Generation of acyclic trienes and cyclic dienes by mild dehydration and dehydrohalogenation in aprotic media

Abstract

Substituted hexa-1,3,5-trienes and cyclohexa-1,3-dienes can be generated under mild conditions in hexamethylphosphoric triamide (HMPT), by dehydration of an appropriately substituted hexadienol or cyclohexenol or by dehydrohalogenation of dienyl or cyclohexenyl bromides, by the action of lithium iodide or methyltriphenoxyphosphonium iodide (MTPI) in HMPT. The MTPI reaction products can be rationalized on the basis of simple E₂-type dehydrohalogenation of intermediary allylic halides; however, the intermediates in the lithium iodide dehydrohalogenations appear to have a greater degree of carbocation character, and to be similar to dehydrohalogenations with silver nitrate in dimethyl sulphoxide.