Zirconocenes cationic complexes

Bis(cyclopentadienyl)zirconocenes, cationic complexes, 4,896 l,l -Bis(dialkylboryl)cobaltocenium cations, complexation behavior, 7, 88... 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Note that the synthesis and reactivity of anionic [77, 78] and cationic [79] hydrido zirconocene(IV) complexes have been investigated but will not be addressed in this chapter. 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

The above reactions are rather slow because the insertion reactivity is directly related to the rate of dissociation of the THF ligand, which is also present as the solvent. The most reactive cationic complexes are therefore base-free, i. e. they do not contain coordinating solvents or ligands. Logically, anion coordination is a problem in such systems. Ion pairing reduces the reactivity of the cationic zirconocenes, but in the presence of B(C6F5)4, RB(C6F5)3 (R = H, Me), or BARF", this interaction is only weak. 

The aforementioned reactive cationic zirconocene species can react with chlorinated solvents by chloride abstraction. However, an even more pronounced property of cationic zirconocenes is their ability to abstract and bind fluoride. Cationic complexes [Cp2ZrMe(thf)]+ are unstable with BF4 and PF6 counterions [14], and the reaction of Cp2ZrCl2 with AgBF4 directly affords Cp2ZrF2 [7]. The use of Cp2ZrCl2/AgC104 as an... 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

Thus, we have shown that, in addition to the known methyl and chloride ancillary ligands in zirconocenes, amido ligands can also be activated by strong Lewis acids such as MAO, producing cationic complexes active in the polymerization of tt-olefins. 

The cationic zirconocene acyl complex (O-inside and O-outside isomers) unusually reacts with vinylchloride or methylvinylketone, affording in good yield a dinuclear dicationic complex, which has been characterized by X-ray diffraction (Scheme 46). ... 

Cationic zirconocene alkyl complexes of general type [Cp2Zr(R)(L) ]+ (n = 0, 1, L = labile ligand) see Labile) react readily with pyridines and other A-heterocycles by coordination and metalation ortho C H activation) to yield... 

Methyl abstraction of Cp2ZrMe2 with a trityl salt of a highly methylated carborane anion, [GPh [closo-HCBuMen], shown in Equation (47), leads to formation of the cationic complex 727 in which the methyl zirconocene cation is closely associated with the methylated carborane anion through the Zr(/x-CH3)B motif.562 The Zr- H3G interaction is maintained in solution, as shown by a change in chemical shift for the methyl group... 

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