Br0nsted acid catalysis

Br0nsted acid catalysis, the substrate electrophile is reversibly protonated in a pre-equilibrium step, prior to the nucleophilic attack (Scheme 2). In general acid catalysis, however, the proton is (partially or fuUy) transferred in the transition state of the rate-determining step (Scheme 2). Clearly, the formation of a hydrogen bond precedes proton transfer. 

X 0, NR Nu Scheme 2 Specific and general Br0nsted-acid catalysis... 

The key feature of Br0nsted acid catalysis is often the choice of a catalyst with the appropriate acidity for particular substrate classes. Whereas less reactive substrates require stronger Brpnsted acids than the widely used phosphoric acids for activation, acid-sensitive substrates tend to decompose under strongly acidic conditions. Thus, weaker Brpnsted acid catalysts may prove beneficial. 

Hydrogen-Bond Catalysis or Br0nsted-Acid Catalysis General Considerations... 

The difference between hydrogen-bond catalysis and Br0nsted-acid catalysis is not always clear in the literatures. In this chapter, the differences and similarities of the hydrogen-bond catalysis and Bronsted-acid catalysis will be addressed. 

Because there is an equilibrium between hydrogen-bond complex 5 and ion pair 6, it is not always easy to differentiate between hydrogen-bond catalysis and Br0nsted-acid catalysis explicitly. 

In summary, the differences between Br0nsted-acid catalysis and hydrogen-bond catalysis were discussed. Because there is a gradual transition from hydro-gen-bond catalysis to Bronsted-acid catalysis, it is not always easy to differentiate the two modes of catalysis. However, the combination of a stronger acid and an imine will be a Bronsted-acid-catalyzed reachon, while the combination of a neutral acid and an aldehyde will be a hydrogen-bond-catalyzed reaction. 

Br0nsted-Acid-Assisted Br0nsted Acid Catalysis... 

Another complement to enamine catalysis is Br0nsted acid catalysis. Blanche showed that a Hg-BlNOL-derived phosphoric acid catalyzes the aldol reaction to give jy -aldols 162-165 from various ketones [175]. Gratifyingly, the method is suitable for acetophenone, fused cyclic aromatic ketones, and a,(3-unsaturated ketones, substrates that are normally challenging donors in enamine catalysis (Chart 3.22). 

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

Until 2006, the activation of simple carbonyl compounds by means of Br0nsted acid catalysis proved to be challenging since the acidity of the known BINOL-derived phosphoric acids were mostly insufficient. Indeed, carbonyl compounds and other less reactive substrates often require... 

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