Rh-catalyzed asymmetric hydrogenation using a furanoside monophosphite second-generation ligand library: Scope and limitations

Article abstract of DOI:10.1016/j.tetasy.2013.12.010

The ligand design of one of the most successful monophosphite ligand classes in Rh-catalyzed hydrogenation was expanded upon by introducing several substituents at the C-3 position of the furanoside backbone. A small but structurally important library of

Full text of DOI:10.1016/j.tetasy.2013.12.010

Tetrahedron: Asymmetry 25 (2014) 258–262
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Rh-catalyzed asymmetric hydrogenation using a furanoside
monophosphite second-generation ligand library: scope and
limitations
a
a,
Sabina Alegre a, Elisabetta Alberico b, , Oscar Pàmies , Montserrat Diéguez
⇑
⇑
a Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Campus Sescelades, C/Marcelꢀlí Domingo, s/n. 43007 Tarragona, Spain
b Istuto di Chimica Biomolecolare, CNR, tr. La Crucca 3, Li Punti, 070 Sassari, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 November 2013
Accepted 12 December 2013
The ligand design of one of the most successful monophosphite ligand classes in Rh-catalyzed hydroge-
nation was expanded upon by introducing several substituents at the C-3 position of the furanoside back-
bone. A small but structurally important library of monophosphite ligands was developed by changing
the substituents at the C-3 position of the furanoside backbone and the substituents/configurations at
the biaryl phosphite group. These new furanoside monophosphite ligands were evaluated in the Rh-cat-
alyzed asymmetric hydrogenation of
a,b-unsaturated carboxylic acid derivatives and enamides. The
results show that the effect of introducing a substituent at the C-3 position of the furanoside backbone
on the enantioselectivity depends not only on the configuration at the C-3 position of the furanoside
backbone and the binaphthyl group but also on the substrate. Thus, the new ligands afforded high to
excellent enantioselectivities in the reduction of carboxylic acid derivatives (ee’s up to >99.9%) and mod-
erate ee’s (up to 67%) in the hydrogenation of enamides.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
successful use of furanoside ligands 1 and 2, derived from D-(+)-
glucose, in the Rh-catalyzed asymmetric hydrogenation of a range
of carboxylic acid derivatives, enamides, and vinyl carboxylates
(Fig. 1).
The asymmetric hydrogenation of functionalized prochiral
olefins catalyzed by chiral transition metal complexes has been
widely used in organic synthesis and some processes have found
industrial applications.1 Many chiral ligands, mainly P- and con-
taining ligands with either C2- or C1-symmetry, have been success-
fully applied.1 For a long time, it was generally accepted that
enantioselective hydrogenation was more effective in the presence
of bidentate ligands.1 Less attention was therefore paid to catalysts
containing monodentate ligands in this process. The first successful
report on the use of monodentate ligands in this process was
reported in 2000 with the application of monophosphonites de-
rived from binaphthol.2 Soon after, Reetz et al. obtained excellent
enantioselectivities with catalyst precursors containing monopho-
Following our interests in carbohydrates as an inexpensive and
highly modular chiral source for preparing ligands,5 and encour-
aged by the results of monophosphite ligands 1 and 2 in asymmet-
ric hydrogenations,4d–g we decided to go one step further and
develop a second generation of furanoside ligands L1–L6a–c
(Fig. 2)6 to be used in the Rh-catalyzed enantioselective hydroge-
nation of a range of carboxylic acid derivatives and enamides.
These ligands differ from previous monophosphite ligands 1 and
2 because new substituents with different electronic and steric
properties have been introduced at the C-3 position of the sugar
backbone. With this library we therefore fully investigated the
effects of systematically varying the configuration of the C-3 car-
bon atom in the sugar backbone L1–L2, the electronic and steric
hindrance of the new substituent at C-3 L2–L6, and the substitu-
ents/configuration of the biaryl phosphite moieties a–c.
sphite ligands, derived from D-(+)-mannitol, in the hydrogenation
of dimethyl itaconate.3 Since then, other successful monodentate
ligands have been developed.4 Research in this field has mainly
focused on the selection/design of new chiral ligands prepared
from readily available, inexpensive/renewable raw materials. For
this purpose, carbohydrates are particularly advantageous thanks
to their low price and easy modular construction.5 In this respect,
Reetz et al.4d and Zheng et al.4e–g have independently reported the
2. Results and discussion
2.1. Asymmetric hydrogenation of dimethyl itaconate S1
Initially, we evaluated furanoside monophosphite ligands L1–
L6a–c (Fig. 2) in the Rh-catalyzed asymmetric hydrogenation of
⇑
Corresponding authors. Tel.: +34 977558780; fax: +34 977559563.
