7'12 Molecules with Multiple Chirality Centers

1
7.12MoleculeswithMultiple Chirality Centers
2
How many stereoisomers?

• maximum number of stereoisomers 2n
• where n number of structural units capable of
  stereochemical variation
• structural units include chirality centers and
  cis and/or trans double bonds
• number is reduced to less than 2n if meso forms
  are possible

3
Example

• 4 chirality centers
• 16 stereoisomers

4
Cholic acid

• 11 chirality centers
• 211 2048 stereoisomers
• one is "natural" cholic acid
• a second is the enantiomer of natural cholic acid
• 2046 are diastereomers of cholic acid

5
How many stereoisomers?

• maximum number of stereoisomers 2n
• where n number of structural units capable of
  stereochemical variation
• structural units include chirality centers and
  cis and/or trans double bonds
• number is reduced to less than 2n if meso forms
  are possible

6
How many stereoisomers?

• 3-Penten-2-ol

R
E
E
S
H
OH
HO
H
R
Z
Z
S
OH
H
H
HO
7
7.13 Chemical Reactions That Produce
Diastereomers
8
Stereochemistry of Addition to Alkenes

• In order to know understand stereochemistry of
  product, you need to know two things
• (1) stereochemistry of alkene (cis or trans Z
  or E)
• (2) stereochemistry of mechanism (syn or anti)

9
Bromine Addition to trans-2-Butene
S
R
Br2
S
R
meso

• anti addition to trans-2-butene gives meso
  diastereomer

10
Bromine Addition to cis-2-Butene
R
S
Br2

S
R
50
50

• anti addition to cis-2-butene gives racemic
  mixture of chiral diastereomer

11
Epoxidation of trans-2-Butene
S
R
RCO3H

R
S
50
50

• syn addition to trans-2-butene gives racemic
  mixture of chiral diastereomer

12
Epoxidation of cis-2-Butene
R
S
RCO3H
S
R
meso

• syn addition to cis-2-butene gives meso
  diastereomer

13
Stereospecific reaction

• Of two stereoisomers of a particular starting
  material, each one gives differentstereoisomeri
  c forms of the product
• Related to mechanism terms such assyn addition
  and anti addition refer tostereospecificity

14
.

• Stereospecific reactions

15
Stereoselective reaction

• A single starting material can give two or
  morestereoisomeric products, but gives one of
  themin greater amounts than any other

H
CH3

H
CH3
32
68
16
7.14 Resolution of Enantiomers

• Separation of a racemic mixture into its two
  enantiomeric forms

17
Strategy
enantiomers
18
Strategy
enantiomers
2P()
diastereomers
19
Strategy
enantiomers
C()P()
2P()
C(-)P()
diastereomers
20
Strategy
C()
enantiomers
P()
C()P()
2P()
C(-)P()
P()
diastereomers
C(-)
21
7.15Stereoregular Polymers

• atactic
• isotactic
• syndiotactic

22
Atactic Polypropylene

• random stereochemistry of methyl groups attached
  to main chain (stereorandom)
• properties not very useful for fibers etc.
• formed by free-radical polymerization

23
Isotactic Polypropylene

• stereoregular polymer all methyl groups
  onsame side of main chain
• useful properties
• prepared by coordination polymerization under
  Ziegler-Natta conditions

24
Syndiotactic Polypropylene

• stereoregular polymer methyl groups alternate
  side-to-side on main chain
• useful properties
• prepared by coordination polymerization under
  Ziegler-Natta conditions

25
7.16Chirality CentersOther Than Carbon
26
Silicon
b
b
a
a
d
d
Si
Si
c
c

• Silicon, like carbon, forms four bonds in its
  stable compounds and many chiral silicon
  compounds have been resolved

27
Nitrogen in amines
b
b
very fast
a
a


N
N
c
c

• Pyramidal geometry at nitrogen can produce a
  chiral structure, but enantiomers equilibrate too
  rapidly to be resolved

28
Phosphorus in phosphines
b
b
slow
a
a


P
P
c
c

• Pyramidal geometry at phosphorus can produce a
  chiral structure pyramidal inversion slower
  than for amines and compounds of the type shown
  have been resolved

29
Sulfur in sulfoxides
b
b
slow
a
a


S
S


O_
O_

• Pyramidal geometry at sulfur can produce a
  chiral structure pyramidal inversion is slow
  and compounds of the type shown have been resolved

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End of Chapter 7