Azatrioxa[8]circulenes: Planar anti-aromatic cyclooctatetraenes

Article abstract of DOI:10.1002/chem.201203113

We describe herein the first synthesis of a new class of anti-aromatic planar cyclooctatetraenes: the azatrioxa[8]circulenes. This was achieved by treating a suitably functionalised 3,6-dihydroxycarbazole with 1,4-benzoquinones or a 1,4-naphthoquinone. We fully characterised the azatrioxa[8]circulenes by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The results of a computational study (NICS) suggest that the central planar cyclooctatetraene is anti-aromatic when the molecules are in neutral or oxidised states (2+), and that the corresponding dianions are aromatic. We discuss the aromatic/anti-aromatic nature of the planar cyclooctatetraenes and compare them with the isoelectronic tetraoxa[8]circulenes. Planar cyclooctatetraenes: The first synthesis of a new class of anti-aromatic planar cyclooctatetraenes is described: the azatrioxa[8]circulenes (see scheme). The azatrioxa[8]circulenes were fully characterised by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The aromatic/anti-aromatic nature of the planar cyclooctatetraenes is discussed and compared with the isoelectronic tetraoxa[8]circulenes. Copyright

Full text of DOI:10.1002/chem.201203113

FULL PAPER
DOI: 10.1002/chem.201203113
Azatrioxa[8]circulenes: Planar Anti-Aromatic Cyclooctatetraenes
Christian B. Nielsen,[a] Theis Brock-Nannestad,[a] Peter Hammershøj,[a]
Theis K. Reenberg,[a] Magnus Schau-Magnussen,[a] Denis Trpcevski,[a] Thomas Hensel,[a]
Roberto Salcedo,[b] Gleb V. Baryshnikov,[c] Boris F. Minaev,[c] and Michael Pittelkow*[a]
Abstract: We describe herein the first
synthesis of a new class of anti-aromat-
ic planar cyclooctatetraenes: the aza-
trioxa[8]circulenes. This was achieved
by treating a suitably functionalised
3,6-dihydroxycarbazole with 1,4-benzo-
quinones or a 1,4-naphthoquinone. We
fully characterised the azatrioxa[8]cir-
culenes by using optical, electrochemi-
cal and computational techniques as
well as by single-crystal X-ray crystal-
lography. The results of a computation-
al study (NICS) suggest that the central
planar cyclooctatetraene is anti-aro-
matic when the molecules are in neu-
tral or oxidised states (2+), and that
the corresponding dianions are aromat-
ic. We discuss the aromatic/anti-aro-
matic nature of the planar cycloocta-
AHCTUNGTRENGNtUN etraenes and compare them with the
isoelectronic tetraoxa[8]circulenes.
Keywords: aromaticity
·
fluores-
cence · heterocycles · polycycles
Introduction
When benzoquinone is treated with strong acid it oligomer-
ises to give a mixture of linear, branched and cyclic benzo-
furan structures.[1] A minor component of this mixture is the
cyclisation product obtained by condensation of four mole-
cules of benzoquinone: tetraoxa[8]circulene (1, R=H; Fig-
ure 1a).[2] The yield of this cyclisation product is significantly
improved by substituting one side of the benzoquinone with
either two alkyl substituents or one tert-butyl substituent, or
by using naphthoquinone.[2a,3] When analysing the reaction
mixture of this polymerisation/tetramerisation reaction with
benzoquinone or naphthoquinone, Hçgberg showed that an
important intermediate was 3,6-dihydroxydibenzofuran (Fig-
ure 1a).[4] When 3,6-dihydroxydibenzofuran was isolated and
allowed to react with a further 2 equiv of benzoquinone or
naphthoquinone, it yielded the tetraoxa[8]circulenes in good
yields (1 and 2).[5] We have recently shown that a statistical
[a] Dr. C. B. Nielsen, Dr. T. Brock-Nannestad, Dr. P. Hammershøj,
Dr. T. K. Reenberg, Dr. M. Schau-Magnussen, D. Trpcevski,
T. Hensel, Dr. M. Pittelkow
Department of Chemistry, University of Copenhagen
Universitetsparken 5, 2100 Copenhagen Ø (Denmark)
Fax : (+45)35320212
E-mail: pittel@kiku.dk
[b] Dr. R. Salcedo
Instituto de Investigaciones en Materiales
Universidad Nacional Autonoma de Mꢀxico
Circuito Exterior s/n, Ciudad Universitaria
Coyoacꢁn 04510 (Mꢀxico D. F.)
