Synthesis of Functional Carbo -benzenes with Functional Properties: The C 2 Tether Key

Article abstract of DOI:10.1055/s-0037-1610269

Beyond demonstration of conceptual relevance and synthetic feasibility of aryl/alkyl-substituted representatives, carbo -benzene molecules started to gain prospects of broader impact through the emergence of alkynyl derivatives. This is first illustrated

Full text of DOI:10.1055/s-0037-1610269

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© Georg Thieme Verlag Stuttgart · New York
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en
K. Cocq et al.
Account
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Synthesis of Functional Carbo-benzenes with Functional
Properties: The C2 Tether Key
OnCnH2n+1
Kévin Cocqa,b
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Cécile Barthesa,b
Arnaud Rivesa,b
Valérie Maraval*a,b
Remi Chauvin*a,b
Ph
Ph
Ph
OnCnH2n+1
OnCnH2n+1
H2N
Et
N
•
•
•
•
R
•
R
•
0
0
0
0-0
0
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3
3
2
nC8H17
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2-4
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9
1-6
3
9
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C2
C2
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a CNRS, LCC (Laboratoire de Chimie de Coordination), 205
Route de Narbonne, BP44099, 31077 Toulouse Cedex 4,
France
18 pz e
FG
•
•
•
•
FG
•
•
Ph
Ph
valerie.maraval@lcc-toulouse.fr
remi.chauvin@lcc-toulouse.fr
b Université de Toulouse, UPS, ICT-FR 2599, 31062 Toulouse
Cedex 9, France
•
•
vs
SiR3
H
nC6H13
nC6H13
R
R
with or without:
versatile effects on stability, solubility, magnetic anisotropy, molecular conductance, columnar mesogenicity,
one-photon and two-photon absorption, p-cooperative surface photosensitization
Received: 11.07.2018
Accepted after revision: 13.08.2018
Published online: 12.10.2018
1 Introduction
The acetylenic spC2 unit is ubiquitous in the structures
DOI: 10.1055/s-0037-1610269; Art ID: st-2018-a0437-a
of organic materials with diverse optical, electrical, mag-
Abstract Beyond demonstration of conceptual relevance and synthetic
feasibility of aryl/alkyl-substituted representatives, carbo-benzene mol-
ecules started to gain prospects of broader impact through the emer-
gence of alkynyl derivatives. This is first illustrated by examples of di-
and hexaalkynyl-carbo-benzenes, a carbo-naphthalene, a carbo-biphe-
nyl, and two carbo-terphenyls. A focus is then given to dialkynyl deriva-
tives by reference to the peripherally C2-extruded parents. In the centro-
symmetric quadrupolar series, the C2 expansion or ethynylogation
effect is more particularly considered for 9H-fluoren-2-yl, tris(O-n-
alkyl)pyrogallyl, indol-3-yl, 4-anilinyl, and tetraphenyl-carbo-phenyl
substituents on the following respective properties: two-photon ab-
sorption, chemical stability, columnar mesogenicity, on-surface photo-
induced charge separation vs single-molecule conductance, and reduc-
tion potential. Topical results and prospects of application are
discussed on the basis of crystallographic, spectroscopic, and electro-
chemical analyses vs DFT-calculated nuclear and electronic structures.
For the sake of the discussion consistency, complementary experimen-
tal and computational results are disclosed in the dianilinyl series. Over-
all, it is shown that combined advances in strategy, protocols, and sub-
strate scope of acetylenic synthesis remain crucial for the development
of yet poorly explored but promising types of molecular materials.
netic, or supramolecular properties. As a linker, the spC2
unit indeed provides rigidity, minimal steric hindrance, and
efficient -conjugation (elliptic torus of doubly degenerated
-orbitals). It most often occurs between -unsaturated
functional groups, in particular aromatic moieties, as illus-
trated by the paradigm of oligo(phenylethynyl)enes (OPEs).
