Transformation of Ethylenic Ion-Radicals

Stilbene derivatives can be reduced with alkali metals in liquid ammonia. The reaction is usually performed in a homogeneous medium to give substituted diphenylethane compounds as a mixture of enantiomeric forms. However, there are compounds (particularly, biologically active ones) for which the stereospecificity of synthesis has decisive importance. A simple modification of the reduction method with an alkali metal in liquid ammonia was found (Collins and Hobbs 1983), which makes it possible to perform the process stereoselectively. The metal is not predissolved, as is usual, but is added in small portions without trying to make the reaction medium homogeneous. Stereoselectivity is ensured by carrying out the reduction on the surface of the metal and not in the solution bulk. 

Compounds containing n systems have an affinity to the metallic surface and are arranged on the surface in a parallel manner. The ethylene bond of a-methyl-p-isopropyl stilbene is responsible for adsorption, and the substituents deviate from the metal surface. It is well known that alkali 

The trans isomer remains unchanged, whereas the cis isomer is converted into the trans form. The mixture thus becomes enriched with the molecules having the trans configuration. In a reference experiment, the treatment of pure cXp-CtrimethylsilyOstyrene with the same zerovalent nickel complex results in a 95% conversion into the trans isomer. 

Fraser and Taube (1959), in their pioneer work, studied the interaction of Cr + or ions with complex salts containing a cation of methylmaleatopentamminocobalt. In acid medium (HCIO4 + H2O), a redox reaction takes place. As a result of this reaction, the positive charge in the complex cation decreases by unity and the complex cation exchanges water for the methylmaleate ligand. 

The case of anethole, An-CH=CH-CH3, is interesting because of the relation between the cation-radical cyclodimerization and reduction processes. Although the rapid cation-radical dimerization of trani -anethole is strongly predominant, the cyclobutadimer formed is also ionized. Therefore, reductive cleavage of the dimer proceeds efficiently (80%). 

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