Dehydration of βγ-unsaturated alcohols in dimethyl sulphoxide

Abstract

The dehydration of 2-methylpent-4-en-2-ol (V) by heating in dimethyl sulphoxide produced mixtures of the isomeric 2-methylpentadienes (I)–(IV), (I) and (II) being primary dehydration products and (III) and (IV) resulting from an acid-catalysed rearrangement of (I). Deuterium labelling experiments have shown that the acidic protons originate from the alcohol and not from dimethyl sulphoxide, and that the kinetic isotope effect for the formation of (II) is 2.4. Similar dehydration of 4-methylpent-4-en-2-ol (VI) produced the dienes (I), (III), and (IV), but not (II). The results suggest that an E2 rather than an E1 dehydration process is involved.