Electrochemical reactions of organic compounds in liquid ammonia. III. Reductive alkylation of quinoline

The electrochemical behavior of quinoline in anhydrous liquid ammonia was investigated by cyclic voltammetry and controlled potential coulometry. In the absence of added alkylating agent, quinoline is reduced in two steps to yield the radical anion and dianion both of which undergo further chemical reaction. The radical anion species dimerizes to form the dimeric dianion which is stable in the medium and can be reversibly reoxidized back to parent compound. The second-order rate constant for this dimerization reaction was found to be 1.5 X I O 2 I./mol sec at -4OOC. In the presence of ethyl bromide or n-butyl bromide, reductive alkylation proceeds via an ECEC mechanism to yield approximately equal quantities of the 1,2-dihydro1,2-dialkyl and 1,4-dihydro1.4-dialkyl derivatives. Differences in product composition between chemical reduction with lithium and electrochemical reduction are explained Recently there has been an upsurge of interest in the study of reductive alkylation reactions in nonaqueous solvents because of their possible applications to organic synthesis. Previously, this technique has been used to prepare a variety of monoand dialkyl derivatives by chemically reducing an unsaturated hydrocarbon with an alkali metal in liquid ammonia, followed by quenching of the reaction mixture with an alkyl halide.'-4 In more recent work, these reactions have been performed in the nonaqueous solvents dimethylformamide (DMF) and acetonitrile (AN) with electrochemical reduction of the parent An advantage i n this procedure over alkali metal reduction is that both reactants can be present in solution simultaneously since selective reduction of the unsaturated compound can be obtnincd by proper adjustment of the electrode potential. With the electrochemical procedure, the scope of the reaction has been broadened to include reduction of unsaturated carbon-oxygen, carbon-nitrogen, and nitrogen-oxygen bonds, while the use of acid chlorides or acid anhydrides as electrophiles has led to the preparation of certain acyl derivatives in addition to the alkyl derivatives available from reaction with alkyl halides. A competing protonation reaction frequently accompanies the alkylation reaction in DMF and AN, producing a product mixture consisting of diprotonated, monoalkylated, and dialkylated derivatives, with the dialkyl derivative often occurring in very low yield. A higher yield of the alkyl derivatives as well as a better understanding of the alkylation reaction would be obtained if protonation could be eliminated. For this reason, a study of this reaction with Smith, Bard / Reductire Alkylation of Quinoline