Carbo-cyclohexadienes vs. carbo-benzenes: Structure and conjugative properties

Article abstract of DOI:10.1039/c4sc02742f

Ideally C_s-/C_2v-symmetric chromophores, constituted by two electro-active groups conjugated through the carbo-mer of the cyclohexa-1,3-diene core, are selectively prepared by the SnCl₂-mediated reduction of tailored hexaoxy-[6]pericyclynes: in the latter substrates, one of the 1,4-dioxybut-2-yne edges is "chemically locked" by two CF₃ substituents preventing complete reduction to the corresponding aromatic carbo-benzenic core, which is expected to be more "π-insulating" between the electro-active ends. The bis-trifluoromethylated carbo-cyclohexadiene products are also shown to be significantly stabilized with respect to their bis-phenylated analogues. Their structural (crystal X-ray diffraction analyses), spectroscopical (NMR and UV-vis spectra), physio-optical (dichromism in solution) and electrochemical (cyclic voltammograms) properties are compared on the basis of the electron-donating/electron-withdrawing nature of the substituents. These properties are also compared with those of their aromatic carbo-benzene and flexible carbo-n-butadiene counterparts.

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Carbo-cyclohexadienes vs. carbo-benzenes:
structure and conjugative properties†‡
Cite this: DOI: 10.1039/c4sc02742f
ab
Arnaud Rives,ab Iaroslav Baglai,abc Cecile Barthes,ab Valerie Maraval,
*
´
´
Nathalie Saffon-Merceron,d Alix Saquet,ab Zoia Voitenko,c Yulian Volovenkoc
ab
*
and Remi Chauvin
Ideally Cs-/C2v-symmetric chromophores, constituted by two electro-active groups conjugated through
the carbo-mer of the cyclohexa-1,3-diene core, are selectively prepared by the SnCl2-mediated
reduction of tailored hexaoxy-[6]pericyclynes: in the latter substrates, one of the 1,4-dioxybut-2-yne
edges is “chemically locked” by two CF3 substituents preventing complete reduction to the
corresponding aromatic carbo-benzenic core, which is expected to be more “p-insulating” between the
electro-active ends. The bis-trifluoromethylated carbo-cyclohexadiene products are also shown to be
significantly stabilized with respect to their bis-phenylated analogues. Their structural (crystal X-ray
diffraction analyses), spectroscopical (NMR and UV-vis spectra), physio-optical (dichromism in solution)
and electrochemical (cyclic voltammograms) properties are compared on the basis of the electron-
donating/electron-withdrawing nature of the substituents. These properties are also compared with
those of their aromatic carbo-benzene and flexible carbo-n-butadiene counterparts.
Received 6th September 2014
Accepted 4th November 2014
DOI: 10.1039/c4sc02742f
www.rsc.org/chemicalscience
DBA derivatives B were shown to be actually quite stable and
could be studied in a systematic manner.7 In passing, they were
Introduction
Until the recent past,1 the chemistry of carbo-mers mainly
focused on carbo-benzenes,2 because of the stability, rigidity
and p-electronic features anticipated to be associated with their
unique aromatic structure,3 as compared to those of acyclic
fragments. Like the C6 ring of benzene, the C18 macrocycle of
carbo-benzenes A (Fig. 1) can be formally divided into two
acyclic components relevant from both the viewpoints
of experimental retro-synthesis4 and theoretical aromaticity
analysis.5 While the ethylene and 1,3-butadiene components for
benzene are stable molecules, the issue for their partial carbo-
mers, dialkynylbutatriene (DAB) and di(alkynylbutatrienyl)-
acetylene (DBA, i.e. the functionality of carbo-n-butadiene)
components, was less obvious (Fig. 1).6 Recently, generic acyclic
found to be much more sensitive than the carbo-benzene
counterparts A to the effects of electro-active substituents on the
maximum UV-vis absorption wavelength (Fig. 1: R ¼ 4-X–C6H4,
X ¼ NO2, CF3, H, OMe, NR02.).7b Whereas the phenomenon
was tentatively attributed to macrocyclic aromaticity (“macro-
aromaticity”) making the C18 ring of A quite p-independent
from its substituents R, the same carbo-benzene ring, which is
anyway three-times smaller and three-times less energetically
aromatic than benzene,3f has at most a weak electrical insu-
lating effect. Very recently, indeed, the single molecule conduc-
tance (SMC) of a functional carbo-benzene A (R ¼ 4-NH2–C6H4)4
measured by STM techniques proved to be much higher than
that of benzenoid or porphyrine parents of similar size (ca. 2
nm), and almost two orders of magnitude higher than the SMC
of the acyclic DBA counterpart B (R ¼ 4-NH2–C6H4) (106 nS vs.
2.7 nS).8 On the basis of NEGF-DFT-calculations, this SMC
difference was correlated with the difference in conformational
freedom between the rigid carbo-benzene A and the freely
rotating DBA derivative B (Fig. 1). The stiffness of A is, however,
also effective in the s-cyclic and p-acyclic carbo-cyclohexadiene C,
which is a rigid version of the non-macro-aromatic DBA B, thus
locked in a cisoid conformation. Access to C (R ¼ 4-NH2–C6H4)
would thus allow an appraisal of the role of the macro-aroma-
ticity of the equally rigid parent A on conduction. More funda-
mentally, the carbo-cyclohexadiene C is also the closest realistic
non-aromatic but cyclic reference for the carbo-benzene A, just
as cyclohexadiene is for benzene.9
aCNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP
44099, F-31077 Toulouse Cedex 4, France. E-mail: vmaraval@lcc-toulouse.fr;
chauvin@lcc-toulouse.fr; Fax: +33 5 61 55 30 03
b
´
Universite de Toulouse, UPS, Institut de Chimie de Toulouse, ICT-FR2599, 118 Route
de Narbonne, F-31062 Toulouse, France
cKiev National Taras Shevchenko University, 60 Volodymlyrska St, 01033 Kiev, Ukraine
d
´
Universite de Toulouse, UPS, Institut de Chimie de Toulouse, ICT-FR 2599, 118 route
de Narbonne, 31062 Toulouse, France
† The investigations presented in this report have been performed within the
framework of the French-Ukrainian GDRI “Groupement Franco-Ukrainien en
´
Chimie Moleculaire” funded by the CNRS.
