Electrochemically initiated cyanomethylations

• Main Author: Mackirdy, Iain Stewart
• Published: University of Edinburgh 1980
• Subjects: 541.37
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.654354

The results of preparative eleotrolyses in aoetonitrile containing tetrealkylammonium salts have confirmed that the hydrodimerisation of cinnamonitrile, benzaldehyde and aorylonitrile by electroreduotion at low current densities can be used to initiate the nuoleophilic cyanomethylation of these compounds. The cyanomethylated adducts, once formed, are capable of regenerating -CH2CN by further proton abstraction. Compounds such as benzophenone and 3-phenyloinnamonitrile which have stable radical-anions cannot be cyanomethylated in this way in pure acetonitrile. The effects on the yield of cyanomethylated products of temperature, concentration of the electroactive species, the presence of acids, water and alkali metal cations, and the use of an undivided cell have been investigated. The results of linear sweep voltammetries (LSV) have shown that the presence of azopyridines in solutions of benzophenone, benzaldehyde or acrylonitrile decreases the peak current of each of these substrates, whereas no significant decrease is obtained for cinnamonitrile or 3-phenyloinnamonitrile. Using ISV, it was found that electroreduced azo compounds in super-dry acetonitrile deprotonate the solvent in a reversible reaction; cyanomethylation of the substrates explains the decreases in peak current. Computer simulation of the voltammetric experiment revealed that the chemical stability of the radical-anion of the substrate has only a small effect on the peak current while the decrease in the peak current is much larger if regeneration of the nucleophilic species occurs. Working curves were produced from which could be estimated the 2nd-order rate constant, k2, for the reaction of an electrochemically generated nucleophilic species with the electroactive substrates k2 for cyanomethylation of benzophenone - 600 lmol-i8 1. Preparative electrolyses of benzenesulphonylacetonitrile in DMF in the presence of acrylonitrile yielded glutaronitrile, adiponitrile and propionitrile; possible mechanisms have been discussed. Cyanomethylated products were not detected in the electroreduction in DMF of benzenesulphonylacetonitrile in the presence of either acetophenone or 3-methylcinnamonitrile. ISV indicated that alkyl aryl ketones undergo nucleophilic addition by a species formed during the electroreduction of benzenesulphonylacetonitrile in DMF.