Chemistry Reference
In-Depth Information
polymer had a (
P
)-helical conformation and that the dextrorotatory polymer had a (
M
) -
helical conformation [251] . Helix - sense - selective polymerization of achiral bulky isocya-
nides in the presence of optially active catalysts has also been developed. Nolte and
Drenth reported the catalyst consisting of Ni(CNR)
4
(ClO
4
)/optically active amines was
effective to synthesize single-handed helical polymers with enantiomeric excess of up to
83% [252,253] . Dinuclear
- allyl Ni(II) complexes (
178
-
180
) were also employed for the
polymerization although the molecular weight of the obtained polymers was rather low
[254]. Takahashi and coworkers developed the helix-sense-selective polymerization of
isocyanides using a dinuclear Pd/Pt complex containing a single-handed oligomeric
isocyanide chain (
183
) prepared from chiral aryl isocyanide
182
with
181
. The complex
183
effi ciently promoted further polymerization of achiral aryl isocyanide (
184
) in a
helix - sense - selective manner (Scheme 13.17 ) [255,256] . This methodology was applied
to the synthesis of polyisocyanides bearing porphyrins as a pendant group. Helix-sense-
selective polymerization of aryl isocyanide
186
with
181
smoothly proceeded even in the
presence of bulky porphyrin groups to give the polymer in which porphyrin units were
regularly arranged face - to - face (Scheme 13.18 ) [257 - 259] . The helical sense of the poly-
isocyanides can be unequivocally determined based on the exciton-coupled CD spectra
of the porphyrin Soret band [260].
π
F
3
C
MeO
O
Ph
N
Ph
MeO
F
3
C
F
3
C
O
O
O
O
O
O
Ni
Ni
Ni
Ni
Ni
Ni
O
O
O
O
O
O
CF
3
OMe
CF
3
CF
3
OMe
Ph
Ph
N
O
178
179
180
m
Ar
1
NC
n
Ar*MC
PR
3
PR
3
Cl
R
3
P
PR
3
R
3
P
PR
3
1
1
Cl
Pd
PR
3
Pt
PR
3
Cl
R
3
P
Pd
C
N
Pt Cl
Cl
Pd
CC
N
Pt Cl
PR
3
R
3
P
N
PR
3
Ar*
n
183
Ar
Ar*
n
181
185
m
Ar
1
=
m
-PrOCOC
6
H
4
Ar
1
=
m
,
m
-(PrOCO)
2
C
6
H
3
Ar* =
m
-C
6
H
4
CO
2
R*
R* = (
−
)-menthyl
Scheme 13.17.
Yashima and coworkers have developed helicity induction of achiral polyisocyanides
bearing a less bulky substituent through acid-base interactions. Although optically inac-
tive poly(
p
-carboxyphenyl isocyanide)s cannot maintain a 4
1
helical conformation, they
can adopt a dynamic single-handed helical conformation upon complexation with chiral
amines [261]. Surprisingly, if the helicity induction is performed in water, the macromo-
lecular helicity is stably memorized after the complete removal of the amines. The