‡ Electronic supplementary information (ESI) available: Experimental details,
spectroscopic and crystallographic data. CCDC 1003439, 951896 and 951897.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4sc02742f
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Fig. 1 The common DBA moiety in three types of carbo-meric molecules related to the carbo-benzene A, generated by reduction of the
hexaoxy-[6]pericyclyne D, via the carbo-cyclohexadiene C. A complete A, B, C series is known for R ¼ 4-MeO–C6H4.10
To date, a single example of carbo-cyclohexadiene has been destabilization (–M or –I effect; the more or less protecting
reported:10a C (R ¼ 4-MeO–C6H4) was incidentally isolated in groups are denoted as PG in Scheme 1). Within this prospect,
low yield as a sub-reduced side product of the reductive triuoromethyl groups are ideal candidates:13 it was indeed
aromatization of a hexaoxy-[6]pericyclyne10b D to the corre- observed that the quite general method for the conversion of
sponding carbo-benzene A. The latter was also obtained by 1,4-dioxybut-2-yne derivatives to the corresponding butatrienes
prolonged reductive treatment of the parent C (Fig. 1). Though by treatment with SnCl2/HCl is not compatible with CF3 sub-
it is quite sensitive in the solid state, this rst carbo-cyclo- stituents.6c Two substituents PG ¼ CF3 at adjacent carbinoxy
hexadiene was found to exhibit a persistent sharp turquoise vertices of a hexaoxy-[6]pericyclyne DF are therefore anticipated
blue-purple dichromism (or dichromatism) in solution.10a It to freeze the reactivity of corresponding 1,4-dioxybut-2-yne edge
thus appears in different colors to the human eye depending on towards reduction and thus optimize the selectivity for the
the length of the optical path crossing the solution,11 which is partly reduced carbo-cyclohexadiene product CF vs. the putative
an unusual physio-optical property giving a further attractive- carbo-benzene AF (Scheme 1).
ness to the class of chromophores C.
The envisaged [8F + 10] and [8 + 10F] strategies to synthesise
In order to guarantee the selective and systematic access to the bis-triuoromethylated pericyclynic precursors DF are
carbo-cyclohexadiene C with various types of substituents R, the inspired from a strategy previously used in the tetraphenylated
control of the reduction step to preserve one of the 1,4-dioxybut- series D and are based on a [8 + 10] cyclization step between a C8
2-yne edges of the precursors D is the synthetic challenge dinucleophile and a C10 dielectrophile, where the index F here
addressed below.
refers to bis-triuoromethylated synthons (Scheme 1). In spite
of the recognized specicity of the chemistry of organouorine
compounds, both in terms of reactivity (due to extreme elec-
tronegativity and hardness) and purication (due to a high
lipophilic character), the general strategic principles developed
in the 1,4-diphenylbut-2-yne series10a are shown to be adaptable
for the 1,4-bis(triuoromethyl)but-2-yne series.
Results and discussion
The sole known carbo-cyclohexadiene C (R ¼ 4-MeO–C6H4) was
obtained by serendipity at low temperature, thus indicating that
the formation of the two rst butatrienic edges of the carbo-
benzene target A (R ¼ 4-MeO–C6H4) was slightly faster than the
formation of the third one.10 Assuming that the mechanism of
action of the reducing system SnCl2/HCl starts with the
formation of a bispropargylic carbenium from the correspond-
1. [8 + 10F] cyclization route to bis-triuoromethylated
hexaoxy-[6]pericyclynes
ing carbinoxy vertex of the hexaoxy-[6]pericyclyne precursor D The uorinated dialdehyde or diynone synthon C10F was tar-
(Fig. 1),12 the two anisyl-stabilized carbenium centers are likely geted through the known triyne intermediate 1 (Scheme 2).6c
to initially drive the formation of the butatrienic edges that are The latter was prepared in two steps from the diol 2 via the
conjugated with the anisyl substituents R, as found in C. The silylated triyne 3, in 75% overall yield (in spite of its volatility) as
phenyl-substituted carbinol ether vertices, though less reactive, a statistical mixture of dl and meso diastereoisomers, identied
remain, however, prone to dissociate under the operating acidic by two 19F NMR singlet signals at ꢀ79.52 and ꢀ79.53 ppm. The
conditions, thus leading to A and making the partial reduction mixture was not resolved before use as either the precursor of
to C difficult to control.10a,12 The selectivity for C should, the C10F synthon here, or the C8F synthon in the [8F + 10] route
however, be improved by increasing the difference in the (see Section 2).
mesomeric donor stabilization (+M effect) of the two types of
Three C10F synthons were prepared in two steps, starting
carbenium centers. Ultimately, this should be improved by with the addition of the dilithium salt of 1 to p-formaldehyde, p-
deliberately changing the two phenyl substituents for substit- anisaldehyde or benzaldehyde, giving the respective diols 4a,
uents exerting opposite mesomeric or inductive acceptor 4b, 4c in 60–93% yields. Subsequent oxidation gave the
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Scheme 1 [8 + 10F] and [8F + 10] strategies to synthesise bis-trifluoromethylated hexaoxy-[6]pericyclynes, envisaged as precursors for the
selective synthesis of the corresponding carbo-cyclohexadienes.
