From metacyclophanes to cyclacenes: Synthesis and properties of [6.8] 3cyclacene

Article abstract of DOI:10.1002/chem.200802297

In this article we show synthetic pathways to [6.8]_ncyclacenes demonstrated by the de novo synthesis of [6.8]₃cyclacene as the first purely hydrocarbon cyclacene and of precursors for [6.8]₄cyclacene. The design of the de

Full text of DOI:10.1002/chem.200802297

DOI: 10.1002/chem.200802297
From Metacyclophanes to Cyclacenes: Synthesis and Properties of
[6.8]3Cyclacene
Birgit Esser, Arkasish Bandyopadhyay, Frank Rominger, and Rolf Gleiter*[a]
Dedicated to Professor Armin de Meijere on the occasion of his 70th birthday
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Chem. Eur. J. 2009, 15, 3368 – 3379

FULL PAPER
Abstract: In this article we show syn-
thetic pathways to [6.8]ncyclacenes
demonstrated by the de novo synthesis
of [6.8]3cyclacene as the first purely hy-
drocarbon cyclacene and of precursors
for [6.8]4cyclacene. The design of the
de novo synthesis by exploring alterna-
tive pathways is discussed and various
precursors are shown. Crucial to the
synthesis of [6.8]3cyclacene were two
cyclization steps. The first is a Wittig
trimerization reaction which yielded
the hexamethyl substituted all-cis-
second cyclization step the methyl
groups were converted to aldehyde
functionalities by two subsequent oxi-
dation steps of N-bromosuccinimide
(NBS) bromination and oxidation with
2-iodoxybenzoic acid (IBX). The final
cyclization of the second set of double
bonds was achieved by a McMurry-
coupling reaction. Towards the synthe-
sis of [6.8]4cyclacene different synthetic
pathways to methyl substituted all-cis-
[24]metacyclophanetetraenes were ex-
plored. Insights into the structures of
[23]metacyclophanetri- and [24]meta-
A
ACHTUNGTRNEpNUNG hanetetraenes were gained by X-
A
a
with planar benzene rings and a forma-
tion of tubular structures in the solid
state.
Keywords: cyclacenes
cycles nanotubes
Wittig reactions
·
macro-
torus ·
·
·
[23]metacyclophanetriene.
For
the
Introduction
Belt-like macrocycles incorporating groups with donor func-
tionalities (e.g. cyclodextrins, cavitands, curcurbiturils) are
well known as hosts in supramolecular chemistry.[1,2] De-
pending on the donor groups and the size of the belt they
are capable of recognizing guests, which vary in size from
small metal ions to complex organic molecules. A number
of belt-like systems consisting of cylindrical p-conjugated
entities are not only of interest because of their specific
host/guest properties, but also because of their physical
properties, such as absorption and emission spectra. These
systems have in common a scaffold that forms a torus in
which the p-orbitals of the p-system are oriented parallel to
the molecular plane of the macrocycle and perpendicular to
the torus axis.[3] The prototype of such belt-like conjugated
molecules are [6]ncyclacenes. They are built from linear an-
nelation of n six-membered, conjugated rings. In Figure 1
[6]10cyclacene (1) is shown. Such systems have been dis-
cussed since 1954[4] and can as well as the related cyclo[10]-
phenacene (2) be regarded as subunits of single walled
carbon nanotubes[5] as shown in Figure 1.
Figure 1. [6]nCyclacenes (1 with n=10) and cyclo[n]phenacenes (2 with
n=10) as subunits of carbon nanotubes.
Efforts to synthesize [6]ncyclacenes started in the 80s of
the last century.[6,7,8] One clear problem was the bending of
the planar p-system into a hoop. This task was approached
by first preparing a partially saturated system utilizing a se-
quence of Diels–Alder reactions of 7-oxanorbornene deriva-
tives. This allowed systems with 16 to 18 six-membered rings
partially bridged by oxygen atoms to be achieved in good
yields. Attempts to remove the ether functionalities,
AHCTUNGTREGhNNNU owever, and to transform the partially hydrogenated sys-
tems into fully conjugated ones have failed so far.
One of the reasons for this failure was unraveled by quan-
tum chemical calculations.[9,10] It was predicted that
[6]ncyclacenes with an even number of n should exhibit
small singlet–triplet gaps.[9] These calculations also revealed
that angularly annelated cyclacenes, such as 2, should have
large singlet–triplet splittings.[10]
The detour by way of bridged saturated systems to
achieve the bending, and the intrinsic problem of small sin-
glet–triplet splittings in evenly numbered [6]ncyclacenes was
circumvented in two ways: Either a cage molecule contain-
ing an angularly annelated cyclacene as substructure was
used as starting point, or such ring structures were incorpo-
rated in the belt, which naturally adopt boat-like conforma-
tions. The first approach was successfully carried out by
[a] Dr. B. Esser, Dr. A. Bandyopadhyay, Dr. F. Rominger,
Prof. Dr. R. Gleiter
Organisch-Chemisches Institut
Universitꢁt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax : (+49)6221-544-205
E-mail: rolf.gleiter@oci.uni-heidelberg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802297.
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R. Gleiter et al.
In this article we report the full details of our recent com-
munication on the synthesis of [6.8]3cyclacene (13) as the
first purely hydrocarbon cyclacene.[14] We are discussing our
synthetic approach via metacyclophane derivatives and ex-
perimental pathways to [6.8]4cyclacene.
Results and Discussion
Scheme 1. Synthesis of C60-embedded cyclo[10]phenacene 5 by Nakamura
et al.
Synthesis of [6.8]3cyclacene: To find a reasonable synthetic
path to [6.8]ncyclacenes (7) we had to choose between a
stepwise procedure (Path A) and a multicomponent one-pot
reaction (Path B), as shown in Scheme 4.
ACHTUNGTRENNUNGNakamura et al. by utilizing methods to reduce the north
and south pole of C60 (Scheme 1).[11]
Our approach uses eight-membered rings as building
blocks. The boat conformation of cyclooctatetraene provides
excellent conditions to incorporate it in a conjugated
medium sized p-system. In Scheme 2 we have shown the
Scheme 4. Retrosynthetic approaches to [6.8]ncyclacenes (7).
We
carried
out
model
studies
on
dibenzo-
AHCTUNGTREG[NNUN a,e]cyclooctatetraene (17) as the synthetic target, as it con-
Scheme 2. Construction of [4.8]n (6) and [6.8]ncyclacenes (7) by annela-
tion of four- and eight-membered rings and six- and eight-membered
rings, respectively.
tains the representative structural unit of [6.8]ncyclacenes.
For a stepwise synthetic approach we found that the
McMurry coupling of dialdehyde 18 yielded 17 (Scheme 5).
cases for linear cyclacenes in which four- and eight-mem-
bered as well as six- and eight-membered rings alternate.[3e]
Recent quantum chemical calculations reveal that 6 (n=3,4)
and 7 (n=3–8) should be thermodynamically stable spe-
cies.[12] The necessary bending of the smaller species is
mainly achieved by a stronger bending of the flexible cyclo-
octatetraene rings.
To achieve
hydrodibenzo[a,e]cyclooctatetraene (8) with [CpCo(CO)2]
(9) or [CpRh(C2H4)2] (10) in one-pot reactions. These yield-
a [4.8]ncyclacene we irradiated tetrade-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ed the benzo-derivatives of metal stabilized [4.8]3cyclacenes
11[13a] and 12[13b] (Scheme 3).
Scheme 5. Synthetic pathways to dibenzoACTHUNRGTNE[NUG a,e]cyclooctatetraene (17) as
ꢀ
model studies; a) TiCl3·DME1.5, Zn Cu, DME, 908C, 21%; b) LiOEt/
EtOH, MeOH, 28%; c) LiOEt/EtOH, DMF, 18%.
An olefin metathesis reaction of the corresponding (Z)-1,2-
bis(2-vinylphenyl)ethene, on the other hand, turned out to
be unsuitable. Corresponding to the one-pot approach for
[6.8]ncyclacenes (7) the best syntheses of our model com-
pound 17 were those reported in the literature by Sargent[15a]
and Peters,[15b] both of which applied a double Wittig reac-
tion. In the first case (2-formylbenzyl)triphenylphosphonium
chloride (19) is dimerized and in the second case phthalalde-
hyde (20) is reacted with (1,2-phenylenebis(methylene))-
bis(triphenylphosphonium) bromide (21).
