Diazadioxa[8]circulenes: Planar antiaromatic cyclooctatetraenes

Article abstract of DOI:10.1002/chem.201303194

In this paper we describe a new class of antiaromatic planar cyclooctatetraenes: the diazadioxa[8]circulenes. The synthesis was achieved by means of a new acid-mediated oxidative dimerization of 3,6-dihydroxycarbazoles to yield the diazadioxa[8]circulenes

Full text of DOI:10.1002/chem.201303194

FULL PAPER
DOI: 10.1002/chem.201303194
Diazadioxa[8]circulenes: Planar Antiaromatic Cyclooctatetraenes
Thomas Hensel,[a] Denis Trpcevski,[a] Christopher Lind,[a] Rꢀmi Grosjean,[a]
Peter Hammershøj,[a] Christian B. Nielsen,[a] Theis Brock-Nannestad,[a]
Bjarne E. Nielsen,[a] Magnus Schau-Magnussen,[a] Boris Minaev,[b]
Gleb V. Baryshnikov,[b] and Michael Pittelkow*[a]
Abstract: In this paper we describe
a new class of antiaromatic planar cy-
clooctatetraenes: the diazadioxa[8]cir-
culenes. The synthesis was achieved by
means of a new acid-mediated oxida-
tive dimerization of 3,6-dihydroxycar-
bazoles to yield the diazadioxa[8]circu-
lenes in high yields. The synthetic pro-
tocol appears to be general, and is
a one-pot transformation in which two
of water. We also present a detailed
characterization of the optical and elec-
trochemical properties of this new class
of stable planar cyclooctatetraenes. The
properties of the diazadioxa[8]circu-
lenes are compared with the properties
of isoelectronic tetraoxa[8]circulenes
and azatrioxa[8]circulenes. We discuss
the antiaromatic nature of the planar
central cyclooctatetraene moiety. The
antiaromatic nature of the planar cy-
clooctatetraenes was studied by using
computational methods (NICS calcula-
tions), and these calculations reveal
that the central eight-membered ring
has antiaromatic character.
Keywords: aromaticity
functional calculations
·
·
density
electro-
chemistry · fluorescence · heterocy-
cles
À
À
C C bonds and two C O bonds are
formed with the loss of two molecules
Introduction
ers.[11] Analysis of the experimental and computational data
collected on these systems suggest that planarized cycloocta-
tetraenes are antiaromatic.[8–10,12]
Antiaromatic compounds that are chemically stable are
rare. An attractive strategy to stabilize antiaromatics is to
fuse them in larger p-conjugated systems, giving structures
that contain formal aromatic- and antiaromatic regions.[1]
The antiaromatic regions in these types of systems often
impart unique electrochemical and optical properties on the
entire conjugated system. This strategy has been applied by
several teams for the preparation of large antiaromatic por-
phyrioids by the groups of Osuka, Sessler, and Latos-Gra-
Hçgberg and Erdtman established that when 1,4-benzo-
quinones and naphthoquinones are treated with strong acid,
tetrameric condensation products of the tetraoxa[8]circulene
type (1) are formed.[13] Hçgberg showed that 3,6-dihydroxy-
dibenzofurans are important intermediates in these conden-
sation reactions, and that isolated 3,6-dihydroxydibenzofur-
ans derived from naphthoquinone or benzoquinones react
with an additional two equivalents of 1,4-quinones to give
mixed tetraoxa[8]circulenes.[14] It has also been shown that
substitution of a 1,4-benzoquinone in the 2- and 3 positions
resulted in significantly improved yields of the tetraoxa[8]-
circulenes.[14] Inspired by these findings we showed that 2-
tert-butyl-1,4-benzoquinone tetramerizes to yield tetra-tert-
butyl-tetraoxa[8]circulene in a one-step procedure (Fig-
ure 1a).[15]
Recently we extended this work to provide the first syn-
thesis of azatrioxa[8]circulenes (2) by condensing a N-alkyl-
2,3-di-tert-butyl-3,6-dihydroxycarbaxole (4) with a benzoqui-
none.[16] We reasoned that an N-alkyl-2,7-di-tert-butyl-3,6-di-
hydroxycarbaxole (4) would be ideal to (through a dimeriza-
tion reaction) form the diazadioxa[8]circulene (3). The two
tert-butyl-substituents serve to prevent polymerization.
