Nanochannel array within a multilayered network of a planarized dehydro[24]annulene

Article abstract of DOI:10.1021/ol1006967

Dehydro[24]annulene 1c adopts an unusual planarized conformation in the crystal. A multilayered stack of hydrogen-bonded grids delineates tightly packed nanotubular channels. The related macrocycles 1a and 1b, on the other hand, have the expected puckered

Full text of DOI:10.1021/ol1006967

ORGANIC
LETTERS
2
010
Vol. 12, No. 10
346-2349
Nanochannel Array within a Multilayered
Network of a Planarized
Dehydro[24]annulene
2
Mitsuharu Suzuki, Angelo Comito, Saeed I. Khan, and Yves Rubin*
Department of Chemistry and Biochemistry, UniVersity of California,
Los Angeles, California 90095-1569
rubin@chem.ucla.edu
Received March 24, 2010
ABSTRACT
Dehydro[24]annulene 1c adopts an unusual planarized conformation in the crystal. A multilayered stack of hydrogen-bonded grids delineates
tightly packed nanotubular channels. The related macrocycles 1a and 1b, on the other hand, have the expected puckered conformations in the
crystal.
4
Acetylenic macrocycles offer a significant advantage in a
synthetic approach to nanosized carbon allotropes and
carbon-rich materials. Each alkyne carbon stores a large
invoked in work on various acetylenic systems. So far, this
approach has been challenging, with the required solid-state
molecular organization necessary for topochemical polym-
erization being achieved mostly by chance. There is strong
potential in using noncovalent self-assembly to drive the
organization of macrocyclic diacetylenes into topochemically
polymerizable arrays. Remarkable single-crystal-to-single-
crystal topochemical polymerization of self-assembled di-
1
amount of energy and is prone to expand its coordination.
2
3
Vollhardt and Bunz have shown that the explosive solid-
state decomposition of dehydroannulenes can afford carbon
nanostructures. However, a downside of this high energy
content is that the transformations usually give a chaotic array
of nanostructures.
An attractive strategy for converting carbon-rich macro-
cycles into well-defined nanostructures lies in a stepwise
nonexplosive topochemical polymerization approach (Figure
(
4) (a) Baughman, R. H.; Yee, K. C. J. Polym. Sci., Part A: Polym.
Chem. 1974, 12, 2467–2475. (b) Day, D.; Lando, J. B. J. Polym. Sci., Part
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Lando, J. B.; Yee, K. C.; Baughman, R. H. J. Polym. Sci., Part B: Polym.
Phys. 1979, 17, 655–662. (e) Yee, K. C. J. Polym. Sci., Part A: Polym.
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5, 263–266.
1), followed by graphitization. Indeed, this strategy has been
(
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3
1
–847. (b) Bunz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc. ReV. 1999, 28,
07–119.
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A. J.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1997, 119, 2052–2053.
(
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U. H. F. J. Am. Chem. Soc. 2002, 124, 13814–13818.
10.1021/ol1006967  2010 American Chemical Society
Published on Web 04/19/2010

