Togati Naveen

Palladium-catalyzed synthesis of benzofurans and coumarins from phenols and olefins.

Benzofuran derivatives are ubiquitous in natural products, agrochemicals, pharmaceuticals, and organic materials. Particularly, 2-arylbenzofurans are widely distributed in nature, and possess a number of biological activities. Over the years, several effective strategies involving transition-metal-catalyzed inter/intramolecular heteroannulation reactions have been reported for synthesizing benzofuran scaffolds. However, synthesis of 2-substituted benzofuran by simply reacting phenol and an unactivated olefin has yet to be reported (Scheme 1).

Efficient and Stereoselective Nitration of Mono- and Disubstituted Olefins with AgNO2 and TEMPO

Nitroolefin is a common and versatile reagent. Its synthesis from olefin is generally limited by the formation of mixture of cis and trans compounds. Here we report that silver nitrite (AgNO2) along with TEMPO can promote the regio- and stereoselective nitration of a broad range of olefins. This work discloses a new and efficient approach wherein starting from olefin, nitroalkane radical formation and subsequent transformations lead to the desired nitroolefin in a stereoselective manner.

Stereoselective Nitration of Olefins with tBuONO and TEMPO: Direct Access to Nitroolefins under Metal-free Conditions

Nitroolefins are essential elements for both synthetic chemistry and medicinal research. Despite significant improvements in nitration of olefin an efficient metal-free synthesis remains elusive so far. Herein, we disclose a new set of reagents to access nitroolefins in a single step under metal-free conditions. A wide range of olefins with diverse functionalities has been nitrated in synthetically useful yields. This transformation is operationally simple and exhibits excellent E-selectivity. Furthermore, site selective nitration in a complex setup makes this method advantageous.

A predictably selective nitration of olefin with Fe(NO3)3 and TEMPO.

Ferric nitrate with catalytic TEMPO has been identified as a useful reagent for regio- and stereoselective nitration of a wide variety of aromatic, aliphatic, and heteroaromatic olefins. This reaction provided nitroolefins in preparatively useful yields with excellent E-selectivity. Due to its mild nature and operational simplicity, the present protocol is expected to find application in synthetic setup.

Palladium‐Catalyzed Annulation of Diarylamines with Olefins through CH Activation: Direct Access to N‐Arylindoles

A palladium-catalyzed dehydrogenative coupling between diarylamines and olefins has been discovered for the synthesis of substituted indoles. This intermolecular annulation approach incorporates readily available olefins for the first time and obviates the need of any additional directing group. An ortho palladation, olefin coordination, and β-migratory insertion sequence has been proposed for the generation of olefinated intermediate, which is found to produce the expected indole moiety.

Radical based strategy toward the synthesis of 2,3-dihydrofurans from aryl ketones and aromatic olefins.

A copper-mediated annulation of aryl ketones with a wide range of aromatic olefins has been developed. This strategy allowed convenient access to 2,3-dihydrofuran derivatives. The versatility of the protocol is shown by synthesizing α-methyl dihydrofurans, which serve as an intermediate for the synthesis of vitamin B1. In addition, the applicability of the protocol in conjugated systems is demonstrated. A radical pathway was presumed and supported for annulation of aryl ketones with olefins.

Divergent Reactivity in Palladium-Catalyzed Annulation with Diarylamines and α,β-Unsaturated Acids: Direct Access to Substituted 2-Quinolinones and Indoles

A palladium-catalyzed CH activation strategy has been successfully employed for exclusive synthesis of a variety of 3-substituted indoles. A [3+3] annulation for synthesizing substituted 2-quinolinones was recently developed by reaction of α,β-unsaturated carboxylic acids with diarylamines under acidic conditions. In the present work, an analogous [3+2] annulation is achieved from the same set of starting materials under basic conditions to generate 1,3-disubstituted indoles exclusively. Mechanistic studies revealed an ortho-palladation-π-coordination-β-migratory insertion-β-hydride elimination reaction sequence to be operative under the reaction conditions.

Copper/P(tBu)3-Mediated Regiospecific Synthesis of Fused Furans and Naphthofurans

A novel [3+2] cycloaddition between a variety of cyclic ketones and diverse olefins or alkynes can be effectively promoted by copper in combination with the tri-tert-butylphosphine [P(tBu)3 ] ligand. This protocol exhibits excellent selectivity and provides an exemplary set of fused heterocycles in good to excellent yields. Present strategy also represents an extremely simple and atom-economic way to construct substituted fused furans and naphthofurans from readily available starting materials under mild reaction conditions. The utility of the method is further demonstrated by the synthesis of chiral furans from (R)-(-)-carvone and (S)-(+)-carvone. A plausible mechanism involving the oxidative radical cyclization has been suggested based on experimental observations.

Palladium‐Catalyzed [3+3] Annulation between Diarylamines and α,β‐Unsaturated Acids through CH Activation: Direct Access to 4‐Substituted 2‐Quinolinones

A C-H activation strategy has been successfully employed for the high-yielding synthesis of a diverse array of 4-substituted 2-quinolinone species by a palladium-catalyzed dehydrogenative coupling involving diarylamines. This intermolecular annulation approach incorporates readily available α,β-unsaturated carboxylic acids as the coupling partner by suppressing the facile decarboxylation. Based on preliminary mechanistic studies, a reaction sequence is proposed, involving ortho palladation, π-coordination, β-migratory insertion, and β-hydride elimination.