[c] G. V. Baryshnikov, Prof. B. F. Minaev
Bohdan Khmelnytsky National University
18031, Cherkasy (Ukraine)
Figure 1. Structures and schematic retrosynthetic analyses of tetraoxa[8]-
circulenes 1 and 2, and azatrioxa[8]circulenes 3 and 4 (R=C11H23, tert-
butyl or H; R1 =propyl).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203113.
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condensation of a 2,3-dialkyl-1,4-benzoquinone and naph-
thoquinone gives a mixture of p-extended tetraoxa[8]circu-
lenes (e.g., 2; Figure 1b).[6] Inspired by the above findings,
we decided to investigate whether a suitably functionalised
N-alkyl-3,6-dihydroxycarbazole could replace 3,6-dihydroxy-
dibenzofuran in the condensation reaction with benzoqui-
nones or naphthoquinones to produce azatrioxa[8]circu-
lenes. Herein we describe the synthesis of the first azatri-
Results and Discussion
The azatrioxa[8]circulenes 3 and 4 were synthesised from
3,6-dihydroxycarbazole
(Scheme 1). Carbazole (6) was alkylated using a two-phase
the
common
precursor
5
ACHTUNGTRENNUNGoxa[8]circulenes 3 and 4 (Figure 1c,d) and compare their
electrochemistry and photophysical properties with their iso-
electronic tetraoxa[8]circulene analogues 1 and 2. The re-
sults of a computational study (nuclear-independent chemi-
cal shift calculations) suggest that the central planar cyclo-
ACHTUNGTRENNUNGoctatetraenes (COTs) are anti-aromatic when the molecules
are in the neutral or oxidised states (2+) and that the corre-
sponding dianions are aromatic.
[n]Circulenes are conjugated cyclic compounds that can
formally be derived from [n]radialenes by connecting the
termini of the semi-cyclic double bonds by etheno bridges.[7]
In heterocyclic [n]circulenes, heteroatoms such as oxygen,
nitrogen, sulfur or selenium replace one or more of the
etheno bridges. Although the parent [8]circulene is predict-
ed to be saddle-shaped,[7b] some of the heterocyclic [8]circu-
lenes have been shown to be planar. Heteroatom substitu-
tion of [8]circulenes has only been achieved in a few instan-
ces since the seminal work on tetraoxa[8]circulenes by Erdt-
man and Hçgberg.[8] These include the contribution of
Christensen and co-workers for the synthesis of soluble tet-
raoxa[8]circulenes,[3a] our own tetraoxa[8]circulenes and p-
extended tetraoxa[8]circulenes,[6] Nenajdenko and co-work-
ers octathia[8]circulene and tetrathiatetraselena[8]circu-
lene,[9,10] and Osuka and co-workers formal tetraaza[8]circu-
lene.[11]
Polycyclic conjugated compounds are complex to under-
stand in terms of aromaticity and anti-aromaticity as the
simple Hꢃckel rules do not apply. From a magnetic point of
view, a diatropic ring current is a characteristic of aromatic
rings, whereas paratropic ring currents are a characteristic of
anti-aromatic rings. In computational chemistry, methods to
measure aromaticity have been developed, and nuclear-in-
dependent chemical shift (NICS) calculations appear to be a
reliable method for studying this kind of phenomena. In
NICS calculations, large negative values indicate aromaticity
and large positive values indicate anti-aromaticity.[12]
Scheme 1. Synthesis of the azatrioxa[8]circulene precursor 5.
protocol to give the alkylcarbazole 7 in high yield. The N-
propylcarbazole (7) was selectively brominated twice para
to the nitrogen to yield the dibromide 8, which was effec-
tively transformed into the dimethoxy derivative 9 by treat-
ment with NaOMe/MeOH and CuI in DMF.
tert-Butylation proved sensitive to the choice of reagents,
but a mixture of anhydrous FeCl3 and tert-butyl chloride
gave a clean conversion to 2,7-di-tert-butyl-3,6-dimethoxy-
carbazole 10. Demethylation with BBr3 in CH2Cl2 gave the
acid- and air-sensitive 3,6-dihydroxy-substituted carbazole 5
in good yield. Treatment of 5 with tert-butylbenzoquinone
or naphthoquinone in CH2Cl2/BF3·OEt2 gave the azatri-
AHCTUNGTREGoNNUN xa[8]circulene 3 and the extended azatrioxa[8]circulene 4
in yields of 65 and 75%, respectively (Scheme 2). In the
case of the tetra-tert-butylazatrioxa[8]circulene 3, only one
isomer with respect to the tert-butyl groups was observed.
