polymeric chiral catalyst design and chiral polymer synthesis (itsuno/polymeric chiral catalyst) ||...

CHAPTER 9

C2 CHIRAL BIARYL UNIT-BASEDHELICAL POLYMERS AND THEIRAPPLICATION TO ASYMMETRICCATALYSIS

TAKESHI MAEDA and TOSHIKAZU TAKATA

9.1 INTRODUCTION

Accurate control of helical conformation is one of the current subjects in polymer

science. Optically active helical polymers can be classified into two classes, dynamic

helices and static helices, according to the stability of the helical conformations in

solution [1–4]. In dynamic helices, helix inversion can occur in solution so that the

excess of a screw sense can be controlled by external stimuli, e.g., temperature,

solvent, and chiral additive [4–7]. Static helices undergoing no helix inversion can be

synthesized by helix-sense-selective polymerization of achiral monomers with chiral

initiators or catalysts [8–12] and by polymerization of chiral monomers. It is

generally difficult to obtain concrete evidences for helical conformations of polymers

because of the uncertainty of the directionality of the junction between their

monomer units, although a limited number of helical conformations of polymers

in solution are determined by X-ray crystallography of oligomers of helix and by

direct observation of helical structures by atomic force microscopy [13, 14].

However, it is of importance to understand detailed helical structures for the

applications of helical polymers to asymmetric catalysts, chiral recognition materi-

als, and so on. Therefore, the structure-definite helical polymers with rigid fixed

conformations are strongly desired for such applications. Rational designs of rigid

helical polymers by means of the fixation of random conformation of a polymer main

chain into desired helical conformation results in a solution for synthesis of helical

polymers possessing well-defined helical structures. C2 chiral biaryl moiety seems a

very suitable candidate as a repeating unit of optically active polymers with

Polymeric Chiral Catalyst Design and Chiral Polymer Synthesis, First Edition. Edited by Shinichi Itsuno.� 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.

267

predictable helical conformations because the C2 chiral biaryl unit with atropisomer-

ism has an inherently helical nature as a helically twisted unit.

Takata et al. have demonstrated a rational design of an artificial helix using C2

chiral moiety [15, 16]. This strategy is based on the consideration of significant

components constituting a helix, as illustrated in Figure 9.1. Namely, the helical

structure can be broken down or disintegrated into two key structural motifs: “chiral

twist” and “planar junction.” These motifs can be embodied as the C2 chiral moiety

like the C2 chiral binaphthyl motif and the planar metal complex like the planar

metallosalen compolex motif. The resulting polymeric architectures built from the

motifs inevitably adopt helical structures because they have no freedom ofmovement

that randomizes their conformations.

Once the synthetic strategy for helical polymer is constructed according to the

above concept, a variety of helical polymers possessing unique functionalities can

be obtained. Because a helical polymer mainly has three possible asymmetric

fields for reaction and recognition as shown in Figure 9.2, functional groups can

be arranged at a desired position such as outside, inside, or groove by rational

design of the helix. Several important reactions using these asymmetric fields of

the helix have been reported so far, which will be introduced at the latter part of

this chapter.

In this chapter the authors discuss the helical polymer syntheses by usingC2 chiral

moiety as a twisted unit in the main chain and by rigidly binding these C2 chiral

moieties for the fixation of the helix conformation, according to the as mentioned

construction protocols of the helix. Furthermore, the asymmetric catalysis using the

unique asymmetric fields of such helical polymers is also described.

FIGURE 9.1. A concept for rational design of an artificial helix derived from disintegration of

the helix into two key structural motifs, “chiral twist” and “planar unit,” by disintegration of a

wooden-plate–integrated helix.

268 C2 CHIRAL BIARYL UNIT-BASED HELICAL POLYMERS AND THEIR APPLICATION

9.2 SYNTHESIS OF C2 CHIRAL UNIT-BASED HELICAL POLYMERS

9.2.1 Use of C2 Chiral Biaryl Moieties as Chirally Twisted Units in thePolymer Main Chain

There are a large number of studies on the optically active polymers formed by

polymerizations of optically active C2 chiral monomers with inherent atropisomer-

ism such as biphenyl and binaphthyl monomers [17] (Figure 9.3). The twists

originating from the atropisomeric monomers in the main chains promise to make

the polymers conformationally chiral as long as the racemization is prevented during

the polymerization process. In these polymers synthesized by the connection of C2

chiral biaryl units through bifunctional and conjugated linkers, the freedom in main-

chain conformation was partially regulated by the rotational constraint at the chiral

axis of the biaryl units. In most cases, a presence of helical structures in main chains

is suggested by significant changes in circular dichroism (CD) spectra and/or specific

optical rotation.

In 1968, Schultz and Jung first reported on the optically active polyamide

with enantiopure binaphthyl units (Scheme 9.1) [18]. The polyamide (R)-poly-1

was prepared by the polycondensation of (þ)-2,20-diamino-bi-2,20-naphthol andterephthaloyl chloride.

Chiral Twisted Unit

C2 - ChiralBinaphthyl

12

3

4

FIGURE 9.3. C2 chiral biaryls as a chiral twisted unit of Figure 9.1.

FIGURE 9.2. Asymmetric fields of the helix for reaction and recognition.

SYNTHESIS OF C2 CHIRAL UNIT-BASED HELICAL POLYMERS 269

Jaycox et al. and Takeishi et al. have demonstrated stimuli-responsive polyamides

(poly-2–4, poly-10) containing C2-chiral biaryls and azobenzene units in the main

chains (Schemes 9.2 and 9.3) [19–24].

