Abstract
It is our great pleasure as Guest Editors of the journal ‘Mini-Reviews in Organic Chemistry’ to present you with a ‘mini-hot topic issue’ on nonmetal asymmetric catalysis. Asymmetric catalysis entails the catalytic, selective, and reproducible generation of a given enantiomer of a chiral product from achiral reactants. Leading the quest of asymmetric catalysis is the need of the pharmaceutical, flavors and fragrances, and agrochemical industries for enantiopure molecules because the different enantiomers or diastereomers of a molecule often have different biological activity. Nearly 85% of new drugs in the market are chiral. Organocatalysis can be used as environmentally-friendly alternatives to transition-metal catalysts as no toxic metals are required. The principle interactions of organocatalyst are non-covalent, such as hydrophobic, hydrogen bonding, van der Waals and electrostatic as in enzymes. These catalysts are often inexpensive to prepare and reaction can be performed under aerobic environments and in wet solvents. The beneficial impact of organocatalytic reactions on a large scale production of chiral intermediates has been demonstrated. Therefore, this special issue aims to review the advances in nonmetal asymmetric catalysis during the last decade, specified in organocatalysis. The first review related to the above mentioned topic is written by Drs. Leow, Shen and their colleagues, who explore organocatalytic enantioselective protonation of enol derivatives. As they mentioned, the proton is the smallest functional group in organic synthesis and it is extremely challenging to control it in terms of enantioselectivity. Traditional methods are using pre-formed enolates obtained from the direct deprotonation of carbonyl compounds by strong bases. The newer strategies involve the usage of silyl enol ethers, tautomerization of enols or in situ generation of transient enolates. They can be generated through either additions to ketenes, conjugate additions to Michael acceptors, rearrangements, or isomerization of double/triple bonds. In the following article, as an alternative of the most popular asymmetric sulfur-Michael addition, Dr. Zhiyong Jiang and his colleagues summarized the advance on asymmetric α-sulfenylation. Two asymmetric protocols were discussed respectively, including transition-metal catalysis and organocatalysis. It was noted that the unprecedented substrate scopes still exist which represents developing rooms, although asymmetric α-sulfenylation has obtained remarkable progress in recent years; and novel and easily prepared sulfenylated reagents are still highly desirable. In the last article in this issue, Dr. Hansen and his colleagues gave an overview over the major advances in asymmetric enamine catalysis since, and including, 2008 with focus on strategic transformations and catalyst development. As they noted, enamine-catalysis has become a powerful enabling technology for enantioselective chemical synthesis, which could be attributed to a rapid evolution of novel catalyst architectures and strategies, combined with the design and discovery of strategically important, asymmetric transformations. This exciting field will continue to expand, fuelled by its unique ability to generate novel and stereodefined structural complexity in the synthesis of organic molecules. We would like to sincerely thank all the reviewers for their valuable contributions that ensured the quality of articles to be published in this special issue. It was a great opportunity for us to cooperate with researchers from all over the world including United States, Norway, Taiwan and Mainland China. We hope that readers will enjoy this issue, obtain useful information, and be inspired with new ideas for future research on nonmetal asymmetric catalysis.
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