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Copper-catalyzed asymmetric 1 4-conjugate addition of dialkylzinc to enones.

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Full Paper
Received: 15 February 2010
Revised: 25 February 2010
Accepted: 26 February 2010
Published online in Wiley Interscience: 13 April 2010
(www.interscience.com) DOI 10.1002/aoc.1651
Copper-catalyzed asymmetric 1,4-conjugate
addition of dialkylzinc to enones
Shaohua Goua∗ , Zhongbin Yea,b , Leiting Shib , Dayong Qinga , Wen Zhanga
and Yuliang Wangc
Asymmetric 1,4-conjugation addition of dialkylzinc (diethylzinc and dimethylzinc) to cyclic enones, chalcone and nitroalkenes
was achieved by a 25 mol% (R)-6,6 -Br2 -BINOL(1f), 25 mol% CuSPh and 100 mol% dicyclohexylmethylamin(Cy2 NMe) catalyst
system. The Cu(I) catalyst system enables the cyclic enone, chalcone and nitroalkene generality with high enantioselectivity (up
c 2010 John Wiley & Sons, Ltd.
to 84% ee) and isolated yield (up to 94%) under mild reaction conditions. Copyright Keywords: addition reaction; conjugation; copper; enones; zinc
Introduction
Appl. Organometal. Chem. 2010, 24, 517–522
Catalyst System Screening
Initially, we investigated the 1,4-conjugate addition reaction of
diethyl zinc (Et2 Zn) to 2-cyclohexen-1-one (2a) in the presence of
15 mol% (R)-BINOL (1a) combined with 15 mol% copper salts (1 : 1
ratio) in dry toluene at 0 Ž C under nitrogen atmosphere (Table 1).
The results in Table 1 showed that these copper complexes
could promote the reactions with good to excellent yields
except for the Cu(acac)2 complex (Table 1, entry 9). However,
the enantioselective excess (ee) values were very low, all copper
complexes affording racemic products (Table 1, entries 1–11).
We then thought of introducing another activator (tertiary
amine) to eliminate or decrease the ‘mono-activity’ and form
a Lewis acid–Lewis base catalyst system (dual-activity) for
the 1,4-conjugate addition based on Shibasaki’s method.[15]
Therefore, we introduced serial tertiary amines into the (R)1a–CuSPh catalyst system (Table 2). A significant improvement
in the ee to 67% and in the yield to 68% was achieved when
15 mol% dicyclohexylmethylamine (Cy2 NMe) was employed in
the conjugate addition of diethylzinc to 2-cyclohexen-1-one
(Table 2, entry 1). The use of NEt3 , DMAP, cinchonine, cinchondine, spartein, AD-Mix-Alpha, i-Pr2 NEt, [2,20 ]bipyridinyl, 3,4,7,8tetramethyl-5,6-dihydro-[1,10]phenanthroline and pyridine gave
lower ee (Table 2, entries 3–12 vs entry 1). Surprisingly, the
Ł
Correspondence to: Shaohua Gou, Southwest Petroleum University, School of
Chemistry and Chemical Engineering, Chengdu 610500, People’s Republic of
China. E-mail: shaohuagou@swpu.edu.cn
a School of Chemistry and Chemical Engineering, Southwest Petroleum
University, Chengdu 610500, People’s Republic of China
b State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,
Southwest Petroleum University, Chengdu 610500, People’s Republic of China
c College of Chemistry, Sichuan University, Chengdu 610064, People’s Republic
of China
c 2010 John Wiley & Sons, Ltd.
