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K. Zeitler
catalysts, “strange to say the investigation of organic compounds concerning their enzyme-like properties has been neglected”, only in recent years have organocatalysts received widespread attention (Dalko
and Moisan 2004; Pellisier 2007). During the last decade it has been
demonstrated that such small (purely) organic molecules can function as
efficient and highly selective catalysts, which are generally non-toxic,
inexpensive to prepare, can easily be linked to solid supports, and allow novel modes of substrate activation (Lelais and MacMillan 2007;
Seayad and List 2005). Hence, asymmetric organocatalysis complements the established fields of (transition)-metal catalysis and biocatalysis (List and Yang 2006).
Referring to a mechanistic classification of organocatalysts (Seayad
and List 2005), currently the two most prominent classes are Brønsted
acid catalysts and Lewis base catalysts. Within the latter class chiral
secondary amines (enamine, iminium, dienamine activation; for a short
review please refer to List 2006) play an important role and can be
considered as—by now—already widely extended mimetics of type I
aldolases, whereas acylation catalysts, for example, refer to hydrolases or peptidases (Spivey and McDaid 2007). Thiamine-dependent enzymes, a versatile class of C–C bond forming and destructing biocatalysts (Pohl et al. 2002) with their common catalytically active coenzyme
thiamine (vitamin B 1 ), are understood to be the biomimetic roots of carbene catalysis, a further class of nucleophilic, Lewis base catalysis with
increasing importance in the last 5 years.
This rapidly growing interest in N-heterocyclic (NHC) carbenes
might be partly due to their important role as ligands for transition
metal complexes (Glorius 2007; Nolan 2006), but is also attributed to
their highly versatile character as organocatalysts (Enders et al. 2007a,b;
Marion et al. 2007; Zeitler 2005). Based on this functional duality
a comparison to phosphines can be drawn. Although some similarities
can be found, NHC compounds have already proven to be not merely
‘phosphine mimics’, but to be important in their own right.
1 This is
especially true as carbene catalysis offers the opportunity to swap tradi1 Some aspects of the different electronic and steric properties of phosphines and carbenes have been summarized in short comparative overviews (Glorius 2007; Kantchev
et al. 2007).
K. Zeitler
catalysts, “strange to say the investigation of organic compounds concerning their enzyme-like properties has been neglected”, only in recent years have organocatalysts received widespread attention (Dalko
and Moisan 2004; Pellisier 2007). During the last decade it has been
demonstrated that such small (purely) organic molecules can function as
efficient and highly selective catalysts, which are generally non-toxic,
inexpensive to prepare, can easily be linked to solid supports, and allow novel modes of substrate activation (Lelais and MacMillan 2007;
Seayad and List 2005). Hence, asymmetric organocatalysis complements the established fields of (transition)-metal catalysis and biocatalysis (List and Yang 2006).
Referring to a mechanistic classification of organocatalysts (Seayad
and List 2005), currently the two most prominent classes are Brønsted
acid catalysts and Lewis base catalysts. Within the latter class chiral
secondary amines (enamine, iminium, dienamine activation; for a short
review please refer to List 2006) play an important role and can be
considered as—by now—already widely extended mimetics of type I
aldolases, whereas acylation catalysts, for example, refer to hydrolases or peptidases (Spivey and McDaid 2007). Thiamine-dependent enzymes, a versatile class of C–C bond forming and destructing biocatalysts (Pohl et al. 2002) with their common catalytically active coenzyme
thiamine (vitamin B 1 ), are understood to be the biomimetic roots of carbene catalysis, a further class of nucleophilic, Lewis base catalysis with
increasing importance in the last 5 years.
This rapidly growing interest in N-heterocyclic (NHC) carbenes
might be partly due to their important role as ligands for transition
metal complexes (Glorius 2007; Nolan 2006), but is also attributed to
their highly versatile character as organocatalysts (Enders et al. 2007a,b;
Marion et al. 2007; Zeitler 2005). Based on this functional duality
a comparison to phosphines can be drawn. Although some similarities
can be found, NHC compounds have already proven to be not merely
‘phosphine mimics’, but to be important in their own right.
1 This is
especially true as carbene catalysis offers the opportunity to swap tradi1 Some aspects of the different electronic and steric properties of phosphines and carbenes have been summarized in short comparative overviews (Glorius 2007; Kantchev
et al. 2007).