Ewoud

Nature, Published online: 22 August 2024; doi:10.1038/d41586-024-02705-x

As the threat of deadly heatwaves rises, scientists are working with cities to introduce low-tech cooling features to protect citizens.

Polyacetal polyols were synthesized via the polycondensation of aldehydes and diols, and subsequently used for the preparation of closed-loop recyclable polyurethanes. The synthesis of acetal-containing polyols via the polycondensation of aldehydes and diols as a platform for closed-loop recyclable polyurethanes is presented. These polyurethane materials show outstanding mechanical properties and can be recycled back into original monomers in excellent yield. The recovered monomers can be used for the preparation of new polyols and polyurethanes with identical properties to the original material.

Abstract

Polyurethanes (PUs) are highly versatile polymers widely utilized across industries. However, chemical recycling of PU poses significant challenges due to the harsh conditions required and the formation of complex mixtures of oligomers upon depolymerization. Addressing this inherent lack of recyclability, we developed closed-loop recyclable PU materials by integrating cleavable acetal groups. We present a sustainable and scalable synthesis method for acetal-containing polyols (APs) through aldehyde-diol polycondensation, utilizing reusable heterogeneous catalysts. Three APs with different hydrolytic stabilities depending on the structure of acetal groups were synthesized from formaldehyde, acetaldehyde, and propionaldehyde with 1,6-hexanediol (H16). These APs were employed alongside 4,4′-methylene diisocyanate (MDI) for preparation of PU materials. The resulting PUs exhibited mechanical properties comparable to or surpassing those of conventional PUs, while demonstrating excellent recyclability under acidic conditions. Notably, hydrolysis of PU materials based on acetaldehyde-derived APs yielded remarkable monomer recovery rates, with 89 % for H16 and 84 % for 4,4′-methylenedianiline, a precursor to MDI. Furthermore, we successfully demonstrated closed-loop recycling by synthesizing APs from recovered H16, resulting in PU materials with identical properties to the original PU. This achievement highlights the potential for establishing a closed-loop recycling system for acetal-containing PUs, contributing to the advancement of a sustainable and circular economy.

A mild strategy for the carbonylation of alkyl halides with various nucleophiles driven by visible light has been developed. This metal-free method is selective and produces diverse esters and amides in good to excellent yields. Additionally, it allows for full ¹³C isotopic incorporation.

Abstract

Herein, we describe a new strategy for the carbonylation of alkyl halides with different nucleophiles to generate valuable carbonyl derivatives under visible light irradiation. This method is mild, robust, highly selective, and proceeds under metal-free conditions to prepare a range of structurally diverse esters and amides in good to excellent yields. In addition, we highlight the application of this activation strategy for ¹³C isotopic incorporation. We propose that the reaction proceeds by a photoinduced reduction to afford carbon-centered radicals from alkyl halides, which undergo subsequent single electron-oxidation to form a carbocationic intermediate. Carbon monoxide is trapped by the carbocation to generate an acylium cation, which can be attacked by a series of nucleophiles to give a range of carbonyl products.

Nature, Published online: 08 August 2024; doi:10.1038/d41586-024-02553-9

Survey of bacteria living inside household and laboratory appliances finds a robust ecosystem.

Light-driven enzymatic degradation of lignocellulose could help boosting the economy of biorefineries by either shortening the bioprocessing time, or accelerating enzymatic kinetics and so possibly spare some precious enzymes. The redox Lytic Polysaccharide Monooxygenases - LPMOs, a key enzyme for cellulose degradation, had been shown to benefit from photobiocatalysis settings thanks to the in-situ photocatalytic reduction of O2 into H2O2, a co-substrate of the enzyme and perhaps through a faster reduction of its catalytically active metal Cu-II to Cu-I. Despite the successful photoactivation of LPMO, a clear gain in saccharification yield during enzymatic hydrolysis of industrial-relevant lignocellulosic substrate was never achieved. The previous attempts had seen the use of externally added photosensitizers i.e. chlorophyllin or melanin, or settings where the lignin already present in the biomass was exploited as photosensitizer, in both cases undergoing an uncontrolled photocatalytic production of the H2O2. Eventually, it appears that the choice of the lignocellulosic pretreatment technology - PT, the ratio between glycoside hydrolases - GHs and LPMO in the enzymatic cocktail, and the light irradiation could all be key to set a yield-gaining light-driven biorefinery. In this work, we successfully found that the reductive catalytic fractionation – RCF, a lignin-first PT, is suitable to fractionate birchwood lignin so to provide lignin-oil, and a remaining lignocellulose consisting of a photoactivable lignin and highly digestible cellulose. Up to 20% yield gain was obtained when conducting the enzymatic hydrolysis under intermitted light irradiation using a lab-made cocktail containing GHs and LPMOs. Moreover, the exchange of photo-excited electrons among the various components had been electrochemically characterized. Eventually, confocal laser scanning microscopy had been extensively used to characterize the oxidative status of the lignin during photo-driven catalysis in the presence of LPMO.

