Origin of Increased Reactivity in Rhenium-Mediated Cycloadditions of Tetrazines

Pyridyl tetrazines coordinated to metals like rhenium have been shown to be more reactive in [4+2] cycloadditions than their un-complexed counterparts. Using density functional theory (DFT) calculations, and in particular distortion/interaction and energy de-composition analysis, we analyze the factors that contribute to this increase in selectivity. The reaction of the complexed tetrazine has a more favorable interaction energy compared to the uncomplexed tetrazine, and the main contributor to this favorable interaction energy an increase in orbital interactions through a lowering of the LUMO in the complexed tetrazine. Additionally, the high regioselectivity of the reaction is shown to be due to a greater degree of charge stabilization in the transition state leading to the major prod-uct. This study suggests that the energy of the FMOs is a good indicator of reactivity in these reactions, and the enhanced [4+2] reactivity upon complexation adds another tool to the toolbox of click reactions.