School of Engineering and Science - Jacobs University

Bu
O
Bu
Bu
OH
OH
OH
racemic (R)-enantiomer (S)-enantiomer
A B
Fig. 1-12. Kinetic resolution of racemic allylic alcohol –A, yielding its (S) enantiomer
(B) [3].
1.3.2 Asymmetric synthesis
There are two basic ways to synthesize a chiral compound. The first way is to begin
with single stereoisomer and to use a synthesis that does not affect the chiral centre.
The other basic method is called asymmetric synthesis. The definition of asymmetric
synthesis is as follows: "Asymmetric synthesis is a reaction in which an achiral unit in
an ensemble of substrate molecules is converted by a reactant into a chiral unit in such
a manner that the stereoisomeric products are formed in unequal amounts" [19].
1.3.3 Asymmetric catalysis
As was mentioned above in order to synthesize a chiral compound another chiral
substance is needed. Before such a chiral substance was considered as a reactant,
however it can also be a chiral catalyst in asymmetrical synthesis. According to the
definition catalyst influences on reaction but keeps its structure unchanged in the end of
reaction. Thus, a small amount of specific chiral catalyst can induce synthesis of chiral
product in large scales. This strategy attracts much attention from industry and science,
in fact, the number of publication on asymmetric syntheses has been increasing
exponentially every year since the 80s [20].
Willian S. Knowles, Ryoji Noyori and Barry Sharpless were awarded the Nobel Prize
for chemistry in 2001 for their contribution in asymmetrical catalysis, sealing the
importance of this field in contemporary chemistry and chemical technology.
Asymmetrical reactions catalyzed with natural and man-made chiral catalysts becomes
main-stream in different fields of industry (chemical, pharmaceutical, etc.).
Asymmetrical catalysis can be divided into two main areas: homogeneous and
heterogeneous. It is clear from terms that in the case of homogeneous catalysis the
important reactions are occurring in one phase, e.g. molecules in a solution. Whereas,
in the case of heterogeneous catalysis reactions are occurring on the interface of two
phases e.g. solid-liquid, solid-gas or in other words, on the surface of solid bodies.
It is important to mention here briefly the important requirements [21] for
enantioselective catalysts, since they are applied either for heterogeneous and
homogeneous catalysts.
Enantioselectivity, expressed as enantiopurity of a system in terms of enantiomeric
excess (ee, %). A catalyst has to be able to induce enantioselectivity in the range of 99
% for further pharmaceutical applications. However ee’s > 80 % are acceptable also if
further enantiopurification could be easily performed, e.g. via recrystallization.
Catalyst productivity, characterized in terms of turnover number (TON). For example,
in case of homogeneous enantioselective hydrogenation reactions the following values
of TON can be used for the catalyst productivity classification. TON’s > 1000 for
small-scale, high-value products and > 50000 for large-scale or less expensive products.
7