Insights into the mechanism of active catalyst generation for the Pd^II(acac)₂/PPh₃ system in the context of telomerization of 1,3-butadiene

The mechanism by which Pd^II precursors are reduced to catalytically active low-valent Pd species has been a subject of interest for developing better catalysts. This process is well understood for catalytic systems employing a combination of palladium(ii) acetate [Pd(OAc)₂] and tertiary phosphines. However, the mechanism of reduction of palladium(ii) acetylacetonate [Pd(acac)₂] in the presence of phosphines has not been thoroughly investigated. This is especially important in the context of the Pd-catalyzed butadiene telomerization process, which uses a combination of Pd(acac)₂ and tertiary phosphines in methanol to produce the commercially valuable precursor 1-methoxyoctadiene (MOD-1). In this work, we elucidate the steps for generating the active Pd⁰ species for this reaction using a combination of Pd(acac)₂ and triphenylphosphine (PPh₃). The investigations presented in this study provide the following key insights: (a) unification of the steps involved in the generation of the active precatalyst [Pd^II(acac)(PPh₃)₂]⁺; (b) elucidation of the mechanism of reduction of the precatalyst to Pd⁰ without MOD-1, which parallels the chemistry of the Pd(OAc)₂/PPh₃ system; and (c) the generation of Pd^II-octadienyl species from the reaction between the precatalyst and MOD-1, the product of the telomerization reaction. A reversible C-O bond cleavage process was identified that leads to the formation of the Pd^II π-octadienyl species as the active catalyst in the commercial telomerization process. These studies provide important insights into the reduction of Pd(acac)₂ into active Pd⁰ species or Pd^II π-allyl species, which have wide implications for both cross-coupling catalysis as well as the telomerization reaction.