Bronsted acid-assisted Lewis acids BLAs

Mixtures of polyphosphoric acid (PPA) or K-10 clay and Bi(OTf)3 xH20 have been reported to be efficient systems for the rearrangement of indanone oximes (Equation 36) [73]. The apparent synergy between PPA and Bi(OTf)3 xH20 might be rationalized as Bronsted acid assisted Lewis acid (BLA) catalysis. 

One possible way to take advantage of such abilities may be to apply a combined acids system [2] to the catalyst design. The concept of combined acids, which can be classified into Bronsted acid-assisted Lewis acid (BLA), Lewis acid-assisted Lewis acid (LLA), Lewis acid-assisted Br0nsted acid (LBA), and Bronsted acid-assisted Bronsted acid (BBA), can be a particularly useful tool for the design of asymmetric catalysis, because combining such acids will bring out their inherent reactivity by associative interaction, and also provide more organized structure, which will allow an effective asymmetric environment to be secured. 

Bronsted acid-assisted Lewis acid catalyst (BLA)... 

Yamamoto and co-workers have introduced a conceptually interesting series of catalysts that incorporate an acidic proton into the active catalyst. Termed Bronsted acid-assisted chiral Lewis acid (BLA), catalyst 14 selectively catalyzes a number of diene-aldehyde cycloadditions reactions (Scheme 16) [67]. While extremely selective for the substrates shown, no aldehydes lacking an a-substitu-ent were reported to be effective in this reaction. This feature was addressed in... 

Bronsted acid assisted chiral Lewis acid catalysts (BLA) are designed for the asymmetric Diels-Alder reaction [11]. Both BLA (17) [11] and (18) [12] were employed in... 

Ishihara and Yamamoto et al. have found that chiral boron ate complexes prepared from chiral tetraols and BHj THF or B(OR)j give new catalysts [Bronsted acid-assist-ed chiral Lewis acids (BLA)] in enantioselective synthesis, which achieve selectivity... 

Since the first report of chiral oxazaborolidine-based Bronsted acid-assisted chiral Lewis acid (BLA) for enantioselective Diels-Alder reactions by Corey and coworkers... 

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

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