Organocatalytic Dearomative Spirocyclization Reaction of Enone-Tethered α-and β-Naphthols and Dearomatization Reaction of In Situ Generated Nitro-Olefin-Tethered α-Naphthols

Herein, we report a catalytic dearomative spirocyclization reaction of new substrates having aryl/alkyl enone tethered α- and β-naphthols and a dearomatization reaction of in situ generated nitro-olefin-tethered α-naphthols. The spirocarbocycles were obtained in moderate to good yields with high diastereoselectivities. A preliminary catalytic asymmetric variant was reported. A few applications such as hydrogenations and epoxidation reaction have also been demonstrated. Theoretical study has also been performed to understand high diastereoselectivity in the triethylamine catalyzed spirocyclization reaction.

Facile Cleavage of Activated Ketones: An Access to Thioethers via In Situ Generation of Anhydrides by Pummerer-Type Rearrangement

Herein, we report an oxygen insertion in activated ketones from simple inorganic carbonates for the synthesis of symmetric aromatic anhydrides. For the first time, Li2CO3 acts as an oxygen source and the in situ generated symmetric aromatic anhydrides undergo Pummerer-type rearrangement to access α-benzoyloxy-thioethers. Attractively, this protocol occurs under metal-, ligand-, and oxidant-free conditions and is compatible with a wide range of substrates. Control experiments reveal the reaction pathway.

Electro-organic green synthesis of dicyano-2-(2-oxoindolin-3-ylidene) malononitriles using molecular iodine as catalyst

The first electrochemical molecular iodine promoted, domino reactions for the green synthesis of biologically relevant dicyano 2-(2-oxoindolin-3-ylidene) malononitriles (11 examples, up to 94% yield) from readily available isatin derivatives, malononitrile, and iodine at room temperature have been presented. This synthesis method showed tolerance towards various EDG and EWG and was completed in a short reaction time at the constant low current density of 5 mA cm-2 in the low redox potential range of -0.14 to 0.07 V. The present study exhibited by-product-free formation, easy operation, and product isolation. In particular the formation of a C[double bond, length as m-dash]C bond was observed at room temperature with a high atom economy. Furthermore, in the present study, the electrochemical behavior of dicyano 2-(2-oxoindolin-3-ylidene) malononitrile derivatives using a cyclic voltammetry (CV) technique in 0.1 M NaClO4 in acetonitrile solution was studied. All the chosen substituted isatin exhibited well-defined diffusion-controlled quasi-reversible redox peaks except 5-substituted derivatives. This synthesis could serve as an alternative strategy to synthesize other biologically important oxoindolin-3-ylidene malononitrile derivatives.

Carbocatalysis: a metal free green avenue towards carbon–carbon/heteroatom bond construction

Indeed, all the heterocycles comprises of either “C–C, C–N, C–S or C–O” bonds in their skeleton and construction of these bonds has laid the foundation stone of organic chemistry. The present researchers are continually attempting to develop new strategies for synthesizing miscellaneous structurally divergent molecular entities and these bond forming reactions are the fundamental tools. As a consequence, a colossal upheaval is witnessed in development of benign and sustainable synthetic routes for green bond-forming reactions envisaging carbon–carbon/heteroatom. This chapter is aimed towards highlighting the recent developments perceived in “C–C, C–N, C–S or C–O” bondconstruction especially emphasising greener perspectives i.e. carbocatalysis.

Regioselective synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds via transition metal-free C-C and C-N bond formation

Few methods are known for the synthesis of nitroindole derivatives. A simple and practical Cs₂CO₃-promoted method for the synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds has been developed. Two new C-C and C-N bonds were formed in a highly regioselective manner under transition metal-free conditions.

One-pot synthesis of 3-substituted-3-hydroxyindolin-2-ones: three component coupling of N-protected isatin, aryne precursor and 1,3-cyclodione under metal-free conditions

A transition-metal free synthetic strategy for the direct synthesis of 3-substituted-3-hydroxy-indolin-2-ones via three component coupling of N-protected isatin, aryne precursor, and 1,3-cyclodione.

Accessing oxy-functionalized N-heterocycles through rose bengal and TBHP integrated photoredox C(sp3)–O cross-coupling

A C(sp3)–O coupling strategy is described involving tautomerizable N-heterocycles (phthalazinone, pyridne, pyrimidinone and quinoxalinone) carbonyl employing rose bengal as the photocatalyst and TBHP.

