Kanai Lab. Literature Seminar (M1 part)2011.6.28 (Tue.)
Junya KAWAI (M1)

Chemical Approach for Carbon Nanotubes~Recent progress in Organic Synthesis~

Chem. Soc. Rev. 2009, 38, 1076

Index: 1. Introduction: What is "Carbon Nanotube" ?
1-1. Identification
1-2. Physical properties and applications
1-3. Difficulties in conventional syntheses
2. Organic synthesis approach
2-1. About "bottom-up template approach"2-2. Synthesis of CPP ~ Prof. Bertozzi's work
2-3. Synthesis of CPP ~ Prof. Itami's work
2-4. Synthesis of CPP ~ Prof. Yamago's work
2-5. Elongation ~ Diels-Alder strategy
3. Summary
11. Introduction: What is "Carbon Nanotube" ?
参考文献:
齋藤弥八、坂東俊治 「 カーボンナノ チューブの基礎」 1998、コロナ社
齋藤理一郎、篠原久典 「 カ ーボンナノ チューブの基礎と 応用」 2004、培風館
Terrones M.

,

419

1-1. Identification
Carbon Nanotube (CN):One of the allotropes of Carbon with a nano-scale cyrindrical structure of a graphene sheet.
First discovered in 1991 by Dr. Iijima (NEC corporation) Iijima S. Nature 1991, 354, 56
cf . Graphene sheet: a 1-atom-thick sheet of graphite (network of sp2-hybridized carbons)
first generated by Dr. Geim and Dr. Novoselov. (2010 Novel prize in physics)

Dr. Sumio Iijima
http://www.nec.co.jp/rd/innovative/cnt/top.html

Some allotropes of carbon:a) diamond; b) graphite; c) lonsdaleite;d-f) fullerenes (C60, C540, C70);g) amorphous carbon; h) carbon nanotube
http://schools-wikipedia.org/wp/c/Carbon.htm(Single-Wall Carbon Nanotube)
http://www.nanotech-now.com/nanotube-buckyball-sites.htm
2History of discovery:
Fullerene, another allotrope of carbon, was first found in 1985, and around 1990-1991 was developed
the large-scale synthesis by using arc discharge method.
Although in those days many researchers were interested only in fullerene synthesis, Dr. Iijima paid attention
to sediment left on cathode after fullerene formation.
Analyzing it by electron microscope, he discovered the first multi-walled carbon nanotubes (MWNTs).

Structural diversity:(1) Wall number
Single-walled nanotube (SWNT): Rolled single graphene sheet
Diameter = 0.5~5 nm (cf . fullerene ~0.7nm)
Length = ca. 1 m (max.~18.5cm Nano Lett. 2009, 9, 3137)
Simpler electrical property than MWNTs
Multi-walled nanotube (MWNT): Rolled multiple number of graphene sheets
Diameter = 4~50 nm, Length >10 m
Stronger materials than SWNTs(2) Chiral vector (Ch)
http://www-ibmc.u-strasbg.fr/ict/vectorisation/nanotubes_eng.shtml= A two-dimentional parameter which indicates diameter and chiral angle of SWNT.
Pure. Appl. Chem. 2006, 76, 1703

Iijima S. Nature 1991, 354, 56

O = A in the rolled structure
a1, a2: elementary vector
Ch: chiral vector (= OA = na1 + ma2 = (n,m))
T: lattice vector (= tube direction)
d: diameter of tube, : chiral angle
ac-c: distance between nearest carbon atoms

T

O
Ch
a1
a2

A
(5,2)

In this case,
Ch = 5a1 + 2a2 = (5,2)
3There are 3 kinds of classificationin the structure of SWNTs.

(8,0)

O

a1
a2

(6,4)

(5,5)

(3) Handedness= directionof helicity
graphenesheet(positive or negative )In the case of chiral SWNTs, two kinds of helicity (a pair of enantiomers) can be defined.

