Phase transfer epoxidation

A limited study has been made of the role of the structure of the catalyst in the phase-transfer epoxidation reaction (77). The catalysts tried were mainly salts of quinine (3a-g), cinchonidine (4), ephedrine (5), and a camphor derivative (6) (Figure 14). The conclusions were as follows ... 

The asymmetric phase-transfer epoxidation of ( )-a, 3-unsaturated sulfones has recently been achieved by Dorow and coworker using N-anthracenylmethyl cinchona alkaloid derivatives as catalysts and KOC1 as an oxidant at low temperature [23]. The screening of several etheral functional groups at the C9( O) position of the catalyst moiety indicated that the steric size and the electronic factor of the ether substituent has a significant effect on both the reaction conversion and the enantioselectivity. 

A typical example of potent and selective inhibitors of cysteine proteases, E-64c 144, was prepared by use of a phase-transfer epoxidation with an aqueous solution of NaOCl (Scheme 4.33). Diastereoselective epoxidation of the enone 145 under the influence of PTC 23 was achieved in 70% yield with a diastereomeric ratio of 5 1. Functionalizations of both terminus of the product furnished the target compound 144. 

The alkene is allowed to react at low temperatures with a mixture of aqueous hydrogen peroxide, base, and a co-solvent to give a low conversion of the alkene (29). These conditions permit reaction of the water-insoluble alkene and minimise the subsequent ionic reactions of the epoxide product. Phase-transfer techniques have been employed (30). A variation of this scheme using a peroxycarbimic acid has been reported (31). 

Reaction of perfluoroaLkenes and hypochlorites has been shown to be a general synthesis of perfluoroepoxides (32) (eq. 7). This appears to be the method of choice for the preparation of epoxides from internal fluoroalkenes (38). Excellent yields of HFPO from hexafluoropropylene and sodium hypochlorite using phase-transfer conditions are claimed (34). 

In addition, NaOMe, and NaNH2, have also been employed. Applieation of phase-transfer conditions with tetra-n-butylammonium iodide showed marked improvement for the epoxide formation. Furthermore, many complex substituted sulfur ylides have been synthesized and utilized. For instance, stabilized ylide 20 was prepared and treated with a-D-a/lo-pyranoside 19 to furnish a-D-cyclopropanyl-pyranoside 21. Other examples of substituted sulfur ylides include 22-25, among which aminosulfoxonium ylide 25, sometimes known as Johnson s ylide, belongs to another category. The aminosulfoxonium ylides possess the configurational stability and thermal stability not enjoyed by the sulfonium and sulfoxonium ylides, thereby are more suitable for asymmetric synthesis. 

Epoxidation of aldehydes and ketones is the most profound utility of the Corey-Chaykovsky reaction. As noted in section 1.1.1, for an a,P-unsaturated carbonyl compound, 1 adds preferentially to the olefin to provide the cyclopropane derivative. On the other hand, the more reactive 2 generally undergoes the methylene transfer to the carbonyl, giving rise to the corresponding epoxide. For instance, treatment of P-ionone (26) with 2, derived from trimethylsulfonium chloride and NaOH in the presence of a phase-transfer catalyst Et4BnNCl, gave rise to vinyl epoxide 27 exclusively. ... 

Interestingly, phase-transfer catalysts including crown ethers have been used to promote enantioselective variations of Darzens condensation. Toke and coworkers showed that the novel 15-crown-5 catalyst derived from d-glucose 33 could promote the condensation between acetyl chloride 31 and benzaldehyde to give the epoxide in 49% yield and 71% A modified cinchoninium bromide was shown to act as an effective phase transfer catalyst for the transformation as well. ... 

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

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. 

The scope of reactions involving hydrogen peroxide and PTC is large, and some idea of the versatility can be found from Table 4.2. A relatively new combined oxidation/phase transfer catalyst for alkene epoxidation is based on MeRe03 in conjunction with 4-substituted pyridines (e.g. 4-methoxy pyridine), the resulting complex accomplishing both catalytic roles. 

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). 

The 9,10-dihydrodiol of 3-MC (24a) was synthesized from 9-hy-droxy-3-MC by Method IV (86). Oxidation of this phenol with Fremy s salt in the presence of Adogen 464, a quaternary ammonium phase transfer catalyst, furnished 3-MC 9,10-dione. Reduction of the qui-none with NaBH -C gave pure 24a in good yield. Treatment of 24a with m-chloroperbenzoic acid was monitored by HPLC in order to optimize the yield of the anti diol epoxide (25 ) and minimize its decomposition. 

The simplest model compound is cyclohexene oxide III. Monomers IV, V and VII represent different aspects of the ester portion of I, while monomers VII and VIII reflect aspects of both the monomer I and the polymer which is formed by cationic ring-opening polymerization. Monomers IV-VII were prepared using a phase transfer catalyzed epoxidation based on the method of Venturello and D Aloisio (6) and employed previously in this laboratory (7). This method was not effective for the preparation of monomer VIII. In this specific case (equation 4), epoxidation using Oxone (potassium monoperoxysulfate) was employed. 

For a similar series of chalcone derivatives the use of aqueous sodium hypochlorite in a two phase system (with toluene as the organic solvent) and the quinine derivative (32) as a chiral phase-transfer catalyst, produces epoxides with very good enantiomeric excesses and yields1981. 

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... 

Excellent enantioselectivities have not been attained in the asymmetric phase transfer catalyzed epoxidation of (Z)-enones in contrast to that of (E)-enones. However, a few promising results168,691 have been reported on the epoxidation of 2-substituted... 

S. Arai, H. Tsuge, T. Shioiri, Asymmetric Epoxidation of a,p-Unsaturated Ketones under Phase-Transfer Catalyzed Conditions , Tetrahedron Lett. 1998,39,7563-7566. 

B. Lygo, P. G. Wainwright Phase-Transfer Catalyzed Asymmetric Epoxidation of Enones using N-Anthracenylmethyl-Substituted Cinchona Alkaloids , Tetrahedron 1999, 55,6289-6300. 

Epoxidation with sodium hypochlorite.1 Ni(salen), is an effective catalyst for oxidation of some alkenes with NaOCl under phase-transfer conditions. Styrenes... 

Tungsten-based catalysts including POMs show high efficiency of H202 utilization [17,18,78-100]. Ishii and coworkers [18] have reported effective H202-based epoxidation of alkenes catalyzed by H3PW12O40 combined with cetyl pyridinium chloride (CPC) as a phase-transfer agent ... 

Two molecules of carbon monoxide were successively incorporated into an epoxide in the presence of a cobalt catalyst and a phase transfer agent [29]. When styrene oxide was treated with carbon monoxide (0.1 MPa), excess methyl iodide, NaOH (0.50 M), and catalytic amounts of Co2(CO)8 and hexadecyltrimethylammonium bromide in benzene, 3-hydroxy-4-phenyl-2(5H)-furanone was produced in 65% yield (Scheme 7). A possible reaction mechanism was proposed as shown in Scheme 8 Addition of an in situ... 

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