Reactions Involving Cationic Zirconocenes

In the following sections, we discuss reactions in which cationic zirconocenes are involved as reagents, intermediates, or catalysts. As already mentioned, polymerization reactions will not be considered. Section 8.2 deals with the use of the Cp2ZrCl2/AgC104 system (or similar combinations) as an activator in glycoside synthesis. In Section 8.3, nucleophi- 

Glycosylations with Cp2ZrCI2/Silver Salt Activators 

Cp2ZrCI2/Silver Salt as a New Activator of Glycosyl Fluorides 

An even more reactive activator of glycosyl fluorides is generated by employing a 1 2 ratio of the metallocene and silver salt in benzene [25]. The zirconium reagent proved 

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The gas-phase reaction of cationic zirconocene species, ZrMeCp2, with alkenes and alkynes was reported to involve two major reaction sequences, which are the migratory insertion of these unsaturated hydrocarbons into the Zr-Me bond (Eq. 3) and the activation of the C-H bond via er-bonds metathesis rather than /J-hydrogen shift/alkene elimination (Eq. 4) [130,131]. The insertion in the gas-phase closely parallels the solution chemistry of Zr(R)Cp2 and other isoelec-tronic complexes. Thus, the results derived from calculations based on this gas-phase reactivity should be correlated directly to the solution reactivity (vide infra). 

In 1978, Negishi et al. reported highly regio- and stereoselective methylalumination of alkynes with Me3Al using a zirconocene catalyst [59]. The involvement of cationic zirconocene species in the activation of carbon—carbon triple bonds was suggested in a reaction mechanism featuring electrophilic activation by aluminum (Scheme 8.30). 

The application of achiral cationic zirconocene compounds for methyl methacrylate polymerisation, e.g. a mixture of [Cp 2ZrMe(THF)]+[BPh4] and Cp 2ZrMe2 in methylene chloride solution, leads to the formation of syndio-tactic poly(methyl methacrylate). The species responsible for propagation are believed to be the bimetallic ones, involving cationic zirconium enolate and neutral zirconocene, which facilitates the process. Propagation is postulated to occur via the Michael reaction between the coordinating monomer and the cationic enolate [537] ... 

It is generally adopted that the catalytically active species in the metallocene-catalysed polymerization is a 14-electron cation. As an example, the mechanism of activation of an unbridged zirconocene catalyst is presented in Fig. 9.5-4, top. In the first two steps the activation by MAO, resulting in the 14-electron cation, is shown. The same cation can be generated by N,N -dimethylanilinium-tetrakis(pentafluorophenyl)borate and methylated metallocenes. As side-products methane and an amine are formed. TiBA can also be involved in the activation, which is not shown in Fig. 9.5-4, bottom. On the other hand, TiBA acts as a scavenger in the polymerization. The above-mentioned reactions take place in the absence of the monomer and are performed before the catalyst is used in the polymerization process. 

The chemistry of group 4 silyl compounds is closely associated with the dehydrogenating coupling of hydrosilanes. This reaction was discovered by Harrod and coworkers who converted PhSiHa with a catalytic amount of Cp2TiMe2 to H(PhSiH) H [370]. Later Tilley and coworkers showed that if zirconocene and hafnocene complexes are used in the reaction a catalytic cycle is involved which consists of o-bond metathesis steps [371]. To enhance o-bond metathesis reactivity with Si-H and C-H bonds Cp2Hf(Me)Si(SiMe3)3 and related compounds were treated with B(C6F5)3 to obtain cationic Hf complexes [372]. 

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