Synthesis of tetraphenyl-substituted [12]cycloparaphenylene: Toward a rationally designed ultrashort carbon nanotube

Article abstract of DOI:10.1021/jo301024g

The first phenyl-substituted [n]cycloparaphenylene (1) has been synthesized. The preparation of this structure addresses several challenges toward a more elaborate phenyl-substituted [n]cycloparaphenylene (2), a molecule that may lead to the homogeneous s

Full text of DOI:10.1021/jo301024g

Brief Communication
pubs.acs.org/joc
Synthesis of Tetraphenyl-Substituted [12]Cycloparaphenylene:
Toward a Rationally Designed Ultrashort Carbon Nanotube
Thomas J. Sisto, Xia Tian, and Ramesh Jasti*
Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
S
* Supporting Information
ABSTRACT: The first phenyl-substituted [n]-
cycloparaphenylene (1) has been synthesized. The preparation
of this structure addresses several challenges toward a more
elaborate phenyl-substituted [n]cycloparaphenylene (2), a
molecule that may lead to the homogeneous synthesis of
armchair carbon nanotubes.
he [n]cycloparaphenylene ([n]CPP) class of molecules
represents the smallest possible segment of an armchair
carbon nanotube (CNT), consisting of n benzene rings linked
at the para position (Figure 1). Similar to other small fragments
toward this challenging synthetic target (2), we report the first
synthesis of tetraphenyl-substituted [12]cycloparaphenylene 1,
the first CPP with phenyl substitution.
T
Before embarking on the formidable syntheses of 2, and
ultimately 3, we surmised that several fundamental questions
must first be addressed on a more tractable model system. In
our previous synthetic approaches to the CPPs, we have utilized
a 3,6-syn-dimethoxycyclohexa-1,4-diene moiety as a masked
aromatic ring in order to provide the curvature and rigidity
necessary for macrocyclization.8,23 These structures are easily
prepared with moderate stereoselectivity by aryllithium
addition (2 equiv) to benzoquinone. We were curious as to
whether this strategy could be translated to a tetraphenyl-
substituted benzoquinone. To address this, we turned to the
synthesis of diiodide 8 (Scheme 1). 1-Bromo-4-n-butylbenzene
(4) was converted to the corresponding boronic acid (5) and
carried on without purification through a 4-fold Suzuki−
Miyaura coupling reaction with tetrabromoquinone.24 The
crude reaction mixture was then treated with DDQ to afford
tetraphenyl-substituted quinone 6 in 54% yield over three steps.
Addition of 2 equiv of lithiated 1,4-diiodobenzene to quinone 6
at −78 °C generated syn diol 7 in 69% yield (3.5:1 syn/anti).
Standard methylation of 7 produced diiodide 8 in excellent
yield. Utilizing this synthetic sequence, we were able to easily
prepare over 25 g of diiodide 8.
With diiodide 8 in hand, we next investigated the preparation
of macrocycle 12 (Scheme 2). We anticipated using a
chemoselective Suzuki−Miyaura coupling strategy, similar to
our recently reported selective synthesis of [7]CPP.14 As
expected, using Pd(PPh3)4 as catalyst allowed for the smooth
coupling of 8 and boronate 914 to produce dichloride 10 in
75% yield with no side reactivity of the aryl chloride
functionality (Scheme 2). Of note, this reaction can be
conducted on a multigram scale. With the more hindered
quinone fragment included in the backbone of dichloride 10,
Figure 1. CNT-inspired targets: (a) [12]cycloparaphenylene, (b)
tetraphenyl-substituted [12]cycloparaphenylene, (c) strategy toward
an ultrashort CNT.
of CNTs,1−3 these structures have garnered considerable
interest because of their potential application in the growth
of homogeneous CNTs of discrete chirality and diameter, a
grand challenge in the field of nanoscience.4−7 First synthesized
by Jasti and Bertozzi in 2008, numerous synthetic routes to the
CPPs have been developed.8−15 Despite access to various
CPPs, there are currently no reports of cycloparaphenylenes
being elongated into carbon nanotubes. Several studies suggest
that a longer CNT fragment such as structure 3 (Figure 1)
might be more amenable as a seed for CNT growth than the
CPPs.5,7,16 Ultrashort CNT (3) may be envisioned as the
product of multiple cyclodehydrogenations of highly phenyl-
substituted [12]CPP 2,17−22 a CPP in which every other
backbone aryl ring is tetraphenyl-substituted. As an initial foray
Received: May 18, 2012
© XXXX American Chemical Society
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The Journal of Organic Chemistry
Brief Communication
in a single macrocyclization/aromatization sequence from 10.
