An air-stable chiral Hf-based catalyst for asymmetric Mannich-type reactions

Article abstract of DOI:10.1016/j.tet.2007.05.115

A new class of isolable, air-stable, storable, and Hf-based catalyst has been developed. In the presence of 10 mol % of the powdered Hf catalyst, the asymmetric Mannich-type reactions of imines with silicon enolates derived from esters proceeded smoothly

Full text of DOI:10.1016/j.tet.2007.05.115

Tetrahedron 63 (2007) 8425–8429
An air-stable chiral Hf-based catalyst for
asymmetric Mannich-type reactions
*
Shu Kobayashi, Ryo Yazaki, Kazutaka Seki and Masaharu Ueno
ꢀ
Department of Chemistry, School of Science, Graduate School of Pharmaceutical Sciences, The University of Tokyo,
The HFRE Division, ERATO, Japan Science Technology Agency (JST), Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received 14 May 2007; revised 31 May 2007; accepted 31 May 2007
Available online 2 June 2007
Abstract—A new class of isolable, air-stable, storable, and Hf-based catalyst has been developed. In the presence of 10 mol % of the pow-
dered Hf catalyst, the asymmetric Mannich-type reactions of imines with silicon enolates derived from esters proceeded smoothly to afford the
corresponding Mannich-type adducts in high yields with high enantioselectivities. Hafnium single crystals for X-ray analysis were obtained,
and the crystals also showed high performance in the asymmetric Mannich-type reactions.
Ó 2007 Published by Elsevier Ltd.
1. Introduction
activity in these reactions, preliminary results showed that
related Ti catalysts had lower activity.6d Then, we have been
interested in catalytic activity of Hf complexes.8
Asymmetric Mannich reactions provide efficient routes to
optically active b-amino carbonyl compounds, which can
be converted to versatile biologically important compounds
including b-amino acids, b-lactams, b-amino alcohols, and
[1,3]oxazinan-2-ones.1 While many chiral catalysts for these
reactions have been developed, recent advances have allowed
use of carbonyl compounds directly in the presence of cata-
lytic amounts of activators.1c,2 However to date, the carbonyl
compounds applicable to this reaction are limited to ketones
in most cases, and for synthetically useful esters preformed
enolates are required. In this context Mukaiyama-type reac-
tions using silicon enolates are the most promising, and sev-
eral chiral catalysts for these reactions have been developed.3
However, a drawback is that most chiral Lewis acids are
moisture sensitive and have to be prepared just before use.
Br
R
N
OH
Hf(Ot-Bu)4
+
+
2
2
OH
N
Br
R
N
Br
Br
Br
Br
N
O
O
O
Hf
N
CH2Cl2
Hexane
O
N
R
We have already reported chiral zirconium-catalyzed enan-
tioselective Mannich-type reactions of imines with silicon
enolates.4,5 While chiral zirconium catalysts have been
widely used in enantioselective reactions,6 the first-genera-
tion catalyst for the asymmetric Mannich-type reactions was
prepared in situ from a zirconium alkoxide, a BINOL deriv-
ative, and an imidazole derivative in dichloromethane just
before use.4 Subsequent to these examples we described
the synthesis of stable chiral zirconium catalysts, which can
be isolable, air-stable, storable, and effective for several
reactions.5,7 Interestingly, while the Zr catalysts have high
In this paper, we describe the synthesis and characterizations
of air-stable, chiral Hf catalysts that promote asymmetric
Mannich-type reactions.
2. Results and discussion
A chiral Hf complex was prepared from Hf(OtBu)4
(1 equiv), (R)-6,60-dibromo-1,10-bi-2-naphthol (6,60-Br-
BINOL, 2 equiv), and N-methyl or N-benzylimidazole
(2 equiv). The three components were first combined in
dichloromethane at room temperature for 1 h, and hexane
was added to form white powder. The Hf contents of the
powder were determined by ICP analysis (Table 1).
Keywords: Hafnium; Asymmetric catalysis; Imines; Mannich-type reac-
tions.
* Corresponding author. Tel.: +81 3 5841 4790; fax: +81 3 5684 0634;
e-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
0040–4020/$ - see front matter Ó 2007 Published by Elsevier Ltd.
doi:10.1016/j.tet.2007.05.115

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S. Kobayashi et al. / Tetrahedron 63 (2007) 8425–8429
Table 1. Preparation of chiral Hf catalysts
The powdered Hf catalyst combined with NMI was used in
asymmetric Mannich-type reactions with other substrates
(Table 2).
