MRV

Nature Chemistry, Published online: 23 March 2021; doi:10.1038/s41557-021-00640-2

Excising hydrogen adjacent to a carbonyl group—one of the most basic and widely employed transformations in organic synthesis—is traditionally achieved using metals and/or stoichiometric oxidants. Now, it has been shown that an electrochemically driven approach removes such requirements, resulting in a more sustainable and easily scalable method with wide substrate scope.

The generation of Ni^III and excited Ni^II intermediates facilitates the reductive elimination step leading to the mild cross-couplings. This review provides an overview of the state-of-the-art approaches for mild C-heteroatom bond formations via homo- and heterogeneous photoredox and nickel dual catalysis, electro- and nickel dual catalysis, as well as conventional zinc and nickel dual catalysis.

Abstract

The formation of C-heteroatom bonds represents an important type of bond-forming reaction in organic synthesis and often provides a fast and efficient access to privileged structures found in pharmaceuticals, agrochemical and materials. In contrast to conventional Pd- or Cu-catalyzed C-heteroatom cross-couplings under high-temperature conditions, recent advances in homo- and heterogeneous Ni-catalyzed C-heteroatom formations under mild conditions are particularly attractive from the standpoint of sustainability and practicability. The generation of Ni^III and excited Ni^II intermediates facilitate the reductive elimination step to achieve mild cross-couplings. This review provides an overview of the state-of-the-art approaches for mild C-heteroatom bond formations and highlights the developments in photoredox and nickel dual catalysis involving SET and energy transfer processes; photoexcited nickel catalysis; electro and nickel dual catalysis; heterogeneous photoredox and nickel dual catalysis involving graphitic carbon nitride (mpg-CN), metal organic frameworks (MOFs) or semiconductor quantum dots (QDs); as well as more conventional zinc and nickel dual catalyzed reactions.

Electrochemical approaches to the direct arylation of carbonyls and alcohols through less‐explored cathodic reduction and convergent paired electrolysis are presented. This protocol features: excellent functional group (including ester, amide, amine, thioether, borate) tolerance, mild conditions (metal catalyst‐ and external reductant‐free), good scalability (>10 gram‐scale), and site‐selectivity.

Abstract

Arylation of carbonyls, one of the most common approaches toward alcohols, has received tremendous attention, as alcohols are important feedstocks and building blocks in organic synthesis. Despite great progress, there is still a great gap to develop an ideal arylation method featuring mild conditions, good functional group tolerance, and readily available starting materials. We now show that electrochemical arylation can fill the gap. By taking advantage of synthetic electrochemistry, commercially available aldehydes (ketones) and benzylic alcohols can be readily arylated to provide a general and scalable access to structurally diverse alcohols (97 examples, >10 gram‐scale). More importantly, convergent paired electrolysis, the ideal but challenging electrochemical technology, was employed to transform low‐value alcohols into more useful alcohols. Detailed mechanism study suggests that two plausible pathways are involved in the redox neutral α‐arylation of benzylic alcohols.

Shine on me! Photocatalytic Giese‐type reactions with alkylsilicates bearing C,O‐bidentate ligands as alkyl radical precursors are reported. Not only primary, secondary, and tertiary alkyl radicals, but also elusive methyl radicals, can be generated under the present reaction system. This radical generation process is investigated by theoretical calculations.

Abstract

Herein, a photocatalytic Giese‐type reaction with alkylsilicates bearing C,O‐bidentate ligands as stable alkyl radical precursors has been reported. The alkylsilicates were prepared in one step from organometallic reagents. Not only primary, secondary, and tertiary alkyl radicals, but also elusive methyl radicals, could be generated by using the present reaction system. The generated radicals were trapped by electron‐deficient olefins bearing various functional groups to give the desired alkyl adducts. The silicon byproduct can be recovered after the photoreaction. The radical generation process was investigated by theoretical calculations, which provided an insight into the facile generation of methyl radicals from methylsilicate bearing C,O‐bidentate ligands.

In cross‐coupling reactions, chemoselectivity becomes a concern when two or more (pseudo)halides are present in the substrates. This minireview surveys cases in which divergent chemoselectivity between electrophiles can be achieved under different reaction conditions. Particular emphasis is placed on discussing the possible mechanistic origins of selectivity control.

Abstract

Chemodivergent cross‐couplings are those in which either one of two (or more) potentially reactive functional groups can be made to react based on choice of conditions. In particular, this review focuses on cross‐couplings involving two different (pseudo)halides that can compete for the role of the electrophilic coupling partner. The discussion is primarily organized by pairs of electrophiles including chloride vs. triflate, bromide vs. triflate, chloride vs. tosylate, and halide vs. halide. Some common themes emerge regarding the origin of selectivity control. These include catalyst ligation state and solvent polarity or coordinating ability. However, in many cases, further systematic studies will be necessary to deconvolute the influences of metal identity, ligand, solvent, additives, nucleophilic coupling partner, and other factors on chemoselectivity.

A Pd⁰‐catalyzed highly regio‐ and enantioselective [3+2] spiroannulation reaction has been developed for rapid assembly of [5,5] spirocyclic carbo‐ and heterocycles. The regioselectivity could be dominated by fine‐tuning the Pd‐π‐allyl intermediate. An array of coupling partners could be well‐tolerated with excellent regio‐, and enantioselectivities. Potential application of the reaction was exemplified by several further transformations.

