Chemistry Reference
In-Depth Information
12.4.3. Enantiomorphous Inorganic Crystal - Induced Asymmetric Autocatalysis
Chiral crystals provide an environment for the discrimination of chiral molecules, so
their possible roles in the origin of biological homochirality have been discussed for a
long time [87]. In the Earth's crust, there are a wide variety of chiral minerals, such as
chiral oxides and silicates, which serve as accessible chiral surfaces in the prebiotic evolu-
tion of chiral organic molecules. However, no apparent asymmetric induction using
chiral minerals has been observed. Only a very small asymmetric induction has been
reported in an adsorption of chiral compounds on quartz [106].
The possible interaction between the chiral surface of minerals and the organic mol-
ecules remains obscure, so we performed an asymmetric autocatalysis triggered by
quartz. When pyrimidine - 5 - carbaldehyde
32
was treated with
i
- Pr
2
Zn in the presence of
d
- quartz powder, (
S
) - pyrimidyl alkanol
33
with 97% ee was obtained in a yield of 95%
(Scheme 12.39) [107]. In contrast, in the presence of
l
- quartz, (
R
) -
33
with 97% ee was
obtained in a yield of 97%. These reproducible results clearly show that the absolute
confi gurations of the pyrimidyl alkanol formed were regulated by the chirality of quartz.
A small enantiomeric imbalance of the initially formed pyrimidyl alkanol zinc alkoxide
induced by quartz was amplifi ed signifi cantly by the subsequent consecutive asymmetric
autocatalysis to produce pyrimidyl alkanol with very high ee.
S
N
OH
N
(
S
)-
33
with high ee
t
-Bu
CHO
N
d
-Quartz
+
Zn
N
2
t-
Bu
R
N
OH
N
(
R
)-
33
with high ee
t
-Bu
I
-Quartz
Scheme 12.39.
Asymmetric autocatalysis utilizing inorganic crystals as an initial source of
chirality.
The achiral inorganic ionic sodium chlorate (NaClO
3
) and sodium bromate (NaBrO
3
),
both of which crystallize in enantiomorphs belonging to the cubic space group
P
2
1
3, also
act as the origin of chirality of asymmetric autocatalysis to provide the enantioenriched
alkanol [108]. The reaction of
i
- Pr
2
Zn to aldehyde
30
and
32
in the presence of
d
- NaClO
3
crystal affords (
S
) - pyrimidyl alkanols
31
and
33
, respectively. On the other hand, (
R
) -
alkanol
31
and
33
are formed in the presence of
l
- NaClO
3
crystal. It should be noted
that
d
- NaBrO
3
has the opposite absolute confi guration of the crystal structure to that of
d
- NaClO
3
. In other words,
d
- NaBrO
3
and
l
- NaClO
3
have the same absolute confi gura-
tion of the crystal structure. Thus,
d
- NaBrO
3
crystal affords (
R
) - alkanol
33
, while
l
-
NaBrO
3
crystal affords (
S
) -
33
.
