Chemistry Reference
In-Depth Information
]
D
over
4,000°). Although the polymers
171
with high molecular weight are insoluble and their
conformation cannot be directly elucidated, the helical structure of
171
has been verifi ed
by NMR analyses [242,243] and crystallographic analyses of the corresponding oligomers
[244] .
as an effective initiator, giving optically active
171
with high optical rotations ([
α
C
8
H
17
CCl
3
CO
2
Me
∗
Ph
∗
∗
O
OLi
n
OLi
LiO
H
1
1
1
1
1
3 - Phenylpropanal (
175
) was found to be a suitable monomer for the synthesis of
polyaldehydes with single-handed screw sense. The asymmetric polymerization of
175
initiated by Grignard reagent/(
−
) -
Sp
complexes such as EtMgBr/(
−
) -
Sp
and OctMgBr/
56 ° , which may
be attributed to a predominant single-handed helical conformation of the main chain
[246]. Detailed investigations of the corresponding oligomers revealed that the polymer
had a 3-phenylpropoxy group at the
(
−
) -
Sp
afforded the polymers with negative rotations of [
α
]
365
−
33 ° to
−
α
-end and a (2-phenylethyl)carbonyl group at the
ω
-end, indicating that the true initiator for the polymerization was not Grignard reagents
but (1-phenylpropoxy)magnesium bromide (
177
) formed by Tishchenko reaction shown
in Scheme 13.16 .
O
MgBr
Sp
O
1) RMgBr/
Sp
2)
175
Ph
H
H
O
175
MgBr
Sp
O
Ph
O
Ph
H
Ph
O
175
Ph
R
Ph
R
O
MgBr
Sp
O
O
O
Ph
Ph
O
+
H
H
n
O
Ph
Ph
R
176
Ph
Ph
177
Scheme 13.16.
13.3.1.4. Polymerization of Isocyanides
13.3.1.4.1. Monoisocyanides
Polyisocyanides are known to have a 4
1
helical conforma-
tion when they have a bulky substituent due to the steric repulsion between the adjacent
substituents [247-249]. The conformation is rigid enough to form two conformational
isomers with right- (
P
) and left - handed (
M
) helices, which usually exist in a 1:1 mixture
even in solution. The fi rst optically active polyisocyanide was obtained by chromato-
graphic resolution of poly(
t -
butyl isocyanide) using poly((
S
) -
s
- butyl isocyanide) as a
chiral stationary phase [250]. The CD spectral analyses revealed that the levorotatory
