Thioether-phosphinite and diphosphinite ligands derived from D-xylose for the copper-catalyzed asymmetric 1,4-addition to 2-cyclohexenone

Article abstract of DOI:10.1016/j.tetasy.2005.05.001

A series of thioether-phosphinite 1-3 and diphosphinite 4 and 5 ligands, derived from inexpensive D-(+)-xylose, were tested for the first time in the Cu-catalyzed asymmetric 1,4-addition to 2-cyclohexenone 6. Good enantioselectivities (up to 72%) and acti

Full text of DOI:10.1016/j.tetasy.2005.05.001

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 2161–2165
Thioether–phosphinite and diphosphinite ligands derived
from D-xylose for the copper-catalyzed asymmetric
1
,4-addition to 2-cyclohexenone
*
Eugeni Guimet, Montserrat Di e´ guez, Aurora Ruiz and Carmen Claver
*
Departament de Qu ı´ mica F ı´ sica i Inorg a` nica, Universitat Rovira i Virgili, C/ MarcelÆl ı´ Domingo s/n, 43007 Tarragona, Spain
Received 20 April 2005; accepted 2 May 2005
Available online 31 May 2005
Abstract—A series of thioether–phosphinite 1–3 and diphosphinite 4 and 5 ligands, derived from inexpensive D-(+)-xylose, were
tested for the first time in the Cu-catalyzed asymmetric 1,4-addition to 2-cyclohexenone 6. Good enantioselectivities (up to 72%)
ꢀ
1
and activities [TOF up to >1225 mol (mol h )] combined with excellent selectivity in the 1,4 product were obtained. Our results
show that activity and selectivity (chemo- and enantioselectivity) depend strongly on the type of functional group at the C-5 position
of the carbohydrate backbone, the steric properties of the substituent in the thioether moiety, the catalyst precursor and the alkyl-
ating agent.
Ó 2005 Elsevier Ltd. All rights reserved.
4
1
. Introduction
enantioselective reactions, there have been few reports
on the highly enantioselective 1,4-addition using these
systems. Notable examples, however, include monopho-
The enantioselective conjugate addition of organometal-
lic reagents to a,b-unsaturated substrates catalyzed by
chiral transition metal complexes is a useful synthetic
process for asymmetric carbon–carbon bond forma-
tion. A prominent position in the rapid development
of this process is occupied by the copper-catalyzed,
ligand-accelerated, 1,4-addition of organozinc reagents.
Trialkylaluminum reagents were tested in only a few
cases but these represent an interesting alternative.
5
3c,5,6
sphonite, monophosphite,
7
and mixed amino-thiol-
ate ligands derived from TADDOL, and furanoside
8
diphosphite ligands. Other carbohydrate ligands, such
11
1
3c,5,6,9
10
as phosphoroamidite
1
and mixed S–O, N–P,
heterodonor ligands, have also been
2
0
11,12
S–P, and P–P
tested with low-to-moderate enantioselectivities.
1
2
Following our interest in carbohydrates as an inexpen-
sive and highly modular chiral source for preparing
ligands, and encouraged by the success of the
8
furanoside diphosphite ligands in this process, we
tested their thioether–phosphinite 1–3 and diphosphinite
4 and 5 counterparts (Fig. 1) in the enantioselective cop-
per-catalyzed 1,4-addition to 2-cyclohexenone. To the
best of our knowledge, diphosphinite and thioether–
phosphinite ligands have not previously been applied
in this process.
The selection of chiral ligands for the highly enantiose-
lective conjugate addition of organozinc reagents to
a,b-unsaturated compounds has mainly focused on P-
4
1
f–h,3
donor and mixed P,N-donor ligands.
Most
phosphorus ligands are of the phosphite (mainly mono-
f–h
1
phosphite) and phosphoramidite type.
phorus ligands have scarcely been used with
Non-phos-
1
f
dialkylzinc reagents. More research is therefore re-
quired to study the possibilities offered by other classes
of ligands in this process. Carbohydrates are particu-
larly advantageous for this purpose because they are
inexpensive and their modular constructions are easy.
Although they have been successfully used in other
2. Results and discussion
2.1. Ligand design
*
Corresponding authors. Tel.: +34 9778780; fax: +34 977559563;
e-mail addresses: montserrat.dieguez@urv.net; mariaaurora.ruiz@
urv.net
Ligands 1–5, which were easily prepared in a few steps
1
3
from inexpensive D-(+)-xylose, consist of chiral 1,2-
O-protected xylo- and ribo-furanoside backbones,
0
957-4166/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.05.001

