Mechanism and Scope of the Base-Induced Dehalogenation of (E)-Diiodoalkenes

A wide range of nucleophiles have induced the elimination of iodine from (E)‐diiodoalkenes to form alkynes under surprisingly mild conditions. The iodide anion is particularly efficient, and can drive the reaction to completion in less than 1 hour at room temperature in a polar aprotic solvent. Detailed investigations have suggested the reaction has a bimolecular polar mechanism. The deiodination reaction can be driven to completion with 1 equiv. of nucleophile and is partially catalytic with substoichiometric amounts of deiodinating reagent. Kinetic analysis of the stoichiometric iodide‐induced reaction indicated an overall pseudo‐first‐order behavior. The reaction exhibited strong solvent effects, with much slower reactions observed in protic solvents than in polar aprotic solvents. The substrate dimethyl (2E)‐2,3‐diiodobutene‐2‐dioate demonstrated orthogonal reactivity for either elimination or hydrolysis, depending on the solvent and nucleophile used. This reaction is a major pathway for all the diiodoalkenes examined, and represents a challenge and an opportunity for using these substrates in organic synthesis. A diiodoalkene diester undergoes hydrolysis on treatment with KOH, but eliminates iodine on reaction with iodide salts. This orthogonal reactivity is just one example of the interesting chemistry that arises from the reactions of diiodoalkenes with Lewis bases. The mechanism for this mild reaction has been studied experimentally.