Organocatalytic processes

For recent reviews of the ability of NHCs to catalyse organocatalytic processes see Enders D, Niemeier O, Henseler A (2007) Chem Rev 107 5606-5655 Marion N, Diez-Gonzalez S, Nolan, SP (2007) Angew Chem Int Ed 46 2988-3000... 

Organocatalytic processes provide the capacity to catalyze known transformations with chemo-, regio-, diastereo-, or enantioselectivities that are orthogonal or complementary to known metal-catalyzed systems. 

Computational Studies of Organocatalytic Processes Based on Hydrogen Bonding... 

Specific mechanistic information on the organocatalytic processes discussed in this book is given in the individual chapters. 

Nucleophilic addition of sulfur ylides to C=0 double bonds is an important means of synthesis of epoxides [198], Because optically active epoxides are widely applied as versatile intermediates in the preparation of, e.g., pharmaceuticals, the asymmetric design of this sulfur ylide-based reaction has attracted much interest [199, 200, 212, 213], One aspect of this asymmetric organocatalytic process which has been realized by several groups is shown in Scheme 6.87A. In the first step a chiral sulfur ylide of type 204 is formed in a nucleophilic substitution reaction starting from a halogenated alkane, a base, and a chiral sulfide of type 203 as organocata-... 

Oxidation reactions - notably alkene epoxidations - were some of the first asymmetric organocatalytic processes to develop into generally useful synthetic methods applicable to a range of substrates [1], This chapter surveys these reactions, with emphasis placed on the most practical and general. Some recent, very useful oxidation reactions involving a-oxidation of carbonyl compounds are covered elsewhere (see Chapter 2). 

The suitability of organocatalytic reactions for larger-scale production processes of chiral building blocks has also already been demonstrated in some cases. Notably, different types of bond formation have been reported, comprising several carbon-carbon bond formations as well as oxidation processes. An overview about asymmetric organocatalytic processes with an industrial impact is given in Table 1. These syntheses comprise asymmetric organocatalytic reactions which have been scaled... 

Synthesis of chiral heterocycles by domino organocatalytic processes has also been intensively studied. In particular, various benzo-fused heterocycles, such as chiral chromans, " thiochromanes, hydro-quinolines, dihydropyranes, or thiopyranes were investigated. These organocatalytic sequence were typically initiated by a hetero-Michael addition of a sulfur, oiqrgen or nitrogen nucleophile, which triggers the formation of an enolate/enamine that adds to the ortho electrophile terminating the cascade reaction. An elimination step or an additional cyclisation step follows (Scheme 8.25). 

The aldol addition is probably one of the most weU-investigated organocatalytic processes. The organocatalytic intramolecular version of this reaction was reported... 

Finally, kinetic studies for the aldol reaction showed that this process was characterized by a well-behaved positive order kinetics, conversely to that observed in other related organocatalytic processes [315]. 

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