Strained cyclooctenes and other activated alkenes react with tetrazines in an inverse electron-demand Diels-Alder followed by a retro [4+2]  cycloaddition (see figure). Like the other reactions of the trans-cyclooctene, ring strain release is a driving force for this reaction. Thus, three-membered and four-membered cycloalkenes, due to their high ring strain, make ideal alkene substrates.
Similar to other [4+2] cycloadditions, electron-donating substituents on the dienophile and electron-withdrawing substituents on the diene accelerate the inverse-demand Diels-Alder. The diene, the tetrazine, by virtue of having the additional nitrogens, is a good diene for this reaction. The dienophile, the activated alkene, can often be attached to electron-donating alkyl groups on target molecules, thus making the dienophile more suitable for the reaction.