Ultraviolet (UV) photoirradiation of the polymer samples to drive the trans-cis

isomerization resulted in an immediate chiroptical response, with CD band intensi-

ties and specific rotation significantly diminished. These effects were fully reversible

and were attributed to the presence of putative one-handed helical conformations in

the trans-azobenzene–modified polymers that were disrupted by the trans-cis

isomerization. This indicates that the spacers in the main chain significantly

influenced their helical conformation.

OO

O

H2N

NH2O

N N COClClOC

5

6

NH2

H2N

7

NH2H2N

8

HN Ar

HN

OC N N

OC

poly-2; Ar = x [(S) -1] + (100 -x) 6

poly-3; Ar = x [(R) -1] + y 6 +(100 -x -y) 7

poly-4; Ar = x [(S) -1] + (100 -x) 8

6

7

8

+ 5n

(R) or (S) -1

NH2

NH2+

SCHEME 9.2.

Cl

Cl

O

O

(R) -1 [α]578

25= +160.6(c = 0.1, THF)

poly-(R) -1Mw 7000 (anaylzed by VPO)

[α]578 25= -15.0

(c = 0.1, THF)

NH2

NH2

n

NH

O

O

HN

SCHEME 9.1.


Optically active conjugated polymers (poly-12–21) with chiral conformation

along the main chains were prepared by the palladium-catalyzed cross-coupling of

the corresponding aryls and enantiopure binaphthyl monomers. (Figure 9.4) [25–33].

The UV spectrum of these polymers showed only a small red shift (�10 nm) of the

absorption maxima when compared with their repeating unit models, indicating

almost no extended conjugation between the repeating units of the polymers. The

present cross-coupling synthesis has a great advantage in design of optically active

polymers. The facility in the design of the resulting conformation of polymer chains

has made a wide range of applications possible [34–39]. However, polybinaphthyls

have single bonds in the junction of the binaphthyl units, which could induce the free

rotation to partly disrupt helical conformation of the main chain. In fact, the CD

spectra of the polybinaphthyls having bulky dendrons (poly-(R)-22), which was

expected to restrict free rotation about the single bonds in the junctions by steric

repulsions, exhibited similar Cotton effects in comparison with those of the

monomeric model dendrimers (Figure 9.5) [40]. These results suggest that helical

conformation was not wholly generated on their main chains, despite the steric

repulsion between the bulky dendrons.

Takata et al. described optically active polycarbonates (poly-(R)-23) prepared by

the anionic ring-opening polymerization of a cyclic carbonate monomer prepared

starting from binaphthol (Figure 9.6) [41].

The rotational freedom around the main chain of poly-(R)-23was not zero but low

because of the direct connection of C2 chiral binaphthyl moieties through the

carbonate bond. Poly-(R)-23a having no alkyl groups was insoluble in common

organic solvents, although racemic poly-(rac)-23a was soluble in some solvents.

The introduction of octyl group in the naphthalene ring improved the solubility of

Br

Br

ORRO +

N N

COPd(PPh3)4, PPh3, DBU

CH2

O

N

ORH2N-Ar -NH2

Ar =

DMF, 115º C

(R) or (S) - 9

a:

b: d:

c:

poly-10a -e

HN

OAr

HN

On

R = Me. Et

e:

RO

SCHEME 9.3.


poly-(R)-23 to make the structural analysis possible. The specific optical rotation and

CD spectra of poly-(R)-23b were significantly different from those of the unit model

compound 24. The opposite signs of Cotton effect and specific rotation of poly-(R)-

23b and 24 suggested the emergence of some special conformation of poly-(R)-23b.

a

RO OR

RO OR

b (m = 1, 2)m

poly-(R) -19a (Ref. 28)poly-(R) -19b (Ref. 29)

poly-(R) -19c (Ref. 30)

Ar Ar

n

RO OR

RO OR

m

m

poly-(R) -12a (m = 1)poly-(R) -12b (m = 2)(Ref. 24)

n

(b) Suzuki-Miyaura coupling polymerization

(a) Sonogashira coupling polymerization

RO OR

RO OR

poly-(R) -13(Ref. 25)

O O

O O

ORRO RO OR

O

O O

O

O

O

O

O

poly-(R) -14

(Ref. 26)

3

3

R R

poly-(R) -15a (R = OC6H13, R1 = H, R2 = NO2, R3 = H)poly-(R) -15b (R = OC6H13, R1, R2 = NO2, R3 = H)poly-(R) -16 (R = N(CH3)C6H13, R1, R2, R3 = F)(Ref. 27)

R2R1

R = (CH2)17CH3

RO ORpoly-(R) -17a (R' = H)poly-(R) -17b (R' = NO2)(Ref. 27)

NO2R'

R = (CH2)17CH3

RO OR

poly-(R) -18(Ref. 27)

R = (CH2)17CH3

NO2

O2N

RMeN NMeR

O2N NO2

poly-(R) -20(Ref. 31)

(c) Heck coupling polymerization

(d) Stille coupling polymerization

N

S

NO

O

N

O

m

em (m = 1, 3)

RO OR'

S Sx

y

hexO Ohex

poly-(R) -21a; R = e1, R' = e1

poly-(R) -21b; R = e3, R' = e3

poly-(R) -21c; R = e1, R' = hex

poly-(R) -21d; R = e3, R' = hex

(Ref. 32)

n

n

n n

n

n

n

R3 R3

S m

c (m = 1, 2, 4)

R = C18H37

Ar =

FIGURE 9.4. Optically active conjugated polymers containing binaphthyl groups in main

chains.