Interestingly, the UV−vis absorption of 1 in dichloromethane
shows a dominant absorption peak at 328 nm, similar to
[12]CPP, along with a secondary peak at approximately 240
nm. This second peak may be attributed to the tetraphenyl
substitution. The fluorescence spectra of 1 displays a red-shifted
fluorescence with a fluorescence maximum at 423 nm (95 nm
shift), similar to unsubstituted [12]CPP. The quantum yield of
1 is 0.81, identical to the parent [12]CPP.14
Scheme 1. Synthesis of Diiodide 8
In conclusion, we have identified highly phenyl-substituted
CPP 2 as a plausible precursor to an ultrashort CNT (3), a
viable seed for the preparation of homogeneous armchair
carbon nanotubes. As an initial exploration of the requisite
methodology to prepare 2, we have synthesized tetraphenyl-
substituted [12]cycloparaphenylene 1. In order to prepare this
structure, we have developed a multigram synthesis of
tetraphenyl-substituted 3,6-syn-dimethoxycyclohexa-1,4-diene
8. This elaborated cyclohexadiene fragment was incorporated
into the backbone of a CPP by a multigram chemoselective
Suzuki−Miyaura coupling to dichloride 10 followed by
macrocyclization. Our reductive aromatization strategy was
proven to be equally effective upon these phenyl-substituted
quinone derivatives. With this knowledge in hand, a synthesis
of more elaborate phenyl-substituted 2 is underway.
Scheme 2. Synthesis of Tetraphenyl-Substituted
[12]Cycloparaphenylene 1
EXPERIMENTAL SECTION
■
1
General Experimental Details. H NMR spectra were recorded
at 500 or 400 MHz, while 13C NMR spectra were recorded at 125 or
100 MHz. All NMR spectra were referenced to TMS. All reagents
were obtained commercially. Tetrahydrofuran, dicholoromethane, and
dimethylformamide were dried by filtration through alumina. Silica
column chromatography was conducted with Zeochem Zeoprep 60
Eco 40−63 μm silica gel, while alumina chromatography utilized
Sorbent Technologies 50−200 μm basic activity II−III alumina. Thin-
layer chromatography (TLC) was performed using Sorbent Tech-
nologies silica gel XHT TLC plates or Sorbent Technologies alumina
TLC plates, respectively. Developed plates were visualized using UV
light at wavelengths of 254 and 365 nm. All glassware was oven- or
flame-dried and cooled under an inert atmosphere of nitrogen unless
otherwise noted. Moisture-sensitive reactions were carried out under
an inert atmosphere of nitrogen using standard syringe/septa
technique. All solvents were degassed by sparging with N2.
Tetraphenyl-Substituted Quinone 6. 1-Bromo-4-n-butylben-
zene (44.1 mL, 250 mmol) was added to a dry flask charged with a stir
bar. THF (750 mL) was added, and the flask was consequently cooled
to −78 °C. Once cooled, nBuLi was added dropwise (110 mL, 275
mmol), resulting in a yellow reaction mixture. The reaction mixture
was stirred for 30 min. Trimethoxyborane was then added as a stream
(35.3 mL, 300 mmol), resulting in a colorless reaction mixture. The
reaction mixture was then warmed to room temperature and stirred for
45 min. A 6 M solution of HCl was added (600 mL), and the reaction
mixture was stirred for 1 h before being transferred to a separatory
funnel and diluted with diethyl ether. The aqueous layer was extracted
three times with diethyl ether, which was combined and subsequently
washed with a saturated solution of sodium bicarbonate followed by
brine. The organic layer was then dried over sodium sulfate and
concentrated to afford the corresponding (4-butylphenyl)boronic acid
as a slightly wet, white solid.