However, since the proposed mechanism requires a Mi-
chael-type reaction between a “hydroquinone”-type moiety
and a “quinone”-type moiety, followed by acid-mediated de-
hydration, an oxidant was needed. We proposed the reaction
pathway depicted in Scheme 1.
[2–4]
´
z˙ynski,
indenoACHTUNGTRENNUNG
[1,2-b]fluorenes by Haley et al.,[5] and
hetero-acenes by the group of Bunz.[6] A particular funda-
mental area that has attracted interest is the studies of the
aromatic/antiaromatic properties of planarized conjugated
cyclooctatetraenes.[7] Several experimental strategies have
been explored and fully conjugated systems have been pre-
pared by Osuka,[8] Nenajdenko,[9] and us.[10] Completely non-
fused systems have been prepared by Komatsu and co-work-
[a] T. Hensel, D. Trpcevski, C. Lind, R. Grosjean, Dr. P. Hammershøj,
Dr. C. B. Nielsen, Dr. T. Brock-Nannestad, B. E. Nielsen,
M. Schau-Magnussen, Dr. M. Pittelkow
Department of Chemistry, University of Copenhagen
Universitetsparken 5, 2100 Copenhagen Ø (Denmark)
Fax : (+45)35320212
E-mail: pittel@kiku.dk
[b] Prof. B. Minaev, G. V. Baryshnikov
Bohdan Khmelnytsky National University
18031, Cherkasy (Ukraine)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303194.
Chem. Eur. J. 2013, 19, 17097 – 17102
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
17097

prepared by using the protocol depicted in Scheme 2. Carba-
zol (5) was benzylated by using a two-phase protocol to give
the N-benzyl carbazol (6) in high yield. The N-benzyl carba-
Scheme 2. Synthesis of N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbaxole
(4Bn).
zol (6) was brominated selectively twice in the position para
to the nitrogen to yield the dibromide (7), which was effec-
tively transformed into the dimethoxy derivative 8 by treat-
ment with NaOMe/MeOH and CuI in DMF. The tert-butyla-
tion proved sensitive to the choice of reagents, but a mixture
of anhydrous FeCl3 and tert-butyl chloride gave a clean con-
version to the 2,7-bis-tert-butyl-3,6-dimethoxy carbazole (9).
The procedure is chromatography free up to this point. Se-
lective demethylation of 9 proved difficult due to competi-
tive debenzylation with most acidic (e.g., BBr3 or AlCl3 in
CH2Cl2) and nucleophilic reagents (e.g., LiI in pyridine,
NaSBu in DMF, PhSeNa in DMF). It turned out that a com-
bination of BCl3 and Bu4NI in CH2Cl2 selectively cleaved
the methyl groups and left the benzyl substituent on the ni-
trogen intact, and thus yielded the desired dihydroxy com-
pound 4Bn.
Figure 1. Structures and schematic retrosynthetic analyses of tetraoxa[8]-
circulenes (1), azatrioxa[8]circulenes (2), and the new diazadioxa[8]circu-
lenes (3).
We were delighted to find that when a N-alkyl-2,7-di-tert-
butyl-3,6-dihydroxycarbaxole was treated with an oxidizing
agent
(e.g.,
2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) or chloranil) and a Lewis acid (e.g., BF3·OEt2,
FeCl3, or AlCl3) diazadioxa[8]circulenes (3) form in 71–85%
yield (Scheme 3).
Unambiguous evidence for the synthetic outcome came
from the single-crystal X-ray crystallography (Figure 2). The
crystals of 3Pro and 3Bn were grown by slow evaporation of
CH2Cl2 from a mixture of CH2Cl2/ethanol (1:1) giving small
pale-yellow needles. The presence of four tert-butyl groups
on the diazadioxa[8]circulenes prevents p-stacking to domi-
nate the crystal packing. Both diazadioxa[8]circulene possess
Scheme 1. Proposed reaction pathway for the oxidative acid-mediated
formation of a diazadioxa[8]circulene (3).
