Isolation of bicyclopropenylidenes: Derivatives of the smallest member of the fulvalene family

Article abstract of DOI:10.1002/anie.200804372

(Figure Presented) Small and stable: A triafulvalene derivative is indefinitely stable at room temperature both in solution and in the solid state under inert atmosphere. The triafulvalene is synthesized by the magnesium-catalyzed coupling of two bis(chlo

Full text of DOI:10.1002/anie.200804372

Angewandte
Chemie
DOI: 10.1002/anie.200804372
Highly Strained Molecules
Isolation of Bicyclopropenylidenes: Derivatives of the Smallest
Member of the Fulvalene Family**
Rei Kinjo, Yutaka Ishida, Bruno Donnadieu, and Guy Bertrand*
Dedicated to Professor Edgar Niecke on the occasion of his 70th birthday
The class of hydrocarbons obtained by formally cross-
conjugating two rings through a common exocyclic double
bond is known as the fulvalenes, a terminology introduced by
Brown.[1] Although the synthesis of compound I (Scheme 1)
was reported as early as 1915 by Courtot,[2,3] the first non-
benzannulated fulvalene, a derivative of pentafulvalene II,
was prepared in 1954.[4] A decade later, derivatives of
triapentafulvalenes III,[5] larger-ring[6] and heterocycle-con-
taining fulvalenes, such as the widely studied tetrathiafulva-
lenes (TTF),[7] were isolated. However, despite intensive
effort,[6,8] the experimental demonstration of the existence of
derivatives of the smallest and most strained member, namely
triafulvalene (bicyclopropenylidene) IV, has not been ach-
ieved to date. Even compounds Va–c,[9,10] which feature
thermodynamically highly stabilizing benzo substituents,[11]
1
were only characterized by H NMR spectroscopy and mass
spectrometry. The difficulty in isolating triafulvalene IV has
been predicted by theoretical calculations,[8] Radom and co-
workers[8a] even concluding that these compounds have an
antiaromatic character. Herein we show that kinetically
protected triafulvalenes can be prepared and isolated as
thermally stable compounds. They feature a small HOMO–
LUMO gap (HOMO = highest occupied molecular orbital,
LUMO = lowest unoccupied molecular orbital), and conse-
quently are very reactive, as exemplified by the spontaneous
=
room temperature addition of water to the central C C bond.
Triafulvalenes IV are formally the dimers of cycloprope-
nylidenes VI. According to the Carter and Goddard formu-
lation,[12] the strength of the C C bond that results from the
=
dimerization of singlet carbenes should correspond to that of
=
a canonical C C bond (usually that of ethene) minus twice
the singlet–triplet energy gap (EST) for the carbene. The
comparison of the calculated EST value for the isolated
diaminocyclopropenylidenes VIa[13] and matrix-characterized
diphenylcyclopropenylidene VIb,[14] 250 and 179 kJmolÀ1,
respectively, suggests that aryl-substituted triafulvalenes
should be more likely to exist. However, the dimerization of
singlet carbenes is believed to follow a non-least-motion
pathway[15] that involves the attack of the occupied in-plane
lone pair of s electrons of one singlet carbene center on the
out-of-plane vacant pp orbital of a second carbene, and in
cyclopropenylidenes VI the latter orbital is difficult to access
because of the 2p-electron system. Thus, there is an energy
barrier for the dimerization of cyclopropenylidenes VI that
arises from electronic factors, this barrier would be enhanced
by the presence of bulky substituents, which are necessary to
kinetically protect the central p bond of triafulvalenes IV.[10]
Based on this analysis, we planned a two-step approach
involving the coupling of two dichlorocyclopropene units
followed by reduction of the resulting bis(chlorocyclopro-
penyl) moiety.
Scheme 1. Benzannulated compound I, pentafulvalene II, triapentaful-
valene III, triafulvalene IV, benzannulated derivatives V, cyclopropenyli-
denes VI, and bicyclopropylidene VII.
[*] Dr. R. Kinjo, Dr. Y. Ishida, B. Donnadieu, Prof. G. Bertrand
UCR–CNRS Joint Research Chemistry Laboratory (UMI 2957),
Department of Chemistry, University of California
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: guy.bertrand@ucr.edu
Homepage: http://research.chem.ucr.edu/groups/bertrand/
guybertrandwebpage/
[**] Financial support from the NSF (CHE 0808825) is gratefully
acknowledged. We also thank the JSPS for Fellowships (R.K. and
Y.I.) and Dr. J. C. Daran for his help with the crystallographic part of
this manuscript.
Starting from the commercially available tetrachlorocy-
clopropene 1, a bulky 2,4,6-triisopropylphenyl group (Tip)
was first incorporated into the framework by using a Friedel–
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804372.
