Dual catalyst system for asymmetric alternating copolymerization of carbon dioxide and cyclohexene oxide with chiral aluminum complexes: Lewis base as catalyst activator and lewis acid as monomer activator

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Dual Catalyst System for Asymmetric Alternating Copolymerization

of Carbon Dioxide and Cyclohexene Oxide with Chiral Aluminum

Complexes: Lewis Base as Catalyst Activator and Lewis Acid as

Monomer Activator

Kiyoshi Nishioka,^†,§Hidetoshi Goto,^†and Hiroshi Sugimoto*^,†,‡

^†Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, 12-1 Ichigaya-Funagawara, Shinjuku,

Tokyo 162-0826, Japan

^‡Chiral Material Research Center, Research Institute for Science & Technology, Tokyo University of Science, Tokyo, Japan

S

* Supporting Information

ABSTRACT: Optically active aluminum complexes such as

Schiﬀ base, binuclear β-ketoiminate, and bisprolinol complexes

were found to promote asymmetric alternating copolymeriza-

tions of carbon dioxide and cyclohexene oxide. The aluminum

Schiﬀ base complexes−tetraethylammonium acetate aﬀorded

isotactic poly(cyclohexene carbonate)s with low enantioselectiv-

ities. Lewis bases having two coordinating sites were utilized to

enhance activity and selectivity based on the binuclear structure

of the aluminum β-ketoiminate clariﬁed by X-ray crystallog-

raphy. [2gAlMe]₂−bulky bisimidazole produced the alternating

copolymer with high enantioselectivity (62% ee). The polymerization is considered to preferentially proceed at more crowded,

enantioselective site owing to coordination of bulky Lewis bases to aluminums in less enantioselective sites. 3₂AlMe−2-picoline

also exhibited a high enantioselectivity (67% ee). Methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) was applied to

perform faster and more enantioselective copolymerizations at low temperature (82% ee). The asymmetric copolymerizations

were found to be signiﬁcantly dependent on size of epoxide, temperature, and kind/amount of activators.

Scheme 1. Copolymerization of CO₂and Epoxide

INTRODUCTION

■

As carbon dioxide (CO₂) has been considered as one of

principal substrates giving rise to global warming since the late

1980s, the reduction of CO₂emission and its concentration in

the atmosphere have been required. On the contrary, CO₂is

the most fundamental substance related in origin of life.

Namely, CO₂is an indispensable carbon source for living

organisms in nature via the transformation of CO₂to useful

chemicals through photosynthesis.¹In the ﬁeld of industrial

chemistry, CO₂has attracted chemists as a C₁feedstock

alternative to fossil resources because CO₂is abundant in

nature, harmless, nonﬂammable, inexpensive, and easy to

handle.²The transformation of CO₂on a commercial scale

has been limited to the productions of urea, methanol, salicylic

acid and carbonates. Therefore, useful processes for converting

CO₂into chemicals are of great attractive and interest.³

Polymeric synthesis using CO₂as a monomer is one of the

most desirable processes to eﬀectively incorporate CO₂into

polymeric main chains and to exhibit remarkable properties.

The ﬁrst example is alternating copolymerization of CO₂and

epoxide reported by Inoue, Koinuma, and Tsuruta in 1969 to

give aliphatic polycarbonates (Scheme 1).^4,5An equimolar

mixture of Et₂Zn and H₂O was used as the catalyst. Since this

discovery, many heterogeneous and homogeneous catalysts

based on various metals were developed for alternating

copolymerization of CO₂and epoxides.⁶⁻³⁴Chemists have

focused their attentions on development of discrete complexes

as catalysts for investigating reaction mechanism or controlling

chemoselectivity, regioselectivity, stereoselectivity (tacticity),

molecular weight, molecular weight distribution, and sequential

arrangement in copolymer.

Because organoaluminum systems are known to be eﬀective

catalytic systems for homopolymerization of epoxide,³⁵

a

Et₃Al−H₂O system (instead of the Et₂Zn−H₂O system) and

a Et₃Al−PPh₃system were utilized for copolymerization of

CO₂and epoxide in the early investigation by Koinuma in 1977

to aﬀord a copolymer with low content of carbonate

linkages.^18aSince then, aluminum complexes have been applied

for the copolymerization by research groups.¹⁶⁻¹⁸Our former

Received: August 10, 2012

Revised: September 25, 2012

A

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group reported the ﬁrst successful alternating copolymerization

of CO₂and epoxide with aluminum porphyrin complex−

quaternary phosphonium or ammonium salts.^16bOur group^17a

and Darensbourg’s group^17bseparately demonstrated alumi-

num salen-type Schiﬀ-base complex−quaternary ammonium

salt or Lewis base systems catalyzing alternating copolymeriza-

tion of CO₂and cyclohexene oxide (CHO). In our system, the

copolymerization proceeded in a living fashion, and the

selectivities for the products (polycarbonate, polyether, and

cyclic carbonate) signiﬁcantly depended on the combination of

the catalyst and cocatalyst.^17aFor instance, an achiral aluminum

salen-type Schiﬀ base complex ((Salophen)AlMe (4AlMe);

Salophen = N,N′-bis(salicylidene)-1,2-phenylenediamine, see

Results and Discussion) in conjunction with tetraethylammo-

nium acetate (Et₄NOAc) or pyridine predominantly generated

the alternating copolymer (polycarbonate), whereas the same

aluminum complex−Et₄NBr induced the production of cyclic

carbonate in 30% against total products.^17a

There are a few examples of CO₂ﬁxation in asymmetric

synthesis though there are a lot of processes, in which CO₂with

substrates is transformed into chemicals, including alternating

copolymerization of CO₂and epoxide. The ﬁrst is a nickel-

catalyzed enantioselective, carboxylative cyclization of bis-1,3-

dienes.³⁶The second is a silver-catalyzed enantioselective CO₂

incorporation into symmetrical bispropargylic alcohols to

generate monopropargylic cyclic carbonates.³⁷The third is an

enantioselective alternating copolymerization of CO₂and meso-

epoxides with enantiopure catalysts to aﬀord polycarbonate-

s.^{14,25a,d,30f,j}These three reactions are categorized as asym-

metric induction transformation. The fourth is an enantiomer-

selective alternating copolymerization or cycloaddition of CO₂

and racemic epoxides with enantiopure catalysts to give

polycarbonates^{25b,c,30a−e,g−i,k}or cyclic carbonates.³⁸This

reaction is classiﬁed into enantiomer-selective transformation

(kinetic resolution). Although these catalyst systems are needed

to exhibit high activities and enantioselectivities, asymmetric

CO₂incorporation is still one of the challenging areas.

binuclear zinc aminoalkoxide complex by Nozaki (Figure

1A).^14a−cThe complex copolymerized CO₂and CHO into

isotactic poly(cyclohexene carbonate) (PCHC) which was

quantitatively converted into trans-1,2-cyclohexanediol with

80% ee (enantiomeric excess) by alkali-catalyzed hydrolysis.

Since then, Coates (Figure 1B),^14dDing (Figure 1C),^14eand Lu

(Figure 1D)^30freported a chiral zinc imine-oxazoline complex

(up to 76% ee), a chiral zinc bisprolinol complex (18% ee), and

a chiral cobalt salen-type Schiﬀ base complex−bis-

(triphenylphosphine)iminium chloride system (up to 38%

ee), respectively. Most recently, Lu and co-workers reported an

unsymmetrical chiral salen cobalt complex−(S)-2-methyltetra-

hydrofran system for the production of PCHC composed of

1,2-cyclohexanediol units with 96%ee (R,R) (Figure 1E).^30j

In this paper, we describe asymmetric alternating copoly-

merization of CO₂and CHO with C₂-symmetric optically active

aluminum complexes easily synthesized from an inexpensive

aluminum reagent and chiral compounds. To our knowledge,

chiral aluminum complexes for asymmetric copolymerization of

CO₂and meso-epoxides have never been reported. The

complexes used in this study were the aluminum salen-type

Schiﬀ base complexes (1aAlMe−1fAlMe),^39,40aluminum β-

ketoiminate complexes ([2aAlMe]₂−[2iAlMe]₂),⁴¹and alumi-

num aminoalkoxide complex (3₂AlMe),⁴²(Figure 2). The

details will be described hereinafter.

RESULTS AND DISCUSSION

■

Synthesis and Characterization of Aluminum Com-

plexes. Three series of optically active aluminum complexes,

aluminum chiral Schiﬀ base complexes (1aAlMe−1fAlMe),^39,40

aluminum chiral β-ketoiminato complexes ([2aAlMe]₂−

[2iAlMe]₂),⁴¹and aluminum prolinol complex (3₂AlMe)⁴²

were synthesized according to Schemes 3, 4, and 5, respectively.

(1R,2R)-1,2-Diphenyl-1,2-ethanediamine⁴³and (1R,2R)-1,2-

cyclohexanediamine⁴⁴were obtained by optical resolutions of

the rac-diamines with ^L-tartaric acid and the subsequent

removal of ^L-tartaric acid with potassium carbonate. The

enantiopure diamines were mixed with salicylaldehyde

derivatives (salicylaldehyde, 3-phenylsalicylaldehyde,⁴⁵and

3,5-di-tert-butylsalicylaldehyde) to aﬀord six salen-type Schiﬀ

base compounds (1aH₂−1fH₂),⁴⁶⁻⁴⁸respectively (Scheme 3).

The reactions of two β-diketones (2,4-pentanedione and 1,3-

diphenyl-1,3-propanedione) with seven amino alcohols (glyci-

nol, ^L-alaninol, L-valinol, L-leucinol, L-isoleucinol, L-phenyl-

alaninol, and ^L-phenylglycinol)⁴⁹generated the corresponding

β-ketoimine compounds (2aH₂−2iH₂),^41,50respectively

(Scheme 4). After N-ethoxycarbonyl ^L-proline methyl ester

An asymmetric alternating copolymerization of CO₂and

CHO being one of asymmetric induction polymerizations

(Scheme 2) was ﬁrst accomplished using an optically active

Scheme 2. Alternating Copolymerization of CO₂and CHO

with Asymmetric Induction

Figure 1. Metal complexes used as catalysts for alternating copolymerization of CO₂and meso-epoxides with asymmetric induction.

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complex having the ﬁve- and six-membered chelate rings. Two

1:1 complexes were then dimerized via μ-O bridges between

two aluminums of each other: the distances in the range of

1.82−1.88 Å (for Al−O), 1.99−2.00 Å (for Al−N), and 1.94−

1.95 Å (for Al−C) and the angles in the range of 105−116°

(for C−Al−O or C−Al−N), 75−91° (for O−Al−O or O−Al−

N, cis), and 138−144° (for O−Al−O or O−Al−N, trans).⁵²

This result exhibited that the aluminum complex adopted a

ﬁve-coordinate, distorted square-pyramidal geometry. Both

methyl groups were connected to the aluminums in a syn

arrangement. The “syn-arrangement” is denoted as an arrange-

ment of the substituents and functional groups in the same

direction against the plane formed by two aluminums and two

tridentate units. In addition, two phenyl groups in [2gAlMe]₂

were also aligned in the syn arrangement the directions whose

were the same as those of the methyl groups on the aluminums.

These observations indicated the bimetallic aluminum complex

adopted a C₂-symmetric structure similar to a truncated

trapezoid (Figure 3B). The R⁵substituents in [2bAlMe]₂−

[2hAlMe]₂(e.g., R⁵= Ph for [2gAlMe]₂) are denoted as “wall”

to regulate the direction of CHO approaching the aluminums.

The spatial region surrounded by the R⁵substituents is

abbreviated as “inside walls” and the opposite region of “inside

walls” against the coordination plane formed by two aluminums

and two [O,N,O] chelating units is named as “outside walls”.

When a nucleophilic reagent (e.g., alkoxide) attacks a C_s-

symmetric meso-epoxide coordinated to the aluminum in the

region “inside walls”, the approaching directions of the regent

to two tertiary carbons of the epoxide are diﬀerentiated by the

phenyl groups attached on the asymmetric tetrahedral carbons.

The asymmetric ring-opening of the meso-epoxide in the

enantioselective site (“inside walls”) may generate the optically

active oxycarbonyloxy-trans-cyclohexane-1,2-diyl repeating

units.

A coordination geometry of [2gAlMe]₂in solution was

investigated by ²⁷Al NMR analysis in noncoordinating solvent

(Figure S2A in the Supporting Information).⁵³Only one signal

assignable to the aluminums of [2gAlMe]₂was observed in the

range of 40−100 ppm at the center of 70 ppm. This suggests

that the four-coordinated complex in the monomeric state was

dimerized to adopt the more stable, ﬁve-coordinated complex

in C₆D₆(Scheme S1, Figure S3 in the Supporting

Information).⁵⁴This result was in good agreement with that

from X-ray crystallographic analysis. Such a formation of

bimetallic ﬁve-coordinated aluminum complexes via dimeriza-

tion was conﬁrmed by Doherty and co-workers:⁴¹a dimeric

structure of a complex [2aAlCl]₂in solution was conﬁrmed by

²⁷Al NMR analysis. Therefore, the structures of other [2AlMe]₂

complexes are considered to be similar to that of the complex

[2gAlMe]₂though the complexes ([2AlMe]₂) except for

Figure 2. Structures of aluminum complexes: aluminum salen-type

Schiﬀ base complexes (1aAlMe−1fAlMe and 4AlX¹s), bimetallic β-

ketoiminate complexes ([2aAlMe]₂−[2iAlMe]₂and [2iAlX²]₂s, and

aminoalkoxide complex (3₂AlMe)).

was synthesized via a spontaneous protection of the amino and

carboxyl groups of ^L-proline, the ester was converted into (S)-

diphenyl(pyrrolidine-2-yl)methanol (3H) by the coupling with

phenylmagnesium bromide and the subsequent base-catalyzed

hydrolysis (Scheme 5).⁵¹Each reaction of 1aH₂−1fH₂, 2aH₂−

2iH₂, or 3H, with trimethylaluminum (Me₃Al) was followed by

¹H NMR spectroscopy; the signals assignable to the phenol (at

13−15 ppm for 1aH₂−1fH₂), enol (at 10−12 ppm for 2aH₂−

2iH₂), or alcohol (at 4.6 ppm for 3H) disappeared, whereas

novel signals assignable to methyl groups vertically connected

to the aluminums (Al−Me) appeared at −2 to −1 ppm. These

results indicated the formation of the aluminum complexes

(1aAlMe−1fAlMe,^39,40[2aAlMe]₂−[2iAlMe]₂,⁴¹and

3₂AlMe⁴²).

