DOI: 10.1002/chem.200600550 Dramatic Enhancement of Enone Epoxidation Rates in Nonionic Microemulsions Thomas Wielpütz, [b] Thomas Sottmann, [b] Reinhard Strey, [b] Friederike Schmidt, [a] and Albrecht Berkessel* [a] Introduction OrganicSynthesisliesattheheartoftheproductionoffine chemicals, pharmaceuticals and other valuable products from readily available and cheap starting materials. Howev- er,reagentincompatibility,thatis,dramaticallydifferentsol- ubility properties are frequently encountered in organic re- actions (i.e., inorganic salts and organic substrate). In many cases,water-solublereactantssuchasinorganicsaltsneedto be reacted with water-insoluble organic compounds. Many reactions, for instance, the alkaline hydrolysis of esters, oxi- dative cleavage of olefins or nucleophilic substitution on benzylhalides with inorganic salts are typical examples. It is currentpracticetosolvetheproblemofthepurephasecon- tactbychoosingassolventspolarproticorpolaraproticliq- uids such as DMSO (dimethyl sulfoxide) and DMF (di- methyl formamide), offering the possibility of solubilizing both the organic and the inorganic compound. These sol- vents are excellent media; however, they have three main drawbacks: they are expensive, toxic and have high boiling points,whichmakesthemdifficulttoremoveoncethereac- tioniscompleted. [1] Asanalternativeapproach,thereactantscanbedissolved in two immiscible solvents, and the contact area between the phases is increased by either intensive mechanical mixingorbetterbytheuseofaphase-transferagent(PTA). These PTAs, in most cases quaternary ammonium salts and crown ethers, act as phase-transfer catalysts (PTCs) and are widelyusedintwo-phasereactions.Suchcompoundsareca- pable of carrying the ionic reactant into the organic phase, where the low degree of solvation increases the reactivity remarkably. PTCs have been intensively used for oxidation reactions in general. [1–3] The catalytic epoxidation of enones, [4] in particular the epoxidation of trans-chalcone using alkaline hydrogen peroxide (Weitz–Scheffer condi- tions) as oxidant, [5] is of special interest in this field. Jew etal. found an enhancement of the enantioselectivity and the reaction rate in epoxidations of trans-chalcone using enantiomeric pure PTAs and catalytic amounts of technical Abstract: The ability of microemul- sions to dissolve polar and non-polar components with a huge internal inter- face can overcome the reagent incom- patibilities frequently encountered in organic reactions. We investigated model epoxidation reactions of a,b-un- saturated enones and alkaline hydro- gen peroxide in different nonionic mi- croemulsions, both in the presence and absence of a phase-transfer agent (PTA). The obtained reaction profiles were compared with those for the cor- responding surfactant-free two-phase systems.Inaddition,wedefinedatime constant t asameasurefortherateof turnover. The epoxidation of trans- chalcone using an n-alkyl-polyoxyethy- lene surfactant based microemulsion wasfastestinthesystemwiththePTA (t = 66min)andslightlyslowerwithout the PTA (t = 77min). It was still slower in the two-phase system with a PTA (t = 114min) and extremely slug- gish without a phase-transfer agent. With n-alkyl b-d-glucopyranoside as thesurfactanttheconversionwastwice as fast than in the former microemul- sion systems, but the PTA did not ac- celerate the reaction further (t = 35 and 33min). The epoxidation of vita- min K 3 , the second model system, was extremely accelerated. It proceeded a factorofapproximately35fasterinthe microemulsion (t = 1.44min) than in the corresponding two-phase system (t = 57min). Keywords: enones · epoxidation · kinetics · microemulsions · phase-transfercatalysis [a] Dr.F.Schmidt,Prof.Dr.A.Berkessel DepartmentofOrganicChemistry,UniversityofCologne Greinstrasse4,50939Kçln(Germany) Fax:(+ 49)221-470-5102 E-mail:berkessel@uni-koeln.de [b] Dipl.-Chem.T.Wielpütz,Dr.T.Sottmann,Prof.Dr.R.Strey DepartmentofPhysicalChemistry,UniversityofCologne LuxemburgerStrasse116,50939Kçln(Germany) Chem.Eur.J. 2006, 12,7565–7575 #2006Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim 7565 FULL PAPER