Epoxidation yields

Epoxides provide another useful a -synthon. Nucleophilic ring opening with dianions of carboxylic acids (P.L. Creger, 1972) leads to y-hydroxy carboxylic acids or y-lactones. Addition of imidoester anions to epoxides yields y-hydroxyaldehyde derivatives after reduction (H.W. Adickes, 1969). 

One of the most significant developmental advances in the Jacobsen-Katsuki epoxidation reaction was the discovery that certain additives can have a profound and often beneficial effect on the reaction. Katsuki first discovered that iV-oxides were particularly beneficial additives. Since then it has become clear that the addition of iV-oxides such as 4-phenylpyridine-iV-oxide (4-PPNO) often increases catalyst turnovers, improves enantioselectivity, diastereoselectivity, and epoxides yields. Other additives that have been found to be especially beneficial under certain conditions are imidazole and cinchona alkaloid derived salts vide infra). 

The catalytic activitira of synfliesized catalysts are given in Table 1. The TS-1 catalyst exhibited the highest epoxide yield and the best catalytic performance for the epoxidation of 1-hexene. The convasion of cyclohexene, however, is the lowest over TS-1. In case of TS-1/MCM-41-A and TS-1/MCM-41-B, the selectivity to epoxide is much hi er than that of Ti-MCM-41. Moreover, the conversion of 1-hexene as well as cyclohexene is found larger on the TS-l/MCM-41-Aand TS-1/MCM-41-B than on other catalysts. While the epoxide yield from 1-hexene is nearly equivalent to that of TS-1, the yield from cyclohexene is much larger than those of the otiier two catalysts. Th e results of olefins epoxidation demonstrate that the TS-l/MCM-41-Aand TS-1/MCM-41-B possess the surface properties of TS-1 and mesoporosity of a typical mesoporous material, which were evidently brou in by the DGC process. 

Epoxide Yields in Reactions of Olefins Oxidation by sec-Decylsulfonic Peracid [40]... 

Intramolecular attack of the tertiary amide anion 73 on the epoxide yielded a p-lactam <00T3209>. 

In contrast to Red-Al reductions, DIBAL-H or LiBH4/Ti(OPr1)4 reduction of epoxides yields 1,2-diols as the major products.32 When treated with DI-BAL-H, ratios of 1,3- to 1,2-diol ranging from 1 6 to 1 13 have been observed. 

Since acidity (Lewis or Brpnsted) impacts adversely on the yield of epoxides, Clerici and Ingallina (204) added basic compounds in low concentrations to TS-1 catalysts during epoxidation of alkenes to inhibit the oxirane ring opening and enhanced the epoxide yields. A comprehensive investigation of the influence of pH on product selectivity in epoxidation of allylalcohol, allylchloride, and styrene catalyzed by various titanosilicates was reported recently by Shetti et al (205). 

In the first of these techniques the lanthanoid complex (33) (5-8 mol%) is used as the organometallic activator in cumene hydroperoxide or tert-butyl hydrogen peroxide-mediated oxidation of chalcone (epoxide yield 99 % 99 % ee) or the ketone (34) (Scheme 20)[1001. 

Taking into account the AErs for epoxide ring opening of the fluorinated compounds, it could be noted that the F-6 structure (20) was most favored for opening of the 1,2-epoxide, yielding a AEr even more exothermic than the unsubstituted molecule. Therefore, a fluorine atom at a highly positively charged site stabilized the carbocation. Fluorine at positions 12 (21) and 14 (23) afforded... 

Equations 1 to 3 show some of fixation reactions of carbon dioxide. Equations la and lb present coupling reactions of CO2 with diene, triene, and alkyne affording lactone and similar molecules [2], in a process catalyzed by low valent transition metal compounds such as nickel(O) and palladium(O) complexes. Another interesting CO2 fixation reaction is copolymerization of CO2 and epoxide yielding polycarbonate (equation 2). This reaction is catalyzed by aluminum porphyrin and zinc diphenoxide [3],... 

Reactions of organolithium species with epoxides, yielding secondary alcohols, are shown in equations 69 and 75 (Section VI.B.l). 

Epoxide yields were around 20% with the heterogeneons Zr catalysts and aronnd 10% with the heterogeneous Hf catalysts. 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

Yudin and Sharpless reported on the utilization of much cheaper, readily available inorganic Re catalysts [Re207, ReOsfOH), ReOs (0.5-1 mol%)] in combination with bis(trimethylsilyl) peroxide as oxidant and 0.5-1 mol% of pyridine (equation 52) . In this oxidation process high epoxide yields (78-96%) were obtained using CH2CI2 or THF as solvent. Traces of water or other protic species have been found to be essential for rapid turnover and accelerate the reaction. 

Substtate Catalyst Oxidant Time (h) Conv. (%) Yield epoxide (%) Yield diol (%) Reference... 

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