A triphenylene-based triptycene with large free volume synthesized by zirconium-mediated biphenylation

Article abstract of DOI:10.1021/jo801643u

(Chemical Equation Presented) Biphenylation using (Li(THF) ₄)₂·Zr(biphe)₃ of hexabromotriptycenes bearing H (1-H) or Bu (1-Bu) at the bridgeheads gave triptycenes with triphenylene blades. The blades extend both perpendicu

Full text of DOI:10.1021/jo801643u

A Triphenylene-Based Triptycene with Large
Free Volume Synthesized by Zirconium-Mediated
Biphenylation
shape of triptycene is credited with improving the mechanical
1
1-13
properties of polymers in which it is incorporated.
Triptycene can be extended by increasing the size of its arene
1
4,15
blades. With a few exceptions,
this has been done by
16
increasing the length of the blade in a direction perpendicular
Cameron L. Hilton, Christopher R. Jamison,
Hannah K. Zane, and Benjamin T. King*
to the 3-fold axis of the triptycene (e.g., benzene blades to
anthracene blades). Extended triptycenes have primarily served
Department of Chemistry, UniVersity of NeVada,
Reno, NeVada 89557-0216
17
as starting materials for the synthesis of higher iptycenes.
Pascal’s dodecaphenyltriptycene is an especially noteworthy
1
8
analogue.
king@chem.unr.edu
We chose to synthesize 6,13-(2,3-triphenyleno)-6,13-dihy-
dro[a,c,l,n]tetrabenzopentacene (1-H), which is a rare example
of a triptycene that has been extended both parallel and
perpendicular to its highest symmetry axis. This extension
generates large voids above the bridgehead sites and between
the blades and gives rise to a large internal molecular free
ReceiVed July 24, 2008
1
2
volume (IMFV). Swager defines IMFV as “the difference in
volume between that which is generated by the geometry of
1
2
the structure and that occupied by the structure itself.” The
3
IMFV of 1-H, from the calculated AM1 geometry, is 710 Å .
3
The free volume of parent triptycene, 74 Å , is 1 order of
magnitude less than that of 1-H but is nonetheless sufficient to
6
permit its use as a stator for molecular gyroscopes and as a
“
barbed-wire” component of polymers, which inhibits chain-
Biphenylation using (Li(THF)
)
4 2
·Zr(biphe)
3
of hexabromot-
12,13
chain slippage and improves mechanical properties.
We feel
riptycenes bearing H (1-H) or Bu (1-Bu) at the bridgeheads
gave triptycenes with triphenylene blades. The blades extend
both perpendicular and parallel to the 3-fold axis and generate
a large intramolecular free volume (IMFV) (1-H, AM1, 710
Å ; cf. triptycene, AM1 71 Å ). Crystals of 1-H could not
be obtained. Triptycene 1-Bu, in which the Bu groups fill
the voids near the bridgehead, was crystalline. X-ray dif-
fraction analysis revealed crystal packing with alternating,
interlocked corrugated and distorted hexagonal layers.
the large IMFV of 1-H and its derivatives may enable similar
applications but more effectively and on a somewhat larger
scale.
The synthesis of 1-H is carried out in two steps. First,
3
3
bromination of triptycene yields 2,3,6,7,12,13-hexabromotrip-
4
tycene (2-H), which is followed by the substitution of three
biphenyl units for the three pairs of o-dibromides using the
1
9
recently reported reagent, (Li(THF)4)2 ·Zr(biphe)3 (3), where
biphe is biphenyl(2,2′-diyl). A traditional Diels-Alder approach
2
0
would require tetrabenzo[a,c,l,n]pentacene, which is difficult
2
1
to prepare, and 2,3-didehydrotriphenylene.
Triptycene is a useful building block in supramolecular
chemistry. Its rigidity, paddlewheel shape, and ease of
The synthesis 1-H was facilitated by the development of an
improved synthesis of hexabromide 2-H from triptycene. Our
improved synthesis provides a one-step, multigram preparation
of 2-H in 64% yield. Hexabromide 2-H may be a versatile
building block for other large iptycenes.