Entry
R
Yield (%)
Hf content (mmol/g)
Calcd
Found
1
2
Bn
Me
98
94
0.73
0.82
0.69
0.82
Thioketene silyl acetal 2a as well as ketene silyl acetal 2b
derived from methyl isobutyrate reacted with some aromatic
imines to afford the desired Mannich-type adducts in high
yields with high enantioselectivities. In the reaction of an
aliphatic imine, a three-component protocol provided the
desired adduct in good yield with good enantioselectivity
(Scheme 2).
The powdered complex prepared from Hf(OtBu)4
(10 mol%), (R)-6,60-Br-BINOL (20 mol%), and N-methyl-
imidazole (20 mol%) was used as a catalyst in the Man-
nich-type reaction of imine 1a with thioketene silyl acetal
2a (Scheme 1). The reaction proceeded in dichloromethane
at –45 ꢀC for 16 h to afford the corresponding Mannich-type
adduct in 85% yield with 78% ee. The yield and the enantio-
selectivity were slightly lower compared with those obtained
using the Hf catalyst prepared in situ from Hf(OtBu)4
(10 mol%), 6,60-Br-BINOL (20 mol%), and N-methylimid-
azole (30 mol%) in dichloromethane (84% yield, 93% ee).
It should be noted that the yield and the selectivity were
improved when the powdered Hf complex (10 mol%) was
combined with NMI (10 mol%) (87% yield, 91% ee).
While a powder was obtained from a dichloromethane solu-
tion of Hf(OtBu)4, (R)-6,60-Br-BINOL, and N-methyl or N-
benzylimidazole by adding hexane, crystals were formed
from a hexane–ethyl acetate solution. The crystals were suit-
able for X-ray crystallographic analysis (Fig. 1).
Interestingly, the X-ray structure shows Hf4(m-BINOLate)6-
(m3-OH)4, wherein four hexa-coordinated Hf atoms and six
BINOL ligands are present. It is worth noting that no
imidazole derivatives were included in the single crystals,
although NMI was included in the powdered Hf catalysts.
While this is the first X-ray crystallographic structure of
Hf–BINOL complexes, the structure is similar to that of
the known Ti and Zr complexes.5a The Hf single crystals
were found to show high catalytic activity for the asymmet-
ric Mannich-type reaction (Scheme 3). In the presence of
10 mol% of the crystals, imine 1a reacted with ketene silyl
acetal 2b in dichloromethane at –45 ꢀC for 132 h to afford
the corresponding Mannich-type adduct in 67% yield with
85% ee. Furthermore, the enantioselectivity was slightly
improved to 89% ee even in the presence of 10 mol% of the
catalyst when NMI (20 mol%) was added.
HO
OSiMe3
N
+
SEt
H
2a (1.2 equiv)
1a
OH
NH
O
Cat. (10 mol%)
HCl/THF
0 °C, 1 h
CH2Cl2, –45 °C,16 h
SEt
Cat. = Powdered Hf: 85% y, 78% ee
It was noted that the yield and the enantioselectivity were
comparable to those obtained using the powdered zirconium
catalyst, albeit longer reaction time was needed. It is also
= Powdered Hf + NMI (10 mol%): 87% y, 91% ee
Scheme 1. Powdered Hf and effect of NMI.
Table 2. Powdered Hf-catalyzed asymmetric Mannich-type reactions
OH
HO
Powdered Hf Catalyst
(10 mol%)
OSiMe3
R3
R2
NH
O
NMI (10 mol%)
HCl/THF
0 °C, 1 h
N
+
R1
R3
R2
CH2Cl2, –45 °C, 16 h
R1
H
R2 R2
(1.2 equiv)
Entry
R1
Ph
R2
R3
Yield (%)
ee (%)
1
2
3
4
5
Me
Me
H
H
H
OMe
OMe
SEt
SEt
SEt
83
93
66
87
77
79
85
82
91
84
1-Naphthyl
Ph
1-Naphthyl
p-ClC6H4
Powdered Hf (10 mol%)
NMI (10 mol%)
MS4A
OH
OH
NH2
(1.0 equiv)
O
OSiMe3
SEt
HCl/THF
0 °C, 1 h
+
+
NH
O
H
CH2Cl2, –45 °C,16 h
SEt
57%, 70% ee
(1.5 equiv)
2a (1.5 equiv)
Scheme 2. Powdered Hf-catalyzed three-component Mannich-type reactions.

