CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

elatively recently that such asymmetric catalysis, with enantiomeric excesses approaching
100%, was achieved with synthetic catalysts. This method involves the conversion of an
achiral substrate to a chiral product with an chiral catalyst. The control of the reaction is
also intermolecular. For example, oxindole 19 was formed in high enantiopurity from 2-
acyloxindole precursor 18 (Scheme I.3.5) by using planar chiral 4-aminopyridine catalyst
20. 20 N
Me
Ph
O
O
R = C(Me) 2 CCl 3
OR
i
Ph
O
N
Me
OR
O
R = C(Me) 2 CCl 3
18 19
N
N
Ph
Ph
Fe
Ph
catalyst 20
Ph
Ph
(i) 5 mol% catalyst 20, CH 2 Cl 2 , 35 o C.
Scheme I.3.5
Chemical catalysts have some advantages over bio-catalysts. First of all, chemical
catalysts can promote reactions that are not known to occur in nature. Secondly, the
chirality of a chemical catalyst can be changed relatively easily by using the antipode as
the starting material in its preparation. In most cases, essential chiral intermediates are
taken from the 'chiral pool' and often both enantiomers are available. Thirdly, the
substrates that are not accepted in enzymatic reactions may be transformed using chemical
catalysts. In many cases, high substrate concentrations can be used and separation and
recovery of the products are easy, compared to biocatalytic reactions. Finally, most of
these catalysts have a greater stability than enzymes, which are often very sensitive to heat
and pH of the medium.
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