The CD spectrum calculated for the MM2-simulated structure of a model decamer of

poly-(R)-23 coincided with the measured one and supported the occurrence of the

stable 41-helical structure. Furthermore, the CD spectra of several oligomers (R)-25

(1, 2, 4, 8-mer), synthesized independently, exhibited a drastic change of Cotton

effect between 3-mer and 4-mer (Figure 9.7). Because the Cotton effects of 4-mer

and 8-mer resembled that of poly-(R)-23, they seemed to hold similar secondary

structures, i.e., 41-helical conformation like the calculated structure of its decamer

model. The prediction that poly-(R)-23 takes a helix structure was finally evidenced

by the X-ray crystal structure of a 1:1 mixture of (R) and (S) model 4-mers

(Figure 9.8) [42]. A similar polycarbonate to poly-(R)-23 could be synthesized

simply by polycondensation of binaphthol and bis(4-nitrophenyl)carbonate [43].

Optically active poly(biphenyl carbonate)s poly-(R)-26 synthesized by the

anionic ring-opening polymerization of optically active cyclic carbonate (R)-26was also expected to adopt a stable helical conformation (Scheme 9.4) [44, 45]. The

intensity of the CD Cotton effect of poly-(R)-26was much larger than that of the unit

model compound 27. Similarly, the Cotton effect of model 8-mer was much stronger

than those of 1–4-mers, suggesting the generation of the stable helical structure of

poly-(R)-26.

D0 = -CH2Ph

O

O

Ph

Ph

OO

O

Ph

Ph

O

O

Ph

PhO

D1 =

D2 =

Br

Br

OROR

(R) -22a; R = D0

(R) -22b; R = D1

(R) -22c; R = D2

Ni(II)/ Zn

Polymer

poly-(R) -22a

poly-(R) -22b

poly-(R) -22c

poly-(R) -22c

Mn (PDI)

3600 (1.8)

5200 (1.6)

7200 (1.6)

19300 (1.4)a

Yield %

62

71

86

a Determined by MALLS technique.

[α]D

-174

-89.7

-30.5

RO OR

RO OR

n

poly-(R) -22a; R = D0

poly-(R) -22b; R = D1

poly-(R) -22c; R = D2

FIGURE 9.5. Optically active polybinaphthyls having bulky dendrons.


Polycondensation of optically active (R)-2,20-dihydroxy-9,90-spirobifluorene (R)-28 and bis(4-nitrophenyl)carbonate yielded the corresponding optically active

polycarbonate poly-(R)-28) (Figure 9.9) [46]. The helical structure, originated from

the rigid spiro structure and consisting of a C2 chiral spirobifluorene (SBF) moiety,

was suggested by the CD spectral analysis.

In addition, the structural feature of optically active polyesters poly-(R)-29obtained from the same C2 chiral monomer (R)-28 and a few homoditopic acid

chlorides (Figure 9.10) was studied by UV-vis and CD spectroscopies [47]. Poly-(R)-

29 exhibited Cotton effects around the absorption regions of terephthaloyl, naphtha-

loyl, and azodibenzoyl linkers, suggesting the generation of some ordered polymer

structure. However, their decrease in CD intensity with an increase in temperature

indicated that poly-(R)-29 takes an unstable helical structure, in comparison with

polycarbonate poly-(R)-28 that exhibited no temperature dependency on CD

intensity.

Thus, the introduction of C2 chiral biaryl units, in addition to the reduction in

rotational freedom by the rigid linkers between the C2 chiral units, effectively

contributes the induction of helical conformation to the corresponding polymer.

O O

O

O O

O

t-BuOK

4-nitrophenylchiroformate

R R R R R R

(R)-23a; R = H[α]D

24 = +440 (c = 1.0, THF)(R)-23b; R = n-C8H17[α]D

24 = –280 (c = 0.15, THF)

poly-(R)-23a; R = Hpoly-(R)-23b; R = n-C8H17

[α]D24 = +530

(c = 0.15, THF)

HO O

O

R R

24; R = n-C8H17[α]D

24 = –21(c = 0.15, THF)

a; R = Hb; R = n-C8H17

HO OH n

(a)

(b) (c)

Top View Side View

20 mer

FIGURE 9.6. Synthesis of optically active polycarbonates by anionic ring-opening poly-

merization of a cyclic carbonate prepared from a binaphthol derivative (a). Simulated structure

(MM2) of a model decamer of poly-(R)-23 (b). Structure of monomeric model 24 (c).


FIGURE 9.8. X-ray crystal structure of a 1 : 1 mixture of (R) and (S) model tetramers of

(R)-25.

R R R R

OO

O

OR'OR'

n

(R)-25; R' =H or SiMe2But

n = 0, 1, 2, 3, 4

2000

1500

1000

500

8 mer

4 mer

2 mer

1 mer

3 mer

3 mer2 mer1 mer

4 mer8 mer

0

–500Δε

/ dm

3 mol

–1

–1000

–1500

–2000200 250

Wavelength / nm

300

solvent : THF

FIGURE 9.7. CD spectra of model oligomers (R)-25.


HO OH O O

O

O O

O

(R)-26

poly-(R)-26[α]D26 = +27.4 (c = 0.1, THF)

triphosgene

t-BuOK

bis(4-nitrophenyl

chloroformate)

HO O OCH3

O

27[α]D24 = +4 (c = 0.1, THF)

SCHEME 9.4.

Bis(4-nitrophenyl)carbonate

DMAP (2eq)

toluene, reflux

O O

O

(R )-

(R )-28[α]D

20 = +27.6poly-(R )-28

[α]D20 = +116 (c = 0.1, THF)

n

(a)

(b)

OH HO

FIGURE 9.9. Synthesis of poly-(R)-28 (a) and MM2-optimized structure of poly-(R)-28 (b).


