Page 1122 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1098 Higher concentrations of DMDO can be obtained by extraction of a 1:1 aqueous
dilution of the distillate by CH Cl CHCl ,orCCl . 88 Another method involves in
3
2
2
4
CHAPTER 12 89
situ generation of DMDO under phase transfer conditions.
Oxidations
O
CH CCH , O
3
3
KOSO OOH
2
) OCH Ph
CH 3 CH CH(CH ) OCH Ph CH CH CH(CH 2 3 2
2
2 3
3
pH 7.8 buffer,
+
–
n-Bu N HSO ,
4
4
The yields and rates of oxidation by DMDO under these in situ conditions depend on
pH and other reaction parameters. 90
Various computational models agree that the reaction occurs by a concerted
mechanism. 91 Comparison between epoxidation by peroxy acids and dioxiranes
suggests that they have similar transition structures.
CH 3
CH 3 O
O CH CH 3
O 3 R O
R
R
R
Kinetics and isotope effects are consistent with this mechanism. 92 The reagent is
electrophilic in character and reaction is facilitated by ERG substituents in the alkene.
A B3LYP/6-31G computation found the transition structures and E values shown in
∗
a
Figure 12.11.
Similarly to peroxycarboxylic acids, DMDO is subject to cis or syn stere-
oselectivity by hydroxy and other hydrogen-bonding functional groups. 93 However
a study of several substituted cyclohexenes in CH CN −H O suggested a domin-
3 2
ance by steric effects. In particular, the hydroxy groups in cyclohex-2-enol and

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