LongLarf

The front cover picture, provided by Vytautas Petkevičius and co‐workers, illustrates the application of Escherichia coli whole cells overexpressing soluble diiron monooxygenase PmlABCDEF as a new type of biocatalyst for the synthesis of aromatic N‐oxides. The introduced method works under mild reaction conditions and is an environment‐friendly alternative to existing chemical procedures. Moreover, this approach features a broad substrate range and holds the potential for the productive up‐scale synthesis, thus surpassing previous enzyme‐based methods. Details of this work can be found in the full paper on pages 2456–2465 (V. Petkevičius, J. Vaitekūnas, D. Tauraitė, J. Stankevičiūtė, J. Šarlauskas, N. Čėnas, R. Meškys, Adv. Synth. Catal. 2019, 361, 2456–2465; DOI: https://doi.org/10.1002/adsc.20180149110.1002/adsc.201801491).

The opening of epoxides typically requires electrophilic activation, and subsequent nucleophilic (S_N2) attack on the less substituted carbon leads to alcohols with Markovnikov regioselectivity. We describe a cooperative catalysis approach to anti-Markovnikov alcohols by combining titanocene-catalyzed epoxide opening with chromium-catalyzed hydrogen activation and radical reduction. The titanocene enforces the anti-Markovnikov regioselectivity by forming the more highly substituted radical. The chromium catalyst sequentially transfers a hydrogen atom, proton, and electron from molecular hydrogen, avoiding a hydride transfer to the undesired site and resulting in 100% atom economy. Each step of the interconnected catalytic cycles was confirmed separately.

Going full circle: Polyhydroxyurethanes, for the preparation of non‐isocyanate polyurethanes, are the best candidate to replace polyurethanes. This Review focuses on sustainable ways to synthesize cyclocarbonates from renewable resources. Different parameters that affect the aminolysis reaction and properties of the resulting materials are also discussed.

Abstract

With a global production of around 18 million tons (6th among all polymers) and a wide range of applications, such as rigid and soft foams, elastomers, coatings, and adhesives, polyurethanes (PUs) are a major polymer family. Nevertheless, they present important environmental and health issues. Recently, new and safer PUs, called non‐isocyanate polyurethanes (NIPUs), have become a promising alternative to replace conventional PUs. Sustainable routes towards NIPUs are discussed herein from the perspective of green chemistry. The main focus is on the reaction between biobased carbonates and amines, which offers an interesting pathway to renewable polyhydroxyurethanes (PHUs). An overview of different routes for the synthesis of PHUs draws attention to the green synthesis of cyclic carbonate (CC) compounds and the aminolysis reaction. Current state‐of‐the‐art of different biobased building blocks for the synthesis of PHUs focuses on CC compounds. Three classes of compounds are defined according to the feedstock: 1) vegetable fats and oils, 2) starch and sugar resources, and 3) wood derivatives. Finally, biobased PHU properties are discussed.

Chemical synthesis typically relies on reactions that generate complexity through elaboration of simple starting materials. Less common are deconstructive strategies toward complexity—particularly those involving carbon-carbon bond scission. Here, we introduce one such transformation: the hydrodealkenylative cleavage of C(sp³)–C(sp²) bonds, conducted below room temperature, using ozone, an iron salt, and a hydrogen atom donor. These reactions are performed in nonanhydrous solvents and open to the air; reach completion within 30 minutes; and deliver their products in high yields, even on decagram scales. We have used this broadly functionality tolerant transformation to produce desirable synthetic intermediates, many of which are optically active, from abundantly available terpenes and terpenoid-derived precursors. We have also applied it in the formal total syntheses of complex molecules.

Abstract

The first B(C₆F₅)₃‐catalyzed deoxygenative reduction of amides into the corresponding amines with readily accessible and stable ammonia borane (AB) as a reducing agent under mild reaction conditions is reported. This metal‐free protocol provides facile access to a wide range of structurally diverse amine products in good to excellent yields, and various functional groups including those that are reduction‐sensitive were well tolerated. This new method is also applicable to chiral amide substrates without erosion of the enantiomeric purity. The role of BF₃ ⋅ OEt₂ co‐catalyst in this reaction is to activate the amide carbonyl group via the in situ formation of an amide‐boron adduct.

Abstract

This review aims to report recent advances on decarboxylative reactions of amino acids catalyzed by transition metals or non‐metals and the growing opportunities they present in the construction of complex chemical scaffolds for applications encompassing natural product synthesis, synthetic methodology development, and functional materials. Different decarboxylative reactions have been highlighted such as radical, photoredox and metal and metal‐free coupling reactions. etc. The review is divided into two main sections: one deals with reactions via radical pathways and the other via azomethine ylide pathways. The reactions have been discussed in more detail with particular emphasis on their useful applications and mechanistic illustrations.