1
-4
5
6
funtionalization
make triptycene a useful rotor or stator
component in nanodevices, a rigid spacer for the construction
7
of host-guest complexes, a framework for the construction
1
,8,9
2
of chelating ligands,
and a subunit of molecular cages.
Electron-rich clefts between the arene blades of triptycene allow
10
it to form inclusion complexes with C60. The rigid paddlewheel
(
(
11) Ohira, A.; Swager, T. M. Macromolecules 2007, 40, 19–25.
12) Tsui, N. T.; Paraskos, A. J.; Torun, L.; Swager, T. M.; Thomas, E. L.
(
(
(
(
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Macromolecules 2006, 39, 3350–3358.
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Thomas, E. L. AdV. Func. Mat. 2007, 17, 1595–1602.
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976, 12, 191–197.
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42, 1641–1654.
(18) Lu, J.; Zhang, J.; Shen, X.; Ho, D. M.; Pascal, R. A., Jr. J. Am. Chem.
Soc. 2002, 124, 8035–8041.
(19) Hilton, C. L.; King, B. T. Organometallics 2006, 25, 4058–4061.
(20) Clar, E.; Kelley, W.; Niven, W. G. J. Chem. Soc. 1956, 1833–1836.
(21) Romero, C.; Pe n˜ a, D.; P e´ rez, D.; Guiti a´ n, E. Chem.sEur. J. 2006, 12,
5677–5684.
(
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(
(
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(
(
9) Chong, J. H.; MacLachlan, M. J. J. Org. Chem. 2007, 72, 8683–8690.
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B. L. Chem. Commun. 1999, 1709–1710.
1
0.1021/jo801643u CCC: $40.75  2009 American Chemical Society
J. Org. Chem. 2009, 74, 405–407 405
Published on Web 11/24/2008

SCHEME 1. Synthesis of Extended Triptycenes 1-H and
-Bu
1
FIGURE 1. Thermal ellipsoid plots for 1-Bu from the corrugated plane
(
left) and the distorted hexagonal plane (right).
The synthesis of 1-H relies on reagent 3 to affect the 3-fold
biphenylation of 2-H. The reaction gives a double ipso substitu-
tion on an o-dihaloarene to form a triphenylene fragment
2
2
Examination of packing of 1-Bu reveals interlocking planes
(
Scheme 1).
organized by π-stacking, but no large voids. A distorted
hexagonal plane contains molecules of 1-Bu aligned with their
bridgehead-bridgehead axis nearly normal to the plane and is
stabilized by triphenylene-triphenylene π-stacking. A cor-
rugated plane contains molecules of 1-Bu with their bridgehead-
bridgehead axis aligned (∼30°) with the plane, and a blade from
each 1-Bu protruding up or down into the voids in the adjacent
hexagonal plane. The hexagonal voids flatten to maximize
π-stacking with the two interdigitated triphenylene blades,
expanding the triphenylene-triphenylene dihedral from the
expected 120° to 138°. One benzene molecule fills, along with
the butyl group, a void above the bridgeheads of the hexagonal
layer. The other benzene molecule fills a void in the corrugated
layer (Figure 2).
2
2
A modification to the reported workup for reactions using
simplifies purification. In the original procedure, protic quench
3
yielded neutral oligophenyls, which were difficult to separate
from the products. In our improved procedure, a CO2 quench
afforded oligophenylcarboxylates, which could be easily sepa-
rated by flash chromatography on alumina. The remaining
mixture contained 1-H, incompletely biphenylated products, and
reduced products and was separated using normal-phase HPLC
to give pure 1-H in 9% yield.
We were particularly interested in obtaining a crystal structure
of 1-H in order to determine its shape and solid-state packing.
Despite exhaustive efforts, crystals of 1-H suitable for single-
crystal X-ray diffraction could not be grown. Attempts to
cocrystallize 1-H with picric acid, trinitrofluorenone, C60, and
adamantane also failed. In every case, only films were obtained.
