Photo-initiated desulfurization of sulfur-containing derivatives such as, native thiols, thioethers, sulfonium salts and xanthates, allows the generation of carbon-based radicals that can be coupled with various partners providing access to a wide range of functionalized molecules by C−C and C-heteroatom bond formation.

Abstract

Thiols constitute an important family among sulfur-containing compounds, with well-established applications in various fields ranging from medicine to material science. For instance, thiol residues are good hydrogen donors which reduce radical species in biological or chemical processes. However, even though the S−H bond activation of thiols for providing access to thiyl radicals has been largely studied, desulfurative processes affording carbon-based radicals by C−S bond activation have been less explored. In recent years, photoredox catalysis has become the prevalent method for the generation of radicals under soft reaction conditions from readily available starting materials under visible light. In this context, recent studies have been devoted to the development of photocatalytic procedures aiming at the desulfurization of thiol derivatives leading to new C−H, C−C or C-Het bond formation reactions. This review will cover the synthetic methodologies and strategies for photo-mediated desulfurization of native thiols, thioethers, sulfonium salts and xanthates to access new organic compounds. This emerging field is especially interesting for new transformations of cysteine and peptide derivatives.

Researchers have extensively investigated photo-catalytic water reduction utilizing Cobalt-based catalysts with poly-pyridyl ligands. While catalysts exhibiting distorted poly-pyridyl ligand demonstrate higher H2 production yields, those with ideal octahedral coordination display poor performance. This outcome suggests the crucial role of ligand framework in catalytic activity, yet reasons behind the disparity in H2 production rates for catalysts with octahedral geometries remain unclear. We theoretically examined the water reduction mechanism of Co-based poly-pyridyl catalyst, CoPy5, having perfect octahedral coordination. We clarified the effect of octahedral coordination by utilizing each intermediate step of ECEC mechanism. We determined spin states, solvent response, electronic structures, and reduction free energies. CoPy5 with perfect octahedral coordination, alongside its distorted counterparts, exhibit similar spin states as the reaction progresses through each intermediate step. However, the first reduction free energy obtained for the CoPy5 is slightly higher than that of its distorted counterparts. Following the second protonation, resulting H2 molecule experiences limited diffusion from the Co center due to the compact structure of the CoPy5, which blocks the Co center for the next H2 production cycle. Catalysts having distorted octahedral geometries facilitate fast removal of H2 into the solvent. Thus, the reaction center becomes immediately available for subsequent H2 production.

We found that supercritical carbon dioxide could mediate multi-scale structure engineering in PdCu/C nanocatalysts, including defect engineering, phase engineering, morphology engineering and substrate engineering. The achieved PdCu/C with amorphous surface, BCC phase, nanoflake morphology and curved carbon substrate shows ultrahigh mass activity for formic acid oxidation as high as 3624 mA/mg_Pd.

Abstract

Nowadays, PdCu alloy nanocatalyst with excellent performance in electrocatalytic formic acid oxidation reaction (FAOR) is believed to have great potential in application of direct formic acid fuel cells. Structural engineering has shown great success in achieving PdCu alloys with high catalytic performance, while achievement of multi-scale structure engineering is still a great challenge. In this work, we found that supercritical carbon dioxide (SC CO₂) could lead to multi-scale structure engineering in PdCu/C nanocatalysts, including surface defect engineering, phase engineering, morphology engineering and substrate structure engineering. With the assistance of SC CO₂, amorphous phase in surface, the transformation from face-centered cubic (FCC) to body-centered cubic (BCC) phase, the morphology of 2D nanoflakes and the curved carbon as substrate all contribute to the ultrahigh mass activity for electrocatalytic FAOR as 3624.3 mA/mg_Pd in PdCu/C nanocatalysts, which is the highest value in PdCu alloy reported up to now. Therefore, this work not only displays the great potential of SC CO₂ in multi-scale structure engineering, but also provides new inspiration of material design to achieve nanocatalysts with ultrahigh catalytic performance.

The synthesis and characterization of two isomers of a homobimetallic rhenium complex is reported. The photo- as well as electrocatalytic activity in the CO₂ to CO transformation was determined and discussed in comparison to the monometallic analogue and bimetallic conformational isomers. Further spectroscopic investigations led to the proposition of a new reaction mechanism involving cooperative CO₂ activation.

