Generation and characterization of highly strained dibenzotetrakisdehydro[12]annulene

Article abstract of DOI:10.1021/ja035079x

We report here the generation of tetrakisdehydro[12]annulene possessing a highly deformed triyne component from the [4.3.2]propellatriene-annelated precursor by its photolysis extruding indan and the characterization of the highly reactive annulene by che

Full text of DOI:10.1021/ja035079x

Published on Web 04/22/2003
Generation and Characterization of Highly Strained
Dibenzotetrakisdehydro[12]annulene
Yoshito Tobe,*,† Ichiro Ohki,† Motohiro Sonoda,† Hiroyuki Niino,‡ Tadatake Sato,‡ and
Tomonari Wakabayashi§
Department of Chemistry, Faculty of Engineering Science, Osaka UniVersity, and CREST, Japan Science and
Technology Corporation (JST), Toyonaka, Osaka 560-8531, Japan, Photoreaction Control Research Center,
National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki
305-8565, Japan, and DiVision of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto
606-8502, Japan
Received March 10, 2003; E-mail: tobe@chem.es.osaka-u.ac.jp
Scheme 1
Dehydroannulenes are compounds that have been studied both
experimentally and theoretically since the 1960s in connection with
the aromaticity-antiaromaticity of cyclic π electron systems.1
Recently, considerable interest has been focused on these com-
pounds because of their new aspects such as materials with
optoelectronic properties2 (e.g., second-order or third-order nonlinear
optical properties), segments of two-dimensional carbon networks3
(e.g., graphyne), precursors of onion- and tube-like closed shell
carbon clusters4 formed by their explosive decomposition, and
Scheme 2
precursors of cyclo[n]carbons5 (themselves members of dehydroan-
nulenes). We became interested in the title compound, dibenzotetra-
kisdehydro[12]annulene (1), because of the following reasons: (1)
It would provide information about the reactivity of the smallest
identified cyclocarbon, C12, because it possesses a remarkably
deformed triyne unit incorporated in a ring structure. (2) Aromaticity
under highly deformed circumstance is a subject of considerable
interest.6 (3) Decomposition or polymerization of 1 would form
novel forms of carbon cluster. (4) Cyclotrimerization7 of the central
a solution of 2 in THF-d8 was irradiated at room temperature with
triple bond of the triyne unit would give a partial structure of
a low-pressure mercury lamp, and the reaction was monitored by
graphyne.
1H NMR spectra. Upon irradiation, the signals of 2 decreased, and,
In this Communication, we report the generation of 1 and its
instead, the signals ascribed to indan appeared, indicating that [2+2]
characterization by the UV-vis and FTIR spectra in an argon matrix
cycloreversion of 2 took place. However, any signals ascribable to
at low temperature. In the geometry of 1 optimized by the density
1 were not observed, but those assigned to a [4+2] adduct of 1
functional theory (DFT) at the B3LYP/6-31G* level, the triple
and 2 were observed in a low intensity (see the Supporting
bonds in the triyne component are highly deformed from linearity;
Information), which was also characterized by LC-MS of the
the bond angle of the most strained triple bond is as small as 150°.8
photolyzate. To trap the reactive intermediate generated by the
To surmount this large strain, we planned to generate 1 by
photolysis of 2, irradiation was then carried out using furan as a
photochemical [2+2] cycloreversion of propellane-annelated de-
solvent. As a result, [4+2] adduct 5 was obtained in 20% yield
hydroannulene 2 extruding indan (Scheme 1), because we have
(Scheme 2).11 X-ray crystallographic analysis12 of 5 revealed that
already reported that a number of highly reactive polyynes were
the indan unit of 2 was replaced by the furan moiety, indicating
generated successfully from the corresponding precursors possessing
that the addition took place at the central, most strained triple bond
the [4.3.2]propellatriene unit under laser desorption as well as
of 1. These results indicate that the photolysis of 2 leads to the
generation of 1, which is too reactive to survive for characterization
in solution.
For spectroscopic characterization of 1, precursor 2 was dispersed
in an argon matrix film under 20 K, and its photolysis was
monitored by UV-vis and FTIR spectroscopic methods. When the
matrix was thick, photolysis of 2 took place only on the surface of
photochemical conditions.5c,9
The precursor 2 was prepared in 49% yield by in situ deprotection
and the cross-coupling reaction of bis(2-iodophenyl)acetylene 310
and diethynyl[4.3.2]propellatriene derivative 45c under the phase-
transfer conditions (Scheme 2).
First, we investigated [2+2] fragmentation of 2 under the laser-
desorption time-of-flight (LD-TOF) mass spectrometric condition.
the matrix presumably because of the filter effect of the products,
In the negative mode spectrum, an intense peak due to 1- formed
resulting in low conversion of 2. Accordingly, the thickness of the
by the loss of an indan fragment from 2 was observed (see the
film par deposition was reduced, and the deposition-irradiation
Supporting Information). Moreover, a peak due to an adduct formed
