Page 1100 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1076 The most widely used reagent for oxidation of alkenes to glycols is osmium
tetroxide. Osmium tetroxide is a highly selective oxidant that gives glycols by a
CHAPTER 12 39
stereospecific syn addition. The reaction occurs through a cyclic osmate ester that is
Oxidations formed by a 3+2 cycloaddition. 40

H
R H R O O
+ OsO 4 Os RCHCHR
R H O O
R H HO OH

The reagent is toxic and expensive but these disadvantages are minimized by methods
that use only a catalytic amount of osmium tetroxide. A very useful procedure involves
an amine oxide such as morpholine-N-oxide as the stoichiometric oxidant. 41

R H
R H OsO H R N + O – R H
4 R 3
O O H R + OsO + R N
4
3
H R H O
2
Os HO OH
O O
t-Butyl hydroperoxide, 42 barium chlorate, 43 or potassium ferricyanide 44 can also be
used as oxidants in catalytic procedures.
Scheme 12.6 provides some examples of oxidations of alkenes to glycols by both
permanganate and osmium tetroxide. The oxidation by KMnO in Entry 1 is done in
4
cold aqueous solution. The reaction is very sensitive to the temperature control during
the reaction. The reaction in Entry 2 was also done by the catalytic OsO method using
4
N-methylmorpholine-N-oxide in better (80%) yield. Note that the hydroxy groups are
introduced from the less hindered face of the double bond. Entries 3 to 5 illustrate
several of the catalytic procedures for OsO oxidation. In each case the reaction is
4
a stereospecific syn addition. Note also that in Entries 4 and 5 the double bond is
conjugated with an EWG substituent, so the range of the reaction includes deactivated
alkenes.
Osmium tetroxide oxidations can be highly enantioselective in the presence of
chiral ligands. The most highly developed ligands are derived from the cinchona
alkaloids dihydroquinine (DHQ) and dihydroquinidine (DHQD). 45 The most effective

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40
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43
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45
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