9.2.2 Synthesis of Stable Helical Polymers by the Fixationof Main-Chain Conformation

9.2.2.1 Helical Polymers Generated via Intramolecular Cyclization. In

the helical polymers prepared by linking C2 chiral biaryl units through single bond

formation, the helix usually tends to become unstable because of the free rotation

around the linker units to randomize the conformation. For example, poly-(R)-22

having single bonds combining C2 chiral binaphthyl units was not considered as a

helical polymer stable in solution, even if it had a bulkier side chain. In fact, the CD

spectrum of poly-(R)-22 was close to that of its unit model compound. The single

bonds in the junctions of the binaphthyl units allow for several main-chain

conformations with almost the same stability. Elimination or a large decrease of

degree of rotational freedom in the main chains results in the formation of highly

stable helical polymers in solution.

The fixation of atropisomeric units by intramolecular cyclization produces a

successful entry into the construction of helical molecules. Grimme, V€ogtle, andcoworkers reported on the extended atropisomeric compound 30 that was prepared

by the intramolecular reactions of a triaryl (Figure 9.11) [48]. Because the rotation

around the aryl-linking single bonds was inhibited by the cyclizative linking of the

aryl moieties, the resulting three possible conformers 30 of the helical terarylophane

Br

BrBr

Br

Na2S

NaH

CO2Et

CO2Et

X

Xbenezene/ orethanol

30a: X = S30b: X = C(CO2Et)2

X

X

P, P helix M,M helix

+ + P,M helix

FIGURE 9.11. Synthesis of helical molecules having an elongated screw shape.

poly-(R)-29a-c

n

O O

O

Ar

OAr = a:

NN

b:

c:

FIGURE 9.10. Optically active polyesters containing a spirobifluorene moiety.


were separated with a chiral high-performance liquid chromatography (HPLC)

column.

In 2004, Pu and Zhang prepared optically active helical polybinaphthyls

(poly-(R)-32) having no single bond, allowing for unrestricted rotation in the main

chain (Figure 9.12) [49]. These helical polymers were obtained according to two-step

synthesis consisting of initial polymerization of two monomers to poly-(R)-31followed by cyclization on the main chain to poly-(R)-32 that eliminate the

freely rotatable single bonds between the monomer units. Effective conversion of

poly-(R)-31 into poly-(R)-32 using Swager’s acid-catalyzed cyclization of arylalk-

ynes to polyaromatics resulted in the formation of a helical polymer. UV, fluores-

cence, and CD spectra of poly-(R)-31 and poly-(R)-32 were measured for the

evaluation of the structural change. Specific rotation of poly-(R)-32 was fluctuated

at �708, which was compared with that of poly-(R)-31 (�115.7). Some molecular

weight-dependency of poly-(R)-32 in CD Cotton effect was confirmed.

9.2.2.2 Helical Polymers Generated via the Metal Complexation ofLinker Moiety. Fixation of main-chain conformation using a rigid framework

OMOMOMOM

B

B

O

O

O

O

+

MOMO OMOM

MOMO OMOM

R

R R

R

Br

Br

R

R

R = Ph-p-OC12H25

Ph(PPh3)4

K2CO3

H2O/THF

poly-(R)-31

CF3CO2H

CH2Cl2, r.t.97% yield

OHOH

OH OH

R

R

R

R

[α]D = -115.7 (c = 0.1, CH2Cl2)

[α]D = -708 (c = 0.1, CH2Cl2)poly-(R)-32

FIGURE 9.12. Helical polybinaphthyls obtained by acid-catalyzed cyclization.


of the metal complex is actually used in both biological and chemical processes.

Protein-folding reactions in either natural and synthetic proteins often occur via

metal complexation, causing activity enhancement [50–52]. Use of complexation

between transition metal and ligands for stabilizing and/or regulating conformation

of molecule has recently been recognized as an effective synthetic protocol in rapidly

growing supramolecular chemistry. Many regulated architectures, foldamers,

stepladders, chains, rings, cages, and dendric macromolecules have been given so

far by the coordination of polytopic ligands to metal ion [53–55]. These complexes

with specific conformation and geometry are well designed by considering binding

sites, metal ions, and ligands involving linkers between the binding sites. Utilization

of metal complex formation should be also advantageous to construct helical

polymers. A successful example can be observed in construction of helicates via

reaction of polytopic ligands and metal ions [56]. A lot of single, double, and triple

helices were obtained through the appropriate designs of ligands including

metal binding sites and spacers between the binding sites and the choice of

metal ions [57, 58]. Katz et al. reported the most straightforward helix synthesis,

i.e., the polymerization of a formyl group-functionalized helicene monomer 33

having a intrinsic helical geometry followed by the complexation with nickel acetate

to nickel salen complex capable of serving as a rigid planar connection between the

helicenes to eventually afford a helical ladder polymer (Scheme 9.5) [59, 60]. This

approach using the fixation of main-chain conformation through the rigid planar

complex formation seems a very efficient protocol that exclusively provides a desired

stable helical polymer.

Takata et al. used a similar metallosalen complex formation for the helicate

synthesis. Namely, according to the discussion in Figure 9.1, a metallosalen complex

was chosen as the planar rigid junction in addition to the choice of (R)-binaphthl

group as the chiral twist (Figure 9.13).

The synthesis of helical poly(binaphtyl-salen complex)es poly-(R)-34was carried

out by the two-step procedure via the initial polymerization of the monomer

components through a [2, 3; a,b] fusion mode followed by the metal complexation

for the rigid junction (Figures 9.14 and 9.15) [15, 16]. The precursor polymer, poly

(binaphthyl-Schiff base) poly-(R)-34, was prepared by the polycondensation of

binaphtyl dialdehyde 34 and a diamine. Treatment of poly-(R)-34 with metal acetate

H2N NH2

MeO CHO

OH

OHC

HO

OMe

ORRO

NN

OONiNi(OAc)2

OMe

MeO

OR

OR

MeO

RO

33

R= -(CH2)2O(CH2)3CH3

SCHEME 9.5.


