Zirconocene hydride addition

Usefid reviews address alkyl zirconocene catalysts for the pol)mierisation of silanes to poly silanes by a a-bond metathesis mechanism, chiral titanates as promoters in aldol reactions, and MeTiCl3 as a reagent for chelate-controlled carbonyl addition reactions. 5 xhe reactions of terminally functionalized alkenes with zirconocene hydrides are reviewed. Thermochemical studies show that while the bond dissociation enthalpies of Zr—C6H13 and Zr— CgHjj in zirconocene systems are comparable, the insertion of cyclohexene into the Zr—bond is more exothermic than the insertion of hexene. 

A further improvement in the cuprate-based methodology for producing PGs utilizes a one-pot procedure (203). The CO-chain precursor (67) was first functionalized with zirconocene chloride hydride ia THF. The vinyl zirconium iatermediate was transmetalated direcdy by treatment with two equivalents of / -butyUithium or methyUithium at —30 to —70° C. Sequential addition of copper cyanide and methyUithium eUcited the /V situ generation of the higher order cyanocuprate which was then reacted with the protected enone to give the PG. 

The addition of Cp2Zr(H)Cl, known as the Schwartz reagent [30], to different alkenes and alkynes is known to be a facile process [31]. Therefore, the hydrozirconation of a variety of readily available enynes 12 is among the first methods developed for the stereoselective preparation of dienyl zirconium reagents 13. This process is both completely chemo- and regioselective with a syn addition of the zirconium hydride across the alkyne [32] (Scheme 5). From the same intermediate, the Zr atom can be isomerized in its internal position such as in 15 via a zirconacyclopropene intermediate 14. Moreover, the addition of trimethylstannyl chloride to 14 led to the stannylated dienyl zirconocene 16 [33] (Scheme 5). 

The P—H bonds of primary phosphines RPH2 add to zirconocene to give unstable zirconium phosphido hydrides CpZr(H)(PHR) from which C—H activation products such as fulvalene complexes [CpZr(/A-PHR)]2(rf r -QoHg) can be isolated.44 The addition of Ph2P(0)H to M(PEt3)3 (M = Pd, Pt) gives (21-X) the Pd derivative catalyzes the hydrophosphinylation of alkynes.45... 

Analogous dienyl zirconium compounds can be prepared by addition of unconjugated dienes that contain enol ethers to 105 (Scheme 19).56-58 These reactions are believed to proceed by initial coordination of the terminal olefin followed by isomerization via zirconocene allyl hydride intermediates to eventually yield zirconacyclopentanes that... 

Reactivity studies reveal that the bis(indenyl)zirconium sandwich complexes serve as isolable, modular precursors to the rich chemistry of divalent zirconocene. Addition of 1 atm of carbon monoxide generates the bis(indenyl)-zirconocene dicarbonyl complex, 307, while treatment with N,N-dimethylaminopyridine results in C-H activation to form the zirconocene pyridyl hydride complex 308. Crystallographic characterization of both complexes as well as multinuclear NMR spectroscopy establishes that the more familiar 77s, 77s hapticity for the indenyl ligands has been restored. Alkyne coupling reactions are also observed as addition of 2-butyne to 302 allows observation of the alkyne complex, 208, and ultimately the zirconacyclopentadiene 309 (Scheme 50).104... 

In addition to zirconocenes, C -symmetric scandocenes and yttrocenes with two dimethylsilyl interannular linkers (38a-c, 39, Figure 4.18) have been synthesized and tested as polymerization catalysts for propylene and 1-pentene." " Owing to the electronics of these group 3 catalysts (14-electron, d electronic configuration) no cocatalysts are required. However, metallocene chlorides cannot be used for polymerization metallocene alkyl or hydride compounds are necessary. 

Analogically to the titanocene(III) hydride catalyzed reactions, the zir-conocene 1 can be used as well. However, in this case the active species is assumed to be a cationic zirconium(IV) hydride generated by the reaction of the zirconocene 1 with MAO. The reaction mechanism copies the one described in Scheme 40. However, the course of the reaction favors antz-Markovnikov addition of the zirconium hydride to the double bond to give solely the methyl-enecycloalkanes 93 [50,51]. 

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