The Mechanism of On‐Water Catalysis

Abstract

Sharpless and co-workers [1] used the term on-water to describe the substantial rate acceleration that is observed when some insoluble organic reactants are stirred in aqueous suspension. We now propose a mechanism that accounts for the phenomenon of on-water catalysis. Three of the observations of Sharpless and co-workers are directly pertinent to the mechanism we propose below. Firstly, the reaction mixture must be heterogeneous, that is, there must be an interface between the organic reactants and water. The presence of some methanol in the aqueous phase made little difference to the rate of the reaction, "but the rate slowed considerably when enough methanol was used to make the reaction homogeneous." Secondly, the interface must be with an aqueous phase. An emulsion formed with the organic reactants in perfluorohexane gave an only slightly enhanced rate. Thirdly, there was a significant solvent isotope effect, with a noticeably slower rate in D 2 O.Our proposed mechanism is supported by two additional considerations. One is that all of the reactions that have been described as accelerated by the on-water effect (with the possible exception of those of metal complexes) are also known to be subject to acid catalysis. [1,2] This suggests that acid-base chemistry at the interface is responsible. Hence the second consideration is our recently developed model that explains the intrinsic charge that develops at the interface of water with low relative permittivity (low dielectric constant) materials.[3] This model accounts for the numerous observations made over many decades that the surface of water at these interfaces, whether they are with gas, liquid or solid, becomes negatively charged by the strong adsorption of hydroxide ions. [4,5] The adsorption equilibrium constant can be estimated from the pH dependence of the zeta potentials of oil drops in water to be at least 10 8 .[6]At first this seems unlikely to account for acid catalysis at the same interface, but a little reflection reveals that this counterintuitive result is to be expected. Consider a substrate that is activated by protonation. Reaction with water at the interface results in the protonated activated substrate and a hydroxide ion that is stabilised by its strong adsorption at the interface. This drives the protonation equilibrium of the substrate [Eq. (1)] strongly to the right and accounts for acid catalysis even in neutral solution.This mechanism accounts for all of the available evidence described above. It requires water, at the interface with the organic reactants, which provides the conditions for the protonation of the substrate, driven by the adsorption of the hydroxide ion product, with the associated deuterium isotope effect, leading to acid catalysis and the enhanced rate. We now describe additional experimental evidence consistent with this mechanism. We chose to examine another Diels-Alder reaction, that between cyclopentadiene and dimethylfumarate (Scheme 1). In addition to having a rate that is convenient to measure, this reactio...

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Metal-free aerobic oxidative coupling of amines to imines

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Photochemical oxidation of water and reduction of polyoxometalate anions at interfaces of water with ionic liquids or diethylether

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