This occurrence benefits from the general applicability of
the Sonogashira sp2C–spC coupling reaction. However, the
C∞-axial dicarbon unit is more than a linker and actually
acts as an ‘expander’, preserving the local valence shell
electron pair repulsion (VSEPR) geometrical relationships
between the connected moieties. As formally proposed
through the carbo-mer concept,1a,b the intrinsic C2-expan-
sion effect should be appraised by comparison of any acety-
lenic molecule with its parent where all the spC2 units have
been formally extruded.1a This would be topically illustrat-
ed by the ethynylogous series OPEs vs OPPs (oligo-para-
phenylenes), but to the best of the authors’ knowledge, lit-
erature lacks report of such systematic comparison for any
particular property.2 Though much less invoked than viny-
logation, the ethynylogation process was, however, formally
invoked in different contexts.1d–h
For now two decades, most efforts in carbo-mer chemistry
have been devoted to ring carbo-mers of benzenic deriva-
tives of type 1, simply named carbo-benzenes (Figure 1).1a–c
The first representatives were reported in 1995 by I. Ueda,
Y. Kuwatani, et al.,3 simultaneously with the general defini-
tion of carbo-merization,1c giving rise to expanded carbon-
and -electron-rich molecules, naturally calling for com-
parison with their nonexpanded parent, at least at the theo-
retical level.4 In particular, the Hückel 4n+2 -electron
1
2
3
4
Introduction
Hexaalkynyl-carbo-benzene
ortho-Dialkynyl-carbo-benzene
para-Dialkynyl-carbo-benzenes
4.1 Bistrimethylsilylethynyl-carbo-benzene
4.2 Bisfluorenylethynyl-carbo-benzene
4.3 Bistrialkoxyarylethynyl-carbo-benzenes
4.4 Bisindolylethynyl-carbo-benzene
4.5 Bisanilinylethynyl-carbo-benzene
5
6
Carbo-oligo(phenyleneethynylene)s
Conclusions
Key words alkyne, C2 tether, carbo-benzene, carbo-mer, -conjuga-
tion
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

B
K. Cocq et al.
Account
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Biographical Sketches
After few months in Prague,
Laboratoire de Chimie de Coor-
dination (LCC). He worked on
the synthesis of carbo-quinoids
(expanded analogue of quinodi-
methane) and carbo-benzenoids
(as small fragment of -gra-
phyne). He then spent ten
months as a postdoctoral fellow
for the synthesis of novel photo-
sensitizers for water-splitting
process. He is currently teacher
at the Université Paul Sabatier in
Toulouse, France and he carries
on the development of catalysts
for efficient water-splitting
technology.
Czech Republic, working on
Helicene chemistry under the
supervision of Dr. Irena G. Stara,
Kévin Cocq came back to
Toulouse, France. He obtained
his PhD in 2015 under the su-
pervision of Prof. Remi Chauvin
and Dr. Valérie Maraval at the
Cécile Barthes obtained
a
Andre Gourdon. Then she
worked six months at Sanofi-
Aventis in the field of research
synthesis. In late 2007 she
joined LFHA for one year with
industrial contract working on
silicon polymers. Since 2008,
she got a position in LCC-CNRS
in 2008 as technician in chemi-
cal synthesis. She is taking part
in various projects in syntheses
forming electron-rich organics
molecules as carbo-mers to
phosphino-carbene complexes.
diploma of advanced technician
in 2003 in synthesis chemistry.
Then she worked in CEMES-
CNRS for 2004–2007 on the
synthesis of polyaromatic com-
pounds under the supervision of
Arnaud Rives was born in
1981 in l’Union, France. He
completed his PhD in 2010 at
the SPCMIB laboratory in
Toulouse under the supervision
of Yves Génisson. After a two-
year postdoctoral fellow stay at
the ‘Laboratoire de Chimie de
Coordination’ in Toulouse with
Valérie Maraval and Remi
Chauvin, he has joined the bio-
technology company AFFICHEM
in Toulouse since 2012, as
chemistry laboratory manager.
Valérie Maraval obtained her
PhD in 2000 under the super-
vision of Jean-Pierre Majoral in
Toulouse, France. In 2001, she
worked as a postdoctoral fellow
with Bernard Meunier and the
Aventis company. She then
came back to Jean-Pierre
Majoral’s team as a CNRS engi-
neer. In 2005, she joined the
group of Remi Chauvin at the
Laboratoire de Chimie de Coor-
dination in Toulouse as a CNRS
research engineer, where she
completed her Habilitation in
2014. Her research activity
focuses on two aspects of acet-
ylenic chemistry: (i) synthesis of
carbon-rich and electron-rich
molecules (carbo-mers) for the
study of their optical, electron-
ic, and mesogenic properties;
(ii) synthesis of biologically ac-
tive chiral acetylenic lipids in-
spired from natural products.
Remi Chauvin, Professor at
the Paul Sabatier University, has
been leading his research at the
Laboratoire de Chimie de Coor-
dination of the CNRS in Tou-
louse since 1998. Before, he was
successively PhD student with
Prof. Henri Kagan in Orsay,
France (1986–1988), postdoc-
toral fellow with Prof. Barry
Sharpless in Cambridge, MA,
USA (1989), then with Prof.
Andrea Vasella in Zürich,
Switzerland (1990), R&D investi-
gator at the Roussel-Uclaf com-
pany in Romainville, France
(1990–1993), and CNRS re-
search associate in Toulouse
(1993–1998). Since 2008 he is
in charge of interuniversity rela-
tionships with research institu-
tions in Ukraine and was
awarded Doctor Honoris Causa of
the Taras Chevtchenko National
University in Kiev in 2015. Since
2015 he is recurrent invited pro-
fessor at Huaqiao University,
Xiamen, China, where he manag-
es a research outpost at the
School of Biomedical Sciences.
