Palladium‐Catalyzed Reactions of Propargylic Compounds in Organic Synthesis

Palladium was used in organic synthesis for long time only as a heterogeneous catalyst for hydrogenating unsaturated compounds. But in recent years palladium compounds have found broad application as homogeneous catalysts. In the last decade, homogeneous catalysis relying on transition metal complexes has led to innovations in organic synthesis. Complexes of various transition metals are now used for organic synthesis, and each metal has its own characteristic features. Palladium complexes are the most versatile and used extensively in organic synthesis, particularly for carbon–carbon bond formation. Palladium‐catalyzed reactions can be classified into several groups based on their substrates: organic halides, allylic compounds, conjugated dienes, alkenes, and alkynes. Propargylic compounds undergo a variety of palladium‐catalyzed transformations and make up one important class of these reactions. It is well known that copper and silver salts promote or catalyze several reactions of propargylic compounds. However, palladium compounds, particularly Pd 0 complexes, show catalytic activities entirely different from those of silver and copper salts. Advances in the research on the palladium‐catalyzed reactions of propargylic compounds in the last decade have made the scope of the reactions clear. The ready availability of numerous propargylic alcohols and their esters by the reaction of terminal alkynes with carbonyl compounds clearly enhances the synthetic utility of their reactions. Palladium complexes, in particular palladium phosphane complexes, are soluble in organic solvents and behave as active catalysts under homogeneous conditions.