Intramolecular Homolytic Substitution Enabled by Photoredox Catalysis: Sulfur, Phosphorus, and Silicon Heterocycle Synthesis from Aryl Halides

Cyclic Sulfinamides

The literature on cyclic sulfinamides (put simply, sultims) published from 1989 to 2022 has been summarized and reviewed. The information is divided into two sections: the analysis of synthetic methods on the preparation of cyclic sulfinamides and the discussion of the chemical properties of cyclic sulfinamides focusing on their reactions and applications. The survey of the reaction conditions, provided in the most detailed way, and a critical view of the reaction mechanisms add an extra dimension to the text. The data presented will be useful to specialists in different areas, especially those who work in the field of synthetic organic and pharmaceutical chemistry.

Thermoinduced Radical Cyclization for the Synthesis of Sultines

A thermoinduced radical homolytic substitution cyclization of alkenyl tethered sulfinate esters was displayed under mild metal-free conditions, enabling the functionalization of alkenes and leading to structurally diverse value-added sultine products. The process utilizes readily available substrates using inexpensive 5% benzoyl peroxide (BPO) as an initiator to generate functionalized sultines with broad functional group tolerance in medium to excellent yields in a highly atom-economical manner. In addition, the obtained sultines could be further readily functionalized toward valuable sultone frameworks in one pot. A thermo-catalytic radical chain process was proposed based on mechanistic studies.

Recent advances in the synthesis of cyclic sulfinic acid derivatives (sultines and cyclic sulfinamides)

The chemistry of cyclic sulfinic acid derivatives (sultines and cyclic sulfinamides) was underdeveloped for a long time due to their inaccessibility. Considering the importance of cyclic sulfinate esters and amides in the fields of chemistry, pharmaceutical science, and material science, synthesis strategies involving cyclic sulfinic acid derivatives have been paid more attention in recent years, and have been widely used in the synthesis of sulfur-containing compounds such as sulfoxides, sulfones, sulfinates and thioethers. Despite the impressive improvements that have been made in last twenty years with the new strategies, to date, no reviews have been published, to the best of our knowledge, dealing with the preparation of cyclic sulfinic acid derivatives. This review summarizes the latest advances in the development of new synthesis methods to access cyclic sulfinic acid derivatives in the last two decades. The synthetic strategies are reviewed by highlighting their product diversity, selectivity and applicability, and the mechanistic rationale is presented where possible. We wish to bring readers a comprehensive understanding of the state-of-play of cyclic sulfinic acid derivative formation and make a contribution to future research.

Gram-Scale Synthesis of the N-Phenyl Phenothiazine Photocatalyst by Benzyne Addition

N-phenyl phenothiazine is one of the most reducing photoredox catalysts. Its synthesis commonly requires transition metal catalyzed cross-coupling reactions. Here we show the syntheses of four aryl phenothiazines via a benzyne route, including a multi-gram scale synthesis of N-phenyl phenothiazine. While yields are modest, the simplicity, low cost, and lack of requirement for cross-coupling catalysts in this synthesis will be attractive to users of this photocatalyst.

Catalytic aerobic photooxidation of triarylphosphines using dibenzo-fused 1,4-azaborines

Although dibenzo-fused 1,4-heteroaromatics are utilized as strongly reducing photocatalysts in organic synthesis and polymerization, they have rarely been employed in catalytic photooxidation. Moreover, to date, their boron-analogs, dibenzo-fused 1,4-azaborines (DBABs), have not been applied in photocatalysis despite their promising potential as photocatalysts. Accordingly, herein, aerobic photooxidation of triarylphosphines (Ar₃P) was performed using DBABs as photocatalysts. The reaction smoothly proceeded in an aprotic solvent, and phosphine oxides were obtained in appropriate yields. Density functional theory calculations suggested that DBAB captured and activated phosphadioxirane intermediates, which were generated by the interaction of Ar₃P with ¹O₂, at the Lewis acidic boron center.