Transition-metal-free oxindole synthesis: quinone-K2CO3 catalyzed intramolecular radical cyclization

A novel and highly efficient transition-metal-free approach for the conversion of α-bromoanilides to 3,3-disubstituted oxindoles is described. This transformation is promoted by catalytic amount of 9,10-phenanthrenequinone (PQ) together with K2CO3,...

One-Pot Construction of Diverse β-Lactam Scaffolds via the Green Oxidation of Amines and Its Application to the Diastereoselective Synthesis of β-Amino Acids

In this study, a simple one-pot construction of β-lactam scaffolds was successfully achieved via 4,6-dihydroxysalicylic acid-catalyzed organocatalytic oxidation of amines to imines using molecular oxygen. Although some imines are highly unstable and difficult to isolate by conventional methods, the organocatalytic oxidation of amines described herein, followed by their direct reaction with acyl chlorides in the presence of a base, afforded a series of new β-lactam derivatives with excellent cis selectivity, which could not be synthesized and isolated by previously reported methods. Thus, this one-pot protocol will be one of the powerful methods applicable to the synthesis of various potential drug candidates and functional molecules. Furthermore, the subsequent hydrolysis of these β-lactams successfully afforded the corresponding β-amino acids as almost single diastereomers in up to 99% yields.

Recent Advances in Transition-Metal-Free Late-Stage C-H and N-H Arylation of Heteroarenes Using Diaryliodonium Salts

Transition-metal-free direct arylation of C-H or N-H bonds is one of the key emerging methodologies that is currently attracting tremendous attention. Diaryliodonium salts serve as a stepping stone on the way to alternative environmentally friendly and straightforward pathways for the construction of C-C and C-heteroatom bonds. In this review, we emphasize the recent synthetic advances of late-stage C(sp2)-N and C(sp2)-C(sp2) bond-forming reactions under metal-free conditions using diaryliodonium salts as arylating reagent and its applications to the synthesis of new arylated bioactive heterocyclic compounds.

Recent developments in selective N-arylation of azoles

Transition-metal based carbon-heteroatom (C-X) bond formation has attracted the attention of synthetic chemists over the past few years because the resultant aryl/heteroaryl motifs are important substructures in many natural products, pharmaceuticals, etc. Several efficient protocols such as Buchwald-Hartwig amination, Ullmann coupling, Chan-Lam coupling and metal-free approaches have proved beneficial in C-X bond formation. Selective arylation of one hetero-centre over other centres without protection/deprotection thus allowing minimum synthetic manipulation has been achieved for several substrates using these protocols. Azoles are one such novel five-membered heterocyclic core with huge pharmaceutical applications. Though N-arylation on azole-bearing analogues has been extensively practised, selective N-arylation either on one N-centre or the exocyclic N-site of the azole ring in competition with other hetero-centres in the framework has been recently explored for azole-carrying systems. Thus, this review would focus on recent advances in chemo- and regio-selective N-arylation (either on one N-centre or the exocyclic N-site of the azole ring) on azole-containing frameworks.

Direct C-H aminocarbonylation of N-heteroarenes with isocyanides under transition metal-free conditions

A C-C bond forming amide synthesis through direct C-H aminocarbonylation of N-heteroarenes with isocyanides was developed. The reaction was mediated by an inorganic persulfate salt under transition metal-free conditions. Mechanistic studies suggested a radical pathway for this reaction without the participation of H₂O and O₂. This method also showed merits of substrate availability, easy operation and atom economy. It provided an efficient route for straightforward synthesis of N-heteroaryl amides.

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.

Transition-metal-free C-S, C-Se, and C-Te Bond Formation from Organoboron Compounds

The present review describes the successful application of organoboron compounds in transition-metal-free C-S, C-Se, and C-Te bond formations. We presented studies regarding these C-Chalcogen bond formations using organoboron reagents, such as boronic acids, boronic esters, borate anions, and several sources of chalcogen atoms/moieties. Moreover, a broad range of transition-metal-free approaches to synthesize sulfides, selenides, and tellurides were described using conventional heating methods, which are sometimes green since they use green solvents, safe reagents, among others. Furthermore, protocols using alternative energy sources, including ultrasound, microwave irradiation, photocatalysis, and electrolytic processes, were also shown to be suitable. These protocols were applied to prepare a broad scope of functionalized chalcogenides with high molecular diversity. These studies and their proposed mechanisms were also reported herein in addition to the reuse of reaction promoters.

