Reaction centers basic principles

This chapter begins with an introduction to the basic principles that are required to apply radical reactions in synthesis, with references to more detailed treatments. After a discussion of the effect of substituents on the rates of radical addition reactions, a new method to notate radical reactions in retrosynthetic analysis will be introduced. A summary of synthetically useful radical addition reactions will then follow. Emphasis will be placed on how the selection of an available method, either chain or non-chain, may affect the outcome of an addition reaction. The addition reactions of carbon radicals to multiple bonds and aromatic rings will be the major focus of the presentation, with a shorter section on the addition reactions of heteroatom-centered radicals. Intramolecular addition reactions, that is radical cyclizations, will be covered in the following chapter with a similar organizational pattern. This second chapter will also cover the use of sequential radical reactions. Reactions of diradicals (and related reactive intermediates) will not be discussed in either chapter. Photochemical [2 + 2] cycloadditions are covered in Volume 5, Chapter 3.1 and diyl cycloadditions are covered in Volume 5, Chapter 3.1. Related functional group transformations of radicals (that do not involve ir-bond additions) are treated in Volume 8, Chapter 4.2. 

The basic principles of the mechanism of this Lewis-base-catalyzed aldol reaction have already been described in Section 6.2.1.1. With regard to the course of the enantio- and diastereoselective formation of aldol adducts with two stereogenic centers, it is proposed that synthesis of anti-products proceeds via a chair-like transition structure. A distinctive feature of the cationic transition state complex is a hexacoordinated silicon atom bearing two chiral phosphoramide molecules as ligands (Scheme 6.30). 

We turn our attention now to chain-growth polymerizations. The reader should recall that the Features which distinguish chain-growth and step-growth polymerizations were summarized in Section 5.2. The present chapter is devoted to the basic principles of chain polymerizations in which the active centers are free radicals. Chain-growth reactions with active centers having ionic character are reviewed in Chapter 9. 

In the present chapter, the basic principles of chain polymerizations in which the reactive centers are free radicals will be considered in detail, focusing on the polymerization reactions in which only one monomer is involved. Copolymerizations involving more than one monomer are considered separately in Chapter 7. Chain-growth polymerizations in which the active centers are ionic are reviewed in Chapter 8. 

Chapter 6 presented the basic principles of oxidative addition. Recall that the term "oxidative addition" refers to reactions that lead to an increase in the oxidation state of the metal center by two units, an increase in the number of valence electrons on the metal center by two, and an increase in the coordination number of the complex by one or two. This term does not refer to a particular mechanism by which this transformation occurs. 

In this chapter we introduced the basic physical chemistry that governs catalytic reactivity. The catalytic reaction is a cycle comprised of elementary steps including adsorption, surface reaction, desorption, and diffusion. For optimum catalytic performance, the activation of the reactant and the evolution of the product must be in direct balance. This is the heart of the Sabatier principle. Practical biological, as well as chemical, catalytic systems are often much more complex since one of the key intermediates can actually be a catalytic reagent which is generated within the reaction system. The overall catalytic system can then be thought of as nested catalytic reaction cycles. Bifunctional or multifunctional catalysts realize this by combining several catalytic reaction centers into one catalyst. Optimal catalytic performance then requires that the rates of reaction at different reaction centers be carefully tuned. 

Nucleophilic addition to the C-O double bond of carbonyl derivatives is one of the most widely used C-C-bond forming reactions in organic synthesis. The synthetic value of this process lies particularly on the stereocontrolled formation of a new C-O-chiral center when aldehydes or ketones are used. On the other hand, carbonyl derivatives themselves can serve as nucleophiles leading to new carbon-carbon bonds and thus expanding the synthetic scope of these functional groups. The explanation of the underlying basic principles in terms of reactivity and stereochemical models is beyond the scope of this book and will not be discussed in the following chapter since many textbooks cover these issues. Instead, we wish to provide the practitioner with examples that detail the delicate relationship of an individual, unique structure with its special reactivity delineated thereof... 

The carbon dioxide molecule exhibits several functionalities through which it may interact with transition metal complexes and/or substrates. The dominant characteristic of C02 is the Lewis acidity of the central carbon atom, and the principle mode of reaction of C02 in its main group chemistry is as an electrophile at the carbon center. Consequently, metal complex formation may be anticipated with basic, electron-rich, low-valent metal centers. An analogous interaction is found in the reaction of the Lewis acid BF3 with the low-valent metal complex IrCl(CO)(PPh3)2 (114). These species form a 1 1 adduct in solution evidence for an Ir-BF3 donor-acceptor bond includes a change in the carbonyl stretching frequency from 1968 to 2067 cm-1. 

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