Iodine-PEG as a unique combination for the metal-free synthesis of flavonoids through iodonium-triiodide ion-pair complexation

An efficient metal-free single-step protocol has been developed for the direct synthesis of flavones from 2-hydroxyacetophenone and substituted benzaldehydes. This chemical transformation is exclusively promoted by the iodonium-triiodide ion couple formed through iodine and PEG-400 complexation. The triiodide anion not only helps in the abstraction of a proton from the acetophenone but also promotes the cyclization of intermediate chalcone to the corresponding flavones. The flavones were obtained in very high yields without using any toxic metal catalysts or harsh reaction conditions. The reaction mechanism was established through a series of test reactions and entrapping of reaction intermediates. The developed protocol provides direct access to flavones in high yields under milder reaction conditions with great substrate compatibility, including hydroxylated derivatives.

Mechanochemical Nickel-Catalyzed Carbon-Sulfur Bond Formation between Aryl Iodides and Aromatic Sulfur Surrogates

The formation of aromatic thioethers from C-S coupling is of great importance in synthetic chemistry. Traditional solution strategies through transition-metal catalysis generally require bulk solution, heat, and longer reaction time. Herein, a mechano-promoted sulfenylation of aryl iodides with nickel catalysis is described. The active aromatic sulfide agents are in-situ generated from aromatic thiol or disulfide and subsequently adapted in the nickel catalytic cycle, with a tolerance of broad substituted groups under optimized conditions. In addition to the gram-scale synthesis that reveals the application potential of the method, the radical trapping and competitive experiments are also conducted for the mechanistic study, thus providing a plausible mechanism rationally. Furthermore, the proposed methodology is certificated as being versatile and following the green principles with ideal calculated values of green chemistry metrics, and the comparison with other approaches for C-S bond formation is also demonstrated.

I2-DMSO-Mediated N-H/α-C(sp3)-H Difunctionalization of Tetrahydroisoquinoline: Formal [2 + 2 + 1] Annulation for the Construction of Pyrrolo[2,1-a]isoquinoline Derivatives

An I₂-DMSO-mediated cascade reaction using methyl ketones and 1,2,3,4-tetrahydroisoquinolines (THIQs) as commercially available substrates has been developed for the construction of pyrrolo[2,1-a]isoquinoline derivatives. This metal-free process involves N-H/α-C(sp³)-H difunctionalization of THIQ. Two C-C bonds and one C-N bond are formed in one pot under mild conditions. Besides, a quaternary carbon center has been constructed in this transformation efficiently.

I2-Promoted site-selective C–C bond cleavage of aryl methyl ketones as C1 synthons for constructing 5-acyl-1H-pyrazolo[3,4-b]pyridines

A novel iodine promoted [1 + 3 + 2] cleavage cyclization reaction for the synthesis of 1H-pyrazolo[3,4-b]pyridines from aryl methyl ketones, 5-aminopyrazoles and enaminones has been established.

Ten Years of Glory in the α-Functionalizations of Acetophenones: Progress Through Kornblum Oxidation and C-H Functionalization

This review article focuses on the α-functionalization of acetophenones involving Kornblum oxidation and C-H functionalizations. Although various other strategies, such as classical approaches, enamine approaches and umpolung strategy are also known for this functionalization, here we discuss mainly the Kornblum oxidation approach and C-H functionalization strategy as they have advantages over the others. In Kornblum oxidation, the reaction uses iodine and dimethylsulfoxide and proceeds through the formation of arylglyoxal as the key intermediate. In C-H functionalization, the reaction requires metal, or metal-free catalyst, and generates radical intermediate in most cases. α-Functionalization of acetophenones is very important because of their huge applications in the synthesis of various natural products and pharmaceuticals and, therefore, a number of research articles have been published in this area. However, no review articles are available so far. In this article, we present a succinct discussion of various important and novel reactions, along with their mechanisms, published since 2012 to date. We believe that this first review article in this field will give readers one-stop information on this topic and encourage further intriguing work in this area.

