Bulletin No. 2, 2019

15 They Stand on the Shoulders of Giants Many existing industrial processes rely on heavy metal catalysts, some of which may result in heavy metal contamination in the processes such as drug synthesis. Prof. Yeung Ying-yeung looks into the design and applications of organocatalysts to develop efficient and metal-free catalytic protocols. ‘Catalysis is the core of many chemical processes. They facilitate the development of new drugs and functional materials, which play a central role in the advancement of human life and technology in the 21st century,’ he explains. Nature contains many catalysts such as enzymes. However, enzymes have a large molecular size and are difficult to modify for various applications. Organocatalysts are designed based on the functional fragments of enzymes and can be considered as biomimetic small molecular machines. ‘It is easier to design and synthesize those small molecular organocatalysts for various catalytic processes.’ Organocatalysis has emerged in recent years because organocatalysts can catalyze reactions similar to those of metal catalysts yet they are metal-free. Many of the protocols are nonetheless inefficient and require high catalyst loading and elevated temperature. Professor Yeung’s team has successfully developed novel zwitterionic organocatalyst systems that are highly efficient in various chemical processes. Zwitterions are an emerging class of bifunctional organocatalysts. However, catalysis using zwitterions are underexploited, partly because of the difficulties in identifying suitable catalyst architectures and preparation of zwitterionic catalysts. The team therefore designed a new class of zwitterionic organocatalysts based on an amide anion/iminium cation GreenChemist pair, which were found to be applicable to the scalable synthesis of biodiesel. ‘The most difficult part is to identify a catalyst that is stable under ambient conditions and concurrently provides sufficient catalytic activity in the designated chemical processes,’ Professor Yeung says. His team has also developed the first proof- of-concept of using the halogen bond as an organocatalyst to catalyze bromo- carbocyclization of cinnamyl substrates. The resulting halogenated tetrahydroquinolines and chromanes are valuable drug cores and natural products scaffolds. Professor Yeung’s work on the development of halogenation reactions, catalyst design and applications and green industry is much facilitated by CUHK’s world- class research infrastructure. He adds, ‘The excellent reputation of CUHK also attracts internationally renowned professors to visit, which can facilitate idea exchange and collaboration.’ He remarks that the ability to design elegant and economical synthesis routes is a major factor in the eventual viability and commercial success of industrial products. As a green alternative to classical metallic catalysis industrial processes, organocatalysis is attractive to chemical industry. The catalytic processes can reduce energy utility, waste production and carbon footprint, which are important elements for sustainable development. ‘The excellent reputation of CUHK also attracts internationally renowned professors to visit, which can facilitate idea exchange and collaboration.’