(4) "Cap"

Roll-up
The terminal of tubes is often closed by a hemispheric structure of fullerene (called "cap").
Each cap has six 5-membered rings that allow the tube to close.
By using proper methods (e.g. oxidation), the cap can be removed and "open" form nanotubes can be
Ch = (5,5)
Ch = (8,0)
Ch = (6,4)
obtained which have carboxylic groups on terminal insteadof fullerene caps.
m=0
the other casen=m
( = 0)(0< | | < / 6)
( = / 6)a) Model of a capped SWNTb) HRTEM image of a capped MWNTc) Model of closed MWNT indicating thelocation of six pentagons

Uncapped MWNT
Iijima S. et al. Nature 1993, 362, 5224

1-2. Physical properties and applications(1) Electrical property
Carbon nanotube can become both metal and semi-conductor !
# About "metal (metallic conductor)" and "semiconductor"
Metallic conductor: No band gap between valence band and conduction one.
The more the temp increases, the lower the conductivity becomes.
Semiconductor: Possesing a smaller band gap.
The more the temp increases, the higher the conductivity becomes.
Insulator: Possesing a bigger band gap. Electrons are forced to stay in valence band.

http://mrsec.wisc.edu/Edetc/background/LED/band.htm

Graphene itself is known as a zero-gap semiconductor due to the lack of band gap.
On the other hand, as the quantum conditions appear due to rolling the graphene sheet,the band gap of CN becomes different from graphene.
quantum condition:
Ch k = 2 q (k = wavenumber, q = integer)n - m = 3q : metaln - m ≠ 3q : semi-conductor

Ch = (5,5)

Saito R. et al. Appl. Phys. Lett. 1992, 60, 2204

metal

Ch = (8,0)

Ch = (6,4)

metal or semiconductor(In this case, both are semiconductor)
5(2) Strength
Current density: 1000 times larger than copper (~ 4× 109 A/cm2)
Science 2001, 292, 2462
Electrical conductivity: larger than copper (in the case of metallic CN)
Thermal conductivity: 7-8 times larger than silver (3000 W/m/K)

Next generation LSI
instead of silicon-based IC

Tough materials(Rope for space elevator ?)Tensile strength: almost same as diamond (~63 GPa for MWNTs)
Science 2000, 287, 637(3) Large surface area (2000 m2/g if opened)
Aspect ratio (length-to-width) : ~ 10 million
Opened SWNT can become clathrate holding small molecules inside.
To dope new elements or to absorb other molecules inside,
physical property of materials can surprisingly change.

Strage of gas molecules
Gas sensor

Development of new materials

Carbon peapod(C60 @ [10,10]SWNT)
Nanotechnology 2006, 17, 5691

Carborane as a guest molecule
Nakamura E. et al. Science 2007, 316, 853
See also: Dr. Morimoto's Lit. Semi. (2007.8.1)(4) Field emission
Electrons are easily emissed from pentagons of CN cap
by putting in an electric field.

Field emission display(5) Poor solubility
Capped CNs are not soluble for most solvents
due to the large molecular weight.
It was known the commercially available
green tea "伊右衛門 濃いめ" can dissolve
SWNTs, probably due to containing
catechin. Chem. Lett. 2007, 36, 1140
C60 fullerene
Mw:
720
Solubility:
good[7,7]SWNT>107
bad"伊右衛門 濃いめ"
(Suntory)

As carbon nanotubes possess a lot of interesting properties and corresponding uses,
required features of each purpose are so various.
ex. gas strage...large-scale, large surface area
LSI...large-scale, chirality control (metallic or semiconducting)
Both large-scale synthesis and selective synthesis are important.

61-3. Difficulties in conventional syntheses
Conventional synthetic methods:

Ref. http://www.nedo.go.jp/
Bertozzi C. R. et al. Chem. Phys. Lett. 2010, 494, 1(1) Arc discharge synthesis (First discovery of CNs in 1991)
While a current is passed between two graphitic electrodes under inert atmosphere,some graphites vaporize and condense on the cathode as CNs.
(2) Laser ablation
A pulsed lasar is used to vaporize a graphite/catalyst target generating CNs on the walls of the reactor.
Yield and selectivity (purity) are better, but costly and not useful for large-scale production(3) Chemical vapor deposition (CVD)
A metal catalyst particle is deposited on a substrate and placed into a high temperature furnace.
A carbon gas source (e.g. CH4, C2H2) is then passed, resulting in growth of CNs on the substrate.
Useful for large-scale production. Several advanced methods are available.
cf . High pressure carbon monooxide method (HiPCO)Super growth CVD method: Highly pure SWNTs Hata K., Iijima S. et al. Science 2004, 306, 1362