Scheme 3. Synthesis of [4.8]3cyclacenes 11 and 12 by Gleiter et al.
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Chem. Eur. J. 2009, 15, 3368 – 3379

Synthesis and Properties of [6.8]3Cyclacene
FULL PAPER
We first explored synthetic routes to precursors of
[6.8]ncyclacenes (7) suitable for multicomponent one-pot re-
actions (Path B in Scheme 4). In view of the results of our
model studies we decided to focus on precursors for Wittig
cyclophanetriene scaffold by means of a Wittig reaction, the
formyl substituents needed to be introduced in a later stage.
For X we chose bromo or methyl substituents (29 and 30, re-
spectively), which can be transformed to aldehyde function-
alities. Disconnection of the second set of double bonds re-
sults in the bromo- and methyl substituted isophthalalde-
hydes 31 and 32, respectively.
ꢀ
reactions and chose CHO and CH2Br as substituents Y1 Y4
(24 and 26, Scheme 6). Our attempted synthesis of 24 start-
In the synthesis of the [23]metacyclophanetriene deriva-
tives 29 and 30 we chose the route via a Wittig trimerization
according to Wennerstrçmꢂs synthesis of the unsubstituted
analogue.[18a] Both synthetic routes started from the substi-
tuted isophthalaldehydes 31[19] and 32 (Scheme 8).[20] By se-
Scheme 6. Syntheses of 1,2,4,5-tetrasubstituted benzene derivatives 23
and 26; a) NBS, dibenzoyl peroxide, CCl4, 808C, 22%; b) DIBAL-H,
THF, RT, 61%; c) PCC, CH2Cl2, RT; d) oxalyl dichloride, DMSO, Et3N,
CH2Cl2; e) DIBAL-H, toluene, ꢀ90!ꢀ408C, 44%; f) PPh3, toluene,
reflux.
Scheme 8. Synthesis of phosphonium bromides 35 and 36; a) NaBH4,
EtOH, RT; b) HBr, HOAc, reflux, 26% (X=Br), 65% (X=CH3);
c) PPh3, toluene, reflux, 91% (X=Br), 99% (X=CH3).
ed from dimethyl 4,5-dimethylphthalate (22),[16] which was
transformed to 23 by N-bromosuccinimide (NBS) bromina-
tion followed by reduction with DIBAL-H to the diol. Oxi-
dation of the hydroxyl groups to 24 failed, as well as the
direct reduction of 22 to 24 with DIBAL-H, because of
lective reduction of one aldehydic group with less than the
equivalent amount of NaBH4 the corresponding mono-
AHCTUNGTREGaNNUN lcohols were generated. Subsequent substitution of the hy-
droxyl groups by bromide yielded the benzyl bromides 33
and 34. Treatment with triphenylphosphine afforded the de-
sired phosphonium bromides 35 and 36.
the formation of
a
lactone. 4,6-Bis(bromoACTHNGUTERNNUmG ethyl)-
isophthalaldehyde (26) was synthesized from diethyl 4,6-
bis(bromomethyl)isophthalate (25),[17] but transformation to
the phosphonium salt 27 failed for reasons of product isola-
tion. As our attempts to synthesize the desired precursors
for multicomponent one-pot reactions were not successful,
we focused on a stepwise approach for the synthesis of
[6.8]ncyclacenes.
For a stepwise procedure (Path A in Scheme 4) we decid-
ed on the McMurry coupling reaction for the closure of the
second set of double bonds (14!7 in Scheme 4), given the
results of our model studies. For the first set of double
bonds (15!14) we found that intermediate 14 corresponds
to an o,o’-substituted metacyclophene. We were inspired by
the work of Wennerstrçm et al.,[18a] Oda et al.,[18b] and Iyoda
et al.[18c] from their studies on meta- and ortho-cyclophane-
trienes and higher oligomers. We chose [6.8]3cyclacene (13)
as our target. Our retrosynthetic concept is shown in
Scheme 7. A first disconnection of three double bonds leads
to the [23]metacyclophanetriene derivative 28 carrying six
formyl substituents. As we planned to synthesize the meta-
Now the stage was set for the intermolecular Wittig reac-
tions to assemble three benzene rings in a cyclic fashion. We
first concentrated on the bromo substituted system. Treat-
ment of bromo derivative 35 in DMF with LiOEt in ethanol
at low temperature yielded the trimeric 37 with all-cis con-
figured double bonds, as the main product in addition to
other higher cyclic oligomers (Scheme 9). The structure of
37 was elucidated by X-ray crystallographic investigations
and will be discussed in a later section. The bromine atoms
in 37 had to be replaced by aldehyde groups for which we
intended a halogen-metal exchange reaction. The Br/Li ex-
change with 37 was investigated under a variety of reaction
conditions. Quenching with DMF followed by acidic hydrol-
ysis did not lead to the desired hexaaldehyde 28. Instead, a
mixture of 38–40 was isolated in which three to four
Scheme 9. Synthesis of bromo substituted metacyclophanetriene 37 and
Br/Li exchange; a) LiOEt/EtOH, DMF, ꢀ408C, 15%; b) i) tBuLi, THF,
ꢀ788C; ii) DMF, ꢀ788C!RT; iii) H3O+.
Scheme 7. Retrosynthetic approach to [6.8]3cyclacene (13).
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R. Gleiter et al.
ACHTUNGTRENNUNGbromine atoms had been re-
placed by CHO. The structure
of 39/40 (in a mixed crystal)
was confirmed by X-ray crys-
tallographic
analysis.
We
assume that steric constraints
are the reason that no sixfold
Br/Li exchange was observed
in 37. As the bromine route
had reached a dead end we set
our attention on the methyl
substituted system.
The Wittig cyclization reac-
tion of the methyl substituted
phosphonium bromide 36 was
also successful. Reaction with
LiOEt/ethanol in DMF yielded
the trimeric systems 41a,b, the
cyclic tetramers 42a–c as well
as larger cyclic oligomers
(Scheme 10). Interestingly, the
Scheme 11. Synthesis of formyl substituted metacyclophanetriene 28; a) NBS, dibenzoyl peroxide, CCl4, reflux,
40%; b) IBX, DMSO, 658C, 37%; c) KOAc, DMF, 708C, 69%; d) CaCO3, dioxane/H2O, reflux, 32%;
e) KOH, H2O, 1008C; f) IBX, DMSO, RT, 45%.
soluble in common solvents, but readily dissolves in DMSO.
Thus, standard oxidation methods failed, such as those that
use pyridinium chlorochromate (PCC) or the Swern oxida-
tion, as they are usually performed in CH2Cl2. An attempt
to improve the yield of alcohol 44 by conversion to the ace-
tate 45 failed in the subsequent hydrolysis step. However,
we were able to abridge the route by direct oxidation of the
benzylic bromide 43 into hexaaldehyde 28 by oxidation with
IBX at elevated temperatures.[23]
Now, as the last step of the synthesis, the second set of
double bonds needed to be linked. For this step we had
chosen a McMurry coupling reaction. We applied the im-
ꢀ
proved procedure with the TiCl3/DME complex and Zn Cu
as reductive agent on aldehyde 28 (Scheme 12).[24] This
Scheme 10. Synthesis of methyl substituted metacyclophanetrienes 41a,b
and metacyclophanetetraenes 42a–c; a) LiOEt/EtOH, DMF, ꢀ108C, 8%;
b) hn, benzene, 58C, 85%.
predominantly formed trimeric isomer 41a had cis,cis,trans-
configured double bonds. Its molecular structure was investi-
gated by an X-ray crystallographic analysis (Figure 5).[14,21]
By irradiation in degassed benzene double-bond isomeriza-
tion to the desired all-cis isomer 41b was possible.[14,21]
The next challenge was the crucial introduction of the al-
dehyde functionalities. In the conversion of the methyl
groups in 41b to aldehyde groups we were successful in two
oxidation steps. The first was a sixfold benzylic bromination
with NBS, yielding the hexa(bromomethyl) substituted
metacyclophanetriene 43 (Scheme 11). This was then hydro-
lyzed to the alcohol 44 on which the second oxidation step
to the aldehyde 28 was performed by using 2-iodoxybenzoic
acid (IBX) as oxidizing agent.[22] Compound 44 is poorly
ꢀ
Scheme 12. Synthesis of [6.8]3cyclacene (13); a) TiCl3·DME1.5, Zn Cu,
DME, reflux, 8%.
coupling reaction was successful and yielded [6.8]3cyclacene
(13) as the only isolable product. The low yield of the reac-
tion might partly be explained by the preferred conforma-
tion of hexaaldehyde 28. X-ray crystallographic analysis and
theoretical calculations revealed that 28 adopts a twisted
conformation, as indicated in Scheme 12.[14,21] The ring clo-
sure to 13, on the other hand, requires a conformational
change to a disfavored bowl-shaped conformation.
