Organocatalysis Diels-Alder reactions

Notz W, Tanaka F, Barbas CF (2004) Enamine-based organocatalysis with proline and diamines the development of direct catalytic asymmetric aldol, Mannich, Michael, and Diels-Alder reactions. Acc Chem Res 37(8) 580-591... 

More recently, MacMillan has introduced the amine catalysts 42 and 45, readily available from L-phenylalanine, methylamine, and acetone or pivalaldehyde, respectively (Schemes 4.15 and 4.16). The broad potential of these materials in enan-tioselective organocatalysis was first proven in Diels-Alder reactions [28] and nitrone cydoadditions [29]. In 1,4-addition of C-nudeophiles MacMillan et al. later showed that Friedel-Crafts reactions of pyrroles with enals can be made highly enantioselective (Scheme 4.15) [30]. 

Subsequent major events, up until the early 1980s, have been reviewed [2], with one of the major reactions involved being that of asymmetric hydrogenation, which is especially useful and efficient. This was first developed using rhodium complexes equipped with chiral mono- or diphosphines [3-6], though many other types of reaction (e.g., hydroformylation, Diels-Alder reaction) are now well controlled in the presence of chiral organometallic catalysts. Over the past few years there has been a clear renewal of interest for organocatalysis [7], and consequently this chapter will review the specific and unusual case of the catalytic enantioselective reduction of C=C, C=0, and C=N double bonds. 

Notz, W., Tanaka, F., Barbas, C. F., III. Enamine-Based Organocatalysis with Proline and Diamines The Development of Direct Catalytic Asymmetric Aldol, Mannich, Michael, and Diels-Alder Reactions. Acc. Chem. Res. 2004, 37, 580-591. 

For reviews on the organocatalytic Diels-Alder reaction see (a) A. Berkessel and H. Groger, Asymmetric Organocatalysis, Wiley-VCH, Weinheim, 2005, 256.(b) A. Berkessel and H. Groger, Asymmetric Organocatalysis, Wiley-VCH, Weinheim, 2005, 256.(c) H. PeUisier, Tetrahedron, 2007, 63, 9267. 

The term aminocatalysis has been coined [4] to designate reactions catalyzed by secondary and primary amines, taking place via enamine and iminium ion intermediates. The field of asymmetric aminocatalysis, initiated both by Hajos and Parrish [5] and by Eder, Sauer, and Wiechert [6] in 1971, has experienced a tremendous renaissance in the past decade [7], triggered by the simultaneous discovery of proline-catalyzed intermolecular aldol [8] and Mannich [9] reactions and of asymmetric Diels-Alder reactions catalyzed by chiral imidazolidinones [10]. Asymmetric enamine and iminium catalysis have been influential in creating the field of asymmetric organocatalysis [11], and probably for this reason aminocatalytic processes have been the object of the majority of mechanistic smdies in organocatalysis. 

Chiral oxazaborolidine catalysts were applied in various enantioselective transformations including reduction of highly functionalized ketones/ oximes or imines/ Diels-Alder reactions/ cycloadditions/ Michael additions, and other reactions. These catalysts are surprisingly small molecules compared to the practically efficient chiral phosphoric acids, cinchona alkaloids, or (thio)ureas hence, their effectiveness in asymmetric catalysis demonstrates that huge substituents or extensive hydrogen bond networks are not absolutely essential for successful as5unmetric organocatalysis. 

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