Enantioselective Henry reaction catalyzed by salen-cobalt complexes

Article abstract of DOI:10.1055/s-2004-829157

The enantioselective Henry reaction catalyzed by the optically active salen-cobalt complexes, proceeded to afford β-hydroxynitroalkanes in good-to-high yields with high enantioselectivity.

Full text of DOI:10.1055/s-2004-829157

PAPER
1947
Enantioselective Henry Reaction Catalyzed by Salen-Cobalt Complexes
Enantioselective
H
o
enry Reactio
u
n Catalyze
d
i
by Sale
c
n-Cobalt
C
h
omplexes i Kogami, Takahiro Nakajima, Taketo Ikeno, Tohru Yamada*
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
Fax +81(45)5661716; E-mail: yamada@chem.keio.ac.jp
Received 9 March 2004; revised 26 April 2004
Ph
N
Ph
N
Abstract: The enantioselective Henry reaction catalyzed by the op-
tically active salen-cobalt complexes, proceeded to afford b-
hydroxynitroalkanes in good-to-high yields with high enantio-
selectivity.
N
O
N
O
Co
Co
O
O
Key words: aldehyde, asymmetric catalysis, salen-cobalt complex-
es, direct nitroaldol reaction, b-hydroxynitroalkanes
1b
1a
Figure 1
The Henry reaction1 is one of the most convenient reac-
tions for direct carbon-carbon bond formation2 without
any pretreatment to afford b-hydroxy-nitroalkanes, and
the resulting b-hydroxynitroalkanes can be further con-
verted to amines by reduction3 to carbonyl compounds by
the Nef reaction4 and to nitroalkenes by dehydration5 etc.6
Since the catalytic enantioselective version of this reac-
tion was first reported by using heterobimetallic lan-
thanide BINOL catalyst systems7 various complex
catalyst systems have been successfully released; e.g.
copper/bisoxazoline complexes,8 dinuclear zinc complex
catalysts,9 etc. While seeking for catalytic activities of ke-
toiminato cobalt complexes, they were found to work as
chiral Lewis acid catalysts for hetero Diels–Alder
reaction10 and carbonyl-ene reaction.11 Although these
type of cobalt complexes were prepared in aqueous
solution12 and their axial sites were both occupied by wa-
ter molecules or oxygen-containing compounds, such as
THF,13 they could be employed as novel Lewis acids com-
patible with Lewis bases, such as water, nitrones,14 and
amines. In the previous communication, it was reported
that enantioselective Henry reaction was catalyzed by
these cobalt complexes in the presence of tertiary amine to
obtain b-hydroxynitroalkanes in high yields with good-to-
high enantioselectivities.15 As the similar salen-cobalt
complexes developed for hydrolytic kinetic resolution16
were also compatible with nucleophilic compounds, such
as water and phenols,17 it is expected that they could be
employed as efficient catalysts for enantioselective Henry
reaction in the presence of tertiary amine bases. In this ar-
ticle, we would like to describe that the enantioselective
Henry reaction was catalyzed by the optically active
salen-cobalt complexes (Figure 1) to afford the b-hydro-
xynitroalkanes with good-to-high enantioselectivities.
Examination of several salen-cobalt complexes revealed
that the commercially available salen-cobalt complex 1a
or its derivative 1b worked as the efficient catalyst for the
enantioselective Henry reaction. Various aldehydes were
then applied to the enantioselective Henry reaction with
nitromethane in the presence of diisopropylethylamine
(Table 1).
Although the nitroaldol reaction of benzaldehyde pro-
ceeded to afford the corresponding b-nitroalcohol in poor
yield with 62% ee (entry 1), p-chlorobenzaldehyde afford-
ed the corresponding b-nitroalcohol with 88% ee (entry
2). It was observed that ortho-halo substitution improved
the enantioselectivities in the reaction of o-chlorobenzal-
dehyde, o-fluorobenzaldehyde, and o-trifluoromethylben-
zaldehyde (entries 3, 5, 7, 9 and 10). When the catalyst 1b
was used in the reaction of o-halobenzaldehyde, the opti-
cal yield was improved compared with the reaction by the
catalyst 1a (entries 4, 6 and 8). In the Henry reaction of
ortho-halo-substituted benzaldehydes, 2,3-dichloroben-
zaldehyde (2g), 2,4-dichlorobenzaldehyde (2h), and
2,3,5-trichlorobenzaldehyde (2i), the corresponding b-ni-
troalcohol was obtained in quantitative yield with high
enantioselectivity (entries 10–12). The alkanal, such as 3-
phenylpropanal, also reacted with nitromethane in the
presence of the cobalt-salen complex 1a to afford the cor-
responding b-hydroxynitroalkane with high enantioselec-
tivity, 93% ee (entry 13).