2162
E. Guimet et al. / Tetrahedron: Asymmetry 16 (2005) 2161–2165
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
P
P
5
P
O O
P
O O
RS
O
O
O
1
4
2
O
O
O
3
O
O
Ph
Ph
O
O
P
1
4
5
R = i-Pr
2
3
R = Me
R = Ph
Figure 1. Furanoside thioether–phosphinite 1–3 and diphosphinite 4 and 5 ligands.
which determines their underlying structure with either
thioether (ligands 1–3) or phosphinite (ligands 4 and
CuCN and [Cu(MeCN) ]BF . Interestingly, the sense
4 4
of the asymmetric induction obtained with these catalyst
precursors was the opposite of that for the catalyst pre-
cursor Cu(OTf) . The nature of the catalyst precursor is
5
) groups at the C-5 position.
2
We studied the influence of various substituents at the
thioether groups using ligands 1–3, which have the same
phosphinite moiety. We then used ligands 4 and 5 to
study how a phosphinite moiety rather than the thio-
ether functionality affected catalytic performance. We
also looked at how the configuration of the C-3 stereo-
genic center of the ligand backbone was affected by com-
therefore also important in determining the
enantioselectivity.
Under the optimized conditions, we then studied how
the thioether substituents affected the catalytic perfor-
mance with ligands 2 and 3. Using ligand 2 with a
methyl substituent in the thioether moiety showed lower
enantioselectivities (entries 9–10). Using ligand 3 with a
phenyl substituent in the thioether moiety showed the
lowest asymmetric induction (entries 11 and 12).
paring ligands and 5, which have opposite
configurations at C-3.
4
2
.2. Asymmetric conjugate 1,4-addition of ZnEt2
The influence of the phosphinite moiety rather than the
thioether functionality was studied with ligands 4 and 5
(entries 13–15). In general, these ligands led to lower
enantioselectivities and chemoselecitivities with higher
activities. Interestingly, the sense of asymmetric induc-
tion was the opposite of that obtained with the catalytic
systems Cu/1–3. Also, if we compare the results ob-
tained with ligands 4 and 5 we can see that the configu-
ration of C-3 of the carbohydrate backbone had no
effect on the catalytic performance of the process (entries
14 and 15).
In the first set of experiments, we tested furanoside
ligands 1–5 in the copper-catalyzed conjugate addition
of diethylzinc to 2-cyclohexenone 6 (Eq. 1). The latter
was chosen as a substrate because this reaction has been
performed with a wide range of ligands with several do-
nor groups enabling the direct comparison of the effi-
1
ciency of the various ligands systems. The catalytic
system was generated in situ by adding the correspond-
ing ligand to a suspension of catalyst precursor. The
results are shown in Table 1.
O
O
Finally, we also studied how the temperature affected
the outcome of the reaction with ligand 1. Lowering
the temperature to 0 °C increased enantioselectivity
[
Cu] / 1-5
ð1Þ
ZnEt2
*
Et
(
up to 72%) (entry 7 vs 16). This phenomenon was also
6
7
observed for related furanoside Cu–thioether–phosphite
1
systems.
2
The effect of several reaction parameters, such as sol-
vent, ligand-to-copper ratio and catalyst precursor, were
studied using ligand 1 (entries 1–8). Our results showed
that the efficiency of the process depended on the nature
of the solvent (entries 1–3). Therefore, the selectivity
2.3. Asymmetric conjugate 1,4-addition of AlEt3
Michael additions of organolithium, Grignard and
diorganozinc reagents to enones have been widely stud-
ied in the last decade but very little attention has been
(
chemoselectivity in the 1,4-product and enantioselectiv-
1
,2
ity) was best when dichloromethane was used. Adding
one-fold excess of ligand led to a higher chemoselectivity
in the 1,4-product and enantioselectivity (entry 4). How-
ever, the outcome of the reaction was not affected when
a greater excess of ligand was added (entry 5).
paid to trialkylaluminum reagents. Bearing in mind
the positive effect when we used triethylaluminum
rather than diethylzinc as the alkylating reagent, we also
studied the use of triethylaluminum. Table 2 shows the
results of the copper-catalyzed conjugate addition of tri-
alkylaluminum to 2-cyclohexenone 6 (Eq. 2) using the
thioether–phosphinite 1–3 and diphosphinite 4 and 5
ligands. The catalytic system was generated in situ by
adding the corresponding ligand to a suspension of cat-
alyst precursor.
Varying the catalyst precursor showed an effect on the
selectivity of the process (entries 6–8). The best trade-
off between chemoselectivities and enantioselectivities
was therefore achieved with the catalyst precursors

