Abstract
Seven structurally distinct pentadienyl type lithium and potassium compounds were
screened against a variety of electrophiles in order to assess the regioselectivity
of the trapping reactions. Organoborates and analogs thereof (fluorodimethoxyborane)
proved to be perfectly regioreliable attacking only unsubstituted terminal positions
and thus providing, after oxidation, exclusively primary allylic alcohols. 2,4-Pentadienyllithiums
or -potassiums, that carry a methyl group at the 1- or 3-position, exhibit the same
extreme regioselectivity towards halotrialkylsilanes or carbon dioxide. Although the
unsubstituted parent compounds combine with such electrophiles still preferentially
at the terminal position, considerable proportions of branched products are concomitantly
formed as well (1/3-attack ratios ranging from 2:1 to > 20:1). Hydroxyalkylating and
alkylating reagents such as formaldehyde, oxirane or butyl iodide invariably afford
regioisomeric mixtures generally varying in composition between 3:1 and 1:3. The condensation
reaction with halotrialkylsilanes appears to follow a concerted (SN2-like) rather than an addition/elimination (ate complex-mediated) mechanism.
Key words
diene metalation - superbase chemistry - organometallic mobility - regioselectivity
- stereoselectivity
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