Selectivity in epoxidations

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). 

The catalyst obtained via anion exchange with the commercial resin Amberlite lRA-900 showed excellent selectivity in epoxidation of acid-sensitive natural terpenes and allylic alcohols [73-75]. The selectivity of 92% at 83% conversion was attained in epoxidation of a-pinene and limonene. The catalytic activity of the reused catalyst was completely maintained after several catalytic cycles, and the filtrate was catalytically inactive [75]. 

In 2002, Wu and Tatsumi reported on the high fraw5-selectivity in epoxidation reactions with Ti-MWW (also known as MCM-22), a titanosilicate that contains both medium and... 

However, the low facial selectivity in epoxidations of unsymmetrical alkylidene- and cy-cloalkylidenecyclopropanes can be a serious drawback. Thus, both 2-cyclopropylidenebicy-clo[2.2.1]heptane (10)48 and 10,15-dicyclopropylidenetrispiro[3.1.3.1.3.1.]pentadecan-5-one (U)4<). so produced mixtures of stereoisomeric cyclobutanones on epoxidation and rearrangement of the resulting oxaspiropentanes. 

A unique titanium(IV)-silica catalyst prepared by impregnating silica with TiCLt or organotitanium compounds exhibits excellent properties with selectivities comparable to the best homogeneous molybdenum catalysts.285 The new zeolite-like catalyst titanium silicalite (TS-1) featuring isomorphous substitution of Si(IV) with Ti(IV) is a very efficient heterogeneous catalyst for selective oxidations with H2C>2.184,185 It exhibits remarkable activities and selectivities in epoxidation of simple olefins.188,304-306 Propylene, for instance, was epoxidized304 with 97% selectivity at 90% conversion at 40°C. Shape-selective epoxidation of 1- and 2-hexenes was observed with this system that failed to catalyze the transformation of cyclohexene.306 Surface peroxotitanate 13 is suggested to be the active spe-... 

Newer studies provide numerous examples of the use of metal catalysts exhibiting high activity and selectivity in epoxidation with various oxidants mainly with H202.1201-1210 Catalysts developed by combinatorial methods1211,1212 and hetero-genized catalysts1213 were also applied. 

Figure 4 Selectivity in epoxidation for a range of substrates plotted against the dissociation enthalpy of the weakest C-H bond in the olefin (m) TS-1 peroxide system (u) silver-oxygen system. 1. 1-octene, 2. 1-butane, 3. 2-butane, 4. gropene, 5. 4-unyltoluene, 6. 1-3 butadiene, 7. styrene, 8. 4-vinylpyridine, 9. ethylene.2...

Freccero, M., Gandolfi, R., Sarzi-Amade, M., Rastelli, A. Facial Selectivity in Epoxidation of 2-Cyclohexen-1-ol with Peroxy Acids. A Computational DFT Study. J. Org. Chem. 2000, 65, 8948-8959. 

Selectivity in epoxidation reactions frequently depends on the reagent chosen and also on reaction conditions. The epoxidation of cholesterol using a range of peroxycarboxylic acids, including MMPP, gives a mixture in which the a-epoxide... 

In the epoxidation of racemic secondary alcohols, there are two stereochemical problems to be considered (i) differentiation of enantiomers (kinetic resolution) and (ii) diastereoface selection in epoxidation. 

Scheme 8.10. Reaction of divinyl carbinol under (+)-AE conditions as an example of enantiotopic group selectivity in epoxidation chemistry. Matched cases of enantiofacial selectivity are shown with bold arrows. Qualitative rate differences are on the order kj k2, ks k4 (without specifying an order for k2 vi. k3 (however, cf. Scheme 8.8b). Note that the products arising from the pairs ki/k3 and k2/k4 are enantiomers.

It has been shown that this catalyst is selective in epoxidation of linear alkenes the linear epoxide yield was two to four times higher than in catalysis by ordinary porphyrin. It was also demonstrated that, in catalysis by the dendrimers, cyclic alkenes are oxidized three times more rapidly than similar linear 1-alkenes are. The catalyst activity decreases only by 10% at a turnover number (TON) of 1000, which is much higher than that for the monomolecular analogue. A cobalt complex with dendrimer phthalocyanine was much more stable, while remaining active, in... 

Very recent literature indicates that Ag clusters of only several tens of atoms, also yield exceptionally high selectivities in epoxidation of alkenes, other than ethane. The preparation of these clusters is currently only possible in very small quantities, and upscaling of the vacuum methodologies is very necessary. Perhaps also atomic layer deposition, recently applied successfully, could provide for a solution, in particular if this becomes feasible in a continuous mode of operation. 

Breslow, R., Brown, A.B., McCullough, R.D. and White, P.W., Substrate selectivity in epoxidation by metal-loporphyrin and metallosalen catalysts carrying binding groups,/. Am. Chem. Soc., 1989, 111, 4517-4518. 

Epoxidation of Alkenes. The use of aromatic N-donor ligands in significant excess (ca. 10-12 1) together with MTO leads to higher activities and selectivities in epoxidation catalysis than with MTO alone. This observation is made for... 

The asymmetric-selective polymerization of epoxyethyltrimethylsilane (ETMS) and of (2,3 epoxypropoxy)trimethylsilane (EPOS) (LX) were studied using a ZnEt2/(0-menthol initiator system. OA polymers were obtained in both cases but the asymmetric selectivity of EPOS polymerization was low, considering the weak optical activity of glycidol obtained from the hydrolysate of the unconverted monomer. The presence of a silicon atom, which is considered to have a stronger interaction with initiators than a carbon atom, did not improve the degree of asymmetric selectivity in epoxide polymerization [150 bis]. 

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