7.12
Molecules
with
Multiple Chirality Centers
Dr. Wolf's CHM 201 & 202
7-1
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
Dr. Wolf's CHM 201 & 202
7-2
Example
O
HOCH2CH—CH—CH—CHCH
OH OH OH
OH
4 chirality centers
16 stereoisomers
Dr. Wolf's CHM 201 & 202
7-3
Cholic acid
HO H
CH3
H
H
Dr. Wolf's CHM 201 & 202
CH2CH2CO2H
H
H3C
HO
CH3
H
OH
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
7-4
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
Dr. Wolf's CHM 201 & 202
7-5
How many stereoisomers?
3-Penten-2-ol
E
R
E
HO H
R
Z
HO
Dr. Wolf's CHM 201 & 202
S
H
Z
H
OH
S
H
OH
7-6
7.13
Chemical Reactions That
Produce Diastereomers
Dr. Wolf's CHM 201 & 202
7-7
Stereochemistry of Addition to Alkenes
C
C
+
E—Y
E
C
C
Y
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)
Dr. Wolf's CHM 201 & 202
7-8
Bromine Addition to trans-2-Butene
Br2
S
R
S
R
meso
anti addition to trans-2-butene gives meso
diastereomer
Dr. Wolf's CHM 201 & 202
7-9
Bromine Addition to cis-2-Butene
S
R
Br2
R
50%
+
S
50%
anti addition to cis-2-butene gives racemic mixture
of chiral diastereomer
Dr. Wolf's CHM 201 & 202
7-10
Epoxidation of trans-2-Butene
S
R
RCO3H
+
R
50%
S
50%
syn addition to trans-2-butene gives racemic
mixture of chiral diastereomer
Dr. Wolf's CHM 201 & 202
7-11
Epoxidation of cis-2-Butene
S
R
RCO3H
R
S
meso
syn addition to cis-2-butene gives meso
diastereomer
Dr. Wolf's CHM 201 & 202
7-12
Stereospecific reaction
Of two stereoisomers of a particular starting
material, each one gives different
stereoisomeric forms of the product
Related to mechanism: terms such as
syn addition and anti addition refer to
stereospecificity
Dr. Wolf's CHM 201 & 202
7-13
cis-2-butene
bromination
anti
2R,3R + 2S,3S
anti
meso
.
trans-2-butene bromination
Stereospecific reactions
cis-2-butene
epoxidation
syn
meso
trans-2-butene epoxidation
syn
2R,3R + 2S,3S
Dr. Wolf's CHM 201 & 202
7-14
Stereoselective reaction
A single starting material can give two or more
stereoisomeric products, but gives one of them
in greater amounts than any other
H
H
CH3
H
CH3
H2
CH3
+
CH2
H
68%
Dr. Wolf's CHM 201 & 202
H
CH3
Pt
CH3
32%
7-15
7.14
Resolution of Enantiomers
Separation of a racemic mixture into its two
enantiomeric forms
Dr. Wolf's CHM 201 & 202
7-16
Strategy
enantiomers
C(+)
Dr. Wolf's CHM 201 & 202
C(-)
7-17
Strategy
enantiomers
C(+)
C(-)
2P(+)
C(+)P(+)
C(-)P(+)
diastereomers
Dr. Wolf's CHM 201 & 202
7-18
Strategy
enantiomers
C(+)
C(-)
2P(+)
C(+)P(+)
C(-)P(+)
C(+)P(+)
C(-)P(+)
diastereomers
Dr. Wolf's CHM 201 & 202
7-19
Strategy
enantiomers
C(+)
C(+)
P(+)
C(-)
2P(+)
C(+)P(+)
C(-)P(+)
C(+)P(+)
C(-)P(+)
diastereomers
Dr. Wolf's CHM 201 & 202
P(+)
C(-)
7-20
7.15
Stereoregular Polymers
atactic
isotactic
syndiotactic
Dr. Wolf's CHM 201 & 202
7-21
Atactic Polypropylene
random stereochemistry of methyl groups
attached to main chain (stereorandom)
properties not very useful for fibers etc.
formed by free-radical polymerization
Dr. Wolf's CHM 201 & 202
7-22
Isotactic Polypropylene
stereoregular polymer; all methyl groups on
same side of main chain
useful properties
prepared by coordination polymerization
under Ziegler-Natta conditions
Dr. Wolf's CHM 201 & 202
7-23
Syndiotactic Polypropylene
stereoregular polymer; methyl groups alternate
side-to-side on main chain
useful properties
prepared by coordination polymerization under
Ziegler-Natta conditions
Dr. Wolf's CHM 201 & 202
7-24
7.16
Chirality Centers
Other Than Carbon
Dr. Wolf's CHM 201 & 202
7-25
Silicon
b
b
a
a
Si
c
d
d
Si
c
Silicon, like carbon, forms four bonds in its stable
compounds and many chiral silicon compounds
have been resolved
Dr. Wolf's CHM 201 & 202
7-26
Nitrogen in amines
b
b
very fast
a
N
c
:
a
:
N
c
Pyramidal geometry at nitrogen can produce
a chiral structure, but enantiomers
equilibrate too rapidly to be resolved
Dr. Wolf's CHM 201 & 202
7-27
Phosphorus in phosphines
b
b
slow
a
P
c
:
a
:
P
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
Dr. Wolf's CHM 201 & 202
7-28
Sulfur in sulfoxides
b
b
slow
a
+S
O_
:
a
:
S+
O_
Pyramidal geometry at sulfur can produce a chiral
structure; pyramidal inversion is slow and
compounds of the type shown have been resolved
Dr. Wolf's CHM 201 & 202
7-29
End of Chapter 7
Dr. Wolf's CHM 201 & 202
7-30