Mechanism of Metal-Free C–H Activation of Branched Aldehydes and Acylation of Alkenes Using Hypervalent Iodine Compound: A Theoretical Study

Photoexcitation of (diarylmethylene)amino benziodoxolones for alkylamination of styrene derivatives with carboxylic acids

The alkylamination of alkenes using pristine carboxylic acids was achieved by the photoexcitation of (diarylmethylene)amino benziodoxolones (DABXs), which serve as both an oxidant and an aminating reagent (an iminyl radical precursor). The developed method is a simple photochemical reaction without the need for external photosensitizers and shows a broad substrate scope for aliphatic carboxylic acids leading to the formation of primary, secondary, and tertiary alkyl radicals, thus enabling the facile synthesis of various structurally complex amines. Mechanistic investigations including transient absorption spectroscopy measurements using a laser flash photolysis (LFP) method disclosed the unique photochemical reactivity of DABXs, which undergoes homolysis of their I-N bonds to give an iminyl radical and ortho-iodobenzoyloxy radical, the latter of which participates in the single-electron oxidation of carboxylates.

Transition-metal-catalyzed C-H bond alkylation using olefins: recent advances and mechanistic aspects

Transition metal catalysis has contributed immensely to C-C bond formation reactions over the last few decades, and alkylation is no exception. The superiority of such methodologies over traditional alkylation is evident from minimal reaction steps, shorter reaction times, and atom economy while also allowing control over regio- and stereo-selectivity. In particular, hydrocarbonation of alkenes has grabbed increased attention due its fundamental ability to effectively and selectively synthesise a wide range of industrially and pharmaceutically relevant moieties. This review attempts to provide a scientific viewpoint and a systematic analysis of the recent developments in transition-metal-catalyzed alkylation of various C-H bonds using simple and activated olefins. The key features and mechanistic studies involved in these transformations are described briefly.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.

Catalyst-Free Visible-Light-Promoted Cyclization of Aldehydes: Access to 2,5-Disubstituted 1,3,4-Oxadiazole Derivatives

An efficient synthesis of a variety of 2,5-disubstituted 1,3,4-oxadiazole derivatives via a cyclization reaction by photoredox catalysis between aldehydes and hypervalent iodine(III) reagents is described. The reaction proceeds under mild conditions and affords various target compounds in excellent yields. The commercially available aldehydes without preactivation and a simple visible-light-promoted procedure without any catalysts make this strategy an alternative to the conventional methods.

A bench-stable N-trifluoroacetyl nitrene equivalent for a simple synthesis of 2-trifluoromethyl oxazoles

ortho-Nitro-substituted N-trifluoroacetyl imino-λ³-iodane is a bench-stable trifluoroacetyl nitrene precursor, in which intra- and intermolecular halogen bonding (XB) plays an important role. Potential synthetic applications of this novel precursor were explored.

Recent advances in acyl radical enabled reactions between aldehydes and alkenes

Radical-mediated functionalization of alkenes has been emerging as an elegant and straightforward protocol to increase molecule complexity. Moreover, the abstraction of a hydrogen atom from aldehydes to afford acyl radicals has evolved as a rising star due to its high atom-economy and the ready availability of aldehydes. Considering the great influence and synthetic potential of acyl radical enabled reactions between aldehydes and alkenes, we provide a summary of the state of the art in this field with a specific emphasis on the working models and corresponding mechanisms. The discussion is divided according to the kind of alkenes and reaction type.

Merging CF3SO2Na photocatalysis with palladium catalysis to enable decarboxylative cross-coupling for the synthesis of aromatic ketones at room temperature

By merging CF₃SO₂Na-mediated photocatalysis with palladium catalysis, an efficient decarboxylative coupling strategy of α-keto acids and aryl boronic acids has been developed for the synthesis of aromatic ketones.

Development of New Radical-mediated Selective Reactions Promoted by Hypervalent Iodine(III) Reagents

In this account, we describe our recent developments on the four-types of hypervalent iodine(III)-mediated radical reactions in organic synthesis. Firstly, the activation of aldehydic C-H bonds can be successfully effected with hypervalent iodine(III) reagents, thereby allowing the synthesis of various ketones with high efficiency. Secondly, the site-selective oxidation of unactivated C(sp3 )-H bonds of hydrocarbon substrates was realized with designer hypervalent iodine(III) reagents. Thirdly, various perfluoroalkyl and α-aminoalkyl radicals can be generated from sodium perfluoroalkanesulfinates and sodium α-aminoalkanesulfinates, respectively, under the influence of hypervalent iodine(III) reagents. Finally, the efficient generation of difluoromethyl radical from hypervalent difluoroacetoxyliodine(III) reagent was realized by photolysis. These four different strategies are illustrated by using various selective radical approaches.

