Exchange of the halogen

When treated with butyllithium in diethyl ether at —50°C, bromobenzene undergoes the permutational exchange of the halogen against the metal only slowly, but rapidly at 0 °C. The transformation is complete after a few seconds if conducted in tetrahydrofuran at —75 °C. lodobenzene reacts instantaneously in any case. 

Searching for other 6-membered "diazido malonyl N-heterocycles" we have selected the pyridazine derivative 25 and diazido barbituric acids 32 as model systems, since they are available easily and in large quantities. Compound 25 is obtained in the usual way by chlorination of 23 with sulfuryl chloride and exchange of the halogen atoms with sodium azide. 

A -Dioxaphospholanes show no inversion at the phosphorus atom, which can be seen in the nonequivalence of the CFj groups (273). A -Dioxaphospholanes 73 can be oxidized with halogens to form A -dioxaphospholanes 74 (228). Exchange of the halogens in 73a-c with suitable reaction partners (e.g., LiNHj) yields substituted A -phospholanes [e.g., 73g (265)] (see also Section III,A,2). 

An interesting preparation is tliat of ethyldi-iodoursims where an exchange of the halogen is made between the e(trre.spunding diehloro-arsine and sodium iocude in dry acetone solution,... 

Finkelstein reaction Equilibrium exchange of the halogen atom in alkyl halides for another halogen atom. 170... 

As the starting diazirines have now become readily available from the exchange of the halogen atom in a 3-halo-3-organooxy-3/f-diazirines for an alkoxy group, this method has gained importance. 

In the series F < Cl < Br < I, iodine is the most easily replaced directly by hydrogen. This reaction is usually easier when the halogen is attached to tertiary than to secondary or primary carbon. Also exchange of the halogen is facilitated by neighboring substituents that, by electron-donor effect on the carbon atom, weaken the carbon-halogen bond. 

Neutral amidinate complexes [RuX /i (A,iV)-CMe(NPf)2 ( "ar i )] 256 have been synthesized by reacting the dimers [ RuX(/i-X)(7 -arene) 2] with the lithium salts of the appropriate ligands (Scheme 20)." The related cationic compounds 257 are obtained by exchange of the halogen atom with the weakly coordinating BAr 4 counteranion. 

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