A series of "Balanced Catalytic Surfactants" (BCS) [(Cn)2N(C1)2]2MoO4 (n = 8, 9, 10, 12) based on amphiphilic double-tailed quaternary ammonium with molybdate as a counterion has been developed for the dark singlet [4 + 2] cyclooxygenation of organic substrates in three-liquid-phase microemulsion systems. These cationic surfactants form three-liquid-phase microemulsion systems at room temperature in the presence of an appropriate organic solvent and water without addition of any cosurfactant or electrolyte. Comparative peroxidation of rubrene points out the specific advantages of these three-phase media over phase transfer catalysis in two phase systems and on conventional one-phase microemulsions based on sodium molybdate: (i) only three constituents, (ii) low amounts of surfactants, (iii) insensitivity to water dilution, (iv) fast separation of the three phases, (v) straightforward recovery of the product and the surfactant from the oil and microemulsion phases, respectively. The preparative peroxidation of alpha-terpinene and 1,4,5-trimethylnaphtalene was performed in the ternary systems [(C8)2N(C1)2]2MoO4/water/tert-butyl acetate or benzene. The reusability of the catalyst, the catalytic nature of the BCS, and the ability of the systems to oxidize poorly reactive substrates were demonstrated showing the broadness of the applicability of such systems.
Formation of stable second-sphere adducts between a water-soluble organometallic complex and a cyclodextrin (CD) is possible by finely designing the structure of the water-soluble phosphane. The key point to obtain such adducts was the synthesis of a water-soluble phosphane which possesses a tert-butylphenyl group recognized by the CD and separated from the phosphorus atom by a phenyl ring to avoid phosphane decoordination during the molecular recognition process between the organometallic complex and the CD. These adducts are able to catalyze the cleavage of water-insoluble carbonate in a biphasic system. Keywords: Biphasic catalysis; cyclodextrins; molecular recognition; palladium; phosphane ligands Cyclodextrins (CDs) are cyclic oligosaccharides composed of six, seven, or eight d-glucopyranose units that are well-known to form inclusion complexes with numerous compounds including organometallic complexes.[1] When a CD includes a part of an organometallic complex into its internal hydrophobic cavity, the CD behaves as a second-sphere ligand, binding non-covalently the first-sphere ligands of the metal center.[2]The effect of such second-sphere ligands on the chemical, electrochemical, and photochemical properties of organometallic compounds, as well as on their geometries in solution and in the solid state, has been intensively investigated and it has been demonstrated that second-sphere coordination can induce unusual behaviors. [3] Although second-sphere coordination adducts of a trimethylphosphane/platinum complex with CDs have been fully characterized, [4] the involvement of CDs as hosts in the second-sphere of transition metals has not been exploited in aqueous organometallic catalysis. This stems from the poor ability of CD to solubilize classical highly hydrophobic organometallic complexes in water and from the strong destabilizing effect of CDs on water-soluble organometallic compounds. For instance, addition of CD to stable water-soluble hydriderhodium complex [RhH(CO)L 3 ] (L¼ water-soluble phosphane ligand) resulted in ligand dissociation and formation of phosphane low-coordinated rhodium species.[5] This phenomenon was attributed to the complexation of the phosphane ligand by the CD. In particular, it was assumed that the phosphorus donor atom located near the CD cavity is not available for coordination to a metal center due to steric crowding.In this paper, we describe the first example of catalytically-active adducts formed between a water-soluble organometallic catalyst and a CD. The key point of our strategy to obtain such adducts was the synthesis of a water-soluble phosphane which possesses a fragment recognized by the CD and distant from the metal center to avoid phosphane decoordination during the molecular recognition process between the organometallic complex and the CD. A triphenylphosphine bearing sulfonate groups and a tert-butylphenyl group on phenyl rings was sought to fulfill the above requirement. Indeed, it is well-known that sulfonate groups are highly hydrophilic groups...
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