The anticipated D3h symmetry of 13 was confirmed by X-
ray structural investigations (Figure 2).[14,25] The benzene
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Chem. Eur. J. 2009, 15, 3368 – 3379

Synthesis and Properties of [6.8]3Cyclacene
FULL PAPER
Structures of [23]metacyclophanetrienes: As mentioned
above, X-ray crystallographic investigations were carried out
on a number of the synthesized [23]metacyclophanetriene
derivatives.[14,21] As representative examples the molecular
structures of 28,[14,21] 37,[30] and 41a[14,21] are shown in
Figure 4 and Figure 5. The all-cis-metacyclophanetrienes
Figure 2. Structure and packing of [6.8]3cyclacene (13) in the solid state.
Hydrogen atoms are omitted for clarity.
units retain their planarity. The bending of the cycloocta-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
This bending of the flexible cyclooctatetraene units leaves a
strain energy of only 6.8 kcalmolꢀ1 per subunit[12] for
[6.8]3cyclacene (13). The mean torsion between the planes
of the double bonds and that of the adjacent aromatic rings
amounts to 728. This indicates about 31% conjugation for
the cyclacene torus.
Figure 4. Molecular structures of 28 (left) and 37 (right) in the crystal
(aromatic hydrogen atoms are omitted for clarity).
The crystal packing of [6.8]3cyclacene (13) in the solid
state is shown in Figure 2. The molecules are stacked in
piles on top of each other building molecular tubes. We
found that CH–p interactions contribute to the molecular
arrangement in the solid state. As shown in Figure 3 each
Figure 5. Molecular structure of 41a in the crystal (hydrogen atoms are
omitted for clarity except for the olefinic ones).
show a twisted conformation in the solid state. The NMR
spectra, on the other hand, point towards C3v symmetrical
structures in solution and show no coalescence down to
ꢀ1058C. To elucidate this behavior we performed quantum
chemical
calculations
on
the
unsubstituted
[23]metacyclophanetriene.[31] These calculations revealed
that the twisted conformer is the thermodynamically most
stable one, the degenerated rearrangement between the dif-
ferent twisted isomers proceeds via a barrier of only
1.3 kcalmolꢀ1 height (B3LYP/6-31G*).
Figure 3. CH–p interactions in the solid state of [6.8]3cyclacene (13).
molecule has CH–p interactions with neighboring molecules.
The distances all amount to 2.88 ꢃ and are slightly smaller
than the sum of the van der Waals radii of C and H
(2.9 ꢃ).[28] The angles CH···C are close to linear (159.88),
which is typical for this type of interaction.[29]
We also measured the UV absorption and emission. In the
absorption spectrum 13 shows a strong band at l=220 nm
with two shoulders at l=278 and 290 nm (log e=4.73, 3.85,
and 3.27, respectively). Emission takes place at l=370 nm,
corresponding to a Stokeꢂs shift of 80 nm.
In the molecular structure of 37 in the solid state we
found intermolecular contacts between bromine centers.
ꢀ
Each molecule shows eight intermolecular Br Br contacts
with distances between 3.52 and 3.67 ꢃ. These are shorter
than the sum of two van der Waals radii (3.7 ꢃ).[28] Intermo-
lecular interactions between neutral halogen centers have
been addressed in the literature.[32] Interactions between
closed-shell systems have at first been interpreted on the
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3373

R. Gleiter et al.
basis of donor–acceptor interactions,[32a] but correlation ef-
fects were later shown to contribute considerably.[33]
E,Z,E,Z-isomer is the most stable, whereas the desired
Z,Z,Z,Z-isomer owns the highest relative energy, 11.5 kcal
molꢀ1 above the former. This explains why this stereochem-
istry is disfavored in the Wittig tetramerization of 36.
To obtain the desired double bond configuration we de-
cided to make use of the cis-selective reduction of triple
Structures of [24]metacyclophanetetraenes and synthesis of
[24]metacyclophanedienediyne: Our synthetic strategy of a
stepwise procedure for the synthesis of [6.8]ncyclacenes was
successful for the trimeric system and should be applicable
in the syntheses of larger cyclacenes. In the case of a
bonds. As
a precursor for an octaformyl substituted
[24]metacyclophanetetraene (46) we chose the dienediyne
47. Our retrosynthetic approach is shown in Scheme 13. For
the cyclization reaction we focused in this case on the
McMurry coupling of dialdehyde 48.
[6.8]4cyclacene,
an
octamethyl
substituted
all-cis-
[24]metacyclophanetetraene would be the precursor. In our
explorations on the synthesis of such a system we investigat-
ed the cis-trans isomerism of [24]metacyclophanetetraenes.
Of those, the methyl substituted derivatives, which were iso-
lated as minor products in the Wittig reaction of phosphoni-
um bromide 36 (Scheme 10), none showed the desired all-
cis stereochemistry. We isolated the E,Z,Z,Z- (42a),
E,Z,E,Z- (42b), and E,E,E,Z-isomers (42c) and elucidated
their structures by NMR-spectroscopy and X-ray crystallog-
raphy. The molecular structures are shown in Figures 6 and
7. Irradiation of each of these isomers at l=254 nm did not
lead to the all-cis isomer, but to a mixture of 42a–c. In each
of these mixtures the ratio of 42a to 42b to 42c was roughly
the same. To illuminate the question why we could not find
the Z,Z,Z,Z-isomer we carried out quantum chemical calcu-
lations on the six cis-trans isomers of the unsubstituted
[24]metacyclophanetetraene.[31] We found out that the
Scheme 13. Retrosynthetic approach to the octaformyl substituted
[24]metacyclophanetetraene 46.
Our synthesis of dialdehyde 48 started from 1,5-diiodo-
2,4-dimethylbenzene (49)[37] (Scheme 14). One of the iodine
atoms was exchanged by an aldehyde group applying a
Figure 6. Molecular structures of 42a (left)[34] and 42b (right)[35] in the
crystal (hydrogen atoms are omitted for clarity except for the olefinic
ones).
Scheme 14. Synthesis of the methyl substituted [24]metacyclo-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
reflux!RT, 11 %.
method by Iida and Mase[38] which led to 50. The alkyne
moiety was introduced by a Sonogashira coupling reaction,
exchanging the remaining iodine atom by TMSA. Subse-
quent deprotection yielded 5-ethynyl-2,4-dimethylbenzalde-
hyde (51). Coupling 51 with 50 under Sonogashira condi-
tions led to dialdehyde 48 as the starting material of the
cyclic dimerization. In the last and crucial step of the syn-
thesis, the intermolecular McMurry homocoupling of 48, we
Figure 7. Molecular structure of 42c[36] in the crystal (hydrogen atoms are
omitted for clarity except for the olefinic ones).
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Synthesis and Properties of [6.8]3Cyclacene
FULL PAPER
tested different reaction conditions. Most successful turned
out to be a mixture of TiCl4, Zn, and CuI in DME,[39] which
led to the [24]metacyclophanedienediyne 47.