In conclusion, it was found that optically active salen-co-
balt complexes effectively catalyzed the enantioselective
Henry reaction of various aldehydes to afford the corre-
sponding b-hydroxynitroalkane in good-to-high yields
with high enantioselectivities.
Melting points were measured on an Electrothermal IA9100 appa-
ratus and are uncorrected. IR spectra were recorded on a JASCO
Model FT-IR-410 spectrometer as KBr pellets or as liquid film on
NaCl. 1H NMR and 13C NMR spectra were measured using a JEOL
Model GX-400 spectrometer with CDCl3 as solvent and TMS as in-
ternal standard. High-resolution mass spectra were obtained with a
Hitachi M-80B. For the thin-layer chromatography (TLC) analyses,
SYNTHESIS 2004, No. 12, pp 1947–1950
Advanced online publication: 21.07.2004
DOI: 10.1055/s-2004-829157; Art ID: F03904SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
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0
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1948
Y. Kogami et al.
PAPER
(R)-1-Phenyl-2-nitroethanol (3a)8b
Table 1 Catalytic Enantioselective Henry Reaction of Various
HPLC: Daicel Chiralpak AD-H (3% EtOH in hexane, flow 1.0 mL/
min, 254 nm), tR = 41.3 min (minor), 43.2 min (major). Configura-
tion assignment: The absolute stereochemistry was assigned as (R)
by comparison of the optical rotation with the following literature
values: Lit8b [a]21D –41.6° (c = 1.03, CH2Cl2), 94% ee, (R)-isomer.
Aldehydes
O
OH
2 mol% catalyst 1a or 1b
+
CH3NO2
NO2
R
H
R
1 equiv i-Pr2NEt,CH2Cl2, –78 °C
2a-j
3a-j
X1
IR (NaCl): 3437, 1554, 1495, 1454, 1379, 1066, 895, 766, 700, 609
cm–1.
1H NMR (CDCl3): d = 3.18 (br s, 1 H), 4.47 (dd, J = 13.1, 2.9 Hz, 1
H), 4.57 (dd, J = 13.1, 9.6 Hz, 1 H), 5.41 (d, J = 9.6 Hz, 1 H), 7.34–
7.41 (m, 5 H).
2a : X=H
R :
2g : X1=X2=Cl, X3=X4=H
2h : X1=X3=Cl, X2=X4=H
2i : X1=X2=X4=Cl, X3=H
X2
X3
X
2b : X=p-Cl
2c : X=o-Cl
2d : X=o-F
2e : X=o-Br
2f : X=o-CF3
X4
2j
Ph
13C NMR (CDCl3): d = 70.9, 81.1, 125.8, 128.8, 128.9, 138.0.
Entry
1c
2
Aldehyde Cat
Time (h) Yield (%)a ee (%)b
(R)-1-(4-Chlorophenyl)-2-nitroethanol (3b)8b
2a
2b
2c
1b
1a
1a
1b
1a
1b
1a
1b
1b
1b
1a
1a
1a
116
98
74
95
64
18
63
94
116
43
78
77
45
36
62
88
92
95
94
98
90
94
79
81
87
77
93
[a]23 –40.4° (c = 0.27, CH2Cl2). HPLC: Daicel Chiralcel OD-H
D
45
(3% EtOH in hexane, flow 1.0 mL/min, 254 nm), tR = 25.8 min (ma-
jor), 31.4 min (minor). Configuration assignment: The absolute ste-
reochemistry was assigned as (R) by comparison of the optical
rotation with the following literature values: Lit8b [a]23D –37.6° (c =
2.03, CH2Cl2), 90% ee, (R)-isomer.
3d
4
quant
quant
91
5
2d
2e
IR (NaCl): 3450, 1554, 1493, 1414, 1379, 1090, 1014, 897, 829,
741, 661, 528 cm–1.