Within the framework of rele-
vant collaborations, his research
interests extend from organo-
metallic main group chemistry
for catalysis (extreme phospho-
carbon ligands) to mathemati-
cal-computational chemistry for
theoretical analysis (quantifica-
tion of chemical concepts, chi-
rality, aromaticity, dativity),
through organic chemistry for
physics (carbo-meric molecules
with optical, electrical or liquid
crystal properties) and organic
chemistry for biology (bioin-
spired lipidic acetylenic mole-
cules with antitumor or
antibacterial properties).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

C
K. Cocq et al.
Account
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count of the C18 ring provides carbo-benzenes with a ‘carbo-
aromatic’ character1b underlying unique chromophoric,
electric, or supramolecular properties, notably different
from those of parent benzenes and nonaromatic -conju-
gated counterparts such as carbo-butadienes 2,5 carbo-
cyclohexadienes 3,6 or carbo-quinoids 4.1,7
2 Hexaalkynyl-carbo-benzene
With six triethylsilylethynyl groups, 1a can be consid-
ered as a Si-protected version of the total carbo-mer of ben-
zene C18(C2H)6,12 completing the features of the peripheral
counterpart,
hexaethynylbenzene,13a
among
other
Carbo-benzenes have been mostly produced by reduc-
tive aromatization of hexaoxy[6]pericyclynes formed by
heteropolar macrocyclization reactions characterized by
patterns of the type [m + (18 – m)], where m denotes the
number of carbon atoms of the target C18 ring brought by
the nucleophilic reactant (m = 8, 9, 11, 14). 8 While the bare
carbo-benzene C18H6 remains unknown (a priori because of
the instability of envisaged synthetic intermediates),9 only
a few partially tetra- or trisubstituted versions have been
reported,3,10 all the other representatives being hexasubsti-
tuted, of generic formula C18R1R2R3R4R5R6, Ri ≠ H. Beside re-
cent examples of alkyl substitution (Ri = n-CnH2n+1, 2 ≤ n ≤
20, i-Pr, t-Bu),11 most of the known carbo-benzenes bear ar-
omatic groups (Ri = 4-X-C6H4, pyrogallyl, indolyl, fluorenyl, ...),
and less frequently alkynyl groups Ri = –C≡C–R'i.1,10a,12
Beyond their postfunctionalization potential, the latter are,
however, basically attractive for the above-emphasized
‘pure’ expander effect of the spC2 unit.
perethynyl [n]annulene derivatives Cn(C2R)n, n = 3–5, 8.13b–p
The target 1a was prepared through a 16-step sequence in-
volving a [8+10] macrocyclization step from the C8-pen-
tayne 5a and the C10-diketone 6 (Scheme 1). Reductive aro-
matization of the resulting hexaalkynylated [6]pericyclyne-
diol 7 could not be performed under the standard
conditions used for the aromatization of arylated counter-
parts, i.e., by treatment with SnCl2 and HCl.1,3a This was
attributed to the destabilization of the six equivalent puta-
tive trialkynyl carbenium primary intermediates: In the ab-
sence of steric hindrance to planar -C+ conjugation, a prop-
argylic C–OR bond is indeed less prone to heterolytic dis-
sociation than benzylic counterparts.14 Stabilization of the
carbenium centers was thus envisaged via Nicholas’ com-
plexes generated in situ by treatment of 7 with two equiva-
lents of Co2(CO)8:15 The anticipated complexes 7·Co2(CO)6 or
7·[Co2(CO)6]2 were not isolated, but treatment with SnCl2/HCl,
followed by oxidative Co-decoordination with ceric ammo-
nium nitrate, afforded 1a with 12% yield over three steps in
one pot.12 While the six SiEt3 groups surrounding the rigid
C30 core provided sufficient solubility for full characteriza-
tion of 1a (including by 13C NMR spectroscopy), attempts at
protodesilylation resulted in the deposit of a black solid,
which was not characterized but tentatively attributed to
the extremely carbon-rich stoichiometry C30H6 (C/H = 5,
equivalent to that of the [4]coronene graphene flake C150H30
made of 61 C6 hexagons).16 The total carbo-mer of benzene
C18(C2H)6 thus remains uncharacterized experimentally,
but was studied in detail at the DFT computational level
(B3PW91/6-31G(d,p)), and compared with the correspond-
ing ring carbo-mer C18H6.4d The effect of C2-tethering of six
H atoms to the C18 core happens to be limited: both the
root-mean-square deviation from the mean C–C bond
length (r) and nucleus-independent chemical shift at the
center of the ring (NICS) increase slightly from r = 0.061 Å
and NICS = –19.7 ppm for C18H6 to r = 0.070 Å and NICS =
–17.8 ppm for C18(C2H)6. These variations reveal a slight
loss of the aromatic character of the C18 ring according to
structural and magnetic criteria, indeed anticipated to be
induced by -conjugation extension over 12 external spC
atoms. The same trend was recently observed in a series of
R6
R5
X
X
R1
R4
R1
R4
R2
R3
R3
R2
carbo-benzene 1
carbo-butadiene 2
CF3
CF3
Ph
Ph
MeO
OMe
R1
R4
Y
Y
R2
R3
Ph
Ph
carbo-cyclohexadiene 3
carbo-quinoid 4
Figure 1 Main prototypes of carbo-mer molecules with extended -
conjugation
Alkynyl-substituted carbo-benzenes are thus surveyed
hereafter from both the standpoints of properties and syn-
thesis. Except the hexaalkynyl representative 1a,12 all other
know examples are tetraarylated with two alkynyl substit-
uents only.
