Oxidation in Three-Liquid-Phase Microemulsion Systems Using “Balanced Catalytic Surfactants” Ve ´ ronique Nardello-Rataj, Laurent Caron, Ce ´ dric Borde, and Jean-Marie Aubry* LCOM, Equipe Oxydation et Physico-Chimie de la Formulation, UMR CNRS 8009, Cite ´ Scientifique, ENSCL, BP 90108, F-59652 VilleneuVe d’Ascq, France Received July 7, 2008; E-mail: jean-marie.aubry@univ-lille1.fr Microemulsions (μem) are thermodynamically stable submicron dispersions of two immiscible liquids stabilized by a surfactant monolayer. 1 They are used as reaction media in organic synthesis to overcome the incompatibility between hydrophilic reagents and hydrophobic substrates. 2 The considerable increase (∼10 5 times) of the oil-water interfacial area and the compartmentalization of reactants often provide a significant rate enhancement and better selectivity compared to the routine Phase-Transfer Catalysis (PTC) or to cosolubilization in polar solvents. 3,4 Single-phase water-in- oil microemulsions have been applied to the peroxidation of fragile organic compounds by singlet oxygen, 1 O 2 ( 1 Δ g ), chemically gen- erated by the H 2 O 2 /Na 2 MoO 4 catalytic system. 4 These microemul- sions suffer two main drawbacks: (i) lengthy recovery of products because of the high concentration of amphiphiles (≈ 20%); (ii) demixing after addition of a certain amount of H 2 O 2 . Two-phase microemulsion systems with an excess oil phase require lower amounts of surfactant (∼5%) and allow easier workups, but they are still sensitive to dilution by water arising from H 2 O 2 dispro- portionation. 5 This work describes the concept of “Balanced Catalytic Sur- factants” (BCSs) giving three-liquid-phase systems that overcome all these drawbacks. The catalyst (MoO 4 2- ) is electrostatically bound to a cationic surfactant moiety leading to a “catalytic surfactant” (CS). Cetyltrimethylammonium bicarbonate may be considered as a CS since it catalyzes the micellar oxidation of aryl sulfides by H 2 O 2 . 6 Furthermore, a CS is named “balanced” if it spontaneously forms a three-liquid-phase system when mixed with an appropriate organic solvent and water. Several types of nonionic surfactants possess this uncommon feature, 7 whereas most ionic surfactants require a cosurfactant and an electrolyte to give a three- phase system. Only a few double-tailed ionic surfactants behave as balanced surfactants. 8,9 A series of CSs suitable for dark singlet oxygenation has been prepared by coupling MoO 4 2- with single-chain [C n N(C 1 ) 3 ] + and double-tailed [(C n ) 2 N(C 1 ) 2 ] + ammonium groups. The latter provide BCSs which play the role of surfactant, cosurfactant, and catalyst simultaneously, leading to a three-liquid-phase system with only three constituents (Figure 1). In this medium, MoO 4 2- is specifically localized at the water-oil interface of the middle-phase micro- emulsion. The hydrophobic substrate partitions between the mi- croemulsion and the excess oil phase, whereas H 2 O 2 partitions between the microemulsion and the excess water phase. The generation of 1 O 2 exclusively takes place in the aqueous micro- domains of the middle-phase microemulsion. As the typical size of these microdomains (ca. 10 nm) is much smaller than the mean travel distance of 1 O 2 in water (ca. 200 nm), this small uncharged and rather hydrophobic excited molecule diffuses freely through the charged interfacial film to the organic compartments where it reacts with the substrate S. In this process, the upper oil and the lower aqueous excess phases can be regarded as reservoirs for the reagents and the products. Under stirring, oxidation products are continuously extracted from the microemulsion to the organic phase and the water arising from disproportionated H 2 O 2 is expelled in the aqueous excess phase without destabilizing the microemulsion (Figure 1). At completion of the reaction, stirring is stopped and the three phases separate within a few seconds. A balanced three-phase microemulsion system is formed when the interfacial film between oil and water is flexible and has a zero mean curvature. 10 To fulfill this condition, the interfacial packing parameter p )V/al, where V, a, l are the alkyl chain volume swollen with oil, the headgroup area, and the alkyl chain length respectively, must be equal to 1. p is related to the structure of the surfactant in solution and to the ability of the oil to penetrate the interfacial film. 11 Single-chain ammonium molybdates [C n N(C 1 ) 3 ] 2 MoO 4 (n ) 12, 14, 16) do not behave as BCSs (p , 1) since they do not form three-phase systems without electrolytes, the anion of which would exchange with molybdate ions. To increase p and to obtain a flexible interfacial film without cosurfactant, double-tailed CSs [(C n ) 2 - N(C 1 ) 2 ] 2 MoO 4 (n ) 8, 9, 10, 12) were prepared and turned out to be balanced in the presence of water and an appropriate oil. The range of experimentally effective chain lengths, i.e. the values of n providing three-phase systems at room temperature, lies between 8 and 12 carbons. The ternary systems based on these CSs have been investigated with various oils of increasing hydrophobicity (Table 1). The three-phase microemulsion system based on one of the BCSs (n ) 8) was applied to the peroxidation of three substrates reacting according to a [4 + 2] cycloaddition, the most specific reaction of 1 O 2 . Rubrene, 1, which is very hydrophobic, bulky, and poorly Figure 1. Dark singlet oxygenation of a substrate S in a three-liquid-phase microemulsion (μem) system based on [(C 8 ) 2 N(C 1 ) 2 ] 2 MoO 4 . Table 1. Microemulsion Systems as a Function of Catalytic Surfactants, [(C n ) 2 N(C 1 ) 2 ] 2 MoO 4 , and Oil Hydrophobicity at 25 °C (Composition is Water/Oil ) 1/1 (v/v) and 5 wt % CS) a a WI, WII, WIII refer to the Winsor systems, i.e. to microemulsions in equilibrium with oil, aqueous, or both excess phases respectively. 5,12 Published on Web 10/15/2008 10.1021/ja805220p CCC: $40.75  2008 American Chemical Society 14914 9 J. AM. CHEM. SOC. 2008, 130, 14914–14915