Epoxidations with aqueous

Treatment of the following epoxide with aqueous acid produces a carbocation intermediate that reacts with water to give a diol product. Show the structure of the carbocation, and propose a mechanism for the second step. 

The second major discovery regarding the use of MTO as an epoxidation catalyst came in 1996, when Sharpless and coworkers reported on the use of substoichio-metric amounts of pyridine as a co-catalyst in the system [103]. A change of solvent from tert-butanol to dichloromethane and the introduction of 12 mol% of pyridine even allowed the synthesis of very sensitive epoxides with aqueous hydrogen peroxide as the terminal oxidant. A significant rate acceleration was also observed for the epoxidation reaction performed in the presence of pyridine. This discovery was the first example of an efficient MTO-based system for epoxidation under neutral to basic conditions. Under these conditions the detrimental acid-induced decomposition of the epoxide is effectively avoided. With this novel system, a variety of... 

The first application of ionic hquids for salen complexes dealt with the epoxidation of alkenes [14]. Jacobsen s Mn complex was immobilized in [bmimjlPFe] and different alkenes were epoxidized with aqueous NaOCl solution at 0 °C. As the ionic solvent sohdified at this temperature, dichloromethane was used as a cosolvent. The recychng procedure consisted of washing with water, evaporation of dichloromethane, and product extraction with hexane. The results (Table 3) were excellent and only a slow decay in activity and enantioselectivity was detected after several cycles. 

Table 11 Olefin epoxidation with aqueous H202/zeolite-Y supported MTO...

Fluorous solvents proved to be highly effective in epoxidation of alkenes. H202 can be used in combination with trifluoroacetone,27 perfluoroacetone,28 or a mixture of perfluoroacetone and hexafluoro-2-propanol.29 In fluorinated alcohols as solvents uncatalyzed epoxidations with aqueous H202 are performed.30,31... 

G. Miscettaneous Nwdeopkilic Additions (1) Azide ion. The ready cleavage of ethylene oxide rings by N. ion is in acoordanoe with its recognized nucleophilic character. Alllums-Ii so far limited to three,6 8-127 -1778 publications devoted to this reaction have explored the behavior of a wide assortment of epoxides. Jt. is customary to characterize 0-azido alcohol obtained on treatmml of epoxides with aqueous sodium azide by reduction to /1-atnirm aa-o-hok.iwo. ins aa shown in Eq. (896). 

Sheldon RA, van Vliet MCA, Arends IWCE (2001) Fluorinated alcohols effective solvents for uncatalysed epoxidations with aqueous hydrogen peroxide. Synlett 2001 248-250... 

It is clear from the preceding discussion that several systems are now available for epoxidations with aqueous H202. The key features are compared in Table 4.5 for epoxidation of cyclohexene and a terminal olefin. They all have their limitations regarding substrate scope, e.g. TS-1 is restricted to linear olefins and... 

The synthesis of the preceding vinyl amino sulfone offers a very good example of a stereospecific sulfide-directed epoxidation (Scheme 76).Oxidation of optically active sulfide alcohol (12 readily made from epoxycyclopentadiene and resolved) with MCPBA affords the corresponding sulfoxide, which is in equilibrium with the sulfenate ester. Treatment with pyridine hydrobromide and then phenyl disulfide and bromine gives the bromodiol (13), which is simply cyclized to the epoxide with aqueous sodium hydroxide solution (83% overall yield from the sulfoxide). Treatment of the P-epoxy sulfone with DBU followed by in situ silylation with f-butyldiphenylsilyl chloride affords an 86% yield of vinyl sulfone (14). Mesylation of the alcohol moiety followed by immediate treatment with dimethylamine produces the amino vinyl sulfone via a syn Sn2 substitution. ... 

Majetich, G., Hicks, R., Sun, G.-r., McGiii, P. Carbodiimide-promoted oiefin epoxidation with aqueous hydrogen peroxide. J. Org. Chem. 1998, 63, 2564-2573. 

M. C. White, A. G. Doyle, E. N. Jacobsen, A synthetically useful, self assembling MMO mimic system for catalytic alkene epoxidation with aqueous H2O2, J. Am. Chem. Soc. 123,7194-7195 (2001). 

Y. Sawada, K. Matsumoto, S. Kondo, H. Watanabe, T. Ozaka, K. Suzuki, B. Saito, T. Katsuki, Titanium-salan-catalyzed asymmetric epoxidation with aqueous hydrogen per-oxide as the oxidant, Hngew. Chem. Int. Ed. 45, 3478-3480 (2006). 

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. 

R. Rinaldi, J. Sepulveda, U. Schuchardt, Cyclohexene and cyclooctene epoxidation with aqueous hydrogen peroxide using transition metal-free sol-gel alumina as catalyst, Adv. Synth. Catal. 346 (2004) 281. 

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. 

Table 3. Comparison of the different catalysts for cyclohexene epoxidation with aqueous hydrogen peroxide.

Whereas several catalytic methods are currently available for manganese-catalyzed epoxidation with aqueous H2O2, high turnover numbers for cis-dihydroxylation reactions so far have only been achieved with osmium compounds 161-165). However, manganese-catalyzed oxidation reactions have a few inherent advantages, such as the low price of the manganese salts and complexes and their non-toxic nature. 

Shitama H, Katsuki T (2007) Synthesis of metal-(pentadentate-salen) complexes asymmetric epoxidation with aqueous hydrogen peroxide and asymmetric cyclopropanation (salenH(2) N,N-bis(salicylidene)ethylene-1,2-diamine). Chem Eur J 13 4849 858... 

Stability of Epoxy Groups During the Epoxidation with Aqueous Peracetic Acid... 

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. 

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