Process Development of a Sonogashira Cross-Coupling Reaction as the Key Step of Tirasemtiv Synthesis Using Design of Experiments

Dihydro-10H-indeno[1,2-b]benzofurans and Tetrahydroindeno[1,2-c]isochromenes via Stereoselective Intramolecular Carbocation Cascade Cyclization

Nazarov cyclization of the (E)-(2-stilbenyl)methanols under the catalysis of p-TsOH immobilized on silica (PTS-Si) proceeded to give the corresponding indanyl cation with the exclusive trans relationship at the two newly formed adjacent stereogenic centers. The ensuing intramolecular nucleophilic addition by the MOM-protected phenol (m = 0) or benzyl alcohol (m = 1) furnished the Indane-fused benzofuran [5/5] or isochroman [5/6] system, respectively, with the exclusive cis stereocontrol at the two-carbon ring junction. Thus, in a single step, from nonchiral starting materials, the intramolecular cascade carbocation cyclization (CCC) furnished the [5/5] or [5/6] oxygen-containing Indane fused-ring systems in moderate to good yields with excellent stereoselectivity on all three contiguous stereogenic centers.

Photochemical Sonogashira coupling reactions: beyond traditional palladium-copper catalysis

Sonogashira coupling is one of the Nobel reactions discovered in 1975 to form a C-C bond using palladium and copper as co-catalysts. Incorporating alkyne functionalities either in macro or micro molecules by using this Sonogashira reaction has already proven its viability and relevance in the sphere of synthetic chemistry. While aiming for sustainable chemistry, in recent years, visible light photoredox catalysts have appeared as an advanced tool in this regard. In this review, we aim to portray a comprehensive summary of modern visible light photo redox catalyzed Sonogashira reaction, which will leave space for the readers to rethink alternative strategies to conduct the Sonogashira reaction. This review briefly describes the implementation of various metal-based nanomaterial photocatalysts, developing either copper or palladium-free photocatalytic methods, and organo-photolytic and bioinspired photocatalysts for the Sonogashira coupling reactions. Besides, this review also gives a concise overview of the mechanistic aspects and highlights selective examples for substrate scope.

Assessing the environmental benefit of palladium-based single-atom heterogeneous catalysts for Sonogashira coupling

The Pd-Cu catalysed Sonogashira coupling of terminal alkynes and aryl halides is a cornerstone synthetic strategy for C-C bond formation. Homogeneous organometallic systems conventionally applied are typically not reusable and require efficient downstream Pd removal steps for product purification, making it challenging to fully recover the precious metal. A holistic cradle-to-gate life cycle assessment (LCA) unveils that process footprint can be improved up to two orders of magnitude from repeated catalyst reuse. New classes of heterogeneous catalysts based on isolated metal atoms (single-atom catalysts, SACs) demonstrate promising potential to synergise the benefits of solid and molecular catalysts for efficient Pd utilisation. Here we show that using Pd atoms anchored on nitrogen-doped carbon permits full recovery of the metal and reuse of the catalyst over multiple cycles. A hybrid process using the Pd-SAC with a homogeneous CuI cocatalyst is more effective than a fully heterogeneous analogue based on a bimetallic Pd-Cu SAC, which deactivates severely due to copper leaching. In some scenarios, the LCA-based metrics demonstrate the footprint of the hybrid homogeneous-heterogeneous catalytic process is leaner than the purely homogeneous counterpart already upon single reuse. Combining LCA with experimental evaluation will be a useful guide to the implementation of solid, reusable catalysts for sustainable organic transformations.