Copyright 517
Catalytic asymmetric carbon–carbon bond formation using
organometallic reagents is a very important transformation
in the field of organic chemistry.[1] Among these reactions,
copper-catalyzed conjugate addition of dialkylzinc to α, βunsaturated compounds has attracted much attention because
of its simplicity and utility for the preparation of biological
active or natural compounds.[1,2] Ever since the discovery
in the early1990s by Alexakis and co-workers of the Cucatalyzed asymmetric conjugated addition of dialkylzinc reagents
approach, considerable effort has been put into this area
during the past decades by many research groups.[2,3a] As a
consequence, diorganozinc reagents have taken a prominent
position in this field, and they have been successfully applied
to many α, β-unsaturated compounds, such as cyclic and
acyclic enones, nitro olefins, amides, lactones, lactones and
malonates.[1,2] The design of chiral ligands is perhaps the key
to attaining high asymmetric induction in the Cu-catalyzed
asymmetric conjugated addition. The most explored chiral ligands
types were phosphorus-based ligands including phosphites,[3]
diphosphites,[4] phosphoramidites,[5] P,O-[6] and P,N-ligands.[7]
Other classes of ligands explored, without a phosphorus atom,
included chiral sulfonamides,[8] diaminocarbenes,[9] oxazolines,[10]
S,O-,[11] N,N-[12] and N,S-[13] ligands. On the other hand, binaphtholbased ligands are some of the most widely studied in 1,4conjugate addition. Many modifications on the binaphthol
backbone have been developed by many research groups.[2a,14]
These ligands have provided excellent enantioselectivities with
a wide range of substrates for cyclic, linear, nitro- olefin and
alkylidenemalonate substrates. However, these bear a chiral
phosphite or phosphoramidite in the asymmetric conjugated
addition. To the best of our knowledge, there has been only
one example of the enantioselective 1,4-conjugate addition
of dialkylzinc to enones using chiral BINOL-ligands without a
phosphorus atom.[11b,c] Thus, it is still necessary to develop new
types of catalytic system for conjugate addition, especially based
on the chiral BINOL without a phosphorus atom. We report here
the enantioselective conjugate addition of diorganozinc (ZnEt2 or
ZnMe2 ) to cyclic enones, chalcone and nitroalkenes using BINOL
and its derivatives (see Fig. 1) combined with copper salt and a
tertiary amine catalytic system.
S. Gou et al.
R1
R
Table 2. Screening of different tertiary amine for the conjugation
addition of diethylzinc to 2-cyclohexen-1-one
OH
OH
R1
Entrya
R
(R )-1a-g
1a: R = R1 = H
1e: R = P(O)Ph2, R1 = H
1b: R = Br, R1 = H
1f: R = H, R1 = Br
1c: R = I, R1 = H
1d: R = SiPh3, R1 = H 1g: R = H, R1 = I
Figure 1. Screening chiral ligands structures.
Table 1. Screening of different metal salts for the conjugation
addition of diethylzinc to 2-cyclohexen-1-one
O
+
ZnEt2
15 mol% (R)-1a
15 mol% CuX
Entrya
1
2
3
4
5
6
7
8
9
10
11
12
O
*
3a
CuX
Yield (%)b
Ee(%)c
CuI
Cu(TfO)2
CuBr
CuCl
CuCN
CuCl
Cu(AcO)2 .H2 O
CuSO4 .5H2 O
Cu(acac)2
CuBrPh3 P
CuSPh
26
88
25
75
96
80
42
63
trace
91
65
2
3
2
3
1
4
3
2
1
3
8(S)
a Conditions: concentration of 2a, 0.2 M in PhCH ; Et Zn, 1.8 equiv. in
3
2
hexane solution; reaction performed for 5 h at 0 Ž C.
b Isolated yield.
c
The ee was determined by chiral GC G-TA column.[3j,16]
configuration of the products were converted from S to R
when employing [2,20 ]bipyridinyl and 3,4,7,8-tetramethyl-5,6dihydro-[1,10]phenanthroline (Table 2, entries 10 and 11). Notably,
similar results were obtained when the (S)-1a was employed in the
same conjugate addition (Table 2, entry 2).
Catalyst System Optimization
518
Having realized the potential of activators in the asymmetric copper-catalyzed 1,4-conjugate addition of diethylzinc to
2-cyclohexen-1-one (2a), the effects of several reaction parameters, such as copper sources, BINOL derivatives 1b–g and reaction
solvent were studied with the aim of optimizing the reaction
conditions and developing more efficient catalytic system.