Formylation and Bromination of Pyrrolo[2,1-a]isoquinoline Derivatives with Bromoisobutyrate and Dimethyl Sulfoxide

We have developed an efficient formylation of pyrroloisoquinolines using bromoisobutyrate and dimethyl sulfoxide as carbonyl reagent. Various formylated pyrroloisoquinolines could be prepared in good yields (up to 94%). This formylation process can be easily scaled up to gram scale with good yield. In most cases of pyrroloisoquinolines without methoxy groups, the combination of bromoisobutyrate and dimethyl sulfoxide could act as a bromination reagent, delivering brominated pyrroloisoquinolines in acceptable to good yields (up to 82%).

Employing Arylacetylene as a Diene Precursor and Dienophile: Synthesis of Quinoline via the Povarov Reaction

A novel I₂-mediated Povarov reaction of arylacetylenes and anilines for the synthesis of 2,4-substituted quinolines has been developed, in which arylacetylene first acts as both a diene precursor and dienophile. This work further develops the Povarov reaction to expand the types of diene precursors. Preliminary mechanistic studies indicate that the I₂/DMSO system realized the oxidative carbonylation of C(sp)-H of arylacetylene and then undergoes a [4 + 2] cycloaddition reaction.

Reactions involving aryl methyl ketone and molecular iodine: a powerful tool in the one-pot synthesis of heterocycles

The preparation of heterocyclic compounds has attracted great attention in organic chemistry because of their extensive application in the field of bioactive molecules, materials science, and natural products.

Fast and high-efficiency synthesis of 2-substituted benzothiazoles via combining enzyme catalysis and photoredox catalysis in one-pot

An efficient and green method, combining enzymatic and visible-light catalysis for synthesis of the widely applicable 2-substituted benzothiazoles, has been developed. This method features a relay catalysis protocol consisting of biocatalytic promiscuity and visible-light-induced subsequent oxidization of 2-phenyl benzothiazolines. The whole reaction process is very high-efficiency, achieving 99% yield in just 10 min, under an air atmosphere, nearly 100% atomic utilization, and the 2-substituted benzothiazole products were obtained in good to excellent yields with a wide range of substrates. This reaction is the other example of combining the non-natural catalytic activity of hydrolases with visible-light catalysis for organic synthesis and the catalytic system does not require additional oxidants or metals, which is good for the environment.

Oxidant/Solvent-Controlled I2-Catalyzed Domino Annulation for Selective Synthesis of 2-Aroylbenzothiazoles and 2-Arylbenzothiazoles under Metal-Free Conditions

A simple and practical domino protocol for the selective synthesis of 2-aroylbenzothiazoles and 2-aryl benzothiazoles catalyzed by I₂ is developed under metal-free conditions. The reaction outcomes are exclusively controlled by the reaction oxidant/medium. With DMSO employed as both the solvent and the oxidant, an oxidation of aromatic methyl ketones takes precedence over the condensation with 2-aminobenzenethiols. On the other hand, when the reaction was carried out in PhNO₂ or in 1,4-dioxane containing PhNO₂, the condensation of aromatic methyl ketones with 2-aminobenzenethiols has priority to form imines which is followed by an oxidation of the methyl group from ketones to afford 2-arylbenzothiazoles as a sole product. The PhNO₂/I₂ co-catalytic system is proposed first time.

Recent progress on selective deconstructive modes of halodifluoromethyl and trifluoromethyl-containing reagents

Halodifluoromethyl and trifluoromethyl-containing compounds are widely employed in organic chemistry, pharmaceuticals and materials science. Therefore, their applications and transformations have received significant attention during the past few decades. The single, double, triple and quadruple cleavage of halodifluoromethyl compounds and various deconstructive modes of trifluoromethyl-containing compounds could generate a variety of synthons to prepare more valuable products. Herein, we summarize the most significant achievements in this field with an intriguing focus on results from the last decade.