A Schlenk flask was charged with tetrabromoquinone (25 g, 59
mmol), Pd(PPh3)4 (3.4 g, 2.95 mmol), and K2CO3 (40.7 g, 295
mmol) and then evacuated and backfilled with N2. Separately, THF
and H2O were sparged with N2. Degassed THF (180 mL) was used to
transfer the freshly made boronic acid from its evacuated and
backfilled (N2) flask into the air-free Schlenk. Degassed H2O (60 mL)
was then syringed into the Schlenk, which was subsequently heated to
90 °C and stirred for 16 h.
we were cognizant that this substitution might have a
deleterious effect on the conformation necessary for cyclization.
Gratifyingly, upon switching to Buchwald’s S-Phos ligand,25
dichloride 10 undergoes tandem Suzuki−Miyaura coupling/
macrocyclization with diboronic pinacol ester 118 to generate
macrocycle 12 in a moderate yield of 24%.
The final step in the synthesis involved the aromatization of
the 1,4-cyclohexadiene moieties to the requisite benzene rings.
Treatment of macrocycle 12 at −78 °C with sodium
naphthalenide, a single-electron reducing agent, induced a
reductive aromatization sequence to generate tetraphenyl-
substituted CPP 1 in 63% (Scheme 2).8 The proton and
carbon NMR spectra were consistent with the target molecule,
and the MALDI-TOF spectrum shows the characteristically
strong CPP peak at 1441 attributed to the radical cation of 1.
Although the yield for the macrocyclization step is modest, the
reactions are scalable, and over 100 mg of 1 has been prepared
B
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The Journal of Organic Chemistry
Brief Communication
The reaction mixture was cooled and concentrated under reduced
pressure, at which point it was diluted with ether. A 6 M solution of
HCl was added slowly until the mixture was pH neutral. The mixture
was then transferred to a separatory funnel and extracted three times
with CH2Cl2. The combined organic layer was dried with brine and
sodium sulfate before being concentrated under reduced pressure to
afford a black oil.
This crude oil was dissolved in CH2Cl2 (200 mL), and DDQ (15.5
g, 68.16 mmol) was added. The reaction mixture was stirred overnight
open to air before being concentrated under reduced pressure. The
reaction mixture was purified through a pad of silica (1:9 CH2Cl2/
hexanes to 4:6 CH2Cl2/hexanes) before being recrystallized in hot
hexanes to afford red needles (20.25 g, 54% over three steps): mp
157−159 °C; IR (neat) 2952, 2928, 2858, 1650, 1607, 1503, 1466,
1296, 1137 cm−1; 1H NMR (500 MHz, CDCl3) δ 6.99 (s broad, 16H),
2.54 (t, J = 7.7 Hz, 8H), 1.54 (tt, J = 7.7, 7.2 Hz, 8H), 1.29 (qt, J = 7.2,
7.2 Hz, 8H), 0.89 (t, J = 7.2 Hz, 12H); 13C (125 MHz, CDCl3) δ
187.25, 142.91, 142.78, 130.82, 130.33, 127.58, 35.38, 33.23, 22.22,
13.93; MALDI-TOF m/z calcd for C46H52O2Na (M)+ 659.4, found
659.4.
Tetraphenyl-Substituted Terminal Dichloride 10. A Schlenk
flask was charged with a stir bar, 8 (2.61 g, 2.43 mmol), 9 (2.4 g, 5.35
mmol), Pd(PPh3)4 (280 mg, 0.28 mmol), and CsCO3 (3.96 g, 14
mmol). The flask was then evacuated and backfilled with N2. Toluene
and methanol were sparged with N2 in separate flasks and
subsequently syringed into the Schlenk flask (143 and 13 mL
respectively). The reaction mixture was heated to 80 °C and stirred for
16 h. Upon cooling, the reaction mixture was diluted with H2O and
transferred to a separatory funnel. The aqueous layer was extracted
with CH2Cl2 (3×), and the combined organic layers were
subsequently washed with brine and dried over sodium sulfate.
