Epoxidation using urea

Bentley et al.m recently improved upon Julia s epoxidation reaction. By using urea-hydrogen peroxide complex as the oxidant, l,8-diazabicyclo[5,4,0]undec-7-ene (DBU) as the base and the Itsuno s immobilized poly-D-leucine (Figure 4.2) as the catalyst, the epoxidation of a, (3-unsaturated ketones was carried out in tetrahydrofuran solution. This process greatly reduces the time required when compared to the original reaction using the triphasic conditions. 

More recent developments in the field of the Pirkle-type CSPs are the mixed r-donor/ r-acceptor phases such as the Whelk-Of and the Whelk-02 phases.The Whelk-Of is useful for the separation of underiva-tized enantiomers from a number of families, including amides, epoxides, esters, ureas, carbamates, ethers, aziridines, phosphonates, aldehydes, ketones, carboxylic acids, alcohols and non-steroidal anti-inflammatory drugs.It has been used for the separation of warfarin, aryl-amides,aryl-epoxides and aryl-sulphoxides. The phase has broader applicability than the original Pirkle phases. The broad versatility observed on this phase compares with the polysaccharide-derived CSPs... 

On a laboratory scale, one particular method of producing in situ peracids for epoxidation of a wide range of substrates under mild conditions is via the use of urea hydrogen peroxide (UHP) in the presence of organic anhydrides.27 The anhydride must be added slowly to the UHP, solvent and substrate to generate the peracid. If the UHP is added to the substrate, solvent and anhydride, the unstable and potentially explosive diacyl peroxides can be formed. Table 3.2 illustrates a number of substrates which have been epoxidized using this reagent system. 

Kureshy, R. I., Khan, N.-u. H., Abdi, S. H. R., Singh, S., Ahmed, I., Shukla, R. S., Jasra, R. V. Chiral Mn " salen complex-catalyzed enantioselective epoxidation of nonfunctionalized aikenes using urea-H202 adduct as oxidant. J. Catal. 2003, 219,1-7. 

Recently, 3,5-bis(trifluoromethyl)benzeneseleninic acid has been used in a tandem catalytic epoxidation." The concept of tandem catalysis has been applied to oxidation reactions by Backvall and co-workers for the direct dihydroxylation of olefins using a couple catalytic system and hydrogen peroxide as the terminal oxidant." In this context, the seleninic acid was used in combination with a trifluoromethyl oxaziridine catalyst (Scheme 17), using urea hydrogen peroxide as the terminal oxidant." This system showed... 

Table 2 shows the results of influence of the nature of the precipitant used for depositing Au on the Ti-MCM-41 support on the propylene epoxidation reactions over supported Au catalysts. The Au/Ti-MCM-41 catalyst prepared by using NaOH as the precipitant showed the best activity and hydrogen efficiency. Catalyst prepared by using urea as the precipitant shows lower activity and hydrogen efficiencies. Catalysts prepared... 

Table 1. Epoxidation reactions using urea-hydrogen peroxide (UHP)...

Since its discovery in 1991, methyltrioxorhenium (MTO, 80) has attracted much interest as one of the most versatile catalysts for oxidation.When it is associated with a stoichiometric amount of H2O2, the system can efficiently transform alkene to epoxide, although formation of undesired diol can occur. Alternatively, water-free conditions, using urea hydrogen peroxide (UHP), allow the formation of the desired epoxide without byproducts. A maj or drawback of the MTO/UHP system is its insolubility in organic solvents, leading to a kinetically slow heterogeneous system. 

Novolacs are thermoplastic materials, that need to be transformed by crosslinking reactions to get insoluble, nonmelting, thermally stable thermosets. Different crosslinking agents have been used for thermal curing, for instance l,3,5,7-tetiaazatricyclo[3.3.3. P Jdecane (hexamethylenetetramine), oxiranes (epoxides), diisocyanates, urea, melamine, urea methanal resins, melamine metfaanal resins, and imide precursors. Furthermore, photoreactive compounds crosslink novolacs with methacrylate and methacryloylchloride resins, respeetively by itradialion [1]. 

Laha SC, Kumar R. (2001) Selective epoxidation of styrene to styrene oxide over TS-1 using urea-hydrogen peroxide as oxidizing agent J. Cat, 204 64—70. 

Obsolete uses of urea peroxohydrate, as a convenient source of aqueous hydrogen peroxide, include the chemical deburring of metals, as a topical disinfectant and mouth wash, and as a hairdresser s bleach. In the 1990s the compound has been studied as a laboratory oxidant in organic chemistry (99,100). It effects epoxidation, the Baeyer-Villiger reaction, oxidation of aromatic amines to nitro compounds, and the conversion of sodium and nitrogen compounds to S—O and N—O compounds. 

Thermosetting Reactive Polymers. Materials used as thermosetting polymers include reactive monomers such as urea—formaldehyde, phenoHcs, polyesters, epoxides, and vinyls, which form a polymerized material when mixed with a catalyst. The treated waste forms a sponge-like material which traps the soHd particles, but not the Hquid fraction the waste must usually be dried and placed in containers for disposal. Because the urea—formaldehyde catalysts are strongly acidic, urea-based materials are generally not suitable for metals that can leach in the untrapped Hquid fractions. Thermosetting processes have greater utiHty for radioactive materials and acid wastes. 

Carbon dioxide gas is used to make urea (used as a fertiliser and in automobile systems and medicine), methanol, inorganic and organic carbonates, polyurethanes and sodium salicylate. Carbon dioxide is combined with epoxides to create plastics and polymers. 

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