Comparative DFT study on the role of conformers in the ruthenium alkylidene-catalyzed ROMP of norborn-2-ene

Comparative quantum chemical calculations on the reaction pathways for the formation of ruthena(IV)cyclobutanes from both 1(st)- and 2(nd)-generation Grubbs catalysts of the general formula RuX(2)(Q(L')(=CH(2)) (L=PCY(3) or 1,3-dimesityl-4,5-dihydroimidazolin-2-ylidene, L'= PCY(3)) and norborn-2-ene (NBE) were carried out on the B3LYP/LACVP** level in dependence on the ligand X = 1, Br, Cl, and F. The mechanism proposed by Straub for the formation of (one) active and (three) inactive NBE-Ru-carbene complexes for non-cyclic alkenes was applied to the cyclic alkene NBE. In RuX(2)(PCY(3))(2)(=CH(2)), the inactive NBE-Ru-carbene complex is energetically more stable than the active one; however, in RuX(2)(MesH(2))(PCYA=CH(2)), the active NBE-Ru-carbene complex is more stable than the inactive one. In due consequence, the possible rate limiting barrier for the conversion of the NBE-Ru-carbene complex into the corresponding metallocyclobutane (MCB) is systematically larger in the case of 1(st)-generation Grubbs catalysts than of 2(nd)-generation Grubbs catalysts due to an additional re-arrangement for the formation of an active pi-complex from the more stable (inactive) conformer. This correlates with the observed reactivity of both types of initiators. There is a strong influence of the ligands L and X on the conformational properties and relative stabilities of the 14-electron intermediates, which has a direct effect on the distribution of the inactive and active conformations of the corresponding Ru-carbene-NBE complexes. A direct correlation between the conformational properties of the 14-electron intermediates and the relative stabilities of the active Ru-carbene-NBE complexes was observed. Copyright (C) 2008 John Wiley & Sons, Ltd.