Research On One-Pot Synthesis Of 1,2,4-Oxadiazole And Benzimidazole

N-heterocyclic compounds are widely used in functional materials,dyes,pharmaceuticals,pesticides and other fields;The position of nitrogen heterocycles is irreplaceable,especially in the field of medicine and pesticide.More than 60%of the medicine and pesticide molecules are nitrogen-containing heterocyclic compounds.Therefore,looking for a green and efficient method for the synthesis of N-heterocyclic compounds from simple substrates has always been a hot topic in the synthesis field.The traditional method of transition-metal-catalyzed and electrically heated synthesis of N-heterocyclic compounds has the problem of metal residue and large energy consumption.Through the synthetic means of metal-free,alkali-assisted and photocatalytic green can effectively alleviate the above problems.In the synthesis of N-heterocyclic compounds,we have studied the green synthesis of 1,2,4-oxadiazole and benzimidazole by above means and achieved the following main achievements:Among the known synthetic strategies of 1,2,4-oxadiazoles,the most common approach is to utilize amidoximes as starting materials or intermediates;the.previous synthetic methods through this approach always need to be proceed under harsh conditions.For example,when carboxylic acid and amidoximes are used as the substrates to prepare 1,2,4-oxadiazoles,strong dehydrating agent are usually needed;When aldehydes and amidoximes are used as the substrates,oxidants or microwave conditions are usually needed.A simple base-mediated one-pot synthesis of 3,5-disubstituted 1,2,4-oxadiazoles from nitriles,aldehydes and hydroxylamine hydrochloride has been developed under of nitrogen atmosphere.This method has several advantages such as the substrates is simple,one-pot synthesis without separating intermediates.A variety of aromatic systems,including heterocyclic nitrile and aldehydes are preferred,the yield can be achieved 93%.Compared with the previous method,this method is environmentally friendly,and has great economic and practical value.We synthesized the 1,2,4-oxadiazole without extra oxidants or additives,and proposed a new reaction mechanism of the synthesis of the compound,Aromatic aldehydes are rarely reported as oxidants.In our reaction,aldehydes act as both substrates and oxidants.This reaction method is cost-effective,environmentally friendly;it is in line with the requirements of green chemistry.Traditionally,synthesis of benzimidazoles usually needed to add strong acid or transition metal as catalyst under heating,the reaction conditions are relatively harsh.Photocatalysis is a typically green synthetic method has the advantages such as room temperature reaction,less amount of by-products,easy to handle.And the application of visible light energy is consistent with the requirements of green chemistry.The synthetic methods of benzimidazole through photocatalysis has been reported before,which o-phenylenediamines and aldehydes or alcohols used as the substrates under photocatalytic conditions.However,aldehydes are easy to be oxidizede,leading to the formation of unwanted by-product.When o-phenylenediamines and alcohols are used as the substrates,transition-metal-catalysts are usually needed.We have developed a organic photosensitizer eosinY-catalyzed method for the synthesis of benzimidazole compounds from o-phenylenediamines and alcohols under metal-free and visible-light induced conditions,in which the oxygen act as oxidants.This reaction include two sequential procedures:oxidative dehydrogenation to alcohol to lead to aldehyde,treatment of aldehyde with o-phenylenediamine to form the corresponding benzimidazole compounds under photocatalyzed and oxygen-oxidized conditions.This method has the many advantages such as mild oxidant oxygen,metal-free catalyst and the mild and green reaction conditions.