Enantioselective hydrogenation of functionalized olefins catalyzed by Rh-chiral bidentatephosphite complexes

Article abstract of DOI:10.1016/j.tetasy.2012.12.001

A novel catalytic system for the hydrogenation of dimethyl itaconate has been developed by using rhodium-diphosphite complexes. These chiral diphosphite ligands were derived from glucopyranoside, d-mannitol derivatives, and binaphthyl or H₈-binaphthyl phosphochloridites. The ligands based on the methyl 3,6-anhydro-α-d-glucopyranoside backbone and (R)- and (S)-binaphthol and/or (R)- and (S)-2,2′-dihydroxy-5,5′,6,6′,7, 7′,8,8′-octahydro-1,1′-binaphthol gave almost complete conversion of the dimethyl itaconate and both enantiomers of dimethyl 2-methylsuccinate with excellent enantioselectivities. The stereochemically matched combination of methyl 3,6-anhydro-α-d-glucopyranoside and H ₈-(S)-binaphthyl in ligand 2,4-bis{[(S)-1,1′-H ₈-binaphthyl-2,2′-diyl]-phosphite} methyl 3,6-anhydro-α- d-glucopyranoside was essential to afford dimethyl 2-methylsuccinate with up to 98% ee. The sense of the enantioselectivity of products was predominantly determined by the configuration of the biaryl moieties of the ligands. An initial screening of [Rh(cod)₂]BF₄ with these ligands in the hydrogenation of (E)-2-(3-butoxy-4-methoxybenzylidene)-3-methylbutanoic acid was carried out. Good enantioselectivity (75% ee) and low yield for (R)-2-(3-butoxy-4-methoxybenzyl)-3-methylbutanoic acid were obtained.

Full text of DOI:10.1016/j.tetasy.2012.12.001

Tetrahedron: Asymmetry 24 (2013) 104–107
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enantioselective hydrogenation of functionalized olefins catalyzed by Rh-chiral
bidentatephosphite complexes
Qing-Lu Zhao a,b, Xiao Miao a,c, Lai-Lai Wang a,
⇑
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b Research Institute of Qilu Branch Co., Sinopec, Zibo 255400, China
c University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 October 2012
Accepted 4 December 2012
A novel catalytic system for the hydrogenation of dimethyl itaconate has been developed by using
rhodium-diphosphite complexes. These chiral diphosphite ligands were derived from glucopyranoside,
D
-mannitol derivatives, and binaphthyl or H8-binaphthyl phosphochloridites. The ligands based on the
methyl 3,6-anhydro-a-D-glucopyranoside backbone and (R)- and (S)-binaphthol and/or (R)- and (S)-
2,20-dihydroxy-5,50,6,60,7,70,8,80-octahydro-1,10-binaphthol gave almost complete conversion of the
dimethyl itaconate and both enantiomers of dimethyl 2-methylsuccinate with excellent enantioselectiv-
ities. The stereochemically matched combination of methyl 3,6-anhydro-
H8-(S)-binaphthyl in ligand 2,4-bis{[(S)-1,10-H8-binaphthyl-2,20-diyl]-phosphite} methyl 3,6-anhydro-
-glucopyranoside was essential to afford dimethyl 2-methylsuccinate with up to 98% ee. The sense
a-D-glucopyranoside and
a
-D
of the enantioselectivity of products was predominantly determined by the configuration of the biaryl
moieties of the ligands. An initial screening of [Rh(cod)2]BF4 with these ligands in the hydrogenation
of (E)-2-(3-butoxy-4-methoxybenzylidene)-3-methylbutanoic acid was carried out. Good enantioselec-
tivity (75% ee) and low yield for (R)-2-(3-butoxy-4-methoxybenzyl)-3-methylbutanoic acid were
obtained.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
the modification of their stereogenic centers. Thus, a systematic
regio- and stereoselective introduction of different functionalities