Iodine-catalyzed α,β-dehydrogenation of ketones and aldehydes generating conjugated enones and enals

A transition metal-free α,β-dehydrogenation of ketones and aldehydes was developed.

An atom-economical and regioselective metal-free C-5 chalcogenation of 8-aminoquinolines under mild conditions

A general and simple metal-free protocol for expedient C-H functionalization leading to the regioselective generation of C-5 chalcogenated 8-aminoquinoline analogues in up to 90% yield at room temperature (25 °C) has been established. This methodology is an eco-friendly approach to the atom-economical utilization of diaryl/dialkyl chalcogenides for direct access to chalcogenated quinolines and is scalable to the gram scale without considerable decrease in the yield of the product. It represents a practical alternative to the existing metal-catalyzed functionalization of 8-aminoquinoline derivatives with broad functional group tolerance. The controlled experiments suggest that the reaction possibly proceeds through an ionic pathway at room temperature. Furthermore, the potentiality for the functionalization of free amines in chalcogenated-8-aminoquinolines provides an attractive perspective for further elaboration of the amine substituent through chemical manipulations. The applicability of the standardized method has been augmented through late-stage antimalarial drug diversification of primaquine analogues.

Iodine-Promoted Domino Oxidative Cyclization for the One-Pot Synthesis of Novel Fused Four-Ring Quinoxaline Fluorophores by sp3 C-H Functionalization

A method for the synthesis of novel fused four-ring quinoxaline skeleton has been described by an I₂ promoted sp³ C-H functionalization between 1,2,3,3-tetramethyl-3H-indolium iodides and 1,2-diamines. This transformation proceeds smoothly under metal- and peroxide-free conditions through a sequential iodination, oxidation, annulation and rearrangement. Moreover, 8,9-dichloro-5,12,12-trimethyl-2-(trifluoromethyl)-5,12-dihydroquinolino[2,3-b]quinoxaline showed good photophysical properties and was used in live cell imaging, indicating the potential value of this skeleton as a fluorophore in probes.

S8-Catalyzed triple cleavage of bromodifluoro compounds for the assembly of N-containing heterocycles

An unprecedented S₈-catalyzed selective triple-cleavage of bromodifluoroacetamides is disclosed for the first time.

An unprecedented S₈-catalyzed selective triple-cleavage of bromodifluoroacetamides is disclosed for the first time. Valuable 2-amido substituted benzimidazoles, benzoxazoles and benzothiazoles were obtained in good to excellent yields in a cascade protocol in this strategy. Mechanistic studies suggested that a C2 source was generated in situ by selective cleavage of three C–X bonds, including two inert C(sp³)–F bonds on bromodifluoroacetamides, while leaving C–C bonds intact. This strategy will undoubtedly further consummate the role of halo difluoro compounds and enrich both fluorine chemistry and pharmaceutical sciences.

Oxidative C(sp3)–H functionalization of methyl-azaheteroarenes: a facile route to 1,2,4-triazolo[4,3-a]pyridines

An oxidative [4 + 1] annulation for triazolo[4,3-a]pyridine–quinoline linked diheterocycle synthesis via functionalization of the sp³ C–H bonds of 2-methyl-azaheteroarenes has been developed.

Recent advances in intramolecular C–O/C–N/C–S bond formationviaC–H functionalization

This review presents the construction of C–X bonds (X = O/N/S) by using intramolecular C–H functionalization for the synthesis of heterocyclic compounds.

