Modifying the reactivity of substrates by encapsulation is a fundamental principle of capsule catalysis. Here we show an alternative strategy, wherein catalytic activation of otherwise inactive quinone "co-factors" by a simple Pd2L4 capsule promotes a range of bulk-phase, radical-cation cycloadditions. Solution electron transfer experiments and cyclic voltammetry show the cage anodically shifts the redox potential of the encapsulated quinone by a significant 1 V. Moreover, the capsule also

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... cts the reduced semiquinone from protonation, thus transforming the role of quinones from stoichiometric oxidants into catalytic single electron acceptors. We envisage that the host-guest induced release of an "electron hole" will translate to various forms of non-encapsulated catalysis that involve other difficult to handle, highly reactive species. The modulation of reactivity using transition state-stabilizing interactions is the dominant mechanism that enzymes employ to achieve incredible rate enhancements. [1] Their remarkable activity has inspired a generation of scientists to create artificial mimics using hollow, substrate-binding constructs. [2] Enzymes, however, do not only change the reactivity of bound substrates but also other species, such as co-factors, which are essential for catalytic activity. The use of encapsulated transition metal catalysts, [3] as exemplified by Toste, Raymond and Bergman, [4] can be viewed as a form of biomimetic holocatalysis. Whilst common in nature, manipulation of redox properties using synthetic hosts is much less common, [5] even less so for catalytic applications. [6] Herein we show that the binding of several commercially available quinones by a simple coordination cage generates catalytic activity not inherent in either component. We have previously shown that cages C-1 and C-2 (Scheme 1) bind and activate quinones, [7a] facilitating Diels-Alder (DA) catalysis. [2h] As activation results from a lowered LUMO, [7b] we expected that encapsulation would increase the quinone redox potential, triggering electron transfer (ET) from a free substrate and thus generating bulk-phase radical-cation reactivity (Scheme 1a,b) . This mode of cage catalysis complements usual forms that involve substrate encapsulation; [2] it allows a wider, size-independent reactant scope and avoids product inhibition [2b,c] yet cannot easily facilitate the types of selectivity attainable using confinement effects. We were also expectant that the cage could transform the intrinsic role of quinones from stoichiometric 2(H + + e − ) oxidants to catalytic one-electron acceptors because the poly-cationic shell should "protect" the semiquinone radical-anion from protonation. This fundamental switch in reactivity would make it distinct from small molecule non-covalent activation of o-quinones. [8] Scheme 1. Supramolecular redox catalysis. (a) Cage and "cofactor" are separately inactive but (b) encapsulation switches on electron transfer (ET) and radical-cation reactivity. (c) Chemical structure of cages C-1 and C-2 used in this study.