A DFT study on the palladium-bisphosphine-catalyzed hydrogenation of alkynes is presented. The theoretical study explores the feasibility of two independent mechanisms, one based on the neutral species Pd(0)(P-2) (where P-2 = 2PH(3) or PH2CH2CH2PH2) and the second based on cationic intermediates of the type [Pd(II)(P-2)(H)](+). The paper compares the theoretical results with experimental observations obtained in a parallel NMR study. The calculations reveal that for the Pd(0) system to achieve useful catalysis a phosphine loss mechanism is necessary with subsequent binding of the alkyne to Pd(P-2) being followed by phosphine loss and H-2 coordination. After hydride transfer, the formation of Pd(PH3)(H)(CH = CH2) is predicted. This species is instrumental in forming Pd(P-2)(eta(2)-CH2 = CH2). Formation of Pd(P-2)(H)(CH2CH3) also proceeds via,phosphine loss, in this case from Pd(P-2)(eta 2-CH2 = CH2). In contrast, the cationic mechanism involves Pd(II)(P-2)(H)(+), which reacts with the alkyne to form Pd(P-2)(CH = CH2)(+) directly. A role for Pd(P-2)(H)(eta(2)-CH2 = CH2)(+) and Pd(P-2)(CH2CH3)(+) in both alkene isomerization and hydrogenation is established. For the cationic cycle, alkene isomerization is predicted to be facile, while reductive elimination of alkane via H-2 coordination involves a higher barrier, in good agreement with experimental observations. For the neutral cycle, both alkene isomerization and alkane formation also involve alkylpalladium species such as Pd(P-2)(H)(CH2CH3) but they now correspond to high-energy processes and are predicted to be less likely. Overall calculations support for the palladium-bisphosphine systems a reaction mechanism based on cationic monohydride precursors.
- PARAHYDROGEN-INDUCED POLARIZATION
- MOLECULAR-ORBITAL METHODS
- PAIRWISE ADDITION