tert-butyl-substituted carbo-benzenes, C18PhntBu6-n
,
in
which magnetic anisotropy was shown to decrease upon
increasing the number n of phenyl substituents.11d
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

D
K. Cocq et al.
Account
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SiEt3
Et3Si
SiEt3
Et3Si
SnCl3
Co(CO)3
Co(CO)3
OH
MeO
Et3Si
Et3Si
SiEt3
HO
SiEt3
MeO
OMe
Et3Si
SiEt3
Et3Si
SiEt3
1a, 12%
1) 2 Co2(CO)8
2) SnCl2/HCl
3) CAN
– 2 CO, – MeOH
Et3Si
MeO
SiEt3
OMe
OH
Et3Si
HO
MeO
SiEt3
OMe
Et3Si
SiEt3
SiEt3
7
[8+10]
Et3Si
H
H
MeO
OMe
MeO
OMe
SiEt3
Et3Si
SiEt3
Et3Si
5a
6
O
O
Scheme 1 Synthesis of the hexaalkynyl-carbo-benzene 1a
3 ortho-Dialkynyl-carbo-benzenes
4 para-Dialkynyl-carbo-benzenes
One example of ortho-dialkynyl-carbo-benzene, 1b, was
recently described within the context of the synthesis of
the carbo-naphthalene 8 relying on the dialkynyl-[6]pericy-
clynediol intermediate 9 (Scheme 2).17 The latter, obtained
by [8+10] macrocyclization from the pentayne 5b and dike-
tone 10, was thus independently submitted to the reductive
aromatization reagent SnCl2/HCl, producing 1b with 16%
yield. Noteworthy is the formal ortho-dialkynyl-carbo-ben-
zenic character 8A of the carbo-naphthalene 8 itself, albeit
diluted in the Kekulé resonance with the ortho-carbo-qui-
noid forms 8B, featuring a 1,16-bridged carbo-[10]annulene.
Most of the dialkynyl-carbo-benzenes exemplified hith-
erto belong to the para series, i.e., with two alkynyl substit-
uents at the 1,10-positions of the macrocycle. This is mainly
due to versatility of a general synthetic route involving a
pivotal [6]pericyclynedione synthon (see section 4.1 and
Scheme 3).
4.1 Bistrimethylsilylethynyl-carbo-benzene
The bistrimethylsilylethynyl-carbo-benzene 1c was ac-
cessed through two alternative [14+4] and [8+10] macrocy-
clization processes (Scheme 3).18,10a The [14+4] route in-
volved the C14-dinucleophile 11 and C4-dielectrophile 12,19
the former being obtained in four steps from the dialdehyde
13, itself prepared in five steps from commercial materials.18
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

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K. Cocq et al.
Account
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Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
8A
8B
Ar
Ar
Ar
Ar
8
Ar
Ar
OH
SiiPr3
Me
O
O
OMe
OMe
O
Ar
Ar
+
OMe
MeO
Ar
Ar
O
SiiPr3
Me
HO
Ar
Ar
10
9
+
H
SiiPr3
Me
O
Ar
SiiPr3
Ar
O
H
SiiPr3
Me
Ar
5b
SiiPr3
Ar
nC5H11
Ar =
1b
Scheme 2 Synthesis of the ortho-dialkynyl-carbo-benzene 1b and carbo-naphthalene 8
Addition of the dilithium salt of 11 to dibenzoylacetylene
12 afforded the [6]pericyclynediol 14a with 43% yield. Sub-
sequent reductive aromatization with SnCl2/HCl afforded 1c
with 10% yield.
material product could be assigned to the terminal acety-
lenic proton of 1d,10a the C2-extruded version of which
(H-C18Ph4-H) has been described in detail.10 Nevertheless,
the C2-extruded version of 1c, i.e., Me3Si–C18Ph4–SiMe3,
with two C18–heteroatom bonds, is still missing and defi-
nitely deserves future synthesis efforts.