dialdehyde 5a or diketones 5b or 5c, using either MnO2 in nucleophiles to its keto groups (Scheme 3). A similar approach
dichloromethane (DCM) at room temperature or IBX in reux- indeed proved to be efficient in the tetraphenyl series for the
ing 1,2-dichloroethane (Scheme 2).
synthesis of carbo-benzenes through the tetraphenyl-[6]peri-
The C10F synthons 5a–c were then treated with the known cyclynedione analogue of 8.4,10a,14b
dilithiated C8 bis-terminal triyne 6 in THF at low temperature
The synthesis of 7a was also attempted through the alter-
under quite diluted conditions (Scheme 3).14 While the bis- native [8F + 10] strategy from the uorinated triyne 1 as the C8F
tertiary [6]pericyclynediols 7b and 7c were obtained from the dinucleophile, and the known dialdehyde 9 as the C10 dielec-
corresponding diketones in 40 and 18% yield, respectively, the trophile,14b but without more success (Scheme 3).
bis-secondary [6]pericyclynediol 7a could not be obtained from
the dialdehyde 5a (the nal reaction mixture contained the
2. [8F + 10] cyclization route to bis-triuoromethylated
hexaoxy-[6]pericyclynes
starting triyne 6 and traces of linear oligomers). The CF3
substituents, replacing the original phenyl substituents,10a are
therefore responsible for the uncontrolled reactivity of the car-
baldehyde groups in the g position. The [6]pericyclynediol 7a
was also targeted as a possible precursor of the [6]peri-
cyclynedione 8, a putative pivotal reactant for the preparation of
1,10-disubstituted carbo-cyclohexadienes by addition of various
In spite of the intriguing failure of the [8F + 10] strategy from the
C10 dialdehyde 9 (Section 1, Scheme 3), the same strategy was
envisaged from C10 diketones. The triynediones 10d–h were
thus prepared using two alternative methods involving the bis-
secondary diols 11d–h as intermediates (Scheme 4). The rst
Scheme 2 Synthesis of the dicarbonyl synthons C10F, 5a–c, via the bis-trifluoromethylated triyne 1, also serving as the C8F synthon (see Fig. 1).
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Scheme 3 The [8 + 10F] route to bis-trifluoromethylated bis-tertiary hexaoxy-[6]pericyclynediols 7b and 7c, and the attempted [8 + 10F] and
[8F + 10] routes to the [6]pericyclynedione target 8 via the putative bis-secondary hexaoxy-[6]pericyclynediol 7a.
Scheme 4 Synthesis of the C10 diketone synthons to be involved in a [8F + 10] cyclization route to bis-trifluoromethylated [6]pericyclynes of type
DF (see Schemes 1 and 5).
method, recently described and consisting of a double addition
The ve C10 diketones 10d–h were then involved in an [8F + 10]
of the bis-terminal triyne 6 to two equivalents of 4-(tri- cyclization process with the same bis-triuoromethylated
uoromethyl)benzaldehyde, gave the diol 11d in 96% yield.7b dinucleophile 1, the dilithium salt of which was prepared from
The second method, involving the triynedial 9 as a dielec- either base, n-butyllithium or lithium hexamethyldisilazane
trophile towards various nucleophiles, led to the diols 11e–h. (LiHMDS) (Scheme 5). In comparison to the use of a stoichio-
The procedure, previously described for the preparation of 11g metric amount of n-butyllithium, which turned out to be inef-
in 96% yield from lithium triisopropylsilylacetylide,4 was thus cient in a few cases, the alternative use of four equivalents of
generalized to other nucleophiles, giving the diols 11e,f,h in 27– LiHMDS afforded 7e,f,h. While 7e proved to be elusive upon
72 % yield (Scheme 4). Isolation of the indolyl- and carbazolyl- chromatography, the [6]pericyclynediols 7d,f–h were nally
substituted products 11e,f required the aqueous treatment of obtained in 13–38% yields, i.e. in the classical range of related
the reaction medium at low temperature. The diols 11d–h were [8 + 10] cyclization processes.14b,15
then oxidized to the corresponding diketones 10d–h using
MnO2 in DCM (Scheme 4).
The [6]pericyclynediols 7d–h were obtained as mixtures of
diastereoisomers (20 in theory): this was evidenced by the
extended ranges of resolved 1H NMR signals of C*(R)OCH3
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3. Reduction of bis-triuoromethylated hexaoxy-[6]
pericyclynes to carbo-cyclohexadienes
Reductive treatment of the [6]pericyclynediols 7b–h with SnCl2/
HCl afforded the carbo-cyclohexadienes 12b–h (Scheme 6).
While the tetraaryl targets 12b–f were readily obtained under
classical conditions, the dialkynyl counterparts 12g,h were
more elusive. The reduction of 7g was not selective, giving a
mixture of undetermined products from which a minute
Scheme 5 [8F + 10] cyclization strategy from triynediones 10 to bis-
trifluoromethylated hexaoxy-[6]pericyclynes 7 of type DF (Scheme 3).
vertices (R ¼ Ph, CF3) and resolved 19F NMR signals of C*(OMe)
CF3 vertices (around ꢀ79 ppm), induced by the rigid (cyclic)
close stereochemical environment (in contrast, the 4-CF3-C6H4
substituents of 7d, remote from the stereogenic centers, reso-
nate as a single broad 19F singlet at ꢀ62.7 ppm: see Fig. 2).
