Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 79

As described in the previous section, chiral bifunctional tertiary amine catalysts have been recognized as a particular class of chiral Brønsted base catalysts and have found a vast number of applications. However, the chiral tertiary amine catalysis still possesses inherent critical limitations; high catalyst loadings and long reaction times are often required, and arguably the range of pronucleophiles and electrophiles that are applicable to the reactions is rather narrow, which stem from the low basicity of tertiary amines. Only highly acidic compounds are applicable as pronucleophiles, and thus the resulting anionic nucleophiles only possess moderate nucleophilicity to react with highly electrophilic compounds. In this context, chiral uncharged organobases having higher basicity than chiral tertiary amines have attracted increased attention. This class of chiral organobases, including conventional chiral guanidines, is often called “chiral organosuperbase” although the definition of the term “organosuperbase” is rather ambiguous [1b]. The application of chiral organosuperbases has significant advantages. Most importantly, the chiral organosuperbases potentially broaden the scope of pronucleophiles to compounds having higher pK _a values. In addition, the chiral organosuperbases increase the concentration of anionic nucleophiles. Consequently, they can dramatically accelerate the bimolecular reactions and decrease the catalyst loading and reaction time. Over the last decade, some new types of chiral organosuperbase catalysts have been developed, which is overviewed in this section.