This 13-step synthesis of 1c was later superseded by a
[8+10] route regarded as optimal for symmetrical para-
disubstituted carbo-benzene targets.8d The strategic effi-
ciency is here due to both the selectivity of the cyclization
process and its late divergent character, the C10-dielectro-
phile 13 being directly prepared from the C8-dinucleophile
15. The [6]pericyclynedione intermediate 16 is also a versa-
tile precursor to which various nucleophiles can be added,
thus allowing a two-step access to a wide range of para-
disubstituted carbo-benzenes.8d Addition of the dimagne-
sium salt of 15 to 13 thus led to the [6]pericyclynediol 17,
which was then oxidized to the diketone 16 with MnO2
(Scheme 3). Addition of trimethylsilylethynyl-magnesium
bromide to 16 gave 14b with an unoptimized yield of 17%.
Ultimate reductive aromatization of 14b gave access to 1c
in 12 steps. Though attempts at isolating the deprotected
diethynyl-carbo-benzene 1d failed, a signal at  = 4.3 ppm
in the 1H NMR spectrum of a soluble extract from the black
4.2 Bisfluorenylethynyl-carbo-benzene
Fluorophore-substituted carbo-benzenes were targeted
with the aim of studying their nonlinear optical (NLO)
properties, in particular their two-photon absorption (TPA)
efficiency, a 3d-order NLO property. On the basis of numer-
ous examples, quadrupolar extended -conjugated systems
have indeed been proposed as systematic TPA chromophore
candidates.20 Fluorenyl moieties, widely used as weakly
donor ends D in D–Π–D TPA systems centered on a -conju-
gated core Π,21 were thus also envisaged for a carbo-p-
phenylene Π core. Two carbo-benzenes, 1e and 1f, differing
by the fluorenyl–C18 tethering mode, were thus prepared in
two steps from the key [6]pericyclynedione 16 (Scheme
4).22 Reaction of 16 with dihexylfluorenyllithium 18a and
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

F
K. Cocq et al.
Account
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Ph
Ph
Ph
Ph
TBAF
Me3Si
SiMe3
H
H
or
K2CO3
Ph
Ph
Ph
Ph
1c
1d
SnCl2/HCl
10%
Ph
O
Ph
O
Ph
Ph
Ph
Ph
12
Me3Si
R1O
OR1
OR2
MeO
MeO
OMe
+
Me3Si
H
H
Me3Si
MgBr
n-BuLi
O
O
OMe
R2O
MeO
43%
17%
[14+4]
MeO
MeO
OMe SiMe3
OMe
OMe
Ph
Ph
Ph
16
Ph
OMe SiMe3
14a: R1 = H, R2 = Me
14b: R1 = Me, R2 = H
Ph
Ph
11
MnO2
Ph
70%
Ph
4 steps
Ph
MeO
Ph
1) n-BuLi
Ph
Ph
MeO
O
OMe
H
OH
H
1) EtMgBr
MeO
H
OMe
H
2) (HCHO)n
2) 13
HO
MeO
3) IBX, Δ
30%
H
H
[8+10]
O
OMe
15
Ph
Ph
13
17
Scheme 3 Alternative strategies for the synthesis of the bistrimethylsilylethynyl-tetraphenyl-carbo-benzene 1c
dihexylfluorenylethynyl-magnesium bromide 18b thus
gave the pericyclynediols 19a and 19b in 41% and 50 %
yield, respectively. Subsequent reductive aromatization
with SnCl2/HCl afforded the target carbo-benzenes 1e and
1f in 55 ± 4% yield.
Double ethynylogation has also a significant effect on
the first reduction and oxidation potential values, changing
from –0.75 V and +1.08 V/SCE in 1e to –0.61 V and +1.17
V/SCE in 1f. Notably, Ered1/2 (1f) = –0.61 V is the highest re-
duction potential value reported to date for a mono-carbo-
benzene derivative.
Insertion of ethyndiyl expanders between the carbo-
benzene core and fluorenyl groups of 1e was found to have
a dramatic effect on TPA, with a negligible effect on one-
photon absorption (OPA; the maximum absorption wave-
lengths remaining at 493 ± 1 nm in 1e and 1f, their extinc-
tion coefficient being also comparable at 355 000 ± 25 000 L
mol–1 cm–1, the highest  value being obtained for 1f).22
The TPEF method being prevented by a very weak fluo-
rescence, the use of the z-scan method with femtosecond
excitation at 800 nm showed that the TPA cross-section of
1e (TPA= 336 GM) is notably increased in 1f, reaching 656
GM.22 On the basis of TDDFT-calculated one-photon- and
two-photon-allowed excited states reproducing experi-
mental values, the twofold enhancement factor of TPA from
1e to 1f estimated by the sum-over-state (SOS) approach is
mainly due to the increase of the first transition dipole
moment of the intermediate OPA transitions (contributing
to 81% of the overall TPAvalue).
