Kinds of Catalyzed Organic Reactions

A fundamental classification of organic reactions is possible on the basis of the kinds of bonds that are formed and destroyed and the natures of eliminations, substitutions, and additions of groups. Here a more pragmatic list of 20 commercially important individual kinds or classes of reactions will be discussed. 

Alkylations, for example, of olefins with aromatics or isoparaffins, are catalyzed by sulfuric add, hydrofluoric add, BF3, and A1C13. 

Condensations of aldehydes and ketones are catalyzed homogeneously by adds and bases, but solid bases are preferred, such as anion exchange resins and alkali or alkaline earth hydroxides or phosphates. 

Cracking, a rupturing of carbon—carbon bonds, for example, of gas oils to gasoline, is favored by silica-alumina, zeolites, and add types generally. 

Dehydration and dehydrogenation combined utilizes dehydration agents combined with mild dehydrogenation agents. Included in this class of catalysts are phosphoric add, silica-magnesia, silica-alumina, alumina derived from aluminum chloride, and various metal oxides. 

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Kinds of Catalyzed Organic Reactions 563 Physical Characteristics of Solid Catalysts 564 Catalyst Effectiveness 565... 

Intermediate processes of catalyzed organic reactions may involve neutral free radicals R , positive ions R+, or negative ions R as short-lived reactants. A classification of catalysts and processes from the point of view of elementary reactions between reagents and catalysts Is logically desirable but has not yet been worked out. However, there is a wealth of practice more or less completely documented, some proprietary but available at a price. The ensuing discussions are classified into kinds of catalysts and into kinds of processes. 

Aldol reactions occur in many biological pathways, but are particularly important in carbohydrate metabolism, where enzymes called aldolases catalyze the addition of a ketone enolate ion to an aldehvde. Aldolases occur in all organisms and are of two types. Type 1 aldolases occur primarily in animals and higher plants type II aldolases occur primarily in fungi and bacteria. Both types catalyze the same kind of reaction, but type 1 aldolases operate place through an enamine, while type II aldolases require a metal ion (usually 7n2+) as Lewis acid and operate through an enolate ion. 

For a review of reactions of H2O2 and metal ions with all kinds of organic compounds, including aromatic rings, see Sosnovsky, G. Rawlinson, D.J. in Swem Organic Peroxides, vol. 2 Wiley NY, 1970, p. 269. See also Sheldon, R.A. Kochi, J.K. Metal-Catalyzed Oxidations of Organic Compounds, Academic Press NY, 1981. 

In recent years, supramolecular chemistry has produced a number of systems which have been shown to be able to effectively catalyze a Diels-Alder reaction. Most systems selectively afforded only one diastereomer because of a pre-organized orientation of the reactants. These systems include cyclodextrines, of which applications in Diels-Alder chemistry have recently been reviewed89. Some other kinds of non-Lewis acid catalyzed Diels-Alder reactions, including catalysis by proteins and ultrasound, have been discussed by Pindur and colleagues90. 

Palladium-catalyzed coupling reactions are very efficient for the introduction of new carbon- carbon bonds onto molecules attached to solid support. The mild reaction conditions and compatibility with a broad range of functionalities and high reaction yields, have made this kind of transformation a very common tool for the combinatorial synthesis of small organic molecules. The literature for synthetic methods of palladium-catalyzed reactions on solid supports has recently been reviewed [237-239]. 

There are two fundamental conditions for life. First, the living entity must be able to self-replicate (a topic considered in Part III) second, the organism must be able to catalyze chemical reactions efficiently and selectively. The central importance of catalysis may surprise some beginning students of biochemistry, but it is easy to demonstrate. As described in Chapter 1, living systems make use of energy from the environment. Many of us, for example, consume substantial amounts of sucrose—common table sugar—as a kind of fuel, whether in the form of sweetened foods and drinks or as sugar itself. The conversion of sucrose to C02 and... 

The transfer of phosphoryl groups is a central feature of metabolism. Equally important is another kind of transfer, electron transfer in oxidation-reduction reactions. These reactions involve the loss of electrons by one chemical species, which is thereby oxidized, and the gain of electrons by another, which is reduced. The flow of electrons in oxidation-reduction reactions is responsible, directly or indirectly, for all work done by living organisms. In nonphotosynthetic organisms, the sources of electrons are reduced compounds (foods) in photosynthetic organisms, the initial electron donor is a chemical species excited by the absorption of light. The path of electron flow in metabolism is complex. Electrons move from various metabolic intermediates to specialized electron carriers in enzyme-catalyzed reactions. 

When an ionic organic reaction (the kind catalyzed by most enzymes) occurs a nucleophilic center joins with an electrophilic center. We use arrows to show the movement of pairs of electrons. Tire movement is always away from the nucleophile which can be thought of as "attacking" an electrophilic center. Notice the first step in the second example at right. The unsaturated ketone is polarized initially. However, this is not shown as a separate step. Rather, the flow of electrons from the double bond, between the a- and (1-carbons into the electron-accepting C=0 groups, is coordinated with the attack by the nucleophile. Dotted lines are often used to indicate bonds that will be formed in a reaction step, e.g., in an aldol condensation (right). Dashed or dotted lines are often used to indicate partially formed and partially broken bonds in a transition state, e.g., for the aldol condensation (with prior protonation of the aldehyde). However, do not put arrows on transition state structures. 

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