Some advanced methods can produce enough "pure" CNs to utilize for several industrial purposes.
For example, ratio of metallic/semiconducting can be controled as 90:10 ~ 5:95.
(Science 2009, 326, 116; Nano Lett. 2008, 8, 2682)
But these methods cannot control the chirality completely (produced "pure" CNs are always mixture), althoughmany applications (conductive films, transistors, nanosensors, chemical research etc.) require chirality-pure CNs.
It is unclear how to extend these empirical results to chirality-selective synthesis of CNs.
7

Conventional purification methods:
Unfortunately, CNs are not dissolved in most solvents due to their large molecular weight.
So it is expected to be difficult to apply general chromatographical purification methods.
Recently, large-scale single chirality separation of SWNTs by using simple gel was reported.
Kataura H. et al. Nature Commun. 2011, 2, 1

"Multicolumn gel chromatography"
using allyl dextran-based gel and SDS dispersion.
Semiconducting SWNTs are separated throughthe system based on the structure-dependentinteraction strength between SWNTs and gel.

13 kinds of semiconducting SWNT were separated
in 39~94% purity.
Each SWNT was determined by analyzing its specificwavelength (for excitation/emission).
http://www.aist.go.jp/aist_j/press_release/pr2011/
pr20110511/pr20110511.html

But generally, purification is an inefficient and stressful process,and so ideally it is the best to produce the chirality-pure CNs without purification steps.
New strategy for chirality-selective CN synthesis is highly required today.

82. Organic synthesis approach

Bertozzi C. R. et al. Chem. Phys. Lett. 2010, 494, 1

2-1. About "Bottom-up template synthesis"~ One of the possible approaches towards chirality-selective CN synthesis
TemplatesynthesisElongationRnmacrocyclic template"monomer"

Nanotube"polymer"

Advantages of organic synthesis approach:(1) Strategy is more reliable because it's based on mechanistically well-understood reactions.
(2) Lower temperature (~200oC) can be applied than current methods (>1000oC).
(3) Structural diversification should be possible. (e.g. Regioselective doping of other elements)Br

11 stepsCl

Only report about organic synthesis of C60
Scott L. T. et al. Science 2002, 295, 1500
There has been NO REPORT about organic synthesis of carbon nanotubes.
Candidates for "templates"

Prof. Carolyn R. Bertozzi
University of California, Berkeley[n]-CPP"carbon nanohoop"

Main theme of this chapter
Bertozzi (2008)
Itami (2009, 2010, 2011)
Yamago (2010, 2011)

Prof. Kenichiro Itami
Nagoya University

Prof. Shigeru Yamago
Kyoto University

9Difficulty in CPP synthesis: Strain energy caused by aryl ring closure[12]CPP:
48.1 (kcal/mol)Itami K. et al. Org. Lett. 2010, 12, 2262

Itami K. et al. ACIE 2009, 48, 6112

Key point: How to overcome such a big strain energy

2-2. Synthesis of CPP ~ Prof. Bertozzi's work
The first report of CPP synthesis Bertozzi C. R. et al. J. Am. Chem. Soc. 2008, 130, 17646
MeO

n1) NaH, THF
I 1) BuLi, THF2) benzoquinone 2) MeI

I

R

3

OMe

4: R = I (34%)1) nBuLi, THF2) i PrOBPin
R

Pd(PPh3)4, Cs2CO3

Unexpectedhomocoupling of 5

4+5
toluene/MeOH (10:1)
80oC

5: R = BPin(82%)

To Introduce some sp3-carbons as
cyclohexa-1,4-diene moieties,
strain energy of aryl ring decreases.

