Quenching halide displacement

Addition of an alkyl group at C17 initially involves conditions fairly similar to those used in the Birch reduction of enones. A solution of the steroid in liquid ammonia and some inert co-solvent is first treated with lithium metal (Scheme 6.20). The resulting anion 20-1 is then quenched with an alkyl halide instead of the alcohol usually employed in Birch reductions. The anion at C17 then displaces the halogen, in the case at hand iodide, from the alkyl group to form a carbon-carbon bond and the 17a-methyl derivative 20-2. 

Quenching methods have been used to analyze inorganic anions. Thus, halide ions have been analyzed using indicator fluorescence quenching, with iodide being the most efficient quencher and chloride the least effective. In the same way, electrophilic bromination of fluorescein to yield the low fluorescent 1,3,6,8-tetrabromofluorescein has been used for bromine determination, and a displacement reaction using a nonfluorescent Hg metal chelate has been described for sulfide determination. 

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