ionic liquid immobilized catalytic system for biomimetic dih

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Ionic liquid-immobilized catalytic system for biomimetic dihydroxylation of olefins Adam Closson, Mikael Johansson and Jan-E. Bäckvall* Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden. E-mail: jeb@organ.su.se; Fax:+46 8 154908; Tel:+46 8 6747178Received (in Cambridge, UK) 4th March 2004, Accepted 16th April 2004 First published as an Advance Article on the web 24th May 2004

A biomimetic catalytic system for dihydroxylation of olefins consisting of OsO4, N-methylmorpholine, and a flavin has been immobilized in an ionic liquid. This immobilized catalytic system is highly efficient for dihydroxylation with 30% aqueous H2O2 and it can be reused (at least 5 times) without loss of activity. The osmium-catalyzed dihydroxylation of olefins is one of the most useful and easily performed oxidation reactions in organic synthesis.1 In this process, osmium(VIII) is reduced to osmium(VI) by reaction with an olefin to yield a vicinal diol. A range of oxidants for recyling Os(VI) to Os(VIII) have been reported.1–4 We have recently developed biomimetic catalytic systems for the dihydroxylation of olefins with hydrogen peroxide.4–6 A stepwise low-energy electron transfer7 via electron transfer mediators (ETMs), in analogy with biological systems, leads to a mild and selective dihydroxylation by hydrogen peroxide. The latter is an inexpensive and environmentally friendly oxidant.8,9 In Scheme 1, N-methyl morpholine (NMM) and an additional ETM was used to carry electrons from Os(VI) to H2O2. Examples of H2O2-activating ETMs in Scheme 1 that we have used are 1,4 2,5 and 36 (Fig. 1). One of the mildest oxidation systems according to Scheme 1 was obtained with flavin 1 as the H2O2-activating ETM. Flavin 1 is an efficient ETM but fragile and attempting to isolate it after the reaction is difficult. Also, the other catalysts of the triple catalytic system (Scheme 1) are difficult to recover and reuse. One way to circumvent this problem would be to immobilize one or more components of the catalytic system. We now report on a successful and novel10 ionic-liquid immobilization of the whole biomimetic system of Scheme 1, where the three-component system can be recovered and reused.

Fig. 1 Electron Transfer Mediators (ETMs) successfully used in a triple catalytic system.

Various methods for immobilization of OsO4 have been reported in the literature.11–13 Recent developments have concerned polymer encapsulation of osmium tetroxide,12a–c chemically anchoring of osmium to a solid support,12d–f and dissolving osmium tetroxide in ionic liquids.13 Ionic liquids have recently attracted much attention.14,15 Changing one or both of the counterions can result in a change in solubility properties, which has given them the name“designer solvents”.16 For example,[bmim]PF6 is hydrophobic.17 We realized that use of ionic liquids presented an especially convenient method of immobilizing not only one, but mult

iple components. Accordingly, we began investigations using[bmim]PF6 and the components of our triple catalytic system with flavin 1 as ETM (Scheme 1). The components of the biomimetic triple catalytic system were immobilized in the ionic liquid ([bmim]PF6) and TEAA and DMAP were added. Acetone was added as co-solvent and reaction of styrene with aqueous 30% H2O2 in this medium for 3h at room temperature afforded the diol product in 85% isolated yield after workup (Scheme 2, Table 1, entry 1). A number of representative olefins were dihydroxylated using these reaction conditions. In this series of reactions, 1.5 equiv. of hydrogen peroxide (30% aqueous solution) was added over a 1.5 h period via syringe pump, followed by a further 1.5 h of stirring. After this time, the acetone was removed under reduced pressure and the reaction mixture was extracted with diethyl ether. The crude product obtained was chromatographed on silica gel to give the diol in good yield. The results are summarized in Table 1. The isolated yields of diol from these simple olefins ranged from 75–88%. After obtaining these promising results, we moved on to testing the reusability of the system. The remaining ionic liquid residues from the reaction of a-methyl styrene and allyl phenyl ether were therefore recharged with 1 equiv. of olefin. Co-solvents and hydrogen peroxide were added as above with the same reaction times. Reaction workup was carried out again by removing volatile solvents under reduced pressure followed by extractions with diethyl ether and silica gel chromatography of the crude extracts. The ionic liquid was recovered and again submitted to further reactions with fresh starting material, co-solvents, and hydrogen peroxide until five cycles were reached. The yields for each of the five cycles were good, although with some minor variations. We were pleased to find that the flavin as well as the osmium and NMM (NMO) were retained within the ionic liquid for multiple runs, allowing for continued H2O2-based dihydroxylation to occur without loss of activity. The results are given in Table 2. Control reactions run without flavin under otherwise identical conditions

DOI: 10.1039/b403333g

Scheme 1 Biomimetic system for dihydroxylation of olefins. Electronic supplementary information (ESI) available: experimental details. See /suppdata/cc/b4/b403333g/Chem. Commun., 2004, 1494–1495

Scheme 2 Biomimetic dihydroxylation in ionic liquid.

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This journal is© The Royal Society of Chemistry 2004

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Table 1 Biomimetic dihydroxylation in ionic liquida Entry 1 Olefin Product Yieldb 85%

Notes and references At the end of the reaction the NMM is in the form of polar NMO and the flavin is in a cationic form. Therefore they are not extracted. 1 Review: H. C. Kolb, M. S. Van Nieuwenhze and K. B. Sharpless, Chem. Rev., 1994, 94, 2483–2547. 2 (a) V. VanRheenen, …… 此处隐藏：9279字，全部文档内容请下载后查看。喜欢就下载吧 ……