The asymmetric catalytic synthesis of optically active functional
molecules and materials is one of the most attractive topics in or-
ganic synthesis. In order to achieve excellent activity and enanti-
oselectivity in these synthetic reactions, the design and
preparation of novel transition-metal-complex catalysts with
appropriate steric/electronic natures and great diversity are cru-
cial. The design and synthesis of efficient chiral ligands for transi-
in the synthesis of phosphite ligands has occurred. In our previous
work, we reported on the design and synthesis of bide-
ntatephosphite ligands 1–4 employing methyl 3,6-anhydro-a-
D-glucopyranoside 5, phenyl 3,6-anhydro-b-
D-glucopyranoside 6,
1,2:5,6-di-O-cyclohexylidene-
D-mannitol 7, and/or 1,2:5,6-di-O-
isopropylidene-D
-mannitol 8 as chiral backbones (Scheme 1).5
They were successfully used in the Cu-catalyzed conjugate addi-
tion of organozinc reagents (ZnMe2, ZnEt2, and ZnPh2) to cyclic
enones bearing different ring sizes, to give 3-phenylcyclohexanone
9e with up to 96% ee; other representative enantioselectivities
were also obtained (Scheme 2).
In an effort to expand upon the scope of these ligands and also to
develop an efficient catalytic system for asymmetric hydrogena-
tions, we herein report the development of a novel catalytic system,
which constituted from a catalytic precursor [Rh(cod)2]BF4 and chi-
ral phosphite ligands 1–4 for the hydrogenation of functionalized
olefins. It was found that the stereochemically matched combina-
tions of various chiral elements in these ligands, including stereo-
genic centers, substituents, and binaphthyl moieties, were critical
to induce high enantioselectivities. The sense of enantiodiscrimina-
tion of the product dimethyl 2-methylsuccinate was mainly
controlled by the configuration of the diaryl moieties of the ligands.
tion-metal-catalyzed
enantioselective
hydrogenations
and
conjugate 1,4-addition reactions, which are extremely versatile
protocols for the synthesis of diverse biologically active com-
pounds, have received much attention in recent decades.1 Phos-
phites have emerged as efficient chiral ligands in Rh-catalyzed
enantioselective hydrogenations2 and Cu-catalyzed asymmetric
1,4-addition reactions.3 This is due to their high efficiency, facile
synthesis, and lower sensitivity to air and moisture.
Carbohydrate backbones such as pyranoses, furanoses, and D-
mannitiol, have been used to synthesize various enantiomerically
pure phosphite ligands in recent years.2–4 A variety of structures
based on these natural chiral synthons are readily available by
⇑
Corresponding author. Tel.: +86 (0) 931 4968161; fax: +86 (0) 931 4968129.
E-mail address: wll@licp.cas.cn (L.-L. Wang).
0957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.12.001

Q.-L. Zhao et al. / Tetrahedron: Asymmetry 24 (2013) 104–107
105
diphosphite ligand:
diOH skeleton:
OPh
O
O
O
OPh
O
O
O
O
OMe
O
O
OMe
O
O
O
O
O
O
O
O
O
OH
OH
OH
O
OH
P
P
P
P
5
6
*
*
O
O
1a-1d
*
2a-2d
*
O
O
O
O
O
OH
OH
OH
OH
*
*
O
O
P
P
O
O
O
O
O
O
O
O
O
O
O
O
O
O
P
P
O
O
O
*
O
O
O
*
4a-4d
7
8
3a-3d
O
O
O
O
O
O
O
DMAP, NEt3
CH2Cl2
ligands
=
or
*
P
Cl
diOH
+
*
1-4
O
5-8
14a-14d
c: (R)ax d: (S)ax
a: (R)ax b: (S)ax
Scheme 1. The synthesis of chiral diphosphite ligands 1–4 derived from a diOH skeleton.
O
O
the 1,2:5,6-di-O-cyclohexylidene-
D
-mannitol
7
skeleton, gave
moderate enantioselectivities of 58–81% (Table 1, entries 9–12).
