Ionic Liquids as a Convenient New Medium for the Catalytic Asymmetric Dihydroxylation of Olefins Using a Recoverable and Reusable Osmium/Ligand
journal contributionposted on 25.06.2004, 00:00 by Luís C. Branco, Carlos A. M. Afonso
The use of room-temperature ionic liquids (RTILs) in the Sharpless catalytic asymmetric dihydroxylation (AD) as a cosolvent or replacement of the tert-butanol was studied in detail by screening 11 different RTILs. The AD reaction is faster in 1-n-butyl-3-methylimidazolium hexafluorophosphate [C4mim][PF6] as a cosolvent than in the conventional system of tert-butanol/H2O. For the range of six substrates tested, comparable or even higher yields and enantiomeric excess (ee) were found using [C4mim][PF6] or 1-n-octyl-3-methylimidazolium hexafluorophosphate [C8mim][PF6] compared to the conventional solvent system. Due to high affinity of the catalytic osmium/quiral ligand system to the ionic liquid, the use of ionic liquid/water (biphasic) or ionic liquid/water/tert-butanol (monophasic) solvent systems provides a recoverable, reusable, robust, efficient, and simple system for the AD reaction. Using 1-hexene and [C4mim][PF6] as RTIL it was possible to reuse the catalytic system for 9 cycles with only a 5% of yield reduction from the first cycle, allowing an overall yield of 87%, TON = 1566, and with similar ee. Additionally, for each cycle, after extraction of the reaction mixture with diethyl ether, the osmium content in the organic phase (containing the AD product) and in the aqueous phase was in the range of the detection limit (≤3%, ≤7 ppb) and 3−6% of initial amount, respectively. In contrast, the ionic liquid phase retained more than 90% of the osmium content of the previous cycle.