Upon concentration under reduced pressure, the reaction mixture was
purified by column chromatography (alumina) with an eluent of 2:8
diethyl ether/hexanes to afford a white solid (2.66 g, 75%): mp 116−
128 °C; IR (neat) 2954, 2930, 1491, 1013, 951, 907, 821, 728 cm−1;
1H NMR (500 MHz, CDCl3) δ 7.73−7.56 (broad, overlap, 4H), 7.58
(d, J = 8.6 Hz, 4H), 7.49−7.40 (broad overlap, 4H), 7.43 (d, J = 8.6
Hz, 4H), 7.36 (d, J = 8.8 Hz, 4H), 7.28 (d, J = 8.8 Hz, 4H), 6.67 (d, J
= 8.3 Hz, 8H), 6.62 (d, J = 8.3 Hz, 8H), 6.16 (d, J = 10.24 Hz, 4H),
6.08 (d, J = 10.24 Hz, 4H), 3.87 (s, 6H), 3.45 (s, 6H), 3.44 (s, 6H),
2.34 (t, J = 7.5 Hz, 8H), 1.37 (tt, J = 7.5, 7.4 Hz, 8H), 1.13 (qt, J = 7.4,
7.4 Hz, 8H), 0.80 (t, J = 7.4 Hz, 12H); 13C (125 MHz, CDCl3) δ
142.53, 142.46, 142.06, 142.03, 140.40, 140.06, 138.34, 136.04, 133.74,
133.39, 132.95, 131.15, 128.49, 128.32 (broad), 127.48, 126.88,
126.72, 126.31, 125.72, 81.44, 74.64, 74.51, 52.14, 52.02, 52.02, 34.98,
35.14, 21.96, 13.91; MALDI-TOF m/z calcd for C99H99Cl2O5 (M −
CH3O)+ 1438.7, found 1438.7.
Tetraphenyl-Substituted Diiodide Diol 7. 1,4-Diiodobenzene
(43 g, 129 mmol) was placed in a flame-dried flask with a stir bar. THF
(380 mL) was added, and the solution was cooled to −78 °C, at which
point nBuLi (46 mL, 115 mmol) was added dropwise. The solution
stirred for 30 min, at which point 6 (30.5 g, 47.9 mmol) was
introduced as a solution in THF (120 mL) over 10 min. The reaction
mixture was stirred for 30 min and then quenched with H2O at −78
°C. The reaction mixture was warmed and then diluted with H2O and
diethyl ether before being transferred to a separatory funnel. The
aqueous layer was extracted three times with ether and then three
times with CH2Cl2. The combined organic layers were washed with
brine and dried over sodium sulfate before being concentrated under
reduced pressure. The reaction mixture was purified by silica gel
chromatography with an eluent mixture gradient of 3:7−8:2 CH2Cl2/
hexanes to afford a white solid (34.6 g, 69%): mp 220−221 °C; IR
Macrocyclic 12. A three-necked flask was charged with a stir bar,
10 (882 mg, 0.6 mmol), 11 (424 mg, 0.78 mmol), Pd2(dba)3 (54 mg,
0.06 mmol), S-Phos (98.4 mg, 0.24 mmol), and K3PO4 (254.4 mg, 1.2
mmol). The flask was fitted with a condenser and septa and
subsequently evacuated and backfilled with N2. The flask was then
additionally purged with N2 for 30 min. Separately, a flask containing
DMF and a flask containing H2O were sparged with N2. The DMF
and H2O were syringed into the reaction flask (540 and 60 mL,
respectively), and the reaction mixture was lowered into a hot oil bath
at 125 °C. The reaction mixture was stirred for 16 h.