N N

OH HOR

RR

R

R2 CHO

R2 CHO

OHOH

(R)-34

N N

O OR

RR

R

MM(OAc)2

diaminea-e

M= Zn(II), Cu(II), Mn(III)-OAc,

H2N NH2 H2N NH2H2N NH2NH2 NH2 NH2 NH2

(S,S) -(R,R) -

a b c d e

diamine N N

OR RO

O OM

R = -(CH)2CH3

poly-(R)-34a-e poly-(R)-34a-e(M)

(R)-35(M)

FIGURE 9.15. Synthesis of helical poly(binaphthyl salen complex)es.

Chiral Twisted Unit Planar Joint

C2 Chiral-Binaphthyl

PlanarMetalComplex

OM

NNA

O+1

2

3

4

[2,3; α–β] fusion

FIGURE 9.13. Molecular design of a helicate constructed by the fusion of C2 chiral

binaphthyl as the chiral twist and metallosalen complex as the planar junction.

ConformationChange

Fix by Coordinationto Metal Ions

PolymerizedUnit Structure

FIGURE 9.14. Schematic representation of synthetic strategy of helical polymers con-

structed a priori.


quantitatively yielded poly(binaphthyl-metallosalen complex) poly-(R)-34M.

Random coil poly-(R)-34 changed its form to a very stable helical poly-(R)-34M

having a 31-helical conformation by the metal insertion so as to form a rigid planar

metallosalen complex structure in the main chain. Therefore, the constrained helical

structure of poly-(R)-34M can be predictable as illustrated in Figure 9.13. The salen

ligand features two covalent and two coordinate sites situated in a planar array,

whereas the two axial sites are open for ancillary ligand and substrate.

Studies on an energy-minimized molecular model (built with MM2) of poly-(R)-

34(Cu) calculated using the crystalline data of (R)-35(Cu) as the initial structure, in

addition to the careful investigation with the Corey–Pauling–Kolturn (CPK) space-

filling model, showed a helical conformation as only possible main chain structure

(pseudo 31-helix, ca.1.6 nm in diameter, an interval of ca. 0.8 nm, a helical cavity of

ca. 0.3 nm in diameter) (Figure 9.16). Moreover, the molecular dynamics (MD)

simulations of a model decamer of poly-(R)-34(Cu) indicated that no break of the

helix occurred at least in tens of picoseconds at 400K. A clear evidence for the

helical conformation was obtained in CD spectra revealing the gradual conversion of

poly(Schiff base) poly-(R)-34 to poly-(R)-34(Cu) depending on the amount of Cu(II)

added, around 401 nm assignable to the p-p* type transitions of C¼N chromophore

and at 400–600 nm assignable to LMCT transitions of themetal complex (Figure 9.16

(C)). In CD spectral change, it was also confirmed that the addition of Cu(II) caused a

drastic change of the Cotton effect at the range from 220 to 380 nm. These clear

(a) Top view

(b) Side view

ca. 6 Å

ca. 13 Å

(c)

(Lm

ol-1

cm-1

)

0 eq

0.13

0.27

0.67

1.00

1.33

1.33

1.00

0.67

0.27

0.13

0 eq

210

140

70

0

-70

-140

-210

0 eq

1.33 eq

200 300 400 600500 700Wavelength

ca. 6 Å

(nm)

FIGURE 9.16. MM2-optimized structures of poly-(R)-34e(Cu) (a, b, 10-mer model) and CD

spectral change of poly-(R)-34e to poly-(R)-34e(Cu) upon mixing with Cu(II) in THF (c).


changes could indicate that naphthalene rings were rearranged along the helical main

chain from the random one.

Successive binding of a C2 chiral SBF moiety (R)-36) with two sets of two

functional groups like (R)-34 and a planar metallosalen moiety, which is similar to

the case of poly-(R)-34(M) (Figure 9.15), resulted in the formation of a zigzag 21-

helix (poly-(R)-36(M), Figure 9.17). Takata et al. found that the C2 chiral SBF

moiety with a completely fixed dihedral angle at 90� absolutely affords the structure-definite 21-helix by the two-point junction using the metallosalen complex [61].

Initial synthesis of poly(Shiff base) (poly-(R)-36) using a diamine was followed by

the metal-introduction to fix the main chain by the formation of a planar metallosalen

complex to the 21-helix. The rigid helical conformation of poly-(R)-36(M) was

suggested by the good thermal stability of the CD Cotton effect, which is similar to

poly-(R)-34(M).

9.3 ASYMMETRIC REACTIONS CATALYZED BY HELICALPOLYMER CATALYSTS

Optically active helical polymers having metal complex moieties can provide their

helical backbones and metal complex moieties as chiral recognition and asymmetric

catalytic sites. In the early 1970s, Hatano et al. reported that the poly-L-lysine–

copper(II) complex catalyzed enantiomer-selectively the oxygen oxidation of 3,4-

dihydroxyphenylalanine (Scheme 9.6) [62]. Copper(II) ion at the catalysis center

was fixed by amino groups of lysine residues of the poly(S)-lysine helix.

OH

HO

CHO

OHC

O

N N

OZn

OH

N N

HOH2N

NH2

CH2Cl2r. t. 1 h

ZnEt2

o-chlorophenolr.t. 48 h

(R)-36 poly(R)-36 poly(R)-37

FIGURE 9.17. Synthetic scheme and MM2 calculated structure of 21-helical poly(spirobi-

fluorene-salen complex) (poly(R)-36(Zn)).

NH2

COOH

HOOH

poly-(S)-lysineCu2+, O2,

NH2

COOH

OO(R)-isomer selective

SCHEME 9.6.


