Divergent reactivity of sulfoxonium ylide with allyl carbonate and allyl carbamate.

A divergent reactivity of sulfoxonium ylide with allyl carbonates and allyl carbamates is demonstrated. Rh(III)-catalyzed C-H activation and cyclization of sulfoxonium ylide with ally esters leads to the formation of a cyclopropane-fused tetralone derivative by cascade C-H activation, (4+2) annulation, and cyclopropanation. A similar reaction of sulfoxonium ylide with allyl carbamates forms a C3-substituted indanone derivative via a rare and interesting domino C-H activation and (4+1) annulation in which allyl carbamate is acting as a C1-synthon.

Visible-Light-Mediated Vicinal Difunctionalization of Activated Alkynes with Boronic Acids: Substrate-Controlled Rapid Access to 3-Alkylated Coumarins and Unsaturated Spirocycles.

A visible-light-mediated difunctionalization of activated alkynes with boronic acid is unveiled to synthesize 3-alkylated coumarins and unsaturated spiro-lactones. The substituent at the para-position of the aryl ring of aryl alkynoate plays a pivotal role in the selective formation of chain-alkylated coumarins or spirocyclic compounds under mild conditions. The reaction employs a hypervalent iodine reagent and ruthenium photocatalyst. The spiro-lactones thus obtained were subjected to another novel mode of visible-light-mediated radical addition cascade cyclization (RACC) to access various new fused spirocyclic compounds.

Rhodium(III)-Catalyzed Cascade Reactions of Imines/Imidates with 4-Hydroxy-2-alkynoates to Synthesize Regioselective Furanone-Fused Isoquinoline Scaffolds.

A regioselective synthesis of furanone-fused isoquinoline heterocycles is developed in a single step using a Rh(III) catalyst. In this reaction, a cascade C-H activation, regioselective annulation, and lactonization occur in one pot. A wide range of alkynoates was examined, which showed good regioselectivity. The regioselectivity was guided by steric bulk at the C4 position of the 4-hydroxy-2-alkynoates. The synthetic utility was exemplified, and the model reaction was scaled up to a 1 g scale.

Rhodium(iii)-catalyzed synthesis of trisubstituted furans via vinylic C-H bond activation.

We report an Rh(iii)-catalyzed one-pot synthesis of trisubstituted furan derivatives through Cvinyl-H activation of α,ß-unsaturated ketones with acrylates. The control study revealed that the Heck-type product obtained undergoes Paal-Knorr type cyclization in the presence of an Ag salt. Hence, the Ag salt plays a dual role of a halide scavenger and a Lewis acid catalyst for Paal-Knorr type cyclization. The furan product can be transferred into the respective alcohol and acid derivatives which are useful intermediates in synthesizing biologically active molecules.

Rhodium(iii)-catalyzed [5+1] annulation of 2-alkenylphenols with maleimides: access to highly functionalized spirocyclic skeletons.

A new edition of [5+1] annulation reaction of maleimides with 2-alkenylphenols has been discovered under a Rh(iii)-catalytic system. The process leads to an efficient synthesis of valued spirocyclic scaffolds bearing an oxygen-containing spiro carbon in a single step and shows a broad substrate scope with good functional group tolerance. The synthetic utility has been exemplified by synthesizing highly functionalized 2,2-disubstituted-2H-chromene skeletons and a gram-scale synthesis with a low catalyst loading.

N-Triflination of pyrazolones: a new method for N-S bond formation.

A simple method, which takes place quickly in 5 min, is developed for the N-triflination of pyrazolones using CF3SO2Na (Langlois reagent) and phenyliodine(iii)bis(trifluoroacetate) (PIFA). This reaction takes place at the imine nitrogen centre instead of the more reactive C4-position, forming a new N-S bond. A variety of pyrazolone derivatives were subjected to the reaction. Unlike the previous reports on sulfenylation or sulfonylation of pyrazolone, wherein the corresponding C-S bond is formed, this new method leads to the formation of the hetero-hetero atom bond (N-S bond) at room temperature.

Dual Role of the Rhodium(III) Catalyst in C-H Activation: [4 + 3] Annulation of Amide with Allylic Alcohols to 7-Membered Lactams.