Although the preparation of 7d–h by the alternative [8 + 10F]
route was not attempted, systematic comparison of the two
routes will deserve special attention, in particular in view of
elucidating the failure of both routes for the target 7a and to
allow the design of a suitable procedure for this target.
Scheme
6 The selective four-electron reduction of bis-tri-
fluoromethylated hexaoxy-[6]pericyclynes to the corresponding
conjugated carbo-cyclohexadienes.
Fig. 2 19F NMR spectrum of the [6]pericyclynediol 7d evidencing the occurrence of a diastereoisomeric mixture, 20 diastereoisomers in theory
(CDCl3, 282 MHz).
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quantity of 12g could be obtained. The reduction of 7h turned spectra of the two isomers of the representative example 12d
out to be selective (with one main spot observed on TLC plates), (without assignment) are shown in Fig. 3.
but the product 12h proved to be unstable in the solid state,
giving instantly a black insoluble material when concentrated to
dryness. The carbo-cyclohexadiene 12h could, however, be
4. X-Ray crystallography of carbo-cyclohexadienes
characterized in solution, using a procedure avoiding the
complete evaporation of the solvent.
Three of the bis-triuoromethylated carbo-cyclohexadienes were
obtained as crystalline solids.16 X-Ray diffraction (XRD) analyses
of selected crystals of 12b, 12c and 12d conrmed the conju-
gated structure of the carbo-cyclohexadiene core. On the basis of
experimental spectroscopic data only, it was not possible a
priori to decipher whether the structure of the previously iso-
lated tetraphenylated carbo-cyclohexadiene product was C (R ¼
4-MeO–C6H4), i.e. the core carbo-mer of the 1,3-cyclohexadiene
parent, instead of the core carbo-mer of the 1,4-cyclohexadiene
isomer (Fig. 1).10a The assignment was proposed on the basis of
a comparison of the experimental UV-vis spectrum with
the theoretical spectra of both the regioisomers, calculated at
semi-empirical ZINDO or TD-DFT levels (only the conjugated
butatriene edges of C give rise to the observed two intense
bands spectrum: see Section 5).10a Since 12b exhibits the same
The diastereoselectivity of the partial reduction process
could not be determined from 1H or 19F NMR spectra of the
crude materials because of a low resolution, likely due to traces
of SnCl2. Aer chromatography, however, the carbo-cyclo-
hexadienes 12b and 12d–h were obtained as mixtures of meso
(cis) and D/L (trans) isomers, identied by pairs of sharp 1H NMR
signals for the OCH3 groups and pairs of sharp 19F NMR signals
for the CF3 groups directly connected to the C18 macrocycle. For
the tetraphenylated carbo-cyclohexadiene 12c, single slightly
broadened OC1H3 and C19F3 NMR signals were observed. In all
cases, except in the case of 12h (and, perhaps, 12c), the mixture
could be resolved by silicagel chromatography. 1H and 19F
Fig. 3 1H NMR (300 MHz, left) and 19F (282 MHz, right) NMR spectra of the resolved meso (cis) and D/L (trans) diastereoisomers of 12d. Top: less
polar (on TLC); bottom: more polar.
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two-band UV-vis pattern, the XRD data of 12b conrms the orbitals of the Gouterman model (HOMO-1, HOMO, LUMO,
LUMO+1), and one-electron excitations from the ground state S0
original assignment to C (R ¼ 4-MeO–C6H4).
XRD analysis also revealed that the three carbo-cyclo- to the rst and third excited states S1 and S3, both centered on
hexadiene crystals correspond to the meso (cis) stereoisomers the conjugated DBA core.10a As the Ph or CF3 substituents at the
12b–d, with similar geometrical features (Fig. 4). The C18 mac- sp3 carbon atoms of the carbo-cyclohexadiene ring are not p-
rocycle is slightly distorted (maximum deviation from the mean conjugated with the DBA motif, the same interpretation can be
˚
˚
˚
plane: 0.43 A for 12b, 0.55 A for 12c, 0.23 A for 12d), with small inferred to apply in the CF series. The variation of the maximum
torsion angles of the endocyclic DBA motif: 5.8ꢁ, 7.4ꢁ and 2.1ꢁ in absorption wavelength (lmax) as a function of the substituents
12b, 12c and 12d, respectively. Nevertheless, in contrast to the follows the same trend as the one previously observed for the s-
quasi-planar carbo-benzenes of type A (Fig. 1), the butatriene acyclic DBA derivatives of type B, with a slight general bath-
and but-2-yne edges of the carbo-cyclohexadienes 12b–d of ochromic shi for the present s-cyclic series of type CF
type CF are mesomerically non-equivalent and exhibit bond (Fig. 1).4,7 In this series, and by reference to the tetraphenylated
lengths close to those reported for the linear (s-acyclic) DBA dye 12c, the largest bathochromic shis are thus observed for
analogues of type B.7 Indexing the DBA motif of 12b–d as the most donating anilinyl-type substituents of 12e and 12f. The
C7]C8]C9]C10(Ph)–C11^C12–C13(Ph)]C14]C15]C16, still p-donating anisyl substituents of 12b and extended phe-
the sequence of the bond lengths from C7 to C12 and from C16 nylethynyl substituents of 12h also induce higher lmax values
˚
to C11 reads (in A): 1.355 (ꢂ0.010), 1.241 (ꢂ0.010), 1.356 than the phenyl substituents of 12c. In contrast, the electron-
(ꢂ0.010), 1.423 (ꢂ0.005), 1.196 (ꢂ0.005). The central and lateral withdrawing p-triuoromethylphenyl substituents of 12d
Csp-Csp bonds exhibiting a signicant difference in length induce a small hypsochromic shi below the reference value of
˚
(D ¼ 0.045 A) are therefore assigned to xed triple and double 12c at 574 nm.
bonds, respectively.