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Crafts reaction, as described by West et al.[16] We were not
able to introduce a second Tip group, but successfully
introduced a mesityl group (Mes). Treatment of cycloprope-
none 3 with phosgene led to 4, which was isolated in 69%
yield with respect to 1 (Scheme 2). To prepare the bis(chlor-
Scheme 2. Synthesis of dichlorocyclopropene 4.
ocyclopropenyl) derivative 5, we first tested zinc as a coupling
reagent, as reported by Breslow et al.,[17] for the diphenyl
analogue, but a complex mixture was formed and we were not
able to isolate any products. However, when a solution of 4 in
THF was treated with an excess of magnesium at room
temperature, we directly obtained the desired triafulvalene
derivatives 6a and 6b (94% total yield), instead of the
expected bis(cyclopropenyl) derivatives 5 (Scheme 3). The
observed E/Z ratio (60:40) reflects the relative thermody-
namic stability of 6a and 6b based on steric repulsion
between the bulky substituents.
Figure 1. X-ray crystal structure of 6a (H atoms are omitted for clarity).
Selected bond lengths [ꢀ] and angles [8]: C1–C16 1.303(5), C1–C2
1.459(5), C1–C3 1.479(5), C2–C3 1.330(5), C2–C10 1.456(5), C3–C4
1.452(5), C16–C17 1.457(5), C16–C18 1.455(5), C17–C18 1.323(5),
C17–C25 1.451(5), C18–C19 1.458(5); C2-C1-C3 53.8(2), C1-C2-C3
63.8(3), C1-C3-C2 62.3(3), C17-C16-C18 54.1(2), C16-C17-C18 62.9(3),
C16-C18-C17 63.1(3).
double bonds, respectively, which confirms the highly local-
ized nature of the p system.
Derivatives 6 are indefinitely stable at room temperature,
both in the solid state and in solution, under an inert
atmosphere,. Crystals of 6a melt at 1308C, and no decom-
position was observed by heating a benzene solution at reflux
for 48 h. Solutions and crystals of 6a are dark purple, and
indeed the UV/Vis spectrum showed a distinct absorption
band at 502 nm (e = 2500). This band is unusually red-shifted
compared to the absorption maxima of ethylene (171 nm),
1,3-butadiene (217 nm), trans-1,3,5-hexatriene (274 nm), and
even that of diaminotriafulvene (298 nm).[20] Ab initio calcu-
lations (at the HF/6-31G(d,p)//B3LYP/6-31G(d) level) per-
formed on the tetraphenyltriafulvalene as a model readily
rationalized this phenomenon. The frontier orbitals are
depicted in Figure 2. The HOMO, a combination of the
Scheme 3. Synthesis of triafulvalenes 6a and 6b.
The signal for the central sp2 carbon atoms in the
13C NMR spectrum was observed at d = 85.0 ppm for 6a and
d = 85.3 ppm for 6b, which is remarkably shifted to higher
field compared to the shifts observed for bicyclopropylidenes
VII (115–120 ppm).[18] Although perfect separation of 6a and
6b was not successful because of their similar and high
solubility in all solvents, a few single crystals of 6a were
obtained from a pentane/THF mixture at À788C. An X-ray
diffraction analysis[19] shows that the triafulvalene skeleton is
almost perfectly planar, the sums of bond angles around the
two central carbon atoms C1 and C16 are 359.648 and 359.488,
and the C2-C1-C16-C18 torsion angle is 3.018 (Figure 1). The
dihedral angles between the cyclopropene rings and each Mes
and Tip groups are between 28.268 and 54.618, which suggests
that, if operative, the conjugative interactions between the
fulvalene skeleton and its substituents are weak. As predicted
by theoretical calculations,[8] there is a considerable fulvalenic
bond length compression, because of the high s character of
the s-bond component of the central double bond; indeed,
the C1–C16 distance is only 1.303(5) ꢀ. The C1–C2 and C1–
C3 (1.459(5) and 1.479(5) ꢀ), as well the C2–C3 bond lengths
(1.330(5) ꢀ) are slightly shorter than typical C–C single and
=
three C C p orbitals, is remarkably high-lying (À5.29 eV),
which reflects the high electron density in the triafulvalene
=
plane. The LUMO, which is a combination of the C C
p* orbitals of the two three-membered rings, is relatively low-
lying.
