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Scavengers that capture reactive chemical substances are used to prevent the decomposition of materials. However, in the field of catalysis, the development of scavengers that inhibit background pathways has attracted little attention, although the concept will open up an otherwise inaccessible reaction space. In catalytic bromination, fast non‐catalyzed background reactions disturb the catalytic control of the selectivity, even when using N‐bromoamide reagents, which have a milder reactivity than bromine (Br2). Here, we developed a trans‐cyclooctene (TCO) bearing a 2‐pyridylethyl group to efficiently retard background reactions by capturing Br2 in bromocyclization using N‐bromosuccinimide. The use of less than a stoichiometric amount of the TCO was sufficient to inhibit non‐catalyzed reactions, and mechanistic studies using the TCO revealed that in situ‐generated Br2 provides non‐catalyzed reaction pathways based on a chain mechanism. The TCO is useful as an additive for improving enantioselectivity and regioselectivity in catalytic reactions. Cooperative systems using the TCO with selective catalysts offer an alternative strategy for optimizing catalyst‐controlled selectivity during bromination. Moreover, it also served as an indicator of Br2 involved in catalytic reaction pathways; thus, the TCO was useful as a probe for mechanistic investigations into the involvement of Br2 in bromination reactions of interest. [ABSTRACT FROM AUTHOR]