Zirconium-98, level scheme

Titanium-tartrate complexes also catalyze the epoxidation of homo- and bishomoaiiyiic alcohols but their reactions are considerably slower as compared with reactions of allylic alcohols [55]. Enantioselectivity also drops to moderate levels (Scheme 10). When the substrates are cis-homoallylic alcohols, use of the zirconium N,iV-dicyclohexytartramide complex as catalyst improves the enantioselectivity to some extent [56]. 

As with the above pyrrolidine, proline-type chiral auxiliaries also show different behaviors toward zirconium or lithium enolate mediated aldol reactions. Evans found that lithium enolates derived from prolinol amides exhibit excellent diastereofacial selectivities in alkylation reactions (see Section 2.2.32), while the lithium enolates of proline amides are unsuccessful in aldol condensations. Effective chiral reagents were zirconium enolates, which can be obtained from the corresponding lithium enolates via metal exchange with Cp2ZrCl2. For example, excellent levels of asymmetric induction in the aldol process with synj anti selectivity of 96-98% and diastereofacial selectivity of 50-200 116a can be achieved in the Zr-enolate-mediated aldol reaction (see Scheme 3-10). 

Addition of organometallic reagents to imines is not limited to allylmetal derivatives. Hoveyda and Snapper have demonstrated that dialkylzinc reagents can add to imines in a one-pot procedure. Using a zirconium complex as metal catalyst and a chiral peptide, diverse enantioenriched aryl, aliphatic and alkynyl amines 142 have been obtained with high levels of enantioselectivity (Scheme 8.60) [136],... 

The generation and subsequent reaction of oxy-functionalized allylic zirconium reagents to a wide range of aldehydes proceeds with excellent anti selectivity [188]. Allylic zirconium reagents can also be prepared from the hydrozirconation of allenes [189]. Very high levels of diastereoselectivity for both simple aliphatic and aromatic aldehydes are observed in these reactions for the production of the anti homoallylic alcohol (Scheme 10-97). 

The metallocene-ATRP route has been expanded by Matsugi etal. [164], to produce polyethylene-b-poly(methyl methacrylate). In the first step, hydroxyl-functionalized polyethylene was successfully prepared through the copolymerization of ethylene with aluminum-capped allyl alcohol, using a specific zirconium metaUocene/methylaluminoxane catalyst system. In the next step, the terminal alcohol was converted to hahde by 2-bromoisobutyryl bromide to obtain bromide-functionalized polyethylene, which could initiate the ATRP of MMA (Scheme 11.40). The block copolymers obtained exhibited unique morphological features that depended on the content of PMMA segment. Moreover, the block copolymers effectively compatibiUzed the corresponding homopolymer blend at the nanometer level. 

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