Tuning the Activity-Stability Balance of Photocatalytic Organic Materials for Oxidative Coupling Reactions

Three materials containing a photoactive unit, 10-phenyl phenothiazine (PTH), have been studied for the visible light-mediated oxidative coupling of amines. In particular, the materials considered are assembled through the condensation of extended polyimine, polyhydrazone, or polytriazine frameworks. These three materials present different stabilities in the presence of strong nucleophiles such as amines, which is a key factor for efficient catalytic performance. In the series of materials reported herein, the triazine-based material shows the optimal compromise between activity and stability when studied for the oxidative coupling of amines, achieving imine products. Accordingly, while significant leaching of molecular active fragments is ruled out for triazine-based polymers, other materials of the series show a significant chemical erosion as a result of the reaction with the amine substrates. Consequently, only a triazine-based material allows performing several catalytic cycles (up to seven) with yields higher than 80%. The applicability of this heterogeneous catalyst has been proven with a variety of substrates, confirming its stability and obtaining diverse imine coupling products with excellent yields.

Facile synthesis of N-aryl phenothiazines and phenoxazines via Brønsted acid catalyzed C-H amination of arenes

N-Aryl phenothiazines and phenoxazines are of significant importance in various disciplines throughout academia and industry. The conventional synthetic strategy for the construction of these structures centers on the transition-metal-catalyzed cross-coupling of aryl halides with phenothiazines or phenoxazines. Here we present an organocatalytic approach to access N-naphthyl phenothiazine and phenoxazine scaffolds through a straightforward C-H amination of arenes as enabled by an azo group. This reaction features operational simplicity, adequate substrate generality and excellent functional group compatibility. Notably, the efficiency of the catalyst could be perfectly preserved after 5 catalytic cycles.

Highly Efficient Production of Heteroarene Phosphonates by Dichromatic Photoredox Catalysis

A new strategy to achieve efficient aerobic phosphorylation of five-membered heteraroenes with excellent yields using dichromatic photoredox catalysis in a gel-based nanoreactor is described here. The procedure involves visible aerobic irradiation (cold white LEDs) of a mixture containing the heteroarene halide, trisubstituted phospite, N,N-diisopropylethylamine (DIPEA) as sacrificial agent, and catalytic amounts of 9,10-dicyanoanthracene (DCA) in the presence of an adequate gelator, which permits a faster process than at the homogeneous phase. The methodology, which operates by a consecutive photoinduced electron transfer (ConPET) mechanism, has been successfully applied to the straightforward and clean synthesis of a number of different heteroarene (furan, thiophene, selenophene, pyrrole, oxazole, or thioxazole) phosphonates, extending to the late-stage phosphonylation of the anticoagulant rivaroxaban. Strategically, employment of cold white light is critical since it provides both selective wavelengths for exciting first DCA (blue region) and subsequently its corresponding radical anion DCA^•- (green region). The resultant strongly reducing excited agent DCA^•-* is capable of even activate five-membered heteroarene halides (Br, Cl) with high reduction potentials (∼-2.7 V) to effect the C(sp²)-P bond formation. Spectroscopic and thermodynamic studies have supported the proposed reaction mechanism. Interestingly, the rate of product formation has been clearly enhanced in gel media because reactants can be presumably localized not only in the solvent pools but also through to the fibers of the viscoelastic gel network. This has been confirmed by field-emission scanning electron microscopy images where a marked densification of the network has been observed, modifying its fibrillary morphology. Finally, rheological measurements have shown the resistance of the gel network to the incorporation of the reactants and the formation of the desired products.

Recent advances in visible light-activated radical coupling reactions triggered by (i) ruthenium, (ii) iridium and (iii) organic photoredox agents

Photoredox chemistry with organic or transition metal agents has been reviewed in earlier years, but such is the pace of progress that we will overlap very little with earlier comprehensive reviews. This review first presents an overview of the area of research and then examines recent examples of C-C, C-N, C-O and C-S bond formations via radical intermediates with transition metal and organic radical promoters. Recent successes with Birch reductions are also included. The transition metal chemistry will be restricted to photocatalysts based on the most widely used metals, Ru and Ir, but includes coupling chemistries that take advantage of low-valent nickel, or occasionally copper, complexes to process the radicals that are formed. Our focus is on developments in the past 10 years (2011-2021). This period has also seen great advances in the chemistry of organic photoredox reagents and the review covers this area. The review is intended to present highlights and is not comprehensive.

