LongLarf

Publication date: 30 July 2021

Source: Tetrahedron, Volume 93

Author(s): Xu-Yan Wang, Yigao Li, Linlin Shi, Xinju Zhu, Xin-Qi Hao, Mao-Ping Song

Publication date: 16 July 2021

Source: Tetrahedron, Volume 92

Author(s): Di Wang, Fu-Rong Cao, Guangying Lu, Jiangmeng Ren, Bu-Bing Zeng

Synthesis
DOI: 10.1055/a-1485-4666

Aryl rings are ubiquitous in the core of numerous natural product and industrially important molecules and thus their facile synthesis is of major interest in the scientific community and industry. Although multiple strategies enable access to these skeletons, metal-catalyzed C–H activation is promising due to its remarkable efficiency. Commercially available organoboron reagents, a prominent arylating partner in the cross-coupling domain, have also been utilized for direct arylation. Organoborons are bench-stable, inexpensive, and readily available coupling partners that promise regioselectivity, chemodivergence, cost-efficiency, and atom-economy without requiring harsh and forcing conditions. This critical, short review presents a summary of all major studies of arylation using organoborons in transition-metal catalysis since 2005.1 Introduction2 Arylation without Directing Group Assistance2.1 Palladium Catalysis2.2 Iron Catalysis2.3 Gold Catalysis3 Arylation with Directing Group Assistance3.1 Palladium Catalysis3.2 Ruthenium Catalysis3.3 Rhodium Catalysis3.4 Nickel Catalysis3.5 Cobalt Catalysis3.6 Copper Catalysis4 Conclusion
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

Article in Thieme eJournals:
Table of contents  |  Abstract  |  Full text

Hydrogenation and deuteration of olefins made easy: A core–shell-structured Ni-nanocatalyst (Ni NPs@C) allows the selective hydrogenation and deuteration of alkenes under ambient conditions (1 bar H₂/D₂, room temperature).

Abstract

A general protocol for the selective hydrogenation and deuteration of a variety of alkenes is presented. Key to success for these reactions is the use of a specific nickel-graphitic shell-based core–shell-structured catalyst, which is conveniently prepared by impregnation and subsequent calcination of nickel nitrate on carbon at 450 °C under argon. Applying this nanostructured catalyst, both terminal and internal alkenes, which are of industrial and commercial importance, were selectively hydrogenated and deuterated at ambient conditions (room temperature, using 1 bar hydrogen or 1 bar deuterium), giving access to the corresponding alkanes and deuterium-labeled alkanes in good to excellent yields. The synthetic utility and practicability of this Ni-based hydrogenation protocol is demonstrated by gram-scale reactions as well as efficient catalyst recycling experiments.

Nature, Published online: 01 June 2021; doi:10.1038/d41586-021-01431-y

Ta-DA! The Diels-Alder (DA) reaction of electron-rich furans with electron-poor alkenes has been a benchmark for the preparation of complex scaffolds such as 7-oxanorbornenes. Recently this technology has been used to prepare aromatics from bio-based furanic platforms. The recently described direct DA reaction of bio-based furfurals expands the scope of bioaromatic preparation to afford key biomass derivatives.

Abstract

The preparation of high value-added chemicals from renewable resources is a crucial approach towards a sustainable economy. One prominent alternative to the production of petroleum-based chemicals from fossil resources is through the sequential Diels-Alder/aromatization reactions of biomass-derived furan platforms. This Concept is focused on the recent boom in bio-based furan DA strategies for aromatization of bio-based platform chemicals, particularly that of furfurals, ranging from indirect use and activation strategies to recent examples of direct DA reaction of these electron-withdrawing biomass-derived furans.

Efficient demethylation of methyl phenyl ethers was achieved by biocatalytic O₂-free methyl transfer to thiols forming thio ethers. Since the methyl group is not cleaved from the sulfur under these conditions, the thiol compound acts like a trap driving the reaction towards completion.

Abstract

Demethylating methyl phenyl ethers is challenging, especially when the products are catechol derivatives prone to follow-up reactions. For biocatalytic demethylation, monooxygenases have previously been described requiring molecular oxygen which may cause oxidative side reactions. Here we show that such compounds can be demethylated anaerobically by using cobalamin-dependent methyltransferases exploiting thiols like ethyl 3-mercaptopropionate as a methyl trap. Using just two equivalents of this reagent, a broad spectrum of substituted guaiacol derivatives were demethylated, with conversions mostly above 90 %. This strategy was used to prepare the highly valuable antioxidant hydroxytyrosol on a one-gram scale in 97 % isolated yield.

A high yielding and scalable metal-free four-step linear domino reaction results in functionalized triarylbenzene (TAB) and opens the door for making a broad variety of synthetically challenging hexaarylbenzene (HAB) molecules with six different aryl groups.

Abstract

Hexaarylbenzene (HAB) derivatives are versatile aromatic systems playing a significant role as chromophores, liquid crystalline materials, molecular receptors, molecular-scale devices, organic light-emitting diodes and candidates for organic electronics. Statistical synthesis of simple symmetrical HABs is known via cyclotrimerization or Diels–Alder reactions. By contrast, the synthesis of more complex, asymmetrical systems, and without involvement of statistical steps, remains an unsolved problem. Here we present a generally applicable synthetic strategy to access asymmetrical HAB via an atom-economical and high-yielding metal-free four-step domino reaction using nitrostyrenes and α,α-dicyanoolefins as easily available starting materials. Resulting domino product—functionalized triarylbenzene (TAB)—can be used as a key starting compound to furnish asymmetrically substituted hexaarylbenzenes in high overall yield and without involvement of statistical steps. This straightforward domino process represents a distinct approach to create diverse and still unexplored HAB scaffolds, containing six different aromatic rings around central benzene core.

Anion catalysis: Hydrofunctionalisations of various substrates, including, imines, esters and CO₂ are effectively initiated by lithium phosphacyclohexadienyl compounds. High selectivities and excellent TOFs were found, rivalling those of previously reported organic and alkoxide anions.

Abstract

The ability of phosphacyclohexadienyl anions [Li(1-R-PC₅Ph₃H₂)] [R=Me (1 a), nBu (1 b), tBu (1 c), Ph (1 d) and CH₂SiMe₃ (1 e)] to initiate hydrofunctionalisation reactions was investigated and compared with simple, commercially available compounds, such as LiOtBu, KOtBu and nBuLi. All compounds are expedient catalysts for the hydroboration of a wide scope of substrates, ranging from aldehydes to imines and esters. In the hydroboration of carbon dioxide, however, only our system was observed to efficiently produce the desired methanol equivalents.

A new, transition-metal-free, effective method for aromatic esterification of amides with organoboronic acids has been developed, leading to a wide range of benzoate derivatives with yields ranging from moderate to good. The catalytic reaction shows broad substrate scope and excellent functional group tolerance.

Abstract

A new, transition-metal-free, effective method for aromatic esterification of amides with organoboronic acids has been developed. A wide range of benzoate derivatives were obtained with yields ranging from moderate to good. The catalytic reaction shows a broad substrate scope and excellent functional group tolerance. Conceptually, DMAP mediates the reaction and is crucial for this transformation.