Catalytic Mechanism of Interfacial Water in the Cycloaddition of Quadricyclane and Diethyl Azodicarboxylate

Abstract

Metrics & MoreArticle Recommendations * sı Supporting Information ABSTRACT: "On-water" catalysis, the unusual activity of water molecules at the organic solvent−water interface, has been demonstrated in many organic reactions. However, the catalytic mechanism has remained unclear, largely because of the irreproducibility of the organic−water interface under the common stirring condition. Here, the interfacial area was controlled by employing adsorbed water on mesoporous silica nanoparticles as the catalyst.Reliable kinetics of the cycloaddition reaction of quadricyclane and diethyl azodicarboxylate (DEAD) at the toluene−water interface within the nanoparticle pores were measured. Data reveal an Eley−Rideal mechanism, wherein DEAD adsorbs at the toluene−water interface via hydrogen bonds formed with interfacial water, which lower the activation energy of the cycloaddition reaction. The mechanistic insights gained and preparation of surface water in silica pores described herein may facilitate the future design of improved "on-water" catalysts.Letter pubs.acs.org/JPCL

1H NMR Shift of Single Molecular Water and Its Ligand Interactions in Organic Solvents

“…Due to the vast aromatic moiety in lignin, such πHB is likely also present in the catalytic functionalization of lignin in aqueous solution and within lignin-based composites impacting on their properties 8 . The water ligand interactions in aromatic organic solvents are the most relevant to the "on water" catalysis mechanism investigation because the organic transformations were generally believed to occur in organic solvents close to the interface 11,12,19 . It is estimated that future systematic investigation of water ligand interactions with aromatic solvents with diversified functional groups via 1 H NMR spectroscopy would be able to contribute to quantifying the πHB effect.…”

“…Quantitative measurement of the structure change in bulky water would be helpful and desirable to a better understanding the mechanism and a future rational control of interfacial water. In addition, in the most recent study 19 , a significant amount (if not all) of water would be "free water" absorbed to mesoporous silica nanoparticles and water impurity might likely be present in the oil phase 20 . To our best knowledge, mechanistic investigation including the water effect in the organic phase is not yet reported, which would likely generate new insights.…”

“…Later study 18 found that the solubility of reactant (azodicarboxylate ester) in water does matter for "on water" catalysis and the good aqueous solubility proceeded fast on water. Meanwhile, a very recent study by employing adsorbed water on controlled interfacial area (mesoporous silica nanoparticles) as the catalyst was reported 19 and the data revealed an Eley-Rideal mechanism (heterogeneous catalysis) and the azodicarboxylate reactant was H-bonded (absorbed) to the toluenewater interface. Combining the both studies, it is estimated that the hydrate ability of water-soluble reactant was positively correlated to its anchored concentration at the water-oil interface driven by the global effect of the hydrogen bonding network of the bulky water.…”

The Origin of Anion−π Autocatalysis

“…For enzymatic catalysis, such participation of water has been widely supported from different points of view, with the impact being more often on protein structure than direct participation in the transformation, unless, of course, water is one of the substrates. − In organic synthesis, reactions in or “on” water have received much attention, with often spectacular rate enhancements being observed, first with Diels–Alder reactions, then many more. − In the context of this study, epoxide-opening polyether cascade cyclizations in water are most relevant. With an essential intramolecular tetrahydropyran template and water as a solvent, two water molecules were proposed to interact with the template to enforce the conformational changes needed to activate the nucleophile by proton transfer. , In heterogeneous catalysis, water bound to the surface of the solid catalyst often contributes to activity. , …”

“…18,19 In heterogeneous catalysis, water bound to the surface of the solid catalyst often contributes to activity. 50,55 Examples of stoichiometric amounts of water integrated as co-catalysts in apolar solvents are less ubiquitous and usually well hidden in the literature. A topical example concerns the epoxide-opening cyclization of 4,5-epoxyhexanols with methoxymethyl substituents on carbon 4 to coordinate the lanthanoid Lewis acid catalyst La(OTf) 3 .…”