(Diacetoxyiodo)benzene-Mediated C-H Oxidation of Benzylic Acetals

A useful oxidation of C-H bond of benzylic acetals has been achieved. This method avoids the use of stoichiometric metals and is compatible with the presence of both electron-donating and electron-withdrawing substituents on the aromatic ring. Oxidation was carried out by rapid microwave irradiation of benzylic acetals with PhI(OAc)₂ as the oxidant. This led to the oxidation of acetals into 2-acetoxy-1,3-dioxolanes. Furthermore, this transformation protocol encompasses a wide range of valuable conversions of these useful synthons into different carboxylic acid derivatives.

Photochemical Hydroacylation of Michael Acceptors Utilizing an Aldehyde as Photoinitiator

The hydroacylation of Michael acceptors constitutes a useful tool for the formation of new C-C bonds. In this work, an environmentally friendly procedure was developed, utilizing 4cyanobenzaldehyde as the photoinitiator and household bulbs as the irradiation source. A great variety of substrates was well-tolerated, leading to good yields, and mechanistic experiments were performed to elucidate the catalyst's possible mechanistic pathway. Moreover, the inherent selectivity challenge regarding α,α-disubstituted aldehydes (decarbonylation problem) was studied and addressed.

The mechanism of ring-opening polymerization of L-lactide by ICl3 catalysts: Halogen bond catalysis or participating in reactions?

The mechanism of ring-opening polymerization of L-lactide by iodine trichloride (ICl₃ ) catalyst has been explored by using density functional theory (DFT) calculations and three catalytic pathways were proposed. The first and second pathways belong to the halogen bond catalysis, and the third pathway involves the ICl₃ catalysts participating in reactions. When the carbonyl group was maintained involved in the reaction and activated catalytically by the halogen bond, there are two possible pathways. The first pathway involves only one transition state, and the second pathway requires two transition states. There is another pathway in which ICl₃ directly participates in the reaction, it is named the third pathway. Two different transition states of the four-membered rings are generated successively, the transfer of I─O bonds determined the progress of the reaction. Theoretical calculations in this work provide the most basic understanding of ring-opening polymerization of L-lactide by ICl₃ catalysts. © 2019 Wiley Periodicals, Inc.

Hydrogen Atom Transfer Reactions via Photoredox Catalyzed Chlorine Atom Generation

The selective functionalization of chemically inert C-H bonds remains to be fully realized in achieving organic transformations that are redox-neutral, waste-limiting, and atom-economical. The catalytic generation of chlorine atoms from chloride ions is one of the most challenging redox processes, where the requirement of harsh and oxidizing reaction conditions renders it seldom utilized in synthetic applications. We report the mild, controlled, and catalytic generation of chlorine atoms as a new opportunity for access to a wide variety of hydrogen atom transfer (HAT) reactions owing to the high stability of HCl. The discovery of the photoredox mediated generation of chlorine atoms with Ir-based polypyridyl complex, [Ir(dF(CF₃ )ppy)₂ (dtbbpy)]Cl, under blue LED irradiation is reported.

Direct Aldehyde Csp2 -H Functionalization through Visible-Light-Mediated Photoredox Catalysis

The development of methods for carbon-carbon bond formation under benign conditions is an ongoing challenge for synthetic chemists. In recent years there has been considerable interest in using selective C-H activation as a direct route for generating reactive intermediates. Herein, the use of visible-light-mediated dual photoredox organocatalysis as a mild and effective method for Csp2 -H activation of aldehydes is reported, resulting in the generation of acyl radicals. These nucleophilic acyl radical species can undergo either addition to electrophilic alkenes or nickel-catalyzed cross-coupling reactions to provide a quick access to broad range of unsymmetrical ketones, which are abundantly found in many organic building blocks.

Alkynylation of Csp2 (O)-H Bonds Enabled by Photoredox-Mediated Hydrogen-Atom Transfer

The development of new hydrogen-atom transfer (HAT) strategies within the framework of photoredox catalysis is highly appealing for its power to activate a desired C-H bond in the substrate leading to its selective functionalization. Reported here is the first photoredox-mediated hydrogen-atom transfer method for the efficient synthesis of ynones, ynamides, and ynoates with high regio- and chemoselectivity by direct functionalization of Csp2 (O)-H bonds. The broad synthetic application of this method has been demonstrated by the selective functionalization of C(O)-H bonds within complex molecular scaffolds.

Diastereoselective Radical Hydroacylation of Alkylidenemalonates with Aliphatic Aldehydes Initiated by Photolysis of Hypervalent Iodine(III) Reagents

Diastereoselective radical hydroacylation of chiral alkylidenemalonates with aliphatic aldehydes is realized by the combination of a hypervalent iodine(III) reagent and UV-light irradiation. The reaction is initiated by the photolysis of hypervalent iodine(III) reagents under mild, metal-free conditions, and is the first example of diastereoselective addition of acyl radicals to olefins to afford chiral ketones in a highly stereoselective fashion. The obtained optically active ketones are useful chiral synthons, as exemplified by the short formal synthesis of (-)-methyleneolactocin.