In the cyclic series CF, the molar extinction coefficient was
found to vary from 9900 to 112900 L molꢀ1 cmꢀ1, the limit
values being achieved for 12f and 12d, respectively. These values
are in the same range as those previously reported in the acyclic
series B.7b The uorophore-substituted carbo-cyclohexadienes
12e and 12f were also found to be emissive at 427 (lexc ¼ 243
nm) and 485 nm (lexc ¼ 348 nm), respectively, namely at lower
wavelengths than their s-acyclic parents in the B series, emit-
ting at around 500 nm.4,7b
Possible solvato-chromism in the carbo-cyclohexadiene
series CF was nally investigated for the selected tetrakistri-
uoromethylated representative 12d, which, contrary to its
congeners, is soluble in a typical range of aprotic solvents,
including pentane (see ESI†). Starting from pentane, aer a rst
step of ca 10 nm, a quasi-negligible bathochromic shi with
dielectric constant (3r) is observed, followed by another step of 4
nm for toluene, an aromatic solvent prone to bind with 12b
through specic p–p-stacking interactions: 416 and 568 nm in
pentane (3r ¼ 1.8), 423 and 578 nm in chloroform (3r ¼ 4.8),
424 and 578 nm in THF (3r ¼ 7.5), 427 and 582 nm in toluene
(3r ¼ 2.4).
5. Absorption spectroscopy of carbo-cyclohexadienes
The carbo-cyclohexadienes 12 are highly chromophoric,
possibly giving intense red (12b,c), blue (12d–f,h) or purple
(12g) solutions in usual organic solvents. The electronic spectra
of 12b–f,h in diluted chloroform solutions are combined in
Fig. 5 (the minute quantities and moderate stability of 12g
prevented full characterization). The spectra exhibit similar
patterns, with two intense bands in the visible region (and a
third intense band in the UV region due to carbazole and indole
substituents for 12e and 12f, respectively). The same two-band
pattern was previously observed for the tetraphenylated
analogue C (R ¼ 4-MeO–C6H4), theoretical spectra of which
were also calculated at the TDDFT and ZINDO levels.10a These
calculations were found to reproduce the observations, with
high accuracy for the absolute transition energies (433 and 615
nm at the ZINDO level, vs. 437 and 602 nm for the experimental
values) and denite agreement for the relative oscillator
strengths (f ¼ 1.60 at 433 nm, and f ¼ 0.77 at 615 nm at the
ZINDO level). The transitions were shown to involve the four
Fig. 4 Molecular views of the X-ray crystal structures of the carbo-cyclohexadienes 12b (left), 12c (middle), and 12d (right) (Scheme 6). 50%
probability level for the thermal ellipsoids. For clarity, all hydrogen atoms, disordered atoms and solvent molecules are omitted. DBA motifs:
C7–C16.
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Fig. 5 Absorption spectra of the carbo-cyclohexadienes of type CF, 12b–f,h (in CHCl3).
In a related context, as previously reported for a tetraphenyl- 602 nm for the C version, and at 442 and 605 nm for the CF
carbo-cyclohexadiene of type C,10a the bis-triuoromethylated version 12b (Fig. 6).
dyes of type CF appear more or less dichromic in diluted solu-
The dianisyl series (Fig. 1, R ¼ 4-MeO–C6H4) is completed by
tion.11 In particular, the dianisyl-carbo-cyclohexadienes of types two additional representatives A and B, where the central core is
C (R ¼ 4-MeO–C6H4) and CF (12b) exhibit the same turquoise- a rigid aromatic carbo-benzene ring in A and a exible s-acyclic
blue/deep purple dichromism,11 and almost superimposable carbo-n-butadiene unit in B. The latter possesses formally the
UV-vis spectra, with two intense absorption bands at 437 and same DBA p-conjugated system as in the s-cyclic versions C or
Fig. 6 UV-vis absorption spectra in CHCl3 (bottom) of all the known dianisyl-substituted carbo-meric cores of the types A, B, C (Fig. 1, for R ¼ 4-
MeO–C6H4) and CF (¼ 12b) (top).
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CF (12b) and exhibits also a two-band absorption spectrum, with voltammetry (CV). The corresponding data are summarized in
similar absorption wavelengths at 443 and 591 nm but with Table 1.
markedly different respective intensities (these intensities are
All the carbo-cyclohexadienes exhibit quasi-identical reduc-
similar in C and CF). This difference is attributed to the much tion behaviour, with three waves. The rst one is reversible
greater exibility of the DBA motif in B by comparison to C and (except for 12e) and occurs between ꢀ0.47 and – 0.67 V (the
CF. TDD-DFT or ZINDO calculations of the absorption spectra in latter limit values being achieved for 12d and 12b, respectively).
the free-rotating series B would thus be much more challenging As previously observed in the carbo-n-butadiene series B, and as
than in the locked series C, CF or A (see above),10a as they would expected,7b the acceptor-substituted derivative 12d of the CF
require full conformational analysis before relevant averaging. series (R ¼ 4-CF3–C6H4) is thus more prone to reduction. The
The effect of the cisoid-locked conformation of the DBA motif in rst reduction potentials are, however, systematically higher (in
C and CF (12b) is thus dramatic, resulting, in particular, in a algebraic value) in the carbo-cyclohexadiene series CF than in
much weaker dichromism of the freely rotating carbo-n-buta- the carbo-n-butadiene series B (italicized values in Table 1),
diene B.
likely because of the greater average stabilization of the anion
In contrast to the B, C and CF representatives, the electronic through the optimally conjugated quasi-planar DBA motif of the
spectrum of the quadrupolar dianisyl carbo-benzene A exhibits CF series. Another similarity between the series CF and B is the
only one main absorption band (at 482 nm), as widely docu- result of the less donating character of the indolylphenyl and
mented in the general carbo-benzene series.2c,4,10a
carbazolylphenyl substituents with respect to the anisyl
substituent, the latter giving the smallest half-wave potential of
the carbo-cyclohexadiene series (E1/2 ¼ ꢀ0.67 V for 12b).
