2-Alkenyl anions and their surprising endo preference. Facile and extreme stereocontrol over carbon-carbon linking reactions with organometallics of the allyl type

On structural equilibration, butenyl sodium, potassium and cesium strongly favor the cis (endo) configuration over the extended form. The cis-trans ratios vary from 93:7 to 96:4 and 99.9:0.1 in the series Na, K, and Cs; the corresponding value for 2-butene is 23:77. Homologous alkenylmetal compds. exhibit less extreme stereoselectivities, but still follow the same trend. The metal effect on the endo-preference is due to the electropositivity and bulk of the metal as well as intramol. H bonding between the alkyl group attached to the inner side of the allyl moiety with the electron excess accumulated at the other allylic terminus. [on SciFinder (R)]