π-electron conjugation effects in antiaromatic dehydro[12]- and aromatic dehydro[18]-annulenes

Article abstract of DOI:10.1039/b208115f

N,N-Dimethylanilino-substituted perethynylated dehydro[12]- and dehydro[18]-annulenes were prepared by oxidative acetylenic coupling of cis-bisdeprotected tetra-ethynylethene derivatives obtained by a new photochemical route; they display strongly bathoch

Full text of DOI:10.1039/b208115f

p-Electron conjugation effects in antiaromatic dehydro[12]- and
aromatic dehydro[18]-annulenes
Frieder Mitzel,a Corinne Boudon,b Jean-Paul Gisselbrecht,b Maurice Grossb and François Diederich*a
a Laboratorium für Organische Chemie, ETH-Hönggerberg, CH-8093, Zürich, Switzerland.
E-mail: diederich@org.chem.ethz.ch
b Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, UMR 7512, C.N.R.S, Université
Louis Pasteur, 4, rue Blaise Pascal, 67000, Strasbourg, France
Received (in Cambridge, UK) 19th August 2002, Accepted 4th September 2002
First published as an Advance Article on the web 18th September 2002
N,N-Dimethylanilino-substituted perethynylated dehy-
dro[12]- and dehydro[18]-annulenes were prepared by
oxidative acetylenic coupling of cis-bisdeprotected tetra-
ethynylethene derivatives obtained by a new photochemical
route; they display strongly bathochromically shifted long-
est-wavelength absorption bands compared to their silyl-
substituted counterparts resulting from efficient intra-
molecular charge-transfer between the peripheral p-electron
donors and the electron-accepting central acetylenic core.
several highly unstable intermediates,3 which has hitherto
prevented the preparation of functionalised derivatives of 1a
and 2a. Here we disclose a novel, much simpler photochemical
route to the donor/acceptor substituted TEEs 3b,c which now
has enabled the preparation and subsequent spectroscopic
investigation of the perethynylated dehydroannulenes 1b and
2b bearing lateral N,N-dimethylanilino substituents.
Pd-catalysed Sonogashira cross-coupling of trans-bisdepro-
tected TEE 4, which is readily available by a short synthetic
route,6 with 4-iodo-N,N-dimethylaniline afforded the trans-bis-
donor-substituted TEE 5 in 85% yield (Scheme 1).7 Irradiation
of 5 in diethyl ether with a medium-pressure Hg-lamp (125 W)
for 2 h led to photochemical equilibration between the cis- and
the trans-isomers.8 The diastereoisomeric mixture was sepa-
rated by column chromatography (SiO2, hexane/CH2Cl2 1+1),
yielding 41% of the desired cis-TEE 6 along with 53% of the
trans-isomer 5 that can be recycled. The same procedure was
applied to generate trans-7 and subsequently the cis-bis(ni-
trophenyl)-substituted TEE 8.†
Although aromatic and antiaromatic dehydroannulenes have
been the subject of numerous investigations,1,2 little is known
about the propagation of p-electron donor–acceptor conjuga-
tion across these macrocyclic perimeters. We had earlier
reported the preparation of the per(silylethynylated) dehy-
droannulenes 1a and 2a by oxidative coupling of cis-bisdepro-
tected tetraethynylethene (TEE) 3a and shown spectroscop-
ically that the smaller [12]annulene perimeter is paratropic
whereas the larger [18]annulene is diatropic.3 With their many
conjugated C(sp)-atoms, the all-carbon cores of these annulenes
are strong electron acceptors as revealed by electrochemical
investigations, with the reduction of antiaromatic 1a expectedly
being facilitated when compared with 2a.4,5 We have for some
time been inspired to functionalise these planar perimeters with
donor/acceptor groups in order to investigate intramolecular
charge-transfer effects in annulenes. However, cis-bisdepro-
tected TEEs, required for the oxidative macrocyclisation, had
only been available by a tedious multistep synthesis including
Silyl-deprotection of 6 was accomplished with Bu4NF in
moist THF. The cis-bisdeprotected TEE deteriorates rapidly in
neat form but is rather stable in solution. Subjecting the
deprotected species to Hay coupling conditions led to the
formation of dehydro[12]annulene 1b and dehydro[18]annu-
lene 2b, both as deep-purple solids. Higher macrocyclic
Scheme 1 Synthesis of the donor-substituted dehydroannulenes 1b and 2b.
Reagents and conditions: i, 4-iodo-N,N-dimethylaniline or 4-iodo-
nitrobenzene, [PdCl2(PPh3)2], CuI, Pri2NH, r.t., 14 h, 85% (5), 99% (7); ii,
Et2O, hn, r.t., 2 h, 41% (6), 39% (8); iii, Bu4NF, THF, 0 °C, 10 min; iv,
CuCl, N,N,NA,NA-tetramethylethylenediamine, O2, acetone, r.t., 2 h, 2%
(1b), 22% (2b).
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oligomers were not detectable in the MALDI-TOF mass
spectrum (matrix: 2-[(2E)-3-(4-tert-butylphenyl)-2-methyl-
prop-2-enylidene]malononitrile (DCTB)) of the crude reaction
mixture. The two macrocycles can be separated by repetitive
flash chromatography (SiO2; hexane/CH2Cl2 (1+1) ? CH2Cl2).
The ratio of the two formed annulenes is concentration-
dependent, however, the isolated yield of the [12]annulene 1b
