Chemistry Reference
In-Depth Information
t
-Bu
m
Ar
2
NC
186
R
3
P
PR
3
Cl
R
3
P
Pd
C
N
Pt Cl
181
PR
3
NH
N
TPP =
t
-Bu
Ar
2
= TPPOCOC
6
H
4
N
HN
CO
2
TPP
n
187
t
-Bu
Scheme 13.18.
authors postulated that hydrophobic and chiral ionic interaction in water might be neces-
sary for the selective helix formation and the memory [262-264].
13.3.1.4.2. 1,2 - Diisocyanobenzenes
Aromatizing polymerization of 1,2 - diisocyanoben-
zenes (
188
) using palladium or nickel initiators, which leads to the formation of
poly(quinoxalin - 2,3 - diyl)s (
189
,
190
), has been developed (Scheme 13.19). Since the
poly(quinoxalin-2,3-diyl)s adopt a stable helical structure, they can be optically active if
the polymerization proceeds with a preference for a single-handed helix. Optically active
poly(quinoxalin-2,3-diyl)s were synthesized for the fi rst time by Ito and coworkers in
1992 [265 - 269] . Highly helix - sense - selective polymerization of 1,2 - diisocyano - 3,6 - di -
p
-
tolylbenzene
(188a
) to form a mixture of diastereomeric pentamers (
192
) was achieved
by using optically active arylpalladium initiator (
191
), followed by the separation into
(+) -
192
and (− ) -
192
by chiral HPLC. The enantiomerically pure
192
can be used as an
initiator for further asymmetric polymerization of
188b
to form single-handed helical
copolymers,
193
and
194
, after the treatment with MeMgBr/ZnCl
2
, which exhibits sym-
metrical CD spectra and the same optical rotations with opposite sign (Scheme 13.20)
[265]. Thorough investigation of the helical stabilities of
189
and
190
revealed that the
rate of racemization depends on (i) the bulkiness of the substituents at 5- and 8-positions
on the quinoxaline rings; (ii) the oligomerization degree; and (iii) the bulkiness of the
terminus group [269]. Furthermore, the enantioselective aromatizing polymerization
R
1
R
1
R
1
MeMgBr
ZnCl
2
R
2
R
2
R
2
NC
N
Me
N
Me
PdL
2
MeBr
R
2
R
2
R
2
NC
N
PdL
2
Br
N
Me
n
n
R
1
R
1
R
1
188a
: R
1
=
p
-MeC
6
H
4
, R
2
= H
188b
: R
1
= Me, R
2
= PrOCH
2
1
1
Scheme 13.19.