However, the enantioselectivities of the reaction did not improve
(at best 80% ee) when employing ligands 4a–4d with 1,2:5,6-di-
Copper salt,
ligand
+ ZnR2
*
R
n
n
O-isopropylidene-
D
-mannitol 8 as the scaffold (Table 1, entries
9a-9e
13–16).
9a n=0, R=Et, 92%ee using ligand 4c
9b n=1, R=Et, 93%ee using ligand 2c or 4c
9c n=2, R=Et, 90%ee using ligand 4c
9d n=1, R=Me, 69%ee using ligand 4c
9e n=1, R=Ph, 96%ee using ligand 2c
Table 1
a
Rh-catalyzed asymmetric hydrogenation of dimethyl itaconate 10
[Rh(cod)2]BF4, ligands
COOMe
Scheme 2. The Cu-catalyzed asymmetric 1,4-addition reactions of organozincs to
cyclic enones.
COOMe
CH2Cl2, 25 o
C
MeOOC
MeOOC
*
10
11
2. Results and discussion
Diphosphite ligands 1–4 were synthesized stereospecifically
according to the literature procedures, and their 1H NMR, 13C
NMR, 31P NMR, and HRMS were consistent with the reported val-
ues (Scheme 1).5–7 In order to check whether ligands 1–4 may also
serve as efficient ligands in other types of asymmetric catalytic
reactions, the Rh-catalyzed asymmetric hydrogenation of dimethyl
itaconate 10 was evaluated in the presence of ligands 1–4. A series
of novel catalysts was readily prepared in situ by mixing
[Rh(cod)2]BF4 with 1.1 equiv of ligand in CH2Cl2 respectively at
ambient temperature under nitrogen. In the first set of experi-
Entry
Catalyst
Ligands
Conv.b (%)
%eeb (conf.)
1
2
3
4
5
6
7
8
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
[Rh(cod)2]BF4
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
4a
4b
4c
4d
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
92 (R)
95 (S)
93 (R)
96 (S)
56 (R)
51 (S)
61 (R)
55 (S)
58 (R)
67 (S)
63 (R)
81 (S)
23 (R)
78 (S)
52 (R)
80 (S)
9
10
11
12
13
14
15
16
ments, ligands 1a–1d derived from the 3,6-anhydro-a-D-glucopy-
ranoside backbone 5 were evaluated in the hydrogenation of
compound 10 (Table 1, entries 1–4). The use of ligand 1a gave
92% ee (R) for dimethyl 2-methylsuccinate 11 in >99% conversion
(Table 1, entry 1). It is noteworthy that the use of ligand 1b, which
is based on the same pyranoside backbone of the glucose as ligand
1a and an oppositely configured binaphthyl moiety, gave >99%
conversion of the reaction and with 95% ee (S) for compound 11
(Table 1, entry 2). The catalytic performance of ligand 1c was the
same as ligand 1a, while ligand 1d was the same as ligand 1b (Ta-
ble 1, entries 3 and 4). Ligands 2a–2d with a b-phenyl at the 1-po-
sition of the pyranoside skeleton of the glucose (Scheme 1, ligand
1a–1d vs ligand 2a–2d) were also employed in the reaction; li-
gands 2a–2d afforded much lower enantioselectivities (51–61%
ee) than ligands 1a–1d (Table 1, entries 5–8). Next, ligands 3–4
a
[Rh(cod)2]BF4 0.005 mmol, ligand/Rh = 1.1, substrate/Rh = 100, p(H2) 10 atm,
CH2Cl2 3 mL, 25 °C, t = 4 h.
b
The data on the conversion and enantiomeric excess were measured by GC with
a gamma-DEX 225 column (30 m ꢀ 0.25 mm ꢀ 0.25
lm film thickness). The abso-
lute configuration of the products was determined by comparison with authentic
samples.
