7.9 Stereochemistry of Chemical Reactions

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Monosubstituted Cyclohexanes
• Substituents can occupy an axial or equatorial
position
• Equatorial is preferred by larger groups. Why?
7.3 Monosubstituted Cyclohexanes. Conformational Analysis
1
1,3-Diaxial Interactions
• 1,3-Diaxial Interactions: van der Waals repulsions between an
axial substituents on a cycloalkane ring
2
4
Problems
1) What is the energy cost of a 1,3-diaxial strain
in methylcyclohexane?
2) What about bromocyclohexane?
3) Which of the two molecules above will have
a higher percentage of its molecules in the
equatorial conformation?
More on Ring Flip
• Note that the “up” substituent remains up and
the “down” substituent remains down after
chair interconversion
7
Disubstituted Cyclohexanes
1-chloro-2-methylcyclohexane
• Trans: the two groups have an up-down
relationship
• Cis: the two groups have a down-down (or upup) relationship
8
Conformational Analysis
• For disubstituted derivatives, the larger group
will preferentially occupy the equatorial
position
7.4 Disubstituted Cyclohexanes
9
Problem
1) Which subsituent will most likely occupy the
equatorial position for a greater amount of
time in cis-1-chloro-2-methylcyclohexane?
2) Prove this with calculation.
Polycyclic Molecules
• Polycyclic molecules: compounds with two or
more rings fused together
– Spirocyclic: two rings that have only a single
common atom
– Bicyclic: two rings that share two or more
common atoms
11
7.6 Bicyclic and Polycyclic Compounds
12
Classification and Nomenclature
• Fused and bridged bicyclic compounds
7.6 Bicyclic and Polycyclic Compounds
13
Naming Bicyclic Systems
• Indicate the number of rings using the prefix “bicyclo-”
• Indicate the bridge lengths
– Number of atoms connecting one bridgehead atom to
another (excluding the bridgehead atoms)
– Separate by full periods and place in square brackets.
• Cited in decreasing order of size (e.g. [3.2.1])
• the name of the hydrocarbon indicating the total number
of skeletal atoms
Problems
• Name the following polycyclic molecules:
Norbornane
Decalin
• If you were just given the IUPAC names for a couple
of bicyclic molecules, how would you tell the
difference between a fused and a bridged variety?
Cis and Trans Ring Fusion: Decalin
7.6 Bicyclic and Polycyclic Compounds
16
• Each ring in cis-decalin can undergo ring flip
• Ring fusion causes trans-decalin to be
conformationally locked
17
Ring Fusion with Small Rings
• Bicyclic compounds with small rings are
restricted to cis ring fusion
• Trans fusion would incur too much ring strain
7.6 Bicyclic and Polycyclic Compounds
18
Steroids
• Organic compounds with 20 carbon,
tetracyclic core
– Variations in substituents dictate biological
activity
7.6 Bicyclic and Polycyclic Compounds
19
Steroids
• Many consist of all trans-fused rings
– No conformational change
• Many have methyl groups at C-10 and C-13
7.6 Bicyclic and Polycyclic Compounds
20
Stereochemistry of Cycloalkene Reactions
Addition Reactions
• Syn-addition:
• Anti-addition:
7.9 Stereochemistry of Chemical Reactions
22
Stereochemistry of Bromine Addition
• Addition of bromine to an alkene is a highly
stereoselective reaction
• Exclusively anti-addition
– Gives all trans products
7.9 Stereochemistry of Chemical Reactions
23
Stereochemistry of Halohydrin Formation
• Exclusively anti-addition
– Gives all trans products
Stereochemistry of Hydroboration-Oxidation
• Hydroboration is a stereospecific syn-addition
7.9 Stereochemistry of Chemical Reactions
26
Stereochemistry of Hydroboration-Oxidation
• The oxidation of organoboranes is a
stereospecific substitution reaction
7.9 Stereochemistry of Chemical Reactions
28
Stereochemistry of Hydroboration-Oxidation
• The two steps of hydroboration-oxidation
result in net syn-addition of H-OH to the
alkene
• Note that the trans designation of the name
has nothing to do with the way H-OH added
7.9 Stereochemistry of Chemical Reactions
29
Stereochemistry of Hydrogenation
• Catalytic hydrogenation is a stereospecific synaddition
7.9 Stereochemistry of Chemical Reactions
30
Problem
• Draw the products for the following reactions
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