Chemistry Reference
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to dynamic kinetic enantioselective hydroamination of racemic allene
112
that possessed
an axially chiral trisubstituted allenyl group [43c], since cationic Au(I) complexes race-
mized the axial chirality of allenes. Indeed, the reaction of
112
with a catalytic 1:2
mixture of
115
and AgClO
4
led to isolation of a 49:14:1.9:1 mixture of (
R
,
Z
) -
114
/(
R
,
E
) -
114
/(
S
,
E
) -
114
/(
S
,
Z
) -
114
in 94% combined yield.
Asymmetric hydroamination of nonactivated alkenes using a main group metal has
been reported by Hultzsch and others [44]. The dilithium salt (
S
,
S
,
S
) -
118
was developed
as a catalyst, which possesses a dimeric structure as revealed by X-ray crystallographic
analysis (Scheme 5.37). Cyclization of
116
proceeded using 2.5 mol % (
S
,
S
,
S
) -
118
(= 10 mol % Li) to give pyrrolidine
117
in 75% ee at 22 ° C.
(
S
,
S
,
S
)-
118
Me
C
6
D
6
NH
2
N
N
Li
NH
Li
116
N
Li
N
N
Me
Me
N
N
N
117
(91%, 75% ee)
Li
Me
(
S
,
S
,
S
)-
118
Scheme 5.37.
5.4.2. Hydroalkoxylation
Although tremendous amounts of hydroamination reactions have been carried out, very
few reports are known for hydroalkoxylation reactions partly due to the diminished
nucleophilicity and the weaker Lewis base character of oxygen nucleophiles compared
with those of amines [45].
Toste and others reported the asymmetric gold(I)-catalyzed hydroalkoxylation of
allenes that utilized a chiral counteranion to convey stereochemical information (Scheme
5.38) [45a]. The reaction of allene
119
with a catalytic 1:2 mixture of the dppm(AuCl)
2
H
OH
Ag-(
R
)-
121
dppm(AuCl)
2
Benzene
O
119
120
(90%, 97% ee)
Ar
O
O
P
O
O
Ag
Ar = 2,4,6-
i
-Pr
3
-C
6
H
2
Ar
Ag-(
R
)-
121
Scheme 5.38.
