Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Thiolation of Terminal Alkynes with Thiuram Disulfide Reagents Using Water as the Hydrogen Source: Stereoselective Synthesis of (Z)-Vinyl Sulfides

A stereoselective and environmentally friendly thiolation of terminal alkynes was reported. Thiuram disulfide reagents (tetramethylthiuram disulfide and tetraethylthiuram disulfide) that reacted with alkynes in dimethyl sulfoxide (DMSO)/H2O could give (Z)-vinyl sulfides in good yields (up to 88%). This protocol features broad substrate scope, good stereoselectivity, high atom economy, good yields, and is transition metal-free. Mechanistic studies revealed that water and DMSO served as hydrogen sources, which greatly highlighted the unique reactivity of this special reaction involving two H-atom donors.

Highly Regioselective Dichalcogenation of Alkenyl Sulfonium Salts to Access 1,1-Dichalcogenalkenes

An unprecedented geminal olefinic dichalcogenation of alkenyl sulfonium salts with dichalcogenides ArYYAr (Y = S, Se, Te) is reported, providing various trisubstituted 1,1-dichalcogenalkenes [Ar¹CH = C(YAr²)₂] in a highly selective manner under mild and catalyst-free conditions. The key process involves the formation of two geminal olefinic C-Y bonds via sequential C-Y cross-coupling and C-H chalcogenation. A mechanistic rationale is further supported by control experiments and density functional theory calculations.

Photochemical halogen-bonding assisted generation of vinyl and sulfur-centered radicals: stereoselective catalyst-free C(sp2)-S bond forming reactions

The combination of photochemistry and halogen bonding interactions has risen in the last few years as a powerful synthetic tool for the creation of radical intermediates under mild conditions. In the formation of carbon-centered radicals, this reactivity has been to date restricted to the employment of aryl and alkyl halides as precursors. We now envisioned that the halogen-bonding initiated formation of highly reactive vinyl radicals would be a feasible process for the photochemical cross-coupling between thiols and alkenyl halides under basic conditions. The reaction shows indeed a very broad functional group tolerance, is stereoselective, simple and scalable. In-depth mechanistic studies point at the formation of vinyl and sulfur-centered radicals as the intermediates of the reaction and DFT calculations support the pre-formation of a halogen-bonding complex as the initiator of the photochemical transformation. Synthetic applications were developed to extend the utility of this methodology.

Copper-catalyzed transformation of alkyl nitriles to N-arylacetamide using diaryliodonium salts

This work reports a simple and efficient method for the copper-catalyzed redox-neutral transformation of alkyl nitriles using eco-friendly diaryliodonium salts and leading to N-arylacetamides. The method features high efficiency, broad substrate scope and good functional group tolerance.

β-C(sp2)–H alkylation of enamides using xanthate chemistry

Access to the γ-amino-β,γ-unsaturated acyl scaffold was established by applying xanthate chemistry to enamides.

Organic amine-mediated free-radical carbocyclization reactions of 2,2,2-trihalogeno-substituted N-(2-alkynylphenyl)acetamides

An efficient method for the synthesis of 3-halogeno-substituted 4-benzoylquinolin-2-(1H)-ones from N-(2-alkynylphenyl)-substituted trihaloacetamides has been developed, in which organic amines (TNPA and DIEA) act as the electron donors. In this carbocyclization reaction, a new C-C bond formation occurred regioselectively via a 6-exo-dig radical cyclization. A variety of useful functional groups are compatible with the reaction conditions. In this process, readily removable organic amines were employed and no heavy metal catalysts were required.

An efficient method for retro-Claisen-type C–C bond cleavage of diketones with tropylium catalyst

We report a new convenient and efficient method utilizing the tropylium ion as a mild and environmentally friendly organocatalyst to mediate retro-Claisen-type reactions.

Regio- and Stereoselective Iron-Catalyzed Oxyazidation of Enamides Using a Hypervalent Iodine Reagent

A novel regio- and diastereoselective iron-catalyzed intermolecular oxyazidation of enamides using various azidobenziodoxolone (ABX) derivatives is presented. A variety of α-N₃ amino derivatives and of α-N₃ piperidines were synthesized in good yields and under mild reaction conditions. The reaction involves a radical process using cheap FeCl₂ as the initiator.