Initially, the effects of various copper sources such as CuI,
Cu(TfO)2 , CuBr, CuCl, CuCN, CuCl2 , CuSO4 ž5H2 O, Cu(acac)2 and
Cu(OAc)2 .H2 O were investigated using (R)-1a combined with the
most promising activator (Cy2 NMe; Table 3). In all cases, except
in entry 5, a combination of CuI, CuCl2 , CuSO4 ž5H2 O, Cu(acac)2
and CuBr gave moderate ee values (Table 2, entries 3, 6, 8, 9,
11 vs 1). Other copper complexes, such as CuCl, Cu(TfO)2 , CuCN,
www.interscience.wiley.com/journal/aoc
Yield (%)b
Ee(%)c
Cy2 NMe
Cy2 NMe
NEt3
DMAP
Cinchonine
Cinchondine
Spartin
AD-Mix-Alpha
i-Pr2 NEt
[2,20 ]Bipyridinyl
3,4,7,8-Tetramethyl-5,6dihydro-[1,10]phenanthroline
Pyridine
68
66
67
40
41
67
11
15
58
10
15
67(S)
65(R)
58(S)
30(S)
32(S)
17(S)
10(S)
7(S)
62(S)
16(R)
18(R)
35
24(S)
a
Toluene, 0 °C, 5h
2a
1
2d
3
4
5
6
7
8
9
10
11
Tertiary amine
Conditions: concentration of 2a, 0.2 M in PhCH3 ; CuSPh, 15 mol%;
tertiary amine, 15 mol%; (R)-1a, 15 mol%; Et2 Zn, 1.8 equiv. in hexane
solution; reaction performed for 5 h at 0 Ž C.
b Isolated yield.
c The ee was determined by chiral GC G-TA column.[3j,16]
d (S)-1a was used under same conditions as entry 1.
Table 3. Screening of different salts and solvent for the conjugate
addition using (R)-1a and Cy2 NMe
Entrya
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
CuX
Solvent
Yield (%)b
Ee(%)c
CuSPh
Cu(TfO)2
CuBr
CuCl
CuCN
CuCl2
Cu(AcO)2 žH2 O
CuSO4 ž5H2 O
Cu(acac)2
CuBrPh3 P
CuI
CuSPh
CuSPh
CuSPh
CuSPh
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
Et2 O
CH2 Cl2
Hexene
68
90
25
85
99
80
58
94
10
91
92
15
49
80
58
67
5
24
14
0
47
11
57
38
3
44
59
25
38
43
a
Conditions: concentration of 2a, 0.2 M in PhCH3 ; CuSPh, 15 mol%;
tertiary amine, 15 mol%; (R)-1a, 15 mol%; Et2 Zn, 1.8 equiv. in hexane
solution; reaction performed for 5 h at 0 Ž C.
b Isolated yield.
c The ee was determined by chiral GC G-TA column, and the
configuration was S.[3j,16]
Cu(OAc)2 .H2 O and Ph3 PCuS gave very bad results (Table 3, entries
2, 4, 5, 7, 10).
Next, we investigated the effect of different solvents (toluene,
CH2 Cl2 , THF, Et2 O and hexane) using the complex of (R)-1a
with CuSPh combined with Cy2 NMe (Table 3, entries 12–15). It
was found that THF gave very low yield with 59% ee, and the
configuration of product was converted (Table 3, entry 12). Et2 O
gave moderate yield and ee (Table 3, entry 13). CH2 Cl2 and hexane
gave moderate ee values with moderate to good yield (Table 3,
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 517–522
Copper-catalyzed asymmetric 1,4-conjugate addition
Table 4. Screening of the ligands 1a–g for the conjugation addition
of diethylzinc to 2-cyclohexen-1-one
Entrya
1
2
3
4
5
6
7
8d
9e
ligand (15 mol%)
Time (h)
Yield (%)b
Ee(%)c
1a
1b
1c
1d
1e
1f
1g
1f
1f
5
10
10
5
5
5
5
5
5
68
30
45
72
91
77
73
75
76
67
16
18
8
5
70
66
28
72
a
Conditions: concentration of 2a, 0.2 M in PhCH3 ; Et2 Zn, 1.8 equiv.
in hexene solution, Cy2 NMe 15 mol%; reaction performed at 0 Ž C,
PhSCu:1f D 1 : 1 (mol%).
b Isolated yield.
c
The ee was determined by chiral GC G-TA column, and the
configuration was S.[3j,16]
d (S)-1f was used.
e Et Zn: 1.8 equiv. in toluene solution.
2
entries 14 and 15). The highest ee of 67% was obtained in toluene
(Table 3, entry 1).
BINOL derivatives (R)-1b-g combined with Cy2 NMe and CuSPh
were also determined under the same conditions with (R)-1a and
Cy2 NMe catalyst systems (Table 4, entries 2–8 vs entry 1). The
data indicated that the catalytic system of (R)-1f (6,60 -Br2 -BINOL)
was the best one (Table 4, entry 6). Other catalytic systems could
catalyze the 1,4-conjugate addition with moderate to excellent
yields in 5–66% ee (Table 4, entries 2–5 and 7). Better results
could be obtained when the Et2 Zn in toluene solution was used in
the catalytic system (Table 4, entry 9).
