Download Oxacyclopropane (Epoxide) Synthesis: Epoxidation by

Peroxycarboxylic acids deliver oxygen atoms to double bonds.
Peroxycarboxylic acids have the general formula:
These compounds react with double bonds because one of the oxygen atoms is
electrophilic.
The resulting products are an oxacyclopropane and a carboxylic acid.
This reaction is referred to as an “epoxidation.” The older common name of an
oxacyclopropane was an “epoxide.”
Commonly used peroxycaraboxylic acids for this reaction are metachloroperoxybenzoic acid (MCPBA) which is somewhat shock sensitive, and
magnesium monoperoxyphthalate (MMPP).
The mechanism of this epoxidation reaction involves a cyclic transition state:
The peroxycarboxylic acid reactivity with double bonds increases with alkyl
substitution, allowing for selective oxidations:
Hydrolysis of oxacyclopropanes furnishes the products of anti
dihydroxylation of an alkene.
Ring opening of oxacyclopropanes with water produces anti vicinal diols.
12-11 Vicinal Syn Dihydroxylation with Osmium
Tetroxide
The reaction of osmium tetroxide with alkenes yields syn vicinal diols in a two step
process:
The reaction mechanism involves the concerted addition of the osmium tetroxide
to the  bond of the alkene:
Catalytic amounts of osmium tetroxide in the presence of an oxidizing agent
(H2O2) to regenerate the spent osmium tetroxide are often used, due to the
expense and toxicity of OsO4.
An older reagent for vicinal syn dihydroxylation of alkenes is KMnO4.
This reagent is less useful than OsO4 because of its tendency towards
overoxidation.
The deep purple KMnO4 is converted into a brown precipitate, (MnO2) during the
reaction, which can serve as a useful test for the presence of alkenes.
12-12 Oxidative Cleavage: Ozonolysis
The mildest reagent capable of breaking both the  and  bonds in a double bond
is ozone, O3. This process is known as “ozonolysis.”
Ozone is produced by an electrical discharge in dry oxygen in a instrument called
an ozonator.
The initial product of the reaction of ozone with an alkene is an ozonide which is
then directly reduced to two carbonyl products.
12-13 Radical Additions: Anti-Markovnikov Product
Formation
Hydrogen bromide can add to alkenes in anti-Markovnikov fashion: a
change in mechanism.
The reaction products from the treatment of 1-butene with HBr depend upon the
presence or absence of molecular oxygen in the reaction mixture:
In the presence of oxygen, a radical chain sequence mechanism leads to the antiMarkovnikov product.
Small amounts of peroxides (RO-OR) are formed in alkene samples stored in the
presence of air (O2).
The peroxides initiate the radical chain sequence mechanism, which is much
faster than the ionic mechanism operating in the absence of peroxides.