The symmetrical nature of 3 was evident in the NMR spec-
tra, in which only two types of aromatic protons and only
two types of tert-butyl groups were observed.
HeteroACHTUNGTRENNUNGcyclic [8]circulenes are good candidates for studying
the aromaticity/anti-aromaticity of planar COTs in polycy-
clic structures that are placed in an environment of aromatic
moieties. The seminal work by Osuka and co-workers on
this topic focused on showing that a planar COT formed in
a tetraporphyrin array with two diagonally metallated por-
phyrins connected by bidentate organic ligands displays
The azatrioxa[8]circulenes 3 and 4 possess cyclic 8p elec-
tron conjugation of the central COT and the structures are
almost completely planar, as can been seen in the single-
crystal X-ray structures of 3 and 4 (Figure 2).[14,15] The crys-
tal structures also confirm the regiochemical outcome of the
synthetic protocol. The crystal structure of 3 suggests there
is little interaction between the p systems of neighbouring
molecules, probably due to the steric bulk of the four tert-
butyl substituents on the azatrioxa[8]circulene structure. The
crystal packing of the p-extended azatrioxa[8]circulene 4 is
dominated by p stacking, with each azatrioxa[8]circulene ex-
hibiting significant p–p overlap with its closest neighbour.
para
ACHTUNGTRENNUNGtropicity by measuring the ring current effects in
1H NMR spectroscopy experiments.[11,13]
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Planar Anti-Aromatic Cyclooctatetraenes
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Figure 3. Packing observed in the X-ray crystal structure of 4.
Scheme 2. Synthesis of the azatrioxa[8]circulenes 3 and 4.
Figure 4. UV/Vis and fluorescence spectra (insert) of 1–4.
emitting diodes (OLEDs).[6] The optical and electrochemical
properties of azatrioxa[8]circulenes 3 and 4 were examined
to assess the possibility of their use in this application. The
UV/Vis spectra of 1–4 are shown in Figure 4 together with
their fluorescence spectra. The emission spectra of the di-
naphthyl-based [8]circulenes 2 and 4 are blue-shifted com-
pared with the all-benzene-based [8]circulenes 1 and 3. An
increase in the conjugation length and the substitution of an
oxygen atom by a nitrogen atom leads to an increase in the
quantum yield (1=0.1, 2=0.5, 3=0.3, 4=0.9). The azatri-
Figure 2. X-ray crystal structures of 3 (top) and 4 (bottom).
In addition, the nitrogen atom of every second azatrioxa[8]-
circulene is pointing in the opposite direction, probably to
accommodate the space-filling tert-butyl substituents
(Figure 3). The planarity of the tetrahetero[8]circulenes with
four benzene rings, three furans and one pyrrole (e.g., aza-
trioxa[8]circulene) was anticipated on the basis of the
Dopper and Wynberg model, which predicts whether [n]cir-
culenes are bowl-shaped, planar or saddle shaped.[16]
AHCTUNGTREGoNNNU xa[8[circulenes, therefore, could prove to be even better
Tetraoxa[8]circulenes 1 and 2 have shown promise as can-
didates for the blue fluorescent component of organic light-
suited for applications in blue OLEDs than the tetraoxa[8]-
circulenes.
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M. Pittelkow et al.
By comparing the absorption and fluorescence properties
it can be seen that the effect of substituting an oxygen atom
with a nitrogen atom outweighs the effect of extending the
conjugation length because a pronounced redshift occurs for
the lowest excited state of 1 compared with 2. This effect is
less dominant for 3 and 4. The absorption spectra also show
bands at higher energies that can be ascribed to transitions
to higher electronic states. In addition, strong couplings to
the vibrational degrees of freedom are evident by the signif-
icant fine structure in the spectra.
GACTHUNGTERNNU(G d,p) level of theory by using geometries obtained from
B3LYP/6-31+G(d) calculations. The results suggest that in
the neutral form, the planar COTs of both tetraoxa[8]circu-
lene and azatrioxa[8]circulene are anti-aromatic (positive
values). Meanwhile, the benzene and furan rings are aro-
matic, which is in accordance with our previous work.[17] In
the case of the doubly oxidised (2+) tetraoxa[8]circulene
and azatrioxa[8]circulene, our calculations indicate that the
COTs retain their anti-aromaticity, whereas in the doubly
reduced (2À) species the COTs are aromatic (large negative
values). The benzene, furan and pyrrole rings appear to
retain their aromaticity in all the oxidation states studied.
