New Mechanistic Insights into the Copper-Free Heck–Cassar–Sonogashira Cross-Coupling Reaction

The copper-free Heck–Cassar–Sonogashira (HCS) reaction, known since 1975, nowadays represents one of the most powerful methods for C–C bond formation in organic synthesis with several industrial applications. Despite its great success, the mechanism is still under discussion, with several reported possible pathways. To clarify the copper-free HCS reaction mechanism, stoichiometric and catalytic reactions were carried out and monitored by 31P/1H NMR spectroscopy, HPLC, and GC chromatography. In particular, the investigation of the role of the base, mimicking the real catalytic conditions, highlighted the fact that secondary amines rapidly induce precatalyst reduction and decrease the energy barrier for the alkyne carbopalladation step. The results supported the mechanism via direct coordination of the terminal alkyne on the oxidative addition complex. Depending on the palladium counterion, and independent of the solvent, aromatic substitutions, temperature, and terminal alkyne substitution, these studies support two different pathways: with halides, a neutral route, and with the triflate, a cationic one.