Carbo-quinoids: Stability and reversible redox-proaromatic character towards carbo-benzenes

Article abstract of DOI:10.1002/anie.201407889

The carbo-mer of the para-quinodimethane core is stable within in a bis(9-fluorenylidene) derivative. Oxidation of this carbo-quinoid with MnO₂ in the presence of SnCl₂and ethanol affords the corresponding p-bis(9-ethoxy-fluoren-9-yl

Full text of DOI:10.1002/anie.201407889

Angewandte
Chemie
DOI: 10.1002/anie.201407889
Aromaticity
Carbo-Quinoids: Stability and Reversible Redox-Proaromatic
Character towards Carbo-Benzenes**
Kꢀvin Cocq, Valꢀrie Maraval,* Nathalie Saffon-Merceron, Alix Saquet, Corentin Poidevin,
Christine Lepetit, and Remi Chauvin*
Abstract: The carbo-mer of
the para-quinodimethane core
is stable within in a bis(9-fluo-
renylidene) derivative. Oxida-
tion of this carbo-quinoid with
MnO2 in the presence of SnCl2
and ethanol affords the corre-
sponding p-bis(9-ethoxy-fluo-
ren-9-yl)-carbo-benzene. The
latter can be in turn converted
back into the carbo-quinoid by
reduction with SnCl2, thus evi-
dencing a chemical reversibil-
ity of the interconversion
between
a
pro-aromatic
carbo-quinoid and an aro-
matic carbo-benzene, and is
reminiscent of the behavior of
the benzoquinone/hydroqui-
none redox couple (in the
red–ox opposite sense).
Figure 1. Illustration of the proaromaticity concept for push-pull quinoids based on 6- and 14-membered
cores (top right), corresponding known and unknown bis(fluorenylidene) quinoids (top left), and
envisaged redox generalization to carbo-quinoids (bottom).
W
hereas quinones are stabi-
=
lized by two strong C O
=
bonds, quinodimethanes (QDMs), possessing weaker C C
bonds, are more elusive species, in particular with respect to
aromatization. While ortho-QDMs are thus transient tet-
raenes in synthetically valuable pericyclic processes,[1] the
para isomers, such as tetracyano-QDM (TCNQ) in organic
conductors, are prone to various redox processes.[2] Although
functional para-QDMs can be handled as stable molecules,[3]
only a few sensitive hydrocarbon representatives have been
isolated,[4] and their reactivity is governed by their propensity
to undergo aromatization to give benzene units. In a related
context, the concept of proaromaticity has been developed by
Diederich et al. for the analysis of the generation of aroma-
ticity-stabilized electronic excited states through limited
internal electron transfer in push-pull p-quinoids (DQ6A;
Figure 1):[5a] the HOMO–LUMO gap is thus lowered through
an increase of the ground-state level by charge pre-separa-
tion, which is balanced by linear delocalization between
strong donor (D) and acceptor (A) ends, thus preventing
cyclic delocalization in the aromatic ring of the minor
zwitterionic form DR6A.[5]
[*] K. Cocq, V. Maraval, A. Saquet, C. Poidevin, C. Lepetit, R. Chauvin
CNRS, LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, BP44099, 31077 Toulouse Cedex 4 (France)
K. Cocq, V. Maraval, A. Saquet, C. Poidevin, C. Lepetit, R. Chauvin
Universitꢀ de Toulouse, UPS, ICT-FR 2599
31062 Toulouse Cedex 9 (France)
Beyond proaromaticity toward 6p-electron benzene
E-mail: vmaraval@lcc-toulouse.fr
cores,
a proaromatic dipolar p-quinoidic chromophore
chauvin@lcc-toulouse.fr
(DQ14A; Figure 1), based on a 14p-electron expanded
radiannulenic core, was also devised.[5] It is noteworthy that
the “proacetylenic” character of the two butatriene edges of
the Kekulꢀ structures DR14A,[6] or, equivalently, the general-
ized “aromatic character” of the four triple bonds,[7] may also
help the molecule to resist complete aromatization.
A natural issue is whether the proaromaticity concept is
generalizable to nonpolar molecules by reference to aromatic
ionized ground states (oxidized or reduced) instead of
electronic excited states.[5] The relevance of the corresponding
N. Saffon-Merceron
Universitꢀ de Toulouse, UPS
Institut de Chimie de Toulouse ICT-FR-2599
118 route de Narbonne, 31062 Toulouse Cedex 9 (France)
[**] We thank the ANR program (ANR-11-BS07-016-01) for a doctoral
fellowship for K.C. and for funding of equipment, along with the
CNRS. We also thank Christian Bijani for NMR characterizations
and Dr. Arnaud Rives for useful discussions and advice.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407889.