Li+6, 7, or 8

THF, -78oC[9]CPP:[12]CPP:[18]CPP:

Only lithium naphthalenide condition led to TM.
(Nucleophilic hydride source, Tin(II), and low-valent titanium didn't give good result.)
Structures were determined by 1H-NMR, 13C-NMR, IR, and MALDI-TOF-MS.
Summary of Bertozzi's work
First report of CPP synthesis (0.2 ~ 1.4% overall yield for 5 steps)
Introduced sp3-carbons to decrease the strain energy
Macrocyclization step is not so satisfying (low yield, no selectivity).
102-3. Synthesis of CPP ~ Prof. Itami's work
(1) Selective synthesis of [12]CPP

Itami K. et al. Angew. Chem. Int. Ed. 2009, 48, 6112
Itami's strategy

aromatization[12]CPP
By introducing cyclohexane moieties,
strain energy decreases to 5 kcal/mol !
Cyclization occurs more easily.

Linear oligoarenes (A) cannot be cyclized
due to the big strain energy (~55 kcal/mol).

I
MOMCl, i Pr2NEt,
CH2Cl2

I

I
OMOM

n

I

I

BuLi (3 eq.),
THF;
then
O

OMOM
3 (98%)

I

BPin

OH

(3 eq.)
O(1 eq.)

3 +4(10eq.)

OH
cis-2 (48%)(cis/ trans=81/19)

PinB(BPin)2, PdCl2(dppf),
KOAc, DMSO
4 (81%)OH
OH

PdCl2(dppf) (0.1 eq.)
NaOH (5 eq.),
H2O (16 eq.),
1,4-dioxane (8mM)
60oC, 24 htrimer 6 (81%)

cyclic tetramer 7 (Not obtained)

Despite the screening of conditions, their first target (cyclic tetramer 7) was not obtained in one pot.
11

BPin
PinB

OH
OH
4 (1.4 eq.)p-TsOH

Pd(OAc)2 (0.2 eq.)
X-Phos (0.2 eq.)
NaOH (5 eq.)
H2O (26 eq.)m-xylene
150oC, w

1,4-dioxane (2mM)
80oC, 24 htrimer 6

cyclic tetramer 7 (51%)

Cy2P i Pr
iiPr

X-PhosPr

2 mM is the best concentration.
No other cyclic oligomer was obtained !
By introducing MOM group to only one monomer,
the purification of 6 and 7 was greatly simplified.

[12]CPP (62%)

Cascade sequence;
1) MOM deprotection2) eight-fold dehydration3) oxidation (aromatization)(2) Selective synthesis of [14], [15], [16]CPP Itami K. et al. Angew. Chem. Int. Ed. 2010, 49, 10202

New strategy(1) Synthesis of several U-shaped units (≒ trimer)(2) Cross-coupling between two U-shaped units

Advantages of this strategy:(1) Odd-numbered CPP can be designed
by using different U-shape units.
(2) It can be a uniform synthetic strategy for[n]CPP (n ≧14), as the ring number of
U-shaped units can be easily increased.

12Br(HO)2B
Br

B(OH)2n

1 (5 eq.), Pd(PPh3)4
Na2CO3

Br
OMOM
OMOM
L-shaped unit
1 (81% for 2 steps)>5g scaleBr

H2O/THF
60oC

U-unit (Br)(BPin)2
Pd2(dba)2
X-Phos, KOAc
1,4-dioxane
Pd(OAc)2
X-Phos
NaOH, H2O

NaHSO4.H2O

1,4-dioxane(2mM 2)m-xylene
DMSO
refluxunder airU-unit (BPin)

Non-microwave condition
DMSO may help in dissolving highly polar intermediate during the aromatization.
First selective synthesis of odd-numbered CPP
Why does hetero-size coupling occur ?
"Arch widths" of U-shaped units
are different.
Coupling does not occur.

After chair-flipping of one unit,"arch widths" of two units nicely match.

DFT calculation of 4b implies"Hetero-size" coupling does occur ! the optimized structure matches
well with the chair-flipped model.

