Chemistry Reference
In-Depth Information
13.3.2.4. Polymerization of Isocyanides
Optically active polyisocyanides with excess
screw sense are obtained from optically active monomers by nickel-catalyzed polymer-
ization. The polymers obtained from (
R
) -
222
, (
R
) -
223
, and (
R
) -
224
have
M
- helical
conformation with helical excess of 62%, 56%, and 20%, respectively [369]. The polym-
erization of optically active (
S
) -
225
also gives a polymer with
M
- helicity. Interestingly,
however, the copolymerization of achiral phenyl isocyanide with (
S
) -
225
afforded a
copolymer with
P
-helix [370,371]. The result is attributed to the difference of the polym-
erizability between two comonomers: The polymerization of bulky (
S
) -
225
is slower than
that of less bulky, achiral isocyanides. In the early stages of the copolymerization, there-
fore, the polymerization of phenyl isocyanide is predominant, and a racemic mixture of
P
- and
M
-helical growing chains is formed. (
S
) -
225
is then preferentially incorporated
into one of the two helices,
M
-helix. Thus, the further growth of the
M
- helix is inhibited
as a result of the slower rate of polymerization of optically active comonomers.
One of the signifi cant contributions to the polyisocyanide chemistry is the polymeriza-
tion of isocyanides bearing amino acids. The nickel-catalyzed polymerization of isocy-
anopeptides (
226
,
227
) afforded novel synthetic analogues of
-helix, that is, the polymers
that fold in a proteinlike fashion to give helical strands in which the peptide chains
are arranged in
β
-sheet structures. The helical structure of poly(isocyanopeptide)s is
stabilized by hydrogen bonds between the amide groups [372,373]. The polymerization
of enantiomerically pure aryl isocyanides bearing chiral functional groups, such as L-
alanine (
228
), L - alaninol (
229
), L - phenylalanine (
230
), and L-lactic acid residues (
231
)
with a long
n
-decyl chain, proceeded diastereoselectively in the presence of NiCl
2
to give
either right- or left-handed helical polymers. The helical senses were signifi cantly depen-
dent on the pendant structure and the reaction conditions such as the solvent polarity
and temperature, which suggests that the “on-off” fashion of the intermolecular hydro-
gen bondings between the amido groups strongly infl uences the outcome [374,375].
The control of helical conformation by electrical stimuli has been investigated. The
helical polyisocyanides bearing ferrocenyl moieties were prepared by polymerization of
optically active isocyanides (
232
) using a dinuclear Pt/Pd complex (
181
). Electrolytic CD
and UV spectra suggested that the poly-
232
exhibited reversible conformational change
between helical structures and disordered structures in response to oxidation and reduc-
tion of the ferrocenyl pendants [376].
β
O
NC
NC
NC
NC
CN
∗
∗
N
H
CO
2
Me
MeO
2
C
226
O
H
N
CN
CO
2
Me
∗
∗
H
∗
222
223
224
225
O
227
NC
NC
NC
NC
NC
R
Fe
O
NH
O
NH
O
NH
O
O
O
O
O
O
O
R = Me,
n
-C
21
H
43
OC
m
H
2m+1
O
n
-C
10
H
21
Ph
O
n
-C
10
H
21
O
n
-C
10
H
21
228
(m = 2, 6, 10, 14)
229
230
231
232
