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and chiral silver phosphonate
121
gave the furan
120
with 97% ee. Au(I) complexes
incorporating chiral phosphines have proven very successful for certain processes but
inadequate for others; this defi ciency is possibly attributable to the linear coordination
geometry of gold, which places the chiral components distant from the substrate. There-
fore, the chiral counterion approach is especially appealing for Au(I) catalysis.
Zhang and Widenhoefer reported the gold(I)-catalyzed enantioselective hydroalkox-
ylation of allenes (Scheme 5.39) [45b]. Employment of catalyst
115
and AgOTs as a
cocatalyst was fortuitous as the yields and enantioselectivities. Hydroalkoxylation of
allene
122
gave the desired furan
123
with 93% ee. Hydroxyallenes that possess an
axially chiral allenyl moiety were also effective substrates under similar reaction condi-
tions to give
E
and
Z
isomers in excellent enantioselectivities, respectively. Yamamoto
and others reported that oxygen nucleophiles underwent catalytic asymmetric hydroalk-
oxylation in a similar manner to hydroamination [42i] . The combination of Pd
2
(dba)
3
· CHCl
3
(10 mol %), PhCOOH (20 mol %), and (
R
,
R
) - renorphos
108
(60 mol %) gave the best
result. Hydroalkoxylation of
124
under the best conditions gave the desired product
125
in 57% yield with 86% ee.
Catalyst
115
OH
O
AgOTs
Toluene
Ph
Ph
Ph
Ph
123
(76%, 93% ee)
122
RENORPHOS
108
Pd
2
(dba)
2
-CHCl
3
Ph
Ph
PhCO
2
H
OH
O
Benzene
1
2
4
1
2
5
(57%, 86% ee)
Scheme 5.39.
5.4.3. Wacker - Type Reaction (Transition Metal - Catalyzed Oxidative Reactions)
Study on enantioselective Wacker-type cyclization has stimulated considerable interest
in the development of effi cient methods for the synthesis of cyclic ethers [46]. Particu-
larly, the oxidative transition metal-mediated carboamination and carboalkoxylation of
alkenes provided the direct method for complex heterocycle synthesis. The successful
Pd(II)-catalyzed enantioselective oxidative tandem cyclization reaction was reported by
Sasai and others (Scheme 5.40) [47a]. Wacker-type tandem cyclization of alkenyl alcohol
126
promoted by chiral Pd(II)-spiro bis(isoxazoline) catalyst
128
to afford bicyclic
product
127
with excellent enantiomeric excess (up to 95% ee). In this reaction, benzo-
quinone was used as an oxidant to regenerate Pd(II) species.
Yang and others reported the enantioselective oxidative tandem cyclizations under
Pd(II) catalysis using molecular oxygen as the sole oxidant (Scheme 5.41) [47b]. Cycliza-
tion of
129
using the chiral Pd(II)/(− ) - sparteine complex afforded
130
in 91% ee. In this
study, the use of the bulky tertiary amine diisopropylethylamine (DIPEA) and activated
3 Å molecular sieves led to pronounced improvements in both the catalytic activity and
