LongLarf

Nature Catalysis, Published online: 17 July 2020; doi:10.1038/s41929-020-0468-3

New strategies: New visible‐light‐photocatalytic trifluoromethylation strategies for (hetero)aromatic compounds and carbon–carbon multiple bonds involve non‐organometallic photocatalysts such as all‐organic semiconductors and small‐molecule sensitizers compatible with the requirements of the pharmaceutical industry.

Abstract

Visible‐light‐photocatalyzed methods employed in synthetic transformations present attractive properties such as environmentally friendly, safety, availability and excellent functional group tolerance. In this regard, research on the visible‐light photocatalytic incorporation of the trifluoromethyl CF₃ moiety into organic substrates, in particular, has contributed to a clear evolution of the field of photocatalysis. Although this particular area is constantly evolving and has been reviewed, the last five years have experienced an outburst of seminal and significant photocatalytic trifluoromethylation examples that are leading the way and opening new synthetic avenues. Recent review articles on Ru‐ and Ir‐based photocatalytic trifluoromethylation reactions have borne witness of this evolution. Although this account will show the new Ru‐ and Ir‐based photocatalytic trifluoromethylations, Sections 2 and 3 will also illustrate other photocatalytic systems, such as organic dyes, organic semiconductors and newly‐developed all‐organic photocatalysts. All the known and reviewed strategies for photocatalytic trifluoromethylation reactions of olefins and (hetero)aromatic compounds will not be discussed but will be summarized in two figures (Figures 4 and 5), and new examples (2015–present) will be presented and discussed.

Diastereomers differ in physical properties: A case of real metal/metal redox isomerism is observed in a dinuclear complex with a particularly short bridging ligand. The two stereogenic complex moieties at W and Ru, respectively, are linked by an alkyne based phosphine thiolate chelate ligand. Oxidation of the like isomer pertains to tungsten, whereas the unlike isomer is oxidised at ruthenium. The diastereomers show opposite redox behaviour!

Abstract

A pair of diastereomeric dinuclear complexes, [Tp′(CO)BrW{μ‐η²‐C,C′‐κ²‐S,P‐C₂(PPh₂)S}Ru(η⁵‐C₅H₅)(PPh₃)], in which W and Ru are bridged by a phosphinyl(thiolato)alkyne in a side‐on carbon P,S‐chelate coordination mode, were synthesized, separated and fully characterized. Even though the isomers are similar in their spectroscopic properties and redox potentials, the like‐isomer is oxidized at W while the unlike‐isomer is oxidized at Ru, which is proven by IR, NIR and EPR‐spectroscopy supported by spectro‐electrochemistry and computational methods. The second oxidation of the complexes was shown to take place at the metal left unaffected in the first redox step. Finally, the tipping point could be realized in the unlike isomer of the electronically tuned thiophenolate congener [Tp′(CO)(PhS)W{μ‐η²‐C,C′‐κ²‐S,P‐C₂(PPh₂)S}Ru(η⁵‐C₅H₅)‐(PPh₃)], in which valence trapped W^III/Ru^II and W^II/Ru^III cationic species are at equilibrium.

The iron‐free catalytic arsa‐Wittig reaction was systematically studied in terms of optimization of catalyst and reaction conditions, scope of substrate, and mechanistic study with computational calculation.

Abstract

Efficient catalytic arsa‐Wittig reactions have been developed by using 1‐phenylarsolane as a catalyst. A wide array of aldehydes was converted to the corresponding olefins in high yields with moderate to excellent E stereoselectivity in the presence of a catalytic amount of 1‐phenylarsolane. Moreover, density functional theory calculations were carried out to afford insight into the E/Z selectivity.

Nature Catalysis, Published online: 13 July 2020; doi:10.1038/s41929-020-0485-2

The first atom‐economical stereoselective oxidative bioconjugation of carboxylic acid functionalities in amino acids and oligopeptides to aldehydes is demonstrated. It is also demonstrated that this stereoselective oxidative bioconjugation provides linker molecules with bioorthogonal handles for further functionalization by merging the oxidative coupling strategy with the click concept.

Abstract

The first stereoselective, near‐equimolar, and metal‐free oxidative bioconjugation of amino acids and oligopeptides to aldehydes is presented. Based on a newly developed organocatalytic oxidative concept, the C‐terminal and side‐chain carboxylic acid functionalities of amino acids and oligopeptides are shown to couple in a stereoselective manner to α‐branched aldehydes catalyzed by a chiral primary amine and a quinone as oxidizing agent. The oxidative coupling generally proceeds in high yield. For aspartic acid, selective coupling of the side‐chain, or the C‐terminal carboxylic acid, is demonstrated depending on the protection strategy. The stereoselective, oxidative bioconjugation concept is extended to a series of oligopeptides where coupling to carboxylic acid functionalities is presented. Bioorthogonal linker molecules for further functionalization are obtained by merging the oxidative coupling strategy with the click concept. It is demonstrated that the configuration of the new stereocenter is determined exclusively by the organocatalyst.

“Lignin‐first” biorefinery: Catalytic valorization of lignin provides a promising and attractive strategy for producing value‐added phenolic monomers. The recent advances in catalytic depolymerization of technical lignin and reductive catalytic fractionation of protolignin are comprehensively summarized. The mechanism of condensation reactions and possible solutions are also discussed.

