Epoxidations transition states

The stereochemistry of the resnlting epoxidation products using chiral ketones, such as ketone 26, could provide new insights about the epoxidation transition states. Studies showed that the epoxidation of trans- and trisubstituted olefins with ketone 26 mainly goes through the spiro transition state (spiro A) (Fig. 10). Planar transition state B competes with spiro A to give the opposite enantiomer [53, 54]. Hence, factors that influence the competition between spiro A and planar B will also affect the enantiomeric excess of the resulting epoxides. Spiro A can be further... 

In 1998, Yang and coworkers reported a series of (7 )-carvone derived ketones (63) containing a quaternary center at and various substituents at (Fig. 22) [119]. The ees of fran -stilbene oxide varied with different para and meta substituents when 63b was used as the catalyst. The major contribution for the observed ee difference is from the n-n electronic repulsion between the Cl atom of the catalyst and the phenyl group of the substrate. The substitution at also influences the epoxidation transition state via an electrostatic interaction between the polarized C -X bond and the phenyl ring on franx-stilbene (Table 6, entries 3-7, 10-14). In 2000, Solladie-Cavallo and coworkers reported a series of fluorinated carbocyclic ketones... 

Peroxy acid and alkene Transition state for oxygen transfer from the OH group of the peroxy acid to the alkene Acetic acid and epoxide ... 

More definitive evidence for the formation of an oxirene intermediate or transition state was presented recently by Cormier 80TL2021), in an extension of his earlier work on diazo ketones 77TL2231). This approach was based on the realization that, in principle, the oxirene (87) could be generated from the diazo ketones (88) or (89) via the oxocarbenes 90 or 91) or from the alkyne (92 Scheme 91). If the carbenes (90) (from 88) and (91) (from 89) equilibrate through the oxirene (87), and if (87) is also the initial product of epoxidation of (92), then essentially the same mixture of products (hexenones and ketene-derived products) should be formed on decomposition of the diazo ketones and on oxidation of the alkyne this was the case. 

Regioselectivity in opening of a-functionalized epoxides by treatment with trimethylamine dihydrofluoride has been observed. Thus, cw-isophorol epoxide gives exclusively 3-fluoro-l,2-diol, whereas from the trans isomer, 2-fluoro-l,3-diol is obtained as the main product together with 3-methylenecyclohexane-l,2-diol. This behavior has been discussed m terms of the influence of a-substituents on the transition state conformations [7 J] (equations 11 and 12). 

This model prediets that tri-substituted and tetra-substituted olefins would also be poor substrates. Thus it was not until 1994 that a study in the epoxidation of higher substituted olefins appeared. Indeed Jaeobsen revealed that tri-substituted olefins, and even tetra-substituted olefins ean be excellent substratesA new model was put forth that encompasses a skewed side-on approach of tri-substituted olefins to the Mn-oxo eomplex. The observation that certain tetrasubstituted olefins undergo epoxidation with good enantioseleetivity suggests that further studies are needed in order to fully understand the transition state geometry of the catalyst and substrate. 

A similar reaction patliway was found for tlie S u2 substitution of an epoxide witli a litliiuni aiprate duster [ 124]. tn contrast to tliat in tlie MeBr reaction, tlie ste-reodieniistry of tlie dectropb dic carbon center is already inverted in tlie transition state, providing tlie reason for tlie prefetLed "rrons-diaxial epoxide-opetiing" widely observed in syntlietic studies. Hie TS for tlie S 2 reaction of cydobexene oxide is shown in Eq. 10.12. 

The hydroxy oxygen of a peracid has a higher electrophilicity as compared to a carboxylic acid. A peracid 2 can react with an alkene 1 by transfer of that particular oxygen atom to yield an oxirane (an epoxide) 3 and a carboxylic acid 4. The reaction is likely to proceed via a transition state as shown in 5 (butterfly mechanism), where the electrophilic oxygen adds to the carbon-carbon n-hond and the proton simultaneously migrates to the carbonyl oxygen of the acid ... 

Evidently, the transition state for acid-catalyzed epoxide opening has an Sn2 -like geometry but also has a large amount of S]v-l-like carbocationic character- Since the positive charge in the protonated epoxide is shared by the more highly substituted carbon atom, backside attack of Br- occurs at the more highly substituted site. 

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