DFT study of the olefin metathesis catalyzed by ruthenium complexes

A theoretical investigation has been carried out at the DFT (B3LYP) level on the mechanism of the metathesis reaction catalyzed by Grubbs' complexes. Two model systems have been used: (a) The first model is formed by one ethylene molecule and the Cl-2(PH3)(2)Ru=CH2 complex (Grubbs' catalyst). (b) In the second model the Cl-2(PPh3)(2)Ru=CH2 species has been considered. The following results are relevant: (i) The "primary" active catalytic species is a metal-carbene (PR3)(2)Cl2Ru=CH2. The corresponding carbenoid complex (PR3)(2)ClRu-CH2Cl is significantly higher in energy (18.45 and 19.26 kcal mol(-1) for the two model systems) and thus cannot represent the starting active species of the process. (ii) The existence of three different reaction pathways has been demonstrated. One of the two most likely reaction channels is characterized by the presence of "secondary" active species of carbenoid type. These species, after olefin coordination, become slightly more stable than the corresponding carbenic forms and play a key role in the formation of the metallacy-clobutane intermediates. Their stability further increases when phenyl rings replace the phosphine hydrogens. (iv) The cyclopropanation is disfavored since it requires the overcoming of larger activation barriers than those found for the metathesis.