The optimum loadings of (R)-1f and Cy2 NMe were investigated
under the best conditions (Table 5). It was found that the best
loading was 25 mol% (R)-1f and 100 mol% Cy2 NMe, respectively
(Table 5, entry 7); higher or lower loadings afforded inferior results
(Table 5, entries 1–4, 6–9). The excess Cy2 NMe may act as a
Lewis base or play an additive role in the asymmetric conjugate
addition.[16] Better enantioselectivity could be obtained when
lowering the concentration of 2a from 0.2 to 0.1 M, although
it led to a slightly drop in reactivity (Table 5, entry 10). In
contrast, a further increase of the concentration of 2a led to less
satisfactory enantioselectivity (Table 5, entry 11). Changing the
reaction temperature could not improve the enantioselectivity
(Table 5, entries 12 and 13).
Substrate Generality
Encouraged by the results obtained from 2a under the optimized
conditions (Table 5, entry 10), several α, β-unsaturated compounds
were evaluated using Et2 Zn or Me2 Zn (Table 6). It was found that
these reactions gave the corresponding product with up to 84% ee
and 94% yield (Table 6, entries 1 and 8). Introducing Me2 Zn to the
1,4-conjugate addition of 2a, similar results were obtained after
longer reaction times (3b, Table 6, entry 2 vs entry 1). Cyclopent-2enone (2b) led to similar results reacting with Et2 Zn and Me2 Zn (3c
and 3d, Table 6, entries 3 and 4 vs 1). When 4,4-dimethyl-cyclohex2-enone (2c) with Et2 Zn was employed in the 1,4-conjugate
addition, moderate ee and yield with converse configuration
were afforded under same conditions (Table 6, entry 5). ee Values
of 67 and 63% were obtained when the cyclohept-2-enone (2d)
and Et2 Zn or Me2 Zn were employed in the 1,4-conjugate addition,
respectively (Table 6, entries 6 and 7). 1,3-Diphenyl-propenone
(2e) and (2-nitro-vinyl)-benzene (2f) gave very low ee, although
excellent yields were obtained (Table 6, entries 8 and 9). These
above results revealed that the copper(I) complex of (R)-1f and
Cy2 NMe catalyst was effective for the addition of dialkylzinc to
various enones.
Catalytic Cycle Considerations
Based on previous works in the field of the 1,4-conjugate addition
dialkylzinc to enones,[2,12] the copper(I) complex might play a
multifunctional role in this reaction. As shown in Fig. 2, the
expected active species I could be generated in addition to Et2 Zn
Table 5. Optimization of the catalytic system for the conjugation addition of diethylzinc to 2-cyclohexen-1-one
Entrya
1
2
3
4
5
6
7
8
9
10
11
12
13
Concentration
of 2a
(R)-1f
(mol%)
Cy2 NMe
(mol%)
Temperature
(Ž C)
Time
(h)
Yield
(%)b
Ee(%)c
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.4
0.1
0.1
15
10
10
15
15
25
25
45
45
25
25
25
25
15
10
100
50
100
25
100
100
150
100
100
100
100
0
0
0
0
0
0
0
0
0
0
0
20
20
5
5
5
5
5
5
5
5
5
10
10
3
15
76
35
74
71
75
69
77
74
68
67
86
61
36
72
38
73
76
79
77
80
78
69
84
65
59
46
Conditions: solvent, PhCH3 ; Et2 Zn, 1.8 equiv. in toluene solution; reaction performed for 5 h at 0 Ž C, PhSCu:1f D 1 : 1 (mol%).
Isolated yield.
c The ee was determined by chiral GC G-TA column, and the configuration was S.[3j,15]
a
b
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
519
Appl. Organometal. Chem. 2010, 24, 517–522
S. Gou et al.
Et
Br
Table 6. Extension the substrates for the conjugate addition of
dialkylzinc to enones
Zn
S Ph
Cu
O
Zn Et
O
25 mol% (R)-1f
25 mol% CuSPh
100 mol% Cy2NMe
O
+
ZnR2
O
Br
Toluene, 0 °C
*
2a-f
Cy
ZnEt2
R
I
N Cy
CH3
O
3a-i
Entrya
Enones
1
2
O
3
4
O
R
Product
Time
(h)
Yield
(%)b
Ee
(%)c
2a
2a
Et
Me
3a
3b
5
10
67
63
84(S)
80(S)
2b
2b
Et
Me
3c
3d
5
10
63
78
81(S)
78(S)
O
2a
H+
Br
Et
Zn
S Ph
O
Cu
O
Zn
(S)-3a
Br
Cy
5
2c
O
Et
3e
5
44
76(R)
N Cy O
CH3
II
Figure 2. The proposed catalytic cycle.