Abbreviations: 1,3‐DCB: 1,3‐dicyanobenzene; 1,4‐DCB: 1,4‐dicyanobenzene; Acr: acridinium; BI: benziodoxolone; Boc: tert‐butoxycarbonyl; CB: conduction band; CBX: cyanobenziodoxolone; CFL: compact fluorescent lamp; DBU: 1,8‐diazabicyclo[5.4.0]undec‐7‐ene; DCA: 9,10‐dicyanoanthracene; DCB: 1,2‐dichlorobenzene; DCE: 1,2‐dichloroethane; DDDS: bis(4‐chlorophenyl) disulfide; DIPEA: diisopropylethylamine; DMA: N,N‐dimethylacetamide; DMF: N,N‐dimethylformamide; DMSO: dimethyl sulfoxide; DNA: deoxyribonucleic acid; dtbbpy: 4,4′‐di‐tert‐butyl‐2,2'‐bipyridine; DTBP: di‐tert‐butyl peroxide; equiv.: equivalent; ET: electron transfer; h: hour; HE: Hantzsch ester; LED: light‐emitting diode; min: minutes; mL: millilitre; MS: molecular sieves; MW: microwave; nm: nanometer; NS: nanosphere; Nu: nucleophile; ODCB: o‐dichlorobenzene; PA‐1: (±)‐1,1′‐binaphthyl‐2,2′‐diyl hydrogen phosphate; PC: photocatalyst; PET: photoinduced electron transfer; PEG: polyethylene glycol; PG: protecting group; Phen: phenanthrene; PTA: phosphotungstic acid 44‐hydrate; RNA: ribonucleic acid; r.t.: room temperature; SET: single‐electron transfer; TBAI: tetrabutylammonium iodide; TBHP: tert‐butyl hydroperoxide; TFA: trifluoroacetic acid; THF: tetrahydrofuran; TMEDA: N,N,N′,N′‐tetramethylethylenediamine; TS: transition state; UV: ultraviolet; VB: valence band; W: watt.

Nature, Published online: 13 May 2019; doi:10.1038/d41586-019-01533-8

Synlett
DOI: 10.1055/s-0037-1611541

An electrochemical method for the deoxygenation of N-heteroaromatic N-oxide to give the corresponding N-heteroaromatics has been developed. Several classes of N-heterocycles such as pyridine, quinoline, isoquinoline, and phenanthridine are tolerated. The electrochemical reactions proceed efficiently in aqueous solution without the need for transition-metal catalysts and waste-generating reducing reagents.
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© Georg Thieme Verlag Stuttgart · New York

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A major challenge in carbon-hydrogen (C-H) bond functionalization is to have the catalyst control precisely where a reaction takes place. In this study, we report engineered cytochrome P450 enzymes that perform unprecedented enantioselective C-H amidation reactions and control the site selectivity to divergently construct β-, -, and -lactams, completely overruling the inherent reactivities of the C-H bonds. The enzymes, expressed in Escherichia coli cells, accomplish this abiological carbon-nitrogen bond formation via reactive iron-bound carbonyl nitrenes generated from nature-inspired acyl-protected hydroxamate precursors. This transformation is exceptionally efficient (up to 1,020,000 total turnovers) and selective (up to 25:1 regioselectivity and 97%, please refer to compound 2v enantiomeric excess), and can be performed easily on preparative scale.

Synlett
DOI: 10.1055/s-0037-1611527

Pyridines are valuable motifs in a number of bioactive and functional molecules. The chemoselective functionalization of these structures from stable and widely available starting materials is a meaningful goal. We have demonstrated selective formation of pyridyl radicals at any position (2-, 3-, 4-pyridyl), through the action of a reducing photoredox catalyst. These radicals readily engage alkenes to deliver high-value products. Alteration of the reaction medium has enabled the use of a diverse range of alkene subtypes in a highly divergent and chemoselective manner.1 Introduction2 Minisci-Type Pyridine Alkylation3 An Alternate Approach – Reductive Radical Formation4 Conjugate Addition of Pyridyl Radicals5 Radical Hydroarylation of Neutral and Rich Olefins6 Solvent-Based Chemoselectivity7 Summary and Outlook
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© Georg Thieme Verlag Stuttgart · New York

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Synlett
DOI: 10.1055/s-0037-1611537

The scope of the triphenylphosphine oxide-catalyzed reduction of conjugated polyunsaturated ketones using trichlorosilane as the reducing reagent has been examined. In all cases studied, the α,β-C=C double bond was selectively reduced to a C–C single bond while all other reducible functional groups remained unchanged. This reaction was applied to a large variety of conjugated dienones, a trienone, and a tetraenone. Additionally, a tandem one-pot Wittig/conjugate-reduction reaction sequence was developed to produce γ,δ-unsaturated ketones directly from simple building blocks. In these reactions the byproduct of the Wittig reaction served as the catalyst for the reduction reaction. This strategy was then used in the synthesis of naturally occurring moth pheromones to demonstrate its utility in the context of natural-product synthesis.
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© Georg Thieme Verlag Stuttgart · New York

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Long‐lived replacements: Ruthenium complexes with polypyridine ligands are very popular choices for applications in photophysics and photochemistry, but ruthenium is rare and expensive, whereas iron is comparatively abundant and cheap. Key concepts to obtain long‐lived charge‐transfer excited states in iron complexes (see figure) are discussed and recent conceptual breakthroughs are highlighted.