Abstract

A novel Pd⁰‐catalyzed highly regio‐ and enantioselective [3+2] spiroannulation reaction has been developed for rapid assembly of a new class of [5,5] spirocyclic carbo‐ and heterocycles. Notably, the regioselectivity could be dominated by fine‐tuning of the Pd‐π‐allyl intermediate. An array of coupling partners could be well‐tolerated with excellent regio‐, and enantioselectivities. Moreover, the potential application of this reaction was exemplified by several further transformations.

Abstract

In recent years, great improvements have been made on palladium‐catalyzed radical reactions. Emerging elegant methodologies of radical involved palladium‐catalyzed transformations provide more and more effective strategies for the construction of complex heterocyclic compounds and applications in drug discovery. It is universally known that Pd(0)/(II)/(IV)‐mediated reactions usually undergo two‐electron transfer processes, while the Pd(I) and Pd(III) involved reactions usually occur via single electron transfer processes. Since our review on palladium radical was published, numurous methodolgies involving palladium radical have sprung up in the past five years. This review further summarized the up‐to‐date transformations toward palladium radical from 2015. For most of these catalytic cycles strategies, plausible mechanisms are demonstrated in detail to give better insight for chemists in need.

Oxidation is a major chemical process to produce oxygenated chemicals in both nature and chemical industry. Currently, industrial deep oxidation processes from polyalkyl benzene are major routes to produce benzoic acids and benzene polycarboxylic acids (BPCAs), while to some extent suffering from the energy-intensive and potentially hazardous drawbacks and the sluggish separation issues. In this report, visible-light-induced deep aerobic oxidation of (poly)alkyl benzene to benzene (poly)carboxylic acids was developed. CeCl3 was proved to be an efficient HAT (Hydrogen Atom Transfer)catalyst in the presence of alcohol as both hydrogen and electron shuttle. Dioxygen (O2) was found as a sole terminal oxidant. In most cases, pure products were easily isolated by simple filtration, showing the advantages of for scaling up. The reaction provides an ideal way to form valuable fine chemicals from abundant petroleum feedstocks.

Arylation of carbonyls, as one of the most common synthetic approaches toward the preparation of alcohols, has received sustained attention since alcohols are important feedstock and building blocks in organic synthesis. Despite recent progress in this field, there remains a considerable challenge to develop an ideal arylation method which features: mild conditions, good functional group tolerance and readily available starting materials. Herein we show that electrochemical arylation can meet this challenge. By taking advantage of synthetic electrochemistry, commercially available aldehydes (ketones) and benzylic alcohols can be readily arylated   to provide a general and scalable access to structurally diverse alcohols (97 examples, >10 gram‐scale). More importantly, convergent paired electrolysis, an ideal but challenging electrochemical technology, has been employed for the first time to transform low value alcohols into more useful alcohols. Detailed mechanistic study suggests that two plausible pathways may be involved in the redox neutral  α ‐arylation   of benzylic alcohols.

Synthesis
DOI: 10.1055/s-0040-1706085

Traditionally, metal-catalyzed cross-coupling reactions rely on stable but expensive metals, such as palladium. However, the recent development of synthetic organic electrochemistry allows for in situ redox manipulations, expanding the use of cheaper, abundant and sustainable metals, such as nickel and copper as efficient cross-coupling catalysts. This short review covers the recent advances in metal-catalyzed electrochemical coupling reactions, with a focus on reactions of sp2 electrophiles and nucleophiles with sp3 coupling partners to form both C–C and C–heteroatom bonds.1 Introduction2 Nickel-Catalyzed C–C sp2–sp3 Coupling Reactions3 Coupling of Aryl Groups with Heteroatomic Nuclei4 Conclusion
[...]

Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart, Germany

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

Abstract

Palladium(II) precatalysts are used extensively to facilitate cross‐coupling reactions because they are bench stable and give high activity. As a result, precatalysts such as Buchwald's palladacycles, Organ's PEPPSI species, Nolan's allyl‐based complexes, and Yale's 1‐tert‐butylindenyl containing complexes, are all commercially available. Comparing the performance of the different classes of precatalysts is challenging because they are typically used under different conditions, in part because they are reduced to the active species via different pathways. However, within a particular class of precatalyst, it is easier to compare performance because they activate via similar pathways and are used under the same conditions. Here, we evaluate the activity of different allyl‐based precatalysts, such as (η³‐allyl)PdCl(L), (η³‐crotyl)PdCl(L), (η³‐cinnamyl)PdCl(L), and (η³‐1‐tert‐butylindenyl)PdCl(L) in Suzuki‐Miyaura reactions. Specifically, we evaluate precatalyst performance as the ancillary ligand (NHC or phosphine), reaction conditions, and substrates are varied. In some cases, we connect relative activity to both the mechanism of activation and the prevalence of the formation of inactive palladium(I) dimers. Additionally, we compare the performance of in situ generated precatalysts with commonly used palladium sources such as tris(dibenzylideneacetone)dipalladium(0) (Pd₂dba₃), bis(acetonitrile)dichloropalladium(II) (Pd(CH₃CN)₂Cl₂), and palladium acetate. Our results provide information about which precatalyst to use under different conditions.