DMDO epoxidation

Scheme 5.65 Summary of reaction outcomes in DMDO epoxidation of glycals.

A typical closed-shell transition structure for DMDO epoxidation is exemplified by the epoxidation of E- and Z-2-butene. Baumstark and Vasquez have reported experimental studies that demonstrate the greater reactivity of Z-alkenes in the DMDO epoxidation of E/Z-pairs of alkenes . As anticipated, approach of the dioxirane ring to the Z-double bond in the less hindered manner, away from the methyl groups of DMDO,... 

The approach of DMDO to -2-butene is spiro in nature with nearly equally developing C—O bond distances (Figure 18). The 0—0 bond in the TS is elongated to 1.879 A. The calculated gas-phase enthalpy of activation (A77 = 13.5 kcalmoD ) is higher than the experimental A77 = 7.4 kcalmoD for the DMDO epoxidation of cyclohexene in acetone solvent while the calculated entropy of activation (—39.7 calmol K ) is in better agreement with experiment (—35.5 calmoD K ). 

DMDO epoxidation of cyclohexene (Table 5) is rednced by 4.1 kcalmoD when a single water molecule is hydrogen-bonded to the distal oxygen of DMDO (a bimolecular process relative to a prereaction clnster of DMDO, H2O) and by 6.3 kcalmoD with two complexed water molecules [B3LYP/6-311+G(d,p)]. The H-bonded DMDO-CH3OH prereaction cluster has a stabilization energy of —6.9 kcalmoD. The calculated barriers for the DMDO epoxidation of -2-butene with and without water catalysis are 11.0 and... 

These composite data strongly suggest that the presence of adventitious water or other hydrogen donors can markedly affect the observed rate of epoxidation. For example, Murray and Gu have reported AH = 5.0 kcalmol" for the DMDO epoxidation of cyclohexene in CDCI3 and 7.4 kcalmol" in acetone as solvent . Curci and coworkers also reported an a value of 9.3 kcalmol" for the DMDO epoxidation of isobutylene in acetone . These barriers are significantly lower than the 13-18 kcalmoD gas-phase barriers reported " at the B3LYP level of theory (Tables 3 and 4). Activation barriers of 12.6,... 

Asouti and Hadjiarapoglou have examined the DMDO epoxidation of hexamethyl Dewar benzene 18, which yields a mixture of the monoepoxide 19 and bis-epoxide 20 (Scheme 10). The ratio of the products depends on the amount of dioxirane employed surprisingly, with 1 equiv of DMDO, epoxide 19 was isolated in 97% yield, while with excess of DMDO the bis-epoxide 20 was quantitatively isolated. Epoxide 19 had been previously postulated <19880SC(VI)39> as a labile intermediate (not even detected due to rapid acid hydrolysis) in the peracid epoxidation of 18 <2000TL539>. 

Tabie 2 Solvent effect on product distribution in DMDO epoxidation... 

The transition state for the DMDO epoxidation of 19 is shown in the margin The faces of the olefin and the dioxirane ring are perpendicular to each other, whereas the faces of the two methyl DMDO substituents are parallel to the olefmic one. In the case of 19 the a-face attack is favored and therefore the p-epoxide p-1 is formed as minor diastereomer in 10 % yield. 

Dimethyldioxirane (DMDO), whose structure is shown below, is another reagent commonly used for alkene epoxidation. Write a mechanism for the epoxidation of (Z)-2-butene by DMDO, including a possible transition state structure. What is the by-product of a DMDO epoxidation ... 

---