13(3) Concise synthesis of [12]CPP

Itami K. et al. Angew. Chem. Int. Ed. 2011, 50, 3244

Problems of previous method: (1) Need of 2 monomers
As a solution(2) Stepwise macrocyclization(3) Use of expensive Pd catalyst
X(4) Low yield (4-10% overall)(1) Need of single monomer(2) 1-step macrocyclization(3) Use of Ni instead of Pd(4) Higher overall yield

X
Nickel mediated biaryl coupling: Too complex mechanism is proposed.
Kochi J. K. et al. J. Am. Chem. Soc. 1979, 101, 7547
2 stepsX
1-step cyclization
f rom single monomer

X

MOMO
OMOM
1a (X = I)
1b (X = Br)

Ni(cod)2 (2.0 eq.)
2,2'-bipyridyl (0.5 eq.)
COD (only for 1a, 1.7 eq.)
THF (16 mM), 60oC
>5g

First X-ray crystal structureof [12]CPP.(Cy)2from CHCl3/cyclohexane

This x-ray structure is closer to D3d form which has higher energy(by 3.7 kcal/mol) than most stable D6d form, indicated by DFT calculation.

NaHSO4.H2O
m-xylene/DMSO
reflux under air

11~13% overall~0.5g

In the macrocyclization step, cyclic trimer (3) was also obtained, which can be transformed to [9]CPP.
Itami K. et al. Chem. Lett. 2011, 40, 423
Ni(cod)2 (2.0 eq.)
2,2'-bipyridyl (2.0 eq.)
1b

THF (5mM), reflux,
24 h(optimized condition)

R = MOM
3 (32%), 2 (23%)

X-ray crystal structureof [9]CPP.(thf)214

(4) Synthesis of [13]CPPN
Itami K. et al. Org. Lett. 2011, 13, 2480

CPPN is a potential template of an [n+2, n+1]SWNT(chiral CN).
They supposed their synthetic methodology of CPP
can be applied to CPPN synthesis.
Br

Br

MOMO
OMOM

Pd(PPh3)4
Na2CO3
PinB
72%
BPin

Pd2(dba)3
X-PhosNaHSO4.H2O
m-xylene/DMSO
reflux under air
35%

rac-[13]CPPN (25%)

According to DFT calculation, racemization energy of [13]CPPN is only 8.4 kcal/mol.
In order to achieve enantioselective synthesis of CPPN, new strategies must be required to avoid racemization.
Summary of Itami's work
4~13% overall yield. Introducing sp3-carbons like Bertozzi's case
Selective synthesis for n = 12, uniform strategy for n ≧ 14 (14, 15, 16 were reported)
Catalytic Pd or stoichiometric Ni (only for n =12)
Crystal structure of [12] and [9]CPP
15

2-4. Synthesis of CPP ~ Prof. Yamago's work
YamagoBertozzi, Itami

reductive
elimination

aromatizationRO[n]CPP

L Pt
LROIntroducing sp3-carbons

Introducing square-planer metal complex(1) Synthesis of [8], [12]CPP

Yamago S. et al. Angew. Chem. Int. Ed. 2010, 49, 757
Yamago S. et al. J. Am. Chem. Soc. 2011, 133, 8354
L
L Pt

L
Pt L

L Pt
L

Pt L
L

reductive
elimination[8]CPPstrain energy = 74 kcal/mol(Higher than any other CPP
ever synthesized)

Square-shaped platinum complex
Complexes with 4,4'-bipyridyl or butadiyne(instead of biphenyl) were reported.

Same strategy has already been utilized for [n]cyclothiophenes synthesis.
(But cyclothiophenes are almost planer molecules) Chem. Commun. 2003, 948
L2Pt

SnMe3

PtL2

PtCl2(cod) (1 eq.)

Br2 (7 eq.)

THF, reflux
(84%)
SnMe3

toluene, 95oC
(49%)
L2Pt

PtL2
4a: L2 = cod
4b: L2 = dppf

dppf (4 eq.)
CH2Cl2, rt (91%)[8]CPP
37% overall yieldfor 3 steps

Ligand effect: dppe, dppp, and xantphos gave only <2% [8]CPP,
even though the corresponding platinum phosphine complexes were obtained.
Reductive elimination: addition of iodine, triphenyl phosphine or additive-free condition didn't lead to good result.
16

L2Pt

SnMe3

PtL2

PtCl2(cod) (1 eq.)

Br2 (7 eq.)

