Formation of C-halogen bonds

Synthetic procedures are available for the preparation of fluoro, chloro, bromo and iodo compounds from the corresponding lithio derivatives. Perchloryl fluoride (FC103), W-chlorosuccinimide, bromine and iodine are examples of reagents which can be used to introduce fluorine, chlorine, bromine and iodine, respectively. 

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Formation of C-X bonds is not normally a problem but the Grignatd route can occasionally be useful when normal halogen exchange fails. Thus iodination of Me3CCH2Cl cannot be achieved by reaction with Nal or similar reagents but direct iodination ai the conesponding Grignaid effects a smooth converskm ... 

This chapter deals with three important classes of biotransformations. Firstly, those enzymes that catalyse the stereoselective formation of carbon-carbon bonds will be examined. These enzymes, whose natural functions often are to degrade carbohydrate-like molecules, have proved to be versatile catalysts for C—C bond synthesis. Secondly, we shall look at those enzymes that mediate the formation of C—X bonds, where X = O, N, S, Hal (halogen). These enzymes are termed lyases (see Table 2.1) and often carry out very simple reactions (e.g. the addition of water to a double bond) with very high stereoselectivity and regioselectivity. Finally, the application of a range of enzymes (including C—C bond formation) to carbohydrate synthesis will be examined. This chapter will conclude with some examples of the ways in which multienzyme reactions can be constructed to enable highly complex molecules to be assembled in an efficient manner. 

Lefevre G, Franc G, Adamo C, Jutand A, Ciofini I (2012) Influence of the formation of the halogen bond ArX.N on the mechanism of diketonate ligated copper-catalyzed amination of aromatic halides. Organometallics 31 914—920... 

I > Br > Cl > F. In nucleophilic aromatic substitution, the formation of the addition intermediate is usually the rate-determining step so the ease of C—X bond breaking does not affeet the rate. When this is the ease, the order of reactivity is often F > Cl > Br > I. This order is the result of the polar effeet of the halogen. The stronger bond dipoles assoeiated with the more eleetronegative halogens favor the addition step and thus inerease the overall rate of reaetion. 

The stable C-halogen bond formed can be used, however, for the formation of block copolymers by reinitiation with stronger Lewis acids. The electronic conditions in the model complex... 

Volume 2, Catalytic Reactions. An account of palladium-catalyzed reactions involving formation of C—C, C—O, C—H, C-halogen, C—N, C—S, or C—Si bonds, and heterogeneous reactions. 

The formation of C-O, C-S, C-N and C-C bonds by nucleophilic substitution is described in subsequent chapters. In this section the synthesis of haloalkanes by halogen-halogen exchange and related reactions are presented. 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

Type of reaction C-halogen bond formation Reaction condition solid-state... 

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