Chiral Bransted Acids

Rauniyar, V. Hall, D. G. Catalytic Enantioselective and Catalyst-Controlled Diastereofacia 1-Selective Additions of Allyl- and Crotylboronates to Aldehydes Using Chiral Bransted Acids. Angew. Chem. hit. Ed. 2006,45,2426-2428. 

Scheme 7.45 Catalysis of chiral Bransted acid-base combined salt.

Scheme 38.48 Chiral Bransted acid catalyzed three

Bransted Acid-Assisted Chiral Lewis Acid (BLA)... 

Yamamoto developed a conceptually new class of chiral boron-Lewis acids 174 (Equation 20) that can readily be assembled from a tetraphenol ligand and B(OMe)3 [93, 94]. These were shown to be especially effective with a wide range of a-substituted enals in enantioselective Diels-Alder reactions. One illustrative example is the cycloaddition between aldehyde 173 and cyclopentadiene to give 175 in 99% ee, > 99 1 exo/endo selectivity, and quantitative yield [93]. Yamamoto suggested that these catalysts 174 were Bransted-assisted chiral Lewis acids (termed BLAs), in which the phenol proton activates the enal substrates by hydrogen bonding, as depicted in the proposed transition state assembly 176. 

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