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

Semi-empirical AM1, MNDO and PM3 calculations of the mechanisms of diazoazo le cycloaddition reactions with ethyne, ynamine, and cyanoethyne explain wh y cyanoethyne does not react. Ethyne reacts by either a [7 + 2] cycloadditi on, directly forming the azolotriazine products, or by a [3 + 2] cycloaddit ion which forms spirobicyclic intermediates. The lowest energy reaction pat h for ynamine was neither the [7 + 2] nor the [3 + 2] concerted cycloadditi on, but a two-step mechanism involving an ate-coupled intermediate. The pro posed mechanism for ynamine/diazoazole cycloaddition predicts both the expe rimentally observed regioselectivity, and the increased reactivity of ynami ne relative to ethyne. The relative reactivities of ynamine, enamine and I, 2-diaminoethene with diazoazoles were also compared. No transition states w ere found for cyanoethyne/ diazoazole cycloadditions, which is why these re actions have not been observed. Ab initio (RHF/6-31G**) methods were used i n certain instances to test the semi-empirical results. (C) 1999 Elsevier S cience B.V. All rights reserved.