Abstract

Mononuclear rhenium complexes have been widely studied as photo- and electrocatalysts. However, dinuclear systems with cooperative properties have rarely been investigated. On the basis of two homobimetallic rhenium complexes, we report the synthesis and characterization of two isomers and their photo- and electrochemical properties. By combining the respective isomer with the photosensitizer [Ir(dFppy)₃] (Ir, dFppy=2-(4,6-difluorophenyl) pyridine)) enhanced CO₂ to CO transformation could be observed and by further spectroscopic investigations the reaction mechanism could be fathomed. The observed enhanced catalytic activity compared to monometallic systems derives from the cooperative Re−Re interaction through two electron reduction on the complex (and thereby formation of an intermediate species with a Re−Re bond). Using LSV measurements the cooperative CO₂ activation was also observed for one of the isomers, cisL1-Re₂Cl₂ , in electrocatalytic measurements. The two isomers have a somewhat lower catalytic activity than earlier prepared geometric isomers, but show better catalytic properties than their mononuclear counterpart.

Syntheses of important classes of (heterocyclic) com­pounds, the sustainable generation of hydrogen, and the use of abundantly available metals are highly desirable. We introduce here a catalytic oxazole synthesis. Our reaction is a regio selective, one-pot reaction and starts from esters and amino alcohols. Both are abundantly and diversely available and inexpensive starting materials. Hydrogen is liberated during the reaction and a molecular earth-abundant metal catalyst, a Mn(I) compound, mediates the reaction most effectively - and more ef­ficiently than Ir and Ru catalysts. None of the oxazole derivatives synthesized, except the screening substrate and an active ingredient of a drug (an application), have been reported in literature yet.

Desilication is identified as valuable tool for increasing the lifetime of ZSM-5 catalysts used in the ethanol-to-aromatic conversion. A thorough solid-state NMR characterization enables disentangling the influence of changes in Si(OH) group density, Brønsted acidity, and mesopore volume on the catalyst's performance in terms of products and lifetime.

Abstract

Herein, desilication in increasingly harsh conditions was used to introduce mesopores into two different industrial ZSM-5 catalysts (Si/Al ratio 11 or 29). For desilicated samples, increasing BET surface areas, mesopore volumes, and Si(OH) densities were noted. Brønsted acid site (BAS) densities increased upon desilication, as formerly inaccessible BAS in blocked pores became available, while the strength of the BAS was maintained upon desilication. Using KOH instead of NaOH as desilication agent can increase the mesopore volume generated per mass loss. The correlations between desilication strength and properties were largely determined by the parent Si/Al ratio. In general the introduced mesopores increased lifetimes in the ETA conversion, while additional Si(OH) groups introduced by desilication reduce the lifetime again. The lifetime is thus determined by a complex interplay of BAS density, improved reactant transport by introduced mesopores and Si(OH) density. There were no additional aromatics formed in desilicated samples during the conversion of ethanol and the samples were, in terms of aromatic yield, outperformed by a microporous parent. However, as result of longer lifetimes less ethanol was lost due to coke formation. It is concluded that desilication should be combined with other post-modifications to increase aromatic production and lifetime.

Chemoselective S-adenosyl-l-methionine (SAM)-dependent methyltransferases (MTs) are a promising alternative to traditional synthetic methylation reactions. In presence of multiple nucleophiles, the enzymatic transfer of the carbon fragment is highly chemoselective for N- and O-MTs. Besides methylation, the generation of SAM derivatives enables the transfer of altered groups onto the substrates increasing the pool of products.

Abstract

Methylation reactions are of significant interest when generating pharmaceutically active molecules and building blocks for other applications. Synthetic methylating reagents are often toxic and unselective due to their high reactivity. S-Adenosyl-l-methionine (SAM)-dependent methyltransferases (MTs) present a chemoselective and environmentally friendly alternative. The anthranilate N-MT from Ruta graveolens (RgANMT) is involved in acridone alkaloid biosynthesis, methylating anthranilate. Although it is known to methylate substrates only at the N-position, the closest relatives with respect to amino acid sequence similarities of over 60 % are O-MTs catalysing the methylation reaction of caffeate and derivatives containing only hydroxyl groups (CaOMTs). In this study, we investigated the substrate range of RgANMT and a CaOMT from Prunus persica (PpCaOMT) using compounds with both, an amino- and hydroxyl group (aminophenols) as possible methyl group acceptors. For both enzymes, the reaction was highly chemoselective. Furthermore, generating cofactor derivatives in situ enabled the transfer of other alkyl chains onto the aminophenols, leading to an enlarged pool of products. Selected MT reactions were performed at a preparative biocatalytic scale in in vitro and in vivo experiments resulting in yields of up to 62 %.

