Recent Advance of ‘Combined Acid’ Strategy for Asymmetric 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 Brønsted acid-assisted Lewis acid (BLA), Lewis acid-assisted Lewis acid (LLA), Lewis acid-assisted Brønsted acid (LBA), and Brønsted acid-assisted Brønsted 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. These combined acid catalysts can be classified as shown in Table 1.1. It should be emphasized thatwe anticipated amore or less intramolecular assembly of such combined systems rather than intermolecular arrangements because the intramolecular assembly should generate a well-organized chiral environment, leading to high stereocontrol. Thus, proper design of the catalyst structure is essential for success. Since we have already summarized examples of combined acid catalysis [2], only recent representative examples of such catalysis are discussed with emphasis on the area of asymmetric catalysis in the following sections.