Ring‐Strain‐Promoted Ultrafast Diaryltetrazole–Alkyne Photoclick Reactions Triggered by Visible Light

By exploiting the scope of dipolarophiles, we discovered that endo‐bicyclo[6.1.0]non‐4‐yn‐9‐ylmethanol (BCN) is an appropriate chemical reporter for the 2,5‐diaryltetrazole (DATet) based photoclick reaction, which proceeds via an ultrafast cycloaddition rate (∼105 M−1 s−1). Introduction of a 2,6‐(CF3)2phenyl moiety on the C5‐position of the tetrazole core brings a significant improvement of the chemical selectivity toward BCN without loss of reactivity. The high selectivity benefits from a combination of the steric and the electrostatic sheltering effect of a nitrile imine (NI) intermediate generated from photolysis of the DATet. In the absence of BCN, the oligothiophene moiety on the N2‐position of the tetrazole was found to induce self‐quenching of the NI intermediate instead of nucleophilic addition by 10 mM glutathione, demonstrating the fidelity of the DATet reagent. We also investigated the bioorthogonality of the DATet‐BCN photoclick chemistry for covalent bioconjugation toward BCN tags on proteins with visible‐light illumination. A versatile photoclick chemistry between ring‐strained alkynes and diaryltetrazoles was explored via visible light triggered photolysis of N2‐oligothiophenyl tetrazoles to yield stable pyrazoles as covalent linkages. The ultrafast and highly selective [3+2]cycloaddition was realized by using an electrostatically and sterically sheltered nitrile imine as the dipole and BCN (bicyclo[6.1.0]non‐4‐yn‐9‐ylmethanol) as the dipolarophile.