Theoretical characterization of olefin metathesis in the bis-dicyclopentadienyltitanium(IV) system by density functional theory

Local and nonlocal density functional theory (DFT) was used to study olefin metathesis in the TiCP2C3H4(R-1,R-2) system, in which the alkyl substituen ts (R-1,R-2) are at the beta-position of the metallocyclobutane ring. The s tructures and stabilities of the metallocyclobutanes were calculated, and t he mechanism of olefin insertion and the process of ring-opening metathesis polymerization were investigated. The predicted geometries of the metalloc yclobutanes agree well with experimental structures, especially those predi cted by nonlocal DFT. The relative stabilities of the metallocyclobutanes w ere studied by calculating the energy change for the following olefin excha nge reaction: TiCP2C3H4(R-1,R-2) + C2H2(R-3,R-4) --> TiCP2C3H4(R-3,R-4) + C 2H2(R-1,R-2) The relative stabilities of the metallocyclobutanes are also s trongly dependent upon the number and steric size of the alkyl group(s) (R- 1,R-2) at the beta-position in the metallocyclobutane ring. In general, non local DFT predicts olefin exchange energies that are in better agreement wi th the experimentally observed free energies of olefin exchange than local DFT. The quantitative agreement between the experimental and calculated Del ta G's for olefin exchange are within 0.8 kcal/mol. The mechanism of metath esis was investigated by calculating the potential energy surface for olefi n elimination from TiCp2C3H5(tBu). No compelling evidence was found for a l ocal minimum corresponding to a titanium-alkylidiene-olefin complex, which is inconsistent with conclusions drawn from experimental mechanistic studie s but is consistent with all prior theoretical calculations on metal assist ed 2 + 2 insertions. The mechanism of cyclopentene and norbornene ring-open ing polymerization was also studied.