DFT Study on the Relative Stabilities of Substituted Ruthenacyclobutane Intermediates Involved in Olefin Cross- Metathesis Reactions and Their Interconversion Pathways Katherine Paredes-Gil, † Xavier Solans-Monfort,* ,‡ Luis Rodriguez-Santiago, ‡ Mariona Sodupe, ‡ and Pablo Jaque* ,† † Departamento de Ciencias Quı ́ micas, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Republica 275, Santiago, Chile ‡ Departament de Quı ́ mica, Universitat Autò noma de Barcelona, 08193 Bellaterra, Spain * S Supporting Information ABSTRACT: DFT (M06-L) calculations have been used to determine the relative stabilities of the metallacyclobutane inter- mediates arising from the cross-metathesis reactions of terminal olefins as well as to get insights into the origin of the nondetection of the α,β- substituted species. For that, we discuss the structures, NMR signatures, stabilities with respect to separated reactants, and experimentally proposed interconversion pathways of all potential metallacyclobutane intermediates arising from propene and styrene homocoupling. For the case of propene, the unsubstituted and mono- and disubstituted metallacycles are lower in Gibbs energy than the separated reactants under the NMR experimental conditions. More- over, for the same number of substituents, regardless of their nature, the metallacycles presenting substituents at the C α carbons are always lower in energy than those presenting substituents at C β , the energy difference being between 1.7 and 8.8 kcal mol −1 . The computed energy 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 previously determined through NMR studies. Cycloaddition and cycloreversion energy barriers are also low, and in fact, there is not a significant difference between the barrier heights of the processes leading to observed or nonobserved intermediates. Therefore, the nondetection of metallacyclobutane intermediates with substituents in C β seems to arise from their lower stability in comparison with the isomers with substituents in C α , which makes their detection not feasible under thermodynamic equilibrium conditions. That is, for cross-metathesis processes involving small terminal alkenes and activated carbenes, the nature of the observed metallacycles is based on thermodynamic control. The preference of having the substituents in C α is attributed to the formation of stronger M−C and C−C bonds during the cycloaddition when the substituents are in an α position due to higher charge transfer from the original alkene fragment to the metal carbene. ■ INTRODUCTION Olefin metathesis is a redistribution of carbon−carbon double bonds that allows the conversion of the original reacting alkenes in new product olefins (Scheme 1). 1−10 Currently, this reaction is widely used in the preparation of new polymeric materials, 7,11 biologically active species, and relevant organic compounds with low energy cost, high yields, and significant selectivities. 12,13 Therefore, it has become one of the most relevant reactions for organic synthesis. 4−6,12,13 The reaction only takes place in the presence of a suitable transition-metal catalyst. The existing molecular catalysts can be divided into the two following groups: the early-metal Mo- and W-based alkylidene complexes also known as Schrock type cata- lysts, 5,10,14−17 and the catalysts generally denoted Grubbs type based on ruthenium carbenes. 6,8,9,18−21 The general formula of Ru-based catalysts is Ru(CHR 1 )(L 1 )(L 2 )(X)(Y), and depending on the nature of L 1 they are classified as first- generation 18,19 (L 1 = phosphine) or second-generation 20,9 (L 1 = N-heterocyclic carbene (NHC)) (Scheme 2). Species shown in Scheme 2 (A−F) 15−24 are catalyst precursors. 25−30 For the phosphine-containing Ru-based complexes, the active species are obtained by the dissociation of the L 2 ligand and a cross-metathesis process that exchanges Received: July 14, 2014 Published: October 15, 2014 Scheme 1 Article pubs.acs.org/Organometallics © 2014 American Chemical Society 6065 dx.doi.org/10.1021/om500718a | Organometallics 2014, 33, 6065−6075