Thermal cyclotrimerization of tetraphenyl[5]cumulene (tetraphenylhexapentaene) to a tricyclodecadiene derivative

Article abstract of DOI:10.1039/b815620d

Tetraphenyl[5]cumulene (tetraphenylhexapentaene) underwent cyclotrimerization in refluxing toluene for 10-15 min to give a tricyclodecadiene derivative in 68% yield. The Royal Society of Chemistry.

Full text of DOI:10.1039/b815620d

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Thermal cyclotrimerization of tetraphenyl[5]cumulene
(tetraphenylhexapentaene) to a tricyclodecadiene derivativew
Nazrul Islam,a Takashi Ooi,b Tetsuo Iwasawa,a Masaki Nishiuchia
and Yasuhiko Kawamura*a
Received (in College Park, MD, USA) 8th September 2008, Accepted 24th November 2008
First published as an Advance Article on the web 15th December 2008
DOI: 10.1039/b815620d
Tetraphenyl[5]cumulene (tetraphenylhexapentaene) underwent
cyclotrimerization in refluxing toluene for 10–15 min to give a
tricyclodecadiene derivative in 68% yield.
thermal and catalytic dimerizations of [5]cumulenes occur
regioselectively.
Cumulenic double bonds1 have a wide variety of potentially
reactive sites, which may lead to cyclic dimers, trimers and
higher oligomers. A variety of carbocyclic systems possessing
double bonds2 exocyclic to the ring have been synthesized
by thermal, photochemical and catalytic dimerization of
allenes and higher cumulenes.3 Although the cyclooligo-
merizations of ethylenes, acetylenes, allenes and butatrienes
have been widely investigated,4 only few examples are
known of cyclotrimerization of cumulenes. Early studies
showed that allenes trimerize at 130–200 1C in benzene to
give isomeric cyclic trimers,5 and butatrienes afford [6]radia-
lenes upon transition metal catalyzed cyclotrimerization.6
However, no reports are known for the trimerization of
hexapentaenes, except for the oxidative cyclotrimerization
of hexapentaene-extended quinocumulene.7 In a broader
sense, thermal reactions of higher cumulenes have been
studied only in limited cases, presumably due to instability
of these highly unsaturated compounds in atmospheric
oxygen. On the other hand, although the reactions of
aryl-substituted allenes8 and butatrienes9 have been investi-
gated extensively, only few reports were found on the
reactions of aryl-substituted higher cumulenes.10 We report
here the thermal cyclotrimerization of tetraphenylhexa-
pentaene to a tricyclodecadiene derivative.11
As the cyclooligomerization of [5]cumulenes has importance
in organic synthesis because this reaction provides access to
novel compounds of potential theoretical and synthetic interests,
therefore, we have studied the thermal reaction of tetraphenyl-
hexapentaene. When a solution of hexapentaene 113 in toluene
was heated at 110 1C for 10–15 min, the color of the solution
changed from deep red to reddish purple due to the formation of
the novel tricyclodecadiene derivative, 2 (Scheme 1).z As shown
in Table 1 a minimum of 13.1 mmol Lꢀ1 of hexapentaene 1 in
solution is needed to afford the trimer 2. The reaction with a
concentration lower than this does not occur even on refluxing
for a longer period of time (entry 4), which leads to uncharac-
terized gummy materials. Experiments with higher than
30.8 mmol Lꢀ1 of hexapentaene show almost the same results
(entry 1).
It has been reported earlier that nickel-catalyzed reaction of
tetraphenylhexapentaene proceeded smoothly at room
temperature to afford the head-to-head (C2–C3) dimer12 3 as
the main product with no formation of a trimer and that of
hexapentaenes bearing bulky alkyl-substituents produces
either the head-to-tail dimers or the dimers at the central
CQC double bonds (C3–C4).13 In contrast, thermal dimeriza-
tion of tetra-tert-butyl[5]cumulene at 200 1C produces a C3–C4
dimer,13,14 whereas tetramethyl[5]cumulene dimerizes at a
lower temperature to give a C1–C6 dimer.15 Thus, the known
The thermal trimer 2 reported here is very stable towards
light and air. However, when a CH2Cl2 solution of trimer
a Institute of Technology and Science, The University of Tokushima,
2-1 Minamijosanjima-cho, Tokushima 770-8506, Japan.
E-mail: kawamura@chem.tokushima-u.ac.jp
b Institute of Health Biosciences, The University of Tokushima,
1-78 Shomachi, Tokushima 770-8505, Japan
w Electronic supplementary information (ESI) available: Reaction
