Mechanism-Guided Development of Bifunctional Cyclooctenes as Active, Practical, and Light-Gated Bromination Catalysts

Most molecular catalysts have been developed employing polar functional groups as catalytic sites. However, the use of non-polar functional groups for catalysis has received less attention due to their modest molecular interactions while the bioorthogonal reactivity of non-polar alkenes as substrates is frequently used in click chemistry. In this study, we conducted mechanistic studies on the catalysis of trans-cyclooctene (TCO) derivatives with the strained olefin as the catalytic site using kinetic and computational analyses to aid the design of more active olefin catalysts. The analysis reveals the significant role of the benzyl substituents in accelerating the generation of bromonium species through dispersion interaction in the rate-determining step. Guided by the mechanistic insights, we developed bifunctional TCO catalysts bearing a functionalized benzyl group, taking advantage of the remarkable substituent effects. Experimental studies confirmed the theoretical model and revealed that TCO with a para-hydroxybenzyl group provided excellent catalytic activity. Furthermore, inclusion of the functionalized benzyl groups allowed more readily available and robust cis-cyclooctenes to be used as active catalysts, expanding the practical utility of the olefin catalysts. By using a photochemically labile masking group on the para-hydroxybenzyl substituent, the first light-gated bromination catalyst was developed, enabling spatiotemporal control of the transformation.