Decelerating effect of alkynes in the oxidative addition of phenyl iodide to palladium(0) complexes in palladium-catalyzed multicomponent reactions and Sonogashira reactions

The oxidative addition of PhI to [Pd-0(PPh3)(4)] in DMF is slower when performed in the presence of the terminal alkynes (PhCdropCH, EtO2C-CdropCH) that are reagents in palladium-catalyzed Sonogashira or multicomponent reactions. The concentration of the reactive [Pd-0(PPh3)(2)] complex decreases because of its partial coordination to the alkyne to form [(eta(2) - RCdropCH)Pd-0(PPh3)(2)] (R = Ph, EtCO2), which is in equilibrium with [PdO(PPh3)(2)]. The complex [(eta(2)-PhCdropCH)Pd-0(PPh3)(2)] was found to be unreactive whereas [(eta(2) -EtO2C-CH)Pd-0(PPh3)(2)] was found to be the unique reactive com-plex at high concentrations of EtO2C-CdropCH. As has been reported for alkenes, terminal alkynes play an unexpected role because they interfere in the oxidative addition before their usual reactions in the carbopalladation and transmetallation steps that follow it. Decreasing the rate of the fast oxidative addition with PhI may promote the efficiency of the catalytic cycle by bringing its rate closer to that of the slower carbopalladation or transmetallation steps. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).