Results and Discussion
To test the hypothesis regarding the synthetic pathway we
used two different N-alkyl-2,7-di-tert-butyl-3,6-dihydroxycar-
baxoles (4, R=propyl or benzyl). The N-propyl (4Pro) we
prepared previously, and the N-benzyl (4Bn) derivative was
cyclic 8p-electron conjugation at the central cycloocta
ACHTUNGTRENNUNGtetra-
AHCTUNGTREGeNNNU ne (COT) and the structure is almost completely planar,
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Diazadioxa[8]circulenes
FULL PAPER
Scheme 3. Synthesis of diazadioxa[8]circulene (3). R=propyl (3Pro) or
benzyl (3Bn). Conditions: oxidant (chloranil or DDQ), Lewis acid
(BF3·OEt2, FeCl3, or AlCl3), CH2Cl2, yield: 71–85%.
&
*
Figure 3. a) UV/Vis and b) fluorescence spectra of 1 ( ; 0N), 2Pro
( ; 1N),
~
and 3Pro
(
; 2N) recorded in CH2Cl2.
Figure 2. Single-crystal X-ray structure of 3Pro (left) and 3Bn (right). The
top pictures show a side-view illustrating the planarity of the diaza-
ACHTUNGTRENNUNGdioxa[8]circulenes and the bottom images show the front view of the
structures.
B3LYP/6-31G(d), B3LYP/cc-pVDZ, and CAM-B3LYP/cc-
pVDZ level by using the response methodology as imple-
mented in the Dalton quantum chemistry program. Addi-
tional calculations were carried out using the TD-DFT
method implemented in Gaussian 03 on 1, 2, and 3Pro at the
B3LYP/6-31G(d) level of theory. The CAM-B3LYP func-
tional is known to describe excited states better than other
density functionals mainly due the increased flexibility in
the exchange functional caused by the “switching on” and
increasing the “exact” Hartree–Fock exchange into the
Kohn–Sham orbitals as the interelectronic distance increas-
es.[17] This functional has been shown to provide accurate de-
scriptions of two-photon absorption cross sections.[18–19]
Previous calculations on comparatively large aromatic sys-
tems have indicated that the cc-pVDZ basis set is sufficient
to describe the excitation energies and it was also shown
that augmented functions and even larger basis sets such as
the cc-pVTZ basis set do not alter the calculated values for
the excitation energies.[7,20] Excitation energies and oscillator
strengths for the different molecules are presented in
Table S1 (see the Supporting Information) along with desig-
nations or relevant symmetries.
and only minor bond-length alterations are observed in the
eight-membered ring. These bond length alterations are sim-
ilar to those observed in tetraoxa[8]circulene (between 1.40
and 1.44 ꢁ).[7]
The UV/Vis and fluorescence spectra of tetraoxa[8]circu-
lene (1), azatrioxa[8]circulene, (2Pro) and diazadioxa[8]circu-
lene (3Pro) are shown in Figure 3. Three common electronic
transitions are centered at ꢀ260 (ꢀ4.8 eV), ꢀ360
(ꢀ3.4 eV), and ꢀ425 nm (ꢀ2.9 eV) with varying intensities.
The oscillator strength and Franck-Condon factors for each
transition are not identical when comparing the transitions
at, for example, ꢀ425 nm. The transition at ꢀ425 nm is
weak in intensity in the spectrum of 1 (extinction coefficient
of ꢀ5000mÀ1 cmÀ1) when comparing to the spectra of 2Pro
and 3Pro (extinction coefficients of ꢀ18000–25000mÀ1 cmÀ1).
The S1 energies were deduced from the UV/Vis and fluores-
cence spectra to be ꢀ2.9 eV for all three heterocyclic [8]cir-
culenes.
To investigate the effect of symmetry and selection rules
for the individual electronic transitions in the heterocyclic
[8]circulenes, the completely unsubstituted compounds were
investigated computationally (1model, 2model, and 3model). Calcu-
lations of the electronic transitions were carried out at the
In validating the performance of the theoretical levels it
was noted that the B3LYP/cc-pVDZ and B3LYP/6-31G(d)
level of theories adequately predicted the observed UV/Vis
spectra. For example, compound 1 has a band centered
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M. Pittelkow et al.
around 3.61 eV/ꢀ360 nm, which corresponds well to the cal-
culated excitation energy of 3.45 eV for 1model. The calcula-
tions on the model compound (1model) predicted that this
energy state was degenerate resulting in total oscillator
strength of 0.48. The oscillator strengths for the correspond-
ing transitions of 2model and 3model both are 0.23, in accord-
ance with the UV/Vis spectra. Calculations carried out at
the CAM-B3LYP level (see the Supporting Information)
consistently overestimate the excitation energies leading to
suggest that the ground state may not be very well described
with this functional.
