Reaction-controlled phase-transfer catalysis

X. Zuwei, Z. Ning, S. Yu, L. Kunlan, Reaction-controlled phase-transfer catalysis for propylene epoxidation to propylene oxide. Science 292, 1139-1141 (2001). 

Key Words Reaction-controlled phase-transfer catalysis, 2-ethylanthrahydroquinone, Hydrogen peroxide. Propylene, Propylene oxide (PO), Cyclohexene, Cyclohexene oxide, Allyl chloride, Epichlorohydrin, Epoxidation. 2008 Elsevier B.V. 

Reaction-Controlled Phase-Transfer Catalysis for Epoxidation of Olefins... 

This chapter will present recent progress made in reaction-controlled phase-transfer catalysis for the epoxidation of olefins, focusing on work with hetero-pol)q5hosphotungstates and quaternary ammonium ions from our group. We have systemically investigated the influence of composition of the heteropoly anion and various quaternary ammonium ions on the catalyst activity. The epoxidation of propylene, allyl chloride, and others olefins and the stability of the catalyst in recycle will be summarized and discussed in detail. 

Cyclohexene oxide is an important intermediate used, for example, in synthesis of pesticides. A process for catalytic epoxidation of cyclohexene to cyclohexene oxide by reaction-controlled phase-transfer catalysis has been commercialized in China since 2003. It is an envirorunent-friendly process compared to the polluting traditional chlorohydrin method. 

K. L. Li, N. Zhou, Z. W. Xi, Effects od solvents and quaternary ammonium ions in heteropoly-oxotungstates on reaction-controlled phase-transfer catalysis for cyclohexene epoxidatio. Chin. J. Catal 23 (2003) 125. 

Ishii, Y., Yamawaki, K., Ura, T., et al. (1988). Hydrogen Peroxide Oxidation Catalyzed by Heteropoly Acids Combined with Cetylpyridinium Chloride. Epoxidation of Olefins And Allylic Alcohols, Ketonization of Alcohols and Diols, and Oxidative Cleavage of 1,2-Diols and Olefins, J. Org. Chem., 53, pp. 3587-3593 Sato, K., Aoki, M., Ogawa, M., et al. (1997). A Halide-Free Method for Olefin Epoxidation with 30% Hydrogen Peroxide, Bull. Chem. Soc. Jpn., 70, pp. 905-915 Xi, Z. W., Zhou, N., Sun, Y., et al. (2001). Reaction-Controlled Phase-Transfer Catalysis for Propylene Epoxidation to Propylene Oxide, Science, 292, pp. 1139-1141 Neumann, R. 

Because the cellulose ether alkoxide is present entirely in the aqueous phase, the rate-limiting step may be the partitioning (phase transport) of the hydrophobic electrophile across the interface from the organic to aqueous phase. If the reaction rate is controlled by diffusion of the electrophile across the interface, then one would expect a correlation between water solubility of the hydrophobe and its alkylation efficiency. The fact that the actual alkylation reaction is probably occurring in the aqueous phase (or at the interface) yet the electrophile itself is principally soluble in the organic phase has important mechanistic ramifications. This type of synthetic problem, in which one reactant is water soluble and the other organic soluble, should be amenable to the techniques of phase transfer catalysis (PTC) to yield significant improvements in the alkylation efficiency. 

The reactivity of complexed haloarenes toward thiolates has been studied, and it has been reported that o-, m-, and p-dichlorobenzenetricarbonylchromium complexes 18a-c react with thiolates (RS R = Me, nBu, tBu Scheme 17, path i) under phase-transfer conditions or in DMSO to give 39 and 40a-c. The ortho- and para-complexes 18a and 18c undergo stepwise substitution of the two Cl atoms in a reaction sequence that can be easily controlled by the amount of added thiolate. The meta complex 18b shows a lower selectivity and gives a mixture of mono- and disubstituted products even in the presence of substoichiometric amounts of thiolate (Scheme 17) [22]. Similarly, LiCH(C02Et)CN and BuSH react with the o-dichlorobenzene complex 18a to give complex 39d and then disubstituted arene 40d, showing that this substitution can be performed with two different nucleophiles (Scheme 17) [23]. Phase-transfer catalysis has also been applied to fluoroarene-Cr(CO)3 complexes, which are more reactive toward thiolates than are the corresponding chloro derivatives [22]. 

The reactivity in phase-transfer catalysis is controlled by (1) the reaction rate in the organic phase, (2) the mass transfer steps between the organic and aqueous phases, and (3) the distribution equilibrium of the quaternary salts between the two phases. The distribution of quaternary salts between two phases directly affects the entire system reactivity [60-62]. On the basis of the experimental data and earlier literature [27,28,63], a generalized approach describing a LLPTC reaction system uses a pseudo-first-order reaction. The rate expression is written as... 

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