Cationic zirconocenes

Then, contrary to what was reported previously, the olefin dissociates from the zirconium metal complex. This conclusion was further supported by other experimental observations. However, it cannot be completely excluded that competition between dissociative and direct rearrangement pathways could occur with the different isomerization processes studied up to now. Note that with cationic zirconocene complexes [Cp2Zr-alkyl], DFT studies suggest that Zr-alkyl isomerizations occur by the classical reaction route, i.e. 3-H transfer, olefin rotation, and reinsertion into the Zr-H bond the olefin ligand appears to remain coordinated to the Zr metal center [89]. 

Scheme 10 Cyclization of dienes using a cationic zirconocene 58 and phenylsilane...

It has since been shown that if less coordinating anions are used, then cationic zirconocene alkyls may serve as highly active single-component catalysts. Hence, treatment of Cp2ZrMe2 with... 

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). 

Cationic r 2-acylzirconocenes [Cp2Zr(COR)(L)+] have been reported to show attractive reactivity [39]. A cationic r 2-acylzirconocene can be generated in quantitative yield by treating a cationic zirconocene complex [Cp2Zr(R)(L)+] (R = alkyl or alkenyl) with carbon monoxide (<23 °C, 1 atm) (Scheme 5.36). The cationic acylzirconocene complexes undergo regiose-... 

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),... 

Scheme 6.33. Stereoselective (ebthi)Zr-catalyzed cycloadditions involving a cationic zirconocene and an unsaturated ester.

Tetrasubstituted alkenes are among the most challenging substrates for catalytic hydrogenation reactions. Towards this end, Buchwald and co-workers recently reported efficient and highly enantioselective Zr-catalyzed hydrogenations of a range of styrenyl tetrasubstituted alkenes (Scheme 6.41) [123]. Precedents based on efficient polymerization reactions promoted by cationic zirconocenes led these workers to consider similar catalyst species, derived from dimethylzirconocene 107, for this purpose. 

Single electron transfer to dialkylzirconocenes Single-electron oxidation of dialkylzirco-nocenes, e. g. with a ferrocinium [13] or silver salt [14,15], induces the following reaction leading to a cationic zirconocene [13] ... 

However transient and elusive cationic zirconocenes may be, they are real and some of their salts have been isolated and characterized, both spectroscopically and by X-ray crystallography. None of these cations is ever completely free they are either coordinated by an additional ligand (typically a solvent molecule) or they interact with their counterions, no matter how non-coordinating these may be [16], Scheme 8.1 shows the typical reactivity pattern of a solvent-stabilized cation [Cp 2ZrR(solvent)]+ (A Cp = q5-C5H4Me)... 

Scheme 8.1. Reaction pathways of cationic zirconocene complexes.

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... 

Scheme 8.2. Synthetic access to base-free 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-... 

Cationic zirconocene species efficiently activate alkenes toward carbon—carbon bond formation via carbometalation, as has been demonstrated in studies of alkene polymerization. Today, some zirconocene catalysts are available that allow single additions of metal-alkyls (mainly aluminum-alkyls) to alkenes or alkynes, thereby forming stable alkyl- or alkenyl-metals that do not undergo any further oligomerization. On the other hand, carbozirconation with Cp2ZrRCl in the presence of stoichiometric or catalytic amounts of activators has also been realized. 

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). 

Whereas zirconium-catalyzed alkylalumination of alkynes is at present limited to some simple alkyl groups, it has been found that carbozirconation with Cp2Zr(R)Cl considerably expands the scope of the alkylmetalation of alkynes. The reactions are initiated by catalytic amounts of additives (typically Ph3C[B(C6F5)4]), which probably serve to generate cationic zirconocene species. Regioselectivity in favor of attack at the more highly substituted carbon is observed [70,71] (Scheme 8.35). 

I 8.5 Cationic Zirconocene Complexes as Lewis Acid Catalysts... 

Diels—Alder reactivity was also reported for a cationic zirconocene alkoxide (A Scheme 8.46) at a 10 mol% level for the substrate combination methyl acrylate/isoprene [84]. Whereas the regioselectivity (para/meta = 96.2 3.8) in this process compared favorably to that with traditional Lewis acids (A1C13 in C6H6 regioselectivity = 95 5), the activity was quite low. The substrates methyl acrylate and cydopentadiene (Scheme 8.46 R =... 

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