The structures of [2gAlMe]₂and 3₂AlMe were investigated

by X-ray crystallography because the complexes were obtained

as suitable single crystals for X-ray crystallographic analysis (the

X-ray data of 3₂AlMe is described before polymerization

results). The single crystal X-ray analysis of [2gAlMe]₂revealed

the formation with the 2:2 stoichiometric ratio of aluminum ion

with the tridentate dianion derived from the β-ketoimine 2gH₂

(Figure 3, Table 1). The chelation of the [O,N,O] atoms in one

dianion to one aluminum ion induced the formation of the 1:1

Scheme 3. Synthesis of Aluminum Chiral Schiﬀ Base Complexes (1aAlMe−1fAlMe)

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Scheme 4. Synthesis of Bimetallic Aluminum β-Ketoiminate Complexes ([2aAlMe]₂−[2iAlMe]₂)

Scheme 5. Synthesis of Aluminum Chiral Aminoalkoxide

Complex (3₂AlMe)

Table 1. Selected Bond Lengths (Å) and Bond Angles (°) for

Complex [2gAlMe]₂

Length/Å

Al(1)−C(1)

Al(1)−O(1)

Al(1)−N(1)

Al(1)−O(2)

Al(1)−O(3)

1.942(9)

1.855(4)

1.985(7)

1.831(5)

1.878(5)

Al(2)−C(2)

Al(2)−O(3)

Al(2)−N(2)

Al(2)−O(4)

Al(2)−O(1)

1.954(9)

1.878(5)

2.003(7)

1.822(5)

1.868(5)

Angle/deg

O(1)−Al(1)−C(1)

C(1)−Al(1)−N(1)

O(2)−Al(1)−C(1)

O(3)−Al(1)−C(1)

O(1)−Al(1)−N(1)

O(2)−Al(1)−N(1)

O(2)−Al(1)−O(3)

O(1)−Al(1)−O(3)

O(2)−Al(1)−O(1)

O(3)−Al(1)−N(1)

Al(1)−O(1)−Al(2)

113.3(4)

109.6(4)

106.9(4)

104.8(4)

81.0(2)

O(3)−Al(2)−C(2)

C(2)−Al(2)−N(2)

O(4)−Al(2)−N(2)

O(1)−Al(2)−C(2)

O(3)−Al(2)−N(2)

O(4)−Al(2)−N(2)

O(4)−Al(2)−O(1)

O(1)−Al(2)−O(3)

O(4)−Al(2)−O(3)

O(1)−Al(2)−N(2)

Al(1)−O(3)−Al(2)

115.8(3)

111.0(3)

105.6(3)

104.1(3)

79.8(3)

90.1(2)

89.8(3)

90.5(3)

91.4(2)

75.3(2)

75.0(2)

139.6(3)

143.8(3)

105.5(2)

138.3(3)

143.2(3)

104.2(3)

Figure 3. (A) Ball-and-stick drawings of [2gAlMe]₂with thermal

ellipsoids at 50% probability (carbon, gray; nitrogen, blue; oxygen, red;

aluminum, pink). Hydrogen atoms are omitted for clarity. (B) Model

of bimetallic aluminum complex [2gAlMe]₂.

Figure 4. Model of aluminum complex 3₂AlMe.

1,2-diphenylethane-1,2-diyl or (1R,2R)-cyclohexane-1,2-diyl

group−Et₄NOAc catalytic systems were ﬁrst employed for

alternating copolymerization of CO₂and CHO with asym-

metric induction (Table 2). The copolymerization was carried

out with the complex 1aAlMe in the presence of Et₄NOAc (1

equiv to 1aAlMe) at 80 °C under 50 atm of CO₂at an initial

mole ratio ([CHO]₀/[1AlMe]₀) of 250 in toluene. The

1aAlMe−Et₄NOAc system gave the alternating copolymer in

73% yield for 24 h (the content of carbonate linkages = 98%,

the number-averaged molecular weight (M_n) = 6800, the

molecular weight distribution (M_w/M_n) = 1.22; Table 2, run 1).

The obtained copolymer was converted into trans-1,2-cyclo-

hexanediol in 85% yield by alkali-catalyzed hydrolysis. The

enantiomeric excess (ee) of the diol was estimated to be 8%

(for (R,R)-isomer) by optical rotation measurement ([α]_D

−3.1° (c 1.4, H₂O)).⁵⁶This value shows the ratio of (R,R)

and (S,S) units ((R,R) units: (S,S) units = 54: 46) in the main

chain of the copolymer.

[2gAlMe]₂were not crystallized as suitable single crystals for X-

ray crystallographic analysis.

In the crystal, the aluminum bisprolinol complex 3₂AlMe

formed a ﬁve-coordinate, C₂-symmetric structure similar to the

reported literature (Figures 4 and S1 and Table S2 in the

Supporting Information).⁴²The complex 3₂AlMe had two

regions: one region is “inside walls” enclosed by two phenyl

and two pyrroridyl groups as “wall” and the other is “outside

walls” surrounded by two remaining phenyl groups alone. Since

phenyl groups and pyrroridyl groups in the complex 3₂AlMe

are denoted as “wall” to regulate the direction of CHO

approaching the aluminum, the spatial region encircled by

“walls” (two phenyl and two pyrroridyl groups) are abbreviated

as “inside walls” and the opposite region of “inside walls”

against the coordination plane formed by one aluminum and

two [N,O] chelating units are named as “outside walls”.

Asymmetric Alternating Copolymerization of CO₂

and CHO with Aluminum Salen-Type Schiﬀ Base

Complexes−Et₄NOAc. A series of optically active salen-type

Schiﬀ base complex^25,30,55(1aAlMe−1fAlMe) having (1R,2R)-

To investigate eﬀect of substituents connected to the

phenolate rings on enantioselectivity of the catalysts, aluminum

complexes (1bAlMe and 1cAlMe) having one phenyl group at

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Table 2. Asymmetric Alternating Copolymerization of CO₂and CHO with Aluminum Salen-Type Schiﬀ Base Complexes−

a

Et₄NOAc.

copolymerization

hydrolysis

yield of diol/%

b

c

d

e

f

run

complex

temp/°C

time/h

yield /%

TOF /h⁻¹

C.L. /%

M_n

M_w/M_n

%ee (R,R)

1

2

1aAlMe

1bAlMe

1cAlMe

1dAlMe

1eAlMe

1fAlMe

1aAlMe

1cAlMe

80

60

40

60

24

6

73

24

29

26

20

31

21

32

58

8

6.2

2.1

2.5

2.2

1.7

2.6

8.8

2.7

6.0

3.3

98

94

97

94

96

94

99

70

97

6800

1900

4400

5300

3100

2800

2000

6700

5800

2200

1.22

1.60

1.14

1.23

1.29

1.40

1.23

1.26

1.98

1.66

85

91

88

90

78

95

93

89

79

8

12

14

5

3

4

5

10

9

6

7

9

8

24

6

13

20

23

9

10

88

a

b

In 1 mL of toluene, 80 °C, 50 atm of CO₂, complex 1AlMe (0.1 mmol), Et₄NOAc (0.1 mmol), and CHO (25 mmol). On the basis of isolated

c

d

copolymer. Turnover frequency = (mol of reacted CHO)·(mol of cat.)⁻¹·h⁻¹. Content of carbonate linkages (C.L.) = (carbonate unit)·((carbonate

e

f

unit) + (ether unit))⁻¹, estimated by ¹H NMR (CDCl₃). Estimated by GPC calibrated with polystyrene standard in THF at 40 °C. Estimated from

optical rotation of trans-1,2-cyclohexanediol in H₂O.

the 3-position (for 1bAlMe) and two tert-butyl groups at the 3-

and 5-positions (for 1cAlMe) of the phenolate ring were

employed for the copolymerization (Table 2, runs 2 and 3).

The introduction of the bulky groups to the phenolate was

expected to improve enantioselectivity. The complexes 1bAlMe

and 1cAlMe produced alternating copolymers with more than

94% content of carbonate linkages (M_n= 1900, M_w/M_n= 1.60

(for 1bAlMe) and M_n= 4400, M_w/M_n= 1.14 (for 1cAlMe))

and slightly higher enantioselectivities (12% ee (for 1bAlMe)

and 14% ee (for 1cAlMe)) than that (8% ee) by 1aAlMe. In

contrast, the turnover frequencies (TOFs) were signiﬁcantly

reduced (2.1 h⁻¹(for 1bAlMe) and 2.5 h⁻¹(for 1cAlMe)),

meaning that the rates of the copolymerizations with 1bAlMe

and 1cAlMe were slower than that by 1aAlMe. These results

indicate that the introduction of the bulky substituents around

the active aluminums in the catalysts led to not only the

expected improvement of the enantioselectivity (H < Ph ≈ ^tBu)

but also the undesirable lowering of the copolymerization rate

tivities in asymmetric reactions are signiﬁcantly dependent on

temperature.⁵⁷Thus, the copolymerizations with 1aAlMe−

Et₄NOAc were performed at 60 and 40 °C (Table 2, runs 8 and

9). The enantioselectivities expectedly increased with lowering

of the temperature; 13% ee (for 60 °C) and 20% ee (for 40

°C). However, the copolymer synthesized at 40 °C had a low

content of carbonate linkages ([carbonate linkages]:[ether

linkages] = 70: 30). The highest enantioselectivity of 23% was

achieved among the asymmetric copolymerizations with the

catalysts 1aAlMe−1fAlMe as the complex 1cAlMe was used at

60 °C instead of 1aAlMe (Table 2, run 10).

In the cases except for runs 1 and 9 in Table 2, the polymer

yields were relatively low (8−32%). This is because the

polymerizations were quenched at the predetermined time

regardless of the conversion of CHO on the basis of the results

in Table 2, runs 1 and 7. In addition, the observed M_nvalues

were smaller than the calculated ones (e.g., Table 2, run 1; the

observed M_n= 6800, the calculated M_n= 13000). This

disagreement is due to the contamination of water into the

reaction system.⁵⁸

Asymmetric Alternating Copolymerization of CO₂

and CHO with Aluminum β-Ketoiminate Complexes−

Et₄NOAc. The chiral aluminum Schiﬀ-base complexes

(1aAlMe−1fAlMe) for the asymmetric alternating copolymer-

ization of CO₂and CHO were considered to be less eﬀective as

described above. Thus, aluminum complexes having chiral

skeletons, which have never been used for the asymmetric

alternating copolymerization of CO₂and epoxide, are needed

to aﬀord a more highly enantioenriched copolymer. Metal

complexes exhibiting catalytic activity for ring-opening

polymerization of lactide are known to be eﬀective for

alternating copolymerization of CO₂and epoxide.⁵⁹In fact,

aluminum salen-type ligand complexes including 1fAlMe have

been known as active catalysts for polymerizations of lactide

and for copolymerization of CO₂and epoxide.⁶⁰Doherty and

co-workers reported that aluminum β-ketoiminate complexes

including [2aAlCl]₂functioned as catalysts for ring-opening

polymerization of rac-lactide by an addition of PO or CHO.⁴¹

In addition, achiral and racemic cobalt β-ketoiminate complexes

were recently reported as eﬀective catalysts for alternating

copolymerization of CO₂and epoxide (in particular, ethylene

oxide) by Yamada.³²However, aluminum β-ketoiminate

complexes have been never employed as catalyst for

t

(H > Ph ≈ Bu).

The eﬀect of diimine backbones ((1R,2R)-1,2-diphenyl-

ethane-1,2-diyldiimino (1aAlMe−1cAlMe) and (1R,2R)-cyclo-

hexane-1,2-diyldiimino (1dAlMe−1fAlMe) groups) on enan-

tioselectivity was next investigated using the complexes

1dAlMe−1fAlMe (Table 2, runs 4−6). The complexes

1dAlMe−1fAlMe showed lower enantioselectivities (5−10%

ee) than those of the complexes 1aAlMe−1cAlMe (8−14% ee),

while other polymerization data (TOF, content of carbonate

linkages, M_n, and M_w/M_n) with 1dAlMe−1fAlMe were similar

to those with 1aAlMe−1cAlMe. These results revealed that the

trans-1,2-diphenylethane-1,2-diyldiimino backbone was more

eﬀective for asymmetric alternating copolymerization of CO₂

and CHO with chiral aluminum complex than the trans-

cyclohexane-1,2-diyldiimino one.