Roelfes and Feringa et al. used a DNA-bound copper catalyst for the asymmetric

Diels–Alder reaction of an enone and cyclopentadiene in water (Figure 9.18). The

catalyst consisted of a DNA double helix and a nonchiral intercalating moiety

incorporating a metal complex, which enables the noncovalent anchoring of the

metal complex to DNA. The Diels–Alder adduct was obtained in more than 99%ee

via the asymmetric induction from a double-strand DNA helix [63–65]. This result

indicated that the chiral environment provided by the DNA double helix could be

transferred to the reaction products, whereas the ligand with the intercalatable part

was achiral. The role of the DNAwas found to act as not only a chiral scaffold but

also as a rate-accelerating field, although it depended on the sequence of the

DNA [66]. Similar DNA-bound catalysts were applicable to asymmetric syntheses

in water such as the Michael reaction and the Friedel–Crafts reaction, in addition to

hydrolytic kinetic resolution [67–69].

As mentioned, optically active helical polymers can be regarded as attractive

chiral recognition materials and asymmetric catalysts because of their specific

asymmetric fields originating from both the main chain helix and the helical arrays

of the side chains.

The chiral environment provided by an artificial stable helical polymer is also

effective for enantioselective reaction. Takata et al. demonstrated the asymmetric

reactions catalyzed by helical polymer complexes consisting of C2 chiral binaphthyl

units and metallosalen complexes, poly-(R)-34M. The metallosalen complex moiety

is placed in the groove of the helix constructed by the regular arrangement of both the

naphthalene moieties and the diamine units, according to the helix-forming concept

shown in Figures 9.13–19.16. The asymmetric addition of diethylzinc to benzalde-

hyde was catalyzed by poly-(R)-34(Zn) (5 unit mol%) to afford 1-phenyl-1-propanol

with 13–95%ee in 19–99% yield by 24 h reaction (Figure 9.19) [16].

Among several solvents, tetrahydrofuran (THF) was the most effective solvent for

achieving favorable yield and enantioselectivity. The enantiomeric excess of the

product increased up to 95% in the reaction at �60�C in which no by-product was

Cu2+

ON

X

+

ON

X

DNA-based catalyst

water

> 99 %ee

N

N

DNA-based catalyst

FIGURE 9.18. DNA-bound catalysts used in the catalytic asymmetric Diels–Alder reaction

of cyclopentadiene with aza-chalcone.

ASYMMETRIC REACTIONS CATALYZED BY HELICAL POLYMER CATALYSTS 283

observed. The enatioselectivity of poly-(R)-34b(Zn) was slightly lower than that of

poly-(R)-34a(Zn), indicating the unfavorable effect of the vic-dimethyl group placed

on the diamine unit. Although enantiomeric excess of products obtained by poly-(R)-

34e(Zn) having a (S,S)-cyclohexanediamine unit was low (13%ee), the reaction

catalyzed by poly-(R)-34d(Zn) having a (R,R)-1,2-cyclohexanediamine unit

afforded to the products with high enantioselectivity (81%ee). These results revealed

that the chiral environment around the metallosalen active site of the catalyst can be

effectively modified by the structure of the diamine unit, suggesting a potential fine-

tuning of the chiral space in the groove of the helical poly(binaphthyl salen zinc

complex)es. In contrast, a polymer unit model (R)-35a(Zn) resulted in a very low

enantioselectivity, indicating that asymmetric catalysis was attributed to the helical

structure of polymers rather than to the chiral repeating unit. Catalysis of helical poly

[N-(4-ethynylbenzyl)ephedrine], a optically active polyacetylene, which predomi-

nantly took a one-handed helical conformation induced by the chiral pendant groups,

in the asymmetric addition of dialkylzinc to benzaldehyde was also reported by

Yashima et al. [70]. The ee of the product obtained with the helical polymer was

lower than that with the monomer unit model catalyst, indicating that the one-handed

helical structure negatively affected the enantioselectivity. The present catalytic

reaction using poly-(R)-34(Zn) was the first report that directly used the secondary

structure of the polymer as the asymmetric field.

It is important for asymmetric synthesis to increase efficiency of the reaction and

the degree of enantiomeric excess with as little amount of chiral auxiliary as possible.

Helical polymer catalysts have a great advantage to reduce the chiral sources with

retaining high enantioselectivity because dominant single-handed helical structures

are generated by the propagation of chirality from optically active comonomers to

achiral comonomer or helix-sence-selective polymerization using optically active

additives and initiators. The propagation of chirality through the polymer backbones

in the polymer complex consisting of binaphthyl units to achiral bisphenyl and

biphenyl units (copoly-37(Zn), copoly-38(Zn)) were studied together with their

PhCHO + Et2Zn

poly-(R)-34(Zn)(5 mol%)

Ph

OH

*

Catalyst

poly-(R)-34a(Zn)

poly-(R)-34a(Zn)

poly-(R)-34a(Zn)

poly-(R)-34b(Zn)

poly-(R)-34d(Zn)

poly-(R)-34e(Zn)

(R)-35a(Zn)

> 99a

> 99

> 19b

> 99

> 99

96

85

ee (Config.) (%)

29 (R)

77 (R)

95 (R)

59 (R)

81(R)

13(R)

5(R)

aSovent: Toluene. bReaction temperature: –40 ºC.

Conv. (%)

THF, r.t., 24 h

N N

O OR

RR

R

Zn

poly-(R)-34(Zn)

FIGURE 9.19. Asymmetric addition of diethylzinc to aromatic aldehydes catalyzed by poly-

(R)-34(Zn).


application to asymmetric addition of diethylzinc to benzaldehyde (Figure 9.20) [71].

