Stable dianions, metal-ammonia reduction

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

The dications and dianions of cot are potentially planar, aromatic systems a thorough study of a variety of substituted cot s as dications in SbFs-SOjClF solution at -78 C has been conducted.The formation of cot dianion from cyclo-octa-1,5-diene with potassium metal is suggested as a good laboratory method (30%), and the use of K and PrCU in THF was also studied, being a milder route. Bi(cyclo-octatetraenyl) easily forms a tetra-anion (with potassium in liquid ammonia) which is quite stable at 0 °C and which maintains orthogonal rings, as indicated by an n.m.r. study. A parallel study of its polarographic reduction reveals firstly a two-electron reduction wave followed by two one-electron reductions. ... 

---