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 uncomplexed
counterparts. Using density functional theory calculations, we found
a more favorable interaction energy caused by stronger orbital interactions
as the origin of this increased reactivity. Additionally, the high
regioselectivity is due to a greater degree of charge stabilization
in the transition state, leading to the major product.