Abstract

The efficient valorization of lignin could dictate the success of the 2^nd generation biorefinery. Lignin, accounting for on average a third of the lignocellulosic biomass, is the most promising candidate for sustainable production of value‐added phenolics. However, the structural alteration induced during lignin isolation is often depleting its potential for value‐added chemicals. Recently, catalytic reductive depolymerization of lignin has appeared to be a promising and effective method for its valorization to obtain phenolic monomers. The present study systematically summarizes the far‐reaching and state‐of‐the‐art lignin valorization strategies during different stages, including conventional catalytic depolymerization of technical lignin, emerging reductive catalytic fractionation of protolignin, stabilization strategies to inhibit the undesired condensation reactions, and further catalytic upgrading of lignin‐derived monomers. Finally, the potential challenges for the future researches on the efficient valorization of lignin and possible solutions are proposed.

Isomerization-based strategies to enable the stereodivergent construction of complex polyenes from geometrically defined alkene linchpins remain conspicuously underdeveloped. Mitigating the thermodynamic constraints inherent to isomerization is further frustrated by the considerations of atom efficiency in idealized low–molecular weight precursors. In this work, we report a general ambiphilic C₃ scaffold that can be isomerized and bidirectionally extended. Predicated on highly efficient triplet energy transfer, the selective isomerization of β-borylacrylates is contingent on the participation of the boron p orbital in the substrate chromophore. Rotation of the C(sp²)–B bond by 90° in the product renders re-excitation inefficient and endows directionality. This subtle stereoelectronic gating mechanism enables the stereocontrolled syntheses of well-defined retinoic acid derivatives.

The fleeting existence of [V₂O]⁺ as a reactive intermediate in the oxidation of [V₂]⁺ by CO₂ to form [V₂O₂]⁺ has been identified; the counter‐intuitive oxidation behavior of [V₂]⁺ versus [V₂O]⁺ and the quite enormous reactivity differences have been traced back to the supportive role of the bridging oxygen atom in [V₂O]⁺.

Abstract

[V₂O]⁺ remains “invisible” in the thermal gas‐phase reaction of bare [V₂]⁺ with CO₂ giving rise to [V₂O₂]⁺; this is because the [V₂O]⁺ intermediate is being consumed more than 230 times faster than it is generated. However, the fleeting existence of [V₂O]⁺ and its involvement in the [V₂]⁺ → [V₂O₂]⁺ chemistry are demonstrated by a cross‐over labeling experiment with a 1:1 mixture of C¹⁶O₂/C¹⁸O₂, generating the product ions [V₂ ¹⁶O₂]⁺, [V₂ ¹⁶O¹⁸O]⁺, and [V₂ ¹⁸O₂]⁺ in a 1:2:1 ratio. Density functional theory (DFT) calculations help to understand the remarkable and unexpected reactivity differences of [V₂]⁺ versus [V₂O]⁺ towards CO₂.

“My favorite saying is ‘diamonds are created under pressure’. … My first experiment was to blow up a tin can using a mixture of sugar and saltpeter . …” Find out more about Berthold Hoge in his Author Profile.

Abstract

The cost‐effective TPPH₂/TBACl‐catalyzed (TPPH₂=dianion of tetraphenyl porphyrin; TBACl=tetrabutyl ammonium chloride) carbon dioxide cycloaddition to N‐aryl aziridines was successful in synthesizing N‐aryl oxazolidin‐2‐ones. A catalytic tandem reaction was also developed, in which N‐aryl aziridines were initially synthesized and then reacted with carbon dioxide without being purified. The procedure occurred with a very high atom economy, molecular nitrogen being the only by‐product of the entire tandem process. In addition, the mechanism of catalytic cycle was investigated by DFT calculations.

Non‐noble‐metal catalysis : The cobalt‐catalyzed hydrogenation of epoxides for the synthesis of anti‐Markovnikov alcohols is reported. This method is suitable for internal, as well as terminal, epoxides and works smoothly even with multi‐substituted derivatives under mild conditions.

Abstract

A straightforward methodology for the synthesis of anti‐Markovnikov‐type alcohols is presented. By using a specific cobalt triphos complex in the presence of Zn(OTf)₂ as an additive, the hydrogenation of epoxides proceeds with high yields and selectivities. The described protocol shows a broad substrate scope, including multi‐substituted internal and terminal epoxides, as well as a good functional‐group tolerance. Various natural‐product derivatives, including steroids, terpenoids, and sesquiterpenoids, gave access to the corresponding alcohols in moderate‐to‐excellent yields.

Direct activation of gaseous hydrocarbons remains a major challenge for the chemistry community. Because of the intrinsic inertness of these compounds, harsh reaction conditions are typically required to enable C(sp³)–H bond cleavage, barring potential applications in synthetic organic chemistry. Here, we report a general and mild strategy to activate C(sp³)–H bonds in methane, ethane, propane, and isobutane through hydrogen atom transfer using inexpensive decatungstate as photocatalyst at room temperature. The corresponding carbon-centered radicals can be effectively trapped by a variety of Michael acceptors, leading to the corresponding hydroalkylated adducts in good isolated yields and high selectivity (38 examples).