always remained low as a result of its marked instability, which
led to a significant loss of product during work-up and
purification. In contrast, the [18]annulene 2b is remarkably
stable and can be handled without problems.† Attempts to apply
the same protocol to the preparation of nitrophenyl-substituted
dehydroannulenes starting from the corresponding TEE-pre-
cursor 8 were not successful.
The UV/Vis spectrum (CHCl3) of [18]annulene 2b (Fig. 1) is
dominated by a strong, broad longest-wavelength absorption
band at lmax = 518 nm (2.39 eV; e = 105200 M21 cm21) and
features an end-absorption (around 700 nm; 1.77 eV) that is
bathochromically shifted by more than 200 nm compared to that
of the corresponding silyl-substituted derivative 2a (480 nm,
2.57 eV).3 Upon acidification with toluene-4-sulfonic acid, the
colour of the solution changes from purple to yellow, which is
the colour of the silyl-substituted derivative 2a. Accordingly,
the intense absorption at 518 nm disappears nearly completely
and the absorption spectrum becomes strikingly similar to that
of 2a with the most intense band now being shifted to 427 nm
(2.90 eV; 2a: 405 nm, 3.06 eV). The residual absorptivity above
500 nm in the acidified solution may be explained by
incomplete hexa-protonation. Neutralisation with triethylamine
regenerates the original spectrum of 2b. This behavior identifies
the strong longest-wavelength absorption band at lmax = 518
nm as a charge-transfer band, resulting from efficient intra-
molecular charge-transfer interactions between the electron-
donating peripheral anilino groups and the electron-accepting
all-carbon core.
[18]annulene 2b are capable of mediating p-electron donor–
acceptor conjugation. However, the intensity of the charge-
transfer band of 1b is significantly weaker than that of 2b even
when the smaller number of donor–acceptor conjugation paths
is taken into account. This is somewhat surprising since one
might expect the charge-transfer to be more efficient in the
[12]annulene as the uptake of electrons reduces the anti-
aromaticity whereas in the case of the [18]annulene, the uptake
of electrons is accompanied by a loss of aromaticity. Clearly,
the nature of the electronic transitions in these compounds is
governed by additional factors that have yet to be elucidated in
theoretical calculations.
The redox properties of the dehydro[18]annulene 2b were
examined by cyclic voltammetry (CV) on a glassy carbon
working electrode in THF solution with 0.1 M Bu4NPF6 as the
supporting electrolyte. The CV features two well-resolved
reversible one-electron reduction steps (21.36 and 21.72 V, vs.
the ferrocene/ferricenium couple) demonstrating the electron-
acceptor properties of the central C30 core. The two reduction
steps occur at more negative potential than those of 2a (21.12
and 21.52 V)4 which is readily explained with the higher
electron density in the all-carbon core inflicted by the six anilino
groups. Additionally, the CV shows an irreversible third
reduction step at 22.27 V (peak potential at 0.1 V s21) and a
reversible, two-electron oxidation step at +0.25 V that can be
attributed to the oxidation of two, out of six, anilino groups. The
remaining anilino groups were oxidised in an irreversible
unresolved peak whose potential was observed at +0.4 V (v =
0.1 V s21). The redox properties of 1b could not be studied due
to its instability.
In summary, a straightforward photochemical access to cis-
bisdeprotected TEEs has enabled the synthesis of novel N,N-
dimethylamino-substituted perethynylated dehydro[12]- and
dehydro[18]-annulenes. As a result of the acetylenic spacers
between the donor groups and the electron-accepting macro-
cyclic perimeters, the novel chromophores adopt a fully planar
geometry and undergo intense intramolecular charge-transfer
interactions.
In agreement with its antiaromaticity, the silylated [12]annu-
lene 1a featured a much lower HOMO–LUMO gap (end-
absorption near 660 nm, 1.87 eV) than the corresponding
aromatic [18]annulene 2a (480 nm, 2.57 eV), with weak bands
between 490 and 620 nm (e = 200–400 M21 cm21) being
responsible for its buckminsterfullerene-type magenta–purple
colour.3 In the donor-substituted [12]annulene 1b, these weak
bands are now completely overlapped by an intramolecular
charge-transfer transition, featuring an absorption maximum at
(lmax = 518 nm (2.39 eV; e = 35100 M21 cm21) and end-
absorption (near 700 nm). Like in the case of 2b, the strong
lowest-energy band disappears nearly completely upon acid-
ification (Fig. 1). Thus, both, the [12]annulene 1b and the
Support by the ETH Research Council and the German Fonds
der Chemischen Industrie is gratefully acknowledged.
Notes and references
† All new compounds were fully characterised by IR, UV/Vis, 1H and 13
C
NMR, mass spectrometry and microanalysis.
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