1H NMR (CDCl3): d = 3.26 (br s, 1 H), 4.48 (dd, J = 13.2, 2.9 Hz, 1
H), 4.55 (dd, J = 13.2, 9.3 Hz, 1 H), 5.42 (dd, J = 9.3, 2.9 Hz, 1 H),
7.32–7.38 (m, 4 H).
6
quant
94
7
8
quant
86
13C NMR (CDCl3): d = 70.2, 80.9, 127.1, 129.1, 134.6, 136.4.
9d
10
11d
12e
13
2f
2g
2h
2i
(R)-1-(2-Chlorophenyl)-2-nitroethanol (3c)8b
[a]21 –54.6° (c = 0.21, CH2Cl2). HPLC: Daicel Chiralpak AD-H
quant
quant
quant
64
D
(3% EtOH in hexane, flow 1.0 mL/min, 254 nm), tR = 28.3 min (mi-
nor), 31.7 min (major). Configuration assignment: The absolute ste-
reochemistry was assigned as (R) by comparison of the optical
rotation with the following literature values: Lit8b [a]21D –52.7° (c =
1.21, CH2Cl2), 91% ee, (R)-isomer.
2j
IR (NaCl): 3514, 1556, 1473, 1440, 1417, 1379, 1213, 1130, 1088,
1034, 899, 760, 739, 661 cm–1.
1H NMR (CDCl3): d = 3.21 (br s, 1 H), 4.44 (dd, J = 13.4, 9.4 Hz, 1
H), 4.66 (dd, J = 13.4, 2.2 Hz, 1 H), 5.83 (d, J =9 .4 Hz, 1 H), 7.26–
7.39 (m, 3 H), 7.66 (dd, J = 7.6, 2.2 Hz, 1 H).
a Isolated yield.
b Determined by HPLC analysis using Daicel Chiralpak AD-H or
Chiralcel OD-H.
c Reaction temperature is –40 °C.
d Catalyst (5.0 mol%) and amine (2.5 equiv) were used.
e Catalyst (10 mol%) and amine (5.0 equiv) were used.
13C NMR (CDCl3): d = 67.8, 79.3, 127.4, 127.5, 130.0, 131.3, 135.4.
(R)-1-(2-Fluorophenyl)-2-nitroethanol (3d)
[a]21 –48.9° (c = 0.24, CH2Cl2). HPLC: Daicel Chiralpak AD-H
(5% EtOH in hexane, flow 1.0 mL/min, 254 nm), tR = 21.6 min (mi-
nor), 24.6 min (major).
D
Merck precoated TLC plates (silica gel 60 F254, 0.25 mm) were
used. The products were purified by preparative column chromatog-
raphy on silica gel 60N. HPLC analyses were performed using a
Shimadzu LC-6A chromatograph with an optically active column
(Chiracel OB-H, OD-H, and Chiralpak AD-H columns, Daicel Ltd.,
Co.); the peak areas were obtained with a Shimadzu Chromatopack
CR-4A or a Varian Dynamax MacIntegrator. Optical rotations were
measured with a JASCO DIP-370 digital polarimeter.
IR (NaCl): 3529, 1556, 1489, 1458, 1379, 1228, 1074, 810, 762
cm–1.
1H NMR (CDCl3): d = 3.18 (br s, 1 H), 4.55–4.64 (m, 2 H), 5.72–
5.74 (m, 1 H), 7.05–7.10 (m, 1 H), 7.20–7.26 (m, 1 H), 7.32–7.38
(m, 1 H), 7.53–7.57 (m, 1 H).
13C NMR (CDCl3): d = 65.4 (d, J = 2.5 Hz), 79.6 (d, J = 1.7 Hz),
115.5 (d, J = 21.6 Hz), 124.7 (d, J = 3.3 Hz), 125.0 (d, J = 13.3 Hz),
127.5 (d, J = 3.3 Hz), 130.3 (d, J = 8.3 Hz), 159.2 (d, J = 245.5 Hz).
MALDI–TOF–MS (Matrix; Dithranol): m/z [M + H]+ calcd for
C8H8FNO3: 186.0566; found: 186.0559.