Benefiting from the synergistic effect of supported bimetallic CeFe which induces higher metal dispersion and higher content of reactive oxygen, and the buffering effect of resin-derived-carbon on free radicals, the aerobic oxidation of cyclohexanone to ϵ-caprolactone (ϵ-CL) is performed over CeFe@RDC-x, and the catalytic efficiency is significantly improved. Especially, when solvent-free and green solvents (such as ethyl acetate) are used, excellent catalytic conversion effects are achieved.

Abstract

Upgrading cyclohexanone to ϵ-caprolactone (ϵ-CL) is of great importance to the synthesis of high value-added downstream chemicals and the reduction of foam plastic. The catalytic synthesis of ϵ-CL from cyclohexanone through O₂/aldehyde method is an environmentally benign one-pot tandem reaction without using peroxy acid, balancing the requirements of safety and efficiency. However, due to lack of elaborate design and collaboration of multiple active sites for catalysts, the catalytic efficiency of O₂/aldehyde method still remains to be improved. Herein, a pitaya-like catalyst (CeFe@RDC-3) with Ce and Fe highly dispersed on resin-derived-carbon is synthesized through high temperature self-assembly. On this bimetallic catalyst, high yield (97 %) of ϵ-CL is achieved through aerobic oxidation of cyclohexanone with only 1.5 equivalent benzaldehyde. Moreover, considerable yields of ϵ-CL, 88.3 % and 74.7 %, respectively, are also obtained over CeFe@RDC-3 with green solvent (EtOAc) or even without solvent. No loss of activity is observed after five successive cycles, demonstrating high stability of CeFe@RDC-3. The mechanism study reveals that the high performance of CeFe@RDC-3 is ascribed to the Ce−Fe bimetallic synergy, uniform metal dispersion, abundant active oxygen and stabilizing effect of resin-derived-carbon to free radicals. This work provides prospect for a green, safe and low-cost strategy for Baeyer-Villiger process.

Engineering enhanced regioselectivity: Directed evolution of an O-methyltransferase resulted in variants with increased regioselectivity for the para-methylation of dihydrochalcones and related catecholic compounds (regioisomeric ratio up to 99 : 1). This allows now the biocatalytic production of the taste active hesperetin dihydrochalcone.

Abstract

Directed evolution of the O-methyltransferase ZgOMT from Zooshikella ganghwensis focusing on active site residues resulted in highly regioselective biocatalysts (regioisomeric ratios up to 99 : 1) for the preparation of the taste active hesperetin dihydrochalcone and related compounds. These newly constructed enzyme variants provide an attractive synthesis route for para-methylation of catechol scaffolds, which is challenging to perform with high regioselectivity utilizing wild-type O-methyltransferases.

Catalytic carbonylation with CO₂ : The latest updates on C1-carbonylative homologation of carbon scaffolds by means of metal catalyzed fixation of CO₂ are collected in the present Review article. Innovative catalytic systems, enabling technologies and mechanistic investigations are contributing to the current developments of this fascinating research field.

Abstract

The utilization of CO₂ as an efficient and environmentally friendly chemical analogue of CO is becoming a solid reality in the chemical scenario. CO₂-based carbonylations have started paralleling the more consolidated carboxylation procedures, opening new horizons and perspectives in the utilization of carbon dioxide as an organic C1-containing building block. The advent of efficient and site-selective metal-catalyzed protocols for the fixation of CO₂ into organic scaffolds, under controlled reductive conditions, contributed substantially to the development of robust, efficient, and convenient protocols. In the present Review article, a collection of the most recent examples of metal-catalyzed CO₂-based carbonylations is documented with a particular emphasis on mechanistic aspects.