The molecular structure of 47 was examined by X-ray
crystallography and is shown in Figure 8.[40] It is very similar
to that of the methyl substituted E,Z,E,Z-metacyclophanete-
traene 42b. In 47 the place of the E-double bonds is taken
by the triple bonds. Both double bonds are cis-configured,
as anticipated.
bending of the cyclooctatetraene moieties that is stronger
than in dibenzoACTHUNTGRNEUNG[a,e]cyclooctatetraene. The benzene rings are
found to be planar. This indicates that higher, less strained
[6.8]ncyclacenes should also be accessible. The strong bend-
ing reduces the conjugation within the torus to 31% com-
pared to a fully conjugated system. Our studies on the syn-
theses and configurations of [24]metacyclophanetetraenes
are important steps on the way to [6.8]4cyclacene.
[6.8]nCyclacenes furthermore present smallest subunits of a
novel type of carbon nanotube with a pattern that comprises
four-, six-, and eight-membered rings. An elongation of
[6.8]5cyclacene along its molecular axis leads to a carbon
nanotube as shown in Figure 9.
Figure 8. Molecular structure of 47 in the crystal (hydrogen atoms are
omitted for clarity except for the olefinic ones).
For a further synthetic path towards cyclacene precursor
46 we think that the functionalization of the methyl groups
of 47 would be the next task. As the cis-trans isomerization
of the double bonds in [24]metacyclophanetetraene deriva-
tives takes place easily, the hydrogenation of the triple
bonds should be carried out at the latest possible stage of
the synthesis.
We carried out UV/vis spectroscopical investigations on
the tetrameric systems 42a–c and 47. The absorptions were
found at l=288 nm (42a), 294 nm (42b), and 300 nm (42c)
(log e=4.51, 4.66, 4.74, respectively). Analysis of 47 shows
absorptions at l=218, 294, 304, and 314 nm (log e=3.26,
3.49, 3.37, 3.33, respectively). All of them also show fluores-
cence emission. For 47 this emission was found at l=
384 nm with a Stokeꢂs shift of 70 nm. 42a–c all emit at l=
412 nm corresponding to large Stokeꢂs shifts of 124, 118, and
112 nm, respectively. This behavior correlates well with the
data found from the irradiation experiments, which indicat-
ed that after photon absorption 42a, 42b, and 42c pass
through the same excited state before returning to a ground
state.
Figure 9.
ring pattern.
ACHTUNGRTEN[NGNU 6.8]nCyclacenes as subunits of carbon nanotubes with a new
Experimental Section
General: Air- and moisture-sensitive reactions were carried out under
argon in flame-dried glassware. Solvents were either distilled or dried
and distilled (DMF: CaH2; CCl4: molecular sieves; DME, THF, toluene:
Na, benzophenone) before use or purchased predried (DMSO: Fluka).
All reagents obtained from commercial sources were used without fur-
ther purification. Thin layer chromatography (TLC) was performed on
precoated silica-gel SIL G/UV254 plates (Macherey-Nagel), detection was
carried out by means of UV light (254 nm). Silica gel 60 (230–400 mesh,
Macherey-Nagel) was used for column chromatography. 1H and
13C NMR spectra were recorded at RT by using a Bruker Avance ARX
300, 500 or 600 spectrometer using the solvent as internal standard. Melt-
ing points are uncorrected and were measured in an open capillary with
an apparatus (Dr. Tottoli) or a B-540 (Bꢄchi). IR spectra were recorded
as KBr pellets by using a Bruker Vector 22 FT-IR instrument. UV/vis
spectra were recorded by using a Hewlett–Packard HP 8452 A spectrom-
eter. Mass spectra were measured by using a JEOL JMS-700 instrument.
Elemental analyses were performed by using a VarioEL (Elementar).
5-(Bromomethyl)-2,4-dimethylbenzaldehyde
(34):
NaBH4
(1.55 g,
41.0 mmol) was added within 2 h in two equal portions to a solution of
4,6-dimethylisophthalaldehyde 32 (33.2 g, 205 mmol) in ethanol
(700 mL). The solution was stirred at RT for 1.5 h. Another portion of
NaBH4 (0.54 g, 14.3 mmol) was added and the reaction mixture was
stirred at RT overnight. The solvent was evaporated in vacuo and boiling
water (300 mL) was added to the remaining yellow oil. The mixture was
extracted with ethyl acetate (300 mL), the aqueous phase was made
acidic with conc. HCl and extracted with ethyl acetate (200 mL). The
aqueous phase was saturated with brine and extracted with another ethyl
acetate (200 mL). The combined organic extracts were washed with
brine, dried over MgSO4 and evaporated to dryness. The raw material
was dissolved in acetic acid (700 mL), hydrobromic acid (80 mL, 47% in
water) was added and the solution was refluxed for 2 h. After cooling to
RT dichloromethane (600 mL) were added and the reaction mixture was
Conclusion
Our synthetic efforts demonstrate that [6.8]ncyclacenes can
be prepared in de novo syntheses by a stepwise strategy. By
applying a threefold Wittig reaction and an intramolecular
threefold McMurry coupling we succeeded in synthesizing
[6.8]3cyclacene, the most strained member of the
[6.8]ncyclacene family. The structural details found for
[6.8]3cyclacene reveal that its strain energy, which was pre-
dicted by quantum chemical calculations, stems from a
Chem. Eur. J. 2009, 15, 3368 – 3379
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3375

R. Gleiter et al.
washed with sat. aq. NaHCO3, water, and brine. The organic extracts
were dried and the solvent evaporated. Column chromatography on SiO2
(n-hexane/ethyl acetate, 20:1) afforded 5-(bromomethyl)-2,4-dimethyl-
benzaldehyde 34 (30.1 g, 65%), as a colorless crystalline solid. Rf =0.58
(n-hexane/ethyl acetate, 7:3); m.p.=628C ; 1H NMR (500 MHz, CDCl3):
d=2.43 (s, 3H), 2.61 (s, 3H), 4.51 (s, 2H), 7.08 (s, 1H), 7.71 (s, 1H),
10.18 ppm (s, 1H); 13C NMR (125 MHz, CDCl3): d=19.0, 19.1, 31.0,
132.6, 133.4, 134.1, 134.4, 141.2, 143.7, 191.7 ppm; MS (EI+): m/z: 228/
226 [M+]; HRMS: m/z calcd for C10H1181BrO: 227.9974, found:
227.9986; calcd for C10H1179BrO: 225.9994, found: 225.9983; IR (KBr):
n˜max =3039 (w), 2955 (w), 2925 (w), 2862 (w), 2758 (w), 1688 (vs), 1608
(s), 1447 (m), 1260 (m), 1223 (s), 1188 (m), 1073 (m), 881 (w), 633 (w),
1H); 13C NMR (75 MHz, CD2Cl2): d=18.5, 18.8, 19.0, 128.7, 129.7, 130.6,
131.1, 131.4, 132.2, 133.0, 133.6, 133.8, 133.9, 134.0, 135.9, 136.5 ppm; MS
(EI+): m/z: 390 [M+]; HRMS: m/z: calcd for C30H30: 390.2347, found:
390.2359; IR (KBr): n˜max =3003 (s), 2970 (m), 2942 (m), 2915 (s), 2858
(m), 1495 (s), 1448 (s), 1376 (m), 1032 (m), 991 (m), 922 (s), 869 (s), 760
(m), 724 cmꢀ1 (m); UV/vis (CH2Cl2): lmax
ACTHNUTRGNE(NUG log e)=282 nm (4.06); elemen-
tal analysis calcd (%) for C30H30: C 92.26, H 7.74; found: C 92.40, H 7.70.
Analytical data for 42a: Rf =0.34 (n-hexane/CH2Cl2, 5:1); m.p.=2488C;
1H NMR (500 MHz, CD2Cl2): d=1.96 (s, 6H), 2.09 (s, 6H), 2.29 (s, 12H),
6.31 (s, 2H), 6.50 (s, 2H), 6.61 (s, 2H), 6.63 (s, 2H), 6.82 (s, 2H), 6.85 (s,
2H), 6.93 (s, 2H), 6.96 ppm (s, 2H); 13C NMR (125 MHz, CD2Cl2): d=
19.1, 19.3, 19.9, 20.0, 128.3, 128.4, 128.6, 128.8, 129.6, 130.4, 131.6, 132.0,
133.7, 134.2, 134.3, 134.4, 134.5, 134.6, 134.9, 135.0 ppm; MS (EI+): m/z:
520 [M+]; HRMS: m/z: calcd for C40H40: 520.3130, found: 520.3121; IR
(KBr): n˜max =3007 (m), 2966 (w), 2942 (w), 2918 (m), 2861 (w), 1496 (m),
1449 (m), 1376 (w), 1032 (w), 972 (m), 920 (m), 872 (w); UV/vis
(CH2Cl2): lmax (log e)=288 nm (4.51).