E-mail address: montserrat.dieguez@urv.cat (M. Diéguez).
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.12.010

S. Alegre et al. / Tetrahedron: Asymmetry 25 (2014) 258–262
259
O
O
O
O
O
P
O
O
O
O
O
O
ax
O
O
a
(R)
b (S)ax
=
O
O
O
O
O
1
2
P
O
O
L*
%ee
1a
1b
2a
2b
86.4 (S)
12.8 (R)
39.2 (S)
23.0 (S)
OCOPh
OCOPh
*
[Rh] / L*
H2
n-C4H9
n-C4H9
Figure 1. Furanoside monophosphite ligands 1–2a–b. The enantioselectivities obtained in the asymmetric hydrogenation of a vinyl carboxylate are shown as examples.
O
O
O
R1
O
R2
O
O
6
5
O
P
O
R
O
O
O
1
O
O
O
O
4
2
=
O
3
R
O
O
O
P
O
R2
R1
O
L1 R = Me
L2 R = Me
L3
c R1 = R2 = tBu
ax
a
(R)
b (S)ax
R = Et
L4 R = iPr
L5
R = Bn
L6 R = Ph
Figure 2. Furanoside monophosphite ligand library L1–L6a–c.
Table 1
dimethyl itaconate S1, which is used as a model substrate. The
catalysts were prepared in situ by adding the corresponding
ligands to the catalyst precursor [Rh(nbd)2]SbF6. For the purpose
of comparison, we chose the same reaction conditions that had
previously been used with ligands 1–2 (i.e., 10 bar of H2, dichloro-
methane as solvent, 1 mol % catalyst precursor, a ligand-to-rho-
dium ratio of 2.2 and room temperature).4e
Selected results for the Rh-catalyzed hydrogenation of S1 using the furanoside
monophosphite ligand library L1–L6a–ca
COOMe
COOMe
COOMe
COOMe
[Rh(nbd)2]SbF6 / L1-L6a-c
*
10 bar H2
CH2Cl2, rt, 20 h
S1
Entry
Ligand
Convb (%)
eec (%)
The results, which are summarized in Table 1, indicated that the
catalytic activity is almost suppressed when using bulky biaryl
substituents, that is ligands L1–L2c and 1–2c (entries 3, 6, 13
and 16). We also found that enantioselectivities were greatly af-
fected by the substituents/configuration at the C-3 position of
the furanoside backbone and the configuration of the binaphthyl
group. The results indicated that the effect on the enantioselectiv-
ity of introducing the new substituent at C-3 depends on the con-
figuration at C-3 of the furanoside backbone and at the binaphthyl
moiety. Thus, for glucofuranoside ligands 1, the introduction of a
methyl substituent at C-3 (new ligands L1) had a positive effect
on the enantioselectivity if an (S)-binaphthyl group b was present
(ligand L1b, ee’s up to >99.9%, entry 2 vs 12). However, the pres-
ence of a methyl substituent at C-3 in combination with an (R)-
biaryl group a had a negative effect on the enantioselectivity (li-
gand L1a; entry 1 vs 11). For allofuranoside ligands 2, the enanti-
oselectivities decreased considerably when a substituent was
introduced at C-3, regardless of the configuration at the binaphthyl
group (ligands L2–L5; entries 4, 7–9 vs 14 and entry 5 vs 15). How-
ever with ligand L6a, which had a phenyl substituent, the enanti-
oselectivities obtained were similar to those reported with ligand
2a (entry 10 vs 14).
1
2
3
4
5
6
7
8
L1a
L1b
L1c
L2a
L2b
L2c
L3a
L4a
L5a
L6a
1a
100
100
<5
100
100
<5
100
100
100
100
100
100
<5
100
100
<5
86 (R)
>99.9 (S)
nde
33 (R)
15 (S)
nde
4 (R)
52 (R)
12 (R)
93 (S)
92.8 (R)d
99.1 (S)d
nde
9
10
11
12
13
14
15
16
1b
1c
2a
2b
93.6 (R)d
77.5 (S)d
nde
2c
a
[Rh(nbd)2]SbF6 (1 mol %), ligand (1.1 mol %), S1 (1 mmol), CH2Cl2 (6 mL), 10 bar
of H2, room temperature.
b
% Conversion measured by GC.