Preparation of b-Nitroalcohol; Typical Procedure
Under an anhyd nitrogen atmosphere, a CH2Cl2 solution (1.0 mL)
of 2-fluorobenzaldehyde (62.1 mg, 0.5 mmol) and nitromethane
(1.0 mL, 18.5 mmol) was added to a CH2Cl2 solution (2.0 mL) of
cobalt complex 1b (7.02 mg, 2.0 mol% against aldehyde) at –78 °C
and the mixture was stirred for 18 h. After removal of the solvent,
the residue was purified by column chromatography18 on silica gel
(hexane–EtOAc, 4: 1) to obtain the corresponding nitroalcohol, 1-
(2-fluorophenyl)-2-nitroethanol (92.8 mg), in quantitative yield.
The enantioselectivity was determined by HPLC analysis to be 98%
(Daicel Chiralpak AD-H, 5% EtOH in hexane, flow 1.0 mL/min,
254 nm); [a]21D –24.8° (c = 0.46, CH2Cl2).
(R)-1-(2-Bromophenyl)-2-nitroethanol (3e)
[a]21 –44.6° (c = 0.21, CH2Cl2). HPLC: Daicel Chiralpak AD-H
D
(5% EtOH in hexane, flow 1.0 mL/min, 254 nm), tR = 20.3 min (mi-
nor), 24.1 min (major).
IR (NaCl): 3529, 2924, 1556, 1468, 1415, 1377, 1211, 1126, 1083,
1024, 899, 760, 733, 611 cm–1.
Synthesis 2004, No. 12, 1947–1950 © Thieme Stuttgart · New York

PAPER
Enantioselective Henry Reaction Catalyzed by Salen-Cobalt Complexes
1949
1H NMR (CDCl3): d = 3.19 (br s, 1 H), 4.42 (dd, J = 13.4, 9.8 Hz, 1
H), 4.68 (dd, J = 13.4, 2.0 Hz, 1 H), 5.79 (d, J = 9.8 Hz, 1 H), 7.20–
7.30 (m, 1 H), 7.38–7.42 (m, 1 H), 7.56 (d, J = 8.3 Hz, 1 H), 7.65
(d, J = 7.8 Hz, 1 H).
13C NMR (CDCl3): d = 69.9, 79.3, 121.3, 127.7, 128.0, 130.1, 132.8,
137.0.
13C NMR (CDCl3): d = 67.9, 78.7, 126.1, 127.9, 130.2, 133.7, 134.0,
138.9.
MALDI–TOF–MS (Matrix; Dithranol–HCCA mixture): m/z [M +
H]+ calcd for C8H6Cl3NO3: 269.9491; found: 269.9491.
(R)-1-Nitro-4-phenyl-2-nitrobutanol (3j)9
Mp 101.5–102.8 °C; [a]23D 11.4° (c = 0.22, CH2Cl2). HPLC: Daicel
Chiralcel OD-H (5% EtOH in hexane, flow 1.0 mL/min, 254 nm),
tR = 19.6 min (minor), 20.9 min (major). Configuration assignment:
The absolute stereochemistry was assigned as (R) by comparison of
Anal. Calcd for C8H8BrNO3: C, 39.05; H, 3.28; N, 5.69. Found: C,
39.07; H, 3.35; N, 5.68.
(R)-1-(2-Trifluoromethylphenyl)-2-nitroethanol (3f)
the optical rotation with the following literature values: Lit9: [a]25
–13.6° (c = 1.32, CH2Cl2), 85% ee, (S)-isomer.
[a]23 –34.8° (c = 0.26, CH2Cl2). HPLC: Daicel Chiralpak AD-H
D
D
(5% EtOH in hexane, flow 1.0 mL/min, 254 nm), tR = 11.7 min (mi-
nor), 15.2 min (major).
IR (KBr): 3390, 2949, 1556, 1385, 1205, 1101, 1041, 935, 910, 881,
758, 729, 704, 646, 586, 513 cm–1.
1H NMR (CDCl3): d = 1.73–1.90 (m, 2 H), 2.69–2.88 (m, 3 H),
4.29–4.41 (m, 3 H), 7.18–7.32 (m, 5 H).
13C NMR (CDCl3): d = 31.3, 35.1, 67.7, 80.5, 126.2, 128.3, 128.5,
140.5.
IR (NaCl): 3523, 1558, 1456, 1419, 1381, 1315, 1167, 1117, 1080,
1059, 1036, 901, 771, 715, 656, 621 cm–1.