549 cmꢀ1 (m); UV/vis (CH2Cl2): lmax
ACTHNUTRGNEU(GN log e)=260 (4.15), 300 nm (3.21); el-
emental analysis calcd (%) for C10H11BrO: C 52.89, H 4.88; found: C
53.06, H 4.89.
(5-Formyl-2,4-dimethylbenzyl)triphenylphosphonium bromide (36): A so-
lution of benzyl bromide 34 (30.1 g, 133 mmol) and triphenylphosphine
(36.6 g, 140 mmol) in toluene (700 mL) was refluxed overnight. The pre-
cipitated phosphonium salt was isolated by suction filtration, washed
with toluene and diethylether and dried in vacuo yielding 64.4 g (99%)
of 36 as a white powder. 1H NMR (300 MHz, [D6]DMSO): d=1.74 (s,
Analytical data for 42b: Rf =0.25 (n-hexane/CH2Cl2: 5/1); m.p.=2898C
(decomp); 1H NMR (300 MHz, CD2Cl2): d=2.18 (s, 12H), 2.26 (s, 12H),
6.61 (s, 4H), 6.72 (s, 4H), 6.94 (s, 4H), 7.58 ppm (s, 4H); 13C NMR
(75 MHz, CD2Cl2): d=19.3, 19.4, 127.7, 128.0, 128.5, 131.8, 133.7, 134.3,
134.7, 135.5 ppm; MS (EI+): m/z: 520 [M+]; HRMS: m/z calcd for
C40H40: 520.3130, found: 520.3137; IR (KBr): n˜max =3008 (m), 1968 (m),
2941 (m), 2918 (m), 2864 (w), 1491 (m), 1455 (m), 1376 (w), 1031 (w),
5
2
3H), 2.58 (d, 3H, JHP =2.1 Hz), 5.18 (d, 2H, JHP =15.0 Hz), 7.15 (s, 1H),
7.42 (d, 1H, 4JHP =2.4 Hz), 7.68–7.82 (m, 12H), 7.94–7.99 (m, 3H),
10.00 ppm (s, 1H); 13C NMR (75 MHz, [D6]DMSO): d=18.9, 20.0, 26.6,
118.3, 125.4, 131.1, 132.9, 133.6, 135.0, 135.1, 136.2, 141.6, 146.0,
192.4 ppm; 31P NMR (101 MHz, [D6]DMSO): d=23.7 ppm; MS (ESI,
MeOH): m/z: 441 [(MꢀBr)++MeOH], 409 [[(MꢀBr)+]; IR (KBr):
n˜max =3053 (w), 3007 (w), 2859 (w), 1692 (vs), 1609 (m), 1587 (m), 1438
(s), 1111 (s), 748 (s), 721 (m), 691 (s), 539 (m), 507 (s), 493 cmꢀ1 (m);
958 (s), 912 (w), 872 cmꢀ1 (w); UV/vis (CH2Cl2): lmax
ACTHNUGRTENUNG(log e)=294 nm
(4.66).
Analytical data for 42c: Rf =0.16 (n-hexane/CH2Cl2, 5:1); m.p.>2608C
(decomp); 1H NMR (300 MHz, CD2Cl2): d=2.22 (s, 6H), 2.34 (s, 6H),
2.45 (s, 6H), 2.48 (s, 6H), 6.68 (s, 2H), 6.98 (s, 4H), 7.04 (s, 2H), 7.08 (s,
2H), 7.09 (s, 2H), 7.30 (s, 2H), 7.64 ppm (s, 2H); 13C NMR (75 MHz,
CD2Cl2): d=18.6, 19.2, 19.5, 21.9, 128.3, 129.6, 130.4, 131.6, 132.3, 132.7,
132.9, 133.2, 133.3, 134.2, 134.3, 134.6, 134.6, 135.2, 135.9, 136.8 ppm; MS
(EI+): m/z: 520 [M+]; HRMS: m/z: calcd for C40H40: 520.3130, found:
520.3122; IR (KBr): n˜max =3006 (w), 2947 (w), 2918 (w), 2863 (w), 1499
UV/vis (CH2Cl2): lmaxACHTUNGTRENNUNG(log e)=250 (4.34), 264 (4.07), 284 (2.82), 298
(2.94), 308 nm (2.85).
Bromo substituted [23]metacyclophanetriene 37: A solution of phospho-
nium salt 35 (2.79 g 4.51 mmol) in dry DMF (40 mL) was cooled to
ꢀ408C. A freshly prepared solution of LiOEt in ethanol [prepared by
dissolving Li (38 mg, 5.5 mmol) in dry ethanol (10 mL)] was added in
portions at ꢀ408C during 4 h until the solution on addition of the base
did not turn red anymore. After stirring overnight the reaction mixture
was diluted with dichloromethane (300 mL) and washed several times
with 5m HCl (total: 120 mL). The aqueous solutions were extracted with
dichloromethane (100 mL). The combined organic extracts were washed
with brine and dried over MgSO4. The solvent was evaporated and the
residue purified by column chromatography on SiO2 (n-hexane/CH2Cl2,
20:1–10:1) yielding bromo substituted [23]metacyclophanetriene 37
(170 mg, 15%), as colorless crystals. Rf =0.55 (n-hexane/CH2Cl2, 5:1);
(w), 1453 (w), 1199 (w), 968 cmꢀ1 (w); UV/vis (CH2Cl2): lmax
ACHTUNGTRENNUNG(log e)=
300 nm (4.74).
Methyl substituted all-cis-[23]metacyclophanetriene 41b: In an irradia-
tion flask of the mixture 41a/41b (961 mg, 2.46 mmol) were dissolved in
degassed benzene (500 mL), magnetically stirred and cooled to 58C.
After 18 h of irradiation at l=254 nm the solvent was evaporated in
vacuo and the residue was purified by column chromatography on SiO2
(n-hexane/CH2Cl2, 10:1) yielding 41b (820 mg, 85%), as colorless crys-
tals. Rf =0.41 (n-hexane/CH2Cl2: 5/1); m.p.=1898C; 1H NMR (500 MHz,
CD2Cl2): d=2.12 (s, 18H), 6.46 (s, 3H), 6.54 (s, 6H), 6.89 ppm (s, 3H);
13C NMR (125 MHz, CD2Cl2): d=19.2, 129.1, 129.4, 131.3, 134.2,
1
m.p.=2218C; H NMR (300 MHz, CD2Cl2): d=6.56 (s, 3H), 6.65 (s, 6H),
7.76 ppm (s, 3H); 13C NMR (75 MHz, CD2Cl2): d=122.6, 130.5, 131.3,
136.2, 136.4 ppm; MS (EI+): m/z: =786/784/782/780/778/776/774 [M+];
HRMS: m/z: calcd for C24H1281Br379Br3: 779.5978, found: 779.5962; IR
(KBr): n˜max =3078 (w), 2922 (w), 2850 (w), 1574 (m), 1450 (s), 1327 (w),
1049 (s), 954 (m), 945 (m), 764 (m), 744 cmꢀ1 (m); UV/vis (n-pentane):
134.5 ppm; MS (EI+): m/z: 390 [M+]; HRMS: m/z: calcd for C30H30
:
390.2348, found: 390.2346; IR (KBr): n˜max =3004 (s), 2970 (m), 2945 (m),
2916 (s), 2860 (w), 1608 (w), 1491 (s), 1453 (s), 1376 (w), 1032 (w), 925
(s), 909 (m), 864 (m), 763 (m), 749 cmꢀ1 (m); UV/vis (n-pentane): lmax
-
AHCTUNGTRENNUNG
lmax
C24H12Br6: C 36.97, H 1.55, Br 61.48; found: C 36.85, H 1.77, Br 61.21.