Weak nonlinear correlations between the content of chiral unit and the enantiomeric

excess were observed in the catalytic reactions probably because of the chirality

induction in achiral comonomer units.

Reggelin and coworkers have reported the asymmetric catalysis of single-handed

helical poly(methacrylate)s with sterically congested methacrylates (poly-39, poly-

40) prepared by helix-sense-selective polymerization [72, 73]. These optically active

helical polymers having no optically active moiety were designed to offer the

coordination sites for organopalladium compounds and the catalytically active site of

pyridine N-oxide (Figure 9.21). Catalytic sites located in the helically oriented side

chain provide efficient reaction fields for an asymmetric C-C bond forming reaction

such as allylic substitution of 1,3-diphenylprop-2-enyl acetate with dimethyl

malonate and allylation of benzaldehyde with allyltrichlorosilane.

In addition to a static helical polymer, dynamic poly(isocyanate)s (poly-41)

with a dominant one-handed helical structure dictated by a smaller amount of

optically active monomer units compared with achiral phosphorus monomer units

can complex with rhodium and catalyze an asymmetric hydrogenation of N-

acetamidocinnamic acid to produce the corresponding saturated product in moderate

enantioselectivity (Scheme 9.7) [74].

Poly(binaphthyl salen manganese complex) catalyzed the epoxidation of alkene

with m-chloroperbenzoic acid (mCPBA) (Figure 9.22) [75]. Although poly-(R)-34a

(Mn) showed no enantioselectivity, poly-(R)-34b(Mn) having a 2,2-dimethyl-1,3-

propanediyl unit as the diamine unit exhibited enentioselectivity in the epoxidation

CHO

OHOH

CHO

R1

R1

n

CHO

OHOH

CHO

R2O

R2O

+ (1-n)

N

O

N

O

R1

R1

OR2

OR2

ZnNH2

n

(1-n)copoly-37(Zn)R1 = n-C8H17

R2 = n-C12H25

NH21.

2. Zn2+

N

O

N

O

n

(1-n)

R1

R1

Zn

copoly-38(Zn)

Ratio of chiral and achiral units (n)

(a)

(b) (c)

FIGURE 9.20. Synthesis of polymer complexes consisting of optically active binaphthyl

units and achiral biphenyl units (copoly-40, 41(Zn)) (a, b). Plots of enantiomeric excess of the

product in the diethylzinc addition to benzaldehyde catalyzed by copoly-40(Zn) versus

content of chiral units (n) (c).


N

N

OO

OO

x y

Pd

poly-39

Ph Ph

OAc

CO2Me

CO2Mecat. poly-39(25 mol%)

Ph Ph

CO2MeMeO2C

99 % yield,60 %ee

NO

Ox

poly-40

NO

Oy

O

O

H + SiCl3

OH

*

*

cat. poly-40iPr2NEt

n-Bu4NICH2Cl2

56 % yield,19 %ee

(a)

(b)

FIGURE 9.21. Asymmetric catalysis of helically chiral palladium complexes in the allylic

substitution reaction (a). Helically chiral organocatalysts for allylation of benzaldehyde with

allyltrichlorosilane (b).

NC N

O

N

OH

Ph2P

yx

poly-41

Ph

COOH

NHAc

H2, cat. [Rh(COD)2]OTf / poly-41

Ph

COOH

NHAc*

quant. 14.5% ee

SCHEME 9.7.

92

40

43

Catalyst

poly-(R)-34a(Mn)

poly-(R)-34b(Mn)

(R)-35b(Mn)

Yield/% ee /%

1

17

1

Om -CPBA

poly-(R)-34(Mn) (8 mol%)N N

O OR

RR

R

Mn+3

poly-(R)-34(Mn)

OAc

FIGURE 9.22. Asymmetric epoxidation of 1,2-dihydronaphthalene catalyzed by poly-(R)-34

(Mn).


of 1,2-dihydronaphthalene. The polymer unit model (R)-35b(Mn) showed almost no

enantioselectivity. Therefore, the chiral space originating from the helical poly-(R)-

34b(Mn) was effective for this reaction, although enantioselectivity was not high.

Helical poly(phenylacetylene)s having oligopeptide groups also catalyzed the

asymmetric expoxidation of the chalcone derivative [76]. The helical array of

oligopeptide pendants was crucial for the enantioselective synthesis of epoxides.

Takata et al. elucidated the potential utility of poly-(R)-34(Cu) as the asymmetric

catalyst for the cyclopropanation of styrene with diazoacetate (Figure 9.23) [77].

Some asymmetric induction was observed in the asymmetric cyclopropanation

catalyzed by poly-(R)-34e(Cu), although the yield was small because of their low

catalytic activity. The reaction catalyzed by the corresponding polymer model (R)-

35e(Cu) afforded cyclopropanes with a low enantiomeric excess. Thus, it is apparent

that the higher enantioselectivity of poly-(R)-34e(Cu) than that of its model catalyst

is attributed to the helical environment. The effect of molecular weight of the

polymer catalyst on enantioselectivity was observed [35%ee (Mn¼ 87000), 19%ee

(Mn¼ 18000)]. This might be attributed to the concentration of polymer terminal

forming incomplete helical fields. Thus, the helical structure of poly(binaphthyl

salen complex) played a significant role in the catalytic asymmetric reactions. The

synergistic effect between the helical array of the salen complexes and the diamine

units was confirmed in all catalytic reactions.

All these catalytic reactions with the helical polymers mentioned previously were

also the epoch-making reports that directly used the secondary structure of a polymer

as the asymmetric field.

