Willgerodt-Kindler Reaction of Benzaldehydes: A Comparative Study for a Sustainable Synthesis of Secondary Thiobenzamides

The synthesis of thiobenzamides from benzaldehydes, elemental sulfur and cyclic secondary amines was carried out by Willgerodt-Kindler reaction following three different protocols, avoiding the harsh conditions usually reported in the literature. Protocol A was solventless, protocol B was in acetonitrile, while protocol C was electrochemical in acetonitrile (using electrogenerated acetonitrile anion). For all protocols the temperature was 60 8C. In all cases, a stoichiometric amount of reagents was used, with waste lessening with respect to literature syntheses. Good to high yields were obtained by chemical or electrochemical methods, neat or in acetonitrile. Advantages and limits of these sustainable syntheses are highlighted. A mechamism hypothesis is reported for the reaction induced by electrogenerated acetonitrile anion.