Aromaticity/antiaromaticity of the different rings in the
diazadioxa[8]circulene was evaluated by using NICS calcula-
tions (Figure 4). Both NICS(0) and NICS(1)zz values were
The maximum extinction coefficient for 2 and 3Pro of the
band centered at ꢀ425 nm is approximately half the extinc-
tion coefficient of the corresponding band for 1. The band
observed at ꢀ2.9 eV/ꢀ425 nm in the spectra of 2 and 3Pro
has higher oscillator strengths than what is observed in the
spectrum of 1. This finding was also confirmed by the calcu-
lations in which the lowest symmetry allowed transitions for
1model is at 3.45 eV, whereas the lowest symmetry allowed
transitions for 2model and 3model are at 3.16 eV. The highest
symmetry groups possible for 1model, 2model, and 3model are
D4h, D2h and C2v, respectively, whereas the symmetry is
lowered when alkyl groups are considered, for example, the
highest symmetry group for 1, 2, and 3Pro is Cs. Calculations
carried out on 1, 2Pro, and 3Pro are presented in the Support-
ing Information. Some transitions rendered forbidden by
symmetry are now allowed. The lowest transitions for 2 and
3Pro are 3.00 and 2.92 eV, respectively, which corresponds
well with the S1 energies deduced from the UV/Vis and fluo-
rescence spectra. However, the transitions moments are
weak, which also explains the comparatively lower quantum
yields observed (0.3 for 2 and 3). We observed the same
phenomenon for 1, in which calculations on 1model give
a value for the energy of S1 to 2.95 eV. According to the cal-
culations, this transition is symmetry forbidden explaining
the low quantum of 0.1 for 1. In previous work, two fluores-
cence lifetimes were found for tetra-tert-butyl-tetraoxa[8]cir-
culene, which can be ascribed the symmetry forbidden
nature of the S1 state for this compound making decay from
the S2 to the ground state viable. Solvent effects were
probed by computations using a PCM model and dielectric
constants for dichloromethane. A negligible shift in excita-
tion energies was observed compared to the values found
from the calculations in vacuo (see the Supporting Informa-
tion).
Figure 4. NICS(0) and NICS(1)zz (lower numbers) values calculated for
diazadioxa[8]circulene (3Me). For comparison, the same values for tet-
raoxa[8]circulene (1) and azatrioxa[8]circulene (2) are given in the Sup-
porting Information.
calculated at the B3LYP/6-311+GACHTUNTRGNEUNG(d,p) level of theory by
using geometries obtained from B3LYP/6-31+G(d) calcula-
tions.[7,20] The results suggest that in the neutral form, the
planarized COTs of 1, 2 and 3 are antiaromatic (positive
values). Meanwhile the benzene, the furan and pyrrole rings
are aromatic, which is in accordance with our previous
work. In the case of the doubly oxidized (2+) 1–3 our calcu-
lations indicate that the COTs retain their antiaromaticity,
whereas in the doubly reduced (2À) species the COTs are
aromatic (large negative values). In all oxidation states stud-
ied, the benzene, furan, and pyrrole rings appear to retain
their aromaticity.
Conclusion
We have presented the first synthetic protocol for a diaza-
dioxa[8]circulene (3) and have compared its properties with
those of its isoelectronic tetraoxa[8]circulene (1) and aza-
trioxa[8]circulene (2).
Experimental Section
General experimental procedures: Thin-layer chromatography (TLC)
was carried out using aluminium sheets pre-coated with silica gel 60F
(Merck 5554). Dry column vacuum chromatography was carried out
using silica gel 60 (Merck 9385, 0.015–0.040 mm). Melting points were de-
termined on a Bꢂchi melting point apparatus and are uncorrected. 1H-
(500 MHz) and 13C NMR (125 MHz) spectra were recorded on a Bruker
instrument. Samples were prepared by using deuterated solvents (CDCl3,
[D6]DMSO, CD2Cl2) purchased from Cambridge Isotope Labs. Fast atom
bombardment (FAB) spectra were obtained on a Jeol JMS-HX 110
Tandem Mass Spectrometer in the positive ion mode using 3-nitrobenzyl
alcohol (NBA) as matrix. EI-MS spectra were recorded on a ZAB-EQ
(VG-Analytical) instrument. Microanalyses were performed by Mrs Bir-
gitta Kegel at the Microanalytical Laboratory at the Department of
Chemistry, University of Copenhagen. UV/Vis spectra were recorded on
a Cary 5E (Varian Inc.) with pure solvent as baseline.