6
7
2d
2d
O
Et
Me
3f
3g
5
10
90
88
67(S)
63(S)
Experimental Section
General Remarks
8
9
O
Ph
Ph
Ph
NO2
2e
Et
3h
5
94
9(R)
2f
Et
3i
5
86
7(R)
a Conditions: concentration of 2, 0.1 M in PhCH ; Et Zn or Me Zn, 1.8
3
2
2
equiv. in PhCH3 solution, 0 Ž C.
b Isolated yield.
c
The ee was determined by chiral GC G-TA, γ -DEX, β-DEX, HPLC-AD
or OD-H column, the absolute configuration of the major product
compared with the reported value of optical rotation.[3j,16]
or Me2 Zn from the mixture of CuSPh and (R)-1f, and the centre
metal (Zn) might coordinate weakly to the S atom of PhSCu. Part of
the Cy2 NMe might form a salt of PhSCu–Zn(Me)Et2 , and another
part might be coordinated to the metal Zn and act as a Lewis base.
When the cyclic enones (2) were added to the mixture, the metal
moiety (Cu and Zn) of complex I might have acted as a Lewis acid
to activate the cyclic enones 2 and engender species II. Then the
product 3a was obtained by work-up with aqueous acid (HCl) and
one catalytic cycle accomplished.
Conclusion
520
In summary, we have developed a new class of catalyst system
for the enantioselective copper-catalyzed 1,4-conjugate addition
of dialkylzinc to various enones. The catalyst system could form
a Lewis acid–Lewis base centre in the 1,4-conjugate addition
reaction of dialkylzinc to enones, based on the bifuntional concept.
The work presented here paves the way for the development of a
new catalytic system for asymmetric reactions. Further studies of
other asymmetry reaction applications of the catalyst system are
in progress.
www.interscience.wiley.com/journal/aoc
Enantiomeric ratios of the products were determined using chiral
GC or HPLC techniques. Analytical thin-layer chromatography
(TLC) was carried out on commercial plates coated with 0.25 mm
of silica gel. Preparative flash silica chromatography was performed
using silica gel 200–400 mesh. All reactions were carried out under
an atmosphere of nitrogen in flame- or oven-dried glassware with
magnetic stirring unless otherwise indicated.
Materials
(R)-1a, (S)-1a, diethylzinc (Et2 Zn), dimethylzinc (Me2 Zn), all tertiary
amine such as DMAP, Et3 N, dicyclohexylmethylamine (Cy2 NMe),
2-cyclohexen-1-one (2a), cyclopent-2-enone (2b), 4,4-dimethylcyclohex-2-enone (2c), cyclohept-2- enone (2d), 1,3-diphenylpropenone (2e) and (2-nitro-vinyl)-benzene (2f), and all copper
salts such as CuSPh, CuI, Cu(TfO)2 , CuBr, CuCl, CuCN, CuCl2 ,
ž
CuSO4 .5H2 O, Cu(acac)2 and Cu(OAc)2 H2 O were commercially
available, and used without further purification. Anhydrous
solvents such as THF, CH2 Cl2 , toluene, hexene and Et2 O were
treated by typical methods. Chiral ligands 1b–g were synthesized
according to the literature procedures.[17]
General Procedure for Copper-catalyzed Asymmetric
1,4-Conjugate Addition
A mixture of CuSPh (10.75 mg, 0.0625 mmol), (R)-1f (27.6 mg,
0.0625 mmol) and Cy2 NMe (0.25 mmol, 54 µl) in dried toluene
(1.30 ml) was stirred under nitrogen atmosphere at 22 Ž C for
10 min, and then Et2 Zn (0.45 mmol, 450 µl, 1.0 M in hexane)
was injected into the mixture and stirred for 1.0 h at same
temperature. Cyclic enone 2 (0.25 mmol) was added after 10 min
at the indicated temperature and the resulting mixture was stirred
for indicated time before being quenched by aqueous HCl (2 ml,
1.0 M). The mixture was extracted with EtOAc (3 ð 20 ml); the
organic phases were combined, dried over Na2 SO4 , filtered and
evaporated to dryness. The crude product was purified by silica
gel column chromatography on silica gel (ethyl acetate–hexane)
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 517–522
Copper-catalyzed asymmetric 1,4-conjugate addition
to give the desired products 3a–i; these known compounds were
characterized by comparing their 1 H, 13 C NMR spectra with those
published in the literature.[5c,16]
3-(S)-Ethyl-cyclohexan-1-one (3a)[3j,16b]
84% ee by GC analysis (Chiracel G-TA column, 110 Ž C, and flow,
2.0 ml/min). Retention times, t1 D 5.4 (minor), t2 D 5.6 min
(major). [a]25 D D 11.2 (c D 1.0, CH2 Cl2 ). fLit.[3j] [a]28 D D 15.6
(c D 1.0, CHCl3 ) for S enantiomer in 99% eeg.