No sample exhibited evidence of crystallinity by microscopy
with crossed polarizers. In addition, 1-H tenaciously retained
solvent. Vacuum (30 mTorr) at elevated temperatures (75 °C)
failed to fully remove acetone or dichloromethane.
The reluctance of 1-H to crystallize was intriguingstri-
phenylene and triptycene crystallize easily from a variety of
solvents. We hypothesized that the free volume of 1-H,
especially around its bridgeheads, frustrates packing and inhibits
crystallization.
To test this hypothesis, we synthesized 6,13-(2,3-triph-
enyleno)-6,13-di-n-butyl[a,c,l,n]tetrabenzopentacene (1-Bu) us-
ing the same synthetic scheme that provided 1-H, except starting
with 9,10-dibutyltriptycene.
We have synthesized two new extended triptycenes, 1-H and
1
-Bu, that bear triphenylene blades. These triphenylene blades
extend the triptycene base both perpendicular and parallel to
its 3-fold axis. The synthesis of extended triptycenes 1-H and
1-Bu was made possible by the recently developed biphenylating
agent 3; this reagent formed six new C-C bonds in one step.
Compound 1-H proved reluctant to crystallize, possibly due to
voids about its bridgeheads that inhibit packing. Compound
1
-Bu, in which a butyl group partially occupies the bridgehead
voids, is easily crystallized. This supports our hypothesis that
the large voids in 1-H inhibit packing.
3
The large IMFV of 1-R (710 Å ) suggest applications in
molecular devices and polymers. Both triptycene 1-H and 1-Bu
provide a new rigid scaffold geometry which may enable their
use in molecular devices and as rigid frameworks for the
construction of chelating ligands. The large electron rich clefts
between their triphenylene blades make them candidates for use
in new inclusion complexes. Compound 1-H may be capable
of modulating the mechanical properties of polymers, perhaps
Dibutyltriptycene was synthesized in three steps. First, 9,10-
anthraquinone was allowed to react with n-butyllithium to yield
a mixture of diastereomeric diols, which was reduced with SnCl2
in AcOH to give 9,10-dibutylanthracene (7 g, 40% over two
steps). Diels-Alder cycloaddition of benzyne gave 9,10-
dibutyltriptycene (70%). Bromination gave 2-Bu (68%). Bi-
phenylation of 2-Bu with 3 gave 1-Bu, but in lower yield (20
mg, 5%).
2
3
1
2,13
to a larger extent than triptycene,
increased IMFV.
due to its significantly
Experimental Section
On our first attempt, we were able to grow crystals of 1-Bu
suitable for single-crystal X-ray diffraction analysis (Figure 1).
The bridgehead butyl groups promote crystallinity.
The asymmetric unit comprised two independent molecules
Synthesis of 2-H. Triptycene (1.06 g, 4.18 mmol) was dissolved
in chloroform (80 mL) in a round-bottom flask. Iron filings (30
mg) were added, and the solution was stirred at 25 °C. Bromine
(1.35 mL, 26.3 mmol) was added, and the solution was refluxed
for 1 h, during which time the initially reddish-brown solution
turned reddish-orange. The flask was removed from heat, and
chloroform and excess bromine were removed under vacuum. The
resulting brown powder was dissolved in chloroform (100 mL) and
flushed through a pad of silica using additional chloroform as eluent
24
of 1-Bu and two molecules of benzene. The IMFV of each,
3
calculated using a C-H bond length of 1.09 Å, was 680 Å
3
3
and 720 Å , bracketing the calculated (AM1) value of 710 Å .
(
22) Hilton, C. L.; Jamison, C. R.; King, B. T. J. Am. Chem. Soc. 2006,
28, 14824–14824.
23) Rimmer, R. W.; Christiansen, R. G.; Brown, R. K.; Sandin, R. J. Am.
Chem. Soc. 1950, 72, 2298.