To further address the question of (anti)aromaticity, the
bond lengths in the inner eight-membered ring of the tetra-
The fact that substitution of an oxygen atom outweighs
the effect of extending the conjugation length is further sup-
ported by the oxidation and reduction potentials of 1–4 (see
Table 1). In previous work, it was found that the first oxida-
AHCTUNGTREGoNNNU xa[8]circulene and azatrioxa[8]circulene were examined
(Table 2) by performing B3LYP calculations. The observed
Table 1. Reduction and oxidation potentials for 1–4 referenced against
the Fc/Fc+ couple obtained by using square wave voltammetry.[a]
Table 2. Selected bond lengths for 1 and 3.[a]
Species
Potential [V] vs. Fc/Fc+
1
2
3
4
À2.71
À2.55
À2.40
À2.39
À2.52
À2.58
1.09
0.72
0.67
0.65
1
3
À2.71
0.97
1.41
1.22
Bond
À2
1.4084
0
+2
À2
0
+2
a
b
c
d
e
1.4319
1.3973
1.3872
1.4376
1.4260
1.4290
1.4109
1.4239
1.4068
1.4352
1.4049
1.4351
1.3978
1.4306
1.3855
1.4483
1.3881
1.4370
1.3865
1.46
1.4229
[a] Conditions: 1 mm in MeCN with Bu4NBF4 as the electrolyte.
[a] See Figure 5 for labels. The bond lengths were determined by B3LYP
calculations.
tion potentials of a series of tetraoxa[8]circulenes decrease
almost exponentially with the number of benzene rings in
the molecule. At the same time, the first reduction poten-
tials for this series of molecules have identical values. This
trend is not observed for the two azatrioxa[8]circulenes
studied, with both the first reduction and oxidation poten-
tials relatively unaffected by the number of benzene rings in
the molecule.
NICS calculations were performed on both the tetra-
ACHTUNGTRENNUNGoxa[8]circulene 1 and the azatrioxa[8]circulene 3 (NH) to
address the question of the aromaticity/anti-aromaticity of
the central planar COTs (Figure 5).[12] Both the NICS(0)
and NICS(1)zz values were calculated at the B3LYP/6-311+
bond-length alternations in the eight-membered ring of 1
are 0.0145, 0.0346, and 0.0505 for the charge states À2, 0,
and +2, respectively. A similar trend is observed for 3 in
which the bond-length alternations are 0.013–0.018, 0.030–
0.037, and 0.046–0.063 for À2, 0, and +2. Therefore the
bond-length alternations were found to be significantly
lower for the aromatic species. Thus, the structural informa-
tion (calculated bond lengths) corroborates the conclusions
regarding (anti)aromaticity inferred from the NICS calcula-
tions.
1
The H NMR data of kekulene suggest that the peculiar
downfield shift of the inner protons is an argument against
viewing kekulene as an annulenoid ring system. Instead, ke-
kulene should be considered as a benzenoid system in which
the magnetic field in the inner ring should be considered as
arising from six linked benzene rings.[18] A similar hypothesis
could in principle be considered for tetraoxa[8]circulene,
that is, the observed (by NICS calculations) anti-aromaticity
in the neutral state is not due to an annulenoid ring system.
The NICS values suggest the anti-aromaticity can be ex-
plained by viewing the molecule as four linked benzene
rings. This hypothesis can, however, be rejected by consider-
ing the magnetic field at a point surrounded by benzene
rings. NICS calculations were carried out in which the
dummy atom was placed at an identical distance from the
benzene ring, as in the NICS calculations on tetraoxa[8]cir-
culene, which resulted in a NICS(0) value of around 1. A
similar calculation was carried out with two benzene rings
Figure 5. NICS(0) and NICS(1)zz (bold) values calculated for tetraoxa[8]-
circulene 1 and azatrioxa[8]circulene 3. The letters a–e refer to the bond
lengths in Table 2.
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with the benzene rings and dummy atom placed in a geome-
try identical to that of two opposite-lying benzene rings in
tetraoxa[8]circulene and the dummy atom placed equidis-
tant from the centre of the two rings. The calculated
NICS(0) value of around 2 is in accordance with the addi-
tion of two magnetic field vectors of the same magnitude. If
the electronic structure of tetraoxa[8]circulene is best de-
currently investigating their properties for use in OLEDs as
well as their electrical conductivity. Our computational re-
sults indicate that the planar COT in the heteroatom-substi-
tuted circulenes are anti-aromatic and become aromatic
upon two-electron reduction.
