Ab Initio Calculations of Propylene Addition to the Metal-Methyl Bond in Titanium and Aluminum Alkyl Complexesстатья
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Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.
Аннотация:The reaction of propylene addition to the aluminum-methyl and titanium-methyl bonds in the H2AlCH3 and H2TiCH3 complexes has been examined by the ab initio SCF MO LCAO method in the valencesplit Gaussian basis set. The structures of the transition state (TS) of the insertion reaction, the heats of the π-complex formation, and the activation energies for the insertion of the coordinated propylene via the mechanisms of 1,2- and 2,1-cis-insertion were calculated. It is shown that a lower value of the free activation energy of the 1,2-addition in comparison with that of the 2,1-addition is determined by lower values of both the activation energy and entropy. It is suggested that the driving force for 1,2-insertion is the polarization of the C=C bond in propylene, due to which the electrostatic interaction in the four-center transition state stabilizes TS-1,2 more strongly than TS-2,1. The calculated results indicate that propylene is characterized by a decrease in reactivity (as compared to ethylene) in the reaction of addition to the M-R bond. This fact may be attributed to a higher position of the π* orbital of propylene (the stronger π bond) as compared to ethylene. It is shown that the energy of the antibonding π* orbital of monomers (acetylene, ethylene, and propylene) can serve as a reactivity index for the reaction of addition to the metal-alkyl bond.