Sections 129 to 1212

1
Sections 12-9 to 12-12

• Lectures 12 and 13

2
Diazomethane, Carbenes and Cyclopropane Synthesis
Section 12-9
Cyclopropanes can be prepared by the addition of
a carbene to the double bond of an alkene. A
carbene has the general structure, R2C, in which
the central carbon is surrounded by six electrons
(sextet), and is thus electron deficient. The
electron-deficient carbene readily adds to an
electron rich alkene.
3
Diazomethane forms methylene (CH2), which
converts alkenes into cyclopropanes. H2C can be
produced from the decomposition of diazomethane
4
Methylene reacts with an alkene to form a
cyclopropane ring. The reaction is
stereospecific, with retention of the original
double bond configuration.
5
Halogenated carbenes and carbenoids also give
cyclopropanes. Treatment of chloroform with
strong base causes an elimination reaction to
form a dichlorocarbene. Dichlorocarbene reacts
with alkenes to produce cyclopropanes.
6
Diazomethane preparation can be dangerous. The
Simmons-Smith reaction uses ICH2ZnI
instead. This is an example of a carbenoid, a
carbenelike substance that converts alkenes into
cyclopropanes stereospecifically.
7
Oxacyclopropane Synthesis Epoxidation by
Peroxycarboxylic Acids
12-10
Oxacyclopropanes contain a single oxygen atom
connected to two carbons in a three-membered
ring. Oxacyclopropanes may be converted into
vicinal diols.
8
Peroxycarboxylic acids deliver oxygen atoms to
double bonds. Peroxycarboxylic acids have the
general formula
9
Peroxyacids react with double bonds because one
of the oxygen atoms is electrophilic. The
resulting products are an oxacyclopropane and a
carboxylic acid.
10
This reaction is referred to as an
epoxidation. A commonly used peroxycaraboxylic
acids for this reaction are meta-chloroperoxybenzo
ic acid (MCPBA)
11
The mechanism of this epoxidation reaction
involves a cyclic transition state
12
The peroxycarboxylic acid reactivity with double
bonds increases with alkyl substitution, allowing
for selective oxidations
13
Hydrolysis of oxacyclopropanes produces the
vicinal diol
14
Vicinal Syn Dihydroxylation with Osmium Tetroxide
12-11
The reaction of osmium tetroxide with alkenes
yields syn vicinal diols in a two step process
15
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.
16
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.
17
Ozonolysis of Alkenes
18
Ozonolysis Mechanism
19
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