3-(S)-Methyl-cyclohexan-1-one (3b)[3j,16b]
80% ee by GC analysis (Chiracel γ -DEX column, 70 Ž C, 30 min,
10 Ž C/min, final temperature 150 Ž C). Retention times, t1 D 36.8
(major), t2 D 38.2 min (minor). [a]25 D D 9.7 (c D 1.0, CH2 Cl2 ).
fLit.[3j] [a]28 D D 13.4 (c D 0.6, CHCl3 ) for S enantiomer in 99%
eeg.
3-(S)-Ethyl-cyclopentan-1-one (3c)[3j]
81% ee by GC analysis (Chiracel γ -DEX column, 70 Ž C, 25 min,
5 Ž C/min, final temperature 120 Ž C). Retention times, t1 D 30.2
(major), t2 D 32.0 min (minor). [a]25 D D 71.4 (c D 1.0, CH2 Cl2 ).
fLit.[3j] [a]29 D D 81.2 (c D 0.5, CHCl3 ) for S enantiomer in 93%
eeg.
3-(S)-Methyl-cyclopentan-1-one (3d)[3j]
78% ee by GC analysis (Chiracel γ -DEX column, 60 Ž C, 20 min,
5 Ž C/min, final temperature 120 Ž C). Retention times, t1 D 24.3
(major), t2 D 24.8 min (minor). [a]25 D D 10.8 (c D 0.5, CH2 Cl2 ).
fLit.[3j] [a]29 D D 14.1 (c D 0.2, CHCl3 ) for S enantiomer in 92%
eeg.
3-(R)-Ethyl-4,4-dimethyl-cyclohexanone (3e)[16b,c]
76% ee by GC analysis (Chiracel G-TA column, 100 Ž C, and flow,
1.0 ml/min). Retention times, t1 D 33.3 (minor), t2 D 34.2 min
(major). [a]25 D D 16.6 (c D 0.5, CH2 Cl2 ). fLit.[16c] [a]29 D D 17.2
(c D 1.38, CHCl3 ) for R enantiomer in 75% eeg.
3-(S)-Ethyl-cycloheptan-1-one (3f)[3j,16b]
67% ee by GC analysis (Chiracel G-TA column, 100 Ž C, and flow,
1.0 ml/min). Retention times, t1 D 26.7(major), t2 D 27.2 min
(minor). [a]25 D D 35.8 (c D 0.5, CH2 Cl2 ). fLit.[3j] [a]30 D D 43.9
(c D 0.75, CHCl3 ) for S enantiomer in 79% eeg.
3-(S)-Methyl-cycloheptan-1-one (3g)[3j]
63% ee by GC analysis (Chiracel β-DEX column, 80 Ž C, 30 min,
5 Ž C/min, final temperature 120 Ž C). Retention times, t1 D 35.3
(major), t2 D 37.1 min(minor). [a]25 D D 4.0 (c D 0.5, CH2 Cl2 ).
fLit.[3j] [a]26 D D 6.2 (c D 1.0, CHCl3 ) for S enantiomer in 88% eeg.
(R)-1,3-Diphenylpentan-1-one (3h)
Appl. Organometal. Chem. 2010, 24, 517–522
7% ee by chiralcel OD-H (DAICEL) analysis (eluent, hexane–2propanol, 95 : 5, 0.7 ml/min). Retention times, t1 D 33.2 (minor),
t2 D 21.1 min (major). The absolute configuration was assigned
by analogy.[11e]
Acknowledgment
The authors gratefully acknowledge the Open Fund (no. PLN0908)
of Key Laboratory of Oil and Gas Reservoir Geology and Exploitation
(Southwest Petroleum University) for financial support.
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