Homogeneous catalysts, stereochemistry

The stereochemistry of reduction by homogeneous catalysts is often controlled by functional groups in the reactant. Delivery of hydrogen occurs cis to a polar functional group. This behavior has been found to be particularly characteristic of an iridium-based catalyst that contains cyclooctadiene, pyridine, and tricyclohexylphosphine as ligands, known as the Crabtree catalyst 6 Homogeneous iridium catalysts have been found to be influenced not only by hydroxy groups, but also by amide, ester, and ether substituents.17... 

The suitable stereochemistry, not focused herein, is described as playing an important role (22). As reported in patents of Firmenich S.A., the ring opening reaction of 18 (Eq. 15.2.7) was preferably carried out with the Lewis acidic homogeneous catalysts such as BF3-etherate (5), Mgl2 (21) or SnC14 (21) with almost complete yield at room temperature in toluene as solvent. These catalysts, however, imply a process limitation to discontinuous liquid phase conditions. Major drawbacks are given by a relatively complicated separation procedure of products and catalyst in addition to corrosion, loss of catalyst and environmental... 

Syndiotactic propagation of propylene is know to be catalyzed by homogeneous vanadium catalyst (1 ). In the polypropylene samples prepared with the homogeneous catalysts, the relative population of iso-, hetero- and syndiotactic triads is in accordance with that predicted from the first order Markov model (25, 26). There is no chiral structure around the homogeneous vanadium species. The stereochemistry of the entering monomer is controlled by the chirality of the growing chain end, in contrast with the isotactic propagation. 

Certain soluble catalysts polymerize propylene to highly syndiotactic polymers that are free of the isotactic form. Natta, Pasquon, and Zambelli (7 8) showed that VCl (or vanadium triacetylacetonate) in combination with AlRnX-type metal alkyls and anisole, polymerize propylene to highly syndiotactic polypropylene. These apparently homogeneous catalysts were used at low temperatures (-40 to -78 °C). Stereochemistry was initially explained by a mechanism involving repulsion between the methyl groups on the last added and new monomer unit to achieve orientation (83. 84). 

The stereochemistry of addition to acetylenes has been studied for various types of homogeneous catalysts. In practice, cis addition seems to be a common feature of hydrosilylation catalyzed by transition metal complexes, although this mechanistic scheme does not require a cis addition. Benkeser (39) found that the addition of trichlorosilane to 1-alkenes in the presence of homogeneous (chloro-platinic acid) catalysts induces a cis approach and results in the trans product. Several series of experiments with optically active trisubstituted silanes have indicated that when catalyzed by platinum complexes, the hydrosilylation process occurs overall with retention of configuration at the asymmetric silicon center. 

We conclude this introductory with a short outline of the sections that follow. In Section 2.3.2, we offer a brief introduction to pressrue and material gap problems that arise when model catalytic systems and conditions are used to emulate working catalytic systems. In Section 2.3.3, we describe the local aspects of the catalytically reactive sites in the section titled Ensemble effects and defect sites . This is followed by four sections on environmental influences on the reactivily entitled Cluster size and metal supports , Cooperativity , Surface moderation by coadsorption of organic molecules and Stereochemistry of homogeneous catalysts . The chapter is concluded with a section on surface kinetics, dealing with surface reconstruction and transient reaction intermediates. 

Stereochemistry of Homogeneous Catalysts. Anti-Lock and Key Concept... 

This chapter will discuss various homogeneous catalyst systems for the cyclopolymerization of nonconjugated diolefins, particularly 1,5-HD, which control the stereochemistry and microstructure and therefore the physical properties of the polymers synthesized. In addition, the effect of metallocene structure on ethylene/1,5-HD and propylene/l,5-HD copolymerizations will be described. 

At the same time, the fact that the homogeneous catalyst precursors are structurally well-defined has provided an extraordinary opportunity to investigate the origin of stereospecificity in olefin polymerization at a level of detail that was difficult if not impossible with the conventional heterogeneous catalysts. For example, NMR analysis of the isotactic polymer produced with HI revealed the stereochemical errors mmmr, mmrr, and mrrm in the ratios of 2 2 1 (Fig.5). This observation is consistent with an enantiomorphic site control mechanism, where the geometry of the catalyst framework controls the stereochemistry of olefin insertion.6 30,31 These results established unambiguously a clear experimental correlation between the chirality of the active site, which could be established by x-ray crystallography of the metallocene catalyst precursor, and the isotacticity of the polymer produced. 

The stereochemistry of reduction by homogeneous catalysts is often controlled by functional groups in the reactant. Homogeneous iridium catalysts have been found to be influenced not only by hydroxyl groups, but also by amide, ester, and ether substituents. 

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