Transition-Metal Free Chemoselective Hydroxylation and Hydroxylation–Deuteration of Heterobenzylic Methylenes

Site-selective benzylic C-H hydroxylation in electron-deficient azaheterocycles

Benzylic C-H bonds can be converted into numerous functional groups, often by mechanisms that involve hydrogen atom transfer as the key bond breaking step. The abstracting species is most often an electrophilic radical, which makes these reactions best suited to electron-rich C-H bonds to achieve appropriate polarity matching. Thus, electron deficient systems such as pyridine and pyrimidine are relatively unreactive, and therefore underrepresented in substrate scopes. In this report, we describe a new method for heterobenzylic hydroxylation-essentially an unknown reaction in the case of pyrimidines-that makes use of an iodine(III) reagent to afford very high selectivity towards electron-deficient azaheterocycles in substrates with more than one reactive position and prevents over-oxidation to carbonyl products. The identification of key reaction byproducts supports a mechanism that involves radical coupling in the bond forming step.

Moisture-resistant radical anions of quinoxalin-2(1H)-ones in aerial dioxygen activation

This study demonstrates the successful formation of a radical anion intermediate in a moist atmosphere, facilitating chemical reactions by activating aerial dioxygen through a single electron transfer (SET) mechanism. Derived from deprotonating quinoxaline-2(1H)-one with KOtBu, it shows potential in oxygenation chemistry. Validation comes from radical scavenging and EPR experiments.

Visible Light Mediated Chemoselective Hydroxylation of Benzylic Methylenes

We have developed a metal-free photocatalytic selective hydroxylation of benzylic methylenes to secondary alcohols. This approach utilizes low-cost eosin Y as photocatalyst, O2 as green oxidant, and inexpensive triethylamine as inhibitor for overoxidation. The mild reaction conditions enable the production of secondary alcohols with 56-95% yields, making it a promising and environmental-friendly method for the synthesis of secondary alcohols from benzylic methylenes.

Growing Utilization of Radical Chemistry in the Synthesis of Pharmaceuticals

Our current unhealthy lifestyle and the exponential surge in the population getting affected by a variety of diseases have made pharmaceuticals or drugs an imperative part of life, making the development of innovative strategies for drug discovery or the introduction of refined, cost-effective and modern technologies for the synthesis of clinically used drugs, a need of the hour. Ever since their discovery, free radicals and radical cations or anions as reactive intermediates have captivated the chemists, resulting in an exceptional utilization of these moieties throughout the field of chemical synthesis, owing to their unprecedented and widespread reactivity. Sticking with the idea of not judging the book by its cover, despite the conventional thought process of radicals being unstable and difficult to control entities, scientists and academicians around the globe have done an appreciable amount of work utilizing both persistent as well as transient radicals for a variety of organic transformations, exemplifying them with the synthesis of significant biologically active pharmaceutical ingredients. This review truly accounts for the organic radical transformations including radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling with metal-complexes and radical cations coupling with nucleophiles, that offers fascinating and unconventional approaches towards the construction of intricate structural frameworks of marketed APIs with high atom- and step-economy; complementing the otherwise employed traditional methods. This tutorial review presents a comprehensive package of diverse methods utilized for radical generation, featuring their reactivity to form critical bonds in pharmaceutical total synthesis or in building key starting materials or intermediates of their synthetic journey, acknowledging their excellence, downsides and underlying mechanisms, which are otherwise poorly highlighted in the literature. Despite great achievements over the past few decades in this area, many challenges and obstacles are yet to be unraveled to shorten the distance between the academics and the industry, which are all discussed in summary and outlook.

One-Pot, Five-Component Condensation Reaction of Isatin, Secondary Amines, Malononitrile, Alcohols, and Molecular Oxygen to Access 3-Functionalized 2-Oxindoles

An efficient five-component condensation reaction of isatin, malononitrile, secondary amines, alcohols, and molecular oxygen was discovered. The reaction was performed in a one-pot fashion, and it does not require any metal catalyst. It gives straightforward access to structurally diverse 2-oxo-3-aminoindoline-3-carboxylates in moderate yields (70-88%). The scope of the reaction was successfully demonstrated by synthesizing a series of 3-functionalized 2-oxindoles by varying the isatin, amine, and alcohol components.

