The Unique Bioorthogonal Chemistry of Isonitriles

The isocyano group is the structurally most compact bioorthogonal group known. It reacts with tetrazines under physiological conditions and has great potential for widespread use in the biosciences. In this account, we highlight the unique properties of the isocyano group as a bioorthogonal functionality. Protecting group chemistry based on the reaction of isonitriles and tetrazines that allows releasing payloads is a particular focus of the article. We further discuss the atypical steric attractions that take place in the transition state of the reaction between isonitriles and tetrazines, which result in an increase in the rate of the reaction with steric bulk of the tetrazine substituents. These findings will open up new possibilities in bioorthogonal chemistry where reactivity and stability are simultaneously desired.