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Oxidative Addition to Palladium(0) Diphosphine Complexes: Observations of Mechanistic Complexity with Iodobenzene as Reactant
Author(s) -
Kurbangalieva Almira,
Carmichael Duncan,
Hii K. K. Mimi,
Jutand Anny,
Brown John M.
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201301937
Subject(s) - chemistry , steric effects , phosphine , ligand (biochemistry) , associative substitution , nuclear magnetic resonance spectroscopy , iodobenzene , oxidative addition , palladium , medicinal chemistry , reaction mechanism , substituent , stereochemistry , proton nmr , van der waals force , photochemistry , organic chemistry , catalysis , molecule , biochemistry , receptor
Using a combination of electrochemical and NMR techniques, the oxidative addition of PhX to three closely related bis‐diphosphine P 2 Pd 0 complexes, where the steric bulk of just one substituent was varied, has been analysed quantitatively. For the complex derived from Me t Bu 2 P, a rapid reaction ensued with PhI following an associative mechanism, and data was also obtained by cyclic voltammetry for PhOTs, PhBr and PhCl, revealing distinct relative reactivities from the related (PCx 3 ) 2 Pd complex (Cx=cyclohexyl) previously studied. The corresponding Et t Bu 2 P complex reacted more slowly with PhI and was studied by NMR spectroscopy. The reaction course indicated a mixture of pathways, with contribution from a component that was [PhI] independent. For the Cx t Bu 2 P complex, reaction was again monitored by NMR spectroscopy, and was even slower. At high PhI concentrations reaction was predominantly linear in [PhI], but at lower concentrations the [PhI] independent pathway was again observed, and an accelerating influence of the reaction product was observed over the concentration range. The NMR spectra of the Et t Bu 2 P and Cx t Bu 2 P complexes conducted in C 6 D 6 shows some line broadening that was augmented on addition of PhI. NMR experiments carried out in parallel show that there is rapid ligand exchange between free phosphine and the Pd 2 Pd complex and also a slow ligand crossover between different P 2 Pd complexes. DFT calculations were carried out to further test the feasibility of C 6 D 6 involvement in the oxidative addition process, and located Van der Waals complexes for association of the P 2 Pd 0 complexes with either PhI or benzene. PhI or solvent‐assisted pathways for ligand loss are both lower in energy than direct ligand dissociation. Taken all together, these results provide a consistent explanation for the surprising complexity of an apparently simple reaction step. The clear dividing line between reactions that give a di‐ or monophosphine palladium complex after oxidative addition clarifies the participation of the ligand in coupling catalysis.