Primary amines as heterogeneous catalysts in an enantioselective [2,3]-Wittig rearrangement reaction

A series of heterogeneous catalysts anchored to different polystyrene-based supports has been prepared and applied in an asymmetric [2,3]-Wittig rearrangement reaction of cyclohexanone derivatives. Among them, primary amino acid-derived (aminomethylated)polystyrene-supported catalysts showed excellent reactivity leading to the formation of rearranged products in good enantioselectivities of both diastereomers. Reusability issues connected to the deactivation of the catalyst were proved to be dependent on the end-capping strategy chosen for the blocking of the unreacted active sites of the resin. This issue of end-capping has not previously been in focus. Using bulkier pivaloyl end-capping moiety, we were able to recycle the catalyst in six consecutive cycles with only marginal deceleration of the reaction. Moreover, the epimerization of the product that occurred while conducting a rearrangement reaction in the presence of a homogeneous catalyst was almost fully eliminated by switching the catalytic system to heterogeneous.

One-step assembly of a MacMillan catalyst-based phenolic-type polymer

We report herein a "bottom-up" approach for the one-step assembly of a MacMillan catalyst-based phenolic-type polymer (Mac-CP). The resulting self-supported polymeric organocatalyst possesses homogeneously distributed and highly concentrated catalytic sites. Furthermore, Mac-CP is soluble in CH₃CN but insoluble in hexane. This unique property can be used to employ the polymer as an efficient catalyst in homogeneous organocatalysis and heterogeneous recycling. As a result, Mac-CP possesses comparable catalytic activity and enantioselectivity to its homogeneous counterpart in the asymmetric Diels-Alder reaction (95% yield, 93% enantiomeric excess (ee) for endo and 92% ee for exo).

Combining iminium and supramolecular catalysis for the [4 + 2] cycloaddition of E-cinnamaldehydes

Herein we describe a method for combining supramolecular catalysis with imininum-based organocatalysis in the Diels-Alder cycloaddition reaction. Both supramolecular host and L-proline are required for the reaction to occur, implying that encapsulation of the substrates and co-catalyst are necessary for the reaction to occur. We explore the substrate scope for a variety of E-cinnamaldehydes and dienes. Finally, we probe the supramolecular assembly processes responsible for the observed catalysis using NMR spectroscopic methods.

SuFEx-Click Approach for the Synthesis of Soluble Polymer-Bound MacMillan Catalysts for the Asymmetric Diels–Alder Reaction

Novel polymeric MacMillan catalysts were prepared from modified chiral imidazolidin-4-one monomers via sulfur(VI) fluoride exchange chemistry. The resulting polysulfates containing chiral imidazolidin-4-one units could be employed as polymeric organocatalysts for the asymmetric Diels–Alder reaction. With the use of these polysulfate catalysts, sufficient catalytic activity and enantioselectivity were obtained, which were similar to those obtained by monomeric catalysts in a homogeneous catalytic reaction. In addition, the polysulfate catalysts could be recovered and reused five times without a considerable loss of activity and selectivity.

Multi-Step Continuous-Flow Organic Synthesis: Opportunities and Challenges

Continuous-flow multi-step synthesis takes the advantages of microchannel flow chemistry and may transform the conventional multi-step organic synthesis by using integrated synthetic systems. To realize the goal, however, innovative chemical methods and techniques are urgently required to meet the significant remaining challenges. In the past few years, by using green reactions, telescoped chemical design, and/or novel in-line separation techniques, major and rapid advancement has been made in this direction. This minireview summarizes the most recent reports (2017-2020) on continuous-flow synthesis of functional molecules. Notably, several complex active pharmaceutical ingredients (APIs) have been prepared by the continuous-flow approach. Key technologies to the successes and remaining challenges are discussed. These results exemplified the feasibility of using modern continuous-flow chemistry for complex synthetic targets, and bode well for the future development of integrated, automated artificial synthetic systems.

Stereoselective organocatalysis and flow chemistry

Organic synthesis has traditionally been performed in batch. Continuous-flow chemistry was recently rediscovered as an enabling technology to be applied to the synthesis of organic molecules. Organocatalysis is a well-established methodology, especially for the preparation of enantioenriched compounds. In this chapter we discuss the use of chiral organocatalysts in continuous flow. After the classification of the different types of catalytic reactors, in Section 2, each class will be discussed with the most recent and significant examples reported in the literature. In Section 3 we discuss homogeneous stereoselective reactions in flow, with a look at the stereoselective organophotoredox transformations in flow. This research topic is emerging as one of the most powerful method to prepare enantioenriched products with structures that would otherwise be challenging to make. Section 4 describes the use of supported organocatalysts in flow chemistry. Part of the discussion will be devoted to the choice of the support. Examples of packed-bed, monolithic and inner-wall functionalized reactors will be introduced and discussed. We hope to give an overview of the potentialities of the combination of (supported) chiral organocatalysts and flow chemistry.

Recent advances in continuous-flow organocatalysis for process intensification

The progresses on continuous-flow organocatalysis from 2016 to early 2020 are reviewed with focus on transition from batch to flow.