ACHTUNGTRENNUNGscribed as four benzene rings linked to a benzenoid system,
a NICS(0) value of around 4 should be expected, whereas a
value of about 8 was found. Invoking such an additivity
Experimental Section
General experimental procedures: TLC was carried out by using alumini-
um sheets precoated with silica gel 60F (Merck 5554). Dry column
vacuum chromatography was carried out with silica gel 60 (Merck 9385,
0.015–0.040 mm). Melting points were determined with a Bꢃchi melting
point apparatus and are uncorrected. 1H (500 MHz) and 13C NMR
(125 MHz) spectra were recorded with a Bruker spectrometer (500 MHz
CryoProbe). Samples were prepared in deuteriated solvents (CDCl3,
[D6]DMSO, CD2Cl2) purchased from Cambridge Isotope Labs. FAB-MS
was performed with a JEOL JMS-HX 110 Tandem Mass Spectrometer in
the positive ion mode with 3-nitrobenzyl alcohol (NBA) as the matrix.
EI-MS spectra were recorded with a ZAB-EQ (VG-Analytical) spec-
trometer. Microanalyses were performed with a FlashEA 1112. UV/Vis
spectra were recorded with a Cary 5E spectrophotometer (Varian Inc.)
with pure solvent as the baseline.
scheme on other fragments (benzoACHTUNTRGNEUNGfuran, dibenzofuran,
phenol and 3-hydroxydibenzofuran) of the tetraoxa[8]circu-
lene molecule resulted in values significantly lower than 8
(the largest value found was about 6).
The electronic structures of tetraoxa[8]circulene and aza-
trioxa[8]circulene are visualised with HOMO and LUMO
density plots (Figures 6 and 7). In particular, the HOMO or-
X-ray crystallography: All single-crystal X-ray diffraction data were col-
lected at 122(1) K on a Nonius–KappaCCD area-detector diffractometer
equipped with an Oxford Cryostreams low-temperature device using
graphite-monochromated MoKa radiation (l=0.71073 ꢄ). The structures
were solved by direct methods (SHELXS97) and refined by using
SHELXL97[19]. All non-hydrogen atoms were refined anisotropically and
hydrogen atoms were located in the difference Fourier map and refined
isotropically as riding on their parent atom in a fixed geometry. The mo-
lecular structure diagrams were prepared by using the Ortep-3[20] and
Mercury[21] 3.0.1 programs.
Figure 6. LUMO (top) and HOMO (bottom) density plots for azatriox-
a[8]circulene with charge state À2 (left), 0 (middle) and +2 (right).
Calculations: The Gaussian 03 suite[22] of programs was used for the DFT
calculations. Initially, geometry optimisations were carried out at the
B3LYP/6–31G(d) level of theory, with all the structures constrained to
C2v symmetry. Frequency calculations were then carried out to assure
that the structures were indeed minima on the potential energy surface
(no imaginary frequencies). TDDFT and single-point calculations on the
anion radicals were then carried out at the B3LYP/6–31G(d) level of
theory on these optimised structures. The Gaussian archive entries for
these calculations are included in the Supporting Information. Gauss-
View 3.0 was used for generating the orbital plots.
Figure 7. LUMO (top) and HOMO (bottom) density plots for tetraox-
Cyclic voltammetry: Tetrabutylammonium tetrafluoroborate (Aldrich,
98%) and acetonitrile (Lab-Scan, HPLC grade) were used as received.
The cell was a cylindrical vial equipped with a Teflon top with holes to
accommodate the platinum working electrode, the platinum counter elec-
trode and Fc/Fc+. The electrochemical equipment was purchased from
CHI instruments (CH630) with iR compensation. The solutions for vol-
tammetry were 1 mm in substrate and made by dissolving an accurately
weighed amount of the substrate in the required volume of a MeCN/
Bu4NBF4 (0.1m) solution. The volumes of the resulting solutions were
typically between 5 and 10 mL. The solutions were purged with nitrogen
saturated with MeCN for at least 10 min prior to the measurements.
a[8]circulene with charge state À2 (left), 0 (middle) and +2 (right).
bitals of azatrioxa[8]circulene in oxidation states 0 and +2
suggest that the electronic structure cannot be represented
by four linked benzene rings as the orbitals indicate that the
electron density is spread over two benzene rings for elec-
trons in the HOMO orbitals.