PIDA/I2-Mediated α- and β-C(sp3)-H Bond Dual Functionalization of Tertiary Amines

The α,β-C(sp³)-H bond dual functionalization of tertiary amines is still a challenging task for both organic and medicinal chemists. Herein a direct, mild, metal-free, and site-specific method mediated by PIDA/I₂ was developed for α,β-C(sp³)-H bond dual functionalization of tertiary amines, and this method can provide facile access to α-keto lactams or rarely studied α,α-diiodo lactams. Moreover, this method was used for the effective syntheses of three natural products [obscurumine C (13), obscurumine O (17), and strychnocarpine (18)] and direct preparation of mimics of the in vivo metabolites of two FDA-approved drugs (imatinib and donepezil) in 36-60% overall yield. The method represents a promising protocol for the late-stage α,β-C(sp³)-H bond oxidative dual functionalization of tertiary amine-containing drugs and complex natural products.

Chalcones and Five-Membered Heterocyclic Isosteres Bind to Alpha Synuclein Fibrils in Vitro

A series of chalcone and heterocyclic isosteres, in which the enone moiety was replaced with an isoxazole and pyrazole ring system, was synthesized and their affinities for alpha synuclein (Asyn), amyloid beta (Aβ), and tau fibrils were measured in vitro. The compounds were found to have a modest affinity and selectivity for Asyn versus Aβ fibrils and low affinity for tau fibrils. Insertion of a double bond to increase the extendable surface area resulted in an increase in affinity and improvement in selectivity for Asyn versus Aβ and tau fibrils. The results of this study indicate that compound 11 is a secondary lead compound for structure-activity relationship studies aimed at identifying a suitable compound for positron emission tomography-imaging studies of insoluble Asyn aggregates in Parkinson's disease.

A metal-free synthesis of 2-aminobenzothiazoles through aminyl radical addition to aryl isothiocyanates

A convenient synthesis of 2-aminobenzothiazoles, starting from aryl isothiocyanates and formamides under metal-free conditions, is described. Various secondary and tertiary amine- and even α-amino acid-derived formamides can be used as amino sources in this process. Mechanistic studies suggest that the reaction is initiated by decarbonylative aminyl radical formation in the presence of n-Bu4NI and TBHP, followed by aminyl radical addition to isothiocyanates and cyclization via sulfur centred radical intermediates.

Molecular iodine-catalysed oxidative CO–C(alkyl) bond cleavage of aryl/heteroaryl alkyl ketones: an efficient strategy to access fused polyheterocycles

An efficient molecular iodine-catalysed one-pot strategy has been accomplished for the construction of various fused heterocycles under metal and oxidant free conditions.

Reagent-controlled regiodivergent intermolecular cyclization of 2-aminobenzothiazoles with β-ketoesters and β-ketoamides

Two regiodivergent approaches to intermolecular cyclization of 2-aminobenzothiazoles with β-ketoesters and amides have been developed, controlled by the reagents employed. With the Brønsted base KOt-Bu and CBrCl3 as radical initiator, benzo[d]imidazo[2,1-b]thiazoles are synthesized via attack at the α-carbon and keto carbon of the β-ketoester moiety. In contrast, switching to the Lewis acid catalyst, In(OTf)3, results in the regioselective nucleophilic attack at both carbonyl groups forming benzo[4,5]thiazolo[3,2-a]pyrimidin-4-ones instead.

Synthesis of 2-acyl-benzo[1,3-d]selenazoles via domino oxidative cyclization of methyl ketones with bis(2-aminophenyl) diselenide

The synthesis of unprecedented 2-acyl-benzo[1,3-d]selenazoles is presented using bis(2-aminophenyl) diselenide and aryl methyl ketones under conventional heating and microwave irradiation.

1,2-Arylalkylation of N-(arylsulfonyl)acrylamides using aliphatic aldehydes as the alkyl source

A metal-free decarbonylative arylalkylation of N-(arylsulfonyl)acrylamides using aliphatic aldehydes as the alkyl radical source was developed.

I2-Catalyzed cross dehydrogenative coupling: rapid access to benzoxazinones and quinazolinones

An efficient and applicable I₂-catalyzed intramolecular dehydrogenative C–O/C–N coupling reaction has been developed for the first time to provide both benzoxazinones and quinazolinones.