Procatalysts chiral

Three types of experiments have proved informative in the mechanistic study of asymmetric hydrogenation. These are, respectively, rate measurements and product analysis, X-ray crystallography and NMR-derived identification of stable and transient species involved in the catalytic cycle. The first two of these have been reviewed elsewhere (2, 3, 4) our own work has been concerned with NMR and has provided a surprising wealth of structural and mechanistic detail. A variety of chiral phosphine procatalysts have been used but the current discussion will be concerned largely with two. Thus (R,R)-1,2-ethanediylbis-[(2-methoxyphenyl)phenylphosphine] (1) (DiPAMP)... 

The symmetry of the metallocene and also the kind of procatalyst metal atom, the nature of the catalyst activator and the polymerisation temperature determine the polypropylene tacticity. The general stereoregulation behaviour of metallocene catalysts may be explained in terms of the local chirality, or chirotopicity, of the catalytic sites bonded to the same metal atom. For this analysis, the structure of metallocenes as catalysts should be considered. 

Figure 3.39 Chiral procatalyst of C2 symmetry containing two -substituents at the cyclopentadienyl rings, rac.-ethylenebis[l-(3-methylindenyl)]zirconium dichloride [rac.-(MeIndCH2)2ZrCl2], which, in combination with [Al(Me)o]x, produces atactic polypropylene. Front side view and side view respectively...

Recent advances of the preparation of novel optically active organoselenimn compounds, mainly organic diselenides, and their application as chiral ligands to some transition metal-catalyzed reactions and also as procatalysts for asymmetric diethylzinc addition to aldehydes are reviewed. Recent results of catalytic reactions using some organoselenimn compounds such as aUylic oxidation of alkenes and its asymmetric version as well as epoxidation of alkenes are also summarized. 

Chiral Diselenides and Selenides as Procatalysts for Diethylzinc Addition to Aldehydes... 

Wirth and co-workers used various chiral nitrogen-containing diselenides 20 - 24, which worked effectively as procatalysts for diethylzinc addition to aldehydes (see Sect. 3.1) [13] and for the catalytic oxyselenenylation-elimination reaction of frans- -methylstyrene (Scheme 26) [30]. Under the reaction conditions reported by Iwaoka and Tomoda [19], the diselenide 20 yields the product with highest enantioselectivity (up to 56% ee). Potassium peroxodisulfate seems to be superior to sodium and ammonium analogues. Effect of metal salts on stereoselectivity in the catalytic reaction using the diselenide 20 was investigated since it is known that metal ions can accelerate the decomposition of peroxo-... 

The insert in Scheme 3 shows the big pile of the prochiral substrate to be refined in the enantioselective hydrogenation and the small amounts of procatalyst and cocatalyst necessary for this transformation. The relationship demonstrates the elegancy of the approach using an in situ catalyst. In laboratory hydrogenations usually molar ratios substrate procatalyst cocatalyst (unidentate) of 200 1 2 are applied. On an industrial scale these can be increased up to 50000 1 2 emphasizing the aspect of multiplication of chirality from a small amount of catalyst into a large amount of product. 

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