(
100 mL). The filtrate was evaporated to dryness. The crude white
powder (2.83 g, 98%) was crystallized from acetone yielding
Br ·(acetone) (0.88 g, 29%), mp >350 °C. The mother liquor
1
(
C H
20 8
6
2
4
06 J. Org. Chem. Vol. 74, No. 1, 2009

residue was dissolved in THF (75 mL) and added dropwise to a
solution of tin dichloride (56.25 g, 250 mmol) in acetic acid (300
mL). The resulting suspension was stirred for 1 day, and the organic
material was extracted with hexane (1 L). The organic extract was
washed with 5% ammonium hydroxide (500 mL) and evaporated
to dryness. The residue was crystallized from 2-propanol to give
1
pure 9,10-dibutylanthracene (4.2 g, 40%): mp 105 °C; H NMR
(
500 MHz, CDCl
Hz, 4H), 1.82 (m, 4H), 1.62 (m, 4H), 1.05 (t, J ) 7 Hz, 6H);
NMR (101 MHz, CDCl ) δ 134.0, 129.6, 125.5, 125.0, 33.8, 28.1,
3.7, 14.3. Anal. Calcd for C22 26: C, 90.98, H, 9.02. Found: C,
1.06, H, 8.91.
3
) δ 8.32 (m, 4H), 7.51 (m, 4H), 3.61 (t, J ) 8
13
C
3
2
9
H
Synthesis of 9,10-Dibutyltriptycene. Anthranillic acid (4.00 g,
8.8 mmol) was dissolved in THF (180 mL) and added dropwise
2
over 4 h to a solution of isoamyl nitrite (4.2 mL, 31 mmol) and
,10-dibutylanthracene (4.00 g 13.8 mmol) in refluxing chloroform
250 mL). After the addition was complete, the reaction was
9
(
refluxed for a further 15 min, and the solvent was evaporated to
dryness. The remaining solids were dissolved in xylenes (200 mL),
and maleic anhydride (1.896 g, 19.32 mmol) was added. This
mixture was refluxed for 15 min and allowed to cool. Aqueous
workup and filtration through a plug of silica gel afforded
1
dibutyltriptycene (3.485 g, 69%): mp 247 °C; H NMR (500 MHz,
CDCl
3
) δ 7.46 (bs, 6H), 7.06 (bs, 6H), 3.03 - 2.94 (m, 4H), 2.27
1
3
-
2.15 (m, 4H), 1.94 - 1.73 (m, 4H), 1.22 (t, J ) 7 Hz, 6H); C
NMR (101 MHz, CDBr
3
) δ 147.33, 123.45, 121.14, 52.15, 27.47,
30: C, 91.75, H, 8.25.
2
6.71, 24.14, 14.01. Anal. Calcd for C28
H
Found: C, 91.34, H, 8.51.
Synthesis of 2-Bu. The procedure for 2-H was followed, but on
a smaller scale: 9,10-dibutyltriptycene (0.425 g, 1.16 mmol,
chloroform (30 mL), iron filings (20 mg), and bromine (0.38 mL,
7
.3 mmol). The crude white powder (0.90 g, 93%) was washed
with acetone yielding pure 2-Bu (0.628 g, 65%): mp 297-300 °C;
1
H NMR (500 MHz, CDCl
3
) δ 7.55 (bs, 6H), 2.80-2.66 (m, 4H),
2
.04-1.95 (m, 4H), 1.85-1.75 (m, 4H), 1.20 (t, J ) 7 Hz, 6H);
1
3
C NMR (126 MHz, CDCl
27.57, 27.16, 24.81, 14.25.
Synthesis of 1-Bu. Li(THF)
3
) δ 147.49, 128.02, 121.66, 52.33,
FIGURE 2. Packing of 1-Bu. Black, distorted hexagonal plane; purple,
corrugated lattice upward; blue, corrugated plane downward. Purple
and blue molecules are crystallographically equivalent. Benzene solvent
and hydrogen atoms are omitted.
4
)
2
·Zr(biphe) (3.41 g, 3.00 mmol)
3
was added as a powder to a solution of 2-Bu (0.419 g, 0.502 mmol)
in toluene (50 mL). The reaction mixture was stirred at room
temperature for 2 days and then quenched by addition of anhydrous
carbon dioxide gas. The mixture was passed through a column of
basic alumina using toluene. The eluent was collected and
evaporated, and the residue was purified using preparative HPLC
was evaporated and the residue was crystallized from acetone to
afford a second crop of crystals (0.97 g, C20
H
8
Br
6
2
·(acetone) , 32%).