Complementary double-helical molecules based on an amidinium-carboxylate

salt-bridge with a predominant single-handed helical conformation have been

demonstrated by Yashima et al. as the first double-helix catalysts 42 that have

no chiral auxiliaries except for dissymmetrical helicity of the strands (Figure 9.24)

[78–80]. They were prepared by the substitution of chiral ligands for platinum

linkages with achiral bridged phosphine ligands. The predominantly double-helical

Ph + N2CHCOOEt

poly-(R)-34(Cu) (1.5 mol%)

14 (64/ 36)

45 (65/ 35)

42 (60/ 40)

42 (68/ 32)

36 (62/ 38)

Catalyst (Mn)a

poly-(R)-34e(Cu)b

poly-(R)-34e(Cu)

poly-(R)-34e(Cu)

poly-(R)-34d(Cu)

(R)-35e(Cu)

Yield/% (trans/cis)

(87000)

(87000)

(18000)

(10000)

Ph

CO2Et

Ph

CO2Et

+

ee /% (trans/cis)

28 / 46

11 / 35

6 / 19

2 / 0

2 / 16

aMns of the precursor poly(Schiff base)s. bReaction temperature 0ºC.

N N

O OR

RR

R

Cu

poly-(R)-34(Cu)

FIGURE 9.23. Poly-(R)-34(Cu)-catalyzed asymmetric cyclopropanation of styrene with

ethyl diazoacetate.


conformation formed in the complex formation of an amidine strand with chiral

ligands with carboxyl strands was memorized and provided suitable reaction fields

for the asymmetric cyclopropanation.

Sanda et al. also reported the catalytic asymmetric reduction of ketone with

optically active polyacetylene, i.e., poly(N-propargylamides) with ruthenium com-

plex moiety as the catalyst site (poly-43) (Scheme 9.8) [81]. Some synergistic effect

of the helical main chain and the optically active pendants played a crucial role in the

asymmetric reduction.

Suginome and Yamamoto have reported on the asymmetric catalysis of optically

active helical poly(quinoxaline-2,3-diyl)s (poly-44) having coordination sites of

phosphines (Scheme 9.9) [82]. The polymers were prepared by the asymmetric living

block copolymerization of an achiral comonomer having no metal-binding sites and

a comonomer with phosphorus donor atoms using the chiral initiator, and they

exhibited high catalytic activity in palladium-catalyzed asymmetric hydrosilylation.

Thus, one-handed static helical structures generated by the helix-sense-selective

PPh2

PtOO

TMS

OO

TMS

NHNTMS

N N TMSPt

Ph2P

Ph2P

PPh2

HHHH

Ph + N2CHCOOEt

cat. 42 (1.5 mol%)

Ph

CO2Et

Ph

CO2Et

+

85% ee 5% ee

95% yield62/38 (trans/cis)

42

FIGURE 9.24. Complementary double-helical catalysts for asymmetric cyclopropanation.

Hn

poly-43

NHO

O

H2N

RuCl

CH3

Ocat. poly-43

48% yield36% ee

CH3

OH

*

SCHEME 9.8.


polymerization using a small amount of chiral sources offered effective chiral

environments for asymmetric synthesis.

Recently, dynamic helical polymers that have no chiral center in repeating units

were applied to asymmetric polymeric catalysts. Yashima et al. have reported on the

catalytic activity of poly(phenyl isocyanide)s with achiral amino pendants (h-poly-

46) that have no chiral functionalities except for macromolecular helicity induced by

the agency of chiral amines followed by complete removal of the chiral amines and

modification of the side-chain groups with achiral amines (Scheme 9.10) [83]. The

dynamic helical polymers promoted the direct aldol condensation to produce

nonracemic aldol derivatives, although the enentiomeric excess was low.

9.4 CONCLUSIONS

This chapter has focused on the syntheses of optically active helical polymers based on

C2 chiral biaryl units as the inherently helically twisted units introduced into the main

chain and helical polymers possessing more rigidly fixed conformations by the rigid

CN

O

OH

CN

O

NNH

CN

O

OH

n

helix inductionand memory

addition removal

Ph

NH2

OH

h-poly-45poly-45

HN NH

condensation agent

n yx

h-poly-46

O2N

H

O

h-poly-46

O

DMSO, 24 h O2N

OH O

* *

11% yield12% ee

CN

O

OH

SCHEME 9.10.

Me

MeN

N HR

R

Me

MeN

NR

R Ar*

MeN

NPPh3

m

n

l

R = CH2OCH2CH2CH3Ar* = chiral initiator

poly-44

+ HSiCl3

[Pd(μ-Cl)(η3−C2H5)]2 ( 0.05 mol%)(S)-poly-44 (0.2 mol%)

SiCl3

*

97% yield85% ee

SCHEME 9.9.

CONCLUSIONS 289

binding of the C2 chiral biaryl units. In addition, there have been discussed on the

asymmetric reactionscatalyzedbyhelicalpolymers, someofwhichuse theasymmetric

field of helix, a secondary structure of polymer.As shown inFigure 9.1, the helix canbe

disintegrated into twomotifs: chiral twisted part and planar part binding the two chiral

twisted parts. According to this protocol,many helical polymers have been prepared to

date, although polymers prepared using onlyC2 chiral biaryls can take the one-handed

helix stable in solution in some cases. Further fixation of helical conformation by using

planar metal complex formation is effective to obtain stable helical conformations

constructed a priori. Meanwhile, it has been demonstrated that the asymmetric fields

providedbyhelicalpolymerseffectivelyworkinvariousasymmetricreactions.Polymer

catalysts having no chiral unit except for its main-chain helicity have also caused the

asymmetric reactions. Thus, the syntheses of helical polymers with predictable con-

formations are of importance from the viewpoint of not only new helical polymer

synthesis but also development of efficient polymer asymmetric catalysts.

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