[4 + 3] annulation of primary and secondary benzamide and cinnamamide derivatives using allyl alcohol as a coupling partner catalyzed by Rh(III) is reported, where Rh(III) is playing a dual role of an oxidant and a catalyst for C-H activation. The Rh-catalyst oxidizes allyl alcohol to its carbonyl derivative, and the in situ-generated carbonyl compound reacts with benzamide in the presence of the Rh-catalyst, forming the corresponding alkylated products. Mechanistic studies show that AgSbF6 is also playing a dual role. Apart from being a halide scavenger, AgSbF6 catalyzes the cyclization of the alkylated product, forming the desired lactam. The current method has good synthetic application and is useful for synthesizing a few biologically active compounds that can act as the dopamine D3 receptor ligand, including berberine-like analogues. The deuteration study and control experiments helped us to propose the mechanism.

Visible light-mediated ipso-annulation of activated alkynes: access to 3-alkylated spiro[4,5]-trienones, thiaspiro[4,5]-trienones and azaspiro[4,5]-trienones.

A novel method for chemoselective difunctionalization of activated alkynes for synthesizing 3-alkylated spiro[4,5]-trienones, thiaspiro[4,5]-trienones and spirolactams has been uncovered using photoredox catalysis under visible light conditions. The rarely used tricarbonyl compounds in photoredox catalysis were used as the alkylating source. A remarkable functional group tolerance was observed, and the application of the method has been showcased by transforming tricarbonylated spirolactams to their corresponding monocarbonylated products, which are difficult to access using traditional methods.

Sulfoxonium-Ylide-Directed C-H Activation and Tandem (4 + 1) Annulation.

Rh(III)-catalyzed C-H activation and cyclization of sulfoxonium ylide with acrylates leads to an efficient synthesis of indanone derivatives. The reaction proceeds under mild and external metal-oxidant-free conditions. The sulfoxonium ylide acts as a traceless directing group as well as an internal oxidant. (4 + 1) Annulation after C-H activation leads to the formation of a carbocyclic ring, and the byproduct obtained is DMSO, which can be easily separated.

Weak Coordinating Carbonyl-Directed Rhodium(III)-Catalyzed C-H Activation at the C4-Position of Indole with Allyl Alcohols.

A weakly coordinating carbonyl-directed coupling of allyl alcohols at the C-4 position of indole derivatives under the C-H activation conditions catalyzed by Rh(III) is reported. This results in alkylation at the C-4 position of indole derivatives exclusively. The obtained product forms a tricyclic derivative under aldol reaction conditions, which can be a potential precursor for synthesizing a few alkaloid molecules such as ergot, hapalindole alkaloids, and related heterocyclic compounds.

Rhodium(III)-Catalyzed C-H Activation: A Cascade Approach for the Regioselective Synthesis of Fused Heterocyclic Lactone Scaffolds.

A Rh(III)-catalyzed cascade C-H activation; regioselective [4 + 2] oxidative annulation; and lactonization of aromatic acids, anhydrides, and acrylic acid derivatives with 4-hydroxy-2-alkynoates have been disclosed. This strategy leads to fused heterocyclic lactone scaffolds in a single step with moderate functional group tolerance and excellent site selectivity. Besides, in one step, an antipode of the cephalosol intermediate natural product that contains a tricyclic isocoumarin framework has been synthesized.

Synthesis of Naphthols by Rh(III)-Catalyzed Domino C-H Activation, Annulation, and Lactonization Using Sulfoxonium Ylide as a Traceless Directing Group.

Sulfoxonium ylide directed C-H activation catalyzed by the Rh(III)-catalyst has been disclosed. In this study, sulfoxonium ylide functions as a traceless directing group, which reacts with an unsymmetrical alkyne, 4-hydroxy-2-alkynoate, to form the corresponding furanone-fused 1-naphthols. The application of the methodology has been illustrated by synthesizing bromo lactones, which are used as an intermediate in synthesizing photochromic dichroic materials. We have also demonstrated the synthesis of an analogue of fimbricalyx lactone A.

Synthesis of Furanone-Fused 1,2-Benzothiazine by Rh(III)-Catalyzed C-H Activation: Regioselective Oxidative Annulation Leading to in Situ Lactonization in One Pot.