Methyl substituted [23]metacyclophanetrienes 41a/b and
ACHTUNGTRENNUNG(log e)=224 (4.32), 276 nm (4.05); elemental analysis calcd (%) for
Bromomethyl substituted [23]metacyclophanetriene 43: A sample of N-
bromosuccinimide (5.21 g, 29.3 mmol) and dibenzoylperoxide (45 mg,
0.186 mmol) were added to a solution of methyl substituted all-cis-
[23]metacyclophantriene 41b (1.43 g, 3.66 mmol) in dry CCl4 (225 mL).
The mixture was stirred at reflux for 5 h, cooled to RT and filtrated to
remove the succinimide. The filtrate was washed with aq. NaOH, water,
and brine, dried (MgSO4) and evaporated to dryness in vacuo. The resi-
due was purified by column chromatography on SiO2 (n-hexane/CH2Cl2,
5:1–1:1) yielding benzylic hexabromide 43 (1.28 g, 40%), as colorless
crystals. Rf =0.39 (n-hexane/CH2Cl2, 1:1); m.p.=2398C (decomp);
1H NMR (500 MHz, CD2Cl2): d=4.41 (s, 18H), 6.63 (s, 3H), 6.83 (s, 6H),
7.32 ppm (s, 3H); 13C NMR (125 MHz, CD2Cl2): d=31.2, 129.0, 131.4,
132.2, 135.3, 136.8 ppm; MS (EI+): m/z: 870/868/866/864/862/860/858
[M+]; HRMS: m/z: calcd for C30H2479Br381Br3: 862.6917, found:
862.6962; IR (KBr): n˜max =3012 (w), 2972 (w), 1635 (b), 1489 (w), 1445
[24]metacyclophanetetraenes 42a–c: A solution of phosphonium salt 36
(47.8 g, 97.3 mmol) in dry DMF (1 L) was cooled to ꢀ108C. A freshly
prepared solution of LiOEt in ethanol [prepared by dissolving Li
(810 mg, 117 mmol) in dry ethanolACTHNUTRGENUGN(120 mL)] was added in portions at
ꢀ108C during 10 h until the solution on addition of the base did not turn
red anymore. After being stirred overnight the reaction mixture was di-
luted with dichloromethane, washed several times with 5m HCl and brine
and dried over MgSO4. The solvent was evaporated and the residue puri-
fied by column chromatography on SiO2 (n-hexane/CH2Cl2, 10:1) yield-
ing the mixture 41a/41b (93:7, 961 mg, 8%), as colorless crystals and
42a–c (<1%).
Analytical data for 41a: Rf =0.41 (n-hexane/CH2Cl2, 5:1); m.p.=1938C;
1H NMR (300 MHz, CD2Cl2): d=1.62 (s, 6H), 2.28 (s, 6H), 2.31 (s, 6H),
6.39 (s, 2H), 6.54 (s, 1H), 6.73 (d, 1H, 3J=11.7 Hz), 6.74 (s, 1H), 6.79 (s,
1H), 6.81 (s, 2H), 6.81 (d, 1H, 3J=11.7 Hz), 6.89 (s, 2H), 7.19 ppm (s,
(w), 1210 (s), 933 (m), 587 cmꢀ1 (m); UV/vis (CH2Cl2): lmax
ACTHUNRTGNE(GNU log e)=236
3376
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3368 – 3379

Synthesis and Properties of [6.8]3Cyclacene
FULL PAPER
(4.75), 288 nm (4.50); elemental analysis calcd (%) for C30H24Br6: C
41.71, H 2.80, Br 55.49; found: C 41.52, H 2.85, Br 55.33.
CD2Cl2): d=19.3, 20.6, 81.1, 81.5, 120.3, 132.3 133.1, 136.1, 141.1, 146.7,
191.4 ppm; MS (EI+): m/z: 158 [M+]; HRMS: m/z: calcd for C11H10O:
158.0731, found: 158.0742; IR (KBr): n˜max =3206 (s), 2093 (w), 1682 (s),
1608 (m), 1378 (w), 1260 (w), 1175 (w), 1060 (w), 903 cmꢀ1 (w); UV/vis
(CH2Cl2): lmaxACTHNUGRTNEUNG(log e)=238 (4.74), 262 (4.28), 308 nm (3.37); elemental
analysis calcd (%) for C11H10O: C 83.51, H 6.37; found: C 83.50, H 6.39.
Formyl substituted [23]metacyclophanetriene 28: To a solution of IBX
(2.36 g, 8.43 mmol)[23] in dry DMSO (13 mL) was added a solution of
benzylic hexabromide 43 (607 mg, 0.703 mmol) in dry DMSO (5 mL).
The resulting yellowish solution was stirred at 658C for 8 h and at RT
overnight. Aqueous Na2S2O3 (30 mL) and aq. sat. NaHCO3 (30 mL) were
added and the mixture was extracted with dichloromethane (3ꢅ50 mL).
The combined organic extracts were washed with brine, dried, and the
solvent was evaporated in vacuo. Column chromatography on SiO2 (n-
hexane/ethyl acetate, 2:1–1:1) yielded hexaaldehyde 28 (125 mg, 37%),
as colorless crystals. Rf =0.58 (n-hexane/ethyl acetate, 1:2); 1H NMR
(300 MHz, CDCl3): d=6.53 (s, 3H), 7.30 (s, 6H), 8.25 (s, 3H), 10.10 ppm
(s, 6H); 13C NMR (75 MHz, CDCl3): d=130.8, 133.1, 133.4, 135.3, 142.6,
190.4 ppm; MS (FAB+): m/z: 475 [(M+H)+]; HRMS: m/z: calcd for
C30H19O6: 475.1181, found: 475.1177; IR (KBr): n˜max =2966 (w), 2938 (w),
2853 (w), 1695 (vs), 1594 (s), 1540 (w), 1237 (m), 1224 (m), 1162 (m), 936
5,5’-(Ethyne-1,2-diyl)bis(2,4-dimethylbenzaldehyde) (48): A solution of
5-iodo-2,4-dimethylbenzaldehyde 50 (25.1 g, 96.5 mmol) in THF
(200 mL) and iPr2NH (80 mL) was degassed by bubbling argon through
the solution. [PdACHTNUTRGNE(UNG PPh3)4] (2.23 g, 1.93 mmol) and CuI (735 mg,
3.86 mmol) were added and the mixture was stirred at RT for 10 min. A
solution of 5-ethynyl-2,4-dimethylbenzaldehyde 51 (15.3 g, 96.8 mmol) in
THF (50 mL) was added dropwise, and the reaction mixture was allowed
to stir for 2 h at RT. After diluting with dichloromethane (200 mL) the
reaction mixture was successively washed with ice cold 2m HCl (3ꢅ
200 mL) and sat. aq. NH4Cl (3ꢅ100 mL). The organic extracts were
dried over Na2SO4, and the solvent was removed in vacuo. The residue
was purified by column chromatography on SiO2 (n-hexane/diethyl ether,
10:1–5:1) yielding 5,5’-(ethyne-1,2-diyl)bis(2,4-dimethylbenzaldehyde) 48
(27.4 g, 98%), as colorless needles. Rf =0.40 (n-hexane/diethyl ether,
(w), 827 cmꢀ1 (w); UV/vis (CH2Cl2): lmax
ACTHNUTRGNE(UNG log e)=244 (5.01), 310 nm
(4.71); elemental analysis calcd (%) for C30H18O6: C 75.94, H 3.82;
found: C 75.54, H 3.79.
