Epoxide reactions limiting mechanism

It is interesting that, until recently, a mechanism was advocated which predicts that the selectivity has a theoretical limit of 86%. This would imply that it does not make sense trying to improve the catalyst much further, because, in practice, plants are already operating at selectivities close to this value. This theory was based on the assumption that the catalyst contains two types of oxygen [95] molecular O2, and atomic oxygen. The productive epoxidation reaction was envisaged as shown in Fig. 5.13. 

Cp atom than at the Ca atom of styrene, consistent with a non-concerted mechanism. The rate-limiting generation of a benzylic radical intermediate has been proposed and it has been suggested that the spin delocalization effect is more important than the polar effect in the epoxidation reactions. The head-on approach model rather than a side-on approach model is implicated in the epoxidation.The dioxoruthenium(VI) complex (5) (Figure 2) catalyses the enantioselective epoxidation of alkenes with enantiomeric excess (ee) up to 11%. Kinetic studies on the epoxidation of para-substituted styrenes suggest the formation of a radical intermediate. ... 

Jacobsen developed a method employing (pybox)YbCl3 for TMSCN addition to meso-epoxides (Scheme 7.22) [46] with enantioselectivities as high as 92%. Unfortunately, the practical utility of this method is limited because low temperatures must be maintained for very long reaction times (up to seven days). This reaction displayed a second-order dependence on catalyst concentration and a positive nonlinear effect, suggesting a cooperative bimetallic mechanism analogous to that proposed for (salen)Cr-catalyzed ARO reactions (Scheme 7.5). 

The mechanism of epoxide formation (Scheme 7) has not been established but the intermediacy of nickel enolates and ensuing aldol type reactions are suspected28 (cf. Zn-mediated formation of furans from a-bromoketones29). A limitation on the synthesis is that R cannot be aryl for these cases, the products are 2,4-diarylfurans (see Section IV,B,1).30... 

Treatment of 1,1-disubstituted epoxides of the gibberellin family with sulfuiyl chloride results in the formation of the corresponding a,3-enal in good yield, as shown in equation (31). Four examples were reported in which alcohols, esters, lactones and sdkenes survive. The postulated mechanism involves an electrophilic opening of the epoxide with elimination, followed by oxidation of the primary chlorosulfate ester. A steroidal 3-spirooxirane also undergoes this reaction, but the yield is poor and several products are obtained, suggesting that the overall scope of this reaction may be limited. 

The compehtion of one-electron pathways is sometimes detectable in the epoxidations catalyzed by transition metal catalysts [67]. However, in the epoxidahon of unhindered olefins on TS-1, the typical radical products are below the detection limits. Their presence could no longer be neglected when the rate of epoxidation is so slow as to become comparable to that of homolytic side reactions, for example with bulky olefins (see also Section 18.11). It is possible that, within these limits only, the epoxide is produced in part through the addition of a radical peroxy intermediate to the double bond [68, 69]. Even so, a homolytic pathway has again been proposed as a generally vahd epoxidation mechanism [7]. 

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