Pentadienyl type lithium and potassium species: The regioselectivity of their reactions with electrophiles

Seven structurally distinct pentadienyl type lithium and potassium compound s were screened against a variety of electrophiles in order to assess the r egioselectivity of the trapping reactions. Organoborates and analogs thereo f (fluorodimethoxyborane) proved to be perfectly regioreliable attacking on ly unsubstituted terminal positions and thus providing, after oxidation, ex clusively primary allylic alcohols. 2,4-Pentadienyllithiums or -potassiums, that carry a methyl group at the 1- or 3-position, exhibit the same extrem e regioselectivity towards halotrialkylsilanes or carbon dioxide. Although the unsubstituted parent, compounds combine with such electrophiles still p referentially at the terminal position, considerable proportions of branche d products are concomitantly formed as well (1/3-attack ratios ranging from 2:1 to > 20:1). Hydroxyalkylating and alkylating reagents such as formalde hyde, oxirane or butyl iodide invariably afford regioisomeric mixtures gene rally varying in composition between 3:1 and 1:3. The condensation reaction with halotrialkylsilanes appears to follow a concerted (S(N)2-like) rather than an addition/elimination (ate complex-mediated) mechanism.