Epoxidation phase-transfer catalysis

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

Alternatively, RC CSiMe3 cleavage can be achieved easily, avoiding the use of TBAF, by employing phase-transfer catalysis the reaction is complete in 5-10 min, and the conditions are compatible with other nucleophically labiele functional groups such as epoxides. 

Heteropoly acids can be synergistically combined with phase-transfer catalysis in the so-called Ishii-Venturello chemistry for oxidation reactions such as oxidation of alcohols, allyl alcohols, alkenes, alkynes, P-unsaturated acids, vic-diols, phenol, and amines with hydrogen peroxide (Mizuno et al., 1994). Recent examples include the epoxidations of alkyl undecylenates (Yadav and Satoskar, 1997) and. styrene (Yadav and Pujari, 2000). 

Epoxidation is another important area which has been actively investigated on asymmetric phase transfer catalysis. Especially, the epoxidation of various (i.)-a,p-unsaturated ketones 68 has been investigated in detail utilizing the ammonium salts derived from cinchonine and cinchonidine, and highly enantioselective and diastereoselective epoxidation has now been attained. When 30 % aqueons H202 was utilized in the epoxidation of various a, 3-unsaturated ketones 68, use of the 4-iodobenzyl cin-choninium bromide 7 (R=I, X=Br) together with LiOH in Bu20 afforded the a,p-epoxy ketones 88 up to 92% ee,1641 as shown in Table 5. The O-substituted... 

Finally, one example of trityl salt analogues in the phase-transfer catalysis is presented. The highly stable triazatriangulenium cations 62 [161, 162] were jnst recently introduced to the phase-transfer chemistry [163], Persistent to strongly basic and nncleophilic conditions, these salts revealed efficient catalytic activity in addition reactions (Scheme 64). Modification of the alkyl side chains on nitrogen allowed matching the fair hydro/lipophilicity with the optimised conditions in the alkylation, epoxidation, aziridination and cyclopropanation reactions. The results are comparable to those of tetrabutylammonium salts and in some cases showed even a better outcome. 

The epoxidation of enones using chiral phase transfer catalysis (PTC) is an emerging technology that does not use transition metal catalysts. Lygo and To described the use of anthracenylmethyl derivatives of a cinchona alkaloid that are capable of catalyzing the epoxidation of enones with remarkable levels of asymmetric control and a one pot method for oxidation of the aUyl alcohol directly into... 

Lygo, B. and To, D.C.M., Asymmetric Epoxidation via Phase-transfer Catalysis Direct Conversion of Allylic Alcohols into a, -Epoxyketones. Chem. Commun. 2002, 2360-2361. 

Reactions with phase-transfer catalysis again proved to be efficient and synthetically valuable. Even the least reactive simple terminal alkenes are converted under mild conditions, in short reaction time to epoxides in high yields 283... 

In the catalytic epoxidation of alkenes by a manganese porphyrin with phase-transfer catalysis and hypochlorite, the yield of epoxide also decreases with decreasing alkene concentration363 dibenzo-18-crown-6 has been shown to have an effect on the reaction364. 

The catalytic asymmetric epoxidation of electron-deficient olefins, particularly a,P-unsaturated ketones, has been the subject of numerous investigations, and as a result a number of useful methodologies have been elaborated [44], Among these, the method utilizing chiral phase-transfer catalysis occupies a unique position in terms of its practical advantages. Moreover, it also allows the highly enantioselective epoxidation of trans-a,P-unsaturated ketones, particularly chalcone. 

Asymmetric epoxidation catalyzed by chiral phase-transfer catalysts is another reaction which has been extensively studied following an initial report by Wynberg [2,44]. Shioiri et al. further improved the enantioselective epoxidation of naphthoquinones under cinchona alkaloid-derived chiral phase-transfer catalysis [45],... 

A major improvement addressing the issue of practicability and safety by avoidance of the direct use of (potentially) explosive diazo compounds was recently reported by Aggarwal and co-workers [82, 83], The direct addition of diazo compounds was replaced by use of suitable precursors which form the desired diazo compound in situ. The Aggarwal group developed this concept for the corresponding sulfur ylide type epoxidation (see Section 6.8) [82], and successfully extended it to aziridination [83]. Starting from the tosylhydrazone salt 66 the diazo compound is formed in situ under conditions (phase-transfer-catalysis at 40 °C) which were found to be compatible with the sulfur ylide type aziridination [82, 83], The concept of this improved method, for which sulfide 67 (Scheme 5.41) is the most efficient catalyst, is shown in Scheme 5.40. 

Phase-transfer catalysis has been widely been used for asymmetric epoxidation of enones [100]. This catalytic reaction was pioneered by Wynberg et al., who used mainly the chiral and pseudo-enantiomeric quaternary ammonium salts 66 and 67, derived from the cinchona alkaloids quinine and quinidine, respectively [101-105],... 

In the metal-free epoxidation of enones and enoates, practically useful yields and enantioselectivity have been achieved by using catalysts based on chiral electrophilic ketones, peptides, and chiral phase-transfer agents. (E)-configured acyclic enones are comparatively easy substrates that can be converted to enantiomeri-cally highly enriched epoxides by all three methods. Currently, chiral ketones/ dioxiranes constitute the only catalyst system that enables asymmetric and metal-free epoxidation of (E)-enoates. There seems to be no metal-free method for efficient asymmetric epoxidation of achiral (Z)-enones. Exocyclic (E)-enones have been epoxidized with excellent ee using either phase-transfer catalysis or polyamino acids. In contrast, generation of enantiopure epoxides from normal endocyclic... 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

Asymmetric Phase Transfer Catalysis Asymmetric Epoxidation Reactions... 

Another important asymmetric epoxidation of a conjugated systems is the reaction of alkenes with polyleucine, DBU and urea H2O2, giving an epoxy-carbonyl compound with good enantioselectivity. The hydroperoxide anion epoxidation of conjugated carbonyl compounds with a polyamino acid, such as poly-L-alanine or poly-L-leucine is known as the Julia—Colonna epoxidation Epoxidation of conjugated ketones to give nonracemic epoxy-ketones was done with aq. NaOCl and a Cinchona alkaloid derivative as catalyst. A triphasic phase-transfer catalysis protocol has also been developed. p-Peptides have been used as catalysts in this reaction. ... 

The reaction involving dichlorocarbene also provides a wide range of epoxides, " including terminal. a.a- and a,p-dialkyl-substituted and tiialkyl-substituted compounds, from a large variety of 3-hy-drcxyalkyl methyl and phenyl selenides (Scheme 162, d). The thallous ethoxide reaction, although it takes place more slowly (especially with phenylseleno derivatives) than under phase transfer catalysis condirions, " has to be in several instances preferred since it avoids the concomitant formation of... 

The Darzens reaction (tandem aldol-intramolecular cyclization sequence reaction) is a powerful complementary approach to epoxidation (see Chapter 5) that can be used for the synthesis of a,P-epoxy carbonyl and a,p-epoxysulfonyl compounds (Scheme 8.32). Currently, all catalytic asymmetric variants of the Darzens reactions are based on chiral phase-transfer catalysis using quaternary ammonium salts as catalysts. 

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

Y. Mahha, L. Salles, J.-Y. Piquemal, E. Briot, A. Atlamsani, J.-M. Bregeault, Environmentally friendly epoxidation of olefins under phase-transfer catalysis conditions with hydrogen peroxide, J. Catal. 249 (2007) 338. 

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