In the oxidation process, the cations of the indolylphenyl-
and carbazolylphenyl- substituted carbo-cyclohexadienes 12e
and 12f were found to deposit on the electrode, giving electro-
active lms. In a complementary manner to what is observed in
the reduction process, the carbo-cyclohexadienes CF are less
6. Electrochemistry of carbo-cyclohexadienes
The electrochemical properties of the carbo-cyclohexadienes of
type CF (excluding the poorly stable dialkynyl derivatives 12g
and 12h) were investigated by square-wave (SW) and cyclic
Table 1 CV and SWV data for carbo-cyclohexadienes of type CF, and comparison with the first reduction and oxidation potentials of the
corresponding carbo-n-butadienes of type B (italicized values). Measurements performed at room temperature in DCM; supporting electrolyte:
[n-Bu4N][PF6] (0.1 M); working electrode: Pt; reference electrode: saturated calomel electrode (SCE, 0.242 V vs. the hydrogen electrode); scan
rate: 0.2 V sꢀ1 unless otherwise noted
Reductions
Oxidations
First reduction
Other reductions
First oxidation
Other oxidations
First reduction C
series
First oxidation
C series
B series4,7b
C seriesd
B series4,7b
C seriesd
a
a
E1/2
E1/2
(DEp)b
RIp
E1/2
Erpede
(DEp)b
RIp
E1/2 (ref. 4 and 7b)
Erpede
c
c
Compound
12b
ꢀ0.67
1.00
ꢀ0.88
ꢀ0.95
ꢀ1.18
ꢀ1.44j
ꢀ1.52j
ꢀ1.61j
ꢀ0.88
ꢀ1.07
ꢀ1.57i
ꢀ0.69k
ꢀ1.84
ꢀ0.80l
ꢀ1.00
ꢀ1.44j
ꢀ1.73j
ꢀ0.85
ꢀ1.00
ꢀ1.43j
ꢀ1.74j
1.12
0.86
0.95
1.68h
1.90
(0.06)f
(0.09)g
12c
ꢀ0.61
0.94
ꢀ0.80
1.33
(0.06)
1.05
0.95
1.60g
1.86
(0.06)
12d
12e
ꢀ0.47
0.96
ꢀ0.65
ꢀ0.75
1.54h
irr.
—
—
—
1.31
1.06
(0.06)
ꢀ0.59irr.
—
1.21h,i
irr.
1.42
1.68
12f
ꢀ0.60
1.17
ꢀ0.75
1.17h,i
irr.
—
1.04
1.60
(0.06)
Half wave potential E1/2 ¼ (Erped + Epox)/2, in V/SCE. b Separation between the two peak potentials: DEp ¼ |Erped – Epox|, in V. c Peak current ratio RIp ¼
a
|Iopx/Irped|. d Irreversible unless otherwise noted. e Ep values measured from CV in V/SCE. f Scan rate: 0.1 V sꢀ1
.
Scan rate: 0.5 V sꢀ1
.
Aer the rst
g
h
oxidation, a product deposited on the electrode. i Formation of an electroactive deposit observed. j Potentials obtained from SWV voltammograms.
k
l
Reversible couple: E1/2 ¼ ꢀ0.69 V/SCE, DEp ¼ 0.07 V, RIp ¼ 0.92. Shoulder of low intensity, which could possibly correspond to an adsorbed
product.
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prone to oxidation than the carbo-n-butadiene counterparts B
(italicized values in Table 1). Only the rst oxidation waves of
the dianisyl- and diphenyl-substituted derivatives 12b and 12c
are reversible, with the most p-donating group, R ¼ 4-MeO–C6H4,
giving the lowest potential of the series (E1/2 ¼ +1.12 V for 12b).
In contrast, the most electron-withdrawing group, R ¼ 4-
CF3–C6H4, induces the highest rst oxidation potential at 1.54 V
for 12d. The quite high rst oxidation potentials in the CF series
make the results difficult to interpret (due to the close oxidation
wave of the solvent), but can be correlated to the corresponding
low rst reduction potentials: following a general trend, mole-
cules that are readily reduced are not easily oxidized.
Notes and references
1 (a) R. Chauvin, Tetrahedron Lett., 1995, 36, 397–400; (b)
V. Maraval and R. Chauvin, Chem. Rev., 2006, 106, 5317–
5343.
2 For experimental syntheses of carbo-benzenes, see for
example: (a) Y. Kuwatani, N. Watanabe and I. Ueda,
Tetrahedron Lett., 1995, 36, 119–122; (b) R. Suzuki,
H. Tsukude, N. Watanabe, Y. Kuwatani and I. Ueda,
Tetrahedron, 1998, 54, 2477–2496; (c) C. Saccavini, C. Sui-
Seng, L. Maurette, C. Lepetit, S. Soula, C. Zou,
B. Donnadieu and R. Chauvin, Chem.–Eur. J., 2007, 13,
4914–4931; (d) C. Zou, C. Duhayon, V. Maraval and
R. Chauvin, Angew. Chem., Int. Ed., 2007, 46, 4337–4341; (e)
I. Baglai, V. Maraval, C. Bijani, N. Saffon-Merceron,
Z. Voitenko, Y. M. Volovenko and R. Chauvin, Chem.
Commun., 2013, 49, 8374–8837.