Figure 1. (top) Dehydroannulenes discussed in this work. (bottom)
Examples of potential topochemical polymerization pathways for
dehydro[24]annulene 1a: (a) stack of puckered rings undergoing
4
-fold polymerization of diacetylene units; (b) a planar conformation
undergoing 2-fold polymerization.
acetylenes has enabled unambiguous characterization of the
products, providing a clear understanding of the reaction
pathway.
5
Figure 2. Crystal structure of dehydro[24]annulene 1a: (a,b)
ORTEP plots drawn with 50% probability ellipsoids. (c) Packing
diagram with space-filling view.
With this in mind, we targeted the topochemical polym-
erization of macrocycles incorporating multiple butadiyne
6
units. Among a number of possible structures, nonbenzoan-
nulated dehydroannulenes seemed best suited for this goal.
Benzoannulation can have a stabilizing effect on a dehy-
droannulene, and the steric encumbrance by aromatic rings
9
yields, respectively. It is worth noting that macrocycle 1a
has not previously been isolated through this route. Single
crystals suitable for X-ray diffraction were obtained by slow
evaporation of a CDCl solution of 1a at -14 °C. After
several attempts at mounting crystals, which exploded easily
upon touching, an amber plate provided the structure shown
4
b,c,7
near reaction sites may impede polymerization.
Con-
3
versely, many simple dehydroannulenes are notoriously
unstable, restricting availability of data on their crystal
6
packing.
1
0,11
in Figure 2.
The 24-membered macrocyclic framework
has the expected tub-shaped conformation. With pucker
To explore noncovalent interactions in the solid state, the
crystal structures of parent dehydro[24]annulene 1a, as well
as its tetrasubstituted derivatives 1b and 1c, were obtained
1
2
angles of 62.5(2)° and 66.6(3)°, it is slightly flatter than
9
the DFT calculated structure (83.0°), which is presumably
(
Figures 1, 2, and 3). We were curious about the crystal
due to distortion by crystal packing forces. Additionally, the
structure of parent compound 1a (Figure 2). Although
CtC-C angles in 1a (Figure 2a) are generally smaller in
the crystal than calculated, whereas the CdC-C angles are
essentially identical between theory and experiment. Thus,
most of the angle strain caused by the planarization is
localized at the most flexible sp-hybridized carbons. A similar
8
Sondheimer et al. prepared this compound in 1970, the
9
crystal structure was never reported, presumably because of
its extreme heat and shock sensitivity (see below). We
synthesized compound 1a through oxidative coupling of cis-
-
2
3
-hexen-1,5-diyne under high-dilution conditions (∼10 M),
which provided tetramer 1a and trimer 2 in 5% and 20%
(
(
9) See Supporting Information for additional details.
10) X-ray data: 1a, yellow plate (CDCl ), space group Ccm2 ; a )
3
1
3
(
5) (a) Lauher, J. W.; Fowler, F. W.; Goroff, N. S. Acc. Chem. Res.
008, 41, 1215–1229. (b) Li, Z.; Fowler, F. W.; Lauher, J. W. J. Am. Chem.
Soc. 2009, 131, 634–643.
6) Spitler, E. L.; Johnson, C. A.; Haley, M. M. Chem. ReV. 2006, 106,
344–5386.
7) (a) Curtis, S. M.; Le, N.; Fowler, F. W.; Lauher, J. W. Cryst. Growth
8.276(8) Å; b ) 20.80(2) Å; c ) 9.536(9) Å; V ) 1641(3) Å ; Z ) 4; T )
156(2) K; Nref ) 1300; R ) 0.0434; wR ) 0.0883; 1b, yellow prism
(CH Cl /Et O) space group Ccca; a ) 12.4522(9) Å; b ) 12.4742(2) Å; c
) 19.6932(15) Å; V ) 3059.0(4) Å ; Z ) 4; T ) 100(2) K; Nref ) 2068;
2
1
2
2
2
2
3
(
1
j
j
) 0.0892; 1c, red prism (THF/MeOH) space group P1 ;
5
R
1
) 0.0352; wR
2
(
a ) 7.1356(9) Å; b ) 7.5011(9) Å; c ) 11.3348(14) Å; V ) 591.17(13)
3
Des. 2005, 5, 2313–2321. (b) Lee, J. H.; Curtis, M. D.; Kampf, J. W.
Macromolecules 2000, 33, 2136–2144. (c) Nakanishi, H.; Matsuda, H.; Kato,
M.; Theocharis, C. R.; Jones, W. J. Chem. Soc., Perkin Trans. 2 1986,
Å ; Z ) 1; T ) 100(2) K; Nref ) 3147; R
1
) 0.0442; wR ) 0.1134.
2
(11) For the crystal structure of dehydro[18]annulene 2, see Supporting
Information.
1
965–1967
8) McQuilkin, R. M.; Garratt, P. J.; Sondheimer, F. J. Am. Chem. Soc.
970, 92, 6682–6683.
.
(12) The pucker angle is defined between a set of two mean planes
passing through each edge of opposing tC-CdC-Ct units, e.g.,
C6′-C5′-C5-C6 and C7′-C8′-C8-C7, Figure 2a.
(
1
Org. Lett., Vol. 12, No. 10, 2010
2347

