Tantalum precatalysts

Mono(siloxy) metalhydrocarbyl species can be converted into bis- or tris(siloxy) metal hydrides by reaction with hydrogen, as shown for zirconium and tantalum. Such species are less susceptible to leaching and this route can be extended to titanium and hafnium surface species that are potential precatalysts for hydrogenolysis of C-C bonds, alkane metathesis and epoxidation reactions. 

Scheme 17 Proposed catalytic cycle for the hydroaminoalkylation of secondary amines with tantalum precatalysts...

Doye s group [81] showed that a dinuclear titanium-sulfonamidate complex (Scheme 21), with a tetrahedral sulfur in the ligand backbone, can be used for intermo-lecular hydroaminoalkylation as well. This system gives mixtures of branched and linear products, although to date there has been no mechanistic rationale provided for the reduced regioselectivity of group 4 metal complexes in this transformation. There has been one report by Zi s group [44] that describes axially chiral bis(sulfonamidate) tantalum and niobium complexes for application as precatalysts for hydroamination and hydroaminoalkylation. Unfortunately, these complexes did not show any reactivity for either of these reactions. 

Figure 23 Phosphoramidate tantalum alkyl precatalyst for room temperature hydroaminoalkylation...

Alkylidene complexes of tantalum and ruthenium were discovered in the late 1970s and early 1990s, respectively. Following these discoveries, many new fuUy characterized alkylidene complexes were shown to be efficient precatalysts for metathesis reactions. In the following sections we discuss the different types of metathesis reactions and their mechanisms. 

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