N-Arylbenzo[b]phenothiazines as Reducing Photoredox Catalysts for Nucleophilic Additions of Alcohols to Styrenes: Shift towards Visible Light

N-Phenylphenothiazines are an important class of photoredox catalysts because they are synthetically well accessible, they allow the tuning of the optoelectronic properties by different substituents, and they have strong reduction properties for activation of alkenes. One of the major disadvantages of N-phenylphenothiazines, however, is the excitation at 365 nm in the UV-A light range. We synthesized three differently dialkylamino-substituted N-phenylbenzo[b]phenothiazines as alternative photoredox catalysts and applied them for the nucleo­philic addition of alkohols to α-methyl styrene. The additional benzene ring shift the absorbance bathochromically and allows performing the photocatalyses by excitation at 385 nm and 405 nm. This type of photoredox catalysis tolerates other functional groups, as representatively shown for alcohols as substrates with C–C and C–N triple bonds.

Transition-Metal-Free Synthesis of N-Arylphenothiazines through an N- and S-Arylation Sequence

An efficient synthetic method of N-arylphenothiazines from o-sulfanylanilines under transition-metal-free conditions is disclosed. An N- and S-arylation sequence of o-sulfanylanilines enabled us to synthesize a wide variety of N-arylphenothiazines. In particular, one-pot synthesis of N-arylphenothiazines was accomplished from easily available modules through preparation of o-sulfanylanilines by thioamination of aryne intermediates and following N- and S-arylation sequence.

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

Generation of Glycosyl Radicals from Glycosyl Sulfoxides and Its Use in the Synthesis of C-linked Glycoconjugates

We here report glycosyl sulfoxides appended with an aryl iodide moiety as readily available, air and moisture stable precursors to glycosyl radicals. These glycosyl sulfoxides could be converted to glycosyl radicals by way of a rapid and efficient intramolecular radical substitution event. The use of this type of precursors enabled the synthesis of various complex C-linked glycoconjugates under mild conditions. This reaction could be performed in aqueous media and is amenable to the synthesis of glycopeptidomimetics and carbohydrate-DNA conjugates.

A case of chain propagation: α-aminoalkyl radicals as initiators for aryl radical chemistry

The generation of aryl radicals from the corresponding halides by redox chemistry is generally considered a difficult task due to their highly negative reduction potentials. Here we demonstrate that α-aminoalkyl radicals can be used as both initiators and chain-carriers for the radical coupling of aryl halides with pyrrole derivatives, a transformation often employed to evaluate new highly reducing photocatalysts. This mode of reactivity obviates for the use of strong reducing species and was also competent in the formation of sp² C-P bonds. Mechanistic studies have delineated some of the key features operating that trigger aryl radical generation and also propagate the chain process.

Phosphoranyl Radical Fragmentation Reactions Driven by Photoredox Catalysis

Photocatalytic generation of phosphoranyl radicals is fast emerging as an essential method for the generation of diverse and valuable radicals, typically via deoxygenation or desulfurization processes. This Perspective is a comprehensive evaluation of all studies using phosphoranyl radicals as tunable mediators in photoredox catalysis, highlighting how two distinct methods for phosphoranyl radical formation (radical addition and nucleophilic addition) can be used to generate versatile radical intermediates with diverse reactivity profiles.

Photocatalysis with organic dyes: facile access to reactive intermediates for synthesis

Organic dyes have emerged as a reliable class of photoredox catalysts. Their great structural variety combined with the easy fine-tuning of their electronic properties has unlocked new possibilities for the generation of reactive intermediates. In this review, we provide an overview of the available approaches to access reactive intermediates that employ organophotocatalysis. Our contribution is not a comprehensive description of the work in the area but rather focuses on key concepts, accompanied by a few selected illustrative examples. The review is organized along the type of reactive intermediates formed in the reaction, including C(sp3) and C(sp 2 ) carbon-, nitrogen-, oxygen-, and sulfur-centered radicals, open-shell charged species, and sensitized organic compounds.