Chemistry Reference
In-Depth Information
Pd cat. (5 mol %
)
CH
2
Cl
2
, -55
°
C
H
t
-Bu
Si
+
Si
t
-Bu
69% ee
17
54% ee
N
Me
BARF
-
Pd
OEt
2
N
Pd cat.
Scheme 12.27.
Stoichiometric
Et
D
OLi
Ph
18
(70% ee)
Ph
Et
H
+
Ph
Et
+
EtLi
PhCHO
OLi
OH
19
17% ee
Chiral catalyst
Et
D
O
Ti
Ph
4
20
(70% ee)
Ph
Et
H
+
Ph
Et
+
Et
2
Zn
PhCHO
OZnEt
OH
21
32% ee
Scheme 12.28.
a reaction, if it exists, the enantiomeric purity of the product would increase as the reac-
tion progresses. Since then, asymmetric autocatalysis has attracted considerable atten-
tion [41] .
Seebach and others recognized the importance of the effect of mixed aggregates of
products (lithium enolates) on enantioselectivity [42]. Alberts and Wynberg reported an
asymmetric autoinduction (Scheme 12.28) in which ethyllithium adds to benzaldehyde
to give
in situ
lithium alkoxide of chiral 1 - phenyl - 1 - propanol
19
with 17% ee in the pres-
ence of a stoichiometric amount of lithium alkoxide of 1 - phenyl - 1 - propanol -
d
1
18
with
the same confi guration [43]. They also described an enantioselective addition (32% ee)
of diethylzinc to benzaldehyde using titanium (IV) tetraalkoxide of chiral 1 - phenyl - 1 -
propanol - l -
d
1
20
. In this reaction, the structures of the chiral catalyst
20
and the product
21
(zinc alkoxide before quenching the reaction) are different [43,44]. Danda and
others reported an asymmetric autoinductive cyanohydrin-forming reaction using