Enantiomeric excess measured by GC.
Data from Ref. 4e.
Not determined.
c
d
e
group). When this latter result is compared with the enantioselec-
tivity obtained with the corresponding Rh/1b catalytic system, it
can be concluded that introducing a methyl group into ligand 1b
at C-3 is advantageous. This result is among the best that have
been reported.1
In summary, the best results were achieved (ee’s up to >99.9%)
with ligand L1b (Table 1, entry 2), which contains the optimal
combination of ligand parameters (substituent at C-3 and configu-
ration at the C-3 position of the furanoside ring and at the biaryl

260
S. Alegre et al. / Tetrahedron: Asymmetry 25 (2014) 258–262
Table 2
Table 3
Selected results for the Rh-catalyzed hydrogenation of
a
-dehydroamino acid esters
Selected results for the Rh-catalyzed hydrogenation of S4 using the furanoside
S2–S3 using the furanoside monophosphite ligand library L1–L6a–ca
monophosphite ligand library L1–L6a–ca
R
R
Entry
Ligand
Convb (%)
eec (%)
[Rh(nbd)2]SbF6 / L1-L6a-c
1
2
3
4
5
6
7
8
L1a
L1b
L1c
L2a
L2b
L2c
L3a
L4a
L5a
L6a
1a
100
100
<5
100
100
<5
100
100
100
100
100
100
<5
100
100
<5
54 (S)
24 (R)
nde
32 (R)
44 (S)
nde
*
10 bar H2
CH2Cl2, rt, 20 h
AcHN
CO2Me
AcHN
CO2Me
R= H
S2
S3 R= Ph
Entry
Ligand
S2
S3
29 (R)
18 (R)
26 (R)
58 (R)
93.9 (S)d
85.5 (R)d
nde
Convb (%)
eec (%)
Convb (%)
eec (%)
9
1
2
3
4
5
6
7
8
L1a
L1b
L1c
L2a
L2b
L2c
L3a
L4a
L5a
L6a
1a
100
100
<5
23 (S)
85 (S)
nde
100
100
<5
100
100
<5
100
100
100
100
100
100
<5
100
100
<5
25 (S)
84 (S)
nde
38 (R)
43 (S)
nde
64 (R)
7 (R)
35 (S)
73 (R)
84 (S)
68 (R)
nde
10
11
12
13
14
15
16
1b
1c
2a
2b
100
100
<5
32 (R)
45 (S)
nde
49.1 (R)d
87.1 (S)d
nde
100
100
100
100
100
100
<5
100
100
<5
29 (R)
10 (R)
34 (R)
58 (R)
87.9 (S)d
79.9 (R)d
nde
2c
9
a
[Rh(nbd)2]SbF6 (1 mol %), ligand (1.1 mol %), S2 (1 mmol), CH2Cl2 (6 mL), 10 bar
of H2, room temperature, 8 h.
10
11
12
13
14
15
16
b
% Conversion measured by GC.
Enantiomeric excess measured by GC.
Data from Ref. 4e.
Not determined.
1b
1c
2a
2b
c
d
26.5 (R)d
5.5 (S)d
nde
30 (R)
7 (S)
e
2c
nde
a
[Rh(nbd)2]SbF6 (1 mol %), ligand (1.1 mol %), S2 (1 mmol), CH2Cl2 (6 mL), 10 bar
of H2, room temperature, 20 h.
new substituent at the C-3 position of the furanoside backbone,
we examined the Rh-catalyzed enantioselective hydrogenation of
several 1,1-disubstituted-a-arylenamides (Eq. 1). The hydrogena-
tion of this substrate class provides access to chiral secondary
amines, which are highly valuable intermediates for preparing chi-
ral pharmaceutical and agricultural products.7
b
% Conversion measured by GC.
Enantiomeric excess measured by GC.
Data from Ref. 4f.
Not determined.
c
d
e
2.2. Asymmetric hydrogenation of
S2–S3
a-dehydroamino acid esters
We also screened monophosphite ligands L1–L6a–c in the
asymmetric reduction of some benchmark -dehydroamino acid
a
Table 4
derivatives, methyl 2-acetamidoacrylate S2 and methyl 2-acetam-
idocinnamate S3. The results, which are summarized in Table 2,
again showed that bulky biaryl substituents c have a detrimental
effect on the catalytic activity (i.e., ligands L1–L2c and 1–2c; en-
tries 3, 6, 13 and 16).
