Epoxides with active hydrogen

The selectivity of the epoxidation, in conjunction with the availability of optically active terpenes from natural sources, has resulted in the application of terpene epoxides as starting materials for the synthesis of several natural products. Both enantiomers of carvone, (10) and (ent-l0), have been used for the synthesis of methyl trans- and c/.v-chrysanthemates 15 and ent-15. i+Hsy Carvone (10) was converted into hydrochlorinated compound 13 and the methylated derivative 11, which were selectively epoxidized with alkaline hydrogen peroxide, and further converted into methyl trum-chrysanthcmate 15. The same route led from (— )-(/ )-carvone ent-10) to m-chrysanthemate ent-1543 ent-13 was converted to ( + )-a-3,4-epoxycaran-2-one 1644. 

M. C. A. van Vliet, I. W. C. E. Arends, R. A. Sheldon, Hexafluoroacetone in hexafluoro-2-propanol A highly active medium for epoxidation with aqueous hydrogen peroxide, Synlett (2001) 1305. 

As mentioned above the discovery of the remarkable activity of titanium silicalite-1 (TS-1) as a catalyst for a variety of synthetically useful oxidations, including epoxidation, with aqueous hydrogen peroxide constituted a major breakthrough in oxidation catalysis [14-20]. The success of TS-1 stimulated the search for related materials [21]. Titanium silicalite-2, with the MEL structure, was discovered by Rat-nasamy and coworkers in 1990 [41] and had properties similar to those of TS-1. 

Epoxies may be cured with a wide variety of substances which usually induce molecule to molecule bonds by cationic initiation. For cationic polymerization of the epoxide, HA is a compoimd with active hydrogen. 

Epoxide groups may homopolymerise or react with active hydrogen atoms in other molecules, usually termed hardeners or co-reactants, to produce copolymers. Cure of an epoxy resin may involve either or both of these reactions, and can be very complex, since reaction of an epoxide with one functional group can produce new functional groups for additional reaction with epoxides, and so on. 

More recently, Beller and coworkers have shown that the ruthenium complex 6 (Fig. 7.9) is an effective epoxidation catalyst, for a variety of olefins, with 3 equiv. of 30% H2O2 at very low catalyst loadings (0.005 mol%). A tertiary alcohol such as fert-amyl alcohol was used as a cosolvent. Based on its high activity and broad scope this system appears to have considerable synthetic potential, which may be adapted to afford effective asymmetric variants in the future. Indeed, a truly effective catalyst, with broad scope, for asymmetric epoxidation with aqueous hydrogen peroxide, preferably in the absence of organic solvents, is still an important and elusive goal in oxidation chemistry. 

Several cortisone derivatives with glucocorticoid effects are most active, if they contain fluorine in the 9or-position together with an Il(9-OH group. Both substituents are introduced by the cleavage of a 9,11 -epoxide with hydrogen fluoride. The regio- and stereoselective formation of the -epoxide is achieved by bromohydrination of a 9,11-double bond and subsequent alkali treatment (J. Fried, 1954). 

Polymerization to Polyether Polyols. The addition polymerization of propylene oxide to form polyether polyols is very important commercially. Polyols are made by addition of epoxides to initiators, ie, compounds that contain an active hydrogen, such as alcohols or amines. The polymerization occurs with either anionic (base) or cationic (acidic) catalysis. The base catalysis is preferred commercially (25,27). 

Specialty Epoxy Resins. In addition to bisphenol, other polyols such as aUphatic glycols and novolaks are used to produce specialty resins. Epoxy resins may also include compounds based on aUphatic, cycloaUphatic, aromatic, and heterocycHc backbones. Glycidylation of active hydrogen-containing stmctures with epichlorohydrin and epoxidation of olefins with peracetic acid remain the important commercial procedures for introducing the oxirane group into various precursors of epoxy resins. 

The great reactivity of the sulfurane prepared by this procedure toward active hydrogen compounds, coupled with an indefinite shelf life in the absence of moisture, makes this compound a useful reagent for dehydrations,amide cleavage reactions, epoxide formation, sulfilimine syntheses, and certain oxidations and coupling reactions. 

Begue and coworkers recently achieved an improvement in this method by performing the epoxidation reaction in hexafluoro-2-propanol [120]. They found that the activity of hydrogen peroxide was significantly increased in this fluorous alcohol, in relation to trifluoroethanol, which allowed for the use of 30% aqueous H202. Interestingly, the nature of the substrate and the choice of additive turned out to have important consequences for the lifetime of the catalyst. Cyclic dis-ubstituted olefins were efficiently epoxidized with 0.1 mol% of MTO and 10 mol%... 

In conclusion, the above summary of oxidation methods shows that there is still room for further improvements in the field of selective olefin epoxidation. The development of active and selective catalysts capable of oxidizing a broad range of olefin substrates with aqueous hydrogen peroxide as terminal oxidant in inexpensive and environmentally benign solvents remains a continuing challenge. 

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