Computational study of carbon - carbon and carbon - heteroatom bond formation.

摘要

Pt-B, Rh/Ir-B pi bonding resulting from pi-back donation from these d6 and d8 metals is not significant as deduced from Metal - Boryl rotational barriers, the results of the geometry optimizations at different levels of theory and calculated 11B NMR properties. The oxygen atoms of the Bcat ligand (and by extension similar ligands) exert greater influence on the boryl structure than the transition metal insofar as pi-bonding is concerned. The ppi electrons on oxygen backdonate to the vacant 2p orbital of the trigonal planar B. The early transition metal W (d2) has stronger W - B pi back donation. However, even in this case the pi bonding does not appear to be commensurate with related alkylidene and imido complexes.; The active species of Pd-phosphine catalysts and phosphine ligands are also studied in this computational work. The two possible (up or down) conformers of phosphine ligands are compared. Energetics derived from ONIOM schemes shows that for t-butyl substituted phosphines, the "down" conformer is more stable than the "up" conformer primarily for electronic reasons. However, the cyclohexyl-substituted ligand is more influenced by sterics. For mono-phosphine palladium complexes QM and MM contributions are larger after coordinating to the metal, reinforcing each other leading to the strong preference of the "down" conformers, whether substituted by t-butyl or cyclohexyl. The possible Pd--H and Pd--C interactions are observed in PdP1-down, PdP4-down and PdP6-down. This Pd - pi interaction further stabilized the down conformation. In the more crowded Pd(phosphine)2 systems, the steric contribution is more substantial, and no Pd - pi interaction is observed to stabilize the ONIOM preferred conformations.; The last part of this study is a comprehensive study of the transmetallation process in the Suzuki biaryl coupling reaction. The investigated factors include transition metal (Pd and Ni), competing pathways (A and B), X substituents (Cl, Br, I and OH) and different phosphine ligands (PH3, P( o-tolyl)3 and P(t-Bu)3). The geometry of the TM-C bond and B-C bond in the TS suggests that the transmetallation is not a concerted process. The total electronic density distribution calculation shows that there is no TM-B bonding in the transmetallation TS.; The isomerization process of the Suzuki coupling reaction is not the rate deterministic step. The ONIOM results support the hypothesis that the elimination of B(OH)2X is not a concerted step. TM-C bond is established prior to the breaking of B-C bond. The steric effect is more significant for the P(t-Bu)3 and P(o-tolyl) 3 models. The reaction pathway B is preferred, according to the ONIOM calculations. The TSs in this study are stabilized by the less e-donating P(o-tolyl)3. The less e-donating halides (more electronegative) are also observed to lower the energy barrier. The ONIOM/DFT calculations on activation energy have shown that Pd is more efficient than the Ni replacement.