S. Kobayashi et al. / Tetrahedron 63 (2007) 8425–8429
8427
Tetramethylsilane (TMS) served as internal standard (0 ppm)
1
for H NMR, and CDCl3 was used as internal standard
(77.0 ppm) for 13C NMR. HPLC was carried out using the
following apparatuses; SHIMADZU LC-10AT (liquid chro-
matograph), SHIMADZU SPD-10A (UV detector), and
SHIMADZU C-R6A or C-R8A Chromatopac. Column chro-
matography was conducted on Silica gel 60 (Merck), and
preparative thin layer chromatography was carried out using
Wakogel B-5F. Commercially available chemicals purchased
from Aldrich, Kanto Chemical, Tokyo Chemical Industry and
Wako Pure Chemical Industry, were purified according to
standard procedures. Hafnium tert-butoxide (Hf(OtBu)4)
was purchased from Strem Chemical Inc. (R)-6,60-Dibromo-
1,10-bi-2-naphthol ((R)-6,60-Br2-BINOL) was synthesized
according to the literature’s method.4f Ketene silyl acetals
were prepared according to the literature’s methods.4f
4.2. Preparation of powdered Hf catalysts
All reactions were carried out under argon atmosphere in
well-dried glassware. A solution of N-methylimidazole
(NMI, 1.0 mmol) in dichloromethane (1.0 mL) was added to
a dichloromethane (1.0 mL) solution of a ligand (1.0 mmol)
and Hf(OtBu)4 (0.50 mmol) at room temperature. After the
mixture was stirred for 1 h at the same temperature, hexane
(100 mL) was added and white precipitates were formed.
The suspended mixture was further stirred overnight. Filtra-
tion of white suspension and washing with hexane afforded
an Hf catalyst as white powder.
Figure 1. Molecular structure of Hf4(m-BINOLate)6(m3-OH)4 crystal with
thermal ellipsoids at the 50% probability level. The disordered solvent mole-
cules (hexane and ethyl acetate) and hydrogen atoms are omitted for clarity.
HO
OSiMe3
N
+
OMe
H
2b (1.2 equiv)
OH
1a
4.3. Determination of the Hf contents in the
powdered Hf catalysts
NH
O
Cat. (10 mol%)
HCl/THF
OMe
CH2Cl2, –45 °C,132 h 0 °C, 1 h
The powdered Hf catalyst (15.0 mg) was placed in a 10 mL
test tube, and sulfuric acid (1.0 mL) was added. The mixture
was heatedat 180 ꢀC for 30 min, and thennitricacid (0.5 mL)
was added. The mixture was further heated for 1 h to give a
clear solution. The solution was diluted with water, and the
amount of the Hf metal was measured by ICP analysis.
Cat. = Hf Crystals: 67% y, 85% ee
= Hf Crystals + NMI (20 mol%): 56% y, 89% ee
Scheme 3. Hf single crystals catalyzed asymmetric Mannich-type reactions.
noteworthy that in the reactions using single crystals high
yields and the selectivities were obtained without the addi-
tion of NMI.
4.4. Typical experimental procedure for asymmetric
Mannich-type reactions using an isolated powdered
chiral Hf catalyst
3. Conclusion
4.4.1. Without NMI. The powdered Hf catalyst
(0.040 mmol) was dissolved in dichloromethane (1.00 mL),
and then aldimine (0.40 mmol) and silicon enolate
(0.48 mmol) in dichloromethane (0.75 mL) were added at
ꢁ45 ꢀC. The mixture was stirred for 16 h, and hexane
(10 mL) was added to quench the reaction. Precipitates
were formed, which were removed by filtration. The filtrate
was concentrated and treated with THF–1 M HCl (10:1) at
0 ꢀC for 1 h. The solution was then basicified with a saturated
aqueous NaHCO3 solution, and the aqueous solution was
extracted with CH2Cl2. The organic layer was dried over
anhydrous Na2SO4. After filtration and concentration under
reduced pressure, the crude product was purified by prepar-
ative thin layer chromatography to afford the desired prod-
uct. The optical purity was determined by HPLC analysis
using a chiral column (vide infra).
In summary, we have developed two types of isolable, air-sta-
ble, and storable Hf catalysts; powdered Hf and Hf crystals.