We have previously observed that the first oxidation po-
tentials of a series of p-extended tetraoxa[8]circulenes de-
crease almost exponentially with the number of benzene
rings in the molecules.[10] At the same time, the first reduc-
tion potentials for this series of molecules have identical
values. This trend is not observed for the series studied here.
The reduction potentials for 2 (À2.52 V) and 3Pro (À1.55 V)
clearly differs from the reduction potential of À2.40 V for 1.
The first oxidation potential for 2 and 3Pro are identical
(0.67 V), whereas the second potential is sensitive to the
number of nitrogen atoms in the ring system.
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 ꢁ). The structures
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Diazadioxa[8]circulenes
FULL PAPER
were solved using direct methods (SHELXS97) and refined using the
SHELXL97 software package. All non-hydrogen atoms were refined ani-
sotropically, hydrogen atoms were located in the difference Fourier map
and refined isotropically as constraint riding their parent atom in a fixed
geometry. The molecular structure diagrams were made with the Ortep-3
and Mercury 3.0.1 programs.
128.79, 137.20, 140.69 ppm; GC-MS: tR =25.874 min (m/z=257.2 [M+]);
elemental analysis calcd (%) for C19H15NC: C 88.68, H 5.88, N 5.44;
found: C 88.58, H 5.53, N 5.59;.
N-Benzyl-3,6-dibromo-9H-carbazole (7): The N-benzyl-9H-carbazole
(64.73 g, 251.5 mmol) was suspended in glacial acetic acid (500 mL) fol-
lowed by a drop-wise addition of bromine (13.5 mL, 41.85 g, 523 mmol)
in glacial acetic acid (500 mL) over the course of one hour at RT. After
addition of half the bromine, the solution turned clear, followed by the
formation of a white precipitate. More glacial AcOH (300 mL) was
added. After stirring for two hours, the reaction product was filtered and
the crystals washed with H2O (200 mL). The mother-liquor was treated
with water (ca. 400 mL) until no more product precipitated. It was again
filtered and washed with H2O (100 mL). The crude product was crystal-
lized from glacial AcOH (800 mL) and washed several times with cold
96% EtOH to yield a white solid. Yield: 102.73 g, 98%. M.p. 151–1528C;
TLC: Rf =0.38 (n-heptane/EtOAc, 5:1); 1H NMR (500 MHz,
[D6]DMSO): d=5.68 (s, 2H), 7.12 (d, 2H, J=7.5 Hz), 7.20–7.28 (m, 3H),
7.59 (d, 2H, J=8.5 Hz), 7.64 (d, 2H, J=8.5 Hz), 8.51 ppm (s, 2H);
13C NMR (125 MHz, [D6]DMSO): d=45.76, 111.62, 111.92, 123.11,
123.52, 126.62, 127.38, 128.62, 128.99, 137.16, 139.23 ppm; GC-MS: tR =
30.159 min (m/z=415.0 [M+]); elemental analysis calcd (%) for
C19H13Br2N: C 54.97, H 3.16, N 3.37; found: C 55.04, H 2.92, N 3.34.