Other conditions (time and temperature) aﬀecting enantio-

selectivity, content of carbonate linkages, and so on were

reﬁned for the copolymerization of CO₂and CHO with the

complexes 1aAlMe−1cAlMe. At 6 h for the copolymerization

with 1aAlMe−Et₄NOAc, the enantioselectivity and content of

carbonate linkages were 9% ee and 94%, respectively (Table 2,

run 7), indicating that enantioselectivity and content of

carbonate linkages are independent of polymerization time. In

general, asymmetric reactions with high enantioselectivities

have been achieved at low temperature because enantioselec-

E

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a

Table 3. Asymmetric Alternating Copolymerization of CO₂and CHO with Aluminum β-Ketoiminate Complexes−Et₄NOAc.

copolymerization

hydrolysis

yield of diol/%

b

c

d

e

f

run

complex

yield /%

TOF /h⁻¹

C.L. /%

M_n

M_w/M_n

%ee (R,R)

1

2

3

4

5

6

7

8

[2aAlMe]₂

[2bAlMe]₂

[2cAlMe]₂

[2dAlMe]₂

[2eAlMe]₂

[2fAlMe]₂

[2gAlMe]₂

[2hAlMe]₂

36

26

16

19

20

28

18

32

15.0

10.8

6.7

97

96

98

96

97

99

98

4200

2700

2500

4900

3100

3600

2000

8800

1.21

1.29

1.22

1.24

1.14

1.27

1.18

1.38

89

88

94

90

79

87

95

93

−

10

15

14

18

20

26

24

7.9

8.3

11.7

7.5

13.3

a

b

In 1 mL of toluene, 60 °C, 6 h, 50 atm of CO₂, complex [2AlMe]₂(0.1 mmol), Et₄NOAc (0.2 mmol), and CHO (25 mmol). On the basis of

c

d

isolated copolymer. Turnover frequency = (mol of reacted CHO)·(mol of cat.)⁻¹·h⁻¹. Content of carbonate linkages (C.L.) = (carbonate

e

unit)·((carbonate unit) + (ether unit))⁻¹, estimated by ¹H NMR (CDCl₃). Estimated by GPC calibrated with polystyrene standard in THF at 40

f

°C. Estimated from optical rotation of trans-1,2-cyclohexanediol in H₂O.

a

Table 4. Asymmetric Alternating Copolymerization of CO₂and CHO with Aluminum β-Ketoiminate Complexes−Lewis Bases.

copolymerization

hydrolysis

yield of diol/%

b

c

d

e

f

run

complex

Lewis base

pyridine

yield /%

TOF /h⁻¹

C.L. /%

M_n

M_w/M_n

%ee (R,R)

1

2

3

4

[2gAlMe]₂

[2hAlMe]₂

12

10

12

16

5.0

4.2

5.0

6.7

99

90

97

99

2900

1900

1600

4300

1.18

2.27

1.26

1.18

89

87

94

91

22

19

39

49

g

N-MeIm

DMAP

a

In 1 mL of toluene, 60 °C, 6 h, 50 atm of CO₂, complex [2gAlMe]₂and [2hAlMe]₂(0.1 mmol), Lewis base (0.2 mmol), and CHO (25 mmol); N-

b

c

MeIm = N-methylimidazole, DMAP = N,N-dimethyl-4-aminopyridine. On the basis of isolated copolymer. Turnover frequency = (mol of reacted

d

CHO)·(mol of cat.)⁻¹·h⁻¹. Content of carbonate linkages (C.L.) = (carbonate unit)·((carbonate unit) + (ether unit))⁻¹, estimated by H NMR

1

e

f

(CDCl₃). Estimated by GPC calibrated with polystyrene standard in THF at 40 °C. Estimated from optical rotation of trans-1,2-cyclohexanediol in

H₂O. 0.5 mmol.

g

(asymmetric) copolymerization of CO₂and epoxide Thus, an

achiral aluminum β-ketoiminate complex [2aAlMe]₂was ﬁrst

attempted for alternating copolymerization of CO₂and CHO.

The copolymerization of CO₂and CHO with [2aAlMe]₂in the

presence of Et₄NOAc (1 equiv to Al) was carried out at 60 °C

for 6 h under 50 atm of CO₂at an initial mole ratio ([CHO]₀/

[[2aAlMe]₂]₀) of 250 in toluene. The [2aAlMe]₂−tetraethy-

lammonium acetate (Et₄NOAc) system gave the alternating

copolymer with the narrow molecular weight distribution in

36% yield (content of carbonate linkages = 97%, M_n= 4200,

M_w/M_n= 1.21; Table 3, run 1). The TOF (15.0 h⁻¹) was larger

than that using 1aAlMe−1fAlMe (1.7−8.8), meaning that the

polymerization with [2aAlMe]₂were faster than those with

1aAlMe−1fAlMe.

generated by using all complexes, and the ees of the diols

obtained by alkali-hydrolysis of the polymers increased with

increasing the sizes of the R⁵substituents (ee = 10%, 15%, 14%,

18%, 20%, 26%; size = Me < ⁱPr < ⁱBu < ^secBu < Bn < Ph). The

TOFs of the copolymerizations with [2bAlMe]₂−[2gAlMe]₂

(6.7−11.7 h⁻¹) were smaller than that with [2aAlMe]₂(15.0

h⁻¹), meaning the slower polymerizations with [2bAlMe]₂−

[2gAlMe]₂than that with [2aAlMe]₂. This is considered to be

due to the steric repulsion between the R⁵groups and CHO in

spite of no observation of relationship between the TOFs and

the sizes of the R⁵s. The M_ns were in the range of 2000−4900,

which were independent of the structures of the R⁵s. The

observed M_ns were signiﬁcantly lower than the calculated

values, resulting in the contamination of water into the

polymerization systems similar to those with the aluminum

salen-type Schiﬀ base complexes (1aAlMe−1fAlMe). Sub-

sequently, a complex [2hAlMe]₂(R⁶= Ph) having a more

bulky β-diketone skeleton than that of [2gAlMe]₂(R⁶= Me)

was applied for the copolymerization of CO₂and CHO to

aﬀord a more highly enantioenriched copolymer (Table 3, run

8). Contrary to expectations, the catalyst showed an

enantioselective ability similar to [2gAlMe]₂(26% (for

[2gAlMe]₂) and 24% (for [2hAlMe]₂) ees). On the other

hand, the TOF of [2hAlMe]₂(13.3 h⁻¹) was high as compared

with that of [2gAlMe]₂(7.5 h⁻¹), meaning the higher catalytic

activity of [2hAlMe]₂than that of [2gAlMe]₂. In addition, the

M_nof the copolymer obtained with [2hAlMe]₂was higher than

that with [2gAlMe]₂. These results indicate that the

introduction of the bulky substituents to the ketoimine skeleton

(R⁶−C(OH)CH−C(N)−R⁶) is valid to improve the

catalytic activity but not the enantioselectivity.

On the basis of the structure of the achiral complex

[2aAlMe]₂being active for the copolymerization, optically

active β-ketoiminate complexes ([2bAlMe]₂−[2hAlMe]₂) were

designed. Chiral β-ketoiminate complexes consist of aluminum

compounds (trialkylaluminums or aluminum halides) and

chiral β-ketoimines readily prepared from β-diketones and

chiral amino alcohols. The amino alcohols were obtained by

reduction of amino acids. Since chiral amino acids are easily

available as enantiomerically pure forms from naturally

abundant sources or by using asymmetric synthetic methods,⁶¹

chiral β-ketoimines and their metal complexes are attractive as

ligands and catalysts. The copolymerizations with the

aluminum chiral β-ketoiminate complexes having diﬀerent R⁵

i

substituents (R⁵= Me ([2bAlMe]₂), Pr ([2cAlMe]₂), Bu

([2dAlMe]₂), ^secBu ([2eAlMe]₂), Bn ([2fAlMe]₂), Ph

([2gAlMe]₂) were performed under the same condition as

that with [2aAlMe]₂(Table 3, runs 2−7). The alternating

copolymers (content of carbonate linkages = 96−99%) were

F

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Scheme 6. Active Propagating Species Generated in Copolymerization of CO₂and CHO with Aluminum β-Ketoiminate

Complexes and Et₄NOAc

Scheme 7. Active Propagating Species Generated in Copolymerization of CO₂and CHO with Aluminum β-Ketoiminate

Complexes and Lewis Bases (Monoamines)

Asymmetric Alternating Copolymerization of CO₂

and CHO with Aluminum β-Ketoiminate Complexes−

Lewis Bases. To ﬁnd out condition suitable for asymmetric

alternating copolymerization of CO₂and CHO with optically

active aluminum β-ketoiminate complexes, N-containing Lewis

bases such as pyridine, N-methylimidazole (N-MeIm), and

N,N-dimethyl-4-aminopyridine (DMAP) instead of Et₄NOAc

were employed as cocatalysts in the presence of the complexes

[2gAlMe]₂and [2hAlMe]₂(Table 4). The utilizations of

pyridine and N-MeIm led to the decreases of the catalytic

activities (TOF: 5.0 h⁻¹(for pyridine), 4.2 h⁻¹(for N-MeIm)

vs 7.5 h⁻¹(for OAc)), the enantioselectivities (ee: 22% (for

pyridine), 19% (for N-MeIm) vs 26% (for OAc)), and the

content of carbonate linkages (C.L.: 90% (for N-MeIm))

(Table 4, runs 1, 2). In contrast, the alternating copolymer

generated with DMAP was comprised of the repeating units

with 39% ee (C.L., 97%; M_n, 1600; M_w/M_n, 1.26) (Table 4, run

3) being the largest value among those obtained with Lewis

bases. Hence, DMAP was used to the copolymerization with

the complex [2hAlMe]₂to aﬀord the alternating copolymer

with high enantioselectivity of 49% ee (C.L., 99%; M_n, 4300;

M_w/M_n, 1.18) (Table 4, run 4). These results suggest that the

pK_aof Lewis base and acetate aﬀects the enantioselective ring-

opening of CHO (pK_a: 4.76 for OAc; 5.19 for pyridine; 7.06 for

N-MeIm; 9.70 for DMAP).⁶²

Complexes. We previously described the following two points

regarding alternating copolymerizations of CO₂and CHO with

an achiral aluminum Schiﬀ base complex (4AlMe)−Et₄NOAc

system. One is that the anion species assignable to the

ammonium salt is coordinated to the aluminum to adopt a six-

coordinated ate complex having two axial ligands (the methyl

anion of the original ﬁve-coordinated aluminum complex and

the counteranion derived from the ammonium salt). The other

is that both two anionic species on the aluminum

independently served as the initiators to allow the propagation

of two polymeric chains on both axial sites.^17aOn the basis of

our knowledge, the initiation and propagation mechanism of

the polymerization with the chiral bimetallic aluminum β-

ketoiminate complexes−Et₄NOAc was proposed as shown in

Scheme 6. The coordination of two acetate ions to two

aluminums in the single complex brings about the formation of

two six-coordinated ate structures having four anionic ligands.

Four anionic species on the aluminums severally function as the

initiators on the axial sites to permit the propagation of four

polymeric chains: two chains are placed in the same direction as

the R⁵substituents and two remaining chains are placed in the

opposite direction as the R⁵s. The enantioselective ring-opening

of CHO most likely proceeds in the region “inside walls” as the

R⁵s regulate the orientation of CHO approaching the

aluminum. On the other hand, the less enantioselective ring-

opening of CHO probably advances in the opposite direction as

the R⁵s.

Mechanism of Asymmetric Alternating Copolymer-

ization of CO₂and CHO with Aluminum β-Ketoiminate

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Furthermore, a mechanism of the polymerization with the

chiral bimetallic aluminum β-ketoiminate complexes−mono-

amines was described in Scheme 7. Both two methyl anionic

species connected to the aluminums independently act as the

initiators to allow the propagation of two polymeric chains. The

behavior of aluminum−alkoxide and −carboxylate bonds in the

growing step of the polymer chains is assumed to cause the

formation of three active species of the aluminum β-

ketoiminate complex ([2gAlMe]₂)−Lewis base system as

follow (Scheme 7): (A) form is a active species having two

alkoxide groups of the chain’s terminal in the region “inside

walls”, (B) form is that having one alkoxide group in “inside

walls” and the other in “inside walls”, and (C) form is that

having two alkoxide groups of the chain’s terminal in “outside

walls”.

Information). The ¹H NMR spectrum of the complex

[2iAlO^tBu]₂showed a single signal at 1.25 ppm due to the

tert-butoxide group in [2iAlO^tBu]₂. At 10 min after mixing of

two aluminum complexes, a new signal appeared at 1.26 ppm

assignable to the methyl protons of the tert-butoxide group in

4AlO^tBu. The intensity of the signal at 1.26 ppm gradually

increased with time and reached a constant value (approx-

imately 25% mole fraction based on the signal area) at 60 min

(Figure 6). This result is in agreement with the exchange

between the polymeric species in more enantioselective and

less enantioselective sites.

As shown in Scheme 7, the exchange between alkoxide and

carboxylate groups on the aluminums is presumed to proceed

during the copolymerization of CO₂and epoxide. This is

because the exchange of alkoxide and carboxylate groups as the

growing species on the aluminum porphyrin was previously

observed in the polymerizations of epoxide or β-lactone.⁶³To

clarify the behavior of the aluminum−alkoxide and −carbox-

ylate bonds in the aluminum complexes, an equimolar mixture

of two aluminum complexes with diﬀerent axial groups from

1

each other, 4AlOAc and [2iAlO^tBu]₂, was investigated by H

NMR analysis (Scheme 8, Figures 5 and S4 in the Supporting

Figure 6. Relationship between the mole fractions of 4AlOAc

(squares) and 4AlO^tBu (circles) and the reaction time in the

equimolar mixture of 4AlOAc and [2iAlO^tBu]₂in CDCl₃at 30 °C.