Conclusion
Since the incidental isolation of the rst example of carbo-
cyclohexadiene resulting from a partial reduction of a [6]peri-
cyclyne,10a the introduction of a triuoromethyl group on two
adjacent vertices of hexaoxy-[6]pericyclynes allowed the selec-
tive synthesis of conjugated carbo-cyclohexadienes. These bore
various types of electroactive substituents at the 1,10 positions
of the endo-macrocyclic DBA motif, with spectator phenyl
substituents at the 4,7 positions. Whereas the tetraaryl bis-tri-
uoromethylated carbo-cyclohexadienes were found to be stable
both in solution and in the solid state, two dialkynyldiphenyl
counterparts appeared less stable. Moreover, the tri-
uoromethylated carbo-cyclohexadienes appear much more
stable than their phenylated analogues, without modifying their
chromophoric and spectroscopic properties, as evidenced in the
anisyl-substituted series. The optical and electronic properties
of this novel type of carbo-meric chromophore deserve system-
atic attention. In particular, as justied in the introduction, the
dianilinyl derivative CF (R ¼ 4-NH2–C6H4, Fig. 1) is currently
being targeted for measurement of its single molecule
conductance (SMC) and comparison with the carbo-benzene A
(R ¼ 4-NH2–C6H4).8 As neither the [8 + 10F] nor the [8F + 10]
strategy proved to be compatible with the NH2 substituents,
further methodological improvements are required. In parallel,
theoretical studies are being undertaken to bring out the
specic, but subtle effects of the CF3 substituents in the CF
series, with respect to the phenyl substituents in the C
series.10a,17
3 For theoretical studies on the aromaticity of carbo-benzenes,
see: (a) C. Godard, C. Lepetit and R. Chauvin, Chem.
Commun., 2000, 1833–1834; (b) C. Lepetit, C. Godard and
R. Chauvin, New J. Chem., 2001, 25, 572–580; (c) C. Lepetit,
B. Silvi and R. Chauvin, J. Phys. Chem. A, 2003, 107, 464–
473; (d) C. Zou, C. Lepetit, Y. Coppel and R. Chauvin, Pure
Appl. Chem., 2006, 78, 791–811; (e) C. Lepetit, C. Zou and
R. Chauvin, J. Org. Chem., 2006, 71, 6317–6324; (f)
R. Chauvin, C. Lepetit, V. Maraval and L. Leroyer, Pure
´ ´
Appl. Chem., 2010, 82, 769–800; (g) J.-M. Ducere, C. Lepetit
and R. Chauvin, J. Phys. Chem. C, 2013, 117, 21671–21681.
4 A. Rives, I. Baglai, V. Malytskiy, V. Maraval, N. Saffon-
Merceron, Z. V. Voitenko and R. Chauvin, Chem. Commun.,
2012, 48, 8763–8765.
5 The “double-cut aromatic cyclic energy”, ACEDC, is a
rationally devised approximate of the exact topological
aromaticity of a ring; for benzene, it is the enthalpy of the
homodesmotic equation: 2 1,3,5-n-hexatriene ¼ benzene +
ethylene + 1,3-butadiene. See: (a) J.-P. Malrieu, C. Lepetit,
M. Gicquel, J.-L. Heully, P. W. Fowler and R. Chauvin, New
J. Chem., 2007, 31, 1918–1927.
6 (a) J.-D. van Loon, P. Seiler and F. Diederich, Angew. Chem.,
Int. Ed. Engl., 1993, 32, 1187–1189; (b) A. Auffrant, B. Jaun,
P. D. Jarowski, K. N. Houk and F. Diederich, Chem.–Eur. J.,
2004, 10, 2906–2911; (c) V. Maraval, L. Leroyer, A. Harano,
C. Barthes, A. Saquet, C. Duhayon, T. Shinmyozu and
R. Chauvin, Chem.–Eur. J., 2011, 17, 5086–5100.
7 (a) A. Rives, V. Maraval, N. Saffon-Merceron and R. Chauvin,
Chem.–Eur. J., 2012, 18, 14702–14707; (b) A. Rives, V. Maraval,
N. Saffon-Merceron and R. Chauvin, Chem.–Eur. J., 2014, 20,
483–492.
Acknowledgements
I. B. thanks the French Embassy in Kiev, Ukraine, for nancial
support. In addition to the GDRI “groupement franco-ukrainien
8 Z. Li, M. Smeu, A. Rives, V. Maraval, R. Chauvin, M. A. Ratner
and E. Borguet, submitted.
´
en chimie moleculaire” funded by the Centre National de la
9 Since the pioneering work of L. P. Hammett (L. P. Hammett,
Physical Organic Chemistry, McGraw-Hill, NewYork, 1940),
the effects of substituents on benzene derivatives have
been thoroughly investigated, in particular by comparison
with cyclohexadienes, which were theoretically evidenced
to be more sensitive to substituent effects than their
Recherche Scientique (CNRS), the ANR program (ANR-11-
BS07-016-01) is acknowledged for the post-doctoral fellowship
of A.R. Thanks are nally due to Dr Evelyne Chelain for her
valuable advice on uorine chemistry, and to Mr Kevin Cocq for
his assistance in studying solvent effects in UV-vis absorption
´
spectroscopy.