Oxidative cross-dehydrogenative coupling between iodoarenes and acylanilides for C–N bond formation under metal-free conditions

A metal-free oxidative cross-dehydrogenative coupling between iodoarenes and acylanilides was developed. It gave highly para-selectivie C–N coupling products with the retention of iodine atom which enables further transformations.

Revisiting applications of molecular iodine in organic synthesis

Molecular iodine contributes significantly to organic transformations in synthetic organic chemistry. It works effectively due to its mild Lewis acidic character, ability as an oxidizing agent, good moisture stability, and easy availability.

Metal-Free C-C Coupling of an Allenyl Sulfone with Picolyl Amides to Access Vinyl Sulfones via Pyridine-Initiated In Situ Generation of Sulfinate Anion

Vinyl sulfones are privileged motifs known for their biological activity and synthetic utility. Synthetic transformations to efficiently access high-value compounds with these motifs are desired and sought after. Herein, a new procedure is described to form vinyl sulfone-containing compounds by selective functionalization of the C(sp³)-H bond adjacent to the pyridine ring of pharmacologically prevalent picolyl amides with an allenyl sulfone, 1-methyl-4-(propa-1,2-dien-1-ylsulfonyl)benzene. The reaction conditions are mild with no metal catalyst or additives required and display good functional group tolerance. Mechanistic studies for this unusual transformation suggest that the reaction operates via a rare pyridine-initiated and p-toluenesulfinate anion-mediated activation of the allenyl sulfone analogous to phosphine-triggered reactions.

Modular synthesis and transition metal-free alkynylation/alkenylation of Castagnoli–Cushman-derived N,O- and N,S-heterocyclic vinyl chlorides

A modular and functional group-tolerant protocol for the transition metal-free coupling of novel N,O- and N,S-heterocyclic vinyl chlorides with terminal acetylenes and styrenes has been developed, leading to the epimerization-free synthesis of fully carbofunctionalized dihydro-1,4-oxazines/thiazines. Bicyclic morpholines have also been prepared through the interrogation of newly synthesized cross-conjugated dienes in Diels–Alder reactions. The use of environmentally benign reaction media endows the current strategy with a practical advantage.

A functional group-tolerant and transition metal-free coupling of novel N,O- and N,S-heterocyclic vinyl chlorides, which affords fully carbosubstituted dihydro-1,4-oxazines/thiazines as well as bicyclic morpholines, is described.

Transition-Metal-Free Addition Reaction for the Synthesis of 3-(Aminobenzylidene/aminoalkylidene)indolin-2-ones and Its Synthetic Applications

A novel and efficient transition-metal-free approach for the exclusive synthesis of Z-3-(aminobenzylidene/aminoalkylidene)indolin-2-ones in high yield from 2-oxindole and aryl/alkyl nitrile in the presence of LiOtBu and 2,2'-bipyridine system is described. In addition, we disclosed a new approach towards the metal-free fluorination using selectfluor and the C═C bond cleavage using CuI and environmentally benign O₂.

Quinazolinone Synthesis through Base-Promoted SNAr Reaction of ortho-Fluorobenzamides with Amides Followed by Cyclization

A transition-metal-free synthesis of quinazolin-4-ones by Cs2CO3-promoted SNAr reaction of ortho-fluorobenzamides with amides followed by cyclization in dimethyl sulfoxide has been developed. The present procedure can provide efficient synthetic methods for the formation of both 2-substituted and 2,3-disubstituted quinazolin-4-one rings depending on the use of easily available starting materials and an efficient, one-pot protocol for the synthesis of the marketed drug product of methaqualone.

Copper-catalyzed synthesis of 2-aminopyridylbenzoxazoles via domino reactions of intermolecular N-arylation and intramolecular O-arylation

A simple and general approach to nitrogen-containing heterocycles via copper-catalyzed domino reaction has been developed, and the corresponding 2-aminopyridylbenzoxazole derivatives were obtained in good to excellent yields using the readily available starting materials. This method possesses unique step economy features, and is of high tolerance towards various functional groups in the substrates.

Direct Carboxylation of Electron-Rich Heteroarenes Promoted by LiO-tBu with CsF and [18]Crown-6

We herein demonstrate that the combination of LiO-tBu, CsF, and [18]crown-6 efficiently promotes the direct C-H carboxylation of electron-rich heteroarenes (benzothiophene, thiophene, benzofuran, and furan derivatives). A variety of functional groups, including methyl, methoxy, halo, cyano, amide, and keto moieties, are compatible with this system. The reaction proceeds via the formation of a tert-butyl carbonate species.