1
7:3); m.p.=1648C; H NMR (500 MHz, CD2Cl2): d=2.55 (s, 3H), 2.63 (s,
[24]
ꢀ
G
3H), 7.17 (s, 1H), 7.91 (s, 1H), 10.18 ppm (s, 1H); 13C NMR (125 MHz,
CD2Cl2): d=19.3, 21.0, 91.4, 121.4, 132.4 133.2, 135.3, 140.8, 146.0,
191.5 ppm; MS (EI+): m/z: 290 [M+]; HRMS: m/z: calcd for C20H18O2:
290.1307, found: 290.1322; IR (KBr): n˜max =2920 (w), 2864 (w), 1701 (s),
1687 (vs), 1609 (s), 1407 (m), 1178 (m), 1100 (w), 903 (w), 866 cmꢀ1 (w);
(1.24 g)[24] in dry DME (35 mL) was stirred under reflux for 2 h. A solu-
tion of 28 (50 mg 0.105 mmol) in dry DME (8 mL) was slowly added to
the black-grey suspension and the reaction mixture was stirred under
reflux for 5 h. After stirring at RT overnight the mixture was filtrated
over a pad of florisil, which was subsequently washed with dichlorome-
thane. The solvent was evaporated in vacuo and the residue was purified
by column chromatography on SiO2 (n-hexane/CH2Cl2, 10:1) affording 13
(3 mg, 8%), as colorless needles. Rf =0.38 (n-hexane/CH2Cl2, 5:1);
1H NMR (500 MHz, CD2Cl2): d=6.36 (s, 6H), 6.73 ppm (s, 12H);
13C NMR (125 MHz, CD2Cl2): d=125.2, 133.1, 136.1 ppm; MS (EI+): m/
z: 378 [M+]; HRMS: m/z: calcd for C30H18: 378.1408, found: 378.1400;
UV/vis (CH2Cl2): lmaxACHTNUTRGENUG(N log e)=258 (4.70), 298 (4.44), 308 nm (4.37); ele-
mental analysis calcd (%) for C20H18O2: C 82.73, H 6.25; found: C 82.53,
H 6.15.
Methyl substituted [24]metacyclophandienediyne 47: To a suspension of
Zn dust (20.8 g, 320 mmol) and CuI (2.13 g, 11.2 mmol) in dry DME
(700 mL) were added TiCl4 (17.6 mL, 160 mmol). The bluish-green sus-
pension was refluxed for 3 h resulting in a dark grey suspension. After
cooling to RT, a solution of 5,5’-(ethyne-1,2-diyl)bis(2,4-dimethylbenzal-
dehyde) 48 (4.64 g, 16.0 mmol) in dry DME (150 mL) was added by
means of a syringe pump within 7 h and the mixture was stirred another
15 h at RT. The reaction mixture was filtered through a pad of Al2O3,
which was subsequently washed with dichloromethane (500 mL). The sol-
vent was evaporated in vacuo, the residue absorbed on Al2O3 (4 g) and
purified by column chromatography on Al2O3 (n-hexane/dichlorome-
thane, 10:1–5:1) yielding methyl substituted[24]metacyclophandienediyne
47 (445 mg, 11%), as colorless needles. Rf =0.22 (n-hexane/CH2Cl2, 5:1);
1H NMR (500 MHz, CDCl3): d=2.19 (s, 12H), 2.34 (s, 12H), 6.69 (s,
4H), 6.91 (s, 4H), 7.59 ppm (s, 4H); 13C NMR (151 MHz, CDCl3): d=
19.9, 20.5, 91.5, 120.2, 130.0, 130.7, 133.3, 133.7, 136.0, 138.2 ppm; MS
(EI+): m/z: 516 [M+]; HRMS: m/z: calcd for C40H36: 516.2817, found:
516.2792; IR (KBr): n˜max =3011 (w), 2944 (w), 2916 (w), 2862 (w), 1495
(m), 1448 (m), 1376 (w), 1032 (w), 969 (w), 908 (w), 873 cmꢀ1 (w); UV/
UV/vis (n-pentane): lmaxACHTUNGTRENNUNG(log e)=220 (4.69), 278 (3.40), 290 nm (3.32).
5-Ethynyl-2,4-dimethylbenzaldehyde (51): A solution of 5-iodo-2,4-dime-
thylbenzaldehyde 50 (32 g, 123 mmol) in THF (250 mL) and iPr2NH
(100 mL) was degassed by bubbling argon through the solution. After
cooling to 08C, [PdACHTUNGTRENNUNG(PPh3)4] (4.26 g, 3.69 mmol) and CuI (1.40 g,
7.35 mmol) were added and the mixture was stirred at RT for 10 min. An
amount of TMSA (21.9 mL, 153 mmol) was slowly added in a dropwise
manner, and the reaction mixture was stirred for 2 h at RT. After diluting
with ethyl acetate (200 mL) the mixture was successively washed with ice
cold 2m HCl (3ꢅ200 mL) and sat. aq. NH4Cl (3ꢅ100 mL). The organic
layer was dried over Na2SO4, evaporated to dryness, and the residue was
purified by column chromatography on SiO2 (n-hexane/ethyl acetate,
20:1) giving 2,4-dimethyl-5-((trimethylsilyl)ethynyl)benzaldehyde (27.4 g,
97%), as white solid. Rf =0.72 (n-hexane/ethyl acetate, 7:3); m.p.=448C;
1H NMR (500 MHz, CD2Cl2): d=0.25 (s, 9H), 2.44 (s, 3H), 2.60 (s, 3H),
7.11 (s, 1H), 7.80 (s, 1H), 10.13 ppm (s, 1H); 13C NMR (125 MHz,
CD2Cl2): d=ꢀ0.3, 19.3, 20.7, 99.1, 102.4 121.4, 132.2, 133.0, 135.7, 140.8,
146.6, 191.5 ppm; MS (EI+): m/z: 230 [M+]; HRMS: m/z: calcd for
C14H18OSi: 230.1127, found: 230.1113; IR (KBr): n˜max =2960 (m), 2155
(m), 1705 (s), 1690 (s), 1606 (m), 1251 (s), 1081 (m), 902 (m), 870 (s), 841
vis (n-pentane): lmaxACTHNUTRGNEU(GN log e)=218 (3.26), 228 (3.24), 294 (3.49), 304 (3.37),
314 nm (3.33).
(s), 761 cmꢀ1 (s); UV/vis (CH2Cl2): lmax
ACTHNUTRGNEU(GN log e)=244 (3.66), 266 (3.27), 276
(3.04), 314 nm (2.16); elemental analysis calcd (%) for C14H18OSi: C
72.99, H 7.88; found: C 72.56, H 7.88.
Acknowledgements
To
a
solution of 2,4-dimethyl-5-((trimethylsilyl)ethynyl)benzaldehyde
We thank the Deutsche Forschungsgemeinschaft for financial support. B.
E. thanks the Studienstiftung des deutschen Volkes for a graduate fellow-
ship and M. Dçrner for preparative assistance.
(22.6 g, 98.1 mmol) in degassed MeOH (1 L) and DCM (100 mL) was
added K2CO3 (27.2 g, 196 mmol), and the resulting mixture was stirred
for 1 h at RT under an argon atmosphere. After removal of the solvent in
vacuo at RT the residue was poured into ice cold water (200 mL) and ex-
tracted with dichloromethane (3ꢅ100 mL). The combined organic ex-
tracts were washed with water (2ꢅ50 mL) and brine. After drying over
Na2SO4, the solvent was evaporated in vacuo at RT. The residue was pu-
rified by column chromatography on SiO2 (n-hexane/diethyl ether, 20:1–
10:1) yielding 5-ethynyl-2,4-dimethylbenzaldehyde 51 (15.3 g, 98%), as
colorless needles. Rf =0.56 (n-hexane/diethyl ether, 7:3); m.p.=888C;
1H NMR (500 MHz, CD2Cl2): d=2.46 (s, 3H), 2.60 (s, 3H), 3.36 (s, 1H),
7.13 (s, 1H), 7.84 (s, 1H), 10.14 ppm (s, 1H); 13C NMR (125 MHz,
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0.44ꢅ0.09ꢅ0.05 mm, crystal system monoclinic, space group P21/c,
Z=4, a=15.3781(6) ꢃ, b=6.8444(3) ꢃ, c=18.2608(7) ꢃ, a=908,
b=90.6650(10)8, g=908, V=1921.89(13) ꢃ3, 1=1.308 gcmꢀ3
, T=
200(2) K, Vmax =22.008, radiation MoKa, l=0.71073 ꢃ, 0.38 w-scans
with CCD area detector Bruker Smart CCD, covering a whole
sphere in reciprocal space; 11783 reflections measured, 2340 unique
(RACTHNUTRGNEUG(N int)=0.0699), 1722 observed (I>2s(I)); intensities were correct-
ed for Lorentz and polarization effects; an empirical absorption cor-
rection was applied using SADABS[41] based on the Laue symmetry
of the reciprocal space, m=0.07 mmꢀ1, Tmin =0.97, Tmax =1.00; struc-
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software package,[41] 273 parameters refined, hydrogen atoms were
treated using appropriate riding models, goodness of fit 1.11 for ob-
served reflections, final residual values R1(F)=0.050, wR(F2)=0.101
for observed reflections, residual electron density ꢀ0.20 to
0.15 eꢃꢀ3. CCDC 676676 contains the supplementary crystallograph-
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P21/c, Z=4, a=13.616(3) ꢃ, b=12.804(3) ꢃ, c=14.385(3) ꢃ, a=
908, b=112.778(4)8, g=908, V=2312.2(9) ꢃ3, 1=2.240 gcmꢀ3, T=
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space, 17926 reflections measured, 4069 unique (RACHTUNTRGNEU(GN int)=0.0572),
3197 observed (I>2s(I)), intensities were corrected for Lorentz and
polarization effects, an empirical absorption correction was applied
using SADABS[41] based on the Laue symmetry of the reciprocal
space, m=10.43 mmꢀ1, Tmin =0.15, Tmax =0.75, structure solved by
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271 parameters refined, hydrogen atoms were treated using appro-
priate riding models, goodness of fit 1.01 for observed reflections,
final residual values R1(F)=0.032, wR(F2)=0.065 for observed re-
flections, residual electron density ꢀ0.81 to 0.67 eꢃꢀ3
.