Synthesis of Naphthalimides through Tandem Pd(II)-Catalyzed C(sp3)-H Oxidation and Diels-Alder Reaction Using a Transient Directing Group Strategy

Naphthalimides have found extensive applications in materials science and pharmaceuticals. It is still highly desirable to develop efficient methods for the synthesis of naphthalimides with structural diversity. In this work, we developed a new approach for the synthesis of naphthalimides via a tandem reaction of o-methylbenzaldehydes and maleimides. The tandem reaction involves Pd(II)-catalyzed benzylic C(sp³)-H oxidation using an amino acid as the transient directing group and Diels-Alder reaction. The subsequent dehydration forms naphthalimides. The reaction introduces the imide moiety and constructs a benzene ring simultaneously, allowing for easy access to a range of naphthalimides with a variety of substituents.

Cobalt(III)-Catalyzed and DMSO-Involved Allylation of 1,3-Dicarbonyl Compounds with Alkenes

Cobalt(III)-catalyzed allylation of 1,3-dicarbonyl compounds has been reported with in situ generated allyl reagents from alkenes and dimethyl sulfoxide (DMSO). This novel protocol enables a high regio- and stereoselective access for a broad range of allyl 1,3-dicarbonyl compounds. In the transformation, DMSO plays the role of a C1 source, and it incorporates with alkenes to form the allyl reagent allylic methyl thioether. Moreover, a multiple-step pathway has been proposed to rationalize the mechanism study, which involves silver-mediated coupling, Co(III)-catalyzed π-allylation, and intermolecular nucleophilic substitution.

Iodine-Promoted Controlled and Selective Oxidation of (Aryl)(Heteroaryl)Methanes

The development of direct and controlled oxidation of C_(sp3)-H bonds is of great importance. Herein, an iodine-catalyzed controlled oxidation of (aryl)(heteroaryl)methanes to (aryl)(heteroaryl)methanols is disclosed under metal-free reaction conditions. A catalytic system comprised of iodine/silyl chloride with HI as an additive in the presence of dimethyl sulfoxide selectively oxidizes the C_(sp3)-H bonds without being overoxidized to corresponding ketones. Therapeutically important aryl heteroaryl methanol derivatives were obtained in good yields. The preliminary mechanistic investigation proves that the primary source of oxygen is DMSO.

Recent Developments for the Deuterium and Tritium Labeling of Organic Molecules

Organic compounds labeled with hydrogen isotopes play a crucial role in numerous areas, from materials science to medicinal chemistry. Indeed, while the replacement of hydrogen by deuterium gives rise to improved absorption, distribution, metabolism, and excretion (ADME) properties in drugs and enables the preparation of internal standards for analytical mass spectrometry, the use of tritium-labeled compounds is a key technique all along drug discovery and development in the pharmaceutical industry. For these reasons, the interest in new methodologies for the isotopic enrichment of organic molecules and the extent of their applications are equally rising. In this regard, this Review intends to comprehensively discuss the new developments in this area over the last years (2017-2021). Notably, besides the fundamental hydrogen isotope exchange (HIE) reactions and the use of isotopically labeled analogues of common organic reagents, a plethora of reductive and dehalogenative deuteration techniques and other transformations with isotope incorporation are emerging and are now part of the labeling toolkit.

Molecular oxygen-mediated oxygenation reactions involving radicals

Molecular oxygen as a green, non-toxic and inexpensive oxidant has displayed lots of advantages compared with other oxidants towards more selective, sustainable, and environmentally benign organic transformations. The oxygenation reactions which employ molecular oxygen or ambient air as both an oxidant and an oxygen source provide an efficient route to the synthesis of oxygen-containing compounds, and have been demonstrated in practical applications such as pharmaceutical synthesis and late-stage functionalization of complex molecules. This review article introduces the recent advances of radical processes in molecular oxygen-mediated oxygenation reactions. Reaction scopes, limitations and mechanisms are discussed based on reaction types and catalytic systems. Conclusions and perspectives are also given in the end.

Base-Mediated Anti-Markovnikov Hydroamidation of Vinyl Arenes with Arylamides

We investigated a base-promoted protocol for the intermolecular anti-Markovnikov hydroamidation of vinyl arenes with arylamides to furnish the arylethylbenzamides with excellent chemo- and regioselectivity. The reaction tolerates an extensive variety of functional groups and has been successfully extended with electronically varied handles, aminobenzamides, electron-rich/electron-deficient heterocyclic amides, and vinyl arenes to afford the hydroamidated products. Excellent chemoselectivity was observed for the amide group over amine. The proposed mechanism and vital role of the solvent was well supported by deuterium labeling studies and control experiments.

Transition-metal-free decarbonylative alkylation towards N-aryl α-hydroxy amides via triple C–C bond cleavages and their selective deuteration

A transition-metal-free decarbonylative alkylation reaction for the synthesis of N-aryl α-hydroxy amides via precise cleavages and reorganizations of three C–C σ bonds has been developed.