Scheme 8. Exchange of Axially-Coordinated Alkoxide

(O^tBu) and Carboxylate Groups (OAc) Between Two

Diﬀerent Aluminum Complexes (4AlOAc and [2iAlO^tBu]₂)

Asymmetric Alternating Copolymerization of CO₂

and CHO with Aluminum β-Ketoiminate Complexes−

Bidentate Lewis Bases. The aluminum β-ketoiminate

complexes ([2aAlMe]₂−[2iAlMe]₂) are presumed to have

kept the dimeric structures even during the copolymerization. If

diamines having two nitrogens in parallel in one molecule are

applied instead of monoamines as Lewis bases, the diamines are

expected to be coordinated to two aluminums in the complexes

with the molar ratio of 1:1 (Scheme 9).⁶⁴The (B) form of the

aluminum β-ketoiminate/diamine complex is the forbidden

structure. In addition, the steric hindrance between the

diamines and the complexes most likely induces the formation

of the (A) form rather than the (C) one to promote the

copolymerization at the more enantioselective site. An

enantioselective reaction by using coordination of a Lewis

base to restrict reaction sites was reported by Inoue et al.⁶⁵On

the basis of our strategy, a bisimidazole (two N-MeIms are

linked at the 2-positions of each other, bis-N-MeIm⁶⁶) was

employed for the copolymerization with the complex

[2gAlMe]₂or [2hAlMe]₂(Table 5, runs 1 and 2).⁶⁷The

complex [2gAlMe]₂or [2hAlMe]₂−bis-N-MeIm systems

produced alternating copolymers (96−97% for contents of

carbonate linkages) with the molar ratios of 78.5: 21.5 (57% ee

(R,R)) and 77: 23 (54% ee (R,R)), respectively. When more

bulky bisbenzimidazole (two N-methylbenzimidazoles are

coupled similarly, bis-N-MeBzIm⁶⁶) was used with the complex

[2gAlMe]₂and [2hAlMe]₂(Scheme 10), the obtained

copolymers had higher contents of carbonate linkages (98%

for [2gAlMe]₂and 96% for [2hAlMe]₂) and higher

enantioselectivities (62% ee for [2gAlMe]₂and 60% ee for

[2hAlMe]₂) than those with bis-N-MeIm (Table 5, runs 3 and

4). In particular, the M_nof the copolymer generated with the

complex [2gAlMe]₂−bis-N-MeBzIm was close to the calculated

one.

1

Figure 5. H NMR spectra of the equimolar mixture of 4AlOAc and

[2iAlO^tBu]₂at 30 °C in CDCl₃at 0 (upper) and 30 (lower) min after

mixing.

H

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Scheme 9. Active Propagating Species Generated in Copolymerization of CO₂and CHO with [2aAlMe]₂−[2iAlMe]₂and Bis-N-

MeIm (Diamine)

Table 5. Asymmetric Alternating Copolymerization of CO₂and CHO with Aluminum β-Ketoiminate Complexes−

a

Bisimidazoles.

copolymerization

hydrolysis

yield of diol/%

b

c

d

e

f

run

complex

Lewis base

yield /%

TOF /h⁻¹

C.L. /%

M_n

M_w/M_n

%ee (R,R)

1

2

3

4

[2gAlMe]₂

[2hAlMe]₂

[2gAlMe]₂

[2hAlMe]₂

bis-N-MeIm

10

20

12

19

4.2

8.3

5.0

7.9

97

96

98

96

3200

5800

3900

5700

1.29

1.17

1.41

1.24

90

97

87

91

57

54

62

60

bis-N-MeBzIm

a

In 1 mL of toluene, 60 °C, 6 h, 50 atm of CO₂, complex [2gAlMe]₂and [2hAlMe]₂(0.1 mmol), Lewis base (0.1 mmol), and CHO (25 mmol); bis-

b

N-MeIm = N-methyl-2-(N′-methylimidazol-2-yl)imidazole, bis-N-MeBzIm = N-methyl-2-(N′-methylbenzimidazol-2-yl)benzimidazole. On the

c

d

basis of isolated copolymer. Turnover frequency = (mol of reacted CHO)·(mol of cat.)⁻¹·h⁻¹. Content of carbonate linkages (C.L.) = (carbonate

e

unit)·((carbonate unit) + (ether unit))⁻¹, estimated by ¹H NMR (CDCl₃). Estimated by GPC calibrated with polystyrene standard in THF at 40

f

°C. Estimated from optical rotation of trans-1,2-cyclohexanediol in H₂O.

To investigate whether the Lewis bases are coordinated to

the bimetallic aluminum β-ketoiminate complexes in solution

or not, the mixture of [2gAlMe]₂and bis-N-MeIm (1 equiv to

[2gAlMe]₂) in C₆D₆was measured by ²⁷Al NMR analysis

(Figure S2B in the Supporting Information). Two signals

assignable to the aluminums of [2gAlMe]₂were observed at 68

and 12 ppm in the range of 40−100 ppm. One signal is due to

the ﬁve-coordinated aluminum species without bis-N-MeIm,

whereas the other is assignable to the six-coordinated aluminum

species generated via the coordination of bis-N-MeIm. In

addition, the observation of two signals means the slow ligand

exchange on the NMR time scale. Moreover, the area of the

former is larger than that of the latter regardless of the

equimolar mixture of [2gAlMe]₂and bis-N-MeIm in C₆D₆.

This suggests a part of the binuclear aluminum complexes

forms the active species with the Lewis base under the

condition.

of CO₂and CHO with optically active aluminum complexes,

lowering temperature is one of eﬀective methods. In addition,

utilization of accelerator allows improvement of yield within

predetermined time because reaction generally tends to become

slow at lower temperature. In our previous research,⁶⁸a ring-

opening polymerization of propylene oxide with an aluminum

porphinate complex was substantially accelerated 460-folds by

addition of 0.5 equiv of methylaluminum bis(2,6-di-tert-butyl-4-

methylphenoxide) (MAD⁶⁹). This is owing to the coordinative

activation of epoxide by MAD. According to this knowledge,

MAD was applied for alternating copolymerization of CO₂and

CHO with [2gAlMe]₂− or [2hAlMe]₂−bis-N-MeBzIm sys-

tems. First, the increase of the feed ratio of MAD to the catalyst

[2gAlMe]₂(0 to 1 equiv) was related to the following changes:

not only the increase of the TOF (4.2 to >10.4 h⁻¹; 1.1 to 2.7-

folds to that with [2gAlMe]₂−bis-N-MeBzIm: 3.9 h⁻¹) but also

the decreases of the ee (61 to 39% ee; 63% ee for [2gAlMe]₂−

bis-N-MeBzIm) and the content of carbonate linkages (93 to

35%; > 98% for [2gAlMe]₂−bis-N-MeBzIm) and the increase

of the M_w/M_n(1.51 to 1.93; 1.31 for [2gAlMe]₂−bis-N-

Asymmetric Alternating Copolymerization with Alu-

minum β-Ketoiminate Complex−Lewis Bases−Lewis

Acid. To enhance enantioselectivities for copolymerizations

I

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Scheme 10. Active Propagating Species Generated in Copolymerization of CO₂and CHO with [2gAlMe]₂− or [2hAlMe]₂−bis-

N-MeBzIm (Bulky Diamine)

a

Table 6. Asymmetric Alternating Copolymerization of CO₂and CHO with [2gAlMe]₂−bis-N-MeBzIm−MAD.

copolymerization

hydrolysis

yield of diol/% %ee (R,R)

b

c

d

e

f

run [2gAlMe]₂/equiv bis-N-Me BzIm/equiv MAD/equiv yield /% TOF /h⁻¹C.L. /%

M_n

M_w/M_n

1

2

3

4

5

6

7

1

0.5

1

0

37

40

3.9

4.2

98

93

90

70

35

95

63

7900

6900

1.31

1.51

1.42

1.91

1.93

1.32

2.01

92

94

86

89

96

93

91

63

61

59

51

39

62

47

0.1

0.3

0.5

1

61

6.4

11 300

13 800

12 600

9100

76

7.9

>99

52

>10.4

5.4

0.3

1

90

9.4

17 500

a

In 1 mL of toluene, 60 °C, 24 h, 50 atm of CO₂, complex [2gAlMe]₂(0.1 mmol), and CHO (25 mmol); bis-N-MeBzIm = N-methyl-2-(N′-

b

methylbenzimidazol-2-yl)benzimidazole, MAD = methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide). On the basis of isolated copolymer.

c

d

Turnover frequency = (mol of reacted CHO)·(mol of cat.)⁻¹·h⁻¹. Content of carbonate linkages (C.L.) = (carbonate unit)·((carbonate unit) +

e

f

(ether unit))⁻¹, estimated by ¹H NMR (CDCl₃). Estimated by GPC calibrated with polystyrene standard in THF at 40 °C. Estimated from optical

rotation of trans-1,2-cyclohexanediol in H₂O.

MeBzIm) (Table 6, runs 1−5). Furthermore, various ratios of

[2gAlMe]₂−bis-N-MeBzIm−MAD were attempted to establish

the catalytic system which produces the copolymer in a high

yield with a high content of carbonate linkages and an

enantioselectivity. The optimized ratio was [2gAlMe]₂/[bis-N-

MeBzIm]/[MAD] = 1/1/0.3. The system produced the

alternating copolymer in 52% yield (TOF: 5.4 h⁻¹; the slight

acceleration) with the content of carbonate linkages (95%) and

the enantioselectivity (62% ee) (Table 6, run 6) similar to

those of the polymer obtained by an MAD-free catalytic system

(98% and 62% ee, respectively) in Table 5, run 3. The eﬀect of

MAD on the copolymerization is described afterward (in

section of copolymerization with 3₂AlMe).

The copolymerizations with the [2gAlMe]₂−bis-N-MeB-

zIm−MAD (1: 1: 0.3) system were performed at lower

temperature (60 °C to 0−40 °C) (Table 7, runs 1−4). The

decrease of the temperature expectedly caused the increase of

the ee value of the trans-(R,R)-1,2-diol. In contrast, the content

of carbonate linkages unexpectedly decreased with lowering of

the temperature. The decrease of the content of carbonate

linkages may be owing to decrease of reactivity of CO₂at low

temperature as shown in Table 2, run 9 or increase of reactivity

of epoxide upon the addition of MAD as observed in Table 7.

For example, at 0 °C the catalytic system produced the

copolymer with a low content of carbonate linkages (42%) (M_n

= 2300, M_w/M_n= 1.93) in 19% yield. The copolymer was

J

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Table 7. Asymmetric Alternating Copolymerization of CO₂and CHO with [2gAlMe]₂or [2hAlMe]₂−bis-N-MeBzIm−MAD at

a

Various Temperatures.

copolymerization

hydrolysis

yield of diol/%

b

c

d

e

f

run

complex

temp. /°C

yield /%

TOF /h⁻¹

C.L. /%

M_n

M_w/M_n

%ee (R,R)

1

2

3

4

5

6

7

8

[2gAlMe]₂

[2hAlMe]₂

60

40

25

0

52

41

37

19

62

49

40

21

5.4

4.3

3.9

2.0

6.5

5.1

4.2

2.2

95

88

59

42

94

86

63

47

9100

6500

6000

2300

10 200

7300

6400

2500

1.32

1.51

1.54

1.93

1.47

1.29

1.60

1.55

93

89

84

80

89

90

85

86

62

69

74

80

64

74

75

80

60

40

25

0

a

In 1 mL of toluene, 24 h, 50 atm of CO₂, complex [2gAlMe]₂or [2hAlMe]₂(0.1 mmol), bis-N-MeBzIm (0.1 mmol), MAD (0.03 mmol), and

CHO (25 mmol); bis-N-MeBzIm = N-methyl-2-(N′-methylbenzimidazol-2-yl)benzimidazole, MAD = methylaluminum bis(2,6-di-tert-butyl-4-

b

c

d

methylphenoxide). On the basis of isolated copolymer. Turnover frequency = (mol of reacted CHO)·(mol of cat.)⁻¹·h⁻¹. Content of carbonate

e

linkages (C.L.) = (carbonate unit)·((carbonate unit) + (ether unit))⁻¹, estimated by H NMR (CDCl₃). Estimated by GPC calibrated with

polystyrene standard in THF at 40 °C. Estimated from optical rotation of trans-1,2-cyclohexanediol in H₂O.

