Organocatalysis process

Even if organocatalysis is a common activation process in biological transformations, this concept has only recently been developed for chemical applications. During the last decade, achiral ureas and thioureas have been used in allylation reactions [146], the Bayhs-Hillman reaction [147] and the Claisen rearrangement [148]. Chiral organocatalysis can be achieved with optically active ureas and thioureas for asymmetric C - C bond-forming reactions such as the Strecker reaction (Sect. 5.1), Mannich reactions (Sect. 5.2), phosphorylation reactions (Sect. 5.3), Michael reactions (Sect. 5.4) and Diels-Alder cyclisations (Sect. 5.6). Finally, deprotonated chiral thioureas were used as chiral bases (Sect. 5.7). 

In conclusion, the process development and scale up already achieved for several organocatalytic reactions have shown that organocatalysis can be a valuable tool... 

We have shown organocatalysis to be a powerful tool for the synthesis of drugs and natural products. Our work and that of others have clearly demonstrated that organocatalytic strategies can cut down the total number of synthetic operations. As organocatalysts are usually non-toxic, air- and moisture-stable and often available from renewable sources, they are destined to have an impact on the development of future sustainable chemical processes. 

More examples for applied organocatalysis are presented by H. Groger, who gives an overview of organocatalytic methods already applied on a technical scale. Based on case studies, he shows several examples that satisfy the criteria of a technically feasible process such as high catalyst activity and stability, economic access, sustainability, atom economy, and high volumetric productivity. 

Fig. 19. Different ways to introduce oxyl radical reactivity nature employs metal bound tyrosyl radicals (19) or high-valent metal oxo fragments in many active sites (65,153). Nitroxyl radicals such as 2,2,6,6,-tetramethylpiperidin-l-oxyl (TEMPO, 20) are reactive species used in organocatalysis (154). The excited states of carbonyl functional groups (21) and metal oxo-fragments (22) display a radical pair character, which may become very attractive for biomimetic photoredox processes upon spectral sensitization (3,5).

Phosphine-Catalyzed Reactions. This ligand has also been shown to be effective in the direct organocatalysis of asymmetric processes. For example, the phosphine-catalyzed [3 -1- 2] annula-tion reaction of ethyl 2,3-butadienoate and isobutyl acrylate produces two cyclopentene regioisomers (1 and 2) (eq 2). Isomer 1 generally predominates and enantiomeric excesses ranging from... 

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