4.3 Bistrialkoxyarylethynyl-carbo-benzenes
Carbo-benzenes attached to pyrogallol n-alkyl ether
groups were recently targeted with the aim of investigating
the ability of the flat, flexible, and empty carbo-aromatic
C18 ring1b to act as a discotic mesogenic core of ‘hollow co-
lumnar’ liquid crystals.23 The synthesis of 1g, where the C18
ring is directly substituted by O,O′,O′′-trisoctadecyl-pyro-
gallyl groups failed, due to the difficulty to generate the
bromomagnesium reagent from 20a required for the prepa-
ration of the pericyclynediol precursor 21a.24 However, re-
course to two spC2 tethers allowed access to 1h and 1i bear-
ing pyrogallylethynyl groups.25 The ethynylpyrogallol tri-
ethers 22a and 22b, differing by the length of the six n-alkyl
chains CnH2n+1 (22a: n = 18, 22b: n = 12), were prepared fol-
lowing known procedures,26 and their lithium or bromo-
magnesium salt added to the [6]pericyclynedione 16 to give
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K. Cocq et al.
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16
M = Li, n = 0, 18a
M = MgBr, n = 1, 18b
[6]pericyclynedione
16
OCnH2n+1
OCnH2n+1
OCnH2n+1
OCnH2n+1
M
n
n = 18, 22a
n = 12, 22b
Br
n = 18, 20a
or
H
C6H13
C6H13
C6H13
+ base
+ Mg
C6H13
OCnH2n+1
OCnH2n+1
Ph
Ph
MeO
OMe
OH
H2n+1CnO
OCnH2n+1
Ph
Ph
MeO
HO
OMe
OCnH2n+1
HO
MeO
OMe
OH
OMe
Ph
Ph
n = 0, 19a, 41%
n = 1, 19b, 50%
H2n+1CnO
H2n+1CnO
C6H13
C6H13
MeO
Ph
Ph
OCnH2n+1
m = 0, n = 18, 21a, 0%
m = 1, n = 18, 23a, base = n-BuLi, 75%
m = 1, n = 12, 23b, base = EtMgBr, 25%
SnCl2/HCl
Ph
Ph
C6H13
C6H13
SnCl2/HCl
Ph
Ph
n
n
H2n+1CnO
2n+1CnO
2n+1CnO
OCnH2n+1
OCnH2n+1
OCnH2n+1
C6H13
C6H13
H
m
m
Ph
Ph
H
n = 0, 1e, 59%, λmax = 492 nm, σTPA (800 nm) = 336 GM (calcd. 227 GM; µ0i = 1.7 au)
n = 1, 1f, 51%, λmax = 494 nm, σTPA (800 nm) = 656 GM (calcd. 349 GM; µ0i = 2.4 au)
Ph
Ph
Scheme 4 Synthesis of fluorenyl-carbo-benzenes 1e and 1f, and TPA
cross-section measured by the z-scan method or estimated by the SOS
approach using excited states calculated at the TD-CAM-B3LYP/6-31G**
level. μ0i= transition dipole moment from the ground state to the main
intermediate one-photon-allowed excited state contributing the overall
m = 0, n = 18, 1g, 0%
m = 1, n = 18, 1h, 18%
m = 1, n = 12, 1i, 58%
Scheme 5 Synthesis of carbo-benzenic mesogens
TPAvalue.
parallel C18 mean planes separated by a 7.02 Å increment
(Figure 2, c): Two phenyl rings pointing from neighboring
columns indeed intercalate between two successive C18
rings (with shortest C···C distances of 3.44 Å between the
two C6 rings, 3.41 Å between the C18 and C6 rings). The
neighboring columns also interact by weak – stacking of
pyrogallol C6 rings (3.42 Å), itself driven by dispersive inter-
action between the respective triads of parallel stretched
C12H25 chains. Interdigitation of dodecyl chains, also evi-
denced in STM images, is thus assumed to occur in the dis-
cotic mesophase, where neighboring columns interpene-
trate by intercalation of C6 rings within channels of C18
rings.
The discotic mesogenic properties of 1i rely first on its
spontaneous ability to adopt a near-planar structure in the
condensed phase (in the crystal, the maximum distance
from a C or O atom to the molecular mean plane is 2.3 Å, for
a diameter of 51.3 Å). This minimal strain can be directly at-
tributed to the spC2 expander allowing minimal steric re-
pulsion between the trialkoxyaryl moieties and the C18Ph4
core.
the respective pericyclynediols 23a and 23b. After reduc-
tive aromatization, the carbo-benzenes 1h and 1i were iso-
lated in 18% and 58% yield, respectively (Scheme 5).
As 1i was found to be more soluble and available in larg-
er amounts than 1h, its ability to give liquid crystalline me-
sophases was investigated in a systematic manner using
powder X-ray diffraction (PXRD), polarized optical micros-
copy (POM), and differential scanning calorimetry (DSC).
