Synthetic and mechanistic studies on ruthenium-catalyzed alkene metathesis

The mechanism of the ruthenium-catalyzed alkene metathesis reaction has been examined, providing insight into the catalytic cycle of this important chemical transformation.;The reactivity of two cationic, monomeric, four-coordinate ruthenium phosphonium alkylidenes ([(H2IMes)(Cl)2Ru=C(H)P iPr3]+ and [(H2IMes)(Cl) 2Ru=C(H)PCy3]+) toward a series of alkenes is compared. Both complexes undergo reactions with alkenes to generate transient, four-coordinate ruthenium alkylidenes (H2IMes)(Cl)2Ru=C(H)R. The triisopropyl-substituted ruthenium phosphonium alkylidene is found to be several orders of magnitude more reactive toward alkenes than the tricyclohexyl-substituted one.;The study of a bicyclic ruthenacyclobutane of relevance to the ring-closing metathesis of dimethyl diallylmalonate is described. The ruthenacyclobutane was generated in high yield in CD2Cl2 solution and was fully characterized at low temperatures through spectroscopic and kinetic studies. 1H and 13C NMR spectroscopy, 13C-labeling studies and theoretical calculations indicate that the more substituted ruthenacycle C-C bond in this intermediate is weaker than the less substituted ruthenacycle C-C bond. Kinetic studies of the reaction of the ruthenacyclobutane with ethene have shed light on the species involved in the ring-closing metathesis of dimethyl diallylmalonate. In the phosphine-free system that was studied, the dissociative loss of the RCM product dimethyl cyclopent-3-ene-1,1-dicarboxylate from the bicyclic ruthenacyclobutane constitutes the rate-limiting step of the catalytic cycle, with a free energy barrier of only 65 kJ mol-1. The reactive, four-coordinate methylidene (H2IMes)(Cl)2Ru=CH2 is shown to be involved in the catalytic cycle. The mechanistic studies evidence the high inherent alkene metathesis reactivity that can be achieved with the Grubbs second generation catalyst platform.;New neutral and cationic ruthenium phosphonium alkylidene complexes, varying in the steric bulk of the phosphorus substituents, have been characterized. The reaction of ruthenium carbide complexes (H2IMes)(PR'R 2)(Cl)2Ru≡C with hydrochloric acid affords five-coordinate trichlorides (H2IMes)(Cl)3Ru=C(H)PR'R2. Abstraction of a chloride anion from the trichlorides proceeds cleanly and quantitatively with B(C6F5)3, leading to ion pairs [(H 2IMes)(Cl)2Ru=C(H)PR'R2]+[ClB(C 6F5)3]-. Depending on the size of the phosphorus substituents, the cations may undergo reversible dimerization in solution to form doubly chloride-bridged, centrosymmetric dimers. Thermodynamic and kinetic parameters for this dimerization process have been obtained. The dimers are not reactive toward ethene, whereas the monomers are very reactive toward ethene, forming a ruthenacyclobutane relevant to alkene metathesis.