molecules will not be covered unless necessary for the general understanding of the
Before 2000, several observations of organocatalysis were reported, although most appeared as
unique isolated examples rather than part of development of a comprehensive methodology.
The first example of the application of small chiral organic molecules as catalysts is attributed to
Bredig and Fiske, who, in 1912, showed that the addition of hydrogen cyanide (HCN) to
much success in asymmetric catalysis.
3 (19)
Figure 1. Structure of catalysts 1-3 and the asymmetric methanolysis of methylphenylketene
by Pracejus.
Along the way, several noteworthy observations were made. As early as 1928, the connection
between the catalytic activity of small organic molecules and enzymes was discussed by
Langenbeck, who also coined the term organic catalysts (organische Katalysatoren).
15
Several
years later, Fischer and Marschall (1931) showed that amino acids are excellent catalysts for the
aldol reaction,
16
and Langenbeck and Borth (1942) later showed that chiral amino acids also can
be used for this purpose.
17
The general mechanism for class I aldolases was uncovered in the
1960s and 1970s, and was shown to proceed through an enamine formation between a lysine
residue in the enzyme and a carbonyl group in the substrate.
18-19
By the 1970s, much
information was already available about how organic molecules
act as catalysts, but the time was
not yet ripe to develop a comprehensive understanding of the area.
The last example in this section relates to hydrogen-bonding catalysis. In 1998, Jacobsen and
co-workers showed that thiourea 4, identified using a library screening, is an efficient catalyst
for the Strecker reaction between N-allylbenzaldimine and HCN to yield the corresponding
adduct in high yield and er (eq. 1, The bond that is formed in the reaction is highlighted with red
color and the new stereocenter is indicated with an asterisk
*
).
20-21
Both thioureas and ureas are
excellent catalysts for a number of asymmetric transformations and have been developed as
bifunctional catalysts, by which both a nucleophile and electrophile can be simultaneously
activated.
22-23