On the Acidity and Reactivity of Highly Effective Chiral Bronsted Acid Catalysts: Establishment of an Acidity Scale

摘要

Chiral phosphoric acid diesters derived from 1,1'-bi-2-naph-thol (binol) and their analogues are a highly efficient class of metal-free Br0nsted acid organocatalysts. Transfer hydro-genation as well as various addition reactions to aldimines and ketimines are among the most important reactions that can be carried out efficiently with these catalysts. The moderate acidity of these catalysts renders them ideal for the activation of basic imines by hydrogen bonding or ion pairing in a dual or bifunctional fashion. However, the activation of less basic carbonyl functionalities is more demanding, and only few enantioselective applications have been reported so far.Therefore, the more acidic chiral binol-derived N-triflylphosphoramides (NTPAs) were introduced and proved suitable for the activation of more challenging substrates.Asymmetric C—C and C—X bond-forming transformations, such as Diels-Alder and [3+2] cycloaddition reactions, the Nazarov cyclization, asymmetric protonation, and Mukaiyama aldol reactions, have been carried out efficiently with chiral AMxiflylphosphoramides in a variety of organic solvents, including halogenated as well as aromatic solvents, and more recently also in aqueous media. Furthermore, Br0nsted acid organocatalysts based on bis-(sulfonyl)imides were introduced by Berkessel et al. (JINGLE, Scheme 1) and Giernoth and co-workers (BIN-BAM).