The ring-opening metathesis of bicyclo[2.2.1]heptene (norbornene) has been carried out with group 6 metal-based catalysts associated with various cocatalysts. The percentage of cis double bonds in the polymer has been determined by 1H NMR and 13C NMR with the precursor complexes W(CO)5L [L = CO, PPh3, C(OC2H5)(C6H5), C(C6H5)2], M(CO)3(Mes) (Mes = mesitylene; M = Cr, Mo, W), M(NO)2Cl2(PPh3)2 (M = Mo, W), WX6 (X = Cl, OPh), and WOCl4. With all the catalysts studied the percentage of cis double bonds was found to be equal to 50 ± 5%, a value corresponding to a random coordination and (or) reaction of the exo double bond of the norbornene to the metallocarbene. The results contrast sharply with those obtained with acyclic olefins on using the same catalysts. A general explanation is given for the origin of the stereoselectivity in metathesis of acyclic and cyclic olefins. If the coordinated olefin has an energy which is smaller than that of the two possible metallacyclobutanes leading to the cis or the trans isomers, the resulting stereoselectivity will be governed by the energy levels of these two metallacycles. If the coordinated olefin has an energy which is higher than that of the cis- and trans-directing metallacyclobutanes, the system will loose its stereoselectivity and have a trans/cis ratio of unity. Most acyclic olefins belong to the first category. Highly strained olefins belong to the second category due to the exothermic strain release which is already partially carried out in the metallacyclobutane intermediate. The ligands do not govern the stereoselectivity by their own steric requirements but rather by their electronic effect. Lewis acids decrease the energy levels of the metallacyclobutane by their ability to coordinate to the metal and/or its ligands. The increase of stereoselectivity observed between W and Cr in the metathesis of acyclic olefins, which parallels the decrease of activity, would be due to the high energy level of the chromiacyclobutane.
ASJC Scopus subject areas
- Organic Chemistry