Isotope effects epoxidations

The ring-closure mechanism of 2-chloroethanol has been studied on the basis of kinetic and equilibrium chlorine isotope effects. Epoxidation of the terminal double bond of farnesyl acetate has been achieved via the bromohydrin, obtained with NBS. A stereospecific method has been elaborated for the preparation of 1-alkynyloxiranes, starting from the monotosylate ester of acetylenic diols. 1-Alkynyloxiranes are also formed from a-hydroxy quaternary ammonium salts in alkaline medium (Eq. 57). ... 

The oxygen rebound mechanism was supported by experimental evidence including (1) high kinetic isotope effects, (2) partial positional or stereochemical scrambling, and (3) allylic rearrangements. For instance, in the presence of [Fe(TPP)Cl] and PhIO, dx-stilbene was stereospecihcally epoxidized. In addition, it was found that cis-stilbene was 15 times more reactive than trans-stilbene in competitive epoxidations. (see Figure 7.20). " ... 

FIGURE 24. Proposed transition stmcture for epoxidation of an aryl-substituted styrene (4-vinyl-biphenyl, left) and the transition stmcture for epoxidation of 1,3-butadiene (right, calculated at the QC1SD/6-31G level). The theoretical isotope effects were calculated at the MP2/6-31G level using the Bell tunneling correction. For a discussion see Reference 19b. Bond lengths are given in A... 

For isotope effect studies epoxidation of cyclohexanone with f erf-butyl hydroperoxide in the presence of DBU in 1,2-dichloroethane (DCE) at 22 °C was chosen (Equation (42)).76... 

The predicted isotope effects for the addition transition states are of about 1.030 at C3 and 1.008 at C2 and small KIEs for remaining carbons, as can be expected for a major bonding change at C3 and minor at C2. Predicted isotope effects for the ring-closure step are 1.012 at C2 near unity at C3. The value of 1.012 is near the low end of primary 13C KIEs, but consistent with relatively low isotope effects observed in other epoxidation reactions (vide infra). Overall, KIEs support the addition of a peroxy anion to the enone as the rate-limiting step. 

Table 3 Experimental and predicted 13C kinetic isotope effects for the epoxidation of cyclohexanone in the presence of DBU...

The analysis of 13C integrations in context of different mechanistic models (concerted, biradical or per epoxide) led authors to the conclusion that 13C integration pattern is consistent with the perepoxide model and supports mechanism proposed earlier from 2H KIEs.90 The following intramolecular 13C isotope effects for formation of 43 were calculated from changes of 13C integrations (Figure 18). 

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