Catalytic Asymmetric Synthesis. Группа авторов

2020, Terada and co‐workers developed “chiral cooperative binary base catalysts” 39, which consist of two different organobase functionalities: a P2‐phosphazene as an organosuperbase and a chiral guanidine as a hydrogen bond donor unit for substrate recognition [108]. The molecular design is based on a conceptually new idea for a distinctive cooperative function by two organobases in a single catalyst molecule: the formation of a chiral cyclic structure with an intramolecular hydrogen bond between the two organobase functionalities in the conjugate acid form, which creates an effective chiral environment around the substrate recognition site by limiting the conformational flexibility (Figure 3.15). The prominent catalytic activity of 39 was demonstrated in the enantioselective direct Mannich‐type reaction of α‐phenylthioacetate as a less acidic pronucleophile (Scheme 3.56a). The P3‐phosphazene‐based catalyst 42 possessing enhanced basicity was also synthesized based on the molecular design. The catalyst could promote the reaction of much less acidic α‐phenylthioacetamide, which did not proceed with P2‐phosphazene‐based catalyst 39, albeit in moderate stereoselectivities (Scheme 3.56b).

Scheme 3.56. Enantioselective direct Mannich‐type reactions catalyzed by chiral cooperative binary base catalysts. Source: Based on [108].

3.5. CONCLUSION AND OUTLOOK

      Over the past two decades, the development of enantioselective reactions under asymmetric Brønsted base catalysis has been intensively explored, and fruitful progress has been made in this field. A variety of useful enantioselective transformations has been achieved based on the rational reaction design as well as the suitable choice of a chiral catalyst. In parallel with the development of new enantioselective reactions, various types of efficient chiral uncharged organobase catalysts have been designed and synthesized, which dramatically accelerated the progress. In particular, the new types of chiral organosuperbase catalysts possessing much higher basicity than that of conventional chiral tertiary amine catalysts have substantially expanded the scope of applicable pronucleophiles. Nevertheless, there is still a large room for improvement in this field. For instance, the development of chiral organosuperbase catalysts is still in its infancy, and the development of new catalysts, particularly with unprecedented catalyst design, is highly desirable to accomplish a variety of enantioselective transformations using a much broader range of pronucleophiles. On the other hand, the mechanism and the origin of the stereoselectivity have not been clarified in many reactions. Therefore, detailed mechanistic studies are also needed for a better understanding of the catalysis and the development of next generation of chiral catalysts and reaction systems. Further progress of the asymmetric Brønsted base catalysis is highly anticipated, which will find valuable applications in various fields of organic chemistry, such as natural product synthesis and drug discovery research.

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