A Na₂WO₄/H₂WO₄‐Based Highly Efficient Biphasic Catalyst towards Alkene Epoxidation, using Dihydrogen Peroxide as Oxidant

Abstract

Abstract:The tungsten-containing biphasic catalytic system [Na 2 WO 4 /H 2 WO 4 /PTR/chloroacetic acid] effectively epoxidizes alkenes with 50% H 2 O 2 as terminal oxidant, under organic solvent-free conditions. The catalytic process is proposed to proceed via a dinuclear tungsten peroxo species with coordinated chloroacetic acid, as suggested by ESI-MS measurements. The catalytic system is suggested to involve tungsten-peroxo and/or peracetic acid type of epoxidation catalyzed by the tungsten(VI) in the presence of an organic acid and H 2 O 2 . The reaction conditions employed for various alkenes for epoxidation are mild compared to the earlier studies and result in high product selectivity and conversion rate.Keywords: alkenes; chloroacetic acid; epoxidation; hydrogen peroxide; tungsten The development of efficient and clean processes for the epoxidation of alkenes on a million-ton-per-year scale remains an important objective in the fine chemical industry. [1 -5] There is an increasing need for the elaboration of selective catalytic systems involving cheap, readily available, and ecologically friendly oxidants, such as hydrogen peroxide. [6,7] In this context, polyoxotungstates have been extensively and successfully used during the past 20 years. [8 -11] A key experiment was performed by Venturello who selectively epoxidized 1-octene with a turnover number (TON, defined as mol product per mol catalyst) of 20 (82% yield based on H 2 O 2 ) using a Na 2 WO 4 -H 3 PO 4 -quaternary ammonium chloride catalyst.[12] Since then, this system has been intensely studied. [13,14] Nevertheless, this catalytic biphasic procedure is less desirable, as it requires the use of chlorinated solvents such as 1,2-dichloroethane. [15 -17] A first breakthrough occurred in 1996 when Noyori [18] reported a significant improvement of the original Venturello system. The epoxidation of terminal alkenes could be carried out within 4 hours, without organic solvents, and with up to 49.5 TON (99% yield, based on the olefin), using a catalytic mixture of Na 2 WO 4 dihydrate, (aminomethyl)phosphonic acid and the phase transfer reagent (PTR) methyltri-n-octylammonium hydrogen sulfate. [19] Recently, outstanding procedures involving tungsten-containing catalysts for the production of propylene oxide [20] and the epoxidation of allylic alcohols [21] were reported. The new catalytic system reported here consists of easily available, low-cost chemicals [Na 2 WO 4 /H 2 WO 4 / Aliquat 336 (PTR)/chloroacetic acid], which can be easily handled (Scheme 1). Typically, 200 mmoles of ciscyclooctene were reacted with 300 mmoles of hydrogen peroxide (50%) at 60 8C. The epoxidation is catalyzed by 0.025 mol % of Na 2 WO 4 and 0.025 mol % H 2 WO 4 with 0.2 mol % of chloroacetic acid in the presence of methyltri-n-octylammonium chloride (0.2 mol %) as PTR (Scheme 1). A maximum TON of as high as 1800 is reached after 4 hours with 99% selectivity. This rate of the epoxidation is compared under similar experimental conditions as above, to the one of Noyori et a...

Ionic Liquids as Micellar Agents in Perrhenate‐catalysed Olefin Epoxidation

“…As a proof of concept, this approach was applied to the tungstate anion which exhibits considerable catalytic activity for epoxidation in different phase transfer catalytic reactions, but, to the best of our knowledge, has not yet been made use of in micellar catalysis. This is an expansion of the perrhenate‐catalysed epoxidation with regard to two aspects: The tungstate ion has a different valence and the activation of H 2 O 2 in this case is known to take place at the W atom via an inner‐sphere mechanism .…”

“…Here, a phase‐transfer agent mediates between the two phases and acts as both reactant shuttle and catalyst across the aqueous‐organic interface. The obvious advantage of this approach is that beyond water, no additional solvent is required, at least in some cases ,…”

Green Oxidation Reactions by Polyoxometalate-Based Catalysts: From Molecular to Solid Catalysts

“…5). This value was the highest among those (0.5-77 h -1 ) reported for the transition-metal catalyzed epoxidation of homoallylic alcohols with H 2 O 2 [54][55][56][57][58][59][60]. Compound IV could easily be recovered in an almost quantitative yield by addition of an excess amount of diethyl ether to the reaction solution and the recovered IV could be reused without loss of its high catalytic activity and selectivity.…”

Epoxidation of Olefins Catalyzed by Polyoxomolybdates Formed in-situ in Ionic Liquids

“…This can be attributed to the fact that the amount of ionic liquid in the solution determines the pH value of the reaction media and therefore the formed polyoxomolybdate species [9,10]. In 2005 Reedijk et al showed that the catalytic activity of a tungstate-containing catalytic system is strongly pH dependent [15]. In recent years several polyoxotungstate-containing ionic liquids have been synthesized and tested in catalysis [21, 26 -28].…”