1

f

a

Table 8. Asymmetric Alternating Copolymerization of CO₂and CHO with 3₂AlMe−Lewis Bases.

copolymerization

hydrolysis

yield of diol/%

b

c

d

e

f

run

Lewis base

yield /%

TOF /h⁻¹

C.L. /%

M_n

M_w/M_n

%ee (R,R)

1

2

DMAP

10

8

0.35

0.28

>99

96

28 800

25 600

1.92

2.10

82

97

59

67

2-picoline

a

In 1 mL of toluene, 60 °C, 72 h, 50 atm of CO₂, complex 3₂AlMe (0.1 mmol), Lewis base (0.1 mmol), and CHO (25 mmol); DMAP = N,N-

b

c

d

dimethyl-4-aminopyridine. On the basis of isolated copolymer. Turnover frequency = (mol of reacted CHO)·(mol of cat.)⁻¹·h⁻¹. Content of

e

carbonate linkages (C.L.) = (carbonate unit)·((carbonate unit) + (ether unit))⁻¹, estimated by H NMR (CDCl₃). Estimated by GPC calibrated

1

f

with polystyrene standard in THF at 40 °C. Estimated from optical rotation of trans-1,2-cyclohexanediol in H₂O.

transformed to the trans-1,2-diol with 80% ee⁷⁰and the oligo-

or polyether chains with an optical activity ([α]_D−16.7° (c 0.6,

benzene)) by alkaline hydrolysis.⁷¹

salen-type Schiﬀ base complexes^16,17showing activity for the

copolymerization of CO₂and epoxide (Table 8, run 1). Thus,

the copolymerization of CO₂and CHO with 3₂AlMe in the

presence of DMAP (1 equiv to 3₂AlMe) was carried out at 60

°C for 72 h under 50 atm of CO₂at an initial mole ratio

([CHO]₀/[3₂AlMe]₀) of 250 in toluene. The obtained

copolymer had perfect carbonate linkages (over 99%) with a

higher enantioselectivity of 59% ee ((R,R) units: (S,S) units

=79.5: 20.5) than those with the complexes 1aAlMe−1fAlMe

and [2aAlMe]₂−[2hAlMe]₂(Tables 2−4). However, the

copolymerization was relatively slower (TOF = 0.35 h⁻¹)

than those with the complexes 1aAlMe−1fAlMe and

[2aAlMe]₂−[2hAlMe]₂(Tables 2−4). Compared with the

calculated M_n(3500), the observed molecular weight was

28800. It is presumed that the large M_nand broad M_w/M_nare

due to action of a part of 3₂AlMe as the catalyst for the

copolymerization, that is, the slow initiation and the fast

propagation of the polymerization. Furthermore, utilization of

2-picoline instead of DMAP led to the enhancement of the

enantioselectivity (67% ee) though the TOF, content of

carbonate linkages, and M_w/M_nwere slightly degraded (Table

8, run 2).

Mechanism of Asymmetric Alternating Copolymer-

ization of CO₂and CHO with Aluminum Aminoalkoxide

Complex−Lewis Bases. When CHO approaches the

aluminum in the region “inside walls” of the complex

3₂AlMe, an enantioselective ring-opening of CHO will likely

advance. In contrast, a coordination of a Lewis base such as

pyridine to the aluminum in “inside walls” brings about the

approach of CHO to the aluminum in “outside walls”, probably

causing a less enantioselective ring-opening of CHO. If Lewis

base having bulky substituents around the coordination site

collides with the walls (two pyrrolidine and two phenyl rings)

Furthermore, the replacement of the aluminum complex in

the catalytic system ([2gAlMe]₂to [2hAlMe]₂) slightly aﬀects

the copolymerization behavior (Table 7, runs 5−8). At 0 °C

the catalytic system generated the copolymer with a low

content of carbonate linkages (47%) (M_n= 2500, M_w/M_n=

1.55) in 21% yield. The copolymer comprised of the trans-1,2-

diol with 80% ee⁷⁰and the oligo- or polyether chains with an

optical activity ([α]_D−17.6° (c 0.6, benzene)).⁷¹These results

indicate the polymerization is signiﬁcantly dependent on

temperature regardless of the kinds of the catalytic systems.

Asymmetric Alternating Copolymerization of CO₂

and CHO with Aluminum Aminoalkoxide Complex−

Lewis Bases. Bulky prolinol derivatives synthesized from ^L-

proline with ease have been used as organocatalysts⁷²and

ligands⁷³of metal complexes for asymmetric synthesis. A bulky

prolinol 3H⁵¹is known to be one of powerful organocatalysts

and ligands. The 3H was applied as [N,O]-ligands and

chelating units of catalysts for asymmetric copolymerization

of CO₂and epoxide by Nozaki^14a−cand Ding^14e,15separately.

The former catalyst was an intermolecular bimetallic zinc

prolinol complex and the latter was an intramolecular bimetallic

zinc or magnesium multidentate bisprolinol complex. These

complexes are binuclear zinc complexes composed of two zincs

and two prolinol units. Although a mixture of 3H and

organoaluminum instead of Et₂Zn was expected to form a

binuclear aluminum complex via μ-C or -O linkage similar to

the complexes^14a−c,e,15and [2AlMe]₂, a mononuclear alumi-

num bisprolinol complex (3₂AlMe) was obtained. The complex

3₂AlMe has two bidentate prolinol skeletons and forms a

pyramidal geometry similar to aluminum porphyrinate and

K

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Scheme 11. Mechanism of Asymmetric Copolymerization with 3₂AlMe−2-picoline

of “inside walls”, the Lewis base may be coordinated to the

aluminum in “outside walls”. The steric repulsion may

accelerate the preferential approach of CHO to the aluminum

in “inside walls”. In fact, the enantioselectivity of the

copolymerization was improved by the utilization of 2-picoline

as the Lewis base based on the concept (Table 8, run 2). The

polymerization is considered to proceed through the process as

described in Scheme 11D.

Asymmetric Alternating Copolymerization with Alu-

minum Aminoalkoxide Complex−Lewis Bases−Lewis

Acid. To perform the fast copolymerization with high

enantioselectivity, MAD was applied for alternating copoly-

merization of CO₂and CHO with 3₂AlMe−Lewis bases on the

basis of [2gAlMe]₂− or [2hAlMe]₂−bis-N-MeBzIm systems

(Figure 7). First, MAD (1 equiv to aluminum) accelerated the

and the increase of the M_w/M_n(3.10 to 5.44; 1.92 for 3₂AlMe−

DMAP) (Table 9, runs 1−5).

Even if MAD alone catalyzes the copolymerization, the large

M_w/M_nis explicable because multiple active species (at least

two kinds (3₂AlMe and MAD)) work as catalysts in the

polymerization system. Therefore, the copolymerization with

MAD alone was carried out in the absence of other aluminum

complexes or additives. MAD alone generated the copolymer

with a low content of carbonate linkages ([carbonate linkages]:

[ether linkages] = 37: 63) and a very broad distribution (M_w/

M_n= 6.43) (Table 9, run 9). The 1:1 mixture of MAD and

DMAP was inactive for the copolymerization of CO₂and CHO

(Table 9, run 10). MAD and DMAP were originally expected

to act as a monomer activator and a catalyst activator,

respectively. These results, however, disclosed that MAD not

only catalyzed the polymerization of CO₂and CHO but also

directly reacted with DMAP. The interactions between CHO

or DMAP and MAD were examined by using ¹³C NMR

spectroscopy. The ¹³C NMR spectrum of an equimolar mixture

of CHO and MAD in CDCl₃at 30 °C showed all signals due to

CHO clearly shifted from those of CHO alone (Figure 8). In

particular, the signal assignable to O−CH of CHO was shifted

most signiﬁcantly (52.1 ppm for CHO alone; 66.1 ppm for

CHO−MAD). This result indicates that CHO is coordinated to

the aluminum of MAD. Furthermore, an equimolar mixture of

MAD and DMAP in CDCl₃at 30 °C was investigated in the

same manner (Figure 9). Two signals due to the carbons at the

2- and 4-positions on the pyridine ring of DMAP were

remarkably shifted: 154.3 to 155.9 ppm (for C at the 2-

position) and 150.0 to 147.6 ppm (for C at the 4-position).

This result suggests that DMAP also coordinates to the

aluminum of MAD.

Figure 7. Coordinative activation of CHO with MAD and chain

growth from active chain terminal to CHO.

Various ratios of 3₂AlMe−DMAP−MAD were attempted to

establish the catalytic system which produces the copolymer in

a high yield with a high content of carbonate linkages and an

enantioselectivity (Table 9, run 1−8). The optimized ratio was

[3₂AlMe]/[DMAP]/[MAD] = 1/3/2 (Table 9, run 6). The

system quantitatively generated the copolymer (TOF = over

10.4 h⁻¹; the degree of acceleration was approximately 30-

folds) with the content of carbonate linkages (95%) and the

enantioselectivity (55% ee) similar to those of the polymer

yielded by an MAD-free catalytic system (>99% and 59% ee,

copolymerization (conversion of CHO = 77%) to produce the

copolymer (content of carbonate linkages = 90%, M_n= 23000,

M_w/M_n= 3.10) in 72% yield for 24 h, meaning the 21-folds-

acceleration of the copolymerization (Table 9, run 1). The

increase of the feed ratio of MAD to the catalyst 3₂AlMe (1 to 5

equiv) was related to the following changes: not only the

increase of the TOF (7.5 to 14.8 h⁻¹; 21- to 42-folds to that

with 3₂AlMe−DMAP 0.35 h⁻¹) but also the decreases of the ee

(53 to 39% ee; 59% ee for 3₂AlMe−DMAP) and the content of

carbonate linkages (91 to 83%; > 99% for 3₂AlMe−DMAP)

L

dx.doi.org/10.1021/ma301696d | Macromolecules XXXX, XXX, XXX−XXX

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a

Table 9. Asymmetric Alternating Copolymerization of CO₂and CHO with 3₂AlMe−DMAP−MAD System.

copolymerization

hydrolysis

b

c

d

e

f

run

3₂AlMe/equiv DMAP/equiv MAD/equiv time/h yield /% TOF /h⁻¹C.L. /%

M_n

M_w/M_n

yield of diol/% %ee (R,R)

1

2

1

0

1

3

2

0

1

2

3

4

5

2

3

1

24

18

12

24

72

70

7.5

9.7

90

91

88

83

85

95

94

89

37

−

23 000

20 600

26 100

24 700

15 500

32 700

27 000

29 200

3500

3.10

3.33

4.92

3.72

5.44

1.74

2.29

2.50

6.43

−

94

96

89

86

94

91

94

90

88

−

53

54

49

40

39

55

48

46

0

3

81

11.3

13.9

14.8

10.4

9.3

4

>99

71

5

6

>99

89

7

8

>99

82

10.4

8.5

9

10

−

a

In 1 mL of toluene, 60 °C, 50 atm of CO₂, complex 3₂AlMe (0.1 mmol), and CHO (25 mmol); DMAP = N,N-dimethyl-4-aminopyridine, MAD =

b

c

methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide). On the basis of isolated copolymer. Turnover frequency = (mol of reacted CHO)·(mol

d

of cat.)⁻¹·h⁻¹. Content of carbonate linkages (C.L.) = (carbonate unit)·((carbonate unit) + (ether unit))⁻¹, estimated by H NMR (CDCl₃).

1

e

f

Estimated by GPC calibrated with polystyrene standard in THF at 40 °C. Estimated from optical rotation of trans-1,2-cyclohexanediol in H₂O.

contents of carbonate linkages and enantioselectivities and

large M_w/M_nvalues (Table 9, runs 7 and 8).

To improve further enantioselectivities in the asymmetric

copolymerization of CO₂and CHO, MAD was added to the

3₂AlMe−2-picoline system producing the copolymer with 67%

ee being the highest value among the ees of the trans-1,2-diols

obtained with MAD-free catalytic systems at 60 °C. The

3₂AlMe/2-picoline/MAD (= 1/3/2, molar ratio) produced the

alternating copolymer in a quantitative yield with a high

enantioselectivity of 66% ee (Table 10, run 1), which was

higher than those with 3₂AlMe−DMAP−MAD system (Table

9, run 6). In addition, the temperature was lowered from 60 °C

to 0−40 °C (Table 8, runs 2−4). Lowering the temperature

expectedly led to the improvement of the enantioselectivities.

On the other hand, the contents of carbonate linkages of the

polymers were signiﬁcantly decreased with the decrease of

temperature. For instance, at 0 °C the catalytic system

generated the copolymer with a low content of carbonate

linkages (37%) (M_n= 2100, M_w/M_n= 2.09) in 15% yield. The

copolymer was alkali-hydrolyzed to aﬀord the trans-1,2-diol

with 82% ee and the oligo- or polyether chains. The

enantiomeric excess of the trans-1,2-diol was the highest

among those of the copolymers synthesized with the catalytic

systems in this work.⁷⁰Furthermore, the polyether exhibited an

apparent optical activity ([α]_D−18.9° (c 0.6, benzene)).⁷¹

These results indicate that the asymmetric ring-opening of

meso-epoxide, CHO, proceeded throughout the copolymeriza-

tion together with no insertion of CO₂to alkoxide terminal.

Asymmetric Alternating Copolymerization of CO₂

and meso-Epoxides by Aluminum Complex−Lewis

Base−MAD Systems. The most enantioselective systems

([2gAlMe]₂− and [2hAlMe]₂−bis-N-MeBzIm−MAD (1: 1:

0.3) and 3₂AlMe−2-picoline−MAD (1: 3: 2)) were applied for

copolymerizations of CO₂and other meso-epoxides such as

cyclopentene oxide (CPO)⁷⁴and cyclooctene oxide (COO) at

60 °C for 24 h (Table 11). CPO was transformed under CO₂

with the catalytic systems into the corresponding alternating

copolymer, poly(cyclopentene carbonate), with 90−97%

contents of carbonate linkages (Table 11, runs 1−3). The

copolymers were independently converted into trans-1,2-

cyclopenatanediol with 15% (for [2gAlMe]₂), 15% (for

[2hAlMe]₂), and 21% (for 3₂AlMe) ee.⁵⁶These data exhibit

that 3₂AlMe−2-picoline−MAD (1: 3: 2) is more preferential

for the copolymerization of CPO and CO₂. On the other hand,

Figure 8. ¹³C NMR spectra of CHO in the absence (A) and presence

(B) of MAD in CDCl₃at 30 °C. [CHO]/[MAD] = 1.

