Epoxidations shape-selective

Mn impregnated into MCM-4i, a silicalite containing uniform mesopores of approximately 22 A, catalyzes TBHP epoxidation of alkenes.88 Over Mn-MCM-41, both cis- and trans-stilbene yield trans-stilbene oxide, which the authors conclude signals a radical mechanism.88 In contrast, over Ti—MCM-41, trans-stilbene cannot be oxidized, only cis-stilbene is epoxidized to the cis-stilbene oxide, which suggest a radical-free mechanism.89 Finally, emphasizing the shape selectivity possibilities, only trans-stilbene (not cis-stilbene) can be epoxidized over Mn-ZSM-5, a zeolite with relatively small pores of 5.1 x 5.4 A (Fig. 6.14).88... 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

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-... 

Dramatic shape selectivities in competitive olefin epoxidation was observed with picnic basket metalloporphyrins312 313 designed to exclude bulky axial ligands on one sterically protected porphyrin face. When oxidized with PhIO in acetonitrile in the presence of the rigid p-xylyl-strapped porphyrin, cis-2-octene reacted selectively versus ds-cyclooctene or 2-methyl-2-pentene, giving >1000 reactivity ratios.313,314 Some immobilized manganese(III) porphyrins proved to be as efficient as their homogeneous equivalents in epoxidation with PhIO.151,315... 

The styrene oxide isomerization is known to be an easy reaction due to the carbonium stabilization by the aromatic nucleus. In the case of H-ZSM-5, taking into account the respective size of this medium-pore zeolite (5.5A) and the kinetic diameter of the styrene oxide molecule (5.9A), it was assumed that the weak external acidic sites are active enough to catalyze the reaction (ref. 16). If this were the case for all zeolites, no shape-selectivity could be obtained for any epoxide rearrangement. Nevertheless, for large-pore zeolites, the contribution of all the acidic sites cannot be excluded. 

The intercalated compounds of hydrotalcite [Mg23.3AlI0(OH)66.6] with M07O24 or W12O4 - catalyze the shape-selective epoxidation of olefins epoxi-dation of 2-hexene was favored over theat of cyclohexene (395). 

Evidence of variables that influence the relative rates of reaction of olefins and alcohols was obtained from experiments with compounds that have both olefinic and alcoholic functions and by the competitive oxidation of mixtures of olefins and alcohols. The data of Table VI show that when the double bond has no substituents, as in allyl alcohol, but-3-en-l-ol, or 2-methylbut-3-en-l-ol, only the epoxide is formed but when the double bond has substituents, the epoxida-tion rate is decreased and ketone and aldehyde products from the oxidation of the OH group are formed. This effect is more pronounced with a greater degree of substitution. Since the double bond and the OH group are part of the same molecule, the difference must arise from the different abilities of the reactants to coordinate and react at the titanium center restricted transition-state shape selectivity is a possibility. The terminal double bond, sterically less hindered, interacts strongly with titanium, preventing coordination of the competing OH... 

Tatsumi, T., Yamamoto, K., Tajima, H. and Tominaga, FI. (1992). Shape selective epoxidation of alkenes catalysed by polyoxometalate-intercalated hydrotalcite. Chem. Lett., 815. 

TS-1 exhibits some quite remarkable activities and selectivities in the epoxidation of many substrates. For example, relatively unreactive olefins such as ethene and allyl chloride are epoxidized in high yield under mild conditions using methanol as the solvent.19 As a result of the shape selectivity effect, larger, more reactive olefins such as cyclohexene are epoxidized very slowly (Table 4.2). 

However, since epoxidation occurs within pores of cross-section comparable to that of the olefin, steric restrictions generally prevail over inductive effects, leading to anomalous reactivity orders. They result from restrictions to diffusion in the pores (reactant shape selectivity) and to the approach of the double bond to the active species (transition state shape selectivity). The first is sufficient to explain... 

The reaction was carried out in acetonitrile at 353 K using TBHP as oxidant. Conversions as high as 80 % were obtained. As shown in Scheme 6, it was postulated that the reaction takes place via epoxidation over Ti sites foUowed by acid catalyzed intramolecular opening of the epoxide ring by the 3-hydroxy group. Ti-6 zeolite gave somewhat lower conversions in addition to the preferential formation of furans over pyrans (ratio of ca. 1.5) due to shape selectivity. Ti-MCM-41 and gave furan to pyran ratios of ca. 0.9, comparable to those obtained by the epoxidase conversion of linalool. 

