Chemistry Reference
In-Depth Information
tions, the chiral catalyst and achiral ligand possesses the same functionalities and similar
catalytic activities in the addition of dialkylzinc. Thus, these observation may bring some
new insights to the mechanism of the
-amino alcohol catalyzed addition of dialkylzincs
to aldehydes, because only monomeric species have been proposed to be catalytically
active in the dialkylzinc addition to aldehydes catalyzed by
β
β
- amino alcohols [133,134] .
12.6. SUMMARY
As described, there have been examples of positive NLE in catalytic asymmetric syn-
thesis. NLE is very important not only for obtaining chiral products with higher ees using
chiral catalysts with lower ees but also for elucidating the structure and mechanism of
asymmetric catalysis.
As to asymmetric autocatalysis, we found that chiral 5-pyrimidyl alkanol, 3-quinolyl
alkanol and 5-carbamoyl-3-pyridyl alkanol are highly enantioselective asymmetric auto-
catalysts for the addition of
i
- Pr
2
Zn to the corresponding aldehydes, respectively. Among
these, 2 - alkynyl - 5 - pyrimidyl alkanol is a highly effi cient asymmetric autocatalyst with
more than 99.5% enantioselectivity. Moreover, asymmetric autocatalysis with amplifi ca-
tion of ee from extremely low ee to more than 99.5% ee was realized for the fi rst time
by consecutive asymmetric autocatalysis without the need for any other chiral auxiliary.
Kinetic analysis of pyrimidyl alkanol suggested that the reaction is second order in the
zinc monoalkoxide of the pyrimidyl alkanol.
Chiral organic compounds with low ee, when exposed to CPL serve as chiral triggers
for asymmetric autocatalysis. The overall process correlates, for the fi rst time, the chiral-
ity of CPL with an organic compound with very high ee. Chirality of the CPL was
directly correlated with the chirality of the pyrimidyl alkanol with high ee by asymmetric
photodegradation of racemic pyrimidyl alkanol in combination with asymmetric auto-
catalysis. Chiral inorganic crystals, such as quartz and sodium chlorate, act as chiral
triggers and regulate the sense of the asymmetric autocatalysis. The process correlates,
for the fi rst time, the chirality of inorganic crystals with an organic compound with very
high ee.
Chiral organic crystals composed of achiral compounds such as cytosine act as the
initial source of chirality of asymmetric autocatalysis to produce the highly enantiomeri-
cally pure product. In this reaction, chiral organic crystals are utilized as a chiral inducer,
not as a reactant. Therefore, these results are the realization of the process, in which the
crystal chirality of achiral organic compounds induces asymmetry in another organic
compound and its chirality was amplifi ed to produce a large amount of an enantiomeri-
cally pure organic compound, pyrimidyl alkanol, in conjunction with asymmetric
autocatalysis.
Spontaneous absolute asymmetric synthesis was described in the formation of enan-
tiomerically enriched pyrimidyl alkanol from the reaction of pyrimidine-5-carbaldehyde
and
i
- Pr
2
Zn without adding a chiral substance in combination with asymmetric autoca-
talysis. The approximate stochastic distribution of the absolute confi gurations of pyrimi-
dyl alkanols strongly suggests that the reaction is a spontaneous absolute asymmetric
synthesis.
It was shown that the asymmetric autocatalysis of chiral pyrimidyl alkanol is the only
possible method to discriminate cryptochiral quaternary saturated hydrocarbons, whose
chirality is not capable of determination by any conventional methods. The discrimina-
tion of chirality due to deuterium substitution is also accessible by the highly sensitive