1
(neat) 2954, 2928, 2857, 1505, 1480, 1389, 1005, 906, 732 cm−1; H
NMR (500 MHz, CDCl3) δ 7.55 (d, J = 8.3 Hz, 4H), 7.10 (d, J = 8.3
Hz, 4H), 6.70 (d, J = 8.2 Hz, 8H), 6.66 (d, J = 8.2 Hz, 8H), 2.42 (s,
2H), 2.35 (t, J = 7.6 Hz, 8H), 1.38 (tt, J = 7.6, 7.2 Hz, 8H), 1.14 (qt, J
= 7.2, 7.2 Hz, 8H), 0.81 (t, J = 7.2 Hz, 12H); 13C (125 MHz, CDCl3)
δ 142.96, 140.74, 140.34, 136.47, 134.14, 131.29, 129.60, 126.90,
92.97, 75.11, 35.00, 33.12, 22.03, 13.88; MALDI-TOF m/z calcd for
C58H62I2O2Na (M)+ 1067.2, found 1067.3.
Upon cooling, the reaction mixture was filtered through Celite, and
the Celite was then washed with CH2Cl2. The filtered solution was
transferred to a separatory funnel, and the organic layer was washed
multiple times (∼15) with H2O until the DMF was removed. Upon
concentration under reduced pressure, the reaction mixture was
purified by column chromatography (silica) with an eluent gradient of
10:90 to 15:85 EtOAc/hexanes to afford a white solid (244 mg, 24%):
mp 185−190 °C; IR (neat) 2953, 2927, 2855, 1491, 1083, 1026, 1006,
Tetraphenyl-Substituted Diiodide 8. Diol 7 (34.6 g, 33 mmol)
was placed into a dry flask with a stir bar, at which point THF (300
mL) was added and the reaction mixture was cooled to 0 °C. A NaH
powder in oil (6.6 g, 165 mmol) was added in three portions over 20
min. The reaction mixture was then stirred for 30 min, and MeI (12.3
mL, 198 mmol) was subsequently added as a stream over 5 min. The
ice bath was allowed to warm to room temperature as the reaction
mixture was stirred for 16 h. The reaction mixture was then quenched
with H2O and diluted with diethyl ether before being transferred to a
separatory funnel. The aqueous layer was extracted three times with
diethyl ether, and the combined organic layers were washed with brine
and dried over sodium sulfate before being concentrated under
reduced pressure. The crude mixture was suspended in 150 mL of
hexanes and stirred at 0 °C for 30 min. The solution was filtered and
the solid was washed with cold hexanes (2 × 50 mL) to afford a white
solid (26.5 g, 75%). The yield may be improved if purified through
silica gel chromatography (1:9 CH2Cl2/hexanes) to afford a white
solid (92%): mp 218 °C; IR (neat) 2955, 2924, 2857, 1507, 1480,
1
951, 820 cm−1; H NMR (500 MHz, CDCl3) 7.64−7.34 (m, 32H),
6.66 (d, J = 8.3 Hz, 8H), 6.62 (d, J = 8.3 Hz, 8H), 6.18−6.12 (m,
12H), 3.84 (s, 6H), 3.46 (s, 6H), 3.45 (s, 6H), 3.44 (s, 6H), 2.34 (t, J
= 7.6 Hz, 8H), 1.37 (tt, J = 7.6, 7.5 Hz, 8H), 1.13 (qt, J = 7.5, 7.5 Hz,
8H), 0.79 (t, J = 7.5 Hz, 12H); 13C (125 MHz, CDCl3) δ 142.53,
142.52, 142.40, 142.37, 142.26, 140.41, 140.25, 140.15, 140.09, 138.76,
136.02, 133.40, 133.35, 133.34, 133.33, 131.15, 128.27, 127.14, 127.13,
126.99, 126.71, 126.39, 126.34, 125.81, 81.45, 74.70, 74.60, 74.59,
52.05, 52.00, 51.98 (2), 34.94, 33.12, 21.93, 13.89; MALDI-TOF m/z
calcd for C119H118O7 (M − CH3O)+ 1658.8, found 1658.2.
Tetraphenyl-Substituted [12]CPP 1. A dry flask was charged
with a glass stir bar, and sodium metal was washed with hexanes to
remove oil (405 mg, 17.6 mmol). Dry THF (15 mL) was added,
followed by naphthalene (1.92 g, 15 mmol) in portions. This reagent
was allowed to stir overnight for complete formation.
