Chalk-Harrod-like mechanism

Based on the above results, we have proposed a classical Chalk-Harrod-like mechanism for the Pd nanocluster catalyzed reaction (Scheme 3.10). " The first step involves the reduction of Pd(OAc)2 to zero-valent palladium nanoparticles followed by stabilization with a polysiloxane network. These particles followed the conventional pathway of oxidative-addition and reductive-elimination cycles to generate sUyl esters. After the transformation was over, these palladium colloids precipitated out of the solution as a black solid supported on a polysiloxane network. The presence of the soluble polysiloxane matrix, which was not fully condensed in the form of silica, allows particle redispersion— thus allowing particles to be reused as catalyst... 

More recent studies have shown that a number of other mechanisms are operative in the hydrosilation process for different metals. Mechanistic proposals for early metals, lanthanides and actinides have been elaborated on. These involve a Chalk-Harrod like initial migratory insertion into a metal-hydride bond, followed by a a-bond metathesis step (Scheme 4). An alternative mechanism, however, was proposed for Group 4 metallocene catalysis, which involves a coordinated olefin, which undergoes a-bond metathesis with the hydrosilane. ... 

In the Chalk-Harrod mechanism, oxidative addition of silane occurs to a Pt(0) complex. The ancillary ligands on the active catalyst when reactions are initiated with "Speier s catalyst" are unknown, but are likely to be the olefin substrate. The ancillary ligands on the active form of Karstedt s catalyst or the related PtfCOD) complex could be the original diene ligands or the olefin substrate. Details on the mechanism of oxidative additions of silanes are provided in Chapter 6. In brief, this reaction occurs by coordination of silane to an open site to form a silane a-complex, followed by cleavage of the Si-H bond to form a silyl hydride species. 

The results summarized under the headings 1 and 2 rather conclusively indicate that, under the conditions used, the reaction must proceed via hydropaUadation. For the other steps, a combination of oxidative addition and reductive elimination, as in the Chalk-Harrod mechanism, is plausible, but other possibilities involving metathetical processes may not be ruled out. It should be noted that the two reactions employed by Brookhart and co-workers " and Hayashi and co-workers are substantially different. It is therefore very likely that the mechanistic conclusions made in their studies are both fundamentally correct and that, as in many other reactions of organopalladium compounds, more than one mechanism operates for a given type or class of organopalladium reactions. 

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