Search

The Ru PR 3 bonds of 1 – 2 ( a - b )- PC , Ru CHPh bonds of 1a - b , 2 - Inact / Act and 1a - b , 2 - RCB were analyzed by charge decomposition (CDA) and natural bond orbital (NBO). We have found that the dissociation step of the Ru PR 3 bond is driven by charge transfer, while the RCB by polarization effects. Furthermore, the π(C ipso )–π*(Ru C) interaction was associated with delocalization effects in the benzylidene ring. Likewise, the nature of the rotameric changes in the carbene was studied through the resonance stabilization energy ( E NLW ). 2 presented a lower Δ E NLW (Inactive → Active) than 1a - b , which confirms that the delocalization effects are related to a low carbene rotameric energy. [ABSTRACT FROM AUTHOR]

DFT(M06-L) calculations have been used to determine the relativestabilities of the metallacyclobutane intermediates arising from thecross-metathesis reactions of terminal olefins as well as to get insightsinto the origin of the nondetection of the α,β-substitutedspecies. For that, we discuss the structures, NMR signatures, stabilitieswith respect to separated reactants, and experimentally proposed interconversionpathways of all potential metallacyclobutane intermediates arisingfrom propene and styrene homocoupling. For the case of propene, theunsubstituted and mono- and disubstituted metallacycles are lowerin Gibbs energy than the separated reactants under the NMR experimentalconditions. Moreover, for the same number of substituents, regardlessof their nature, the metallacycles presenting substituents at theCαcarbons are always lower in energy than thosepresenting substituents at Cβ, the energy differencebeing between 1.7 and 8.8 kcal mol–1. The computedenergy barriers associated with the olefin and carbene rotation processes,two of the experimentally proposed pathways for the metallacycle interconversion,are low and are in excellent agreement with the values previouslydetermined through NMR studies. Cycloaddition and cycloreversion energybarriers are also low, and in fact, there is not a significant differencebetween the barrier heights of the processes leading to observed ornonobserved intermediates. Therefore, the nondetection of metallacyclobutaneintermediates with substituents in Cβseems to arisefrom their lower stability in comparison with the isomers with substituentsin Cα, which makes their detection not feasible underthermodynamic equilibrium conditions. That is, for cross-metathesisprocesses involving small terminal alkenes and activated carbenes,the nature of the observed metallacycles is based on thermodynamiccontrol. The preference of having the substituents in Cαis attributed to the formation of stronger M–C and C–Cbonds during the cycloaddition when the substituents are in an αposition due to higher charge transfer from the original alkene fragmentto the metal carbene. [ABSTRACT FROM AUTHOR]