Chiral Br0nsted acid catalysis

Scheme 5.39 Br0nsted acid-assisted chiral Br0nsted acid catalysis...

Akiyama T, Itoh J, Fuchibe K (2006) Recent Progress in Chiral Br0nsted Acid Catalysis. Adv Synth Catal 348 999... 

Hodous BL, Fu GC. Enantioselective addition of amines to ketenes catalyzed by a planar-chiral derivative of PPY possible intervention of chiral Br0nsted-acid catalysis. J. Am. Chem. Soc. 2002 124 10006-10007. 

As protons are the simplest and most easily available Lewis acids available to catalyze organic transformations, the use of a chiral Br0nsted acid combines the potential of proton catalysis with asymmetric induction achieved through the choice of the proper coimteranion. It is without doubt that chiral phosphoric acids have been the most successfully used chiral Br0nsted acids so far (369, 371). With respect to the chiral backbone employed therein, binaphthyls have been found to be the most promising substances for this purpose. 

Br0nsted acid catalysis, especially with chiral phosphoric acid type catalysts, has become one of the most successful subfields of organocatalysis [9, 10]. However, numerous reactions are still elusive, and the smallest substrates frequently give the poorest selectivity. A probable cause could be traced to a limited interaction of the small substrate with the catalysts structure when approaching its open active site (Scheme 21). With large substrates, the steric interaction is more pronounced, resulting in higher selectivity. 

Wanner, M. J., Claveau, E., van Maarseveen, J. H., Hiemstra, H. (2011). Enantioselec-tive syntheses of corynanthe alkaloids by chiral Br0nsted acid and palladium catalysis. Chemistry - A European Journal, 17, 13680-13683. 

Rueping et al. reported achiral Br0nsted acid assisted chiral Bronsted acid catalysis in the direct Marmich reaction of acyclic ketones. The reaction of N-aryl imines with acetophenone was conducted using a chiral phosphoric acid in the presence of acetic acid as the co-catalyst and the resulting products were obtained in moderate yields [11]. 

I 77 Br0nsted Acids Chiral Phosphoric Acid Catalysis in Asymmetric Synthesis... 

A successful asymmetric organocatalytic based C=0 reduction with the Hantzsch ester was not reported until very recently. Terada and Toda developed a relay catalysis that combined Rh(ll) and a chiral phosphoric acid catalyst in a one-pot reaction (Scheme 32.15). In this reaction sequence, a rhodium carbene (I) forms in the first step and is followed with an intramolecular cyclization to afford carbonyl ylide intermediate II or oxidopyrylium III. These intermediates are protonated by 7 to yield the chiral ion pair between isobenzopyrylium and the conjugate base of 7 (IV). Intermediate IV is further reduced in situ by Hantzsch ester Id to produce the isochroman-4-one derivative 67, which is finally trapped with benzoyl chloride to afford the chiral product 68. Surprisingly, the reaction sequence proceeds well to give racemic product even without the addition of chiral 7, while giving rise to the desired product with high enantioselectivity in the presence of chiral Br0nsted acid 7 [38]. 

SCHEME 2.92 Asymmetric a-allylation of branched aldehydes via Pd/chiral Br0nsted acid cascade catalysis. 

SCHEME 5.90 Enantioselective relay catalysis using an Ru complex/chiral Br0nsted acid binary system. 

While carboxylic acids have been recognized as one of the most promising acid catalysts in organic synthesis, they had been scarcely utilized for asymmetric Br0nsted acid catalysis. This situation was presumably due to the relatively poor reactivity of the carboxylic acid and the difficulty associated with creating a discrete chiral environment around the acidic functionality. Maruoka addressed this problem by designing chiral l,T-binaphthyl-2,2 -dicarboxylic acid derivatives 27 (Figure 7.5). 

The enantioselective organocatalytic functionalization of carbonyl compounds at the Y position represents a highly challenging and persistent problem for asymmetric synthesis [33]. Recently, Melchiorre and coworkers [34] reported a chiral Br0nsted acid-assisted dienamine catalysis for the direct asymmetric y-alkylation of a-substituted linear a,P-unsaturated aldehydes (Scheme 43.22). In this reaction, the chiral phosphoric acid 106 can induce the formation of a chiral contact ion-pair from alcohol 103 [35], which may synergistically engage in a matched combination with the chiral covalent dienamine intermediate derived from chiral primary... 

First Highly Enantioselective Br0nsted Acid Catalyzed Strecker Reaction Use of C-Nucleophiles in Chiral Ion Pair Catalysis... 

Br0nsted Base Catalysis. It is noteworthy that although the focus of attention in chiral catalysis has been on Lewis acidic species, naturally occurring alkaloids present the opportunity for highly enan-... 

Abstract The organocatalytic asymmetric Mannich reaction and the related aza-Morita-Baylis-Hillman have been reviewed. The activities in this field have been snbdivided based on the types of catalysts that have been ntilized, which includes catalysis by enamine-forming chiral amines, chiral Br0nsted bases, chiral Brpnsted acids, and phase-transfer catalysts. 

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