Comparative behaviour of cyclic versus acyclic olefins in the stereochemistry of the metathesis reaction with group VI metal-based catalysts

Metatheses of cis-2-pentene and norbornene have been carried out with Group VI metal-based catalysts associated with various co-catalysts. With cis-2-pentene the stereoselectivity has been determined by the trans/cis ratios of 2-butenes and 3-hexenes extrapolated to zero percent conversion. With norbornene, the percentage of double bonds in the polymer has been determined by ¹H NMR and ¹³C NMR. The tungsten precursor complexes used were of the type W(CO)₅L, W(CO)₄L₂, W(CO)₃(arene), W(CO)₄X₂, WCl₆, W(OPh)₆, WOCl₄ and Mo(CO)₃(arene), MoCl₅, Mo(NO)₂X₂L₂ and (Mo(NO)₂X₂)_n. The chromium precursor complex was Cr(CO)₃(arene). The co-catalysts used were Me₃Al₂Cl₃, EtAlCl₂ (with or without O₂, C₂H₅OH, C₂H₅OCP₂H₅, 2,6-dichlorophenol, 2,6-diisopropylphenol, 1,3-diphenyl-2-propanol), SnMe₄, SnPh₄, CCl₄ + hν, AlX₃. With norbornene, the percentage of cis double bonds was found to be equal to 50 ± 5% corresponding to a random coordination and/or reaction of the exo double bond of the norbornene to the metallacarbene. The value obtained was the same whatever the transition metal, the ligands coordinated to the precursor complex with the co-catalyst. These results contrast sharply with those obtained with acylic olefins using the same or similar catalysts. For example, with cis-2-pentene, there is a strong effect of the transition metal on the resulting stereochemistry. The retention of the configuration of the starting olefin increases in the order W < Mo < Cr. It seems also that there is a slight but meaningful effect of ligand, especially with molybdenum. A general explanation is given on the origin of stereoselectivity in the metathesis of acyclic and cyclic olefins, which is based on the relative stabilities of the coordinated olefin and of the metallacyclobutanes. If the centages of cis and trans protons assuming the same relaxation time for both types of protons. In any case the repetition time was large enough. The results obtained by both methods in the same sample gave us the same percent of cis double bonds.