Figure 9. ¹³C NMR spectra of DMAP in the absence (A) and presence

(B) of MAD in CDCl₃at 30 °C. [DMAP]/[MAD] = 1.

respectively) in Table 8, run 1. On the other hand, the

copolymers obtained with other catalytic ratios had low

M

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Table 10. Asymmetric Alternating Copolymerization of CO₂and CHO with 3₂AlMe−2-Picoline−MAD at Various

a

Temperatures.

copolymerization

hydrolysis

yield of diol/%

b

c

d

e

f

run

temp/°C

yield /%

TOF /h⁻¹

C.L. /%

M_n

M_w/M_n

%ee (R,R)

1

2

3

60

40

25

0

>99

69

10.4

7.2

91

67

52

37

26 600

14 100

6000

2.11

2.01

1.57

2.09

97

89

96

95

66

67

74

82

28

2.9

g

4

15

0.9

2100

a

In 1 mL of toluene, 24 h, 50 atm of CO₂, complex 3₂AlMe (0.1 mmol), 2-picoline (0.3 mmol), MAD (0.2 mmol), and CHO (25 mmol); MAD =

b

c

methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide). On the basis of isolated copolymer. Turnover frequency = (mol of reacted CHO)·(mol

d

of cat.)⁻¹·h⁻¹. Content of carbonate linkages (C.L.) = (carbonate unit)·((carbonate unit) + (ether unit))⁻¹, estimated by H NMR (CDCl₃).

1

e

f

g

Estimated by GPC calibrated with polystyrene standard in THF at 40 °C. Estimated from optical rotation of trans-1,2-cyclohexanediol in H₂O. 60

atm of CO₂and CHO (15 mmol).

Table 11. Asymmetric Alternating Copolymerization of CO₂and meso-Epoxides with Complex [2gAlMe]₂, [2hAlMe]₂, and

a

3₂AlMe.

copolymerization

Hydrolysis

yield of diol/%

b

c

d

e

f

run

complex

epoxide

yield /%

TOF /h⁻¹

C.L. /%

M_n

M_w/M_n

%ee (R,R)

1

2

3

4

5

6

[2gAlMe]₂

[2hAlMe]₂

3₂AlMe

CPO

COO

27

30

24

10

11

7

1.1

1.3

1.0

0.4

0.5

0.3

97

90

91

94

92

88

1500

1700

1000

910

1.41

1.37

1.83

1.27

1.53

1.99

91

93

85

90

89

91

15

21

23

10

[2gAlMe]₂

[2hAlMe]₂

3₂AlMe

880

1300

a

In 1 mL of toluene, 60 °C, 24 h, 50 atm of CO₂, complex [2gAlMe]₂or [2hAlMe]₂(0.1 mmol), bis-N-MeBzIm (0.1 mmol), MAD (0.03 mmol),

and CHO (25 mmol) or complex 3₂AlMe (0.1 mmol), 2-picoline (0.3 mmol), MAD (0.2 mmol), and epoxide (10 mmol); MAD = methylaluminum

b

bis(2,6-di-tert-butyl-4-methylphenoxide), bis-N-MeBzIm = N-methyl-2-(N′-methylbenzimidazol-2-yl)benzimidazole. On the basis of isolated

c

d

copolymer. Turnover frequency = (mol of reacted CHO)·(mol of cat.)⁻¹·h⁻¹. Content of carbonate linkages (C.L.) = (carbonate unit)·((carbonate

e

f

unit) + (ether unit))⁻¹, estimated by ¹H NMR (CDCl₃). Estimated by GPC calibrated with polystyrene standard in THF at 40 °C. Estimated from

optical rotation of trans-1,2-cyclohexanediol in EtOH or CHCl₃.

COO was converted into poly(cyclooctene carbonate) with

88−94% contents of carbonate linkages (Table 11, runs 4−6)

under the same conditions as those of CPO and CHO. The

copolymers had the repeating units composed of trans-1,2-

cyclooctanediol with 21 (for [2gAlMe]₂), 23 (for [2hAlMe]₂),

and 10% (for 3₂AlMe) ee. These show that [2hAlMe]₂−bis-N-

MeBzIm−MAD (1: 1: 0.3) is more superior for the

copolymerization of COO and CO₂. The contents of carbonate

linkages (88−97%) of the copolymers synthesized from CPO

or COO and CO₂were on the same level as those (91−95%)

from CHO, whereas the ees of the trans-1,2-diols converted

from the copolymers are signiﬁcantly smaller than those from

PCHC: 15−21% (for CPO) and 10−23% (for COO) vs 62−

66% (for CHO) ee.⁵⁶This result suggests that the asymmetric

copolymerization is signiﬁcantly dependent on the structure

(size, conformation, etc.) of epoxide. In addition, the TOF data

(1.0−1.3 (for CPO) and 0.4−0.5 (for COO) vs 5.4−10.4 (for

CHO)) indicate that the reactivity of meso-epoxide decreased in

order of CHO ≫ CPO > COO. This tendency was similar to

the reactivity of homopolymerization of meso-epoxide with

Et₃Al reported by Bacskai.⁷⁵

ketoiminate complex disclosed by single crystal X-ray analysis,

Lewis bases having two coordinating nitrogens coordinated to

two aluminums were designed and employed as catalyst

activator. In particular, bis-N-MeBzIm assisted the preferential

production of the copolymer with high enantioselectivity (up to

62% ee). In addition, the aluminum bisprolinol complex−2-

picoline system generated the copolymer with high enantiose-

lectivity (up to 67% ee) despite of the relatively slow

copolymerization. MAD was subsequently employed as

monomer activator in order to perform faster and more

enantioselective copolymerizations at low temperature. The

enantiomeric excess of trans-1,2-diol comprising the copolymer

was improved up to 82% ee though the copolymer partially

included oligo- or polyether sequences showing optical

activities. The design and utilization of catalyst and monomer

activator for asymmetric alternating polymerization are

expected to allow improvement of not only asymmetric ring-

opening polymerizations but also asymmetric syntheses related

to ring-opening of cyclic substrates. The related work is under

investigation in our laboratory.

EXPERIMENTAL SECTION

■

CONCLUSION

Materials. The reactions involving air- and/or moisture-sensitive

compounds were performed under nitrogen. Toluene, THF, and C₆D₆

were distilled over sodium benzophenone ketyl in a nitrogen

atmosphere. CDCl₃was distilled over CaH₂under nitrogen. 1,2-

Epoxycyclohexane (CHO) and 1,2-epoxycyclopentane (CPO) was

dried over KOH pellet and distilled over CaH₂and KOH under

reduced pressure and stored under nitrogen prior to use. 1,2-

Epoxycyclooctane (COO) was recrystallized from benzene. N-MeIm,

pyridine, and 2-picoline were distilled over CaH₂under reduced

■

We have explored a series of optically active aluminum

complexes (salen-type Schiﬀ base, β-ketoiminate, and amino-

alkoxide complexes) for alternating copolymerization of CO₂

and CHO with asymmetric induction. The aluminum salen-

type Schiﬀ base complex−Et₄NOAc systems aﬀorded alternat-

ing copolymers with low enantioselectivities (up to 23% ee).

On the basis of the binuclear structure of the aluminum β-

N

dx.doi.org/10.1021/ma301696d | Macromolecules XXXX, XXX, XXX−XXX

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pressure and stored under nitrogen. tert-Butanol was distilled over

Mg/I₂and stored under nitrogen. Acetic acid was distilled over P₂O₅

and stored under nitrogen. Me₃Al was distilled under reduced pressure

and stored in a nitrogen atmosphere. Et₄NOAc was azeotropically

dehydrated with benzene using a Dean−Stark apparatus. DMAP was

recrystallized from toluene. CO₂was used without further puriﬁcation.

Other reagents were used as received. Following materials were

commercially obtained: 2,4-pentanedione, 1,3-diphenyl-1,3-propane-

dione, tert-butanol, pyridine, N-MeIm, DMAP, 2-picoline, CHO, COO

(from TCI); Me₃Al, Et₄NOAc (from Aldrich). Following materials

were synthesized according to the reported literatures: salen-type

Schiﬀ bases [(1R,2R)-N,N′-bissalicylidene-1,2-diphenylethane-1,2-dia-

mine (1aH₂),⁴⁶(1R,2R)-N,N′-bis(3-phenylsalicylidene)-1,2-diphenyl-

ethane-1,2-diamine (1bH₂),⁴⁷(1R,2R)-N,N′-bis(3,5-di-tert-butylsalicy-

lidene)-1,2-diphenylethane-1,2-diamine (1cH₂),⁴⁶(1R,2R)-N,N′-bissa-

licylidene-1,2-cyclohexanediamine (1dH₂),⁴⁷(1R,2R)-N,N′-bis(3-

phenylsalicylidene)-1,2-cyclohexanediamine (1eH₂),⁴⁷and (1R,2R)-

N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine

(1fH₂)⁴⁸], amino alcohols [(S)-2-amino-1-propanol, (S)-2-amino-3-

methyl-1-butanol, (S)-2-amino-4-methyl-1-pentanol, (2S,3S)-2-amino-

3-methyl-1-pentanol, (S)-2-amino-3-phenyl-1-propanol, (S)-2-amino-

2-phenylethanol, and (R)-2-amino-2-phenylethanol],⁴⁹β-ketoimines

[4-(2-hydroxyethylimino)pent-2-en-2-ol (2aH₂)⁴¹and 4-((S)-1-hy-

droxypropan-2-ylimino)pent-2-en-2-ol (2bH₂)⁵⁰], (S)-diphenyl-

(pyrrolidine-2-yl)methanol (3H),⁵¹aluminum complexes [1cAlMe,³⁹

1dAlMe,⁴⁰1fAlMe,³⁹[2aAlMe]₂,⁴¹3₂AlMe,⁴²and 4AlMe^17a], bis-N-

MeIm,⁶⁶bis-N-MeBzIm,⁶⁶MAD,⁶⁹and CPO.⁷⁴

137.9, 129.3, 129.2, 128.5, 126.5, 95.5, 64.7, 57.8, 39.1, 28.4, 18.9, 18.2.

FAB−MS m/z: 234 [for C₁₄H₂₀NO₂[M + H]⁺].

4-((S)-2-Hydroxy-1-phenylethylimino)pent-2-en-2-ol (2gH₂).

This was puriﬁed by recrystallization from ethanol to aﬀord a white

crystal (88% yield). [α]_D−878° (c 1.6, CHCl₃). ¹H NMR (CDCl₃): δ

11.43 (br, 1H), 7.37−7.28 (m, 5H), 5.03 (s, 1H), 4.73−4.68 (m, 1H),

3.87−3.78 (m, 2H), 3.25−3.21 (t, 1H), 2.07 (s, 3H), 1.87 (s, 3H). ¹³C

NMR (CDCl₃): δ 195.4, 163.9, 139.3, 128.9, 128.6, 127.7, 126.5, 96.4,

67.2, 60.1, 28.5, 19.4. FAB−MS m/z: 220 [for C₁₃H₁₈NO₂[M + H]⁺].

4-((R)-2-Hydroxy-1-phenylethylimino)pent-2-en-2-ol was also pre-

pared from ^D-phenylglycinol; [α]_D+860° (c 1.5, CHCl₃).

3-((S)-2-Hydroxy-1-phenylethylimino)-1,3-diphenylprop-1-

en-1-ol (2hH₂).⁷⁶L-Phenylglycinol (2.19 g, 10.0 mmol) and

dibenzoylmethane (2.24 g, 10.0 mmol) were dissolved in toluene

(50 mL) in a 100-mL round-bottomed ﬂask equipped with a reﬂux

condenser. After one drop of formic acid was added to this solution,

the mixture was heated under reﬂux for 48 h. The mixture was

concentrated under reduced pressure to leave a yellow oil. The crude

oil was puriﬁed by silica gel column chromatography with ethyl acetate

to aﬀord a yellow viscous oil (1.65 g, 48% yield). [α]_D−366° (c 1.8,

1

CHCl₃). H NMR (CDCl₃): δ 12.16−11.98 (br, 1H), 7.91−7.85 (d,

2H), 7.42−7.11 (m, 13H), 5.75 (s, 1H), 4.67−4.58 (m, 1H), 4.04−

3.90 (br, 1H), 3.87−3.81 (m, 2H). ¹³C NMR (CDCl₃): δ 188.9, 167.4,

140.1, 139.6, 135.4, 130.9, 129.5, 128.8, 128.4, 128.3, 127.8, 127.6,

127.2, 126.5, 94.6, 67.4. FAB−MS m/z: 344 [for C₂₃H₂₂NO₂[M +

H]⁺]. Compound 2iH₂was also synthesized according to a procedure

similar to that used for 2hH₂.

3-(2-Hydroxyethylimino)-1,3-diphenylprop-1-en-1-ol (2iH₂).