An iron catalyst-initiated photocatalytic strategy including LMCT, HAT, and β-scission processes was developed as a general method for efficient cleavage of lignin C _α −C _β bond and formation of alkyl radical, leading to a series of functionalities assembly. This method offers easy access to various functionalization molecules from cleavage of lignin linkages through efficient capture of alkyl radical species with diversiform functionalization reagents and also exhibits highly selective and efficient fragmentation of β-O-4 linkages in native lignin with high monomers recovery, showing great potential for lignin valorization with practicability and feasibility.

Abstract

The photocatalytic conversion of lignin into value-added chemicals especially those functionalized molecules represent one of the most important strategies for sustainable and environmental-friendly development. Cleavage of C−C bonds in lignin under mild photocatalytic conditions for refining lignin into useful molecules is meaningful but challenging. Meanwhile, the assembly of diverse functional groups into active lignin fragments during the depolymerization is of great challenging. Herein, using cheap iron catalysts under visible light irradiation, the highly selective and efficient cleavage of C _α −C _β bond in lignin is realized via ligand-to-metal charge transfer (LMCT) and hydrogen atom transfer (HAT) processes. The subsequent divergent functionalization of generated lignin fragment-based radical intermediates enables an efficient formation of diverse functionalized molecules. This method is also effective for cleavage of C _α −C _β bond in native lignin, yielding two identified benzaldehyde monomers in a total yield of 8.7 wt %.

The first example of gold-catalyzed alkoxy-carbonylation of aryl and vinyl iodides is reported by utilizing a (P,N)-ligand-enabled Au(I)/Au(III) redox catalysis. This transformation operates under mild reaction conditions with low CO pressure (balloon) and exhibits excellent chemoselectivity for C(sp²)−I bond over other C(sp²)−X bonds (X=F, Cl, Br, OMs, and OTf).

Abstract

Herein, for the first time, we disclose the gold-catalyzed alkoxy-carbonylation of aryl and vinyl iodides utilizing ligand-enabled Au(I)/Au(III) redox catalysis. The present methodology is found to be general, efficient, employs mild reaction conditions and showcases a broad substrate scope even with structurally complex molecules. Density functional theory (DFT) calculations revealed mechanistic pathways distinct from those of conventional transition metal-catalyzed carbonylation reactions.

Nature Synthesis, Published online: 29 May 2024; doi:10.1038/s44160-024-00558-w

Automated iterative small-molecule synthesis has generally been limited to around one carbon–carbon bond-forming step per day. Now, a next-generation automated synthesizer enables rapid, automated, iterative synthesis of a variety of small molecules. Improvements to chemistry and automation leads to a tenfold decrease in reaction time over previous automated platforms.

Nature Catalysis, Published online: 28 May 2024; doi:10.1038/s41929-024-01165-w

Being able to selectively derive desired compounds from biomass feedstock is very challenging. Now the selectivity of Pt–Co catalysts for the electroreduction of guaiacol and other lignin-derived substrates is shown to depend on the Co speciation and preferential C–OH cleavage can be obtained, retaining the C–OR group.

This study describes an accessible approach to various solvent-free transition metal-catalyzed C–H and C–X functionalizations. Conditions from eight different Rh-, Ru-, Ir- and Pd-based mechanochemical reactions are adapted to proceed with generally high efficiency by substituting automated ball milling with pestle and mortar grinding and subsequent heating. This is a low-cost, operationally simple method that requires no specialized equipment and offers similar sustainability benefits. The reaction scope encompasses 10 heterocycle types, many pendant functional groups and is demonstrated on the late-stage modification of a variety of bioactive compounds.

“What makes this development even more thrilling is that the same catalyst proves effective in multiple C−C bond formation reactions and click reactions.” This and more about the story behind the research that inspired the Cover image is presented in the Cover Profile. Read the full text of the corresponding research at 10.1002/cssc.202301588. View the Front Cover here.

Abstract

Invited for this issue's cover are researchers from Tallinn University of Technology (TalTech). The image depicts the lignin chemical evolution route from raw biomass through a greener chloromethylation procedure developed by the research team. It showcases the transformation into lignin-supported metal nanoparticles, serving as a catalyst for various chemical reactions in both batch and continuous flow conditions. The Research Article itself is available at 10.1002/cssc.202301588.