, 88e
/
94 (R)
98 (R)
28 (R)
95
/
a All reactions were carried out in ethanol (5 mL) for 6 h at room temperature under 10 bar of H2.
Substrates (1.0 mmol) was used and the catalyst was prepared in situ from 0.25 mol % of
[Ru(cymene)Cl]2 and 0.55 mol% L1 in ethanol (3 mL) without protection, unless otherwise
specified.
b The yields are based on enones and refer to the amount of isolated product.
c Determined by HPLC analysis, see supporting information.
d The absolute configurations were assigned by comparison of the optical rotations with literature
values or assuming analogous reaction pathways.
e
S/C/B: 100/1/5; A mixture of enol and saturated alcohol were obtained and unseparated using

1
chromatography. H-NMR was used to determine the conversion (>95%) and the ratio of them
(88/12).
Under the optimized reaction conditions, a series of α,β-unsaturated ketones
were hydrogenated to demonstrate the scope and limitations of the Ru-L1 catalyst
system. The results are summarized in Table 2. The hydrogenation results of aryl
substituted enones (entries 1-16) revealed that changing the substitution pattern of the
olefinic moiety and ketonic moiety has no obvious effect on conversion but has
important effect on enantioselectivity. All the reactions gave full conversion after 6 h
under a H2 pressure of 10 bar and products were allylic alcohols with >98%
chemselectivity. The hydrogenation of enones 1b-f indicated that
electron-withdrawing group at benzene ring of olefinic moiety has positive effect on
the enantioselectivity, leading to higher ee (Table 2, entries 2-6, 92-97% ee) than that
of 1a (entry 1, 90% ee). In addition, the position of the substituent has influence on
the enantioselectivity. Substrates 1b-d with Cl-group at orth-, meta- and para-position
of benzene ring were hydrogenated with 96, 92 and 93% ee, respectively. CF3-group
is more electron deficient than Cl-group, so the substrate 1e and 1f with CF3-group at
orth- and para-position of benzene ring were hydrogenated with 97% and 94% ee
(entries 5-6). Hydrogenation of substrate 1g and 1h with electron-donating group CH3
and CH3O at para-position of benzene ring gave the products 2g and 2h with the
similar enantioselectivity as 2a (entries 7 and 8, 90% ee for both). 2-Furyl and
2-thienyl substituted enones 1i and 1j, were also good substrates for this reaction,
affording the corresponding allylic alcohol 2i and 2j in 82% (entry 9) and 84% ee
(entry 10), respectively. Substrate 1k with 1-Naphthyl on the olefinic moiety was
hydrogenated smoothly to the desired allylic alcohol with 91% ee (entry 11). When
electro-withdrawing Cl-group was located at the benzene ring of the ketonic moiety
(substrate 1l), slightly higher ee of 92% (entry 12) was obtained compared with
H-group (entry 1). F-group has no effect on the reaction (entry 13) and 89% ee was
achieved (entry 13). When substrate 1n with electron-donating CH3O-groups at the
para-position of benzene ring of the ketonic moiety was hydrogenated, complete
conversion was obtained, albeit with a lower enantioselectivity (entry 14, 86% ee).

The reaction of 1o with both olefinic moiety and ketonic moiety having Cl-group at
the para-position of benzene ring gave the product 2o with 94% ee in high yield (entry
15). Hydrogenation of substrate 1p with orth-CF3-group at the benzene ring of the
olefinic moiety and para-Cl-group located at the benzene ring of the ketonic moiety
gave the product 2p with the highest enantioselectivity of 98% and high yield (entry
16).
For the enone 1q with alkyl substituent methyl-group at the ketonic moiety, the
Ru-L1 catalyst system is also effective, giving the product 2q in high yield, but with
much lower ee (28%, entry 17) compared with 2a, which indicated that the aromatic
ring of the ketonic moiety may have important effect on the enantioselectivity. When
alkyl substituent i-propyl is located at olefinic moiety, hydrogenation of such enone
1r gave uncompleted conversion under the same conditions. Increasing the catalyst
loading from 0.5 to 1.0 mol%, full conversion was achieved and a mixture of
unsaturated (E)-4-methyl-1-phenyl-2-penten-1-ol (88%, 95% ee) and saturated
(E)-4-methyl-1-phenyl-1-pentanol (12%, 76% ee) were obtained (entry 18). For the
cyclic 3,5,5-trimethylcyclohex2-enone, the Ru-L1 gave no conversion (entry 19).
From the above results, it permitted us to conclude that aryl substituted enones are
good substrates for chemo- and enantioselective hydrogenation with Ru-L1 catalyst
system.
For the mechanism of this reaction, with trans-RuCl2(phosphine)2(1,2-diamine)
complex containing N-H structure as the catalyst, a metal-ligand bifunctional catalysis
process through a pericyclic six-membered transition state is proposed7. With Ru-L1
catalyst system without N-H structure, the reaction may proceed through a classic
[2+2] mechanism and N-H effect is not approved here.
In conclusion, we discovered a stable and robust Ru-PNNP catalyst system for
the highly enantioselective hydrogenation of α,β-unsaturated ketones to allylic
alcohols. This Ru-PNNP catalyst system can tolerate water and air, which provides a
practical approach for the synthesis of allylic alcohols with high yield and
enantioselectivity under mild conditions.

Experimental section
General Procedure for Hydrogenation: [Ru(cymene)Cl]2 (1.6 mg, 0.0025 mmol),
ligand L1 (3.7 mg, 0.0055 mmol) and LiOH·H2O (6.3 mg, 0.15 mmol) were placed in
a 5 mL vial equipped with a stirrer bar. To the mixture was added ethanol (3 mL) and
then stirred without any protection for 2 minutes. Then substrate 1 (1.0 mmol) and
ethanol (2 mL) was added and then the vial was put into a steel autoclave. The
autoclave was then closed, purged three times with hydrogen (less than the pressure
needed) and finally pressurized to the value needed. The reaction mixture was stirred
for the indicated period of time, and then the hydrogen gas slowly released. The
1
conversion of the substrate was determined by H NMR spectroscopy of the crude
reaction mixture, and the product was purified by chromatography using a petrolic
ether/ethyl acetate mixture (10:1-5:1). Enantiomeric ratios were determined with
HPLC using a Chiralcel column. The HPLC conditions and the spectral data of all
compounds are provided in the Supporting Information.
Acknowledgment. We thank the National Natural Science Foundation of China,
(NSFC Grant No. 20902032 and 20921092).
Supplementary data
Supplementary data associated with this article can be found in the online version.
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