Crystal-structure data
Compound 3Bn. C54H54N2O2; Mr =762.99; monoclinic; a=23.778(8), b=
15.476(4), c=11.484(4) ꢁ, a=90, b=98.82(2), g=908; V=4176(2) ꢁ3;
T=122 K; space group Cc; Z=4; mACHTNUGRTNEUNG
(MoKa)=0.07 mmÀ1; 38033 reflections
measured, 7318 independent reflections (Rint =0.120). The final R1 values
were 0.062 [F2 >2s(F2)]. The final R1 values were 0.079 (all data). The
final wR(F2) (all data) values were 0.127. The goodness of fit on F2 was
1.11. CCDC-945390 contains the supplementary crystallographic data for
this compound. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Compound 3Pro
: C46H54N2O2; Mr =666.91; triclinic; a=10.951(2), b=
14.036(2), c=14.394(3) ꢁ;
a=110.48(2),
b=103.361(14),
g=
105.763(16)8; V=1859.6(8) ꢁ3; T=122 K; space group P1; Z=2;
¯
mACHTUNGTRENNUNG
(MoKa)=0.07 mmÀ1; 42204 reflections measured, 6539 independent re-
flections (Rint =0.113). The final R1 values were 0.064 [F2 >2s(F2)]. The
final R1 values were 0.1018 (all data). The final wR(F2) (all data) values
were 0.123. The goodness of fit on F2 was 1.12. CCDC-945393 contains
the supplementary crystallographic data for this compound. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
N-Benzyl-3,6-dimethoxy-9H-carbazole (8): The N-benzyl-3,6-dibromo-
9H-carbazole (35.0 g, 84 mmol) was dissolved in dry DMF (500 mL)
under a nitrogen atmosphere. CuII (20 mol%, 3.21 g, 17 mmol) and
NaOMe in methanol (840 mL, 25% in MeOH) were added at once, re-
spectively. The reaction mixture was kept under nitrogen and heated at
reflux overnight. The resulting suspension was treated with aqueous am-
monia (ca. 180 mL, 24% in H2O) to give a brown suspension, which
turned blue after the addition of water (300 mL). The suspension was ex-
tracted with dichloromethane (3ꢃ150 mL) and the combined organic ex-
tracts were washed with brine (2ꢃ80 mL). The organic phase was dried
over anhydrous MgSO4, filtered and the solvent was removed under re-
duced pressure. The crude product was crystallized from methanol (ca.
250 mL) to give a white crystalline solid. Yield: 25 g, 94%. M.p. 134–
1358C; TLC: Rf =0.38 (n-heptane/EtOAc, 5:1); 1H NMR (500 MHz,
[D6]DMSO): d=3.85 (s, 6H), 5.56 (s, 2H), 7.03 (dd, 2H, J=9, 2 Hz),
7.11 (d, 2H, J=7.5 Hz), 7.18–7.25 (m, 3H), 7.47 (d, 2H, J=9 Hz),
7.75 ppm (d, 2H, J=2 Hz). 13C NMR (125 MHz, [D6]DMSO): d=45.68,
55.60, 103.22, 110.26, 114.94, 122.48, 126.61, 127.10, 128.46, 135.61, 138.12,
152.98 ppm; GC-MS: tR =28.669 min (m/z=317.2 [M+]); elemental anal-
ysis calcd (%) for C21H19NO2: C 79.47, H 6.03, N 4.41; found: C 79.55, H
5.78, N 4.59.
Calculations: The Gaussian 03 suite of programs was used for the DFT
calculations. Initially, geometry optimizations were carried out at the
B3LYP/6–31G(d) level of theory, in which all the structures were con-
strained 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). TD-DFT calculations and single-
point calculations on the anion radicals were then carried out at the
B3LYP/6–31G(d) level of theory on these optimized structures. The
Gaussian archive entries for these calculations are included in the Sup-
porting Information. GaussView 3.0 was used for generating the orbital
plots.
Cyclic voltammetry: Tetrabutylammonium tetrafluoroborate, Bu4NBF4
(Aldrich, 98%) and acetonitrile, MeCN (Lab-Scan, HPLC grade) were
used as received. The cell was a cylindrical vial equipped with a Teflon
top with holes to accommodate the Pt working electrode, the Pt counter
electrode and the Fc/Fc+. The electrochemical equipment was from CHI
instruments (CH630), with iR-compensation. The solutions for voltam-
metry 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 volume of the resulting solutions was typi-
cally between 5 and 10 mL. The solutions were purged with nitrogen sa-
turated with MeCN for at least 10 min prior to the measurements.