The epoxidation of alkenes by sodium hypochlorite in the presence of manganese porphyrins under phase-transfer conditions has been thoroughly studied. Kinetic studies of this reaction revealed a Michaelis-Menten rate equation. As in Scheme 12, the active oxidant is thought to be a high-valent manganese( V)-oxo-porphyrin complex which reversibly interacts with the alkene to form a metal oxo-alkene intermediate which decomposes in the rate determining step to the epoxide and the reduced Mn porphyrin. Shape selective epoxidation is achieved when the sterically hindered complex Mn(TMP)Cl is used as the catalyst in the hypochlorite oxidation. ... 

An added advantage of the TS-1 catalyst, which could have commercial benefits, is the possibility for accomplishing shape-selective epoxidations. Owing to the limited dimensions (5.6 A X 5.4 A) of its micropores, linear olefins are epox-idized much faster than branched or cyclic olefins, e.g., 1-hexene is smoothly epoxidized while cyclohexene is virtually unreactive [45]. This reactivity is completely the opposite to that observed with the metal catalyst-alkyl hydroperoxide reagents (see earlier). It could be utilized in, for example, the selective epoxidation of linear olefins in mixtures of linear and branched or cyclic olefins. 

Thomas[16] reported direct grafting of an organometallic complex onto the inner walls of mesoporous silica MCM-41 to generate a shape-selective catalyst with a large concentration of accessible, well spaced, and structurally well defined active sites. Attachment of a titanocene-derived catalyst precursor to the pore walls of MCM-41 produces a catalyst for the epoxidation of cyclohexene and more bulky cyclic alkenes. Through the grafting route, titanocene complex on mesoporous silica exhibits high activity for catalytic conversion of bulky molecules... 

J. P. Collman, J. 1. Brauman, B. Mcunicr, T. Hayashi, T. Kodadek, S. A. Raybuck, Epoxidation of olefins by cytochrome P-450 model compounds Kinetics and stereochemistry of oxygen atom transfer and origin of shape selectivity, J. Am. Chem. Soc. 107 (1985) 2000. 

Last but not least, the discovery of titaniiun silicalite and its activity in an increasing number of new oxidation reactions not only opens promising prospects of future discoveries with respect to reactions, but also suggests that materials with new catalytic properties will be discovered. Their distinctive features should logically be a control of coordination of the active atom or atoms that zeolites permit more easily than other structures, and shape selectively. Two recent papers review the potential of zeolites as oxidation catalysts [21,22] and very stimulaing comments are made in a third one concerning the oxidation (mainly epoxidation) of large molecules [23]. 

Shape Selective Epoxidation of Crotyl Alcohol with HjOj in the Presence of TS-1... 

Although TS-1 has been investigated for the epoxidation of a range of molecules, e.g. butene, pentene, hexene, allyl chloride and allyl alcohol, little attention has been given to the effect of shape selectivity in the MFI zeotype framework in these reaetions. In this paper we address this aspect and exemplify the shape. selective epoxidation using a range of allylic alcohols. In particular, the shape selective epoxidation of crotyl alcohol is compared and contrasted with the reaction of allyl alcohol in a range of solvents. 

The most widely accepted mechanism for TS-1 catalysed epoxidation is the peradd-like mechanism in which the active epoxidising species acts as the electrophile. In addition to the mild conditions TS-1 offers the advantage of shape selectivity which results from the active sites being situated in a pore system of approximately 0.55 nm in diameter. The branched and cyclic alkenes react much more slowly than the Unear alkenes... 

Collman et al. reported a series of picket basket porphyrins (Figure 1.14) which show extremely high shape selectivity (>1,000 for cis-2-octene vs cw-cyclooctene) and relatively high enantioselectivity in olefin epoxidation (ee around 70-85%) Chiral, binaphthyl straps between adjacent o-aminophenyl groups have afforded very good enantiomeric selectivities for styrene epoxidations. ... 

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