Selected results for the Rh-catalyzed hydrogenation of enamides S5–S8 using the Rh/
L1a and Rh/L6a catalytic systema
NHCOMe
NHCOMe
*
[Rh(cod)2]BF4 / L1a or L6a
10 bar H2
R
R
CH2Cl2, rt, 8 h
As for S1, the effect on the enantioselectivity of introducing a
new substituent at C-3 depends on the configuration at C-3 of
the furanoside backbone and at the binaphthyl moiety. For gluco-
furanoside ligands, this again led to a matched combination for
glucofuranoside ligand L1b, containing an (S)-binaphthyl moiety
(entry 2 vs 12), and a mismatched combination for L1a with an
(R)-binaphthyl group (entry 1 vs 11). However, the effect of the
substituents for allofuranoside ligands is slightly different than
for S1. Thus, regardless of the configuration of the binaphthyl
group, the introduction of a new substituent at C-3 generally had
a positive effect on the enantioselectivity (i.e., entries 4–5 vs 14–
15, respectively). Again, for the allofuranoside ligands, the highest
enantioselectivity was achieved when using ligand L6a (i.e., for S3,
the ee’s increased from 30% to 73% by introducing a phenyl substi-
tuent at C-3; entry 10 vs 14).
Entry
Ligand
Ligand
Convb (%)
eec (%)
NHCOMe
1
2
L1a
L6a
100
100
53 (S)
57 (R)
S5
F
NHCOMe
3
4
L1a
L6a
100
100
59 (S)
61 (R)
S6
NHCOMe
5
6
L1a
L6a
100
100
56 (S)
59 (R)
S7
MeO
In summary, the results again show that the introduction of a
methyl substituent at the C-3 position in a glucofuranoside ligand
containing an (S)-binaphthyl moiety has a positive effect on the
enantioselectivity (ligand L1b; i.e., for S3, ee’s increased from
68% to 84%; Table 2, entry 2 vs 12).
NHCOMe
7
8
L1a
L6a
100
100
64 (S)
67 (R)
S8
2.3. Asymmetric hydrogenation of enamides S4–S8
a
[Rh(cod)2]BF4 (1 mol %), ligand (1.1 mol %), substrate (0.5 mmol), CH2Cl2 (6 mL),
30 bar of H2, room temperature.
b
% Conversion measured by GC.
Enantiomeric excess measured by GC.
To further expand upon the scope of monophosphite ligands
L1–L6a–c and further investigate the influence of introducing a
c

S. Alegre et al. / Tetrahedron: Asymmetry 25 (2014) 258–262
261
(Z)-N-acetylaminocinnamate S39 and enamides S4–S810 were pre-
pared following the literature. All other reagents were used as
commercially available. 1H NMR spectra experiments were re-
corded using a 400 MHz spectrometer.
[Rh(cod)2]BF4/L1-L6a-c
*
R
NHAc
R
NHAc
H2
S4 R= C6H5
S5 R= 4-F-C6H4
ð1Þ
S6
R= 4-Me-C6H4
4.2. General procedure for asymmetric hydrogenation
S7 R= 4-MeO-C6H4
S8 R= 2-Naphthyl
In a typical run, [Rh(nbd)2]SbF6 (5.2 mg, 0.01 mmol), and the
corresponding ligand (0.022 mmol, 2.2 equiv) were dissolved in
dichloromethane (6 mL) and the resulting solution was stirred at
room temperature for 30 min. The catalyst solution was then
transferred to a steel autoclave equipped with a glass liner already
containing the substrate (1 mmol). The autoclave was purged five
times with hydrogen gas. Next, it was pressurized to the desired
pressure. After the desired reaction time, the autoclave was
depressurized and the solvent evaporated off. The residue was dis-
solved in Et2O (2 mL) and filtered through a short plug of Celite.
The enantiomeric excess was determined by chiral GC and conver-
sions were determined by GC and confirmed by 1H NMR. The enan-
tiomeric excesses of the hydrogenated products were determined
by GC.11
First, we used N-(1-phenylvinyl)-acetamide S4 as a substrate in
order to study the potential of the ligand library. The results are
summarized in Table 3. In general, the catalytic activity and enanti-
oselectivity are affected by the same parameters that affect the
,b-unsaturated carboxylic acid derivatives S1–S3.