Both catalysts have high activity for asymmetric Mannich-
type reactions of imines with silicon enolates to afford the
corresponding Mannich-type adducts in high yields with
high stereoselectivities. The X-ray structure shows Hf4(m-BI-
NOLate)6(m3-OH)4, which is similar to the corresponding Zr
complex. Further investigations aimed at developing chiral
Hf-catalyzed enantioselective reactions are now in progress.
4. Experimental
4.1. General
1H and 13C NMR spectra were recorded on a JEOL ECX400,
or ECX600 spectrometer in CDCl3, unless otherwise noted.
4.4.2. With NMI. A solution of N-methylimidazole (NMI,
0.040 mmol) in dichloromethane (1.0 mL) was added to

8428
S. Kobayashi et al. / Tetrahedron 63 (2007) 8425–8429
the powdered Hf catalyst (0.040 mmol), and the mixture was
used for the reaction.
d 0.81 (d, 3H, J¼6.4 Hz), 0.88 (d, 3H, J¼6.6 Hz), 1.26 (t,
3H, J¼7.4 Hz), 1.35 (dd, 2H, J¼6.6, 7.7 Hz), 1.63 (7, 1H,
J¼6.7 Hz), 2.25 (s, 3H), 2.64 (dd, 1H, J¼8.0, 15.9 Hz),
2.80 (dd, 1H, J¼4.2, 15.8 Hz), 2.92 (q, 2H, J¼7.5 Hz),
3.62 (dq, 1H, J¼4.2, 7.1 Hz), 6.66 (dd, 1H, J¼0.6,
7.4 Hz), 6.76 (dd, 1H, J¼1.4, 7.8 Hz), 6.88 (t, 1H,
J¼7.7 Hz). 13C NMR (CDCl3): d 14.6, 18.1, 22.0, 23.2,
23.6, 25.1, 44.5, 48.4, 52.5, 113.3, 121.8, 124.6, 131.2,
132.5, 151.9, 200.3. HPLC: Daicel Chiralpak OD,
hexane/iPrOH¼60/1, flow rate¼1.0 mL/min, tR¼14.9 min
(3S), tR¼17.2 min (3R).
4.4.2.1. Methyl 2,20-dimethyl-3-(2-hydroxyphenyl)-
amino-3-(10-naphthyl)propionate.4f Mp 135–136 ꢀC. IR
1
(neat): 3052, 1691, 1604, 1578, 1270, 798 cmꢁ1. H NMR
(CDCl3): d 1.18 (s, 3H), 1.25 (s, 3H), 3.66 (s, 3H), 5.62 (s,
3H), 6.28–6.62 (m, 4H), 7.22–8.00 (m, 7H). 13C NMR
(CDCl3): d 19.9, 25.1, 48.4, 52.4, 57.8, 113.4, 114.2,
117.9, 121.2, 122.1, 123.2, 125.2, 125.3, 125.4, 126.1,
128.1, 129.1, 133.6, 135.3, 135.5, 144.1, 177.9. HPLC: Dai-
cel Chiralcel OD, hexane/iPrOH¼19/1, flow rate¼0.8 mL/
min, tR¼25.8 min (3S), tR¼29.0 min (3R).
4.5. Preparation of the single crystals
4.4.2.2. Methyl 2,20-dimethyl-(2-hydroxyphenyl)-
The powdered Hf catalyst containing N-benzylimidazole
(20 mg) was dissolved in AcOEt (2 mL). This solution was
allowed to stand in a sealed vial filled with hexane. After
standing the solution for 1 day, colorless single crystals
were formed.
amino-3-phenylpropionate.4f
Mp 112.5–114 ꢀC. IR
(KBr): 3401, 1709, 1611, 1514, 1453, 1391 cmꢁ1 1H
.
NMR (CDCl3): d 1.21 (s, 3H), 1.24 (s, 3H), 3.68 (s, 3H),
4.57 (s, 1H), 6.36–6.76 (m, 4H), 7.21–7.28 (m, 5H). 13C
NMR (CDCl3): d 19.9, 24.2, 47.3, 52.3, 64.3, 113.2, 114.1,
117.6, 120.8, 127.3, 127.9, 128.3, 135.6, 138.9, 144.0,
178.0. HPLC: Daicel Chiralpak AD, hexane/iPrOH¼9/1,
flow rate¼1.0 mL/min, tR¼9.3 min (3R), tR¼16.0 min (3S).
