Polycycles lithium metal reduction

Dibenzo[b,fjpentalene (indeno[2,la]indene dianion) (32 ) has attracted much interest82). Its preparation via lithium metal reduction of 3 and from 32 by deprotonation with BuLi is very easy. NMR studies ( H, 13C and 7Li) of this system yield information on the electron distribution and the ion pair structure 60,81,82). The intermediate step, i.e. the radical anion 3 affords the estimation of charge densities. The starting material, viz. 3, is a 4nn system and is an excellent example of the Randic approach of polycyclic system s8). The H NMR spectrum of 32 shows the balance... 

The reduction process of polycycles by lithium metal converts the neutral atoms to anions. The electron transfer is best achieved in ethereal solvents. This enables the stabilization of the lithium cation by coordination to the oxygen atoms of the solvent. The hydrocarbon anion and the cation are linked together by electrostatic forces in which the solvent molecules are also involved, therefore the ion-solvation equilibrium should be considered8. The limiting cases in this equilibrium are free ions and contact ion-pairs (CIP), and in between there are several forms of solvent separated ion-pairs (SSIP)9. In reality, anionic species of aromatic hydrocarbons in ethereal solvents exist between CIP and SSIP. Four major factors influence the ion-solvation equilibrium of lithium-reduced 7T-conjugated hydrocarbons, as observed by H and 7Li NMR spectroscopies8,10. 

The metal reduction of the polycyclic system is usually carried out in an ether solvent and by an alkali metal at low temperature (—78 °C). When potassium metal is applied it is best to prepare a metal mirror. Sodium and lithium react, either directly in the form of a metal wire, or after treatment in an induction furnace. Cesium is prepared by thermolysis of cesium azide. It has recently been found that the application of an ultrasonic bath facilitates the reaction and avoids side reactions. The reaction can be carried out in a modified NMR tube or in an ESR cavity. Diamagnetic ions are prepared in extended NMR tubes to which the metal is extruded and sealed under vacuum. Reaction rates are difficult to compare as the electron-transfer process depends on various experimental conditions such as concentration, temperature, the presence of impurities, the solvent and the nature of the metal surface. It may take from minutes to days to form the first radical-anion the second step then follows and can sometimes be rather slow 10 13). 

The dissolving metal reduction of mono- or polycyclic 4-piperidinones affords the corresponding pipcridinols generally with high selectivity. A series of monocyclic 4-piperidinones has been reduced stereoselectively with lithium/ammonia, e.g., l,3-dimethyl-4-piperidinone affords the corresponding 4-piperidinol with high equatorioselectivity (98 2). 

Reduction of polycyclic quinones. This hydride in DMF is the most efficient of various metal hydrides for reduction of polycyclic quinones to hydroquinones (90 95% yield) with the exception of anthraquinone. With this quinone, lithium aluminum tri-t-butoxyhydride is more efficient (757n yield of the hydroquinone diacetate). ... 

In this chapter we concentrate on reduction processes of carbon-rich systems. The formation of anions and radical anions of -conjugated monocyclic systems, cyclophanes, bowls and fullerenes is described. Carbon-rich compounds can be reduced directly by contact with alkali metals Li, Na, K, Rb and Cs, which have a low reduction potential. Proton, carbon and lithium NMR and EPR spectroscopies are the main methods used to gain a better understanding of the mono- and polycyclic systems in solution. Special attention will be given to modes of electron delocalization, aromaticity, anti-aromaticity, as well as aggregation, bond formation and bond cleavage processes of diamagnetic electron transfer products. Electrochemical reductions will be briefly discussed. 

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