Heterogeneous NHC-catalysis is an effective synthetic platform for the production of bio-based furan derivatives.

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

The application of N‐heterocyclic carbene (NHC) catalysis to the polycondensation of diols and dialdehydes under oxidative conditions is herein presented for the synthesis of polyesters using fossil‐based (ethylene glycol, phthalaldehydes) and bio‐based (furan derivatives, glycerol, isosorbide) monomers. The catalytic dimethyl triazolium/1,8‐diazabicyclo[5.4.0]undec‐7‐ene couple and stoichiometric quinone oxidant afforded polyester oligomers with a number‐average molecular weight (Mn) in the range of 1.5–7.8 kg mol−1 as determined by NMR analysis. The synthesis of a higher molecular weight polyester (polyethylene terephthalate, PET) by an NHC‐promoted two‐step procedure via oligoester intermediates is also illustrated together with the catalyst‐controlled preparation of cross‐linked or linear polyesters derived from the trifunctional glycerol. The thermal properties (TGA and DSC analyses) of the synthesized oligoesters are also reported.

Heterogeneous NHC-catalysis in batch and flow modes is an effective synthetic platform for the production of monoacylglycerols.

A strategy for the immobilization of the valuable triazolium carbene Rovis catalyst onto polystyrene and silica supports is presented. Initially, the catalyst activity and recyclability were tested under batch conditions in the model stereoselective intramolecular Stetter reaction leading to optically active chromanones. Good results in terms of yield (95%) and enantioselectivity (ee 81–95%) were detected for the polystyrene-supported catalyst (10 mol %), while poorer results were collected for the silica-supported analogue. Also, continuous-flow experiments were performed by fabricating the corresponding polystyrene monolithic microreactors (pressure-resistant stainless-steel columns) to prove the benefits of the heterogeneous catalysis and the flow regime observing a high stability of the catalytic bed (48 h) with unaltered conversion efficiency and stereoselectivity.

Conspectus The introduction of circular principles in chemical manufacturing will drastically change the way everyday plastics are produced, thereby affecting several aspects of the respective value chains in terms of raw feedstock, recyclability, and cost. The ultimate aim is to ensure a paradigm shift toward plastic-based (consumer) materials that overall can offer a more attractive and sustainable carbon footprint, which is an important requisite from a societal, political, and eventually economical point of view. To realize this important milestone, it is vitally important to control the polymerization processes associated with the creation of novel sustainable materials. In this respect, we realized that expanding the portfolio of biomass-derived monomers may indeed create an impetus for atom circularity; however, the often sterically congested nature of biomass-derived monomers minimizes the ability of previously developed catalysts to activate and transform these precursors. Our motivation was thus spurred by an apparent lack of catalysts suitable for addressing the conversion of such biomonomers, as we realized the potential that new catalytic processes could have to advance and contribute to the development of sustainable materials produced from polycarbonates and polyesters. These two classes of polymers represent crucial ingredients of important and large-scale consumer products and are therefore ideal fits for implementing new catalytic protocols that enable a gradual transition to plastic materials with an improved carbon footprint. When we started our research expedition, the field was dominated by metal catalysts that incorporated preferred, and in some cases even privileged, ligand backbones (such as salens) able to mediate both ring-opening and ring-opening copolymerization manifolds. One major drawback of these aforementioned catalysts is their rather rigid nature, a feature that reduces their ability to act as adaptive systems, especially in cases where bulky monomers are involved. While our initial focus was on the utilization of sustainable metal salen complexes (M = Zn, Fe) for the activation of small cyclic ethers, which are privileged monomers for polyester and polycarbonate production, we were rapidly confronted with severe limitations related to their inability to activate a wider range of complex epoxides and oxetanes, which was imparted by the planar coordination geometry of the salen ligand in most of its applied metal complexes. In our quest to find a catalytically more effective metal complex with the ability to electronically and sterically tune its substrate-binding and substrate-activation potential, we identified aminotriphenolates as structurally versatile, easily accessible, and scalable ligands for various earth-abundant metal cations. Moreover, the ligand backbone allows for switchable coordination environments around the metal centers, thus offering the necessary adaptation in substrate activation events. This Account describes how Al­(III)- and Fe­(III)-centered aminotriph...

Fatty acid epoxies serve as valuable starting materials for the development of bio-based polyesters. Here we present a new and efficient catalytic process that allows for the copolymerization of fatty acid-based epoxides and various cyclic anhydrides under attractive process conditions affording functional polyesters. The degree of functionalization and the nature of the polymer backbone can be modulated via monomer design. Postpolymerization cross-linking processes were examined to create rigid macromolecular networks that build on orthogonal polyester functionality, creating possible entries for materials with switchable thermal and mechanical properties.

A new N-heterocyclic carbene (NHC)-catalyzed strategy for the regioselective monoesterification of isosorbide (IS) at either the endo (5-OH) or exo (2-OH) position is described. Site-selective acylation proceeds under oxidative conditions in the presence of a quinone oxidant using aldehydes as mild acylating agents. Experimental evidences suggest a role of the stereoelectronic features of the acyl azolium salt intermediate in determining the selectivity of the acylation process. The solvent effect was also investigated, considering conventional and sustainable solvents. Aromatic aldehydes, including bio-based furfural and 5-hydroxymethyl furfural, together with α,β-unsaturated aldehydes proved to be effective reaction partners affording monoacyl-isosorbides with satisfactory levels of regioselectivity (exo/endo: 5.3−3.5; endo/exo: 5.3−3.3). Additionally, the exo-selective triazolium salt promoter was successfully transferred into the heterogeneous phase and applied to continuous-flow catalysis. In particular, the polystyrenesupported version of the selected NHC showed a catalytic activity comparable to that of the homogeneous counterpart in terms of both conversion efficiency (turnover number = 108) and regioselectivity (exo/endo up to 5.3). Also, the corresponding packed-bed mesoreactor was operated with long-term stability (ca. 110 h on stream) to produce 2-benzoyl-IS (1.32 mmol h −1 mmol cat −1 ), which is the key intermediate in the synthesis of a commercial active pharmaceutical ingredient, namely, the vasodilator isosorbide-5mononitrate.