Allylic Halogenation Synthetically Useful Reactions

The transition states for abstraction of hydrogen from an alkane by chlorine and bromine radicals are very different from each other. In the transition states for abstractions by bromine, the radical on carbon is far more developed than it is in the abstractions by chlorine, which means that differences in the stabilities of different radicals will be far more important in determining the major product of the bromine abstractions than in the chlorine abstractions (Fig. 11.50). 

When questions of selectivity arise, it is always important to consider what the transition state looks like. Selective radicals will have more product-like transition states in any abstraction reaction, and therefore the factors influencing the stability of those products will be important in the transition state as well. 

Photobromination more closely approaches the goal of perfect selectivity because the bromine atom abstracts tertiary hydrogens much more readily than secondary or primary hydrogens. But even this reaction is not without its problems. It is a slow process and the side products of di- and polybromination are formed. 

FIGURE 11.51 When low concentrations of bromine are photolyzed in the presence of the substituted cyclohexene, the product is exclusively the allylic bromide shown. 

The mechanism is a typical radical chdn reaction in which a low concentration of bromine atoms is first produced by breaking of the weak bromine-bromine bond (initiation step). Abstraction of hydrogen from the cyclohexene is the first propagation step, but there are three possible kinds of hydrogen that could be removed H, (vinylic), (allylic), and H (methylene) (Fig. 11.52). 

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Other synthetically useful reactions are allylic amination using TsN=Se=NTs and TsN=S=NTs/ photolysis of a/S-unsaturated ketones in the presence of U02Cl2-methanol [e.g. to give (6) cf. Vol. 6, p. 10], " halogenation of enol silyl ethers/ terminal double bond (e.g. geranyl cyanide, carvone) bromination with (7)... 

Another synthetically useful reaction is catalytic allylic alkylation (Eq. 12.81). With this reaction, the X group can be a halogen, as well as other commonly available substrates such as acetate or carbamate. The synthetic utility derives mostly from stereochemical control, which is briefly introduced in the Connections highlight below. This reaction is widespread in organometallic chemistry and has been found to be catalyzed by a variety of metals, including nickel, palladium, platinum, rhodium, iron, ruthenium, molybdenum, and tungsten. 

Metallations are more useful synthetically when the substrate contains only one reactive site. The syntheses of benzyl-, allyl-, pentadienyl- and more delocalized group-IA organometallics and of group-IA acetylides are best performed by reaction (a). These preparations are not complicated by secondary coupling reactions or by contamination of the reaction mixture with other metals. Metallations of halogen-containing substrates at low T give carbenoids which are difficult to prepare by other routes. 

The most useful synthetic processes which have been developed around the chemistry of nickel involve the coupling of halides. Allyl halides react with nickel carbonyl, Ni(CO)4, to give 7r-allyl complexes. These compounds can react with a variety of halides to replace the halogen with an allyl group.These reactions... 

The introduction of halogen substituents by free-radical substitution was discussed in Section 12.3 of Part A. Halogenation is a fairly general method for functionalization but is limited to the site in the molecule which is most reactive toward halogen atoms. Halogenations at benzylic and allylic positions are therefore the most useful synthetic reactions. 

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