Ligands variation

By contrast, the synthesis of syndiotactic PP, s-PP, is generally catalyzed by Cs-symmetry ansa- metallocenes. For example, (16)/MAO affords PP with a pentad (rrrr) content of 86% at 25 °C.77 The stereoselectivity is highly sensitive to ligand variation. For example, substitution at the 3-position of the Cp ring with a methyl group affords heterotactic PP,78 whilst the Bu analog favors i-PP production.50,75,79... 

We have summarized our research findings in a systematic study of lanthanide-transition metal-amino acid clusters. Several factors of influence that affect the assembly, such as the presence of a secondary ligand, variation of lanthanide, crystallization conditions, the ratio of metal ions to amino acids, and the choice of... 

This was in contrast to the enzymatic reactions known then, where enzymes were believed to be very substrate specific. As we now know, there is no general catalytic system to perform asymmetric hydrogenations and even within a small class of substrates, some ligand variation is required to achieve optimal results. 

These findings imply that, in the case of secondary radicals, the trapping with a second equivalent of Cp2TiCl can compete with tetrahydrofuran formation, and that -hydride elimination can kinetically compete with protonation of Ti - C bonds under our protic conditions [65,66,73,74], It remains to be seen how the reaction can be completely driven toward the tricyclic system 60 by ligand variation of the catalyst. 

The combination of metal/ligand variation in these hybrids not only allows alteration of the ET energetics but also provides a means to study mechanistic questions, such as whether ET is direct or involves a hopping mechanism. 

Because metal substitution and ligand variation affect ET kinetics without perturbing the structure of the ET complex, such changes were used to probe mechanistic aspects of ET. The data show that both photoinitiated and thermal ET are direct processes. The measurements of k( in the [ZnP, Fe (H20)P] hybrids provided the first detection of ET tuimelling in a structurally-defined protein system. 

In transition metal chemistry, ligand variation has proven to be the key to obtaining highly active polymerization catalysts. In particular, sterically hindered monocationic alkyl complexes with an empty site seem to be well suited for polymerization. The steric bulk prevents (associative) -hydrogen transfer, while the positive charge destabilizes the free hydride and thus opposes (dissociative) /(-elimination. 

The following section shows specific examples of association of dicoordinated mononuclear gold units which have been classified according to the nuclearity of the aggregate. Despite this general classification, the different polymorphic forms of a specific compound are discussed together, as well as the structures of complexes with small ligand variations. 

Ligand variation has resulted in considerable improvement of the reaction s efficiency (Scheme 38). With 9-0-(9 -phenanthryl) (PHN) ethers and 9-0-(4 -methyl-2 -quinolyl) (MEQ) ethers of dihydroquini-dine and dihydroquinine as ligands, olefins of various substitution patterns, mono-, gem-di-, trans-di-, and certain trisubstituted substrates can be dihydroxylated with high enantioselectivity (89). Some examples of these reactions are given in Scheme 38. The reaction occurs at either ambient or ice-bath temperature with the solid, nonvolatile Os(IV) salt, K20s02(0H)4, instead of 0s04. The substrate/catalyst ratio for this reaction is 2000 for terminal olefins. 

Ligand Variations in the Rh-Catalyzed Hydrogenation of Dehydro-Amino Acids and Esters... 

The electrochemical behavior of [Ru(trpy)(bpy)(OH2)]2+ in aqueous solution and when incorporated in Nation coatings has been studied in detail by McHatton and Anson (59). The effect of ligand variation on the redox potentials of other Ru(IV)=0 systems is summarized in Table II. For [Ru(trpy)(diamine)(0)]2+, the E° values fall in the order of L TMEA < 4,4 -(MeO)2bpy < bpy < 6,6 -Cl2-bpy, which is the order of the relative cr-donor strength of L. 

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