Substituted tetrazines have been found to undergo facile inversed electron demand Diels-Alder reactions with “tunable” reaction rates. By varying the substituents on tetrazine, cycloaddition rate variations of over 200 fold have been achieved with the same dienophile. Coupled with the availability of different dienophiles, such as norbornene, the reaction rate difference can be over 14,000 folds. These substituted tetrazines can be very useful for selective labeling under different conditions. This finding paves the way to utilize tetrazine conjugation reactions for not only DNA but also stage labeling work.

Carbon monoxide (CO) belongs to the gasotransmitter family of signalling molecules in the mammalian systems with importance on par with that of NO and H₂S. Studies have shown that endogenous production of CO has anti-inflammatory, anti-proliferative, and anti-apoptotic effects in mammalian system. Besides of the conventional metal-based carbon monoxide releasing molecules (CORMs) to deliver CO for therapeutic purposes, organic CO prodrugs represent a new direction. Here we report the “click and release” approached to release CO. Unlike the metal-based CORMs, our system does not contain transition metal and liberates CO with controllable manner and possesses potential tunable releasing rate property under physiological conditions.