4.5.1. Crystal data of Hf crystal. C136H92Br12Hf4O24:
Fw¼3783.02, Dcalcd (g cmꢁ3)¼1.566, crystal system¼cubic,
˚
˚
space group¼F432, a(A)¼31.7745(8), b(A)¼31.7745(8),
˚
c(A)¼31.7745(8), a(deg)¼90.000, b(deg)¼90.000, g(deg)¼
3
o
˚
˚
4.4.2.3. S-Ethyl 3-(2-hydroxyphenyl)amino-3-phenyl-
propanethioate.4f Mp 79.5–80.5 ꢀC. IR (KBr): 3396,
90.000, V(A )¼32080.3(14), Z¼8, T( C)¼–180, l(A)¼
0.7107, R factor [all data]¼0.0472, R1 factor [I>2.0
s(I)]¼0.0428, Rw factor [all data]¼0.1362, goodness of
fit¼1.136.
1
1647, 1608, 1520, 1449, 1362 cmꢁ1. H NMR (CDCl3):
d 1.67 (t, 3H, J¼7.3 Hz), 2.83 (q, 2H, J¼7.3 Hz), 2.97 (dd,
1H, J¼5.4, 14.9 Hz), 3.07 (dd, 1H, J¼8.1, 14.9 Hz), 4.81
(dd, 1H, J¼5.4, 8.1 Hz), 6.44–6.71 (m, 4H), 7.20–7.33
(m, 5H). 13C NMR (CDCl3): d 14.4, 23.6, 51.4, 56.1,
114.4, 114.6, 118.8, 121.1, 126.3, 127.4, 128.6, 134.9,
141.7, 144.7, 198.4. HPLC: Daicel Chiralpak AS,
hexane/iPrOH¼19/1, flow rate¼1.0 mL/min, tR¼26.6 min
(3S), tR¼38.2 min (3R).
4.6. Procedure for asymmetric Mannich-type
reactions using single crystals
4.6.1. Without NMI. The crystal Hf catalyst (8.6 mg,
0.010 mmol) was suspended in dichloromethane
(0.25 mL). After the mixture was stirred for 1 h at room
temperature, aldimine (0.10 mmol) and silicon enolate
(0.12 mmol) in dichloromethane (0.25 mL) were added at
–45 ꢀC. The reaction mixture was stirred for 132 h, and hex-
ane (10 mL) was added to quench the reaction. Precipitates
were formed, which were removed by filtration. The filtrate
was concentrated and treated with THF–1 M HCl (10:1) at
0 ꢀC for 1 h. The solution was then basicified with a saturated
aqueous NaHCO3 solution, and the aqueous solution was
extracted with CH2Cl2. The organic layer was dried over
anhydrous Na2SO4. After filtration and concentration under
reduced pressure, the crude product was purified by pre-
parative thin layer chromatography to afford the desired
product.
4.4.2.4. S-Ethyl 3-(2-hydroxyphenyl)amino-3-(10-
naphthyl)propane thioate.4f Mp 40.5–41.5 ꢀC. IR (KBr):
3380, 1645, 1605, 1514, 1446, 1362 cmꢁ1 1H NMR
.
(CDCl3): d 1.18 (t, 3H, J¼7.3 Hz), 2.86 (q, 2H, J¼7.3 Hz),
3.03 (dd, 1H, J¼9.4, 14.8 Hz), 3.20 (dd, 1H, J¼3.3,
14.8 Hz), 5.68–5.70 (m, 1H), 6.30–6.71 (m, 4H), 7.29–8.23
(m, 7H). 13C NMR (CDCl3): d 14.4, 23.7, 50.7, 51.9,
113.5, 114.4, 118.1, 121.3, 122.1, 123.1, 125.6, 125.7,
126.5, 128.0, 129.2, 130.3, 134.0, 135.3, 136.7, 143.9,
198.5. HPLC: Daicel Chiralpak AD, hexane/iPrOH¼9/1,
flow rate¼1.0 mL/min, tR¼15.5 min (3S), tR¼21.0 min (3R).