In all cases, it was found that ligands bearing H8-binaphthyl
moieties induced higher enantioselectivites than their parent li-
gands with binaphthyl substitutes. This observation suggests that
the better stereocontrol capability could be attributed to the larger
torsion dihedral angle of H8-binaphthyl moieties.8 It is noteworthy
that phosphite ligand 1d was the most efficient ligand in this
based on the
D-mannitol skeleton were evaluated under the same
reaction conditions. The use of ligands 3a–3d, which contained

106
Q.-L. Zhao et al. / Tetrahedron: Asymmetry 24 (2013) 104–107
reaction. In our opinion, the stereochemically matched combina-
tion of the methyl 3,6-anhydro- -glucopyranoside backbone
diphosphite ligands 1–4 in the hydrogenation of (E)-2-(3-butoxy-
4-methoxybenzylidene)-3-methylbutanoic acid 12 was carried
out (Scheme 3). The conversion was measured according to 1H
NMR, and the ee of compound 13 was determined by HPLC (Daicel
Chiralcel AD, hexane/ethanol/acetic acid = 95/5/0.1, 1.0 mL/min at
20 °C, detected at 230 nm). When using ligands 1–2, 10–28% ee
(R) for compound 13 and low conversion (<23%) in the reaction
were obtained. Good enantioselectivity (75% ee) for compound
13 was obtained in the presence of ligand 3b. However, we are un-
able to improve upon the low conversion of the reaction by opti-
mizing the reaction conditions.
a-D
and the H8-(S)-binaphthyl in ligand 1d was essential in affording
the best enantioselectivity (96% ee) for dimethyl 2-methylsucci-
nate (Table 1, entry 4 vs entries 5–8). These results indicated two
notable cooperative effects of the stereochemistry of ligands on
the enantioselectivity of the reaction: one between the type of
the glucoside substituent and the axially chiral binaphthyl and/or
H8-binaphthyl moieties; another between the stereogenic centers
of the glucopyranoside and the chiral biaryl substituents. Further-
more, the sense of the enantiodiscrimination of products 11 was
mainly determined by the configuration of the diaryl phosphite
moieties. As for the chiral skeleton of the glucopyranoside, methyl
3. Conclusion
3,6-anhydro-a-D-glucopyranoside 5 was more efficient than phe-
nyl 3,6-anhydro-b-D-glucopyranoside 6 and afforded higher
enantioselectivities.
In conclusion, we have described the development of a novel
catalytic system, which is constituted readily from a catalytic
precursor [Rh(cod)2]BF4 and ligands 1–4 in the Rh-catalyzed
hydrogenation of functionalized olefins giving rise to dimethyl 2-
methylsuccinate with up to 98% ee (S) and (R)-2-(3-butoxy-4-
methoxybenzyl)-3-methylbutanoic acid with up to 75% ee (R).
Using the most efficient catalyst [Rh(cod)2]BF4/1d, the effects of
different reaction parameters (i.e., solvent, pressure of H2, and the
ratio of Sub./Rh) were investigated. An obvious effect of the solvent
i
on the reaction was observed. Employing THF, hexane, and PrOH
as the solvent gave low to moderate enantioselectivities (38–
69%) (Table 2, entries 2–4). The reaction in Et2O solvent afforded
racemic dimethyl 2-methylsuccinate (Table 2, entry 5). These data
indicated that the efficiency of the process depended strongly on
the nature of the solvent; thus, CH2Cl2 was screened as the most
appropriate solvent (Table 2, entries 1–5). Further optimization
of the reaction conditions, such as the ratio of Sub./Rh and pressure
of H2 brought about no significant improvement of the enantiose-
lectivity (Table 2, entries 6 and 8). Up to 98% ee and almost com-
plete conversion were achieved under 5 atm pressure of H2 in
CH2Cl2 (Table 2, entry 7). For practical applications, the asymmetric
hydrogenation of dimethyl 2-methylsuccinate on a gram scale
could be carried out by using lower catalyst loading (S/C = 1000)
and still give 88% ee (Table 2, entry 9).