The HOMO of tetraoxa[8]circulene in the +2 oxidation
state also indicates that the electronic structure is better rep-
resented by an annulene electronic structure inasmuch as
there is a mirror plane intersecting the heteroatoms.
Synthesis of azatrioxa[8]circulene 3: 2-tert-Butyl-1,4-benzoquinone
(697 mg, 4.24 mmol) and 5 (500 mg, 1.41 mmol) were dissolved in di-
chloromethane (40 mL) in a flame-dried flask under an atmosphere of ni-
trogen. Freshly distilled BF3·OEt2 (1 mL) was added to the solution and
the reaction mixture was stirred over night at room temperature. Then
2m aq. HCl (30 mL) was added and the phases separated. The aqueous
phase was extracted with CH2Cl2 (3ꢅ30 mL) and the combined organic
extracts were dried over MgSO4, filtered and concentrated. The crude
product was purified by dry column chromatography on silica eluting
with heptane/toluene mixtures (from pure heptane to pure toluene with a
10% gradient). Yield: 575 mg, 65%; m.p. 303–3048C; 1H NMR
(500 MHz, CDCl3): d=0.95–1.05 (m, 3H), 1.67 (s, 18H), 1.69 (s, 18H),
Conclusions
We have developed the first protocol for the synthesis of
azatrioxa[8]circulenes. The two azatrioxa[8]circulenes pre-
pared have excellent blue emission properties and we are
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M. Pittelkow et al.
1.94–2.07 (m, 2H), 4.45 (t, J=7.5 Hz, 2H), 7.34 (s, 2H), 7.67 ppm (s,
2H); 13C NMR (126 MHz, CDCl3): d=12.11, 23.31, 30.36 (2C), 35.16,
35.25, 45.42, 105.07, 107.23, 112.21, 115.01, 116.56, 116.94, 133.93, 134.47,
136.98, 150.77, 151.10, 152.71 ppm; MS (MALDI-TOF): m/z: 625; ele-
mental analysis calcd (%) for C43H47NO3: C 82.52, H 7.57, N 2.24; found:
C 82.50, H 7.41, N 2.35.
2H), 4.15 (t, J=7.3 Hz, 2H), 7.20 Hz, (d, J=8.9 Hz, 2H), 7.48 (dd, J=
8.9 Hz, J=2.1 Hz, 2H), 8.07 ppm (d, J=2.1 Hz, 2H); 13C NMR
(126 MHz, CDCl3): d=139.40, 129.01, 123.44, 123.26, 111.95, 110.44,
44.88, 22.23, 11.73 ppm; MS (GCMS): m/z: 367.0; elemental analysis
calcd (%) for C15H13Br2N: C 49.08, H 3.57, N 3.82; found: C 48.92, H
3.68, N 3.96.
Synthesis of 3,6-dimethoxy-N-propylcarbazole (9): Dibromocarbazole 8
(50 g, mmol) was dissolved in dry DMF (200 mL) under an atmosphere
of nitrogen. NaOMe in methanol (500 mL, 25% in MeOH) was added
over 10 min during which time CuII (20 mol-%) was added in small por-
tions. The reaction mixture was maintained under nitrogen and heated at
reflux overnight. The resulting brown suspension was treated with aque-
ous ammonia (24% in H2O) to give a blue suspension. The suspension
was extracted with dichloromethane (3ꢅ500 mL) and the combined or-
ganic extracts were washed with water (2ꢅ200 mL). The organic phase
was dried over anhydrous MgSO4, filtered and the solvent removed
under reduced pressure. The crude product was crystallised from metha-
nol. Yield: 32.3 g, 88%; m.p. 73–748C; 1H NMR (500 MHz, [D6]DMSO):
d=0.85 (t, J=7.5 Hz, 3H), 1.76 (sext., J=7.5 Hz, 2H), 3.88 (s, 6H), 4.29
(t, J=7.5 Hz, 2H), 7.06 (d, J=9.0 Hz, 2H), 7.47 (d, J=9.0 Hz, 2H),
7.73 ppm (s, 2H); 13C NMR (126 MHz, [D6]DMSO): d=152.67, 135.57,
122.14, 114.80, 109.98, 103.07, 55.60, 43.81, 21.99, 11.36 ppm; MS
(GCMS): m/z: 269.2; elemental analysis calcd (%) for C17H19NO2: C
75.81, H 7.11, N 5.20; found C 75.67, H 6.89, N 5.30.
