Abstract
The hydrophobic central cavity of a water‐soluble M8L12 cubic coordination cage can accommodate a range of phospho‐diester and phospho‐triester guests such as the insecticide “dichlorvos” (2,2‐dichlorovinyl dimethyl phosphate) and the chemical warfare agent analogue di(isopropyl) chlorophosphate. The accumulation of hydroxide ions around the cationic cage surface due to ion‐pairing in solution generates a high local pH around the cage, resulting in catalysed hydrolysis of the phospho‐triester guests. A series of control experiments unexpectedly demonstrates that—in marked contrast to previous cases—it is not necessary for the phospho‐triester substrates to be bound inside the cavity for catalysed hydrolysis to occur. This suggests that catalysis can occur on the exterior surface of the cage as well as the interior surface, with the exterior‐binding catalysis pathway dominating here because of the small binding constants for these phospho‐triester substrates in the cage cavity. These observations suggest that cationic but hydrophobic surfaces could act as quite general catalysts in water by bringing substrates into contact with the surface (via the hydrophobic effect) where there is also a high local concentration of anions (due to ion pairing/electrostatic effects).
Highlights
The cavities of self-assembled molecular container molecules provide a fertile environment for the study of catalysis in confined spaces.[1,2,3,4,5,6,7] The relatively rigid, hydrophobic cavities arise from the self-assembly process of relatively simple metal and ligand components into hollow, pseudo-spherical arrays and show some similarities to the binding pockets of enzyme active sites
The species that we initially investigated as possible guests (Scheme 1) were 2,2-dichlorovinyl dimethyl phosphate (‘dichlorvos’), 2-nitrophenyl dimethyl phosphate (2NDP) and di(isopropyl) chlorophosphate (DICP)
Any hydrophobic electrophile which binds in the cavity can be brought into close contact with a high local concentration of any desired anion as a reaction partner
Summary
The cavities of self-assembled molecular container molecules provide a fertile environment for the study of catalysis in confined spaces.[1,2,3,4,5,6,7] The relatively rigid, hydrophobic cavities arise from the self-assembly process of relatively simple metal and ligand components into hollow, pseudo-spherical arrays and show some similarities to the binding pockets of enzyme active sites. We recently reported an example of the highly efficient (2 105-fold rate acceleration) catalysis of the Kemp elimination (base-promoted reaction of benzisoxazole to form 2-cyanophenolate) in the cavity of an octanuclear, approximately cubic, coordination cage Hw (Figure 1) which has a charge of 16 + .[5] The catalysis was attributed to the accumulation of hydroxide ions from aqueous solution around the highly positive surface of the cage, such that the bound benzisoxazole experiences a very high local concentration of base even when the bulk pH is relatively low This ion-pairing effect allows other anionic bases (phenolates) to participate in the cage-catalysed Kemp elimination as they are less well solvated than hydroxide ions and so preferentially accumulate around the cage surface.[6] Related examples come from Raymond and Bergman using a tetrahedral cage with tris-catecholate vertices that has a 12À charge: the high negative charge on the host facilitated protonation of bound guests, such that acid-catalysed reactions can occur in the cage cavity even under basic conditions.[7]. The catalysis, based on bringing together two components in water using orthogonal interactions, will be potentially general and could be used for a very wide range of substrate/anion combinations
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