Light Harvesting for Rapid and Selective Reactions: Click Chemistry with Strain-Loadable Alkenes

Intramolecular strain is a powerful driving force for rapid and selective chemical reactions, and it is the cornerstone of strain-induced bioconjugation. However, the use of molecules with built-in strain is often complicated as a result of instability or selectivity issues. Here, we show that such strain, and subsequent cycloadditions, can be mediated by visible light via the harvesting of photochemical energy. Through theoretical investigations and molecular engineering of strain-loadable cycloalkenes, we demonstrate the rapid chemoselective cycloaddition of alkyl azides with unstrained cycloalkenes via the transiently (reversibly) formed trans-cycloalkene. We assess this system via the rapid bioconjugation of azide-functionalized insulin. An attractive feature of this process is the cleavable nature of the linker, which makes a catch-and-release strategy possible. In broader terms, we show that conversion of photochemical energy to intramolecular ring strain is a powerful strategy that can facilitate complex chemical transformations, even in biomolecular systems. Probing, isolating, and/or manipulating biologically relevant macromolecules is central to the study of their function in living systems. However, the synthetic tools available for performing the chemistry necessary for such studies are often difficult to use or limited in utility. In the approach presented here, light is converted to molecular strain energy, which can in turn be used for performing rapid and highly selective chemistry on macromolecular systems. Because it involves chemically stable and chemoselective reactions, this research not only opens up new possibilities for biomolecular functionalization and manipulation but also promises to make such experiments accessible to a broader class of researchers. The central concept of strain-loadable alkenes is general and provides a firm foundation for light-activated chemistry in complex environments. Strain-loadable alkenes are cycloalkenes that, when irradiated in the presence of a visible-light-absorbing photocatalyst, undergo double-bond isomerization. Because of engineered geometrical constraints, this isomerization results in significant molecular strain. Weaver and colleagues exploit this strain to dramatically accelerate the cycloaddition with azides, which are otherwise unreactive, in mixed molecular environments.