Synthesis of azatrioxa[8]circulene 4: Naphthoquinone (4.24 mmol) and 5
(500 mg, 1.41 mmol) were dissolved in dichloromethane (40 mL) in a
flame-dried flask under an atmosphere of nitrogen. Freshly distilled
BF3·OEt2 (1 mL) was added to the solution and the reaction mixture was
stirred over night at room temperature. Then 2m aq. HCl (30 mL) was
added and the phases separated. The aqueous phase was extracted with
CH2Cl2 (3ꢅ30 mL) and the combined organic extracts were dried over
MgSO4, filtered and concentrated. The crude product was purified by dry
column chromatography using heptane/toluene mixtures as the eluent.
Yield: 651 mg, 75%; m.p. 285–2868C; 1H NMR (500 MHz, CDCl3): d=
1.11 (t, J=7.4 Hz, 3H), 1.86 (s, 18H), 2.13 (sext., J=7.4 Hz, 2H), 4.59 (t,
J=7.4 Hz, 2H), 7.53 (s, 2H), 7.69–7.75 (m, 4H), 8.62–8.68 ppm (m, 4H);
13C NMR (126 MHz, CDCl3): d=150.09, 148.67, 148.61, 137.26, 133.65,
125.99, 125.82, 122.13, 121.67, 120.79, 120.61, 117.17, 113.90, 112.93,
112.08, 104.47, 45.46, 35.41, 30.59, 23.24, 12.14 ppm; MS (MALDI-TOF):
m/z: 613; elemental analysis calcd (%) for C43H35NO3: C 84.15, H 5.75, N
2.28; found: C 84.35, H 7.85, N 2.22.
Synthesis of dihydroxycarbazole 5: Under an atmosphere of nitrogen, 10
(5.0 g, 13.1 mmol) was added to dry dichloromethane (50 mL) in a flame-
dried round-bottomed flask. The solution was cooled to À788C in an ace-
tone/dry-ice bath. At this temperature, BBr3 (27 mL, 27 mmol, 1m in
CH2Cl2) was added dropwise over the course of 1 h. After stirring for 2 h
at À788C, the reaction mixture was allowed to warm to room tempera-
ture with stirring overnight. The reaction was quenched with 2m aq. HCl
(40 mL) and the resulting two-phase system extracted with dichlorome-
thane (3ꢅ50 mL). After drying over anhydrous MgSO4 and filtration, the
solvent was removed under reduced pressure and the crude product was
purified by dry column chromatography (EtOAc/heptane mixtures) to
yield a white powder that turns slightly pink when left in air. Yield:
3.94 g, 85%; m.p. 213–2148C; 1H NMR (500 MHz, [D6]DMSO): d=0.87
(t, J=7.5 Hz, 3H), 1.47 (s, 18H), 1.76 (sext., J=7.5 Hz, 2H), 4.21 (t, J=
7.5 Hz, 2H), 7.20 (s, 2H), 7.24 (s, 2H), 8.85 ppm (s, 2H; OH); 13C NMR
(126 MHz, [D6]DMSO): d=11.67, 22.10, 29.75, 34.95, 43.46, 105.55,
106.16, 119.37, 134.49, 134.71, 148.88 ppm; MS (GCMS): m/z: 353.2; ele-
mental analysis calcd (%) for C23H31NO2: C 78.15, H 8.84, N 3.96;
found: C 78.30, H 8.85, N 3.91.
Synthesis of 2,7-di-tert-butyl-3,6-dimethoxy-N-propylcarbazole (10):
Under an atmosphere of nitrogen, dimethoxycarbazole
37.1 mmol) was dissolved in dry dichloromethane (100 mL). Anhydrous
iron(III) chloride (6 g, 37.1 mmol) was then added to the solution. The
9 (10 g,
AHCTUNGTRENNUNG
reaction mixture was cooled to 08C and 2-chloro-2-methylpropane
(40 mL) was added. The reaction mixture was stirred overnight. Another
portion of ironACTHNUGRTNEUNG(III) chloride (3 g) was then added. The reaction was stir-
red under an atmosphere of nitrogen for another 2 h. The reaction was
quenched with 2m aq. HCl (100 mL) and the phases were separated. The
aqueous phase was extracted with dichloromethane (3ꢅ100 mL) and the
combined organic phases were dried over anhydrous MgSO4, filtered and
concentrated. The crude product was crystallised from 96% ethanol and
the resulting off-white crystals were washed several times with cold 96%
ethanol. Yield: 10.6 g, 75%; m.p. 256–2588C; 1H NMR (500 MHz,
CDCl3): d=0.90 (t, J=7.4 Hz, 3H), 1.41 (s, 18H), 1.81 (sext., J=7.4 Hz,
2H), 3.90 (s, 6H), 4.07–4.23 (m, 2H), 7.18 (brs, 2H), 7.39 ppm (brs, 2H);
13C NMR (126 MHz, CDCl3): d=152.69, 137.10, 135.59, 120.13, 106.67,
102.48, 55.83, 53.43, 44.28, 35.48, 30.18, 11.99 ppm; MS (GCMS): m/z:
381.3; elemental analysis calcd (%) for C25H35NO2: C 78.70, H 9.25, N
3.67; found: C 79.02, H 9.43, N 3.48.
