PET Cats! Recent development of heterogeneous catalysts for chemical recycling of polyethylene terephthalate (PET) is reviewed. PET is commercially used to produce bottles, film, packaging materials and synthetic fibers. Its chemical recycling produces renewable products, mitigating climate and environmental challenges as an alternative to using petroleum.
Polyethylene terephthalate (PET) is a non-degradable single-use plastic and a major component of plastic waste in landfills. Chemical recycling is one of the most widely adopted methods to transform post-consumer PET into PET's building block chemicals. Non-catalytic depolymerization of PET is very slow and requires high temperatures and/or pressures. Recent advancements in the field of material science and catalysis have delivered several innovative strategies to promote PET depolymerization under mild reaction conditions. Particularly, heterogeneous catalysts assisted depolymerization of post-consumer PET to monomers and other value-added chemicals is the most industrially compatible method. This review includes current progresses on the heterogeneously catalyzed chemical recycling of PET. It describes four key pathways for PET depolymerization including, glycolysis, pyrolysis, alcoholysis, and reductive depolymerization. The catalyst function, active sites and structure-activity correlations are briefly outlined in each section. An outlook for future development is also presented.
Plastic Waste Recycling: Polyolefins (polyethylene, polypropylene) represent one of the largest groups of daily used polymers and also a particularly challenging substrate for non-mechanical recycling. In this review the current state for chemical polyolefin recycling is discussed, concentrating on procedures using different catalytic approaches and transformations facilitated at or below 300 °C reaction temperature.
Polyolefins and especially polyethylenes (LLDPE, LDPE and HDPE) and polypropylene (PP) contribute a great deal to the growing amounts of plastic waste with a combined production share of almost 50 % by mass. While being almost universally applicable, they are mainly used for short-lived packaging materials that constitute over 60 % of annual post-consumer plastic waste. Thus, disproportionately high amounts of polyolefins end up as post-consumer waste (PCW) and waste management strategies for these particularly inert plastics are needed. This necessity has promoted a great research effort dealing with valorization of these discarded but, nevertheless, valuable materials. This review aims to highlight the scientific advances made in chemical polyolefin recycling in recent years, focusing, though not exclusively, on catalytic processes to recycle polyolefin waste by various means at more moderate temperatures compared to pyrolysis such as deconstructing the polymer with the objective of upcycling in mind or by catalytic transformation to give access to functional chemicals.
A new class of polystyrene-supported aminocatalysts with a triazole linker from diphenyl prolinol were developed. Their catalytic activity, substrate scope, and reusability were demonstrated in asymmetric α-amination of aldehydes.
A new class of polystyrene-supported aminocatalysts with a triazole linker from diphenyl prolinol were developed. Their catalytic activity was tested in the asymmetric α-amination of aldehydes. The substrate scope of this catalysis was studied under the optimized reaction conditions. The reusability of the polystyrene-supported aminocatalyst was demonstrated up to four cycles. Based on the reaction results, a reaction mechanism was proposed for the asymmetric α-amination of aldehydes.
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In the spotlight! Ferredoxin-dependent enzymes often rely on a complicated electron-transfer chain to enable the catalysis of challenging chemical reactions. In this review, we showcase several Fd-dependent enzymes and their potential biocatalytic applications, while particularly focusing on Rieske oxygenases as a class of enzymes with huge application potential in synthetic organic chemistry that are nonetheless still not very well understood.
Enzymes that depend on sophisticated electron transfer via ferredoxins (Fds) exhibit outstanding catalytic capabilities, but despite decades of research, many of them are still not well understood or exploited for synthetic applications. This review aims to provide a general overview of the most important Fd-dependent enzymes and the electron transfer processes involved. While several examples are discussed, we focus in particular on the family of Rieske non-heme iron-dependent oxygenases (ROs). In addition to illustrating their electron transfer principles and catalytic potential, the current state of knowledge on structure–function relationships and the mode of interaction between the redox partner proteins is reviewed. Moreover, we highlight several key catalyzed transformations, but also take a deeper dive into their engineerability for biocatalytic applications. The overall findings from these case studies highlight the catalytic capabilities of these biocatalysts and could stimulate future interest in developing additional Fd-dependent enzyme classes for synthetic applications.
A new protocol for diastereoselective cyclopropanation from unactivated alkenes and acidic carbon pronucleophiles is reported. The method is scalable and amenable to synthesis of medicinally relevant molecules.
Cyclopropanes are desirable structural motifs with valuable applications in drug discovery and beyond. Established alkene cyclopropanation methods give rise to cyclopropanes with a limited array of substituents, are difficult to scale, or both. Herein, we disclose a new cyclopropane synthesis through the formal coupling of abundant carbon pronucleophiles and unactivated alkenes. This strategy exploits dicationic adducts derived from electrolysis of thianthrene in the presence of alkene substrates. We find that these dielectrophiles undergo cyclopropanation with methylene pronucleophiles via alkenyl thianthrenium intermediates. This protocol is scalable, proceeds with high diastereoselectivity, and tolerates diverse functional groups on both the alkene and pronucleophile coupling partners. To validate the utility of this new procedure, we prepared an array of substituted analogs of an established cyclopropane that is en route to multiple pharmaceuticals.
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Synthesis
DOI: 10.1055/s-0042-1751433
Herein, a facile and environmentally friendly oxidative coupling reaction to obtain unsymmetrical disulfide compounds, without using metal or organic dyes as photocatalyst and only using a catalytic amount of iodine, was developed. Oxygen is used as oxidant in the reaction and the substrate scope is wide. A gram-scale synthesis also highlights the synthetic practicality of this photochemical strategy. Moreover, our mechanistic studies support the formation of disulfide products through free-radical coupling.
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Table of contents | Abstract | Full text
Cysteine desulfurization by a phosphine can be efficiently achieved under aerobic conditions in hydrogen carbonate buffer by micromolar concentrations of iron. The desulfurization mechanism likely involves iron(III) species produced through the oxidation of iron(II)-carbonate complexes by molecular oxygen and is reminiscent of iron-catalyzed oxidation phenomena occurring in natural waters.
One pillar of protein chemical synthesis based on the application of ligation chemistries to cysteine is the group of reactions enabling the selective desulfurization of cysteine residues into alanines. Modern desulfurization reactions use a phosphine as a sink for sulfur under activation conditions involving the generation of sulfur-centered radicals. Here we show that cysteine desulfurization by a phosphine can be effected efficiently by micromolar concentrations of iron under aerobic conditions in hydrogen carbonate buffer, that is using conditions that are reminiscent of iron-catalyzed oxidation phenomena occurring in natural waters. Therefore, our work shows that chemical processes taking place in aquatic systems can be adapted to a chemical reactor for triggering a complex chemoselective transformation at the protein level, while minimizing the resort to harmful chemicals.
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