The applicability of two types of transition-metal carbonyl complexes as appropriate candidates for computationally derived Tolman’s ligand electronic parameters were examined with density functional theory (DFT) calculations employing the B97D3 functional. Both Pd(0)L2(CO) and HRh(I)L2(CO) complexes correlated well with the experimental Tolman Electronic Parameter scale. For direct comparison of the electronic effects of diphosphines with those of monophosphines, the palladium-containing system is recommended. The trans influence of various phosphines did not show a major difference, but the decrease of the H-Rh-P angle from linear can cause a significant change.
The oxidative addition of 4-substituted iodobenzenes on Pd(0) catalysts under CO atmosphere was investigated by means of density functional calculations employing the M06//B97-D3 level of theory. The 18-electron triphenylphosphine-tricarbonyl complex was found to be the global minimum. Several coordinatively unsaturated species are predicted to be present both in N,N-dimethylformamide and toluene solution. In terms of activating iodobenzene, bis(triphenylphosphine)palladium(0) was proved to be the most active. However, due to its lower thermodynamic stability, it is slightly inferior to the Pd-triphenylphosphine-carbonyl complex, which is predicted to react with a free energy of activation of 23.2 kcal mol with respect to the initial resting state tetrakis(triphenylphosphine)palladium(0). The effect of 4-substituents of iodobenzene on reaction energetics is also discussed. The activity of the Pd(0) catalyst was found to be governed by the donor-acceptor strength of the ancillary ligands: the barrier decreases with increasing basicity and decreasing back-donating capability.
Para
-substituted iodobenzenes were reacted with
tert
-butyl isocyanide and piperidine as nucleophiles in
the presence of palladium–diphosphine catalysts. Both single
and double insertion of the isocyanide was observed and the corresponding
amidines and ketimine–amidines were obtained in yields of practical
interest. With the increase of the
tert
-butyl isocyanide/iodobenzene
ratio, 100% chemoselectivity toward the ketimine–amidine was
achieved. The formation of the products was rationalized on the basis
of a catalytic cycle analogous to that of the aminocarbonylation reactions.
Clear connection was found between the activity and the electronic
structure of the proposed catalyst Pd(diphosphine) by computational
studies, as the more negative partial charge on palladium resulted
in higher conversion. Among five isocyanide substrates, only
tert
-butyl isocyanide was proved to be active.
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