All the analytical data confirmed the formation of a colum-
nar rectangular mesophase at 115 °C. STM images of the 2D
self-assembly of the ‘carbo-mesogen’ 1i on a HOPG surface
also evidenced a rectangular arrangement (48 × 43 Å2) com-
parable to the axial projection of the 3D columnar assembly
(48.5 × 45 Å2, Figure 2, a and b).25 In the crystal state, the
packing is monoclinic (prismatic P21/c space group), i.e.,
also truly rectangular with a long cell axis, of length a =
49.45 Å, close to the PXRD or STM values and to the diame-
ter of the molecular skeleton (51.3 Å). Noteworthy is the ab-
sence of direct – stacking between C18 rings forming hex-
agonal columns parallel to the crystallographic c axis, slant-
ed by 17.4° with respect to the normal of the constituting
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

H
K. Cocq et al.
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Figure 2 a) Cartoon representation of the rectangular columnar 3D packing of carbo-mesogen 1i; b) STM image showing the rectangular 2D arrange-
ment of 1i over an HOPG (highly oriented pyrolytic graphite) surface; c) single-crystal packing of 1i, view along the C18 column direction, corresponding
to the crystallographic axis c (= 7.35 Å; b = 15.29 Å, a = 49.45 Å; P21/c space group).
4.4 Bisindolyl(ethynyl)-carbo-benzene
envisaged in the indol-3-yl- and indol-3-ylethynyl- carbo-
benzenes 1j and 1k.28 Their synthesis was tackled by the
classical two-step process from the [6]pericyclynedione 16
and either the bromoindole 24 or ethynylindole 25 (Scheme
6).28,29 Direct anchoring was performed from 24 by bro-
mine–lithium exchange and addition to 16, followed by re-
duction with SnCl2/HCl. The diindolyl-carbo-benzene 1j
was thus isolated with 3% yield from 16. X-ray diffraction
analysis of single crystals of 1j evidenced a limiting -con-
jugation between the indole and carbo-benzene -systems
due to steric repulsion (dihedral angle: spC–C–C–CN =
30.0°).
With the view to outlining prospects of electrooptical
applications of carbo-benzene derivatives (SMC, NLO, TPA,
OPV, ...), the effect of substitution on both the UV-visible
absorption and redox behavior was considered for various
types of nitrogen-containing -donating groups. In a series
of 4-anilinyl derivatives, where the N atom is thus connect-
ed to the C18 ring through an aromatic, so somewhat -in-
sulating, 1,4-phenylene spacer, a bathochromic shift of the
maximum absorption was observed upon increasing the
donating character (NMe2 > NH2 > indol-1-yl > carbazol-1-
yl).27 A more -conducting N···C18 -junction was then
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I
K. Cocq et al.
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Br
Et
Ph
Ph
Ph
Ph
Ph
OMe Ph
N
H
25
Et
1)
Et
24
Ph
N
Ph
OMe
N
Ph
Ph
Ph
Et
Et
Et
HO
+ 2 nBuLi
+ 2 LiHMDS
N
N
16
N
N
SnCl2
HCl
2) SnCl2/HCl
Et
OH
OMe
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
OMe Ph
Ph
1j, 3%
1k, not observed
Et
26, 61%
SnCl2/HCl
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Et
Et
Et
Cl
Cl
O
N
N
N
H
+
+
H
H
N
N
N
Cl
O
O
Et
Et
Et
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1m
1l
1n
SiO2, H2O traces
SiO2, H2O traces
Scheme 6 Synthesis of indolyl-substituted carbo-benzenes
Targeting the indol-3-ylethynyl-carbo-benzene 1k, the
[6]pericyclynediol 26 was prepared in 61% yield by addition
of 25 to the diketone 16 in the presence of LiHMDS. Subse-
quent treatment of 26 with SnCl2/HCl, however, did not al-
low isolation of 1k, but, instead, produced a mixture of
three chromophores, among which the dialkenyl-carbo-
benzene 1l was the main product. Its formation resulted
from a regio- and stereoselective addition of HCl onto the
two external triple bonds of 1k. This peculiar reactivity at
room temperature was attributed to promotion by the
combined effects of the adjacent -electron-rich C18 ring
and -donating enamine group, inducing ‘-frustration’ of
the triple bonds. The dialkenyl-carbo-benzene 1l was found
to react with traces of water on silica gel to give the 2-
oxoalkyl-carbo-benzenes 1m and 1n resulting from the
hydrolysis of one or two chloroalkenyl units of 1l, respec-
tively (Scheme 6).28 The propensity of the chloroalkene
groups of 1l to give the corresponding methylene ketones
(or enol forms) is unusual, as such transformations general-
ly require harsher acidic conditions.30 This observation can
also be ascribed to the double bond ‘-frustration’ induced
by the -donating character of the enamine substituent
facing the -electron-rich carbo-benzene core.
umn chromatography.27 The crude material was thus di-
rectly treated with SnCl2/HCl, then with NaOH, to deliver the
reduced protodesilylated target 1o with 26% yield from 16.
