Chemistry Reference
In-Depth Information
than the corresponding BLA
1a
or
2a
since 10-20 mol % of BLA is generally required
for the optimum results. It is believed that the greater turnover effi ciency of LLA
2
rela-
tive to BLA
2a
may be the result of greater steric screening of the catalytic boron site
by the adjacent AlBr
3
subunit and diminished product inhibition (Scheme 3.13).
H
Ph
Ph
N
O
B
Br
3
Al
Me
LLA
2
O
H
CO
2
CH
2
CF
3
O
LLA
2
(4 mol %)
CH
2
Cl
2
, -78°C
OCH
2
CF
3
O
99% yield
88:12
endo
selective
99% ee (
endo
)
Other examples
O
O
O
H
H
H
H
Me
CO
2
CH
2
CF
3
TIPSO
I
OMe
Me
Me
H
O
O
99% yield
99% ee
99% yield
94:6
endo
selective
95% ee
99% yield
<99:1
endo
selective
88% ee
98% yield
91% ee
Scheme 3.13.
The utility of LLA
2
has been further extended to highly enantioselective [2 + 2]
cycloaddition reactions of trifl uoroethyl acrylate with enol ethers [26]. The protonated
BLA
1a
was shown to be inferior to the LLA
2
in catalyzing the [2 + 2] cycloaddition
reactions due to side reactions involving the enol ether component. The absolute ste-
reochemical outcome could be predicted by the pretransition complex involving the
same type of
-CH hydrogen-bonded complex of catalyst with trifl uoroethyl acrylate
proposed earlier for D-A reactions as shown in Figure 3.1. Furthermore, Corey and his
coworkers demonstrated the utility of the resulting bicyclic [2 + 2] adducts for further
synthetic elaboration (Scheme 3.14 ).
A new chiral bis-Ti oxide complex, LLA
3
, was successfully prepared by Maruoka et
al. and applied to the activation of aldehydes, thereby allowing a new catalytic enanti-
oselective allylation of aldehydes with allyltributyltin (Fig. 3.2) [27]. They proposed that
the high reactivity of LLA
3
might be ascribed to the intramolecular coordination of one
isopropoxy oxygen to the other titanium, thus enhancing Lewis acidity of the original Ti
center for the carbonyl activation, which is a typical example of the LLA activation.
Alternatively, a dual activation of the carbonyl group by the simultaneous coordination
of two Ti centers was proposed as the origin of the high reactivity (Scheme 3.15).
α
