Catalyst-epoxide complex

A catalytic cycle starting from the formation of a catalyst-epoxide complex that further reacts with the thione tautomer of the thiobenzoic add 17 (Scheme 3.7) was computed. Full molecule of the catalyst was used in the PCM-M06-2X/6-31H-G V/M06-2X/6-31G computations. 

The zinc cation gives by far the most active catalyst. Iron, cobalt, and nickel cations also gave salts with considerable catalytic activity. Cadmium, because of its chemical similarity to zinc, and aluminum, because of its use in other epoxide polymerization catalysts, were considered as likely candidates to give active catalysts. However, complexes of the salts of these cations were only slightly catalytic. The salts used as cation sources in catalyst preparations also affected catalytic activity. Zinc salts, especially zinc chloride and zinc bromide, were retained in considerable amounts in the finished complexes, and the use of these salts gave the most active catalysts. 

The first isolated and characterised species that could be envisioned as intermediates in the initiation step for the coordination polymerisation of epoxides when using metal carboxylate catalysts were complexes formed between cadmium carboxylates, solubilised in organic solvents by the tris-3-phenylpyrazole hydroborate ligand, and epoxides such as propylene oxide and cyclohexene oxide [68]. Other epoxide complexes with various metal derivatives have also been reported in the literature [69-72],... 

The selectivity to epoxide is determined by the realtive rates of reaction of the catalyst-hydroperoxide complex with the olefin [Eq. (311)] in competition with its homolytic decomposition [Eq. (312)]. 

A wide variety of solvents has been used for epoxidations, but hydrocarbons are generally the solvent of choice 428 Recently, it has been shown434 that the highest rates and selectivities obtain in polar, noncoordinating solvents, such as polychlorinated hydrocarbons. Rates and selectivities were slightly lower in hydrocarbons and very poor in coordinating solvents, such as alcohols and ethers. The latter readily form complexes with the catalyst and hinder both the formation of the catalyst-hydroperoxide complex and its subsequent reaction with the olefin. 

The retarding effect of alcohols on the rate of epoxidation manifests itself in the observed autoretardation by the alcohol coproduct.428,434 446,447 The extent of autoretardation is related to the ratio of the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes. This ratio will vary with the metal. In metal-catalyzed epoxidations with fe/T-butyl hydroperoxide, autoretardation by tert-butyl alcohol increased in the order W < Mo < Ti < V the rates of Mo- and W-catalyzed epoxidations were only slightly affected. Severe autoretardation by the alcohol coproduct was also observed in vanadium-catalyzed epoxidations.428 434 446 447 The formation of strong catalyst-alcohol complexes explains the better catalytic properties of vanadium compared to molybdenum for the epoxidation of allylic alcohols.429 430 452 On the other hand, molybdenum-catalyzed epoxidations of simple olefins proceed approximately 102 times faster than those catalyzed by vanadium.434 447 Thus, the facile vanadium-catalyzed epoxidation of allyl alcohol with tert-butyl hydroperoxide may involve transfer of an oxygen from coordinated hydroperoxide to the double bond of allyl alcohol which is coordinated to the same metal atom,430 namely,... 

Two research groups496 497 have recently studied the autoxidation of cyclohexene at 60° to 65°C in the presence of a mixture of a low-valent Group VIII metal complex, e.g., RhCl(Ph3P)3 or (Ph3P)2Pt02, and an epoxidation catalyst (molybdenum complexes). Cyclohexen-l-ol and cyclohexene oxide are formed in roughly equimolar amounts. The results could be explained by a scheme involving two successive catalytic processes ... 

The macrocyclic chemistry of tetradentate Schiff base complexes has been known for long time. However, the successful use of such a complex as an enantioselective catalyst in epoxidation reactions is a relatively recent finding. In these reactions complex 9.9 or an analogue is used. One of the possible routes for the synthesis of intermediate 9.2 of Table 9.1 involves the use of a similar catalyst. While complex 9.9 works well with unfunctionalized alkenes, for the epoxidation of allylic alcohols, dialkyl tartarates, 9.10, are the preferred ligands. As we shall see, the mechanisms of epoxidation in these two cases are different. Also for the tartarate-based system titanium is the metal of choice (see Section 9.3.3). 

C imlBr Iron-porphyrin complex H202 CH2C12 as co-solvent product isolated by decantation catalyst recycled 4 times, slow decrease in activity excess oxidant leads to degradation of the catalyst epoxide-selectivity is substrate-dependent. [36]... 