E. Guimet et al. / Tetrahedron: Asymmetry 16 (2005) 2161–2165
2163
a
Table 1. Cu-catalyzed asymmetric 1,4-addition of diethylzinc to 2-cyclohexenone 6 using ligands 1–5
b
c
d
Entry
Ligand
Solvent
Catalyst precursor
% Conv (h)
1,4-Product (%)
% ee
1
2
3
1
1
1
1
1
1
1
1
2
2
3
3
4
4
5
1
CH Cl
2
2
Cu(OTf)
Cu(OTf)
Cu(OTf)
Cu(OTf)
Cu(OTf)
CuI
2
2
2
2
2
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
97 (1.30)
97 (1)
55
45
25 (S)
10 (S)
12 (S)
34 (S)
33 (S)
55 (R)
64 (R)
63 (R)
26 (R)
53 (R)
2 (R)
Toluene
THF
45
e
4
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
2
2
2
2
2
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
60
59
f
5
e
6
100
100
100
85
e
7
e
CuCN
8
[Cu(MeCN)
CuCN
4
4
4
]BF
]BF
]BF
4
4
4
e
9
e
10
11
12
13
14
15
16
[Cu(MeCN)
CuCN
100
98
e
e
[Cu(MeCN)
CuCN
100
38
15 (R)
17 (S)
25 (S)
29 (S)
72 (R)
[Cu(MeCN)
[Cu(MeCN)
CuCN
4
]BF
]BF
4
69
68
4
4
96 (1)
e,g
63 (2)
100
a
Reaction conditions: catalyst precursor (0.025 mmol), ligand (0.025 mmol), ZnEt
temperature.
Measured by GC using undecane as internal standard. Reaction time in hours shown in parentheses.
2
(3.5 mmol), substrate (2.5 mmol), solvent (6 mL), room
b
c
d
e
f
Chemoselectivity in 1,4-product determined by GC using undecane as internal standard.
Determined by GC using Lipodex-A column. Absolute configuration drawn in parentheses.
L/Cu = 2.
L/Cu = 4.
T = 0 °C.
g
a
Table 2. Cu-catalyzed asymmetric 1,4-addition of triethylaluminum to 2-cyclohexenone 6 using ligands 1–5
b
c
d
Entry
Ligand
Solvent
Catalyst precursor
% Conv (min)
1,4-Product(%)
% ee
1
2
3
1
1
1
1
1
1
1
2
2
3
3
4
4
5
5
5
3
CH Cl
2
2
Cu(OTf)
Cu(OTf)
Cu(OTf)
Cu(OTf)
CuI
2
2
2
2
100 (20)
100 (20)
100 (20)
100 (20)
100 (20)
100 (20)
100 (20)
100 (20)
100 (20)
100 (20)
100 (20)
91 (5)
65
60
9 (R)
8 (S)
3 (R)
Toluene
THF
55
e
4
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
2
2
2
2
2
2
2
2
2
2
100
95
9 (R)
1 (R)
5 (S)
e
5
e
6
CuCN
94
e
7
e
[Cu(MeCN)
Cu(OTf)
[Cu(MeCN)
Cu(OTf)
[Cu(MeCN)
Cu(OTf)
4
4
4
]BF
]BF
]BF
4
4
4
100
95
10 (R)
15 (S)
33 (S)
27 (R)
36 (S)
12 (S)
24 (S)
32 (S)
14 (S)
20 (S)
48 (S)
8
2
e
9
100
95
e
10
11
12
13
14
15
16
17
2
e
100
48
2
[Cu(MeCN)
[Cu(MeCN)
[Cu(MeCN)
[Cu(MeCN)
[Cu(MeCN)
4
4
4
4
4
]BF
]BF
]BF
]BF
]BF
4
4
4
4
4
94 (5)
97 (5)
100
100
65
e
91 (10)
93 (10)
100 (60)
f
62
100
e,g
a
Reaction conditions: catalyst precursor (0.025 mmol), ligand (0.025 mmol), AlEt
temperature.
3
(3.5 mmol), substrate (2.5 mmol), solvent (6 mL), room
b
c
d
e
f
Measured by GC using undecane as internal standard. Reaction time in minutes shown in parentheses.
Chemoselectivity in 1,4-product determined by GC using undecane as internal standard.
Determined by GC using Lipodex-A column. Absolute configuration shown in parentheses.
L/Cu = 2.
L/Cu = 0.5.
T = 0 °C.
g
O
mum trade-off between chemoselectivities and enantio-
selectivities was obtained when dichloromethane was
used as solvent, the ligand-to-copper ratio was 2, and
Cu(OTf) and [Cu(MeCN) ]BF were used as catalyst
O
[
Cu] / 1-5
ð2Þ
AlEt3
*
Et
2
4
4
6
7
precursor (entries 4 and 7).
Our preliminary investigations into the solvent effects,
the ligand-to-copper ratio and catalyst precursor using
ligand 1 (Table 2, entries 1–7) indicated that the opti-
Under optimized conditions, the results with ligands 1–3
(Table 2, entries 4 and 7–11) indicate that the chemose-
lectivities and enantioselectivities followed a different