Direct Functionalization of C(sp2)-H Bond in Nonaromatic Azaheterocycles: Palladium-Catalyzed Cross-Dehydrogenative Coupling (CDC) of 2H-Imidazole 1-Oxides with Pyrroles and Thiophenes

The C(sp²)-H bond functionalization methodology was first applied to carry out the palladium-catalyzed oxidative C-H/C-H coupling reactions of 2H-imidazole 1-oxides with pyrroles and thiophenes. As a result, a number of novel 5-heteroarylated 2H-imidazole 1-oxides, which are of particular interest in the design of bioactive molecules and advanced materials, have been synthesized in yields up to 78%. The detailed H/D-exchange experiments have also been performed to elucidate some mechanistic features of this cross-dehydrogenative coupling process.

Metal-free, radical 1,6-conjugated addition of cyclic ethers with para-quinone methides (p-QMs)

An efficient method for metal-free C–C bond formation between p-quinone methides (p-QMs) and cyclic ethers via a radical pathway to afford substituted diarylmethanes and triarylmethanes or to effect the α-alkylation of the cyclic ethers has been developed.

Nitrosonium ion catalysis: aerobic, metal-free cross-dehydrogenative carbon-heteroatom bond formation

Catalytic cross-dehydrogenative coupling of heteroarenes with thiophenols and phenothiazines has been developed under mild and environmentally benign reaction conditions. For the first time, NOx+ was applied for catalytic C-S and C-N bond formation. A comprehensive scope for the C-H/S-H and C-H/N-H cross-dehydrogenative coupling was demonstrated with >60 examples. The sustainable cross-coupling conditions utilize ambient oxygen as the terminal oxidant, while water is the sole by-product.

Transition-Metal-Free Synthesis of N-Hydroxy Oxindoles by an Aza-Nazarov-Type Reaction Involving Azaoxyallyl Cations

A novel transition-metal-free method to construct N-hydroxy oxindoles by an aza-Nazarov-type reaction involving azaoxyallyl cation intermediates is described. A variety of functional groups were tolerated under the weak basic reaction conditions and at room temperature. A one-pot process was also developed to make the reaction even more practical. This method provides alternative access to oxindoles and their biologically active derivatives.

Transition-Metal-Free C(sp³)⁻H Oxidation of Diarylmethanes

An efficient direct C(sp3)–H oxidation of diarylmethanes has been demonstrated by this study. This method employs environment-friendly O₂ as an oxidant and is promoted by commercially available MN(SiMe₃)₂ [M = K, Na or Li], which provides a facile method for the synthesis of various diaryl ketones in excellent yields. This protocol is metal-free, mild and compatible with a number of functional groups on substrates.

Transition-Metal-Free Regiospecific Aroylation of Nitroarenes Using Ethyl Arylacetates at Room Temperature

A novel regiospecific C(sp³)-C(sp²) coupling between ethyl arylacetates and nitroarenes has been developed to deliver biaryl ketones in excellent yields. The protocol is metal-free, mild, and compatible with a number of functional groups on both of the reacting partners.

Evidence of single electron transfer from the enolate anion of an N,N'-dialkyldiketopiperazine additive in BHAS coupling reactions

A designed N,N'-dialkyldiketopiperazine (DKP) provides evidence for the role of DKP additives as initiators that act by electron transfer in base-induced homolytic aromatic substitution reactions, involving coupling of haloarenes to arenes.

Cesium carbonate promoted cascade reaction involving DMF as a reactant for the synthesis of dihydropyrrolizino[3,2-b]indol-10-ones

A cesium carbonate promoted cascade reaction for the synthesis of dihydropyrrolizino[3,2-b]indol-10-ones has been developed.

Intramolecular Direct C-H Arylation via a Metallocenic Radical Pathway: Stereospecific Approach to Planar-Chiral Ferrocenes

Transition metal-free synthesis of planar-chiral 1,2-fused ferrocenes via intramolecular direct C-H bond arylation was achieved. The C-H arylation reactions promoted by a catalytic amount of 1,10-phenanthroline highlighted a unique planar-chiral metallocenic radical intermediate, generated from iodoferrocenes via a single-electron transfer process.