Ph.D. Thesis_AS_Publishing version for IRC_12 ... - Jacobs University

Chapter 1
1.4.1 Generic mechanism of bifunctional catalysts
Bifunctional organocatalysts bear two main parts: a nucleophilic nitrogen and a second part
which attracts the electrophile. For aminocatalysts and specifically for diamines, a primary or
secondary amine serves to form an enamine nucleophilic, while a protonated tertiary amine acts
as the H-bonding site for the electrophile. To explain the mechanism of bifunctional catalysis, let
us consider the general example of ketone addition to an unsaturated nitro compound (Scheme
1.12).
R
O
R'
Ar
NO 2
R 1
H
N
NR 2
R
O
R'
Ar
NO 2
Scheme 1.12. General example of organocatalytic Michael addition.
The nucleophilic nitrogen (primary or secondary) on a bifunctional organocatalyst attacks the
carbonyl (aldehyde or ketone) forming an iminium ion. The iminium ion can readily rearrange to
an enamine. Meanwhile, the protonated tertiary nitrogen (on the catalyst) serves as a hydrogen
bonding site to activate the electrophile (nitroalkenes, maleimides, etc.) and to simultaneously
bring it close to the enamine. This spacial arrangement of the enamine and the electrophile allows
practical reaction rates that are less often achievable with purely steric based catalysts (Scheme
1.13).
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