Synthesis of N-alkylated 2-pyridones through Pummerer type reactions of activated sulfoxides and 2-fluoropyridine derivatives

Nickel-Catalyzed Reductive Arylation of α-Bromo Sulfoxide

A nickel-catalyzed cross-electrophile coupling of aryl iodides with α-bromo sulfoxide to access a diverse array of aryl benzyl sulfoxides has been discovered. These reactions occurred under mild conditions with excellent functional group tolerance so that optically enriched sulfoxides could be coupled with aryl iodides, generating corresponding sulfoxides with excellent stereochemical integrity. Furthermore, the scalability of this transformation was demonstrated. Initial mechanistic studies revealed that the reaction undergoes a radical pathway.

Mechanistic Insights into Tf2O-Promoted Electrophilic Activation of 2-Propynamides and a New Synthesis of 2,4-Disubstituted Quinolines

Direct evidence explaining why 2-propynamides have never been used as substrates in Tf₂O-promoted electrophilic activations was obtained. Furthermore, a new method for the synthesis of structurally special 2,4-disubstituted quinolines was developed, by which the substituent at position 2 of quinolines can be diversified easily.

Coordination effect enabled palladium-catalyzed regioselective O-alkylation of 2-pyridones

A novel tactic for the regioselective O-alkylation of 2-pyridones has been realized through palladium catalysis in moderate to high yields. The coordination effect between palladium and nitrogen on the pyridine ring plays a versatile role.

Rapid Photoracemization of Chiral Alkyl Aryl Sulfoxides

The photoracemization of chiral alkyl aryl sulfoxides with a photosensitizer has not been sufficiently investigated thus far. Therefore, in this study, a rapid photoracemization reaction of enantiopure alkyl aryl sulfoxides using 1 mol % 2,4,6-triphenylpyrylium tetrafluoroborate (TPT⁺) was developed. Various substitution patterns were tolerated and every racemization reaction proceeded extremely fast (k₂ = 1.77 × 10⁴-6.08 × 10¹ M^-1 s^-1, t_1/2 = 0.4-114 s). Some chiral sulfoxides with easily oxidizable functional groups are not appropriate for this photoisomerization. The electrochemical potentials of the functional groups, determined via cyclic voltammetry, are useful for predicting the reactive or nonreactive groups in this photoracemization reaction. A theoretical study was conducted to clarify the sp²-like nature of S of the sulfoxide cation radical, which makes photoracemization easier.

Synthesis of 1-(1-Arylvinyl)pyridin-2(1H)-ones from Ketones and 2-Fluoropyridine

We describe a simple and efficient procedure for the synthesis of N-vinyl-substituted pyridones from ketones and 2-fluoropyridine in the presence of trifluoromethane sulfonic anhydride, followed by a base treatment. Various ketones with electron-donating or electron-withdrawing groups at the benzene rings can be used in this reaction. A preliminary mechanistic study indicates that it is not very likely that both vinyl triflates and vinyl cations play major roles as intermediates in this transformation.

Rhodium(III)-Catalyzed C-H Bond Functionalization of 2-Pyridones with Alkynes: Switchable Alkenylation, Alkenylation/Directing Group Migration and Rollover Annulation

Cp*Rh(III)-catalyzed chelation-assisted direct C-H bond functionalization of 1-(2-pyridyl)-2-pyridones with internal alkynes that can be controlled to give three different products in good yields has been realized. Depending on the reaction conditions, solvents and additives, the reaction pathway can be switched between alkenylation, alkenylation/directing group migration and rollover annulation. These reaction manifolds allow divergent access to a variety of valuable C6-alkenylated 1-(2-pyridyl)-2-pyridones, (Z)-6-(1,2-diaryl-2-(pyridin-2-yl)vinyl)pyridin-2(1H)-ones and 10H-pyrido[1,2-a][1,8]naphthyridin-10-ones from the same starting materials. These protocols exhibit excellent regio- and stereoselectivity, broad substrate scope, and good tolerance of functional groups. A combination of experimental and computational approaches have been employed to uncover the key mechanistic features of these reactions.

α-C–H difluoroalkylation of alkyl sulfoxides via intermolecular Pummerer reaction

A simple and practical intermolecular Pummerer reaction using difluoroenol silyl ethers as nucleophiles is described. The protocol allows for highly efficient α-difluoroalkylation of a wide spectrum of alkyl sulfoxides.

Rhodium(i)-catalyzed C6-selective C-H alkenylation and polyenylation of 2-pyridones with alkenyl and conjugated polyenyl carboxylic acids

A versatile Rh(i)-catalyzed C6-selective decarbonylative C-H alkenylation of 2-pyridones with readily available, and inexpensive alkenyl carboxylic acids has been developed. This directed dehydrogenative cross-coupling reaction affords 6-alkenylated 2-pyridones that would otherwise be difficult to access using conventional C-H functionalization protocols. The reaction occurs with high efficiency and is tolerant of a broad range of functional groups. A wide scope of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, are amenable to this transformation and no addition of external oxidant is required. Mechanistic studies revealed that (1) Boc2O acts as the activator for the in situ transformation of the carboxylic acids into anhydrides before oxidative addition by the Rh catalyst, (2) a decarbonylation step is involved in the catalytic cycle, and (3) the C-H bond cleavage is likely the turnover-limiting step.

Synthesis of N-Alkylpyridin-4-ones and Thiazolo[3,2-a]pyridin-5-ones through Pummerer-Type Reactions

N-Alkylated 4-pyridones were obtained through a one-pot procedure involving either normal or interrupted Pummerer reactions between triflic anhydride-activated sulfoxides and 4-fluoropyridine derivatives, followed by hydrolysis. On the other hand, triflic anhydride-activated benzyl 6-fluoro-2-pyridyl sulfoxide could react with alkenes or alkynes to afford thiazolo[3,2-a]pyridin-5-ones, via the pyridinium salt intermediates.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Sulfur-Mediated Electrophilic Cyclization of Aryl-Substituted Internal Alkynes

A sulfur-mediated electrophilic cyclization reaction of aryl-tethered internal alkynes has been developed. Triflic anhydride-activated sulfoxides induced the electrophilic cyclization and then demethylation with triethylamine in one pot, affording 3-sulfenyl-1,2-dihydronaphthalenes and related types of products in yields of ≤96%.

Environmentally benign nucleophilic substitution reaction of arylalkyl halides in water using CTAB as the inverse phase transfer catalyst

An environmentally benign, scalable and highly selective C-arylalkylation of active methylene compounds is developed using CTAB as the inverse phase transfer catalyst. The methodology is also applicable to the regioselective synthesis of N-aralkyl/alkyl 2-pyridones.

Sulfur mediated propargylic C–H alkylation of alkynes

Sulfur mediated propargylic C–H alkylation was realized through a one-pot procedure with modest yields and substrate scope. Hopefully, the community could find a better solution to this challenge by some novel metal-catalyzed reactions.