Oxacyclopropane Epoxide Synthesis: Epoxidation by Peroxycarboxylic Acids

1
Oxacyclopropane (Epoxide) Synthesis Epoxidation
by Peroxycarboxylic Acids
12-10
Oxacyclopropanes contain a single oxygen atom
connected to two carbons to form a three membered
ring. Oxacyclopropanes may be converted into
vicinal anti diols.
2
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.
3
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 meta-chloroperoxybenzoic
acid (MCPBA) which is somewhat shock sensitive,
and magnesium monoperoxyphthalate (MMPP).
4
The mechanism of this epoxidation reaction
involves a cyclic transition state
The peroxycarboxylic acid reactivity with double
bounds increases with alkyl substitution,
allowing for selective oxidations
5
Hydrolysis of oxacyclopropanes furnishes the
products of anti dihydroxylation of an
alkene. Ring opening of oxacyclopropanes with
water produces anti vicinal diols.
6
Vicinal Syn Dihydroxylation with Osmium Tetroxide
12-11
The reaction of osmium tetroxide with alkenes
yields syn vicinal diols in a two step process
7
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.
8
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.
9
Oxidative Cleavage Ozonolysis
12-12
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.
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The mechanism of ozonolysis proceeds through a
molozonide which breaks apart into two fragments
which then recombine to form the ozonide