The backbone of methyl 3,6-anhydro-a-D-glucopyranoside in li-
gands 1a–1d was critical in affording excellent enantioselectivity.
It was found that among the stereogenic centers, the substituents
in the skeleton and the axially chiral diaryl moieties of the ligands
had an obvious synergic effect on the enantioselectivity of the
reaction. The sense of enantiodiscrimination of the product was
mainly determined by the configuration of the diaryl moieties.
The combination of carbohydrates and artificial chiral moieties
provides an easily accessible tool-box for fine tuning efficient li-
gands. Their application to other transition-metal-catalyzed asym-
metric reactions is currently ongoing in our laboratory.
4. Experimental
4.1. General
Table 2
Rh-catalyzed asymmetric hydrogenations of dimethyl itaconatea
All non-aqueous reactions and manipulations were performed
under an N2 atmosphere with standard Schlenk techniques, and
monitored by thin layer chromatography (TLC, silica gel GF254
plates). Column chromatography separations were conducted
on silica gel (200–300 mesh). THF, Et2O, NEt3, and toluene were
distilled with Na using benzophenone as an indicator, and
CH2Cl2 was dried over CaH2 and degassed by standard methods
and stored under an N2 atmosphere prior to use. All other chem-
icals were obtained commercially and used without further
purification.
Entry Sub./Rh Solvent
p(H2) (atm) t (h) Conv.b (%) %eeb (conf.)
1
2
3
4
5
6
7
8
9c
100
100
100
100
100
1000
100
100
CH2Cl2
THF
10
10
4
4
4
4
4
>99 96 (S)
78 69 (S)
62 38 (S)
75 50 (S)
>99 Racemic
14 90 (S)
>99 98 (S)
>99 95 (S)
96 88 (S)
Hexane 10
iPrOH
Et2O
10
10
10
5
1
5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
4
12
12
24
1000
a
Reaction conditions: [Rh(cod)2]BF4 0.005 mmol, ligand 1d/Rh = 1.1, solvent
3 mL, 25 °C.
4.2. Typical procedure for the Rh-catalyzed hydrogenation of
dimethyl itaconate 10
b
The data on the conversion and enantiomeric excess were determined by GC
with a gamma-DEX 225 column (30 m ꢀ 0.25 mm ꢀ 0.25
l
m film thickness). The
absolute configuration of the products was determined by comparison with
After stirring a solution of [Rh(cod)2]BF4 (0.005 mmol, 2 mg)
and ligand 1d (0.0055 mmol, 4.5 mg) in degassed CH2Cl2 (1 mL)
authentic samples.
c
Dimethyl itaconate (7.5 mmol, 1186 mg), CH2Cl2 (25 mL).
at rt, for 1 h,
a solution of dimethyl itaconate (0.5 mmol,
79.1 mg) in degassed CH2Cl2 (2 mL) was added. The mixture was
transferred via syringe into a stainless autoclave that had been pre-
viously purged with argon. The hydrogenation was carried out in
the autoclave at rt, for 4 h. After releasing H2, the reaction mixture
(R)-2-(3-Butoxy-4-methoxybenzyl)-3-methylbutanoic acid 13
could also be used as a starting material for the synthesis of anti-
hypertensive agents.9 Herein, an initial screen [Rh(cod)2]BF4 with
Me(CH2)3O
MeO
COOH
Me(CH2)3O
MeO
COOH
[Rh(cod)2 ]BF4, ligand 3b
H2 (34 atm), 12/Rh = 33
CH2Cl2, rt. 24h
13
up to 75% ee (R)
12
Scheme 3. The Rh-catalyzed asymmetric hydrogenation of 12.

Q.-L. Zhao et al. / Tetrahedron: Asymmetry 24 (2013) 104–107
107
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