A series of well-defined homopolymer-stabilized Pd colloids with varying metal particle size was used to study the Heck coupling reaction between aryl halides and olefins. Correlation of initial reaction rates and particle sizes determined by transmission electron microscopy demonstrates that the Heck reaction is a sensitive probe of metal surface structure. The presence of the colloid-stabilizing homopolymer affords a much more stable catalyst than that obtained using the colloid−precursor metal complexes alone. Extremely high total turnover numbers (TON, moles of substrate/moles of Pd; TON = 100 000) and turnover frequencies (TOF = TON/h; TOF > 80 000) are easily attainable with poly(vinylpyrrolidone)-stabilized colloidal palladium in the coupling of p-bromobenzaldehyde with butyl acrylate.

Experimental kinetic studies of the coupling of p-bromobenzaldehyde (1) with butyl acrylate (2) using the dimeric palladacycles complex (4) with chelating nitrogen ligands were carried out together with kinetic modeling using a reaction rate expression based on the mechanism shown in Scheme 2. The oxidative addition product of 1 was found to be the resting state within the catalytic cycle. The formation of dimeric Pd species external to the catalytic cycle helped to rationalize a non-first-order rate dependence on catalyst concentration. Theoretical modeling showed how the relative concentrations of the different intermediate species within the catalystic cycle can influence the observed rate dependence on Pd concentration. It was shown how conventional kinetic studies may give reaction orders in substrates which differ from those which would be observed under practical synthetic conditions. Comparison between phosphine- and nonphosphine-based palladacycles suggests that they follow the same reaction mechanism. The role of water in accelerating the initial formation of the active catalyst species is noted.

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