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“redox-proaromaticity” is
addressed below for 9-flu-
orenylidene ends (F;
Figure 1) and a sufficiently
large core satisfying the
4n + 2
Hꢁckel
rule.
Whereas FQ6F escaped
all attempts at isolation
because of the strong aro-
maticity of the benzene
ring (n = 1),[8] its carbo-
mer FQ18F can be envis-
aged because of the
weaker aromaticity of the
carbo-benzene ring, for
example, in the oxidized
form [FR18F]2+ (n = 4).[7]
In the difluorenylidene
[4n + 2] quinoid series,
the carbo-quinoid FQ18F
is next after the term n = 3
exemplified by Tykwinski
et al. as FQ14F, where ben-
Scheme 2. Outcome of the reaction of a fluorenyllithium with the [6]pericyclynedione 2 in THF (XRD data of 5
and 6 in the Supporting Information).[15]
zannelation
guarantees
Scheme 3. Synthesis of the carbo-quinoids 7a,b by treatment of 3a,b with SnCl2. DCM=CH2Cl2.
the absence of macrocy-
clic aromatization.[9] In
contrast, the C18 carbo-
benzene ring is definitely aromatic, but acyclic versions
thereof are also stable.[10] These balancing factors make both
the carbo-benzene FR18F(OH)2 (= [FR18F]2+,2OHꢀ) and
carbo-quinoid FQ18F a priori reasonable targets.
of X-ray crystallography (Scheme 2 and the Supporting
Information).[15]
Treatment of a pure sample of 3a with SnCl2 afforded the
partly reduced product 4a in 40% yield, while formation of
the FQ18F prototype 7a could not be evidenced (Scheme 3).
Nevertheless, prolonged treatment of either 3a or 4a with
SnCl2 gave a blue precipitate, which was assigned as 7a by
MALDI-TOF MS (m/z 852.3); insolubility prevented addi-
tional characterization. When applied to 3b, the same treat-
ment led, in 50% yield, to the soluble carbo-quinoid 7b,
which was stable (XRD molecular view in Figure 2).[15]
The conversion of 7b into 8 (R = H) and 9 (R = Et) was
triggered by treatment with MnO2 in the presence of ROH,
and SnCl2 as a Lewis acid. The diethoxylated product 9 was
found more stable than 8, but both of them
could be characterized by crystallography, in
spite of the limited stability in the solid
state.[15,16] 1H NMR spectroscopy revealed
a dramatic change in the electronic structure
through the classical deshielding of the ortho
1H nuclei of the phenyl substituents of the
diatropic C18 ring (from d = 8.04 ppm in 7b to
d = 9.41 ppm in 9).[16]
Two equivalents of the lithium salt of either 9-trimethyl-
silyl-9H-fluorene 1a,[11] or the 2,7-dialkyl-homologue 1b,[12]
were thus added to the known [6]pericyclynedione
2
(Scheme 1).[13] In diethylether, the reaction cleanly afforded
the difluorenylidene-[6]pericyclynes 3a (46%) and 3b
(51%). In THF, the reaction led to an intractable mixture,
which, when treated with SnCl2, led to the partly reduced
main product 4a, along with small amounts of the dissym-
metrical trifluorenylidene quinoid 5 and tetrafluorenylidene
macrocyclic alleno-acetylenics 6,[14] both identified by means
Reduction of 9 to 7b could also be
performed by treatment with SnCl2. The
carbo-quinoid to carbo-benzene interconver-
sion can be monitored visually with the
reaction medium evolving from blue to
orange in about 15 minutes during the oxida-
tion of 7b, and from orange to blue in about
1.5 hours during the reduction of 9 (also
affording polymerization side products).
Scheme 1. Synthesis of pericyclynic precursors of difluorenylidene carbo-quinoids in Et2O.
TMS=trimethylsilyl.
2
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Chemie
the standard deviation of the crystallo-
graphic bond lengths of the C18 ring is
significantly larger in 7b (0.090 ꢂ) than
in 8 and 9 [(0.075 ꢂ 0.001) ꢂ]. Regarding
the energetic criterion, the electronic
structure in the crystal geometry of 7b
and 9 was calculated at the B3PW91/6-
31G** level of theory on the model
fragments 7c and 9c, which were
obtained by truncation of the heptyl
chains to methyl groups. The near-fron-
tier out-of-plane p orbitals of 9c are thus
found to be much more localized on the
C18 ring than those of 7c, and the
HOMO–LUMO gap is 0.5 eV smaller
in 7c than in 9c (Figure 4).[16] Regarding
the magnetic criterion, while the
NICS(0) values of the C18 ring of 9c
and 7c are calculated at ꢀ14.3 ppm and
+ 0.3 ppm, respectively, the NICS(0)
values of the phenyl substituents are
shifted downfield in 7c (ca. ꢀ4.5 ppm)
