English

• Organocatalysis has become one of the most important frontiers in exploratory organic synthetic research. Because of its connection with reduced energy consumption, greenhouse gas emissions, and waste generation, organocatalysis is increasingly used in pharmaceutical, chemical, food, energy, and agricultural industries. In the past decade, organocatalysis has emerged as a mature strategy for addressing modern day challenges in chemistry.Ithasbecomewidely appreciated that small molecule organic catalysts can hold a wide range of practical advantages relative to macromolecular or precious metalcatalysts, including air and water stability, lowcost, availability from renewable resources, and relative nontoxicity. However, one of the biggest challenges of organocatalysis is controlling the selectivity. This special column on organocatalysis contains 11 articles which can be broadly grouped into three categories, including developing of catalysts, exploring catalysis for synthesis, investigatory studies mechanistic and selectivity on catalysis.

  1. Developing new catalysts

  Song's group (https://doi.org/10.1016/j.cclet.2018.01.045) summarized the very recent advances in asymmetric reactions catalyzed by chiral phosphoric acids (CPAs), a family of versatile catalysts that catalyze a broad range of reactions to afford diverse chiral molecules. They have emerged as one of the fastest growing fields of organic catalysts and will continue to be a research hottopic in the future. In addition, Luo's group (https://doi.org/10.1016/j.cclet.2018.02.009) summarized the evolution of quinone catalysts, including the corresponding mechanism and catalytic applications. Quinoproteins are an important type of redox enzymes for biological oxidation processes. This review has significantly promoted the discovery of new type reactions and catalysts. Chiral organobases occupy a significant position in asymmetric organocatalysis. The chemistry of these organobases is full, several groups have summarized each kind of chiral organobases in different reviews. Feng and Liu (https://doi.org/10.1016/j.cclet.2018.05.041) had also provided a brief introduction of these chiral organobases in asymmetric catalysis. Several selected asymmetric catalytic reactions are presented to elucidate the catalytic characteristics, new types of chiral organobases and different kinds of substrates are anticipated in future development.

  2. Exploring catalysis for synthesis

  Ye and Sun's group (https://doi.org/10.1016/j.cclet.2018.03.003) reported a chiral N-heterocyclic carbene-catalyzed [4 + 2] annulation of α-chloroaldehydes and aurones, which gives the corresponding benzofuran–fused dihydropyranones in good to high yields with good diastereoselectivities and excellent enantioselectivities. A catalytic asymmetric brominated dearomatization reaction of benzofuran derivatives was achieved by using hydroquinidine 1, 4-phthalazinediyl diether [(DHQ)₂PHAL] as the catalyst by the You's group (https://doi.org/10.1016/j.cclet.2018.01.039). Yan's group (https://doi.org/10.1016/j.cclet.2018.01.047) recently completed an enantioselective one-pot synthesis of β-sulfonyl ketones and trisubstituted tetrahydrothiophenes, which is via β- elimination and cycloaddition sequence. In addition, three derivatives of _L-histidine with different alkyl chain lengths, NIPCA, UIPCA, and TIPCA, were synthesized and their self-assembly with citric acid in aqueous solutions was investigated by Song's group (https://doi.org/10.1016/j.cclet.2017.12.024). All _L-histidine derivatives can form hydrogels with citric acid, and exhibit the increasing gelation ability and mechanical strength with the increasing chain length. In addition, Chen's group (https://doi.org/10.1016/j.cclet.2018.06.007) described a tetrahydrobenzo[5]helicenediol (THB-[5]HDIOL) derivatives as additives for the efficient proline-catalyzed asymmetric List-Lerner-Barbas (LLB) aldol reactions of bulky aldehyde substrates. The good yields and excellent enantioselectivities of the corresponding reactions were obtained. The results presented can expand an opportunity for the helical diols to be used in asymmetric catalysis, in particular for bulky substrates involved reactions.

  3. Investigatory studies mechanistic and selectivity on catalysis

  Zhang's group (https://doi.org/10.1016/j.cclet.2018.01.003) reported a computational study elucidating reaction mechanism of the classic stepwise and a concerted acyl substitution mechanism for amide bond formation from esters and amines catalyzed by acetic acid. The concerted acyl substitution mechanism is proposed to be probable that may be widely operating in a variety of related amidation reactions with different types of coupling reagents. A DFT study was conducted to understand the geometric factors and electronic effects of FLPs on reversible H₂ activation by Ke's group (https://doi.org/10.1016/j.cclet.2018.02.007). The results should provide deeper insight to understand the relation between FLPs structure and reactivity, which is critical for the rational design of more efficient FLPs catalysts for reversible H₂ activation. DFT calculations were also used to investigate the mechanism and enantioselectivity of phosphoric acid-catalyzed asymmetric conjugate addition of indolizines to α, β-unsaturated ketones by Lan's group (https://doi.org/10.1016/j.cclet.2018.03.018). The calculation results showed that this transformation occurs via a reaction pathway involving nucleophilic addition from the indolizine to the hydrogen-bondactivated olefin, deprotonation–aromatization, and tautomerization. Optimized structure and NCI analyses showed that the main origin of the enantioselectivity is bond-rotation strain in the transition states of the deprotonation–aromatization step. Investigatory studies mechanistic and selectivity on catalysis may be widely operating in a variety of related reactions with different types of reagents.

  This special column features a number of papers focused on organocatalysis. Increasing attention is being paid to organocatalysis, driven by their growing importance in synthesis and applications. There is a great deal of interesting science in this collection of papers that point the way to organocatalysis in exploratory organic synthetic research. Undoubtedly, this diverse collection of contributions speaks to the vibrancy of the organocatalysis field and provides a nice overview of current activity in the discipline.

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