1H NMR (CDCl3): d = 3.23 (br s, 1 H), 4.50–4.52 (m, 2 H), 5.87–
5.90 (m, 1 H), 7.47–7.50 (m, 1 H), 7.63–7.70 (m, 2 H), 7.84 (d, J =
7.8 Hz, 1 H).
13C NMR (CDCl3): d = 66.6 (q, J = 2.5 Hz), 80.6, 123.9 (q, J = 273.1
Hz), 125.9 (q, J = 5.5 Hz), 126.8 (q, J = 30.4 Hz), 127.9, 128.9,
132.7 (m), 136.7 (m).
Anal. Calcd for C10H13NO3: C, 61.53; H, 6.71; N, 7.18. Found: C,
61.71; H, 6.84; N, 6.98.
References
(R)-1-(2,3-Dichlorophenyl)-2-nitroethanol (3g)
[a]23 –48.5° (c = 0.32, CH2Cl2). HPLC: Daicel Chiralpak AD-H
D
(1) Ono, N. The Nitro Group in Organic Synthesis; Wiley-VCH:
New York, 2001, Chap. 3, 30.
(5% EtOH in hexane, flow 1.0 mL/min, 254 nm), tR = 15.7 min (mi-
nor), 17.1 min (major).
(2) (a) Yamada, Y. M. A.; Yoshikawa, N.; Sasai, H.; Shibasaki,
M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1871. (b) List,
B.; Lerner, R. A.; Barbas, C. F. III J. Am. Chem. Soc. 2000,
122, 2395. (c) Trost, B. M.; Ito, H.; Silcoff, E. R. J. Am.
Chem. Soc. 2001, 123, 3367. (d) Northrup, A. B.;
MacMillan, D. W. C. J. Am. Chem. Soc. 2002, 124, 6798.
(3) Watanabe, M.; Murata, K.; Ikariya, T. J. Org. Chem. 2002,
67, 1712.
(4) Chikasita, H.; Morita, Y.; Itoh, K. Synth. Commun. 1987, 17,
677.
(5) Anbazhagan, M.; Kumaran, G.; Sasidharan, M. J. Chem.
Res., Synop. 1997, 9, 336.
(6) (a) Rosini, G.; Ballini, R. Synthesis 1988, 833. (b) Pinnick,
H. W. Organic Reactions, Vol. 38; Paquette, L. A., Ed.;
Wiley: New York, 1990, Chap. 3.
(7) (a) Sasai, H.; Suzuki, T.; Arai, S.; Arai, T.; Shibasaki, M. J.
Am. Chem. Soc. 1992, 114, 4418. (b) Shibasaki, M.;
Yoshikawa, N. Chem. Rev. 2002, 102, 2187.
(8) (a) Christensen, C.; Juhl, K.; Jørgensen, K. A. Chem.
Commun. 2001, 2222. (b) Evans, D. A.; Seidel, D.; Rueping,
M.; Lam, H. W.; Shaw, J. T.; Downey, C. W. J. Am. Chem.
Soc. 2003, 125, 12692.
IR (NaCl): 3519, 2922, 1556, 1452, 1421, 1377, 1340, 1288, 1207,
1180, 1159, 1095, 1043, 901, 874, 789, 756, 735, 715, 652, 472
cm–1.
1H NMR (CDCl3): d = 3.34 (d, J = 4.4 Hz, 1 H), 4.42 (dd, J =1 3.6,
9.3 Hz, 1 H), 4.67 (dd, J = 13.6, 2.1 Hz, 1 H), 5.85 (dt, J = 7.8, 2.1
Hz, 1 H), 7.26–7.34 (m, 1 H), 7.44–7.51 (m, 1 H), 7.56–7.62 (m, 1
H).
13C NMR (CDCl3): d = 68.2, 79.0, 125.6, 127.9, 129.5, 130.5, 133.3,
137.7.
MALDI–TOF–MS (Matrix; Dithranol–HCCA mixture): m/z [M +
H]+ calcd for C8H7Cl2NO3: 235.9881; found: 235.9881.
(R)-1-(2,4-Dichlorophenyl)-2-nitroethanol (3h)
[a]23 –47.6° (c = 0.95, CH2Cl2). HPLC: Daicel Chiralpak AD-H
D
(10% EtOH in hexane, flow 1.0 mL/min, 254 nm), tR = 8.50 min
(major), 11.6 min (minor).
