Random single-displacement reaction

Note that the ternary complex EAB can be formed in two different ways. If the formation of EAB can occur with either substrate binding first, the reaction is known as a random single-displacement reaction. Many reactions catalyzed by phosphotransferases are of this type. If a particular substrate must bind first with the enzyme before the second substrate can bind, the reaction is known as an ordered single-displacement reaction. Many reactions catalyzed by dehydrogenases are of this type. The values for Km and Vmax for each substrate can be obtained from experiments in which the concentration of one substance is held constant at saturating levels while the concentration of the second substrate is varied. Kinetic analyses can distinguish between these types of reactions. 

In single displacement reactions both substrates A and B simultaneously must be present on the active site of the enzyme to yield a ternary complex EAB in order that the reaction may proceed. Single displacement reactions take place in two forms, random and ordered, and they are distinguished by the way the two substrates bind to the enzyme. 

As mentioned earlier, a random and statistical cyclization with two different and regio-defined Ti-compounds would produce a synthetically unattractive mixture of ten different zirconacycles. In reality, however, there are a few factors that can be exploited to produce a single desired zirconacycle. A systematic investigation has revealed that there are several discrete types offive-membered zirconacycle formation, as shown in Scheme 1.53 [88,89] (Generalization 20). In the Type I reaction, the cross-selective cyclization is kinetically favored. Presumably, little ethylene is displaced during the reaction. Type I reactions cannot be readily observed with ZrCp2 complexes with 1-butene. In contrast, Type II cyclization must be thermodynamically controlled, as 1-butene is readily displaced by a number of better Ti-ligands. It is predicted, however, that the cross-combination of the two Ti-com-... 

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