Polyfunctional catalysis

Polyfunctional catalysis and intramolecular catalysis 12.1 POLYFUNCTIONAL CATALYSIS 

A polyfunctional catalyst possesses two or more reactive groups which interact with different parts of the substrate. The classical (and still most important) example of polyfunctional catalysis is the mutarotation of glucose in benzene solution under the action of 2-hydroxypyridine. According to the finding of Swain and Brown [275], the reaction is first-order in the substrate and first-order in the catalyst. A 10-3 M solution of the catalyst is 7000 times more active than a mixture of 10 3 M pyridine and 10 3 M phenol. 

There is much evidence that enzymes are polyfunctional catalysts. In an 

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Photocatalytic oxidation 405,445 Polyfunctional catalysis 367,487 Potential measurement 321... 

Some heterogeneous catalytic reactions proceed by a sequence of elementary processes certain of which occur at one set of sites while others occur at sites which are of a completely different nature. For example, some of the processes in the reforming reactions of hydrocarbons on platinum/ alumina occur at the surface of platinum, others at acidic sites on the alumina. Such catalytic reactions are said to represent bifunctional catalysis. The two types of sites are ordinarily intermixed on the same primary particles ( 1.3.2) but similar reactions may result even when the catalyst is a mixture of particles each containing but one type of site. These ideas could, of course, be extended to crea te the concept of polyfunctional catalysis. 

Thus a low-level "side product may play a role in allowing polystep and polyfunctional catalysis to arise, in that this side-product possesses the potential properties of a quasi-4niermediate. 

It seems we have matured to the realization that any influential effects must imply involvement through some sort of force fields between catalyst and reaction partners we acknowledge these forces to be electronic and therefore chemical in nature, and thus we imply the existence of at least temporary chemical complex or bond formation with the catalyst. Clearly then, the n-body problem of chemistry (e.g., of A and B) becomes at least an (2n- - l)-body problem (A, B X AX, BX) of catalytic chemistry (even before we worry about such strictly additional problems as energy heterogeneity of sites, polyfunctional catalysis, side reactions, etc.). 

Enzyme-substrate complex Polyfunctional catalysis Proximity Medium effects... 

The impact of nucleophilic and electrophilic groups of the active center on the substrate at the contact area in the enzyme-substrate complex (the effect of synchronous intramolecular catalysis). The polyfunctional catalysis involves a great many processes push-pull mechanisms, processes involving a relay charge transfer, as well as a general acid-base catalysis. Presumably, the enzyme in the initial state of the enzymatic reaction already contains structural elements of the transition state and in this case the reaction must be thermodynamically more advantageous. 

As a result of the above effects, a sharp increase in the absolute reaction rate occurs, since little probable higher-order reactions requiring the creation of three or more molecules are substituted by highly effective first-order reactions, i.e., reactions of intramolecular polyfunctional catalysis. All of the... 

Problems of Polyfunctional Catalysis by Immobilized Metal Complexes... 

When the physical modification method is used, PS is modified by mechanical stirring with various synthetic rubbers such as polybutadiene, polybutadiene styrene, polyisopropene, polychloropropene, polybutadiene styrene-acrylonitrile copolymers. In the chemical modification, PS is modified with polyfunctional modificators in the presence of cationic catalysis. 

Enzyme specificity is often explained in terms of the geometric configuration of the active site of the enzyme. The active site includes the side chains and peptide bonds that either come into direct contact with the substrate or perform some direct function during catalysis. Each site is polyfunctional in that certain parts of it may hold the substrate in a position where the other parts cause changes in the chemical bonding of... 

Some of the reactions covered here are in principle also possible under strongly acidic or strongly basic conditions or as concerted pericydic reactions at high temperatures [3] for polyfunctional substrates in the synthesis of complex products the advantage of transition metal catalysis under neutral conditions and at low temperatures is obvious. 

The formation of arylzinc reagents can also be accomplished by using electrochemical methods. With a sacrificial zinc anode and in the presence of nickel 2,2-bipyridyl, polyfunctional zinc reagents of type 36 can be prepared in excellent yields (Scheme 14) . An electrochemical conversion of aryl halides to arylzinc compounds can also be achieved by a cobalt catalysis in DMF/pyridine mixture . The mechanism of this reaction has been carefully studied . This method can also be applied to heterocyclic compounds such as 2- or 3-chloropyridine and 2- or 3-bromothiophenes . Zinc can also be elec-trochemically activated and a mixture of zinc metal and small amounts of zinc formed by electroreduction of zinc halides are very reactive toward a-bromoesters and allylic or benzylic bromides . ... 

Catalysis of D—H exchange in ketones and aldehydes by bi- and polyfunctional compounds [a>aminoalkanoic acids [82], cu-dimethylamino-alkylamines [83] and poly(ethylenimines)] has been extensively studied by Hine and coworkers as a model of various enzyme-catalysed transformations entailing analogous steps (see below). The main goal of this work lies in the determination of the steric requirements of intramolecular catalysis. Since the principal results have been recently reviewed (Hine, 1978), only a short survey will be presented here. 

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