Chemistry Reference
In-Depth Information
3.3. LLA CATALYSIS
The Corey-BAKSHI-SHIBATI (CBS) reduction [23] of prochiral ketones is a well-
known process that utilizes a chiral oxazaborolidine as a catalyst and borane as a stoi-
chiometric reductant. It is believed that the active catalytic species is generated from
coordination of boron reagent to the nitrogen in the oxazaborolidine, which makes the
boron atom more Lewis acidic. Similarly, in 2005, the Yamamoto group reported that a
new Lewis acid-assisted chiral Lewis acid (LLA
1
), derived from chiral valine-based
oxazaborolidine and SnCl
4
as an activator, is a highly effi cient catalyst for enantioselec-
tive D-A reactions of various classes of substrates [24]. Of interest was that, almost the
same enantioselectivities of D-A adducts were observed even in the presence of large
excess of SnCl
4
, which implies the higher reactivity of the chiral LLA
1
than that of
SnCl
4
. In addition, this catalyst system can be tolerant to a small amount of Lewis bases,
such as moisture and alcohol without any signifi cant loss of enantioselectivity. They
proposed that the coordination of the achiral Lewis acid (SnCl
4
) to the nitrogen atom
of the chiral oxazaborolidine would increase the Lewis acidity of the boron atom
(Scheme 3.12 ).
Ph
Ph
Ph
Ph
SnCl
4
O
N
N
O
B
B
Cl
4
Sn
LLA
1
CHO
LLA
1
(1 mol %)
CH
2
Cl
2
, -78°C
CHO
>99% yield
68:32
exo
selective
95% ee (
exo
)
98% ee (
endo
)
Other examples
O
H
O
H
H
H
CO
2
Et
COEt
H
O
96% yield
99:1
endo
selective
95% ee
90% yield
99:1
endo
selective
96% ee
94% yield
<99:1
endo
selective
99% ee
93% yield
92:8
endo
selective
95% ee
Scheme 3.12.
In 2007, the Corey group reinvestigated Lewis acid activation of chiral oxazaboro-
lidines instead of the activation by strong Brønsted acids [25]. With only very strong
Lewis acid AlBr
3
, complete complexation of oxazaborolidine with Lewis acid took place
to generate useful catalysts for D-A reactions. Excellent D-A adducts were obtained
with only 4 mol % of LLA
2
, which indicates that LLA
2
is considerably more effi cient
