Chemistry Reference
In-Depth Information
p
-Tol
N
Me
1)
188b
2) MeMgBr
188a
(+)-
192
N
OPr
Pd(L*)
2
MeBr
191
5
[
α
]
D
=
−
343°
p
-Tol
OPr
Me
p
-Tol
n
N
Me
193
[
α
]
D
=
+
165°
p
-Tol
N
Pd(L*)
2
Br
5
Me
N
p
-Tol
1)
188b
2) MeMgBr
192
N
L* = PPh[(
S
)-2-methylbutyl]
2
(
−
)-
192
PrO
PrO
5
p
-Tol
[
α
]
D
=
+
233°
Me
n
194
[
α
]
D
=
−
171°
Scheme 13.20.
could be achieved by using the initiators based on chiral binaphthyl groups (
195
-
197
)
to give optically active polymers [270-273]. In particular, the polymerization of
188b
in
the presence of
197
was found to give excellent helix-sense selectivity of over 95% [271].
The binaphthyl moiety served as both an effi cient chiral initiator for the asymmetric
polymerization and a stabilizer to prevent racemization of the resulting helical polymers.
Effective initiators based on the phenyl groups possessing optically active
ortho
- substit-
uents, such as chiral aminocarbonyl, oxazoline, and imidazoline groups, have been
developed [274] .
p
-Tol
N
R
OMe
R=
MeO
H
H
N
Pd(PMe
2
Ph)I
p
-Tol
195
196
197
13.3.1.5. Polymerization of Isocyanates
Polyisocyanate is a synthetic polymer that
has a rigid main chain based on the structure of 1-nylon [275,276]. Although no asym-
metric carbon center is contained in the main chain, polyisocyanate can be optically
active since it possesses a dynamic helical conformation [277,278]. Asymmetric anionic
polymerization of achiral isocyanates (
198
) was successfully performed in the presence
of various optically active anionic initiators (
199
-
205
) (Scheme 13.21 ) [279 - 281] . Among
the initiators, lithium amide
199
showed the best chiral-inducing ability:
199
gave the
polymers derived from
198a
and
198d
with optical rotations of [α ]
365
+416 ° and [
]
365
+819°, respectively. The optical activity of the polymers was presumably attributed to
the helicity of the polymer main chains induced by a chiral initiator residue, which was
attached to the end of the chain, rather than to a chiral initiator residue itself. Chiral
discrimination of racemic amino acid derivatives using a single-handed helical polyiso-
cyanate bearing a crown ether moiety was reported [282]. A single-handed helicity
induction of achiral polyisocyanates by means of chiral acid-base interaction or host-
guest complexation has recently been developed [236,283 - 286] .
α
