Centers in catalysts

We present some recent results obtained in the Princeton University Chemistry Department on the nature of the active center in catalysts derived from sodium type faujasite catalyst (17, 18, 23, 25, 26, 27, 28, 29, 30, 31). A general survey of the structural characteristics has been given by one of us and by others (1, 4, 9, 25, 33). 

Interestingly, despite the numerous accessible C-H bonds in the empty coordination site below the trigonal plane, the ruthenium center in catalyst 4 does not appear to be stabilized... 

Preparation of enantiomerically enriched materials by use of chiral catalysts is also based on differences in transition-state energies. While the reactant is part of a complex or intermediate containing a chiral catalyst, it is in a chiral environment. The intermediates and complexes containing each enantiomeric reactant and a homochiral catalyst are diastereomeric and differ in energy. This energy difference can then control selection between the stereoisomeric products of the reaction. If the reaction creates a new stereogenic center in the reactant molecule, there can be a preference for formation of one enantiomer over the other. 

The importance of the o-hydroxyl moiety of the 4-benzyl-shielding group of R,R-BOX/o-HOBn-Cu(OTf)2 complex was indicated when enantioselectivities were compared between the following two reactions. Thus, the enantioselectivity observed in the reaction of O-benzylhydroxylamine with l-crotonoyl-3-phenyl-2-imi-dazolidinone catalyzed by this catalyst was 85% ee, while that observed in a similar reaction catalyzed by J ,J -BOX/Bn.Cu(OTf)2 having no hydroxyl moiety was much lower (71% ee). In these reactions, the same mode of chirality was induced (Scheme 7.46). We believe the free hydroxyl groups can weakly coordinate to the copper(II) ion to hinder the free rotation of the benzyl-shielding substituent across the C(4)-CH2 bond. This conformational lock would either make the coordination of acceptor molecules to the metallic center of catalyst easy or increase the efficiency of chiral shielding of the coordinated acceptor molecules. 

As follows from the table, each of these compounds affects both reactions in practically the same way. This may indicate that both reactions proceed on the same sites of the surface of the catalyst and that each of the compounds involved is adsorbed on these centers in only one way. The experimentally found effect of the products and intermediate product could be best expressed by extending the equations (17) to the forms ... 

Two-component systems are obtained by the interaction of transition metal compounds of groups IV-VIII of the periodic system with or-ganometallic compounds of groups I-III elements (Ziegler-Natta catalysts). An essential feature of the formation of the propagation centers in these catalysts is the alkylation of the transition metal ions by an organo-metallic cocatalyst. 

The value of Kp as a measure of the reactivity of the active centers in the propagation reaction is the most fundamental characteristic of polymerization catalysts. The conclusions on the polymerization mechanism based on the correct values of N and Kp are much more unambiguous than those made when considering only the data on the polymerization activity and molecular weight of a polymer. 

To determine the number of propagation centers in one-component catalysts, in principle the same methods used to study two-component catalysts of olefin polymerization may be applied Qsee (18, 160, 160a) ]. The most widely used methods for the determination of the number of propagation centers in polymerization catalysts are ... 

B. Number of Propagation Centers in One-Component Catalysts 1. The Chromium-Oxide Catalysts... 

Several determinations of the number of propagation centers by the quenching technique have been carried out (98, 111). As a quenching agent methanol, labeled C14 in the alkoxyl group, proved to be suitable in this case. The number of active centers determined by this technique at relatively low polymerization rates (up to 5 X 102 g C2H4/mmole Cr hr at 75° and about 16 kg/cm2) (98, 111, 168) in catalysts on silica was about... 

The reactivity of the propagation centers in oxide polymerization catalysts depended on the nature of the transition metal, support, activation temperature of the catalyst, and type of reducing agent (168a). 

Anatomy of Catalytic Centers in PhUlips Ethylene Polymerization Catalyst... 

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