4-((S)-1-Hydroxy-3-methylbutan-2-ylimino)pent-2-en-2-ol

(2cH₂).⁵⁰Na₂SO₄(2.5 g) was added to acetylacetone (1.0 g, 10

mmol) in methanol (10 mL) in a 100-mL two-necked round-

bottomed ﬂask equipped with a dropping funnel. (S)-2-Amino-3-

methyl-1-butanol (1.03 g, 10.0 mmol) in methanol (20 mL) was added

dropwise over 1 h while the solution turned yellow, and the resulting

mixture was then stirred for 12 h at room temperature. After dilution

with CH₂Cl₂(10 mL), the mixture was ﬁltered and concentrated to

dryness under reduced pressure to leave a yellow solid. The residue

was puriﬁed by silica gel column chromatography with ethyl acetate/

hexane (2/3, vol/vol) to aﬀord a colorless oil (1.44 g, 78% yield). [α]_D

This was puriﬁed by recrystallization from ethanol to aﬀord a white

1

crystals (87% yield). H NMR (CDCl₃): δ 12.11−11.93 (br, 1H),

7.91−7.85 (d, 2H), 7.42−7.11 (m, 8H), 5.78 (s, 1H), 4.01−3.95 (t,

2H), 3.54−3.41 (t, 2H). ¹³C NMR (CDCl₃): δ 194.4,189.5, 177.6,

164.8, 160.7, 140.7, 139.2, 136.7, 133.2, 131.1, 129.0, 128.8, 127.9,

127.0, 126.8, 126.0, 125.5, 69.3, 68.7, 65.1, 58.8, 58.2. FAB−MS m/z:

268 [for C₁₇H₁₈NO₂[M + H]⁺].

Complex 1aAlMe.^39,40After 1aH₂(0.21 g, 0.50 mmol) was

dissolved in dry toluene (10 mL) in a 50-mL two-necked round-

bottomed ﬂask equipped with a reﬂux condenser under dry nitrogen,

the mixture was cooled at 0 °C. Me₃Al (0.06 mL, 0.6 mmol) was

carefully added to the solution by a hypodermic syringe in a nitrogen

stream. After gas evolution ceased, the resulting mixture was stirred at

0 °C for 30 min and then heated under reﬂux for 2 h. The mixture was

cooled at room temperature, and the volatile components were then

removed under reduced pressure to leave a yellow powder in an

almost quantitative yield. The crude product was puriﬁed by

recrystallization form CH₂Cl₂/hexane to aﬀord a pale yellow powder

(0.20 g, 87% yield). ¹H NMR (CDCl₃): δ 8.32 (s, 2H), 7.40−7.10 (m,

14H), 6.90 (d, 2H), 6.77 (d, 2H), 4.73 (s, 2H), −1.01 (s, 3H).

Compounds 1bAlMe and 1eAlMe were also synthesized according to

the similar procedure to 1aAlMe.

1

−14° (c 3.9, CHCl₃). H NMR (CDCl₃): δ 10.9−10.8 (br, 1H), 4.98

(s, 1H), 4.26−4.02 (br, 1H), 3.61−3.56 (t, 1H), 3.45−3.40 (m, 1H),

1.96−1.95 (d, 6H), 1.88−1.82 (m, 1H), 0.99−0.91 (d, 6H). ¹³C NMR

(CDCl₃): δ 194.1, 164.8, 95.4, 63.8, 61.0, 29.7, 28.1, 19.6, 19.3, 17.6.

FAB−MS m/z: 186 [for C₁₀H₂₀NO₂[M + H]⁺]. Compounds 2dH₂−

2fH₂were also synthesized according to the similar procedure to

2cH₂.

4-((S)-1-Hydroxy-4-methylpentan-2-ylimino)pent-2-en-2-ol

(2dH₂). This was puriﬁed by silica gel column chromatography with

ethyl acetate/hexane (5/1, vol/vol) to aﬀord a colorless viscous oil

1

(62% yield). [α]_D−31° (c 5.5, CHCl₃). H NMR (CDCl₃): δ 10.8−

10.7 (br, 1H), 4.96 (s, 1H), 4.00−3.63 (d, 2H), 3.51−3.46 (t, 1H),

1.99−1.95 (d, 6H), 1.68−1.66 (m, 1H), 1.36−1.33 (t, 2H), 0.93−0.89

(d, 6H). ¹³C NMR (CDCl₃): δ 194.3, 164.0, 95.3, 66.1, 53.9, 41.1,

28.4, 24.8, 23.1, 22.2, 19.4. FAB−MS m/z: 200 [for C₁₁H₂₂NO₂[M +

H]⁺].

1

Complex 1bAlMe. 93% yield as a pale yellow powder. H NMR

(CDCl₃): δ 8.54 (s, 2H), 7.65−7.40 (m, 8H), 7.37−7.30 (m, 4H),

7.23−7.11 (m, 12H), 6.87 (m, 2H), 4.78 (s, 2H), −0.97 (s, 3H).

1

Complex 1eAlMe. 92% yield as a pale yellow powder. H NMR

(CDCl₃): δ 8.47 (s, 2H), 7.70−7.10 (m, 16H), 6.94 (m, 2H), 6.85−

6.60 (m, 2H), 3.49−3.34 (m, 2H), 2.11−1.29 (m, 8H), −1.02 (s, 3H).

Complex [2bAlMe]₂.⁴¹After 2bH₂(0.16 g, 1.0 mmol) was

dissolved in dry THF (10 mL) in a 50-mL two-necked round-

bottomed ﬂask equipped with a three-way stopcock under dry

nitrogen, the mixture was cooled at −30 °C. Me₃Al (0.12 mL, 1.2

mmol) was carefully added to the solution by a hypodermic syringe in

a nitrogen stream. After gas evolution ceased, the resulting mixture was

allowed to warm to room temperature and then stirred for 2.5 h. The

volatile components were then removed under reduced pressure to

leave a white solid. The crude product was puriﬁed by recrystallization

form THF/hexane to aﬀord a colorless crystal (0.18 g, 90% yield). ¹H

NMR (CDCl₃): δ 5.00 (s, 2H), 3.78−3.69 (m, 2H), 3.63−3.43 (m,

4H), 2.02 (s, 6H), 1.93 (s, 6H), 1.25−1.19 (d, 6H), −0.88 (s, 3H),

−0.95 (s, 3H). Complexes [2cAlMe]₂−[2cAlMe]₂were also

synthesized according to the similar procedure to [2bAlMe]₂.

4-((2S,3R)-1-Hydroxy-3-methylpentan-2-ylimino)pent-2-en-

2-ol (2eH₂). This was puriﬁed by silica gel column chromatography

with ethyl acetate/hexane (5/1, vol/vol) to aﬀord a colorless viscous

oil (75% yield). [α]_D−51° (c 2.2, CH₂Cl₂). H NMR (CDCl₃): δ

1

10.8−10.7 (br, 1H), 5.01 (s, 1H), 4.00−3.63 (d, 2H), 3.51−3.46 (t,

1H), 1.99−1.95 (d, 6H), 1.68−1.66 (m, 1H), 1.36−1.33 (t, 2H) 0.95−

0.90 (d, 6H). ¹³C NMR (CDCl₃): δ 194.4, 163.9, 95.4, 63.7, 60.7, 41.1,

28.4, 24.8, 19.6, 15.2, 11.4. FAB−MS m/z: 200 [for C₁₁H₂₂NO₂[M +

H]⁺].

4-((S)-1-Hydroxy-3-phenylpropan-2-ylimino)pent-2-en-2-ol

(2fH₂). This was puriﬁed by silica gel column chromatography with

ethyl acetate/hexane (3/1, vol/vol) to aﬀord a slightly yellowish

viscous oil (58% yield). [α]_D−310° (c 4.7, CHCl₃). ¹H NMR

(CDCl₃): δ 10.89 (br, 1H), 7.29−7.07 (m, 5H), 4.82 (s, 1H), 4.14−

3.67 (m, 3H), 3.62−3.58 (m, 1H), 2.92−2.87 (d, 1H), 2.74−2.68 (t,

1H), 1.95 (s, 3H), 1.60 (s, 3H). ¹³C NMR (CDCl₃): δ 194.6, 164.0,

O

dx.doi.org/10.1021/ma301696d | Macromolecules XXXX, XXX, XXX−XXX

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Complex [2cAlMe]₂. 86% yield as a colorless solid. ¹H NMR

(CDCl₃): δ 4.78 (s, 2H), 3.77−3.61 (t, 2H), 3.55−3.47 (m, 2H),

1.97−1.94 (d, 12H), 1.85−1.83 (m, 2H), 1.02−0.90 (d, 12H), −0.78

(s, 3H), −0.82 (s, 3H).

mixture. The autoclave was pressurized to 50 atm of CO₂, and left

stirred at 60 °C for 24 h. Following operations were identical with the

copolymerization in the absence of MAD (1.8 g, 52% yield).

Alkali-Catalyzed Hydrolysis of Poly(cyclohexene carbona-

te)..^14a,bPCHC (0.14 g, corresponding to 1 mmol of diol) obtained

with [2gAlMe]₂−bis-N-MeBzIm−MAD (Table 7, run 1) was

dissolved in THF (30 mL) and methanol (10 mL) in a 100-mL

round-bottomed ﬂask equipped with reﬂux condenser. After NaOH-

(aq) (2 M, 5 mL) was added to the solution, the resulting solution was

heated to reﬂux for 6 h. When the reaction mixture was neutralized

with 1 M HCl(aq), a white precipitate was generated. The precipitate

was ﬁltered oﬀ and the ﬁltrate was then concentrated to approximately

20 mL under reduced pressure. The solution diluted with water (10

mL) was washed with ethyl acetate (10 mL) three times. After the

combined organic layer was dried over MgSO₄, the solvent was

evaporated to dryness to aﬀord a white solid. The solid was puriﬁed by

silica gel column chromatography with ethyl acetate as an eluent

(0.093 g, 93% yield). The obtained diol was subjected to an optical

rotation measurement. [α]_D−24° (c 1.0, H₂O). (lit.⁵⁶[α]_D²⁰−39° (c

1.6, H₂O)). ¹H NMR (CDCl₃): δ 4.30 (s, 2H), 3.35 (s, 2H), 1.95 (m,

2H), 1.69 (m, 2H), 1.26 (m, 4H). ¹³C NMR (CDCl₃): δ 75.6, 33.0,

24.5. IR (KBr, cm⁻¹): 3371, 3278, 2934, 1632, 1470, 1404, 1351, 1233,

1165, 1080, 1058, 930, 866, 668. Poly(cyclopentene carbonate) and

poly(cyclooctene carbonate) were hydrolyzed with the same

procedure, respectively.

1

Complex [2dAlMe]₂. 79% yield as a colorless crystal. H NMR

(CDCl₃): δ 5.01 (s, 2H), 4.09−3.88 (d, 4H), 3.31−3.20 (t, 2H), 1.99−

1.93 (d, 12H), 1.47−1.39 (m, 2H), 1.26−1.19 (t, 4H), 1.02−0.89 (d,

12H), −0.84 (s, 3H), −0.96 (s, 3H).

Complex [2eAlMe]₂. 77% yield as a colorless solid. ¹H NMR

(CDCl₃): δ 5.12 (s, 2H), 3.98−3.63 (d, 4H), 3.21−3.10 (t, 2H), 2.01−

1.92 (d, 12H), 1.71−1.66 (m, 2H), 1.29−1.23 (t, 4H), 0.94−0.80 (d,

12H), −0.91(s, 3H), −1.01 (s, 3H).

1

Complex [2fAlMe]₂. 69% yield as a colorless powder. H NMR

(CDCl₃): δ 7.40−7.02 (m, 10H), 5.01 (s, 2H), 4.21−3.67 (m, 6H),

3.60−3.50 (m, 2H), 3.00−2.87 (d, 2H), 2.84−2.69 (t, 2H), 2,00 (s,

6H), 1.40 (s, 6H), −0.65 (s, 3H), −0.88 (s, 3H).

1

Complex [2gAlMe]₂. 90% yield as a colorless crystal. H NMR

(CDCl₃): δ 7.50−7.20 (m, 10H), 5.21 (s, 2H), 4.70−4.39 (m, 2H),

3.90−3.61 (m, 4H), 3.10−2.99 (t, 2H), 2.00 (s, 6H), 1.80 (s, 3H),

−0.99 (s, 3H), −1.10 (s, 3H).

1

Complex [2hAlMe]₂. 80% yield as a colorless powder. H NMR

(CDCl₃): δ 8.00−7.97 (d, 4H), 7.60−7.01 (m, 26H), 5.86 (s, 2H),

4.67−4.59 (m, 4H), 3.90−3.81 (m, 4H), −0.88 (s, 3H), −0.97 (s,

3H).

1

Complex [2iAlMe]₂. 77% yield as a colorless crystal. H NMR

(CDCl₃): δ 7.90−7.82 (d, 4H), 7.32−7.00 (m, 16H), 5.87 (s, 2H),

Hydrolysis of Poly(cyclopentene carbonate). 91% yield as a

white solid (Table 11, run 1). [α]_D−3.2° (c 0.11, CHCl₃). (lit.⁵⁶

4.20−4.15 (t, 4H), 3.50−3.40 (t, 4H), −0.97 (s, 6H).

Complex [2iAlO^tBu]₂.^17aAfter [3iAlMe]₂(0.614 g, 1.00 mmol)

was dissolved in dry toluene (10 mL) in a 50-mL two-necked round-

bottomed ﬂask equipped with a three-way stopcock under dry

nitrogen, tert-butanol (5 mL) was added to the mixture at room

temperature. After the mixture was heated under reﬂux for 12 h, the

volatile compounds were removed under reduced pressure to obtain a

slightly yellow powder. The powder was puriﬁed by recrystallization

from THF/hexane to aﬀord a slight yellow crystal (0.46 g, 62% yield).