Abstract

Ruthenium complexes with polypyridine ligands are very popular choices for applications in photophysics and photochemistry, for example, in lighting, sensing, solar cells, and photoredox catalysis. There is a long‐standing interest in replacing ruthenium with iron because ruthenium is rare and expensive, whereas iron is comparatively abundant and cheap. However, it is very difficult to obtain iron complexes with an electronic structure similar to that of ruthenium(II) polypyridines. The latter typically have a long‐lived excited state with pronounced charge‐transfer character between the ruthenium metal and ligands. These metal‐to‐ligand charge‐transfer (MLCT) excited states can be luminescent, with typical lifetimes in the range of 100 to 1000 ns, and the electrochemical properties are drastically altered during this time. These properties make ruthenium(II) polypyridine complexes so well suited for the abovementioned applications. In iron(II) complexes, the MLCT states can be deactivated extremely rapidly (ca. 50 fs) by energetically lower lying metal‐centered excited states. Luminescence is then no longer emitted, and the MLCT lifetimes become much too short for most applications. Recently, there has been substantial progress on extending the lifetimes of MLCT states in iron(II) complexes, and the first examples of luminescent iron complexes have been reported. Interestingly, these are iron(III) complexes with a completely different electronic structure than that of commonly targeted iron(II) compounds, and this could mark the beginning of a paradigm change in research into photoactive earth‐abundant metal complexes. After outlining some of the fundamental challenges, key strategies used so far to enhance the photophysical and photochemical properties of iron complexes are discussed and recent conceptual breakthroughs are highlighted in this invited Concept article.

Air‐stable source of phosphorous: Synthetic methods that involve a reductive rearrangement to convert air‐stable pentavalent phosphorus compounds to reactive trivalent phosphorus compounds constitute a powerful tool. The works focuses on tetraphenyldiphosphine disulfide, which is a shelf‐stable solid, and its reductive rearrangement triggers vic‐bisthiophosphinylation of a variety of alkenes and alkynes when exposed to light without requiring any catalyst, base, or additive (see picture).

Abstract

The facile synthesis of organophosphorus compounds is of great importance for the development of new synthetic methods by using air‐stable sources of phosphorus. In this respect, a synthetic method that is based on a reductive rearrangement and is capable of converting air‐stable pentavalent phosphorus compounds into reactive trivalent phosphorus compounds is a powerful tool. Tetraphenyldiphosphine disulfide, which is a shelf‐stable solid, was the focus of this study, and it was shown to undergo reductive rearrangement to trigger the bisthiophosphinylation of a variety of alkenes, such as terminal, cyclic, internal, and branched alkenes, 1,3‐dienes, and terminal alkynes when exposed to light without any catalyst, base, or additive.

Visible-light photoredox catalysis offers a distinct activation mode complementary to thermal transition metal catalyzed reactions. The vast majority of photoredox processes capitalizes on precious metal ruthenium(II) or iridium(III) complexes that serve as single-electron reductants or oxidants in their photoexcited states. As a low-cost alternative, organic dyes are also frequently used but in general suffer from lower photostability. Copper-based photocatalysts are rapidly emerging, offering not only economic and ecological advantages but also otherwise inaccessible inner-sphere mechanisms, which have been successfully applied to challenging transformations. Moreover, the combination of conventional photocatalysts with copper(I) or copper(II) salts has emerged as an efficient dual catalytic system for cross-coupling reactions.

Who is this Lewis anyway? Lewis base‐COS adducts are used for the first time as organocatalysts for the cyclization of COS with propargylic amines/amides to form functionalized 1,3‐thiazolidine‐2‐ones, and 1,3‐thiazolidine‐2,4‐diones derivatives under mild conditions. The generality and functional group tolerance of this protocol is demonstrated.

Abstract

The organocatalytic cyclization of propargylic amines/amides with carbonyl sulfide (COS) was firstly achieved by employing COS adducts of Lewis base (LB) as organocatalysts, affording various functionalized 1,3‐thiazolidine‐2‐ones, and 1,3‐thiazolidine‐2,4‐diones derivatives in a highly chemo‐ and stereoselective manner. The isotope labeling and stoichiometric experiments suggested the LB‐COS adducts preferentially mediated basic ionic pair mechanism. Furthermore, the practical application of this methodology was highlighted by the highly efficient synthesis of rosiglitazone using COS as sulfur source.