4.4.2.5. S-Ethyl 3-(40-chlorophenyl)-3-[(20-hydroxy-
phenyl)amino]propanethioate.4f IR (neat): 3412, 1665,
4.6.2. With NMI. N-Methylimidazole (1.64 mg,
0.02 mmol) was added to a white suspension of crystals of
the Hf catalyst (8.6 mg, 0.010 mmol) in dichloromethane
(0.25 mL). After stirring for 3 h, the crystals were partially
dissolved, and the slightly suspended mixture was used for
the reaction.
1
1610, 1516, 1447, 742 cmꢁ1. H NMR (CDCl3): d 1.21 (t,
2H, J¼7.4 Hz), 2.87 (q, 2H, J¼7.4 Hz), 2.96 (dd, 1H,
J¼5.1, 14.9 Hz), 3.05 (dd, 1H, J¼8.3, 14.9 Hz), 4.78 (dd,
1H, J¼5.1, 8.3 Hz), 6.39–6.78 (m, 4H), 7.22–7.28 (m,
4H). 13C NMR (CDCl3): d 14.5, 23.7, 51.2, 55.6, 114.5,
115.0, 119.3, 121.2, 127.8, 128.9, 133.2, 134.6, 140.3,
144.7, 197.8. HPLC: Daicel Chiralpak AD, hexane/iPrOH¼
9/1, flow rate¼1.0 mL/min, tR¼19.5 min (3S), tR¼24.3 min
(3R).
Acknowledgements
This work was partially supported by Grant-in-Aid for Sci-
entific Research from the Japan Society of the Promotion
of Sciences (JSPS). We thank Mr. Yoshiki Hasegawa for
his contribution to the early stage of this work.
4.4.2.6.
S-Ethyl
3-[(2-hydroxy-6-methylphenyl)-
amino]-5-methylhexanethioate.4f IR (neat): 3356, 2957,
2871, 1681, 1588, 1472, 1366 cmꢁ1. H NMR (CDCl3):
1

S. Kobayashi et al. / Tetrahedron 63 (2007) 8425–8429
8429
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Komiyama, S.; Hasegawa, Y.; Kobayashi, S. J. Am. Chem. Soc.
2000, 122, 762; (f) Ishitani, H.; Komiyama, S.; Kobayashi, S.
Angew. Chem., Int. Ed. 1998, 37, 3186; Allylation of imines:
(g) Gastner, T.; Ishitani, H.; Akiyama, R.; Kobayashi, S.
Angew. Chem., Int. Ed. 2001, 40, 1896; Mukaiyama aldol reac-
tions: (h) Ishitani, H.; Yamashita, Y.; Shimizu, H.; Kobayashi,
S. J. Am. Chem. Soc. 2000, 122, 5403; (i) Yamashita, Y.;
Ishitani, H.; Shimizu, H.; Kobayashi, S. J. Am. Chem. Soc.
2002, 124, 3292; Hetero Diels–Alder reactions: (j) Yamashita,
Y.; Saito, S.; Ishitani, H.; Kobayashi, S. J. Am. Chem. Soc.
2003, 125, 3793; (k) Yamashita, Y.; Saito, S.; Ishitani, H.;
Kobayashi, S. Org. Lett. 2002, 4, 1221; [3+2] Cycloaddition:
(l) Yamashita, Y.; Kobayashi, S. J. Am. Chem. Soc. 2004, 126,
11279; (m) Kobayashi, S.; Shimizu, H.; Yamashita, Y.;
Ishitani, H.; Kobayashi, J. J. Am. Chem. Soc. 2002, 124, 13678.
7. (a) Seki, K.; Ueno, M.; Kobayashi, S. Org. Biomol. Chem. 2006,
5, 1347; (b) Kobayashi, S.; Saito, S.; Ueno, M.; Yamashita, Y.
Chem. Commun. 2003, 2016.
8. Other asymmetric reactions using Hf complex: (a) Akullian,
L. C.; Porter, J. R.; Traverse, J. F.; Snapper, M. L.; Hoveyda,
A. H. Adv. Synth. Catal. 2005, 347, 417; (b) Matsumoto, K.;
Watanabe, A.; Uchida, T.; Ogi, K.; Katsuki, T. Tetrahedron
Lett. 2004, 45, 2385; (c) Hanawa, H.; Kii, S.; Asao, N.;
Maruoka, K. Tetrahedron Lett. 2000, 41, 5543.
5. (a) Saruhashi, K.; Kobayashi, S. J. Am. Chem. Soc. 2006, 128,
11232; (b) Kobayashi, S.; Ueno, M.; Saito, S.; Mizuki, Y.;