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´
benzenic counterparts: (a) T. M. Krygowski and B. T. St˛epien, 14 (a) L. Maurette, C. Tedeschi, E. Sermot, M. Soleilhavoup,
Chem. Rev., 2005, 105, 3482–3512; (b) T. M. Krygowski,
F. Hussain, B. Donnadieu and R. Chauvin, Tetrahedron,
2004, 60, 10077–10098; (b) L. Leroyer, C. Zou, V. Maraval
and R. Chauvin, C. R. Chim., 2009, 12, 412–429.
´
M. A. Dobrowolski, M. K. Cyranski, W. P. Oziminski and
P. Bultinck, Comput. Theor. Chem., 2012, 984, 36–42.
10 (a) L. Leroyer, C. Lepetit, A. Rives, V. Maraval, N. Saffon- 15 C. Saccavini, C. Tedeschi, L. Maurette, C. Sui-Seng, C. Zou,
Merceron, D. Kandaskalov, D. Kieffer and R. Chauvin,
Chem.–Eur. J., 2012, 18, 3226–3240; (b) original reference
M. Soleilhavoup, L. Vendier and R. Chauvin, Chem.–Eur. J.,
2007, 13, 4895–4913.
on [6]pericyclynes: L. T. Scott, G. J. DeCicco, J. L. Hyun and 16 (a) Crystallographic data for 12b: C48H30F6O4, M ¼ 784.75,
˚
G. Reinhardt, J. Am. Chem. Soc., 1985, 107, 6546–6555.
11 Dichromism, or dichromatism, is a property of some
materials or solutions, which makes them appear in two
different colors to the human eye depending on the
concentration of the absorbing dye and on the thickness of
the traversed medium. For references, see: (a)
H. Cartwright, J. Chem. Educ., 1986, 63, 984–987; (b)
S. Kre and M. Kre, Naturwissenschaen, 2007, 94, 935–
939; (c) J. P. Launay, unpublished report, Toulouse (private
communication); (d) J. P. Launay, La vision des couleurs,
Monoclinic, space group C2/c, a ¼ 13.07527(8) A, b ¼
ꢁ
˚
˚
16.18867(10) A, c ¼ 19.01027(12) A, b ¼ 100.1002(6) , V ¼
3
3
˚
3961.57(4) A , Z ¼ 4, crystal size: 0.15 ꢃ 0.15 ꢃ 0.25 mm ,
35692 reections collected (3771 independent, Rint
¼
0.0213), 262 parameters, R1 [I > 2s(I)] ¼ 0.049, wR2 [all
data] ¼ 0.065, largest diff. peak and hole: 0.61 and ꢀ0.21
e.Aꢀ3, CCDC 1003439; (b) crystallographic data for 12c:
˚
C
48H24F12O2, CHCl3, M ¼ 980.04, Monoclinic, space group
˚
˚
˚
P21/c, a ¼ 12.8182(16) A, b ¼ 35.815(5) A, c ¼ 9.7626(14) A,
3
ꢁ
˚
b ¼ 92.841(6) , V ¼ 4476.4(11) A , Z ¼ 4, crystal size: 0.20
´
“Regards Croises” conference at the Interdisciplinary
ꢃ 0.20 ꢃ 0.04 mm3, 59064 reections collected (7551
Doctoral Workshop 2014-2015 of the IUF on the theme
“The color”, University of Toulouse, November 4, 2014.
12 L. Leroyer, V. Maraval and R. Chauvin, Chem. Rev., 2012, 112,
1310–1343.
independent, Rint
¼
0.1994), 653 parameters, 102
restraints, R1 [I > 2s(I)] ¼ 0.0829, wR2 [all data] ¼ 0.2414,
largest diff. peak and hole: 0.573 and –0.326 e.Aꢀ3, CCDC
˚
951896; (c) crystallographic data for 12d: C46H26F6O2,
˚
ꢁ
ꢀ
13 See for example: (a) A. D. Allen and T. T. Tidwell, in Advances
in Carbocation Chemistry, ed. X. Creary, JAI Press, Greenwich,
CH2Cl2, M ¼ 809.59, Triclinic, P1, a ¼ 12.5119(17) A, b ¼
˚
˚
12.6648(17) A, c ¼ 14.9565(19) A, a ¼ 70.805(5) , b ¼
3
ꢁ
ꢁ
˚
¨
¨
1989, vol. 1, pp. 1–44; (b) T. Surig, H.-Fr. Grutzmacher,
66.621(5) , g ¼ 67.665(5) , V ¼ 1967.5(5) A , Z ¼ 2, crystal
size: 0.20 ꢃ 0.10 ꢃ 0.04 mm3, 28934 reections collected
(7350 independent, Rint ¼ 0.0906), 669 parameters, 489
restraints, R1 [I > 2s(I)] ¼ 0.0672, wR2 [all data] ¼ 0.1983,
´
´
J.-P. Begue and D. Bonnet-Delpon, Org. Mass Spectrom.,
1993, 28, 254–261; (c) K. K. Laali, M. Tanaka,
S. Hollenstein and M. Cheng, J. Org. Chem., 1997, 62,
largest diff. peak and hole: 0.212 and –0.314 e.Aꢀ3, CCDC
˚
´
´
7752–7757; (d) M. Gruselle, B. Malezieux, R. Andres,
H. Amouri, J. Vaissermann and G. G. Melikyan, Eur. J.
951897.
Inorg. Chem., 2000, 359–368; (e) J. P. Richard, T. L. Amyes 17 H. Hamdani, C. Lepetit and R. Chauvin, unpublished
and M. M. Toteva, Acc. Chem. Res., 2001, 34, 981–988.
results.
This journal is © The Royal Society of Chemistry 2014
Chem. Sci.