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Mechanism of Olefin Metathesis with Catalysis by Ruthenium Carbene Complexes: Density Functional Studies on Model Systems
Author(s) -
Vyboishchikov Sergei F.,
Bühl Michael,
Thiel Walter
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020902)8:17<3962::aid-chem3962>3.0.co;2-x
Subject(s) - chemistry , carbene , catalysis , dissociation (chemistry) , associative substitution , ruthenium , metathesis , medicinal chemistry , density functional theory , double bond , catalytic cycle , ligand (biochemistry) , stereochemistry , computational chemistry , polymer chemistry , organic chemistry , polymerization , biochemistry , polymer , receptor
Gradient‐corrected (BP86) density functional calculations were used to study alternative mechanisms of the metathesis reactions between ethene and model catalysts [(PH 3 )(L)Cl 2 RuCH 2 ] with L=PH 3 ( I ) and L=C 3 N 2 H 4 =imidazol‐2‐ylidene ( II ). On the associative pathway, the initial addition of ethene is calculated to be rate‐determining for both catalysts (Δ G $\rm{^{\ne }_{298}}$ ≈22–25 kcal mol −1 ). The dissociative pathway starts with the dissociation of phosphane, which is rather facile (Δ G $\rm{^{\ne }_{298}}$ ≈5–10 kcal mol −1 ). The resulting active species (L)Cl 2 RuCH 2 can coordinate ethene cis or trans to L. The cis addition is unfavorable and mechanistically irrelevant (Δ G $\rm{^{\ne }_{298}}$ ≈21–25 kcal mol −1 ). The trans coordination is barrierless, and the rate‐determining step in the subsequent catalytic cycle is either ring closure of the π complex to yield the ruthenacyclobutane (catalyst I , Δ G $\rm{^{\ne }_{298}}$ =12 kcal mol −1 ), or the reverse reaction (catalyst II , ring opening, Δ G $\rm{^{\ne }_{298}}$ =10 kcal mol −1 ), that is, II is slightly more active than I . For both catalysts, the dissociative mechanism with trans olefin coordination is favored. The relative energies of the species on this pathway can be tuned by ligand variation, as seen in (PMe 3 ) 2 Cl 2 RuCH 2 ( III ), in which phosphane dissociation is impeded and olefin insertion is facilitated relative to I . The differences in calculated relative energies for the model catalysts I – III can be rationalized in terms of electronic effects. Comparisons with experiment indicate that steric effects must also be considered for real catalysts containing bulky substituents.