Therefore for this class of compounds, the presence of a very
bulky biphenyl substituent in the phosphite moiety of the catalyst
ligand completely suppresses activity (see Table 3, entries 3, 6, 13,
16). However, in all cases except for ligand L6a (entry 10 vs 14), the
introduction of a substituent at C-3 has a negative effect on the
enantioselectivity (i.e., entries 1–2 vs 11–12, and entries 4–5 vs
14–15). As observed for previously reported ligands 1–2, both
enantiomers of the hydrogenation product can be obtained using
diastereomeric ligands L1a and L6a (entries 1 and 10).
a
Based on this first screening, the best performing ligands L1a
and L6a among the novel synthesized ones were tested in the
Rh-catalyzed hydrogenation of other enamides with different aryl
substituents. The results, which are shown in Table 4, indicate that
the catalytic performance (activity and enantioselectivity) is hardly
affected by the presence of either electron-donating or electron-
withdrawing groups at the para position of the aryl group. How-
ever, the enantioselectivity was highest when N-(1-(2-naph-
thyl)vinyl)-acetamide S8 was used as the substrate (ee’s up to
67%; Table 4, entries 7 and 8).
S1. Enantioselectivity determined using a Chiraldex b-DM col-
umn (100 kPa H2, Isotherm at 60 °C). tR 25.6 min (R); tR
26.5 min (S).
S2. Enantioselectivity determined using a L-Chirasil-Val column
(100 kPa H2, Isotherm at 100 °C). tR 4.8 min (R); tR 5.7 min (S).
S3. Enantioselectivity determined using a L-Chirasil-Val column
(150 kPa H2, Isotherm at 150 °C). tR 8.7 min (R); tR 9.8 min (S).
S4. Enantioselectivity determined using a Chiraldex CB column
(80 kPa H2, temperature program: 125 °C for 4 min–3 °C/min –
140 °C for 5 min – 20 °C/min – 180 °C). tR 15.4 min (S); tR
15.7 min (R).
S5. Enantioselectivity determined using a Chiraldex CB column
(80 kPa H2, temperature program: 100 °C for 5 min – 3 °C/min –
155 °C for 5 min – 20 °C/min – 180 °C). tR 29.0 min (S); tR
29.5 min (R).
S6. Enantioselectivity determined using a Chiraldex CB column
(80 kPa H2, temperature program: 125 °C for 4 min – 3 °C/min –
140 °C for 5 min – 20 °C/min – 180 °C). tR 26.4 min (S); tR
27.1 min (R).
S7. Enantioselectivity determined using a Chiraldex CB column
(80 kPa H2, temperature program: 100 °C for 5 min – 3 °C/min –
155 °C for 5 min – 20 °C/min – 180 °C). tR 38.4 min (S); tR
38.8 min (R).
3. Conclusion
We have expanded upon the ligand design of one of the most
successful monophosphite ligand classes in Rh-catalyzed hydroge-
nations by introducing several substituents at the C-3 position of
the furanoside backbone. Thanks to the modular nature of carbo-
hydrate feedstocks, these modifications were easily made by using
D
-(+)-glucose as a readily available chiral source. These new furan-
oside monophosphite ligands were evaluated in the Rh-catalyzed
asymmetric hydrogenation of a range of ,b-unsaturated carbox-
a
ylic acid derivatives and enamides. The effect that these new sub-
stituents have on the enantioselectivity generally depends not only
on the configuration at the C-3 position of the furanoside backbone
and at the binaphthyl moiety but also on the substrate. Thus for
S8. Enantioselectivity determined using a Chiraldex CB column
(80 kPa H2, temperature program: 150 °C for 5 min – 1 °C/min –
155 °C for 5 min – 20 °C/min – 180 °C). tR 54.7 min (S); tR
55.2 min (R).
a,b-unsaturated carboxylic acids, enantioselectivities improved
when a methyl substituent was introduced at the C-3 position in
a glucofuranoside ligand containing an (S)-binaphthyl group (li-
gand L1a). Enantioselectivities were therefore increased to
>99.9% ee and 85% ee in the asymmetric reduction of dimethyl itac-
onate and dehydroamino acid derivatives, respectively. However,
in the reduction of enamides, the introduction of substituents at
the C-3 position of the furanoside backbone had a negative effect
and enantioselectivities were only moderate (ee’s up to 67%).
Acknowledgments
We would like to thank the Spanish Government for providing
Grant CTQ2010-15835, the Catalan Government for Grant
2009SGR116, and the ICREA Foundation for providing M. Diéguez
and O. Pàmies with financial support through the ICREA Academia
awards. E. Alberico acknowledges financial support from the Regi-
one Autonoma della Sardegna, L.R. 7 Agosto 2007, no. 7.
4. Experimental
4.1. General
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