A tertiary phosphine that is too bulky: preparation of catalytically less active carbene and vinylidene ruthenium(II) complexes

Feasibility of associative mechanism in enyne metathesis catalyzed by grubbs complexes

The mechanism of enyne metathesis catalyzed by first and second generation Grubbs complexes has been computationally explored at the DFT level. The relative reactivity and the regioselectivity for the reaction of differently substituted alkenes and alkynes with model Ru complexes has been studied. The usually accepted dissociative mechanism for the alkene metathesis has been explored for alkynes, and compared with an associative pathway involving initial coordination of the alkyne to the 16-electron catalyst. Our results show that an associative mechanism would be the preferred pathway for the reaction of phosphine-based (first generation) Ru carbenes, at least for small phosphines such as PMe(3), whereas for the more reactive complexes containing a heterocyclic carbene as ligand (second generation catalysts), the dissociative process is far more favourable.

Preparation of hydrido(vinylidene)ruthenium(II) complexes and a one-pot synthesis of Grubbs-type ruthenium carbenes

Treatment of the hydrido(dihydrogen) compound [RuHCl(H2)(PCy3)2] 1 with alkynes RC[triple bond, length as m-dash]CH (R=H, Ph) afforded the hydrido(vinylidene) complexes [RuHCl(=C=CHR)(PCy3)2] 2, 3 which react with HCl or [HPCy3]Cl to give the corresponding Grubbs-type ruthenium carbenes [RuCl2(=CHCH2R)(PCy3)2] 4, 5. The reaction of 2 (R=H) with DCl, or D2O in the presence of chloride sources, led to the formation of [RuCl2(=CHCH2D)(PCy3)2] 4-d1. Based on these observations, a one-pot synthesis of compounds 4 and 5 was developed using RuCl3.3H2O as the starting material. The hydrido(vinylidene) derivative 2 reacted with CF3CO2H and HCN at low temperatures to yield the carbene complexes [RuCl(X)(=CHCH3)(PCy3)2] 6, 7, of which 7 (X=CN) was characterized crystallographically. Salt metathesis of 2 with CF3CO2K and KI led to the formation of [RuH(X)(=C=CH2)(PCy3)2] 8, 9. The bis(trifluoracetato) and the diiodo compounds [RuX2(=CHCH3)(PCy3)2] 10, 11 as well as the new phosphine P(thp)3 12 (thp=4-tetrahydropyranyl) and the corresponding complex [RuCl2(=CHCH3){P(thp)3}2] 14 were also prepared. The catalytic activity of the ruthenium carbenes 4-7, 10, 11 and 14 in the olefin cross-metathesis of cyclopentene and allyl alcohol was investigated.

Mechanism and Activity of Ruthenium Olefin Metathesis Catalysts: The Role of Ligands and Substrates from a Theoretical Perspective

The reaction mechanism of olefin metathesis by ruthenium carbene catalysts is studied by gradient-corrected density functional calculations (BP86). Alternative reaction mechanisms for the reaction of the "first-generation" Grubbs-type catalyst (PCy(3))(2)Cl(2)Ru=CH(2) (1) for the reaction with ethylene are studied. The most likely dissociative mechanism with trans olefin coordination is investigated for the metathesis reaction between the "first-" and the "second-generation" Grubbs-type catalysts 1 and (H(2)IMes)(PCy(3))Cl(2)Ru=CH(2) (2) with different substrates, ethylene, ethyl vinyl ether, and norbornene, and a profound influence of the substrate is found. In contrast to the degenerate reaction with ethylene, the reactions with ethyl vinyl ether and norbornene are strongly exergonic by 8-15 kcal/mol, and this excess energy is released after passing through the metallacyclobutane structure. While the metallacyclobutane is in a deep potential minimum for degenerate metathesis reactions, the energy barrier for the [2+2] cycloreversion vanishes for the most exergonic reactions. On the free energy surface under typical experimental conditions, the rate-limiting steps for the overall reactions are then either metallacyclobutane formation for 1 or phosphane ligand dissociation for 2.