Metathesis transformations of terpenes. Computational modeling of (−)-α-pinene ring opening by ruthenium and tungsten carbene catalysts

The ring opening metathesis reaction of (−)-α-pinene mediated by the second generation Grubbs catalyst, tungsten based Schrock and Fischer type metalacarbenes has been studied at PBE0/LACV3P*//PBE0/LACVP* level of theory. The calculation demonstrated the importance of the steric factor in both the metathesis catalyst and the monomer substrate. Thus, the lowest activation and reaction energies were found for methylene metalacarbenes. In spite of low steric hindrances in fluorometalacarbene 1d, it shows relatively high activation energy due to strong metal–carbene bond in this catalyst. The successful catalyst for (−)-α-pinene metathesis should have small substituents at metal, carbene atoms, the large active site and the small energy difference between the molecular orbital involving an olefin bond and the carbene atom, thus facilitating the formation of a new Cdouble bond; length as m-dashC bond. The oxidation state of the metal center plays important role in the reactivity of tungsten containing carbene complexes.