IR (NaCl): 3519, 3095, 2924, 1591, 1556, 1471, 1415, 1381, 1286,
1213, 1144, 1088, 1049, 897, 864, 825, 785, 690, 658, 565 cm–1.
1H NMR (CDCl3): d = 3.28 (br s, 1 H), 4.42 (dd, J = 13.7, 9.3 Hz, 1
H), 4.64 (dd, J = 13.7, 2.0 Hz, 1 H), 5.79 (d, J = 9.3 Hz, 1 H), 7.30–
7.43 (m, 2 H), 7.56–7.65 (m, 1 H).
(9) Trost, B. M.; Yeh, V. S. C. Angew. Chem. Int. Ed. 2002, 41,
861.
13C NMR (CDCl3): d = 67.4, 79.0, 127.8, 128.4, 129.3, 131.9, 134.0,
(10) Kezuka, S.; Mita, T.; Ohtsuki, N.; Ikeno, T.; Yamada, T.
Bull. Chem. Soc. Jpn. 2001, 74, 1333.
(11) Kezuka, S.; Kogami, Y.; Ikeno, T.; Yamada, T. Bull. Chem.
Soc. Jpn. 2003, 76, 49.
(12) Yamada, T.; Nagata, T.; Sugi, K. D.; Yorozu, K.; Ikeno, T.;
Ohtsuka, Y.; Miyazaki, D.; Mukaiyama, T. Chem.–Eur. J.
2003, 9, 4485.
(13) Ohba, S.; Nagata, T.; Yamada, T. Acta Crystallogr., Sect. E:
Struct. Rep. Online 2001, E57, 124.
(14) Kezuka, S.; Ohtsuki, N.; Mita, T.; Kogami, Y.; Ashizawa,
T.; Ikeno, T.; Yamada, T. Bull. Chem. Soc. Jpn. 2003, 76,
2197.
(15) Kogami, Y.; Nakajima, T.; Ashizawa, T.; Kezuka, S.; Ikeno,
T.; Yamada, T. Chem. Lett. 2004, 33, 614.
135.1.
MALDI–TOF–MS (Matrix; Dithranol/HCCA mixture): m/z [M +
H]+ calcd for C8H7Cl2NO3: 235.9881, found: 235.9855.
(R)-1-(2,3,5-Trichlorophenyl)-2-nitroethanol (3i)
[a]23 –37.7° (c = 0.53, CH2Cl2). HPLC: Daicel Chiralcel OD-H
D
(1% EtOH in hexane, flow 1.0 mL/min, 254 nm), tR = 29.5 min (mi-
nor), 31.4 min (major).
IR (NaCl): 3521, 3080, 2924, 1556, 1415, 1342, 1286, 1225, 1115,
1286, 1225, 1115, 1047, 937, 897, 868, 822, 754, 710, 660, 559
cm–1.
1H NMR (CDCl3): d = 3.44 (br s, 1 H), 4.41 (dd, J = 13.7, 9.5 Hz, 1
H), 4.67 (dd, J = 13.7, 2.0 Hz, 1 H), 5.82 (d, J = 9.5 Hz, 1 H), 7.47–
7.51 (m, 1 H), 7.60–7.65 (m, 1 H).
Synthesis 2004, No. 12, 1947–1950 © Thieme Stuttgart · New York

1950
Y. Kogami et al.
PAPER
product could be obtained as follows: To the CH2Cl2
solution of the contaminated product (ca. 0.5 mmol), silver
hexafluoroantimonate (34.3 mg, 0.1 mmol) was added at
–78 °C. After stirring for 30 min, the mixture was purified by
a short column chromatography to afford the pure product.
(16) Schaus, S. E.; Brandes, B. D.; Larrow, J. F.; Tokunaga, M.;
Hansen, K. B.; Gould, A. E.; Furrow, M. E.; Jacobsen, E. N.
J. Am. Chem. Soc. 2002, 124, 1307.
(17) Ready, J. M.; Jacobsen, E. N. J. Am. Chem. Soc. 1999, 121,
6086.
(18) In case of contamination with the cobalt-salen complex after
the purification by the column chromatography, the pure
Synthesis 2004, No. 12, 1947–1950 © Thieme Stuttgart · New York