Synthesis of N-propylcarbazole (7): 9H-Carbazole (50.0 g, 280 mmol) was
suspended in
a mixture of toluene (400 mL) and 12m aq. NaOH
(400 mL). After stirring for 10 min, tetrabutylammonium iodide (11.2 g,
30 mmol) was added to the orange reaction mixture. Propyl bromide
(100 mL) was added and the reaction mixture was heated at reflux for
16 h. The reaction mixture was cooled to room temperature, the phases
were separated and the aqueous phase extracted with toluene (3ꢅ
150 mL). The combined organic phases were dried over anhydrous
MgSO4 and filtered. The solvent was removed under reduced pressure
and the resulting yellow crude product was recrystallised from ethanol.
Yield: 61.3 g, 98%; m.p. 47–488C; 1H NMR (500 MHz, CDCl3): d=1.02
(t, J=7.4 Hz, 3H), 1.97 (sext., J=7.4 Hz, 2H), 4.32 (t, J=7.4 Hz, 2H),
7.25–7.30 (m, 2H), 7.46 (d, J=8.3 Hz, 2H), 7.48–7.53 (m, 2H), 8.15 ppm
(d, J=7.2 Hz, 2H); 13C NMR (126 MHz, CDCl3): d=140.52, 125.56,
122.81, 120.34, 118.71, 108.70, 44.64, 22.34, 11.85 ppm; MS (GCMS): m/z:
209.2; elemental analysis calcd (%) for C15H15N: C 86.08, H 7.22, N 6.69;
found: C 86.00, H 7.11, N 6.82.
Acknowledgements
We are grateful to The Lundbeck Foundation for a “Young Group
Leader Fellowship” (M.P.) and The Danish Research Council for Inde-
pendent Research for a “Steno Fellowship” (M.P.), a postdoctoral fellow-
ship (T.B.N.) and an instrument grant (#09-066663). We thank Mrs Birgit-
ta Kegel at the Microanalytical Laboratory at the Department of Chem-
istry, University of Copenhagen for microanalyses.
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Planar Anti-Aromatic Cyclooctatetraenes
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14.8632(12), b=20.273(3), c=10.728(4) ꢄ, b=108.780(11)8, V=
3060.4(13) ꢄ3, T=122(1) K, space group P21/c, Z=4, m
ACHTUNGTRENNUNG(MoKa)=
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[14] a) Crystal data for 3: C43H47NO3, M=625.82, triclinic, a=11.277(5),
b=17.084(5), c=19.853(14) ꢄ, a=93.293(36), b=104.813(40), g=
3
¯
105.77(4)8, V=3525(3) ꢄ , T=122(1) K, space group P1, Z=4, m-
A
Received: September 3, 2012
Published online: && &&, 0000
(MoKa)=0.073 mmÀ1, 102729 reflections measured, 12375 independ-
Chem. Eur. J. 2013, 00, 0 – 0
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M. Pittelkow et al.
Aromaticity
C. B. Nielsen, T. Brock-Nannestad,
P. Hammershøj, T. K. Reenberg,
M. Schau-Magnussen, D. Trpcevski,
T. Hensel, R. Salcedo,
G. V. Baryshnikov, B. F. Minaev,
M. Pittelkow* ................. &&&& — &&&&
Planar cyclooctatetraenes: The first
synthesis of a new class of anti-aro-
matic planar cyclooctatetraenes is
described: the azatrioxa[8]circulenes
(see scheme). The azatrioxa[8]circu-
lenes were fully characterised by using
optical, electrochemical and computa-
tional techniques as well as by single-
crystal X-ray crystallography. The aro-
matic/anti-aromatic nature of the
planar cyclooctatetraenes is discussed
and compared with the isoelectronic
tetraoxa[8]circulenes.
Azatrioxa[8]circulenes: Planar Anti-
Aromatic Cyclooctatetraenes
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Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!