The existence of a small HOMO–LUMO energy gap not
only rationalized the unusual UV/Vis spectrum, but also
suggests that triafulvalenes should be very reactive. Indeed,
triafulvalenes 6 are extremely air-sensitive, and decompose
within a minute to give a complex mixture of unidentified
products. When a mixture of 6a and 6b was treated with
=
oxygen-free water in THF, H2O addition to the central C C
bond occurred to afford the corresponding adducts 7a and 7b
as racemic mixtures in 82% overall yield (Scheme 4). The
structure of these compounds was determined by 1H and
13C NMR spectroscopy and X-ray crystallography for 7b. This
is certainly one of the very rare examples of spontaneous
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Synthesis of dichlorocyclopropene 4: Tetrachlorocyclopropene
(2.1 g, 11.7 mmol) was added to a solution of aluminum trichloride
(1.6 g, 11.7 mmol) in CH2Cl2 (20 mL) at À788C. The solution was
warmed to room temperature and stirred for 2 h. 1,3,5-triisopropyl-
benzene (2.5 g, 12.0 mmol) was added at À788C to this solution and
the reaction mixture stirred at room temperature overnight. 1,3,5-
trimethylbenzene (2.0 g, 16.6 mmol) was then added at À788C, and
after stirring at room temperature overnight, water (30 mL) was
added. The organic layer was washed with water and dried over
MgSO4. After filtration, the volatile components were removed under
vacuum to give 3[21] as a white powder, which was used without further
purification. A 20% solution of phosgene in toluene (5 mL) was
added to a solution of 3 in CH2Cl2 (20 mL) at 08C, and the reaction
mixture was stirred at room temperature overnight. The volatile
components were removed under vacuum, and the solid residue
washed with hexane (5 mL) to give 4 as a white powder (3.5 g, 69%);
m.p. 98.08C; 1H NMR (300 MHz, CDCl3): d = 1.19 (d, J = 6.9 Hz,
12H), 1.24 (d, J = 6.6 Hz, 6H), 2.00 (s, 3H), 2.50 (s, 6H), 2.77 (sept,
J = 6.9 Hz, 2H), 3.24 (sept, J = 6.6 Hz, 1H), 6.61 (s, 2H), 7.19 ppm (s,
2H); 13C NMR (75 MHz, C6D6): d = 21.5 (CH3Mes), 22.0 (CH3Mes),
24.4 (CH3Tip), 25.0 (CH3Tip), 32.1 (CHTip), 35.1 (CHTip), 64.5
(CCl2), 121.6 (ipso), 129.5 (ipso), 132.3 (CringTip) 122.0 (m), 132.1
(CringMes), 130.1 (m), 141.1 (o), 142.1 (p), 149.6 (o), 151.5 ppm (p).
Synthesis of triafulvalenes 6a and 6b: THF (15 mL) was added to
a
mixture of 4 (2.15 g, 5.0 mmol) and magnesium (250 mg,
10.3 mmol), and the solution was stirred at room temperature for
18 h. After evaporation of THF, the residue was extracted with
hexane (50 mL). The remaining excess magnesium and magnesium
salts were removed by filtration, and the solvent was removed under
vacuum to afford a black powder (1.68 g, 94%). Recrystallization of
the residue from THFand pentane gave 6a as dark purple crystals. 1H
and 13C NMR signals of 6a and 6b were assigned by 2D NMR
techniques. 6a: m.p. 130.08C; 1H NMR (300 MHz, C6D6): d = 1.15 (d,
J = 6.7 Hz, 24H), 1.23 (d, J = 6.8 Hz, 12H), 2.09 (s, 6H), 2.38 (s, 12H),
2.81 (sept, J = 6.8 Hz, 2H), 3.56 (sept, J = 6.7 Hz, 4H), 6.75 (s, 4H),
7.19 ppm (s, 4H); 13C NMR (75 MHz, C6D6): d = 21.4 (CH3Mes), 21.5
(CH3Mes), 23.7 (CH3Tip), 24.6 (CH3Tip), 31.2 (CHTip), 35.4
Figure 2. Molecular orbitals of tetraphenyltriafulvalene calculated at
the HF/6-31G(d,p)//B3LYP/6-31G(d) level of theory.
=
(CHTip), 85.0 (C C), 120.9 (m), 126.4 (ipso), 126.7 (ipso), 126.8
(CringTip), 127.1 (CringMes), 129.3 (m), 138.5 (p), 138.8 (o), 149.5 (o),
150.1 ppm (p); UV/Vis (hexane): lmax 502 nm; 6b: 1H NMR
(300 MHz, C6D6): d = 1.15 (d, J = 6.7 Hz, 24H), 1.23 (d, J = 6.8 Hz,
12H), 2.09 (s 6H), 2.32 (s, 12H), 2.81 (sept, J = 6.8 Hz, 2H), 3.63
(sept, J = 6.7 Hz, 4H), 6.71 (s, 4H), 7.24 ppm (s, 4H); 13C NMR
(75 MHz, C6D6): d = 21.4 (CH3Mes), 21.6 (CH3Mes), 23.9 (CH3Tip),
Scheme 4. Spontaneous addition of water to triafulvalenes 6a/6b.
=
24.6 (CH3Tip), 31.1 (CHTip), 35.3 (CHTip), 85.3 (C C), 121.0 (m),
124.9 (CringTip), 126.2 (ipso), 126.4 (ipso), 127.4 (CringMes), 129.1 (m),
138.5 (p), 138.7 (o), 149.9 (p), 150.0 ppm (o).
=
addition of water to the C C bond of a hydrocarbon
derivative.
Received: September 4, 2008
Published online: December 12, 2008
More than half a century after the discovery of the first
fulvalene derivative, this work demonstrates that by providing
=
sufficient kinetic protection of the inter-ring C C bond,
Keywords: carbenes · cyclopropenylidenes · fulvalenes ·
hydration · strained molecules
triafulvalenes can also be isolated. The high degree of strain
means that these molecules should feature unique chemical
reactivity. Moreover, because of the similarities with tetra-
.
=
thiafulvalenes (TTF), namely an electron-rich C C bond and
a small HOMO–LUMO gap, the availability of triafulvalenes
opens a new avenue for the discovery of materials with useful
electrochemical and photochemical properties.
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