A sulfoximine-directed C-H activation strategy catalyzed by a Rh(III)-catalyst leads to an efficient synthesis of furanone-fused 1,2-benzothiazine. In this reaction, cascade C-H activation, regioselective annulation, and lactonization occur in one pot. 4-Hydroxy-2-alkynoates, as coupling partners, form unsymmetrical alkynes, which undergo lactonization after C-H activation and regioselective annulation. The method shows a good scope with a wide-range of sulfoximine and alkynoates and displays regioselectivity in forming single regioisomers in good yields.

Weak Coordinating Carboxylate Directed Rhodium(III)-Catalyzed C-H Activation: Switchable Decarboxylative Heck-Type and [4 + 1] Annulation Reactions with Maleimides.

A weakly coordinating carboxylate directing group assisted C-H activation with maleimides leading to novel and switchable decarboxylative Heck-type and [4 + 1] annulation products catalyzed by Rh(III) has been reported. In these reactions, solvents play a vital role in switching the selectivity. An aprotic solvent, THF, leads to the decarboxylative Heck-type product while the protic solvent, TFE, results in the [4 + 1] annulation product. The methodology shows high functional group tolerance.

Rh(III)-Catalyzed Oxidative Annulation of Sulfoximines with Arylalkynyl Silanes via Desilylation.

Rh(III) catalyzed oxidative synthesis of 1,2-benzothiazine derivatives using sulfoximine as a directing group under C-H activation strategy is reported. In this study, we utilized arylalkynyl silanes as an alternate for terminal alkynes to obtain the 1,2-benzothiazine derivatives via desilylation pathway. The developed methodology shows a good range of functional group tolerance and furnished the products in moderate yields.

Visible-Light-Mediated Direct Decarboxylative Acylation of Electron-Deficient Heteroarenes Using α-Ketoacids.

Acylation of electron-deficient heteroaromatic compounds has been developed using visible light. α-Ketoacids have been used as an efficient source of acyl radicals under photoredox conditions. The in situ generated acyl radicals from α-ketoacids have been coupled to a wide variety of electron-deficient heteroaromatic compounds in a Minisci type reaction. This method would be attractive to access biologically attractive molecules.

Cobalt(III)-Catalyzed [4 + 2] Annulation of N-Chlorobenzamides with Maleimides.

A Co(III)-catalyzed novel [4 + 2] annulation of N-chlorobenzamides with maleimides has been reported. Mostly, maleimides are known to furnish the Michael-type or 1,1-type cyclized products while treating with amides. In this reaction, maleimides furnished [4 + 2] annulated products in good yields at room temperature while being treated with the internal oxidizing N-chlorobenzamide as a directing group. The developed methodology is compatible with a variety of functional groups. The [4 + 2] annulated products obtained are featured in some biologically active molecules.

Rh(iii)-Catalyzed ortho-C-(sp2)-H amidation of ketones and aldehydes under synergistic ligand-accelerated catalysis.

Rh(iii)-Catalyzed ortho-C-H amidation of ketones and aldehydes under cooperative metal organocatalysis has been utilized for synthesizing various ortho-amidocarbonyl analogs, and the reaction for the aldehyde proceeds at ambient temperature. The aniline derivative 3,5-bis(trifluoromethyl)aniline promotes the amidation reaction via a transient imine directing group. The efficient amidation agent is dioxazolone. The synthetic utility has been demonstrated in the synthesis of quindolinone alkaloids.

Employing Water as the Hydride Source in Synthesis: A Case Study of Diboron Mediated Alkyne Hydroarylation.

We present an approach to utilize water as the hydride source via Pd(II)/Pd(0) catalysis. As a case study, we have achieved a diboron mediated Pd(II)-catalyzed hydroarylation of alkynes using arylboronic acids. This approach not only complements conventional reactivity of Pd via Pd(0)/Pd(II) cycle for the hydroarylation but also utilizes water as the hydride source. We believe this would particularly be beneficial in utilizing water as a reagent.

Rhodium(iii)-catalyzed C-H activation at the C4-position of indole: switchable hydroarylation and oxidative Heck-type reactions of maleimides.

A Rh(iii)-catalyzed C-H activation of indole at the C4-position leading to novel and switchable functionalization has been reported by employing a weakly co-ordinating COCF3 group as a directing group. An additive plays an important role in switching the selectivity between 1,4-addition products and Heck-type products. An acid additive led to the formation of 1,4-addition products whereas a base additive promotes the formation of Heck-type products. Deuteration studies and control experiments were helpful to propose the mechanism.