Scheme 1. Synthesis of Tetrasubstituted Derivatives 1b and 1c
degree of planarization has been observed for a dehydro[16]-
1
3
annulene derivative.
Compound 1a crystallizes without solvent inclusion and
9
has a number of short C···C contacts. As a result, the crystals
are highly unstable and decompose in minutes at 25 °C,
turning black upon standing or detonating violently upon
8
mechanical shock. This extreme instability needed to be
addressed for a controlled polymerization strategy. A simple
way to attenuate solid-state reactivity is through the introduc-
tion of substituents that effectively separate molecules from
each other. However, too much steric bulk would not be
conducive to achieving the molecular arrangement necessary
for efficient polymerization. Thus, the derivatives 1b and
1c possessing four relatively small substituents were prepared
(
Scheme 1).
The butadiyne linkages in 1b and 1c were formed in two
oxidative dimerization steps to avoid the formation of
4
f,8
unsymmetrical isomers as well as [18]annulenes.
Wittig
14
15
reaction of 3-(triisopropylsilyl)propynal and ylide 3 gave
bromoenyne ester 4, which afforded alcohol 5 by reduction
with DIBAL (1:9 E/Z mixture). The two diastereomers were
separated by flash chromatography on silica gel (hexanes/
AcOEt, 20:3). Sonogashira coupling of bromoenyne (Z)-5
with trimethylsilylacetylene (TMSA) afforded enediyne 6 in
97% yield. After removal of the TMS group from 6 followed
by Hay coupling, the resulting dimer 7 was converted to
bisacetate 8 (quant). Desilylation of 8 and subsequent
-3
oxidative coupling under high dilution conditions (∼10 M)
afforded macrocycle 1b in 41% yield as a bright yellow
crystalline solid. The tetraol 1c was obtained by saponifica-
tion of 1b in 97% yield.
The X-ray structure for 1b is shown in Figure 3a-d.
Whereas corresponding carbon-carbon bonds within mac-
Figure 3. Representations of the crystal structures of 1b and 1c:
a,b) ORTEP plots of 1b with 50% probability ellipsoids. Packing
diagram for 1b showing (c) rings interlocked by acetate substituents,
(
(
13) Nishinaga, T.; Kawamura, T.; Komatsu, K. J. Org. Chem. 1997,
2, 5354–5362.
14) Tovar, J. D.; Jux, N.; Jarrosson, T.; Khan, S. I.; Rubin, Y. J. Org.
Chem. 1997, 62, 3432–3433.
15) Kayser, M. M.; Zhu, J.; Hooper, D. L. Can. J. Chem. 1997, 75,
315–1321.
and (d) superposition of two layers. (e,f) ORTEP plots of 1c with
6
5
0% probability ellipsoids. (g) Packing diagram of 1c; inset: detail
(
of hydrogen-bonding motif. (h) Packing diagram of 1c viewed down
(
the nanochannels. Disordered solvent shown in purple.
1
2348
Org. Lett., Vol. 12, No. 10, 2010

