Types of Organocatalysts

With regard to catalysts interacting with the reagents participating in a given reaction by weaker interactions, the most important and relevant activation mechanism is that involving the formation of catalyst-substrate complexes by 

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Roles that are normally associated with metals as Lewis acids and as redox agents [4,5], can be emulated by organic compounds. This review will introduce the reader to the research field of Lewis acid organocatalysts. This field, compared to other types of organocatalysts, which are highlighted in the other chapters of this volume, is still limited. The number of asymmetric catalyzed examples is small, and the obtained enantiomeric excess is sometimes low. Therefore, this review will also cover a number of reactions promoted by achiral catalysts. Nevertheless, due to the broad variety of possible reactions, which are catalyzed by Lewis acids, this research field possesses a large potential. 

Interestingly, completely different types of organocatalyst have been found to have catalytic hydrocyanation properties. Among these molecules are chiral diketo-piperazine [4, 5], a bicydic guanidine [6], and imine-containing urea and thiourea derivatives [7-13]. All these molecules contain an imino bond which seems to be beneficial for catalyzing the hydrocyanation process. Chiral N-oxides also promote the cyanosilylation of aldimines, although stoichiometric amounts of the oxides are required [14]. 

In addition to proline, other types of organocatalyst have been found to catalyze the Mannich-type reaction efficiently. The Jacobsen group developed an elegant and highly enantioselective route to N-Boc-/i-amino acid esters via nucleophilic ad-... 

Another type of organocatalyst, which is suitable for the Mannich reaction with ketene silyl acetals, is a chiral binaphthyl phosphoric acid [38c]. Very recently, it has been reported that high enantioselectivity of up to 96% ee can be obtained with this type of catalyst [38c]. 

The suitability of a different type of organocatalyst, chiral N-oxides, for asymmetric allylation was discovered by the Nakajima group [176]. On the basis of the knowl-... 

This chapter has presented Lewis acid organocatalysts. They can catalyze, as do their metal counterparts, a broad range of reactions. However, in most cases the asymmetric induction is low or only racemic product can be obtained so far. Organic Lewis acids are either very active or in case of the stable ionic liquids less active. The more active they are, the more moisture sensitive they are. This research is still at the beginning in terms of asymmetric induction and much room for improvement is left compared to the other well-established types of organocatalysts. 

Takemoto and coworkers [32] elaborated bifunchonal catalyst 27, which was found, after this initial report, to be highly versahle in promohng a large variety of trans-formahons. The combinahon of a thiourea and a terhary amine separated by a chiral scaffold, (l ,l )-l,2-cyclohexyldiamine, was studied to build a new type of organocatalyst. Aminothiourea 27 was first examined as catalyst for the enanhose-lective addition of malonate to nitroaUcenes (Scheme 34.5). The thiourea moiety of catalyst 27 guides and activates the nitroolefin while the terhary amine part deprotonates the malonate. 

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