Cryptand-22 as an Efficient Ligand for the Palladium-Catalyzed Mizoroki-Heck Reaction under Air

Palladium-catalyzed cross-coupling reaction between aryl halides and terminal acetylenes (commonly referred to as the Mizoroki-Heck reaction) is a highly versatile, powerful and popular tool for the synthesis of variously substituted olefins, dienes and precursors to conjugated polymers. This reaction has also been utilized for the preparation of natural products,and pharmaceuticals. The traditional Mizoroki-Heck reaction is typically performed with 1-5 mol% of a palladium catalyst along with a phosphine or phosphorusligands in the presence of a suitable base. However, phosphine ligands are toxic, expensive, unrecoverable, and unstable at high temperatures; thus the development of a phosphine-free palladium catalyst is a topic of enormous interest. Moreover, the Mizoroki-Heck reaction still show some limitations, especially in relation to the use of certain reagents. In particular, a major restriction of palladium-catalyzed coupling processes has been the poor reactivity of cheaper and readily available aryl chlorides and aryl bromides in comparison with more active aryl iodides. Therefore, the search for efficient catalysts for the cross-coupling of olefins with deactivated aryl bromide and towards activated aryl chloride is under progress. Herein, we present a Pd(II) catalyst system based on a commercially available diazacrown ether or cryptand-22, PdCl₂-C22, that is very active homogeneous catalyst under aerobic conditions. This catalytic system is efficiently used for the Mizoroki-Heck cross-coupling reactions of aryl iodides, bromides as well as chlorides with various olefins. The remarkable chelating effect of this N- and O-containing ligand and its flexibility assists in stabilizing the reactive palladium intermediates, and suppresses the Pd black formation even at high temperature under air.