2164
E. Guimet et al. / Tetrahedron: Asymmetry 16 (2005) 2161–2165
trend to those for the copper-catalyzed conjugate addi-
tion of diethylzinc to 2-cyclohexenone 6 (Table 1, entries
4. Experimental
7
3
–12). Enantioselectivity was therefore best with ligand
, which contains a phenyl substituent in the thioether
4.1. General
moiety, while ligand 1 provided the lowest asymmetric
induction in this case. Note that with ligands 2 and 3,
the best catalyst precursor was [Cu(MeCN) ]BF .
All syntheses were performed using the standard
Schlenk techniques under argon atmosphere. Solvents
were purified by standard procedures. Thioether–phos-
4
4
1
3b
13a
phinites 1–3
and diphosphinites 4 and 5
were
In general, diphosphinite ligands 4 and 5 showed much
prepared by previously described methods. All other
reagents were used as commercially available.
ꢀ1
higher reaction rates [TOF up to >1225 mol (mol h )]
but lower enantioselectivities than the Cu–thioether–
phosphinite ligand systems (entries 12–16). Unlike thio-
ether–phosphinite ligands, with diphosphinite ligands
excess of ligand decreased activity, chemoselectivity
and enantioselectivity (entry 14 vs 15). The same effect
was observed by decreasing the catalyst loading (entry
4.1.1. Typical procedure for the catalytic conjugate
addition of diethylzinc to 2-cyclohexenone, 6. In a typi-
cal procedure, a solution of copper-catalyst precursor
(0.025 mmol) and furanoside ligand (0.025 mL) in
dichloromethane (3 mL) was stirred for 30 min at room
temperature. After cooling to 0 °C, diethylzinc (1 M soln
in hexanes, 3.5 mL, 3.5 mmol) was added. A solution of
2-cyclohexenone (0.24 mL, 2.5 mmol) and undecane as
GC internal standard (0.25 mL) in dichloromethane
(3 mL) was then added at the corresponding reaction
temperature. The reaction was monitored by GC. The
reaction was quenched with HCl (2 M) and filtered twice
through flash silica. Conversion, chemoselectivity and
enantioselectivity were obtained by GC using a Lipo-
dex-A columm.
1
6). If we compare entries 13 and 14, we can see that
the configuration at C-3 affected the product outcome.
Therefore, ligand 5, with an (R)-configuration at C-3,
obtained a better enantioselectivity than the catalytic
system Cu/4.
We studied how temperature affected the outcome of the
reaction with ligand 3. Lowering the temperature to
0
1
°C increased the enantioselectivity (up to 48%) (entry
1 vs 17).
If we compare the results of using AlEt3 with the
results of using ZnEt , we can conclude that with
4.1.2. Typical procedure for the catalytic conjugate
addition of triethylaluminum to 2-cyclohexenone, 6. In
a typical procedure, a solution of copper-catalyst pre-
cursor (0.025 mL) and furanoside ligand (0.025 mL) in
dichloromethane (3 mL) was stirred for 30 min at room
temperature. Triethylaluminum (1 M soln in hexanes,
3.5 mL, 3.5 mmol) was added at the corresponding tem-
perature. A solution of 2-cyclohexenone (0.24 mL,
2
thioether–phosphinite ligands, activity was higher but
enantioselectivity was lower using AlEt . However, for
3
diphosphinite ligands AlEt3 produced much higher
activities and chemoselectivities in the 1,4-product than
ZnEt with similar enantioselectivity.
2
2
.5 mmol) and undecane as GC internal standard
3
. Conclusions
(0.25 mL) in dichloromethane (3 mL) was then added.
The reaction was monitored by GC. The reaction was
quenched with HCl (2 M) and filtered twice through
flash silica. Conversion, chemoselectivity and enantio-
selectivity were obtained by GC using a Lipodex-A
columm.
A series of thioether–phosphinite 1–3 and diphosphinite
and 5 ligands, derived from inexpensive and readily
4
available D-(+)-xylose, were tested in the Cu-catalyzed
asymmetric 1,4-addition to 2-cyclohexenone 6. Good
enantioselectivities (up to 72%) and activities [TOF up
ꢀ
1
to >1225 mol (mol h )] combined with high chemo-
and regioselecitivity in 1,4 product were obtained.
Systematically varying the functional groups at the C-
Acknowledgment
5
position (thioether and phosphinite) and different sub-
We thank the Spanish Ministerio de Educaci o´ n, Cultura
y Deporte for their financial support (BQU2001-0656).
stituents in the thioether moieties had a strong effect on
the rate and enantioselecitivity. The best enantioselectivity
was achieved with the catalyst precursor containing the
thioether–phosphinite ligand 1, which has an isopropyl
substituent in the thioether moiety. However, the activ-
ity was best with the diphosphinite ligands 4 and 5.
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