Catalytic reactions involving ethylene mechanism

Practically all the heavy transition metals can be made to eatalyze olefin isomerization, presumably through transient formation of metal hydrides. A stable platinum hydride has been shown to react with ethylene to form a cT-CjHjPt complex which can eliminate ethylene to regenerate the hydride. The commercially successful processes for the conversion of ethylene to acetaldehyde and ethylene to vinyl acetate via PdClj catalysis have stimulated enormous interest in the mechanism of these reactions, their application to other conversions, and their extension to other catalytic systems. The various stages in the conversion of ethylene are quite well-understood and an important step in the reaction involves hydride migration. The exact role of Pd in the migration has not yet been elucidated. It seems almost certain that the phenomenal interest in the whole area of transition metal isomerization in the last several years will be more than matched by the wealth of work that is certain to pour out of research laboratories in the next few years. 

Catalytic transformations involving the C-H bonds of thiophene are rare, but recently there has been a report on the catalytic addition of the C(2)-H bond of thiophene across ethylene to form 2-ethylthiophene <20040M5514>. Reaction of the ruthenium complex TpRu(CO)(NCMe)(Me) (where Tp = hydrido tris(pyrazolyl)borate) with thiophene produces the 2-thienyl complex 249 and methane. This complex catalyzes the formation of 2-ethylthiophene from a solution of thiophene and ethylene (Equation 121). The mechanism of this reaction has been explored. 

Analyzing the mechanism of the catalytic reaction allows the identification of the major factors that affect the reactor design. The reaction kinetics is not sensitive to the concentration of the acetic acid, but the presence of some water is necessary to activate the catalyst. On the contrary, ethylene and oxygen are involved in kinetics through a complex adsorption/surface reaction mechanism. The catalyst manifests high activity and selectivity. The power-law kinetics involves only ethylene and oxygen [8] ... 

Impressive calculations for several sequences of reaction steps of the catalytic hydroboration of ethylene by HB(OH)2 using a model Wilkinson catalyst, RhCl(Ph3)2, have been done by Musaev et al. [64]. Different mechanisms were compared involving more than 30 intermediates and transition states. Full-geometry optimizations were performed at the MP2 level. This study shows that it is now possible to understand catalytic reactions at a very detailed level. The hydroboration reaction is just one example, although a particularly spectacular one, of the type of detailed studies that have been performed by Morokuma et al. For earlier work along similar lines, see Ref. 5. 

The activity of complex [lT2(CH3CN)(H)3(p-H)(P Pr3)2(p-Pz)2] as a catalyst for the hydrogenation of diphenylacetylene and ethylene contrasts with its inactivity when employed in the hydrogenation of A -benzylideneaniline. However, when transformed into its protonated derivative, for example, [lr2(CH3CN)(H)2(H2) ( 4-H)(P Pr3)2(p-Pz)2]BF4 by reaction with HBF4, the new complex becomes a very active catalyst for C=N hydrogenation [111]. The catalytic cycle involves fast elementary steps of hydride and proton transfer according to an ionic outer sphere mechanism that takes place at one of the iridium centers of the binuclear complex (Scheme 27). 

It is important to note that the hydrocarboxylation and carbonylation catalytic cycles involve common intermediates, but there are clear differences. In the hydrocarboxylation reaction, there is no oxidative addition or reductive elimination step, and all the intermediates have Rh. Other Fe-, Ru-, Co-, Rh-, Ir-, Pd-, and Pt-based hydrocarboxylation and/or hydroesterification catalysts are also known. Eastman Chemical has reported a Mo(CO)g-catalyzed ethylene hydrocarboxylation process that involves a radical mechanism. 

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

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