The allylic alcohol binds to the remaining axial coordination site where stereochemical and stereoelec-tronic effects dictate the conformation shown in Figure 5. The structural model of catalyst, oxidant and substrate shown in Figure 5 illustrates a detailed version of the formalized rule presented in Figure 1. Ideally, all the observed stereochemistry of epoxy alcohol and kinetic resolution products can be rationalized according to the conq>atibility of their binding with the stereochemistry and stereoelectronic requirements imposed by this site. A transition state model for the asymmetric epoxidation complex has been calculated by a frontier orbital preach and is consistent with the formulation portrayed in Figure... 

When the alcohol is secondary, the possibility for kinetic resolution exists if the titanium tartrate complex is ctqxiUe of catalyzing the enantioselective oxidation of the amine to an amine oxide (or other oxidation product). The use of the standard asymmetric epoxidation complex, i.e. Ti2(tartrate)2, to achieve such an enantioselective oxidation was unsuccessful. However, modification of the complex so that the stoichiometry lies between Ti2(tartrate)i and Ti2(tartrate)i.s leads to very successful kinetic resolutions of p-hydroxyamines. A representative example is shown in equation (13). " The oxidation and kinetic resolution of more than 20 secondary p-hydroxyamines provi s an indication of the scope of the reaction and of some structural limitations to good kinetic resolution. These results also show a consistent correlation of absolute configuration of the resolved hydroxyamine with the configuration of tartrate used in the catalyst. This correlation is as shown in equation (13), where use of (+)-DIPT results in oxidation of the (5)-P-hydroxyamine and leaves unoxidized the (/ )-enantiomer. 

With chiral ligands, the transition-metal catalyst-hydroperoxide complex yields optically active oxiranes. " One of the most significant advances in the formation of chiral epoxides from allyl alcohols has recently been reported by the Sharpless group. Using (-l-)-tartaric acid, ferf-butylhydroperoxide, and titanium isopropoxide, they were able to obtain chiral epoxides in very high enantiomeric excess. The enantiomeric epoxide can be obtained by employing (—)-tartaric acid (Eq. 33a). 

The catalyst-hydroperoxide complexes are more stable for the metals of Group B than for those of Group A. Epoxidation of the olefins proceeds more easily and more selectively. " They are particularly significant in the industrially important epoxidation of propylene. Reference should be made to the very selective epoxidation of cyclohexene with a Mo complex and to publications relating to the epoxidation of ethylene, hexene-1, and octene-1 with a Cr complex. 

The mechanism of the Jacobsen HKR and ARO are analogous. There is a second order dependence on the catalyst and a cooperative bimetallic mechanism is most likely. Both epoxide enantiomers bind to the catalyst equally well so the enantioselectivity depends on the selective reaction of one of the epoxide complexes. The active species is the Co(lll)salen-OH complex, which is generated from a complex where L OH. The enantioselectivity is counterion dependent when L is only weakly nucleophilic, the resolution proceeds with very high levels of enantioselectivity. 

In addition, Katsuki and co-woi kers <9719541> have shown that this axial ligand can serve as a source of chiral induction. Thus, when chromene 13 was epoxidized at 0°C in the presence Df salen-Mn catalyst 15 and chiral amine ligand 16. a 30% enantiomeric excess was observed even though the catalyst itself lacked asymmetry. The results are rationalized on the basis of a preferred conformation of the catalyst-ligand complex which, in turn, influences the approach of he olefin substrate. 

Olefin epoxidation has many features in common with biological monooxygenation. This reaction requires heterolysis of the peroxide bond. The presence of carbonyl groups facilitates heterolysis in peracids. Epoxidation occurs through electrophilic attack, so di- and tri-substituted double bonds are more easily epoxidized. In the presence of metal complex catalysts, epoxidation proceeds under mild conditions because in the transition state the 0-0 bond is more easily heterolyzed (Eq. 12-29). 

In order to observe rapid rates and high epoxide selectivity, the conditions under which reaction (226) is run must be within fairly restricted limits. In most instances, an excess of olefin over hydroperoxide will result in more efficient use of hydroperoxide and thus in greater selectivity [370]. In general, the lower the temperature, the less radical decomposition of hydroperoxide and the higher the selectivity. The maximum temperature at which each metal complex may be run without a large amount of radical decomposition varies with the metal center. For molybdenum catalysts epoxide selectivities of 98% can be achieved at 100 °C but fall to 75-80% at 130°C. For vanadium complexes the maximum temperature for selective operation is 80 °C and for chromium it is below 60 [370]. 

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