Mechanistic Evaluation of Bioorthogonal Decaging with trans-Cyclooctene: The Effect of Fluorine Substituents on Aryl Azide Reactivity and Decaging from the 1,2,3-Triazoline.

Bioorthogonal prodrug activation/decaging strategies need to be selective, rapid and release the drug from the masking group upon activation. The rates of the 1,3-dipolar cycloaddition between a trans-cyclooctene (TCO) and a series of fluorine-substituted azido-PABC self-immolative spacers caging two model drugs, and subsequent release from the 1,2,3-triazoline are reported. As the number of fluorine substituents on the PABC linker increases from one to four, the rate of cycloaddition increases by almost one order of magnitude. Using a combination of fluorescence, 1H/19F NMR, and computational experiments, we have been able to determine how substituents on the PABC ring can influence the degradation rates and also the product distribution of the 1,2,3-triazoline. We have also been able to determine how these substituents influence the rate of imine hydrolysis and 1,6-self-immolation decaging rates of the generated anilines. The NMR and computational studies demonstrate that fluorine substituents on the aromatic ring lower the transition state energy required for converting the triazoline to the imine or aziridine intermediates via extrusion of diatomic nitrogen, and that in the case of a tetrafluoro substituted aromatic ring, it is the imine hydrolysis and 1,6-self-immolation that is rate-limiting. This knowledge further enhances the understanding of factors which influence the stability of triazolines, and enables potential applications of fluorinated aromatics, in particular, perfluorinated aromatics, in synthetic chemistry and sustained-release drug delivery systems.