Zirconium compounds alkyl derivatives

The polymerization of a-olefins, accelerated by homogeneous Ziegler-Natta catalysts derived from bis(cyclopentadienyl)titanium(IV) or analogous zirconium compounds and aluminum alkyls, occurs simultaneously with... 

Very few sulfur-bonded zirconium or hafnium compounds have been reported. Bradley (71) in attempting to dissolve dialkylamine compounds, M(NR2)x> where M = Zr or Hf, and R = methyl, ethyl, or w-propyl, in carbon disulfide, found that a vigorous reaction occurred with formation of A, A -dialkylthiocarbamates, M(S2CNR2)4. The dimethyl derivatives are very insoluble and nonvolatile, suggesting that they might be polymeric, but the higher alkyl derivatives are soluble and monomeric. The ligand appears to be bidentate. 

In the case of zirconium and hafnium the lack of accessible dichlorides means that alkyl derivatives are generated via the corresponding homoleptic alkyl intermediates. With bulky 2,6-di-ferf-butylphenoxide the tris-benzyls [(ArO)ZrR3] can be isolated and studied. " The dialkyls [(ArO)2MR2] (M = Zr, Hf R = Me, benzyl) can also be isolated for bulky aryloxides. In the case of 2,6-diphenylphenols the monomethyl species [(ArO)3ZrMe] are readily formed by simple addition of parent phenol to the dimethyl compound. 

Zirconium enolates of various carbonyl compounds have also been investigated for Mannich-type reactions with different electrophiles. According to Shibasaki s method,148 the coupling reaction between a 3-acetoxy-4-alkyl-/3-lactam and the in r(/ -generated zirconium enolate 96 of a cyclohexanone derivative was realized as a key step during the total synthesis of an anitibiotic (Scheme 42).117,149... 

Three classes of hybrid HPA are known to be stable to hydrolysis 1. Organometallic derivatives of the type RM (M = Si, Ge, Sn, Pb and R = alkyl or aryl). 2. Cyclopentadienyl-titaninm derivatives. 3. Zirconium alkoxide or phosphate derivatives, all of which are illustrated in Figure 2. We have tested phenyl model compounds of all of these for stability by boiling them in 6M HCl or H2O2 solution. This study showed that only PhP-O-HPA moieties are stable under conditions likely to be encountered in a fuel cell. Never the less we continue to study model compounds of the type RSi-O-HPA due to the large diversity of available ethoxy- and chloro- silanes. 

In modern organic chemistry, silyl enol ethers, as well as the corresponding titanium, tin, boron, or zirconium derivatives, are widely employed as nucleophilic components in enolate alkylation reactions. Their usefulness prompted the elaboration of numerous methods for the selective production of isomeri-cally pure enol ethers from almost any type of carbonyl compounds. 

Examples of homogeneous catalytic systems in propylene polymerization are very few. Zambelli et al. obtained syndiotactic polypropylene by working at low temperatures with catalytic systems based on some vanadium compounds and aluminum alkyls, while Giannini et al. were the first to prepare isotactic polypropylene with some benzyl derivatives of titanium or zirconium. 

Dialkyl and diaryl Ti and Zr derivatives (18) are somewhat less stable than their monosubstituted analogs. They are generally prepared by alkylation of the corresponding dichloro compounds (17 equation 1 1).i3,29,30 jjjg dichloro derivatives themselves can be prepared in turn by reaction of tetrachlorides with the required amount of organometallic species. For example, zirconocene dichloride is prepared by treating zirconium(IV) chloride with cyclopentadienylsodium. ... 

The instability of Ti reagents with branched alkyl groups is due to their tendency to decompose by 3-elimination. For this type of compounds the corresponding zirconium reagents are more suitable, since they do not undergo decomposition. For example, r-butylzirconium tributoxide (58b) is readily prepared from r-butyllithium and chlorozirconium tributoxide (58a) (equation 24). Alkyl- and aryl-zirco-nium(lV) tributoxides add to carbonyl compounds (Table 3) in the same fashion as Ti derivatives, with the advantage of low basicity, as is shown by their reactions with enolizable carbonyl compounds (Section 1.5.3.1.2). 

In most mixed alkyl silyl zirconium derivatives, CO insertion occurs preferentially into the Zr-C bond83,85,252,253. However, in sterically congested complexes, insertion occurs into the Zr-Si bond to give silaacyl alkyl compounds such as CpJZr [q2-COSiHMes2]Me (48)85 and CpCp Zr[ 2-COSi(SiMe3)3]Me (49)250,253. Complexes 48 and 49 readily and cleanly isomerize to the hydrido, silyl-substituted enolates... 

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