THF, reflux(20%)

toluene, 95oC
(58%)

SnMe3

L2Pt

PtL2
5a: L2 = cod
5b: L2 = dppf

dppf (4.3 eq.)
CH2Cl2, rt (96%)

11% overall yieldfor 3 steps(2) Random synthesis of [n]CPP (n = 8 ~ 13)
SnMe3
PtCl2(cod)dppf (2 eq.)
CH2Cl2, rt;
DCE, 50oC
20-32 hPtCl2(cod)SnMe3then Br2 (2 eq.)
toluene, 95oC
observed [n]CPP[8], [9], [10], [11], [12], [13]mixtureThey first expected the selective synthesis of [10]CPP, but as a result mixture of six [n]CPPs were obtained.
This must be because the equilibrium during transmetallation and/or ligand exchange reaction.
Summary of Yamago's work
~ 37% overall yield (highest) for 3 steps (shortest)
Using square-planer metal complex to reduce the strain effect
Selective synthesis for only [4m]CPP (m = 2, 3 are reported.)
Smallest numbered [n]CPP synthesis ([8]CPP)

17

2-5. Elongation ~ Diels-Alder strategy

Bertozzi C. R. et al. Chem. Phys. Lett. 2010, 494, 1
Scott L. T. et al. Polycycl. Aromat. Comp. 2010, 30, 247

elonged template

template

As additional carbon sources, acetylene (or its derivative) is used.
Repetitive 2-step cascade (bay region Diels-Alder and dehydrogenation) will lead to armchair (or chiral) CNs.
Advantages: [n]CPP or its derivatives are possible candidates of this substrate.
1 mg of a template grown to 1 mm long CNs would produce over 1 ton of CNs.
Ideally, no oxidant is required for dehydrogenation step because it's highly exothermic.

H

H

- H2

[8]CPP
H
H
Disadvantages: Zigzag nanotubes cannot be constructed.
Both acetylene and poryarene are not so reactive for Diels-Alder reaction.

Reactivity of polyarenes

Scott L. T. et al. J. Am. Chem. Soc. 2009, 131, 16006

H

H

H

H
Intermediate
after Diels-Alder

They expected the [4+2] reactivity of polyarenes are related to how low the activation energy is.
As the number of benzene rings is increased, aromaticity is not so decreased even in the intermediate.

18<50% conv.
100% conv.
44%

E = CO2Et
Mesityl group is introduced bothto avoid the internal cycloaddition
and to increase the solubility.

12%

It is confirmed 4 is much reactive
for bay region Diels-Alder than 2.

Promissing example using nitroethylene

Scott L. T. et al. Angew. Chem. Int. Ed. 2010, 49, 6626

In order to achieve cycloaddition/rearomatization sequence...
H

H

EtO2C

Poorly reactive

CO2Et

Reactive but need decarboxylation
O

NO2

nitroethylene
Reactive dienophile"masked acetylene"

HO

NO2

O

generated in situO
via dehydration of 2-nitroethanol
Proposed mechanism:

NO2(in situ)

Cycloaddition did not occur.
triphenylene

Reactivity: 1 > 3 > triphenylene

1d
NO2
84%
(in situ)

anthracene

H NO2
no elimination
H H
130oC, 39 h
Cl2CDCDCl2

HONO elimination requires nitrogroup on benzylic position.
4d

58%
(NMR)

The auther says,"Diels-Alder cycloadditions of CPP are expected to be difficult"due to the strain effect and electronic consideration.
19

3. Summary
Introduction ~ suffering problems:
Carbon nanotubes have a lot of interesting potentials such as unique electrical and physical properties.
Numerous kinds of industrial applications are being developed today.
A problematic point lies in the absence of methodology to obtain chirality-pure carbon nanotubes,
and "bottom-up" organic synthesis approach may become one of the solutions for this point.
Template synthesis:
Three groups have reported the synthesis of cycloparaphenylene (CPP).
They utilized either introduction of sp3-carbons or square-planer metal complex as an intermediate,
in order to reduce the strain energy.
They could synthesize [n]CPP (n≧8), but there has been no success for much smaller CPP synthesis.
Perhaps, different methodologies should be prefered for narrower carbon nanotube synthesis.
Elongation methods:
Diels-Alder strategy seems to be promissing, but no successful data were obtained in the case of cycloarene.
Problems lie in poor reactivity and strain effect of CPP.
Higher consideration must be required to achieve the great goal of carbon nanotube synthesis.

20