Chiral zirconocene-containing

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

The a-olefin insertions proceed regiospecrfically in the Zr-C bond, affording 1,2-insertion product, in which a stereogenic center is present at the -position (relatively to Zr) of the metallacycle. Starting from chiral zirconocene complexes (as rac-[(ebi)Zr( -pyridyl)]+), propene insertion occurs with a high level of diastereoselectivity in cases where the -pyridyl ligand contains a six-substituent. 

Phosphorus-containing Ring Systems. - A range of new chiral oxazaphospholidine oxides 266 and 267 have been synthesised and used as catalysts in asymmetric reductions of ketones with diborane. Mannich-type cyclisation reactions of 5-amino-3-benzylthio-4-cyano(ethoxycarbonyl)pyrazoles with dichlorophenylphosphine and aromatic aldehydes in the presence of cation exchange resin have been used to prepare a number of 6-oxo-6-phospha-4,5,6-trihydroimidazolo[l,2-b]pyrazoles, e.g. 268. Some of these compounds have herbicidal activity and this report is typical of a number of similar ones in the Chinese literature. A number of metallocycles, e.g. 269, have been reported as products from reactions of transient zirconocene-benzyne intermediates with phosphaimines followed by sulfuration or selenation. ... 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

Three classes of catalysts have been studied for the asymmetric hydrogenation of imines. One class of catalyst is generated from late transition metal precursors and bisphosphines. These catalysts have typically been generated from rhodium and iridium precursors. A second class of catalyst is based on the chiral titanocene and zirconocene systems presented in the previous section on the asymmetric hydrogenation of unfunctionalized olefins. The third class of catalyst is used for the transfer hydrogenation of imines and consists of ruthenium or rhodium complexes containing diamine, amino tosylamide, or amino alcohol ligands. " ... 

These measurements were direct proof that a chiral (pure enantiomer) soluble zirconocene/MAO catalyst inserts the propene units in the same stereospecific manner and produces optically active oligomers and, by a continued insertion, isotactic polymers. Such polymer chains contain a vinyl group at the end formed by the main termination reaction, and all methyl groups are found at the top or the bottom. Using a racemic mixture of the zirconocene, 50% of the polymer chains show the methyl groups at the top and the other 50% at the bottom. 

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