Catalytic metathesis active catalyst concentrations

Catalyst initiation and overall catalytic activity are not always opposing effects. For example, complexes that contain chelating ligands, such as bidentate phosphines [90], Schiff bases [91], and tris(pyrazolyl)borates [92], generally show both low rates of initiation and low overall olefin metathesis activity (Fig. 4.34). Both effects appear to result from slow rates of ligand dissociation from the starting complexes, which leads to low concentrations of the catalytically active 14-electron species in solution. 

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. 

For the Mo/H-Beta zeolites, the formation of the Al2(Mo04)3 phase and the decrease in the concentration of Brpnsted acid sites explains the low catalytic activity of Mo/H-Beta in metathesis of ethylene and 2-butylene to propylene [149]. A promoting effect of Mg was revealed in the Mo/H-Beta-Al203 catalyst for cross-metathesis of ethene and butene-2 to propene [150]. The stability is improved at the Mg content of l-2wt.% due to the elimination of weak acid sites and suppression of the side olefin oligomerization reaction. 

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