para-Toluenesulfonic Acid Catalyzed Synthesis of Indenes via a Tandem Friedel–Crafts Alkylation/Hydroarylation of Tertiary Propargylic Alcohols with Electron-Rich Arenes

p-Toluenesulfonic acid-promoted organic transformations for the generation of molecular complexity

Since the beginning of this century, p-toluenesulfonic acid (p-TSA) catalysed organic transformations have been an active area of research for developing efficient synthetic methodologies. Often, catalysis using p-TSA is associated with many advantages, such as operational simplicity, high selectivity, excellent yields, and ease of product isolation, which make organic synthesis convenient and versatile. Notably, p-TSA is a non-toxic, commercially available, inexpensive solid organic compound that is soluble in water, alcohols, and other polar organic solvents. p-TSA is a strong acid compared to many protic or mineral acids and its high acidity helps activate different organic functional groups. p-TSA-promoted conversions are fast, have a high atom and pot economy, and feature a multiple bond-forming index. Therefore, the utilization of p-TSA enables the synthesis of many important structural scaffolds without any hazardous metals, making it desirable in numerous applications of sustainable and green chemistry. Recently, this emerging area of research has become one of the pillars of synthetic organic chemistry to synthesise biologically relevant, complex carbocycles and heterocycles. This study provides a comprehensive summary of methods, applications, and mechanistic insights into p-TSA-catalysed organic transformations, covering the literature reports that have appeared since 2012.

Rhodium-Catalyzed Asymmetric Annulation of Unactivated Alkynes with 3-(ortho-Boronated Aryl) Conjugated Enones: Enantioselective Synthesis of 2,3-Disubstituted Indenes

A rhodium-catalyzed tandem arylation/cyclization reaction of 3-(ortho-boronated aryl) conjugated enones with unactivated alkynes is reported. By using a rhodium(I)/chiral-diene complex as the catalyst, the protocol was processed smoothly to provide various 2,3-disubstituted indene compounds in high yields with excellent regioselectivities and enantioselectivities. The approach outlined herein is appealing, as simple diarylalkynes, diakylalkynes, and alkyl(aryl)alkynes are the starting materials.

A Brønsted Acid Catalyzed Easy Access to Conjugated Diene-­Diones from Tertiary Propargyl Alcohols and 1,3-Dicarbonyl ­Compounds

A simple protocol catalyzed by a Brønsted acid, trifluoromethanesulfonic acid (CF3SO3H), to synthesize conjugated diene-diones from easily accessible tertiary propargyl alcohols and 1,3-dicarbonyl compounds is described. Reactions are performed in an air atmosphere by using undried solvents with water being the environmentally benign side product of the process. Preliminary synthetic applications of the conjugated diene-diones in the formation of conjugated dieneones and furans are also reported herein.

Chemoselective and Regioselective Synthesis of Spiroisoindolinone Indenes via an Intercepted Meyer-Schuster Rearrangement/Intramolecular Friedel-Crafts Alkylation Relay

A Brønsted acid-catalyzed reaction between isoindolinone-derived propargylic alcohols and external aromatic nucleophiles for the construction of spiroisoindolinone indenes is described. The reaction proceeds rapidly with a broad range of substrates to generate spiroindenes chemoselectively and regioselectively in moderate to high yields. Key to the success of this transformation is an intercepted Meyer-Schuster rearrangement/intramolecular Friedel-Crafts alkylation relay that offers a modular approach in the synthesis of target compounds.