1
The combined yield was 1.85 g, 61%: H NMR (500 MHz, CDCl
3
)
13
3
δ 7.63 (s, 3H), 5.24 (s, 1H); C NMR (101 MHz, CDBr ) δ 141.7,
(
°
8
silica/ 15:85 CH
C; H NMR (400 MHz, C D
2
Cl
2
/hexane) yielding 1-Bu (22 mg, 5%): mp >350
, 72 °C) δ 8.89 (s, 6H), 8.77 (d, J )
Hz, 6H), 8.46 (d, J ) 8 Hz, 6H), 7.53 (t, J ) 7 Hz, 6H), 7.43 (t,
1
2
3
27.1, 119.9, 48.6; UV-vis λmax/nm(log ꢀ): 229 (4.95), 285 (3.49),
1
6 6
95 (3.83). Anal. Calcd for C20
3.17, 1.19, 65.68.
H
8
Br
6
: 33.01, 1.11, 65.88. Found:
J ) 7 Hz, 6H) 3.53 - 3.45 (m, 4H), 2.67-2.52 (m, 4H), 2.20-2.10
Synthesis of 1-H. Li(THF)
4
)
2
·Zr(biphe)
3
(3.41 g, 3.00 mmol)
13
(
m, 4H), 1.31 (t, J ) 7 Hz, 6H); C NMR (126 MHz, CDBr
3
) δ
was added as a powder to a solution of 2,3,6,7,12,13-hexabromo-
triptycene (0.364 g 0.50 mmol) in toluene (50 mL). The reaction
mixture was stirred under argon, at room temperature, for 2 days
and then quenched by addition of anhydrous carbon dioxide gas.
The mixture was passed through a column of basic alumina using
toluene. The eluent was evaporated, and the resulting residue was
1
2
8
44.1, 127.9, 127.8, 125.2, 124.9, 124.8, 121.4, 121.0, 115.0, 51.5,
6.6, 26.3, 23.8, 13.3; HRMS calcd for C64
16.3734.
H48 816.3756, found
Acknowledgment. This work was supported by the National
Science Foundation (CHE-0449740; diffractometer, CHE-
0226402; NMR spectrometers, CHE-0521191; mass spectro-
meter, CHE-0443647) and the NIH (MALDI-MS, P20-RR-
016464).
purified using preparative HPLC (silica, 40:60 CHCl
3
/hexane)
yielding 1-H (29 mg, 9%): mp >350 °C; H NMR (500 MHz,
Cl ) δ 8.87 (s, 6H), 8.74 (d, J ) 5 Hz, 6H), 8.63 (d, J ) 5
Hz, 6H), 7.72 (m, 6 H), 7.65 (m, 6 H), 6.19 (s, 2H); C NMR
125 MHz, CDCl ) δ 146.7, 133.3, 133.2, 131.4, 130.3, 130.2,
26.6, 121.7, 58.0; UV-vis λmax/nm (log ꢀ) 263 (5.12) sh 273;
HRMS calcd for C56 32 704.2504, found 704.2451.
1
C
2
D
2
4
1
3
(
3
Supporting Information Available: Synthetic procedures,
NMR spectra, full thermal ellipsoid plots, crystallographic
parameters, and crystallographic information files for 9,10-
dibutyltriptycene, 2-H·acetone, 2-H, 2-Bu, and 1-Bu. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
H
Synthesis of 9,10-Dibutylanthracene. Anthraquinone (10.4 g,
0 mmol) was added to a stirred solution of n-butyllithium (273
5
mmol) in hexane (100 mL) and anisole (100 mL) at room
temperature. The reaction was stirred for 1 day and quenched by
addition of ammonium chloride. The organic layer was separated,
washed with water (2 × 200 mL), and evaporated. The resulting
JO801643U
J. Org. Chem. Vol. 74, No. 1, 2009 407