Chemistry Reference
In-Depth Information
formation of 3-chloro-2-butanol, where the
cis
and
trans
epoxides are converted to the
erythro
and
threo
trans
2,3-epoxybutanes take place with inversion of configuration when aluminum 5,10,15,20-
tetraphenylporphine and zinc 5,10,15,20-tetraphenyl-21-methylporphine catalysts are used. To
explain the inversion, Inoue and coworkers proposed a linear transition state mechanism that involves
a simultaneous participation of two porphyrin molecules [
19
]. One porphyrin molecule
accommodates a coordinative activation of the epoxide and the other one serves as a nucleophile to
attack the coordinated epoxide from the back side.
Potassium hydroxide
-chlorohydrins. Inoue and coworkers [
19
] found, however, that polymerizations of
cis
and
alkoxide
polymerizes racemic propylene oxide with better than 95%
regioselectivity of cleavage at the bond between oxygen and the carbon substituted by two hydrogens.
The product, however, is atactic. Both (
or
) propylene oxides react at the same rate. This shows
that the initiator is unable to distinguish between the two enantiomers of propylene oxide. When
t
R
) and (
S
-butyl ethylene oxide is polymerized by KOH it yields a crystalline product. This product is different
in its melting point, X-ray diffraction pattern, and solution-NMR spectra from the typical isotactic
polymers. It contains alternating isotactic and syndiotactic sequences [
31
]. It was suggested [
34
] that
this may be a result of the configuration of the incoming monomer being opposite to that of the
penultimate unit. Chelation of the paired cation (K
L
) with the last and the next to the last oxygen is
visualized. Geometry of such a chelate is dictated by the requirement that the penultimate
-butyl
group be in an equatorial conformation. This makes it reasonable to postulate that the necessary
preference for the incoming monomer is to be opposite to that of the penultimate unit [
31
]:
t
R
H
H
R
H
H
H
O
R
O
O
R
+
O
R
O
K
K
O
O
O
H
R
When phenyl glycidyl ethers are polymerized under the same conditions, the steric arrangement is
all isotactic rather than isotactic-syndiotactic [
31
]. Price explained that on the basis of the oxygen in
C
6
H
6
-O-CH
2
seeking to coordinate potassium ions in the transition state [
31
]. In the case of
t
-butylethylene oxide, on the other hand, the tertiary butyl group tends to be as far as possible
away from the potassium ion [
34
]. This is supported by the observation that
p
-methoxy and
p
-methyl
groups on phenyl glycidyl ether increase the crystalline portion of the polymer, while the
p
-chloro
substituent decreases it [
31
].
Most stereoselective coordination catalysts polymerize propylene oxide to yield polymers that
contain high ratios of isotactic to syndiotactic sequences. Large portions of amorphous materials,
however, are also present in the same materials. These amorphous portions contain head to head units
that are imperfections in the structures [
29
,
30
]. For every head to head placement, one (
R
) monomer
is converted to an (
) unit in the polymer [
23
]. This shows that at the coordination sites abnormal ring
openings occur at the secondary carbon with an inversion of the configuration and results in head to
head placements [
23
,
31
]. Also,
S
erythro
and
threo
isomers units are present. The isotactic portion
consists almost exclusively of the
erythro
isomer while other amorphous fraction contains 40-45%
erythro
[
31
]
All the above information is indirect evidence that a typical catalyst, such as (C
2
H
5
)
2
Zn-H
2
O
contains isotactic and amorphous sites. The isotactic sites are very selective and coordinate either
with (
and 55-60%
threo
R
) or with (
S
) monomers. The amorphous sites, on the other hand, coordinate equally well with
both (
) monomers. In addition, there is little preference for attack on either the primary or the
secondary carbons during the ring-opening reactions [
23
].
R
) and (
S
