| Transition metal catalyzed cyclization of 1,6-dienes attracted widely interest for its great application prospect in organic synthesis over the past few years. In this thesis, we have carried out a theoretical investigation at the DFT (B3LYP) level on the mechanisms of the ruthenium carbene complexes mediated cycloisomerisation reaction of 1,6-diene. The model system is formed by 2,2-diallyl dimethyl malonate molecule acting as the 1,6-diene and the [(NHC)Ru=CH2Cl2] complex which emulates the Grubbs'catalyst. Computational study on Ru(Ⅱ) catalyzed ring closing metathesis reaction of 1,6-diene was reported. The calculations, together with available experimental data, explained the phenomenon that ring closing metathesis reaction occurs at low temperature, but exo-methylene-cyclopentane product was obtained in the same catalyst at a much higher temperature.First,theoretical research on metathesis between ruthenium carbene and olefin was carried out at DFT (B3LYP) level. The energy barrier of this step comes out to be 2.9 kcal/mol. This strongly indicates that the ring closing metathesis reaction theoretically can occur at a low temperature. Besides, transforming paths from the ruthenium carbene catalyst precursor to [(NHC)RuCl2] and [(NHC)RuHCl] are theoretically studied. The most facile path includes formation of a three-membered ring through degradation of the ruthenium carbene catalyst. Different reaction pathways leading to the exo-methylene-cyclopentane product were also studied from the perspective of energy. A generic hydrometallation mechanism, an oxidative cyclometallation mechanism and a generic allylic C–H insertion mechanism were proposed and calculated with the aid of DFT (B3LYP) method. Our results indicate that the highest energy barrier of the hydrometallation route is 25.4 kcal/mol. And it's the most favored reaction channel to obtain the product since it requires the overcoming of the lowest activation barrier than those found for the other mechanisms.
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