rocycles 1a and 1b have very similar lengths (Figures 2a
and 3a), the pucker angle is larger for 1b (74.5(0)°) than for
9
1
a. Compound 1b has a smaller inner angle at the vinyl
carbons (C2-C3-C4; 122.9(1)°) compared to the corre-
sponding angles at C1, C6, C7, and C12 for 1a (124.7(2)-
1
25.9(3)°), which is the cause of the larger pucker angle.
The crystal packing diagram of 1b (Figure 3c) shows that
the macrocycles are intertwined through their acetate sub-
stituent “arms”, forming a linear chain of repeating units.
The chains stack in a staggered fashion (Figure 3d), and each
diacetylene unit is well separated from others even though
the molecules are tightly packed without any incorporated
solvent. This explains the much higher solid-state stability
of 1b compared to that of 1a: crystals of 1b have been stored
over 2 months without any decomposition, in striking contrast
to 1a. In addition, crystals of 1b did not explode either by
rubbing or heating up to 150 °C in air, although they
gradually darkened between 120-130 °C. Thus, the intro-
duction of relatively small substituents effectively improves
the solid-state stability of otherwise highly unstable dehy-
droannulenes.
-5
Figure 4
1
.
UV-vis spectra of 1b (- - -, 1.7 × 10 M, MeCN) and
-5
c (s, 1.9 × 10 M, MeOH). Inset: solid-state UV-vis spectra.
The energetic effect of the planarization was calculated
by density functional theory (BHandHLYP/6-31G*). The
nonplanar and planar conformers of 1c were optimized with
9
9
C
2
and C
i
symmetry constraints, respectively. The energy
difference between the two conformers is only 1.92 kcal/
mol after zero-point energy correction. This value is much
smaller than the calculated energy barriers for the ring flip
process of cyclooctatetraenes (g10 kcal/mol) and is so
small that ring planarization can be easily compensated by
van der Waals interactions and the formation of multiple
We expected that tetraol 1c would adopt a very different
molecular packing mode from that of 1a or 1b, owing to its
ability to form hydrogen bonds. Interestingly, 1c forms deep
red crystals, whereas it is bright yellow in solution. This is
in contrast to compounds 1a and 1b, which are yellow both
in solution and in the crystalline state. A similar deep red
color is characteristic of cyclobutene-fused dehydro[24]-
annulenes that are forced to adopt a planar conformation
19
20
hydrogen bonds per molecule. Experimentally, crystalline
samples of 1c are as thermally stable as tetraacetate 1b: no
significant change in appearance is observed up to 110 °C
in air, at which point crystals start to slowly turn brown.
In summary, the use of noncovalent interactions has
provided a step toward achieving the desired ordering of
dehydroannulenes in the solid state and their controlled
topochemical polymerization. Simple, small substituents
effectively stabilize the dehydro[24]annulene system and
significantly affect macrocycle conformation as shown
between tetraacetate 1b and tetraol 1c. The planarization of
an otherwise tub-shaped π-conjugated macrocycle results in
an intriguing exaltation of its paratropic nature. We are
exploring a variety of derivatives of tetraol 1c to obtain an
optimal molecular arrangement favoring the targeted mul-
tifold topochemical polymerization.
16
through small-ring annulation. Additionally, the solid-state
UV-vis spectrum of 1c shows an absorption edge around
5
60 nm (Figure 4, inset) that is in good agreement with the
reported data for planar cyclobutene-fused dehydro[24]-
1
6a
annulenes. This was a strong indication that macrocycle
1
c has a planar conformation in the crystalline state.
The planarity of 1c was confirmed by its single crystal
X-ray structure (Figure 3e-h). The average deviation from
the mean plane defined by the 24 carbons on the macrocycle
perimeter is only 0.034(1) Å, with the maximum deviation
being 0.068(1) Å. This near-perfect planarity is unusual
1
6,17
considering that no annulation
constraints are present in the molecule. In addition, tetraol
c forms a network of hydrogen bonds in the crystal: a square
or intra-annular bridging
18
1
motif of O···H contacts leads to a corrugated sheet built from
the planar macrocycles (Figure 3g). The sheets stack into
an array of nanochannels walled by the highly polarizable
π-electrons of the butadiyne units (Figure 3h). Each nanochan-
nel is filled with disordered solvent molecules.
Acknowledgment. We are grateful to the National Science
Foundation for support of this work through individual (NSF-
CHE-0617052) and instrumentation grants (NSF-CHE-
9
974928; NSF-CHE-9871332).
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for compounds 1a-c, cal-
culation details, and crystallographic information in CIF
format. This material is available free of charge via the
Internet at http://pubs.acs.org.
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