Mechanism of first generation catalysts

In order to design superior catalyst systems and expand the applications of these first generation catalysts, it was necessary to understand the fundamental mechanism of ruthenium-catalyzed olefin metathesis reactions. Initial investigations focused on the activity of 1 and its derivatives for the catalytic RCM of diethyl diallylmalonate (Eq. 4.14) [86]. These studies revealed that, in all cases, the overall catalytic activity was inhibited by the addition of free phosphine, and that the turnover rate was inversely proportional to the concentration of added phosphine. This indicated that phosphine dissociation was required for catalytic activity, and further suggested that olefin metathesis may be initiated by the substitution of a phosphine ligand with an olefin substrate. 

These initial mechanistic studies with bis(phosphine) catalysts were able to distinguish between overall associative and dissociative possibilities [86]. As shown in pathway (a) of Fig. 4.30, an associative mechanism involves coordination of an olefin directly to the 16-electron pre-catalyst, to form an 18-electron olefin complex that then undergoes [2 -I- 2] cycloaddition. In contrast, pathways (b) and 

More recent mechanistic studies have been able to distinguish between pathways (b) and (c), and all results indicate that (c) is operative [87]. The initial ligand dissociation and substitution steps have been studied using NMR magnetization transfer experiments, NMR and UV-vis kinetics, and mass spectrometry [87,88]. These investigations indicate that both phosphine/phosphine (Fig. 4.31a) and phosphine/olehn (Fig. 4.31b) substitution reactions in (L)(PR3)(X)2Ru= CHR complexes proceed by a dissociative mechanism involving a 14-electron intermediate (L)(X)2Ru=CHR (A). Although this proposed intermediate has not been observed in solution, presumably due to its low concentration, it has been identihed in the gas phase [88]. 

An overall catalytic cycle for olehn metathesis reactions catalyzed by (PCy3)2(Cl)2Ru=CHPh (1) and its derivahves is summarized in Fig. 4.32. The hrst 

Transition Metal-Carbene Complexes in Olefin Metathesis 

---