cycle was repeated to achieve enough intensity of the IR spectra.
by the [4+2] ion molecule reaction of 1 and 2 was also detected.
The UV-vis spectral change upon irradiation is shown in Figure
As a preliminary study of the photochemical [2+2] cycloreversion,
1. After the first irradiation, the absorption band of 2 at 359 nm
disappeared, and a new absorption band appeared at 355 nm. After
each deposition, the absorption band at 359 nm was newly added,
and, upon subsequent irradiation, this band decreased, while the
† Osaka University and CREST.
‡ AIST.
§ Kyoto University.
9
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10.1021/ja035079x CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
level predict two IR-active CtC stretching bands at 2191 and 2208
cm-1 for 2 and three bands at 2089, 2150, and 2188 cm-1 for 1.
Therefore, it should be highly possible that the observed three weak
bands are ascribed to 1. For C-H in-plane and out-of-plane bending
modes, DFT calculations predict IR modes at 1458 and 744 cm-1
for 1. Because these bands should overlap with those of 2 and indan
in the observed IR spectra (2a and 3a) after irradiation, we subtract
(b) and (c) from (a) with appropriate scaling factors to eliminate
the contribution from the latter two, yielding the spectra in 2d and
3d of Figure 2. As shown in spectra 2d and 3d of Figure 2, the
conspicuous bands at 1480 and 756 cm-1 are in good agreement
with the predicted C-H bending modes of 1. Thus, the observed
IR bands in (d) are, in total, in good agreement with the theoretical
IR spectrum of 1. This should be direct spectroscopic proof of the
generation of 1 in the argon matrix.
In conclusion, photolysis of propellane-annelated dehydro[12]-
annulene 2 led to the generation of dibenzotetrakisdehydro[12]-
annulene 1, a highly reactive, unsaturated annulene. Compound 1
was isolated in an argon matrix at 20 K or in a glass matrix of
MTHF at 77 K, and, under those conditions, it was characterized
by FTIR and UV-vis spectra.
Figure 1. UV-vis absorption spectra of precursor 2 upon irradiation with
fourth harmonic generation (FHG) pulses of Nd:YAG laser (λ ) 266 nm)
after deposition (thin line), after irradiation (bold line).
Acknowledgment. Support to I.O. from the 21st century COE
program “Integrated Ecochemistry” is gratefully acknowledged.
Supporting Information Available: Detailed procedures of the
synthesis of 2 and its photolysis in THF-d8, furan, MTHF, and argon
1
matrix, LD-TOF mass spectra, H NMR spectra upon irradiation in
THF-d8, UV-vis spectra in MTHF matrix, whole scale FTIR spectra
in argon matrix, and computational data (PDF); crystallographic
information of furan adduct 5 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
References
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Figure 2. Selected FTIR spectra of C-C stretching of acetylenes (column
1), in-plane C-H bending vibration (column 2), and out-of-plane C-H
bending vibration (column 3). (Top) Observed spectra: (a) after irradiation,
(b) precursor 2, (c) indan, and (d) differential spectra subtracting (b) and
(c) from (a). (Bottom) Theoretical IR spectra calculated at the B3LYP/
6-31* level: precursor 2 (green), 1 (blue), and indan (red).
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(8) See the Supporting Information for the predicted geometry of 1 optimized
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(11) The starting material 2 was recovered in 50% yield.
(12) Crystal data for furan adduct 5: C24H12O, M ) 316.36, monoclinic, space
group P21/n, a ) 5.9175(1) Å, b ) 17.8060(1) Å, c ) 15.1418(1) Å, â
) 93.1104(6), V ) 1593.11(2) Å3, Z ) 4, Fcalcd ) 1.319 g/cm3, final R
indices (I > 2σ(I)) R ) 0.067, Rw ) 0.100, GOF ) 0.76. Detailed data
are shown in the Supporting Information.
band at 355 nm increased. The deposition-irradiation cycle was
repeated seven times, accumulating enough quantity of 1 for IR
measurement. Irradiation of 2 in a glass matrix of 2-methyltetrahy-
drofuran (MTHF) at 77 K resulted in the growth of a similar new
band at 361 nm which was lost when the matrix was thawed by
warming to room temperature and then refrozen at 77 K (see the
Supporting Information).
The FTIR spectra in argon matrix observed after irradiation are
shown in Figure 2a together with those of unphotolyzed precursor
2 and indan (Figure 2b and c). In spectra 2a and 3a, two IR bands
at 1481 and 737 cm-1 are appearing, which are ascribed to C-H
in-plane and out-of-plane bending modes of the photoproducts,
respectively. In the high-frequency region (spectra 1a), three weak
IR bands at 2117, 2149, and 2170 cm-1 appeared and are attributed
to the stretching modes of C(sp)-C(sp) bonds. These frequencies
are slightly lower than those observed for the precursor 2 (2199
and 2222 cm-1).
It has been established that theoretical IR spectra by the DFT
calculation can well reproduce the experimental spectra and can
be used to identify the unusual molecules isolated in low-
temperature matrixes.13 Theoretical spectra at the B3LYP/6-31G*
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Sato, T.; Arulmozhiraja, S.; Niino, H.; Sasaki, S.; Matsuura, T.; Yabe, A.
J. Am. Chem. Soc. 2002, 124, 4512-4521.
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