with respect to 9c (ca. ꢀ6.8 ppm). These
data confirm the much higher macro-
aromatic character of 9c with respect to
7c (see the Supporting Information for
details).
Figure 2. Reversible redox conversion of a carbo-quinoid into carbo-benzenes (top: only one of
the equivalent Kekulꢀ structures of 8 or 9 is depicted). XRD molecular views of 7b (bottom,
left), and 9 (bottom, right) (XRD data for 8 can be found in the Supporting Information).[15]
While the UV-vis absorption spectrum of 9 is similar to
those of other carbo-benzenes, with one intense band at lmax
=
Redox properties of 4a, 7b, and 9 were investigated by
square-wave (SWV) and cyclic (CV) voltammetries (Table 1
and the Supporting Information, all potentials given versus
SCE). In the reduction process, four waves are observed for
4a and 7b and the first two are reversible at very close
potentials (E1/2: ꢀ0.74 and ꢀ0.90 V for 4a, ꢀ0.67 and ꢀ0.80 V
for 7b), thus suggesting quasi-simultaneous reductions of the
two remote exocyclic double bonds with the lower E1/2 values
of 7b being attributed to proaromaticity. With a classical
carbo-benzene reduction comportment,[16] 9 is first reversibly
reduced at ꢀ0.84 V, then irreversibly at ꢀ1.30 V. For the three
compounds, an irreversible oxidation process occurs in the
454 nm (Figure 3), the spectrum of 7b displays two intense
bands at l = 435 and 591 nm. The extinction coefficients of 9
at lmax (e ꢁ 324400 Lmolꢀ1 cmꢀ1, estimated assuming that the
conversion of 7b!9 is quantitative, 9 being not stable enough
to be dried and weighted precisely), is twice that of 7b (e =
145500 Lmolꢀ1 cmꢀ1). This pattern is reminiscent of the
carbo-cyclohexadienes also featuring a conjugated but non-
aromatic macrocyclic core between two indentical ends.[16]
The pro-aromatic character of 7b can be assessed by
comparison of some of its aromaticity indices (structural,
energetic, and magnetic criteria) with those of 8 or 9 (see the
Supporting Information). Regarding the structural criterion,
Table 1: CV data for 4a, 7b, and 9 on a Pt microdisk at RT in CH2Cl2,
0.1m [nBu4N][PF6]; SCE (reference); scan rate: 0.2 Vsꢀ1
.
Reduction
Oxidation
[c]
E1/2[a] [V] DEp [V][b] RIp
Number of eꢀ Epred [V] Epox [V]
4a ꢀ0.74
0.06
0.06
1.08
1.02
2
2
1.29[d]
1.52
ꢀ0.90
ꢀ1.37
ꢀ1.75
7b ꢀ0.67
ꢀ0.80
0.06
0.05
0.05
1.00
1.10
1.00
2
2
2
1.14[d]
ꢀ1.24
ꢀ1.53
ꢀ1.30
9
ꢀ0.84
0.07
1.07
–
1.15[d,e]
[a] Half-wave potential E1/2 = jEpred +Ep j/2. [b] Separation between the
ox
two peak potentials: DEp =Ep ꢀEpox. [c] Peak current ratio RIp = jIp
/
red
ox
red
Figure 3. UV-vis spectra of the carbo-quinoid 7b and carbo-benzene 9
in toluene (e values in Lmolꢀ1 cmꢀ1).
Ip j. [d] The product deposited on the electrode. [e] Becomes reversible
at high scan rates.
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Keywords: alkynes · aromaticity · heterocycles ·
.
quinodimethanes · X-ray diffraction
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Whereas the quinodimethane FQ6F escaped all attempts
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how carbo-merization can circumvent the instability of
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benzoquinone versus hydroquinone is shown to be relevant in
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carbo-benzenes 8 and 9, albeit in the red-ox opposite sense
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Received: August 2, 2014
Revised: October 19, 2014
Published online: && &&, &&&&
4
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These are not the final page numbers!

Angewandte
Chemie
Communications
Aromaticity
K. Cocq, V. Maraval,*
N. Saffon-Merceron, A. Saquet,
C. Poidevin, C. Lepetit,
R. Chauvin*
&&& — &&&
Carbo-Quinoids: Stability and Reversible
Redox-Proaromatic Character towards
Carbo-Benzenes
Back and forth: The carbo-mer of the
para-quinodimethane core is stable
within a bis(9-fluorenylidene) derivative.
Oxidation of the carbo-quinoid with MnO2
in the presence of SnCl2 and ethanol
affords p-bis(9-ethoxy-fluoren-9-yl)-carbo-
benzene, which is converted back into the
carbo-quinoid by reduction with SnCl2,
thus evidencing a chemical reversibility of
the interconversion between a pro-aro-
matic carbo-quinoid and an aromatic
carbo-benzene.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
These are not the final page numbers!