Palladium‐Based Catalytic Systems for the Synthesis of Conjugated Enynes by Sonogashira Reactions and Related Alkynylations

Conjugated alkynes are recurring building blocks in natural products, a wide range of industrial intermediates, pharmaceuticals and agrochemicals, and molecular materials for optics and electronics. The palladium-catalyzed cross-coupling between sp(2)-hybridized carbon atoms of aryl, heteroaryl, and vinyl halides with sp-hybridized carbon atoms of terminal acetylenes is one of the most important developments in the field of alkyne chemistry over the past 50 years. The seminal work of the 1970s has initiated an intense search for more general and reliable reaction conditions. The interest in the catalytic activation of demanding substrates, the need to minimize the consumption of depletive resources, and the search for easy access to an increased variety of functionalized enynes has led to the current generations of high-turnover catalysts. This Review gives an overview of the highly efficient palladium catalyst systems for the direct alkynylation of C(sp(2)) halides with terminal alkynes, both in homogeneous and heterogeneous phases.