Macrocycle 12 (240 mg, 0.14 mmol) was placed in a dry flask along
with a glass stir bar. THF (40 mL) was added, and the reaction
mixture was cooled to −78 °C. Freshly prepared sodium naphthalide
(7 mL, 7 mmol) was added dropwise, and the reaction mixture
immediately changed color to a dark purple solution. The reaction
mixture was stirred for 40 min, at which point the excess reagent was
quenched with a 1 M iodine solution in THF. A saturated solution of
sodium thiosulfate was added, and the reaction mixture was allowed to
warm to room temperature while stirring.
1
1458, 1387, 1096, 1006, 907, 758, 733 cm−1; H NMR (500 MHz,
CD2Cl2) δ 7.62−7.08 (broad, 8H), 6.60 (d, J = 8.2 Hz, 8H), 6.51 (d, J
= 8.2 Hz), 3.76 (s, 6H), 2.28 (t, J = 7.6 Hz, 8H), 1.31 (tt, J = 7.6, 7.2
Hz, 8H), 1.08 (qt, J = 7.2, 7.2 Hz, 8H), 0.75 (t, J = 7.2 Hz, 12H); 13C
(125 MHz, CDCl3) δ 143.05, 142.36, 140.66, 136.35, 135.54, 130.96,
129.85 (broad), 126.83, 92.56, 81.14, 52.10, 34.95, 33.10, 21.95, 13.90;
MALDI-TOF m/z calcd for C59H63I2O (M − CH3O)+ 1041.3, found
1041.3.
C
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The Journal of Organic Chemistry
Brief Communication
The reaction mixture was transferred to a separatory funnel and
diluted with CH2Cl2 and H2O. The aqueous layer was extracted three
times with CH2Cl2 and the combined organic layers were washed with
brine and dried over magnesium sulfate before being concentrated
under reduced pressure. The reaction mixture was then purified
through column chromatography (silica) with an eluent of 25:75
CH2Cl2/hexanes to afford a pale yellow solid (127 mg, 63%): mp
>225 °C; IR (neat) 3025, 2955, 2927, 2856, 1482, 906, 809, 729 cm−1;
1H NMR (500 MHz, CDCl3) 7.66−7.61 (m, 28H), 7.53 (d, J = 8.5
Hz, 4H), 7.19 (d, J = 8.5 Hz, 4H), 6.89 (d, J = 8.5 Hz, 4H), 6.85 (d, J
= 8.5 Hz, 4H), 6.61 (d, 8.0 Hz, 4H), 6.48 (d, J = 8.0 Hz, 4H), 2.17 (t, J
= 7.6 Hz, 8H), 1.24 (tt, J = 7.6, 7.3 Hz, 8H), 1.02 (qt, J = 7.3, 7.3 Hz,
8H), 0.70 (t, J = 7.3 Hz, 12H); 13C (125 MHz, CDCl3) δ 140.36,
140.25, 140.12, 139.66, 139.16, 138.60, 138.59, 138.56, 138.55, 138.50
(2), 138.29, 137.91, 137.70, 137.61, 132.41, 130.71, 127.77, 127.34−
127.29 (6), 127.18, 126.77, 126.32, 125.46, 34.88, 33.14, 22.02, 13.84;
MALDI-TOF m/z calcd for C112H96 (M)+ 1441.7, found 1441.5.
(18) Ormsby, J. L.; Black, T. D.; Hilton, C. L.; Bharat; King, B. T.
Tetrahedron 2008, 64, 11370.
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̈
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(23) A similar strategy had been suggested previously for the
synthesis of cyclophynes; see: Srinivasan, M.; Sankararaman, S.; Hopf,
H.; Varghese, B. Eur. J. Org. Chem. 2003, 660.
(24) Ullah, I.; Khera, R. A.; Hussain, M.; Villinger, A.; Langer, P.
Tetrahedron Lett. 2009, 50, 4651.
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Am. Chem. Soc. 2005, 127, 4685.
ASSOCIATED CONTENT
* Supporting Information
■
S
NMR spectra of all compounds, MALDI-TOF, and optical
characterization of 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jasti@bu.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by generous startup funds provided
by Boston University. We gratefully acknowledge Han Xiao for
experimental assistance.
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