N-Benzyl-3,6-dimethoxy-2,7-di-tert-butyl-9H-carbazole (9): Under a nitro-
gen atmosphere, the N-benzyl-3,6-dimethoxy-9H-carbazole (1.00 g,
3.2 mmol) was dissolved in dry dichloromethane (15 mL). IronACHTUNGTRENNUNG(III) chlo-
ride (anhydrous, 0.40 g, 2.5 mmol) was added to the solution. The reac-
tion mixture was cooled to 08C followed by a very slow drop-wise (four
hours) addition of 2-chloro-2-methylpropane (20 mL, 181.5 mmol) at
N-Benzyl-9H-carbazole (6): The 9H-carbazole (95%, 50.0 g, 280 mmol)
was suspended in toluene (350 mL) and 12m NaOH (400 mL) was added
to the solution. After stirring for 10 min, tetrabutylammonium iodide
(11.2 g, 30 mmol) was added to the orange reaction mixture under con-
stant stirring at room temperature. Benzylbromide (40 mL, 336 mmol)
was added at once, which resulted in a yellow reaction mixture after one
hour. The reaction mixture was left stirring at room temperature for two
hours. The conversion was determined by TLC on silica. The phases were
separated and the aqueous phase was extracted with dichloromethane
(3ꢃ150 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 recrystallized from 96% etha-
nol (200 mL). The crystals were washed with cold 96% ethanol
08C. After stirring overnight, another portion of ironACTHNUGRTENUNG(III) chloride (anhy-
drous, 0.40 g, 2.5 mmol) was added. The reaction was left to stir under
a nitrogen atmosphere for another two hours and the conversion was de-
termined by GC-MS. The reaction mixture was quenched with 2m HCl
(30 mL) and the phases where separated. The aqueous phase was extract-
ed with dichloromethane (3ꢃ30 mL). The combined organic phases were
dried over anhydrous MgSO4 and filtered. Thick purple/black oil was ob-
tained after removal of the solvent under reduced pressure. The crude
product was crystallized in 96% ethanol (ca. 25 mL) and the resulting
off-white solid was washed several times with cold 96% ethanol (ca.
15 mL). Yield: 0.98 g, 73%. M.p. 168–1698C; TLC: Rf =0.64 (n-heptane/
EtOAc, 5:1); 1H NMR (500 MHz, [D6]DMSO): d=1.39 (s, 18H), 3.92 (s,
6H), 5.56 (s, 2H), 7.18–7.23 (m, 3H), 7.28 (t, 2H, J=7 Hz), 7.34 (s, 2H),
7.70 (s, 2H); 13C NMR (125 MHz, [D6]DMSO): d=30.01, 35.10, 45.64,
55.79, 103.03, 107.00, 120.00, 126.90, 127.11, 128.94, 135.06, 136.29, 138.41,
152.40 ppm; GC-MS: tR =31.058 min (m/z=429.3 [M+]); elemental anal-
ysis calcd (%) for C29H35NO2: C: 81.08, H 8.21, N 3.26; found: C 81.14,
H 8.43, N 3.22.
(ꢀ80 mL) until white needle-shaped crystals of N-benzyl-9H-carbazole
were achieved. Yield: 70.1 g, 96%. M.p. 114–1168C. TLC: Rf =0.67 (n-
heptane/EtOAc, 4:1); 1H NMR (500 MHz, CDCl3): d=5.53 (s, 2H), 7.15
(d, 2H, J=7.5 Hz), 7.23–7.29 (m, 5H), 7.37 (d, 2H, J=7.5 Hz), 7.44 (t,
2H, J=7.5 Hz), 8.15 ppm (dt, 2H, J=7.5, 0.7 Hz); 13C NMR (125 MHz,
CDCl3): d=46.58, 108.91, 119.22, 120.41, 123.05, 125.86, 126.43, 127.46,
Chem. Eur. J. 2013, 19, 17097 – 17102
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
17101

M. Pittelkow et al.
N-Benzyl-3,6-dimethoxy-2,7-di-tert-butyl-9H-carbazole (4): Under a nitro-
gen atmosphere, the N-benzyl-3,6-dimethoxy-2,7-di-tert-butyl-9H-carba-
zole (500 mg, 1.13 mmol) and tetra-n-butylammonium iodide (838 mg,
2.27 mmol) were placed into a flame-dried round-bottomed flask. Boron
trichloride (11 mL, 11.00 mmol, 1m in CH2Cl2) was added over two mi-
nutes at room temperature. The reaction mixture was left stirring for
55 min and afterwards quenched with 1m HCl (80 mL). The resulting
two-phase system was extracted with dichloromethane (4ꢃ70 mL). After
drying the combined organic phases over anhydrous Na2SO4 and filtra-
tion, the solvent was removed under reduced pressure and the off-white
foam-like crude product was purified by using dry column chromatogra-
phy (MeCN/toluene, 1:40) to yield a white powder. Yield: 0.400 g, 86%.