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Organometallics 2003, 22, 47–58) in a Sommelet reaction following
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[21] CCDC 676676 (28), 676674 (41a), 676673 (41b), and 676675 (43)
contain the supplementary crystallographic data for this paper.
3378
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3368 – 3379

Synthesis and Properties of [6.8]3Cyclacene
FULL PAPER
PA, 2003, method using the B3LYP functional in combination with
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Pople, J. Chem. Phys. 1980, 72, 650–654.
for observed reflections, residual electron density ꢀ0.21 to
0.18 eꢃꢀ3
.
[36] Crystallographic data for 42c: Colorless crystals (polyhedron), di-
mensions 0.26ꢅ0.16ꢅ0.15 mm, crystal system monoclinic, space
group P21/c, Z=4, a=21.7693(1) ꢃ, b=16.6745(3) ꢃ, c=
8.2746(2) ꢃ, a=908, b=94.9370(10)8, g=908, V=2992.47(9) ꢃ3, 1=
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1.156 gcmꢀ3
,
T=200(2) K, Vmax =21.528, radiation MoKa
,
l=
0.71073 ꢃ, 0.38 w-scans with CCD area detector, covering a whole
sphere in reciprocal space, 17181 reflections measured, 3439 unique
(RACTHNUTRGNEUG(N int)=0.1351), 2200 observed (I>2s(I)), intensities were correct-
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2674.
ed for Lorentz and polarization effects, an empirical absorption cor-
rection was applied using SADABS[41] based on the Laue symmetry
of the reciprocal space, m=0.06 mmꢀ1, Tmin =0.98, Tmax =0.99, struc-
ture solved by direct methods and refined against F2 with a Full-
matrix least-squares algorithm using the SHELXTL-PLUS (5.10)
software package,[41] 370 parameters refined, hydrogen atoms were
treated using appropriate riding models, goodness of fit 1.03 for ob-
served reflections, final residual values R1(F)=0.058, wR(F2)=0.125
for observed reflections, residual electron density ꢀ0.22 to
[34] Crystallographic data for 42a: Colorless crystals (needle), dimen-
¯
sions 0.36ꢅ0.22ꢅ0.14 mm, crystal system triclinic, space group P1,
Z=2, a=7.8036(10) ꢃ, b=13.3424(18) ꢃ, c=15.320(2) ꢃ, a=
73.608(3)8, b=81.289(3)8, g=85.850(3)8, V=1511.9(3) ꢃ3, 1=
1.144 gcmꢀ3
,
T=200(2) K, Vmax =28.358, radiation MoKa
,
l=
0.71073 ꢃ, 0.38 w-scans with CCD area detector, covering a whole
0.23 eꢃꢀ3
.
sphere in reciprocal space, 16047 reflections measured, 7473 unique
(RACHTUNGTRENNUNG(int)=0.0353), 4694 observed (I>2s(I)), intensities were correct-
[37] P. Lulinski, L. Skulski, Bull. Chem. Soc. Jpn. 1997, 70, 1665–1669.
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Kurata, M. Oda, Angew. Chem. 2007, 119, 1104–1106; Angew.
Chem. Int. Ed. 2007, 46, 1086–1088.
[40] Crystallographic data for 47: Colorless crystals (needle), dimensions
0.37ꢅ0.08ꢅ0.05 mm, crystal system monoclinic, space group C2/c,
Z=4, a=16.282(2) ꢃ, b=13.810(2) ꢃ, c=13.013(2) ꢃ, a=90.08, b=
98.207(3)8, g=90.08, V=2896.0(7) ꢃ3, 1=1.185 gcmꢀ3, T=200(2) K,
Vmax =26.378, radiation MoKa, l=0.71073 ꢃ, 0.38 w-scans with CCD
area detector, covering a whole sphere in reciprocal space, 12972 re-
ed for Lorentz and polarization effects, an empirical absorption cor-
rection was applied using SADABS[41] based on the Laue symmetry
of the reciprocal space, m=0.06 mmꢀ1, Tmin =0.98, Tmax =0.99, struc-
ture solved by direct methods and refined against F2 with a Full-
matrix least-squares algorithm using the SHELXTL-PLUS (5.10)
software package,[41] 536 parameters refined, hydrogen atoms were
treated using appropriate riding models, goodness of fit 1.08 for ob-
served reflections, final residual values R1(F)=0.073, wR(F2)=0.149
for observed reflections, residual electron density ꢀ0.17 to
0.19 eꢃꢀ3
.
[35] Crystallographic data for 42b: Colorless crystals (polyhedron), di-
mensions 0.50ꢅ0.10ꢅ0.08 mm, crystal system monoclinic, space
group P21/c, Z=8, a=9.2735(2) ꢃ, b=21.0321(2) ꢃ, c=
30.5409(7) ꢃ, a=908, b=91.6290(10)8, g=908, V=5954.3(2) ꢃ3, 1=
flections measured, 2963 unique (RACTHUGNETRNNU(G int)=0.0460), 2330 observed
(I>2s(I)), intensities were corrected for Lorentz and polarization
effects, an empirical absorption correction was applied using
SADABS[41] based on the Laue symmetry of the reciprocal space,
1.162 gcmꢀ3
,
T=200(2) K, Vmax =21.518, radiation MoKa
,
l=
m=0.07 mmꢀ1
, Tmin =0.98, Tmax =1.00, structure solved by direct
0.71073 ꢃ, 0.38 w-scans with CCD area detector, covering a whole
sphere in reciprocal space, 36055 reflections measured, 6846 unique
(RACHTUNGTRENNUNG(int)=0.1644), 3319 observed (I>2s(I)), intensities were correct-
methods and refined against F2 with a Full-matrix least-squares algo-
rithm using the SHELXTL (6.12) software package,[41] 253 parame-
ters refined, hydrogen atoms were refined isotropically, goodness of
fit 1.20 for observed reflections, final residual values R1(F)=0.071,
wR(F2)=0.133 for observed reflections, residual electron density
ed for Lorentz and polarization effects, an empirical absorption cor-
rection was applied using SADABS[41] based on the Laue symmetry
of the reciprocal space, m=0.06 mmꢀ1, Tmin =0.97, Tmax =0.99, struc-
ture solved by direct methods and refined against F2 with a Full-
matrix least-squares algorithm using the SHELXTL-PLUS (5.10)
software package,[41] 737 parameters refined, hydrogen atoms were
treated using appropriate riding models, goodness of fit 0.98 for ob-
served reflections, final residual values R1(F)=0.058, wR(F2)=0.107
ꢀ0.17 to 0.21 eꢃꢀ3
.
[41] G. M. Sheldrick, Bruker Analytical X-ray-Division, Madison, Wis-
consin 2006.
Received: November 5, 2008
Published online: March 11, 2009
Chem. Eur. J. 2009, 15, 3368 – 3379
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3379