Halogen/metal displacement

Iodine is a bluish-black, lustrous solid, volatizing at ordinary temperatures into a blue-violet gas with an irritating odor it forms compounds with many elements, but is less active than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulfide to form beautiful purple solutions. It is only slightly soluble in water. 

For the halogen-metal exchange reaction of bulkier halopyrimidines, steric hindrance retards the nucleophilic attack at the azomethine bond. As a consequence, halogen-metal exchange of 5-bromo-2,4-di-r-butoxypyrimidine (43) with n-BuLi could be carried out at -75 °C [20]. The resulting lithiated pyrimidine was then treated with n-butylborate followed by basic hydrolysis and acidification to provide 2,4-di-f-butoxy-5-pyrimidineboronic acid (44). 5-Bromopyrimidine 43 was prepared from 5-bromouracil in two steps consisting of a dehydroxy-halogenation with phosphorus oxychloride and an SnAt displacement with sodium r-butoxide. 

The typical reactions of halogenothiophenes can be classified under the following headings nucleophilic displacement, halogen-metal exchange, reductive dehalogenation and formation of thienyl radicals. 

Chlorine reacts readily with hydrogen to form hydrogen chloride, with metals and many non-metals to give chlorides, with metal oxides to give chlorides or oxychlorides, and with many salts of metals to give chlorides. These include the iodides and bromides, whose halogen is displaced by chlorine. 

We should include the somewhat similar cyclisation onto a phosphine oxide here, even though no carbonyl group is involved. In this strange reaction, deprotonation of 88 a to phosphorus protects this position while halogen-metal exchange generates an aryllithium which displaces PhLi from phosphorus to give 89.50... 

Side reactions that can occur in these syntheses include metal-halogen exchange , displacement of organic groups from tin -, e.g. ... 

The synthesis of thieno[2,3- f]imidazole illustrates again the selectivity in halogen-metal exchange processes in imidazoles, hi this sequence a vinyl was used as A-protecting group, and it includes a nucleophilic displacement of bromine from the 4-position, activated by the 5-aldehyde. 

Most displacement reactions fit into one of three subcategories hydrogen displacement, metal displacement, or halogen displacement. 

Procedures for preparation of phenylsodium and vinylsodium from the halides have been described. These procedures involve slow addition of the halide to finely divided dispersions of sodium metal. In this way, the halide concentration is kept low. Since aryl and vinyl halides are relatively unreactive toward displacement, halogen-metal exchange is faster than Wurtz coupling. The organic derivatives of the higher... 

In a displacement reaction, an ion (or atom) in a compound is replaced by an ion (or atom) of another element Most displacement reactions fit into one of three subcategories hydrogen displacement, metal displacement, or halogen displacement. 

Activity series rank elements in order of ability to displace each other. A more reactive metal can displace (reduce) hydrogen ion or the ion of a less reactive metal from solution. A more reactive halogen can displace (oxidize) the ion of a less reactive halogen from solution. 

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