Iodide-Promoted Transformation Of CO₂ And Construction Of N-Heterocycles

CO₂ is a non-toxic and rich carbon resource.Using CO₂ as raw material to synthesize chemicals shows both economic and environmental values.Due to the high thermodynamic stability of CO₂ and the high reaction temperature or the use of noble-metal-catalyst in some conversions,the utilizations of CO₂ are limited.Therefore,it is still highly desirable to develop conversions of CO₂ promoted by non-transition-metal catalysts under mild conditions.1,3,4-Oxadiazol-2?3H?-one and 1,2,4-triazole are important nitrogen-heterocyclic structures,which are widely exist in some chemicals.Many compounds containing1,3,4-oxadiazol-2?3H?-one or 1,2,4-triazole skeletons show good pharmaceutical activities and biological activities.In the early years,many of the synthesis of1,3,4-oxadiazol-2?3H?-ones involved the use of highly toxic phosgene or CO.Although a variety of new synthetic methods which avoid the use of phosgene have been developed in recent years,the disadvantages such as harsh reaction conditions,the use of transition-metal catalysts or multi-step reactions still exist in the some syntheses.In addition,the synthesis of1,2,4-triazole compounds also involves high reaction temperature or the use of transition metal catalysts.Electrochemical methods have been widely introduced into organic synthesis.By adding halides,especially iodides into electrochemical systems,researchers have successfully completed many types of organic transformations.Many studies have shown that the iodine-active-substances produced from iodide under chemical or electrochemical conditions can effectively promote transformations.Our group has also been working on the studies of chemical or electrochemical organic synthesis under the action of iodide.In this paper,CO₂ instead of CO or phosgene was used to synthesize1,3,4-oxadiazol-2?3H?-ones through chemical or electrochemical method,which not only realized the conversion and utilization of CO₂ but also provided a simple and efficient new way for the synthesis of 1,3,4-oxadiazol-2?3H?-ones.Moreover,we also developed a novel electrochemical way to synthesize 1,2,4-triazole compounds.The details are summaried as following:?1?KI-catalyzed synthesis of 1,3,4-oxadiazol-2?3H?-ones from aryl hydrazines,aldehydes and CO₂ in one step.The method features simple reaction process,easily availiable starting materials and broad substrate scope.Mechanism studies showed that IO^-ions gernerated by KI and TBHP ensured the smooth proceeding of the transformation and the obtaintion of target products with high yields.In addition,commercial herbicide Oxadiazon could be successfully synthesized by this method,revealing the potential application value of this method.?2?KI-catalyzed reaction of aryl hydrazines with?-oxocarboxylic acids for the synthesis of 1,3,4-oxadiazol-2?3H?-ones in the presence of CO₂.This process could effectively utilize CO₂ from the decarboxylation of?-oxocarboxylic acids.The approach also features mild conditions,short reaction time,easily available raw materials.A good variety of substituted phenylhydrazines and?-oxocarboxylic acids are favorable for this method.Researches on the mechanism indicated that this transfrormation did not involve radical process and that nitrile imines was generated as 1,3-diploes.Similarly,IO^-ions generated in situ played important role in the process.?3?Electrochemical synthesis of 1,3,4-oxadiazol-2?3H?-ones from aryl hydrazines,polyformaldehyde and CO₂.The reaction was conducted in an undivided cell with a Ni foil cathode and a graphite rod anode at room temperature.The target products were gained with high yields through this constant current electrosynthesis.Iodine generated in situ at anode promoted the transformation.This method makes use of CO₂ effectively at room temperature and avoids the use of toxic reagents and strong oxidants.It provides a green and efficient electrochemical way for the synthesis of 1,3,4-oxadiazol-2?3H?-ones.?4?Electrochemical synthesis of 1,5-disubstituted 1,2,4-triazoles or 1-aryl-1,2,4-triazoles.With aryl hydrazines,polyformaldehyde and ammonium acetate as raw materials,this transformation was carried out in an undivided cell with a Ni cathode and a graphite anode at room temperature.The solvent alcohols participated in the transformation as well.NH₃ generated in situ at cathode provided the nitrogen source in buliding of the heterocyclic ring.With easily available and low toxic raw material,broad substrate scope and the mild reaction conditions,this simple synthetic method offers a new efficient way for the synthesis of substrated 1,2,4-triazoles.