Modeling the isomerization of lactones and the role of catalysts in Hetero-Diels-Alder reactions

Abstract

In this study, the isomerization of a strigolactone analogue and the catalyst effect on the hetero-Diels-Alder reactions is investigated by the quantum mechanical tools. In the first part, the interconversion of the strigolactone analogue between its E and Z isomers as well as the atropisomers in an effort to interpret the nature of the isomeric mixture is studied. Density functional theory calculations reveal that a mixture of E and Z isomers can exist concurrently as a result of being formed in the same environment. To the extent of barrier heights which can be surmounted at room temperature, E atropisomers render an affordable interconversion with lower activation energies. On the other hand, Z atropisomers display higher barriers which can be possibly overcome in time. In the second part, transition metal catalyst effect on 1,2-diazine and siloxy alkyne cycloaddition reactions is investigated. Silver(I) as a transition metal plays a crucial role in lowering the activation barriers of 1,2-diazine and siloxy alkyne cycloadditions, which otherwise require thermal conditions. Substitution effect on 1,2-diazine and siloxy alkyne is also taken into account. Relativistic density functional theory calculations prove that the primary role of a metal catalyst in an inverse electron demand Hetero-Diels-Alder reaction is to facilitate the overlap between HOMO of the dienophile and LUMO of the diene by reducing the energy gap. Elucidating the role of catalysis in the 1,2-diazine and siloxy alkyne cycloadditions under mild conditions, possible reaction mechanisms are proposed to shed light on the behavior of the hetero-Diels-Alder reaction.