M.p. 209–2128C; TLC: Rf =0.27 (n-heptane/EtOAc, 3:1); 1H NMR
(500 MHz, [D6]DMSO): d=1.40 (s, 18H), 5.47 (s, 2H), 7.18–7.29 (m,
5H), 7.23 (s, 2H), 7.24 (s, 2H), 8.87 ppm (s, 2H); 13C NMR (125 MHz,
[D6]DMSO): d=29.69, 34.94, 45.66, 105.56, 106.61, 119.66, 126.91, 127.03,
128.43, 134.60, 134.74, 138.62, 149.18 ppm; GC-MS: tR =26.368 min
(m/z=401.4 [M+]); elemental analysis calcd (%) for C27H31NO2: C 80.76,
H 7.78, N 3.49; found: C 80.55, H 7.84, N 3.43.
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BisACHTUNGTRENNUNG(N-benzyl)diazadioxatetra-tert-butyl[8]circulene (3Bn): Under a nitro-
gen atmosphere, the N-benzyl-3,6-dihydroxy-2,7-di-tert-butyl-9H-carba-
zole (180 mg, 0.45 mmol) and 2,3,5,6-tetrachloro-1,4-benzoquinone
(55 mg, 0.22 mmol) were dissolved in dichloromethane (20 mL) in
a flame-dried round-bottomed flask. To the solution BF3·OEt2 (0.06 mL,
0.45 mmol) was added. After 45 min, 1.5, and 3.5 h, 2,3,5,6-tetrachloro-
1,4-benzoquinone (55 mg, 0.22 mmol, 0.5 equiv) was added and also
(after 1.5 h) a onetime portion of BF3·OEt2 (0.06 mL, 0.45 mmol). The re-
action mixture was left stirring for another 30 min and was afterwards
quenched with 1m HCl (50 mL). The resulting two-phase system was ex-
tracted with dichloromethane (4ꢃ60 mL). After drying the combined or-
ganic phases over anhydrous Na2SO4 and filtration, the solvent was re-
moved under reduced pressure and the crude product was boiled with
KOH in toluene (10 mL, 100 equiv) and ethanol (10 mL) overnight. The
reaction mixture was quenched with 1m HCl (50 mL) and the separated
aqueous phase was extracted with dichloromethane (3ꢃ50 mL). After
drying the combined organic phases over anhydrous Na2SO4 and filtra-
tion, the solvent was removed under reduced pressure and the crude
product was purified by dry column chromatography (toluene/n-heptane,
1:12) to yield a yellow solid. Yield: 0.121 g, 71%. M.p. 301–3038C (de-
composition). TLC: Rf =0.61 (n-heptane/toluene, 2:1); 1H NMR
(500 MHz, CDCl3): d=1.73 (s, 36H), 5.80 (s, 4H), 7.26–7.33 (m, 5H),
7.43 ppm (s, 2H); 13C NMR (125 MHz, CDCl3): d=30.43, 35.08, 47.44,
104.95, 112.75, 116.80, 126.61, 127.39, 128.77, 133.35, 136.97, 138.02,
149.61 ppm; MALDI (method: TJS10de, polarity: positive): m/z=
762.084 [M+]; elemental analysis calcd (%) for C54H54N2O2: C 85.00, H
7.13, N 3.67; found: C 84.83, H 7.83, N 3.11.
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BisACHTUNGTRENNUNG(N-propyl)diazadioxatetra-tert-butyl[8]circulene (3Pro): Procedure sim-
ilar to that of 3Bn. Yield: 0.82 g, 85%. 1H NMR (500 MHz, CDCl3): d=
0.98 (t, J=7.3 Hz, 6H), 1.92 (s, 36H), 1.97–2.05 (hex, J=7.3 Hz, 4H),
4.44–4.54 (m, 4H), 7.40 ppm (brs, 4H); 13C NMR (125 MHz, CDCl3): d=
12.13, 23.23, 30.56, 35.09, 45.28, 104.63, 112.41, 116.75, 133.01, 136.81,
149.39 ppm; elemental analysis calcd (%) for C46H54N2O2: C 82.84, H
8.16, N 4.20; found: C 82.93, H 8.26, N 4.11.
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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.) and an instrument
grant (No. 09–066663).
Received: August 13, 2013
Published online: November 5, 2013
17102
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 17097 – 17102