A semi-empirical and ab initio study of diazoazole cycloaddition reactions with ethyne,ynamine and cyanoethyne

摘要

Semi-empirical AMI,MNDO and PM3 calculations of the mechanisms of diazoazole cycloaddition reactions with ethyne,ynamine,and cyanoethyne explain why cyanoethyne does not react.Ethyne reacts by either a[7+2]cycloaddition,directly forming the azolotriazine products,or by a[3+2]cycloaddition which forms spirobicyclic intermediates.The lowest energy reaction path for ynamine was netither the[7+2]nor the [3+2]concerted cycloaddition,but a two-step mechanism involving an azo-coupled intermediate.The proposed mechanism for ynamine/diazoazole cycloaddition predicts both the experimentally observed regioselectivity,and the increased reactivity of ynamine relative to ethyne.The relative reactivities of ynamine,enamine and 1,2-diaminoethene with diazoazoles were also compared.No transition states were found for cyanoethyne/diazoazole cycloadditions,which is why these reactions have not been observed.Ab initio(RHF/6-31G~(**))methods were used in certain instances to test the semi-empirical results.