details and compound characterization data. CCDC 701607. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b815620d
Scheme 1
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Table
different concentrations
1
Reaction of hexapentaene in refluxing toluene at
Entry
1/mmol Lꢀ1
Time
Yield of 2 (%)
1
2
3
4
43.6
30.8
13.1
6.5
10–15 min
10–15 min
3.0–3.5 h
Overnight
68
67
25
—
2 was subjected to prolonged irradiation of UV-Vis light, it
resulted in slow decomposition of the compound. The electro-
nic spectrum of the trimer shows a broad, strong absorption
(lmax = 555 nm, log e = 4.64) tailing up to 700 nm,
corresponding to the deep purple color in dichloromethane
solution, indicating elongation of p-conjugation. The
1H-NMR spectrum12 of the trimer 2 shows an upper-field
shift of the aryl protons (dH 6.5–7.6 ppm) due to the shielding
effect of the closely situated neighbouring benzene ring. The
13C-NMR spectrum also supports this phenomenon, as well as
indicating the presence of allenic carbon in the molecule. The
IR spectrum16 confirms the presence of a CQCQC group, and
contains a characteristic band at 1922 cmꢀ1. In order to
determine the unique molecular structure of the trimer, its
crystal structure was determined by X-ray diffraction analysis.
Fig. 1 shows the molecular structure of 2 in the crystal;
particularly noteworthy is the near-coplanarity of the central
three fused rings (tricyclodecadiene). Molecular models of the
trimer also demonstrate the planarity of the central three fused
rings. It also shows that the compound has a C2v symmetry
Scheme 2
and that each of the benzene rings is located at a very short
distance, which is in turn in agreement with 1H-NMR and
13C-NMR spectra.
From the X-ray crystal structure analysis, the allene residues
are not linear (CQCQC angle is 163.671) with CQC bond
lengths of 1.29 and 1.32 A, and a dihedral angle of 177.151
between the planes of its terminal CQCz groups. The two
phenyl rings substituted at the different vinyl carbons are at a
114–1161 angle to each other.
The formation of trimer can possibly be explained as due to
a tandem process, i.e., dimerization of hexapentaene 1 at the
central CQC bond to afford an extended [4]radialene 4, a
subsequent [4 + 2] cycloaddition reaction, and electrocycliza-
tion (Scheme 2). Molecular orbital calculations17 with opti-
mized structures (B3LYP/6-31G*) suggest that the HOMO
(ꢀ0.199 eV) of the radialene 4 and the LUMO (ꢀ0.091 eV) of
1 are very close to each other, so 4 may react with another
molecule of 1 in a [4 + 2] manner18 to give an intermediate
which rearranges to trimer 2.
These results suggest that thermal dimerization of tetra-aryl-
hexapentaenes, as the trimer can form only through dimeriza-
tion, follows the same regioselectivity of bulky alkyl-substituted
[5]cumulenes13,14 and may differ from transition metal catalyzed
cyclodimerization of tetraphenylhexapentaene which takes place
via a lateral cumulenic CQC bond to form unsymmetrical
radialenes.12
The study of the chemical properties of the trimer 2 is
currently underway and will be reported in due course.
Notes and references
z Experimental: 1,1,6,6-tetraphenyl-1,2,3,4,5-hexapentaene 1 (0.586 g,
1.54 mmol) was refluxed in toluene (50 ml) for 10–15 min. Evaporation
of the solvent, separation by column chromatography and recrystal-
lization from EtOAc–hexane afforded nice purple crystals of 2, 0.392 g
(0.343 mmol, 67% yield), mp 168–170 1C (decomp.).
Crystal data for trimer 2: empirical formula, C90H60ꢂC6H14: M =
1227.64, crystal dimensions 0.60 ꢃ 0.50 ꢃ 0.40 mm, monoclinic, space
group P21/n, a = 13.1788(5), b = 15.5047(5), c = 33.564(1) A, a =
90, b = 93.516(1), g = 901, V = 6845.3(4) A3, Z = 4, T = 123.1 K,
Dc = 1.191 g cmꢀ3, m(Mo-Ka) = 0.067 mmꢀ1, F(000) = 2600,
2ymax = 55.001, l = 0.7107 A, 67 937 reflections collected, 15 612
unique (Rint = 0.029), 10 774 observed (I 4 2.0s(I)) reflections, 925
refined parameters, R = 0.0690, wR = 0.1990, S = 1.348. The
structure was solved by direct methods and expanded using Fourier
techniques.19
Fig. 1 ORTEP representation of the trimer 2. Hydrogen atoms and
one hexane molecule contained as a pair with 2 are omitted for clarity.
Thermal ellipsoids are drawn at the 50% probability level.
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576 | Chem. Commun., 2009, 574–576