Molecular complexes catalyst preparation

W02005/080456 Al, Lanxess Inc. Process for the preparation of low molecular weight hydrogenated nitrile rubber EP2028194 Al, Lanxess Deutschland GmbH Metathesis of nitrile rubbers in the presence of transition metal complex catalysts Press release Lanxess - New York Conference, New York City, USA, Monday August 28, 2006, Address by Dr. Ulrich Koemm LANXESS Concepts in Rubber ... 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

By proper choice of molecular organometallic complex, catalysts (or catalyst precursors) for several reactions can be prepared by SOMC. This section outlines the most important developments in this field. 

Preparation and Properties of High-Molecular-Weight Poly (propylene oxide). Figure 4 shows a typical conversion-time plot for polymerization of propylene oxide by a hexacyanometalate salt complex catalyst. This reaction is characterized by an initial period during which almost no conversion occurs, followed by a period of rapid polymerization. The initial period, termed the induction period, is highly... 

This short review has tried to emphasize the molecular aspects of catalyst preparation, the role of the support and the different interactions taking place between the metal complex and the support. The case of ion exchange has been presented in more detail, because of the possibilities of varying such interactions in a large domain, ranging from weak physical interactions to stronger chemical interactions. 

Another observed difference between the polymerization of butadiene and pentadiene concerns the influence of the solvent on the type of polymer obtained. In polymerizing pentadiene, cis-1,4 polymers have been obtained only in aromatic solvents, while in aliphatic solvents, at least with catalysts prepared from A1(C2H5)2C1, polypentadienes having a 1,2 syndiotactic structure have been obtained (9). (The catalyst systems prepared from Al(C2H5)Cl2 complexes with pyridine or thiophene are practically insoluble in aliphatic solvents and yield only small amounts of low molecular weight polymer, probably of cationic origin.)... 

Table I reports the results of typical polymerization runs of pentadiene by cobalt catalysts prepared from Al(C2H5)Cl2 complexed with thiophene or pyridine. The crude polymerization products obtained by these systems have a cis-1,4 content of about 75-80%. Fractions having a higher cis-1,4 content (about 85%) could be isolated by dissolving the crude polymers in benzene and reprecipitating with methylethyl ketone (MEK). This solvent dissolves only the low molecular weight polymers, which, in this case, have also a low cis-1,4 unit content.

The reaction is rapid at first, but the catalyst is quickly exhausted. p-HydrOxybenzoic acid gives 4-hydroxycyclohexanecarboxylic acid (4Sf%) and cyclohexanecarboxylic acid (27%). Less success is achieved in the preparation of aminocyclohexanecatboxylic acids. Rates of hydrogenation of eleven phenyl-substituted aliphatic acids have been studied. With increased molecular complexity, higher pressures and larger amounts of catalyst are required. ... 

Enantioselective Aziridination of Alkenes. Copper complexes with neutral methylenebis(oxazoline) ligands (1) and (2) have also been employed as enantioselective catalysts for the reaction of alkenes with (Al-tosylimino)phenyliodinane, leading to A-tosylaziridines. The best results have been reported for cinna-mate esters as substrates, using 5 mol % of catalyst prepared from CuOTf and the phenyl-substituted ligand (2) (eq 6). The highest enantiomeric excesses are obtained in benzene, whereas in more polar and Lewis basic solvents, such as acetonitrile, the selectiv-ities are markedly lower. The chemical yield can be substantially improved by addition of 4X molecular sieves. Both Cu - and Cu"-bisoxazoline complexes, prepared from Cu or Cu triflate, respectively, are active catalysts, giving similar results. In contrast to the Cu-catalyzed cyclopropanation reactions discussed above, in which only Cu complexes are catalytically active, here Cu complexes are postulated as the actual catalysts. ... 

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