Mizoroki–Heck reactions catalyzed by palladium dichloro-bis(aminophosphine) complexes under mild reaction conditions. The importance of ligand composition on the catalytic activity

Abstract

Dichloro-bis(aminophosphine) complexes of palladium with the general formula [(P{(NC₅H₁₀)_3−n(C₆H₁₁)_n})₂Pd(Cl)₂] (where n = 0–2) are easily accessible, cheap and air stable, highly active and universally applicable C–C cross-coupling catalysts, which exhibit an excellent functional group tolerance. The ligand composition of amine-substituted phosphines (controlled by the number of P–N bonds) was found to effectively determine their catalytic activity in the Heck reaction, for which nanoparticles were demonstrated to be their catalytically active form. While dichloro{bis[1,1′,1′′-(phosphinetriyl)tripiperidine]}palladium (1), the least stable complex (towards protons) within the series of [(P{(NC₅H₁₀)_3−n(C₆H₁₁)_n})₂Pd(Cl)₂] (where n = 0–3), is a highly active Heck catalyst at 100 °C and, hence, a rare example of an effective and versatile Heck catalyst that efficiently operates under mild reaction conditions (100 °C or below), a significant successive drop in activity was noticed for dichloro-bis(1,1′-(cyclohexylphosphinediyl)dipiperidine)palladium (2, with n = 1), dichloro-bis(1-(dicyclohexylphosphinyl)piperidine)palladium (3, with n = 2) and dichloro-bis(tricyclohexylphosphine)palladium (4, with n = 3), of which the latter is essentially inactive (at least under the reaction conditions applied). This trend was explained by the successively increasing complex stability and its ensuing retarding effect on the (water-induced) generation of palladium nanoparticles thereof. This interpretation was experimentally confirmed (initial reductions of 1–4 into palladium(0) complexes of the type [Pd(P{(NC₅H₁₀)_3−n(C₆H₁₁)_n})₂] (where n = 0–3) were excluded to be the reason for the activity difference observed as well as molecular (Pd⁰/Pd^II) mechanisms were excluded to be operative) and thus demonstrates that the catalytic activity of dichloro-bis(aminophosphine) complexes of palladium can – in reactions where nanoparticles are involved – effectively be controlled by the number of P–N bonds in the ligand system.