Hypervalent bromine(III) compounds: synthesis, applications, prospects

Hypervalent compounds play a prominent role in homogeneous oxidation catalysis. Despite the higher reactivity of hypervalent bromine compounds when compared to their isoelectronic iodine analogues, the corresponding λ3-bromanes are much less explored. This can be attributed to the discernible lack of convenient strategies for their synthesis. This short review highlights the available methods for the synthesis of various organo-λ3-bromanes, with a major focus on the recent developments and reactivities in the last few years. Additionally, limitations and future prospects of hypervalent bromine chemistry are discussed

Alkene reactions with superoxide radical anions in flow electrochemistry

Alkenes were cleaved to ketones by using dioxygen in a flow electrochemical set-up. The pressurised system allowed efficient gas-liquid mixing with a stabilised flow. This mild and straightforward approach avoids the use of transition metals and harsh oxidants

Electrocatalytic continuous flow chlorinations with iodine(I/III) mediators

Electrochemistry offers tunable, cost effective and environmentally friendly alternatives to carry out redox reactions with electrons as traceless reagents. The use of organoiodine compounds as electrocatalysts is largely underdeveloped, despite their widespread application as powerful and versatile reagents. Mechanistic data reveal that the hexafluoroisopropanol assisted iodoarene oxidation is followed by a stepwise chloride ligand exchange for the catalytic generation of the dichloroiodoarene mediator. Here, we report an environmentally benign iodine(I/III) electrocatalytic platform for the in situ generation of dichloroiodoarenes for different reactions such as mono- and dichlorinations as well as chlorocyclisations within a continuous flow setup

Copper mediated decarboxylative direct C-H arylation of heteroarenes with benzoic acids

Decarboxylative coupling reactions to date require a stoichiometric oxidant (such as copper and silver salts) for decarboxylation purposes along with a metal catalyst (e.g. palladium) for cross-coupling. In this communication, an economic and sustainable approach by using a simple copper salt was developed in the presence of molecular oxygen as the sole oxidant. A wide range of 5-membered heteroarenes undergo aryl–heteroaryl cross-coupling with electron deficient aryl carboxylic acids

Photosensitized intermolecular carboimination of alkenes through the persistent radical effect

An intermolecular, two‐component vicinal carboimination of alkenes has been accomplished by energy transfer catalysis. Oxime esters of alkyl carboxylic acids were used as bifunctional reagents to generate both alkyl and iminyl radicals. Subsequently, addition of the alkyl radical to an alkene generates a transient radical for selective radical–radical cross‐coupling with the persistent iminyl radical. Furthermore, this process provides direct access to aliphatic primary amines and α‐amino acids by simple hydrolysis

Oxidative cleavage of alkenes by photosensitized nitroarenes

Oxidative cleavage of alkenes into carbonyl molecules mainly relies on either ozonolysis or Lemieux-Johnson oxidation involving high valent transition metal oxides. Safety, technical concerns and highly oxidizing conditions of both these procedures limited their adoption in streamlined synthesis. Like ozone, photosensitized nitroarenes can deliver similar types of [3+2] cycloaddition products with alkenes through biradical formation and the resulting “N-doped” ozonides can safely be converted to the corresponding carbonyl compounds through hydrolysis. The high prevalence of nitroarenes with diverse electronic and steric profiles combined with the mild oxidizing power allow to modulate site-selectivity and tolerate highly sensitive functional groups ideal for application in complex molecular setup

Cooperative catalysis: a strategy to synthesize trifluoromethyl-thioesters from aldehydes

A cooperative catalysis between a photoredox and a hydrogen atom transfer (HAT) catalyst has been developed to synthesize trifluoromethylthioesters from aldehydes. This reaction is mild, highly selective, operationally simple, works under redox neutral condition, and exhibits a broad substrate scope with high functional group tolerance. The synthetic utility of this method is demonstrated by the late-stage functionalization of bioactive molecules, making it amenable for drug discovery

Metal-mediated deformylation reactions: synthetic and biological avenues

No two ways about it: The title reaction is immensely important in synthesis and biology. Whereas biological systems oxygenate aldehydes to generate formate and alkanes or alkenes, synthetic deformylation reactions primarily rely on rapid oxidative addition into the C(O)H bond and subsequent rate‐determining extrusion of CO

Recent Developments in Hydrodecyanation and Decyanative Functionalization Reactions

Nitrile is one of the ubiquitous functional groups in natural products and polymer industry. It is often used as a versatile building block in synthetic chemistry. Classically, nitrile group is used for alpha functionalization, ortho−C−H activation and as a precursor for amine or carbonyl functionalities. With the development of various transition metal catalyzed methods, in the last two decades, nitrile group has emerged as a source of carbon synthons through C−CN bond activation despite of its high bond energy. In this review we have summarized all recent developments involving the carbon synthons arising from the cleavage of C−CN bonds. Depending on the fate of the carbon center after cleavage, all the reactions are classified in two major categories for the ease of discussion: 1) decyanation (removal of the nitrile group) and 2) decyanative functionalization (replacement of the nitrile group). Finally, current limitations in C−CN bond activation strategies and future prospects are discussed. © 2021 Wiley-VCH Gmb

Group 9 Transition metal‐catalyzed C−H halogenations

The high importance of organic halides as synthetic precursors has led to the development of milder and environmentally benign methods for their synthesis. In this regard, transition metal catalyzed C−H activation has emerged as one of the most promising methods for the synthesis of organic halides with high atom economy and excellent stereo‐ and regio‐control. Despite the dominance of palladium and copper catalysts in the field of C−H halogenation reactions, iridium‐, rhodium‐ and cobalt‐complexes have also recently been employed as highly efficient catalysts for the formation of carbon‐halogen bonds. This review describes the current state of the art in the field of C−H halogenation reactions using group nine transition metal (Co, Rh, Ir) catalysts