¹H NMR (CDCl₃): δ 7.93−7.86 (d, 4H), 7.45−7.30 (m, 16H), 5.86

(s, 2H), 4.01−3.95 (t, 4H), 3.64−3.63 (m, 4H), 1.25 (s, 18H).

Complex 4AlOAc.^17aAfter 4AlMe (0.349 g, 1.00 mmol) was

dissolved in dry toluene (10 mL) in a 50-mL two-necked round-

bottomed ﬂask equipped with a three-way stopcock under dry

nitrogen, acetic acid (60 μL, 1.0 mmol) was added to the mixture at

room temperature. After stirred for 12 h, the mixture changed to a

yellow suspension. The yellow solid was collected by ﬁltration of the

suspension, washed with cooled toluene, and dried in vacuo to aﬀord a

[α]_D−21° (c 1.0, CHCl₃)). Mp: 54−55 °C (lit.⁵⁶54−56 °C

20

1

(racemic), 47−51 °C (enantiopure)). H NMR (CDCl₃): δ 4.35 (s,

2H), 3.92 (t, 2H), 2.05−1.90 (m, 2H), 1.81−1.62 (m, 2H), 1.59−1.40

(m, 2H). ¹³C NMR (CDCl₃): δ 79.0, 31.2, 19.6. IR (KBr, cm⁻¹): 3382,

3261, 3201, 2924, 1674, 1455, 1374, 1230, 1155, 1100, 1028, 918, 886.

Hydrolysis of Poly(cyclooctene carbonate). Puriﬁcation by

distillation to yield a colorless oil, which was spontaneously solidiﬁed

to give a white solid (90%) (Table 11, run 4). [α]_D−3.2° (c 0.10,

EtOH). (lit.⁵⁶[α]_D+16.9° (c 1.33, EtOH) for (1S,2S)-1,2-

22

cyclooctanediol). Mp: 31−32 °C (lit.⁵⁶32 °C). H NMR (CDCl₃):

1

δ 3.75 (s, 2H), 3.70 (s, 2H), 1.92−1.20 (m, 12H). ¹³C NMR (CDCl₃):

δ 76.2, 31.9, 26.2, 23.8. IR (neat, cm⁻¹): 3350, 3220, 2919, 1620, 1429,

1410, 1323, 1251, 1175, 1083, 930, 876, 845, 660.

Alkali-Catalyzed Hydrolysis of Ether-Containing Copolymer.

The copolymer (0.28 g (52% content of carbonate linkages),

corresponding to 1 mmol of diol) obtained from the complex

[2gAlMe]₂−bis-N-MeBzIm−MAD (Table 10, run 3) was dissolved in

THF (30 mL) and methanol (10 mL) in a 100-mL round-bottomed

ﬂask. After NaOH(aq) (2 M, 5 mL) was added to the solution, the

resulting solution was heated to reﬂux for 12 h. After the reaction

mixture was neutralized by 1 M HCl(aq), the aqueous solution diluted

with water (20 mL) was washed with CHCl₃(20 mL) three times.

After the combined organic layer was washed with water (10 mL)

twice, the solvent was evaporated to approximately 2 mL under

reduced pressure. The residue was poured into methanol (100 mL) to

yield a white suspension. The white solid was collected by ﬁltration

and dried in vacuo at 120 °C for 6 h (0.13 g). [α]_D−14.3° (c 0.8,

1

colorless crystal (0.22 g, 54% yield). H NMR (CDCl₃): δ 8.58 (s,

2H), 7.65−7.00 (m, 12H), 2.08 (s, 3H).

X-ray Crystallography. The aluminum complexes [2gAlMe]₂(R-

form was used) and 3₂AlMe were obtained as X-ray-quality crystals.

Crystallographic data for [2gAlMe]₂and 3₂AlMe were summarized in

the Supporting Information.

Representative Copolymerization of CO₂and CHO. The

aluminum complex [2gAlMe]₂(51.8 mg, 0.10 mmol), bis-N-MeBzIm

(26.2 mg, 0.10 mmol, 1 equiv to Al), toluene (1.0 mL), and CHO (2.5

mL, 25 mmol) were placed in a well-dried 150-mL stainless autoclave

in a nitrogen stream. The autoclave was pressurized to 50 atm of CO₂,

and left stirred at 60 °C for 24 h. The reactor was cooled to ambient

temperature and the CO₂pressure was slowly released. A small aliquot

of the resulting mixture was subjected to analysis. To the remaining

mixture was added methanol. The mixture was dissolved in CHCl₃(30

mL) and washed with HCl(aq) (1 M, 10 mL), water (10 mL). The

organic layer was concentrated to about 3 mL. This solution was

added to methanol (300 mL) with stirring to precipitate the white

solid. This solid was collected and dried in vacuo at 120 °C for 6 h (1.3

g, 37% yield).

Copolymerization in the Presence of MAD. The aluminum

complex [2gAlMe]₂(51.8 mg, 0.10 mmol), bis-N-MeBzIm (26.2 mg,

0.10 mmol, 1 equiv to Al), toluene (0.5 mL), and CHO (2.5 mL, 25

mmol) were placed in a well-dried 150-mL stainless autoclave in a

nitrogen stream and the mixture was stirred for 30 min. A solution of

MAD (14.4 mg, 0.030 mmol) in toluene (0.5 mL) was added to the

1

benzene). H NMR (C₆D₆): δ 4.10−3.76 (br, 2H), 2.34−2.12 (br,

1

4H), 1.80−1.59 (br, 4H). H NMR (C₆D₆): δ 78.9−73.0, 32.0−30.0,

23.3−21.0. IR (KBr, cm⁻¹): 3371, 3278, 1470, 1461, 1350, 1165, 1080,

1063, 912, 836, 768.

The ﬁltrate was evaporated to dryness and the residue was dissolved

in water (10 mL). The insoluble part was removed by ﬁltration and the

ﬁltrate was extracted with ethyl acetate (10 mL) three times. The

combined organic layer was dried over MgSO₄and the solvent was

evaporated to yield a white solid, which was puriﬁed by silica gel

column chromatography with ethyl acetate as an eluent (0.11 g, 96%

yield). [α]_D−28.8° (c 0.11, H₂O). (lit.⁵⁶[α]_D−39° (c 1.6, H₂O)).

20

1

The H NMR (CDCl₃), ¹³C NMR (CDCl₃), IR (KBr), and mp data

were identical with those described above.

¹H NMR Observation of Exchange of Axial Ligands Between

Two Types of Aluminum Complexes.⁶³A CDCl₃(10 mL)

solution of 4AlOAc (100 mg, 0.25 mmol) was mixed to a CDCl₃(5.0

P

dx.doi.org/10.1021/ma301696d | Macromolecules XXXX, XXX, XXX−XXX

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Article

mL) solution of [2iAlO^tBu]₂(77 mg, 0.125 mmol) with stirring in a

50-mL ﬂask equipped with a three-way stopcock at room temperature.

An aliquot (0.5 mL) of the above reaction mixture was transferred into

a glass tube (o.d. = 5 mm) by a syringe under N₂. After, the glass tube

for ²⁷Al NMR measurements of [2gAlMe]₂. H.S. thanks

Nippon Petrochemicals Co., Ltd. for ﬁnancial support.

REFERENCES

1

■

was then sealed under N₂, H NMR spectrum of the sample was

(1) Reszko, A. E.; Kasumov, T.; Pierce, B. A.; David, F.; Hoppel, C.

L.; Stanley, W. C.; Rosiers, C. D.; Brunengraber, H. J. Biol. Chem.

2003, 278, 34959−34965.

measured at predetermined times.

Measurements. ¹H and ¹³C NMR measurements were performed

with CDCl₃as the solvent at 30 °C on a Bruker DPX-300 (300 MHz)

or DPX-400 spectrometer (400 MHz). Chemical shifts were

determined with respected to tetramethylsilane (δ = 0.00 ppm) and

(2) For comprehensive reviews on utilization of CO₂as feedstock,

see: (a) Inoue, S.; Yamazaki, N. Organic and Bioorganic Chemistry of

Carbon Dioxide: Kodansha: Tokyo, Japan, 1981. (b) Utilization of

Greenhouse Gases; Liu, C.-J., Mallinson, R. G., Aresta, M., Eds.; ACS

Symposium Series 852; American Chemical Society: Washington, DC,

2003. (c) Advances in CO₂Conversion and Utilization; Hu, Y. H. ed.;

ACS Symposium Series 1056; American Chemical Society: Wash-

ington, DC, 2010. (d) Carbon Dioxide as Chemical Feedstock; Aresta,

M., Ed.; Wiley-VCH Verlag: Weinheim, Germany, 2010.

1

CHCl₃(δ = 7.26 ppm) for H NMR, and CDCl₃(δ = 77.0 ppm) for

¹³C NMR as the internal standard. ²⁷Al NMR measurements were

performed with external referencing method using the saturated C₆D₆

solution of the sample ([2gAlMe]₂or [2gAlMe]₂−bis-N-MeIm (1:1))

in the outer tube and the D₂O solution (0.5 M) of Al(NO₃)₃(δ = 0.0

ppm) as the external reference in the inset tube at 30 °C on a Bruker

DPX-400 spectrometer (400 MHz). IR spectra were recorded with a

Horiba FT-720 spectrometer. Optical rotations were measured on a

Horiba SEPA-300 polarimeter using 1-dm cell at room temperature

(25−27 °C). Mass spectra were recorded on a JEOL JMS-SX102A

spectrometer. Single crystal X-ray diﬀraction was performed on a

Bruker AXS SMART APEX system at 100 or 173 K. The intensity data

were collected on a Bruker SMART CCD diﬀractometer. The

structure was solved by direct method and reﬁned with SHELXTL

program package (SAINT-Plus, version 6.02, SHELXS-97: Program for

Crystal Structure Solution, SHELXL-97: Program for Crystal Structure

Refinement, SHELXTL, version 5.0).⁷⁷All non-hydrogen atoms were

reﬁned with anisotropic displacement parameters. Hydrogen atoms

were placed in ideal positions with isotropic thermal parameters. Gel

Permeation chromatography (GPC) was performed at 40 °C on a

Tosoh model HLC-8220 high-speed liquid chromatograph equipped

with two series-connected TSK-GEL GMH_HR-H columns, a diﬀer-

ential refractometer detector with THF as an eluent at a ﬂow rate of

1.0 mL min⁻¹. The molecular weight calibration curve was obtained

with standard polystyrenes (TSK standard polystyrene from Tosoh

Co.); M_ws by light scattering (M_w/M_nby GPC) were as follows: 1.89

× 10⁵(1.04), 9.89 × 10⁴(1.01), 3.72 × 10⁴(1.01), 1.71 × 10⁴(1.01),

9.83 × 10³(1.02), 5.87 × 10³(1.02), 2.5 × 10³(1.05).

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ASSOCIATED CONTENT

■

S

* Supporting Information

1

Experimental procedures, spectral data of the copolymer, H

NMR study of alkoxide and carboxylate exchange, a ²⁷Al NMR

study, optimized structures, and crystallographic data and tables

giving full details of the complexes [2gAlMe]₂and 3₂AlMe

including crystallographic information ﬁles (cif). This material

is available free of charge via Internet at http://pubs.acs.org.

AUTHOR INFORMATION

■

Corresponding Author

*E-mail: hrssgmt@rs.kagu.tus.ac.jp.

Present Address

^§Sumitomo Seika Chemicals Company, Limited, 1, Irifune,

Shikama, Himeji, Hyogo 672-8076, Japan.

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Notes

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The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

We thank Mr. Hiroaki Ohtsuka for his active collaboration and

Prof. Shohei Inoue for his generous discussion. We are also

grateful to Prof. Kazuo Miyamura and Mr. Yuji Soneta

(Department of Science of our university) for their kind

instructions for X-ray crystallographic analyses of [2gAlMe]₂

(R-form) and 3₂AlMe and to Dr. Yukishige Kondo (Depart-

ment of Engineering of our university) for his kind instructions

Q

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T.; Sakurai., M.; Nonoshita, K.; Yamamoto, H. J. Am. Chem. Soc. 1988,

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(70) Our research on this theme has preliminarily been presented in

an international symposium: Nishioka, K. and Sugimoto H.

preliminarily presented at Paciﬁchem 2005, Honolulu (HI),

December, 2005. Since then, we have prepared our manuscript.

Before submission of our manuscript, Lu, X.-B. and co-workers

reported the synthesis of poly(cyclohexene carbonate) composed of

96%ee (R,R) of 1,2-cyclohexanediol units via the alternating

copolymerization of CO₂and CHO with enantiopure cobalt(III)-

salen complexes in optically pure (S)-2-methyltetrahydrofuran as the

solvent.^30j

(71) Oligo- or poly(cyclohexene oxide) comprised of enantiometi-

cally pure monomeric units ((1R,2R)- or (1S,2S)-oxycyclohexane-1,2-

diyl units) has never been reported though asymmetric synthesis

polymerization of meso-epoxide (CHO) with chiral initiator systems

was performed. This is owing to no way for estimating the

enantiomeric excess of the monomeric units. We were also not able

to investigate the ees of the repeating units composing of oligo- or

U

dx.doi.org/10.1021/ma301696d | Macromolecules XXXX, XXX, XXX−XXX

Article Doi

Dual catalyst system for asymmetric alternating copolymerization of carbon dioxide and cyclohexene oxide with chiral aluminum complexes: Lewis base as catalyst activator and lewis acid as monomer activator

DOI: 10.1021/ma301696d

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Full text of DOI:10.1021/ma301696d

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