Pd(PhCN)2Cl2
PtBu3, CuI
iPr2NH
TMS
TMS
TMS
TMS
Br
N
E
N
TMS
H
27a
E = TMS, 29
E = H, 27b
K2CO3
MeOH, 0 °C
TMS
N
TMS
Ph MeO
MeO
Ph
Ph
27a + Mg
HO
16
or
OH
27b + LiHMDS
MeO
Ph MeO
TMS
N
n = 0, 28a
n = 1, 28b
TMS
1) SnCl2/HCl
2) aq. NaOH
Ph
Ph
4.5 Bisanilinylethynyl-carbo-benzene
H2N
NH2
n
The para-dianilinyl-carbo-benzene 1o and diethyny-
logue 1p were prepared by the classical two-step method
from the [6]pericyclynedione 16 (Scheme 7). The synthesis
of 1o required the reaction of 16 with two equivalents of the
Grignard reagent of N,N-bistrimethylsilyl-4-bromoaniline
(27a). The resulting [6]pericyclynediol 28a was, however,
found to be unstable upon attempt at purification by col-
n
Ph
Ph
n = 0, 1o, 26%
n = 1, 1p, 20%
Scheme 7 Synthesis of the anilinyl(ethynyl)-carbo-benzenes 1o and 1p
(the preparation and characterization of 27b and 1p are described in
ref. 31 and 32).
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K. Cocq et al.
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Likewise, N,N-bistrimethylsilyl-4-ethynylaniline (27b)
was required for the synthesis of 1p. It was prepared in two
steps from 27a, the latter being first submitted to Sonogas-
hira coupling with trimethylsilylacetylene to give the pri-
mary product 29. Careful optimization of the reaction
conditions allowed selective C-desilylation of 29 with
K2CO3/MeOH, while avoiding N-desilylation (Scheme 7).31
Addition of 27b to 16 afforded the unstable pericyclynediol
28b, which was directly treated with SnCl2/HCl, then with
aqueous NaOH, to give 1p with 20% yield from 16.32
As previously evidenced for fluorenylethynyl-carbo-
benzenes (1e and 1f, see section 4.2.),22 1o and 1p were
found to display almost superimposable UV-vis spectra,
with exactly the same maximum absorption wavelength at
493 nm, thus confirming the negligible effect of C2-tether-
ing on OPA.27,32 As in the fluorenyl(ethynyl) series, C2 teth-
ering was found to induce a slight increase of the extinction
coefficient ( = 131 000 L mol–1 cm–1 for 1p vs 105 000 L
mol–1 cm–1 for 1o). Notably, however, increase of the -con-
jugation extent from 1o to 1p proved to have a significant
effect on the redox properties, 1p being both easier to re-
duce by 0.14 V and more difficult to oxidize by 0.16 V than
1o.32 A similar shift of the first redox potentials was ob-
served in the fluorenyl series, 1f being easier to reduce by
0.14 V and more difficult to oxidize by 0.09 V than 1e (see
Scheme 4 and section 4.2).22
less, salient electronic effects of the ethynylogation of 1o to
1p are the anodic shift by 0.15 V, up to –0.67 V/SCE for the
reduction of 1p (and to +0.74 V for oxidation), and polariza-
tion of the HOMO→LUMO transition along the N···N direc-
tion (x axis): While the LUMO spreads out over the phenyl
substituents (y axis) in 1o, it expands over the C2 tethers
and anilinyl ends in 1p (see Figure 3 and Supporting Infor-
mation). This polarization is also apparent from the corre-
sponding components of the traceless quadrupole moment
(Qxx = 146.0 D.Å and Qyy = -23.8 D.Å for 1o, Qxx = 189.2 D.Å
and Qyy = –33.8 D.Å for 1p) with a difference in quadrupolar
dissymmetry Δ(Qxx – Qyy) = 53.2 D.Å.
The dianilinyl-carbo-benzene 1o is also a highly effi-
cient molecular conductor, with an unprecedented single-
molecule conductance (SMC) as determined by the STM
break junction method:33 SMC(1o) = 106 nS over 2 nm
(N···N distance measured by X-ray diffraction of a single
crystal). This SMC value, much higher than those measured
for other dianilinyl derivatives with similar N···N distances,
highlights the carbo-benzene core as an attractive motif for
the design of carbon-rich molecular device ruled by the
spatial extent and quantum efficiency of -electronic com-
munication/excitation (mixed valence complexes, photo-
voltaic cells, light-emitting diodes, tunneling diodes, field-
effect transistors, or any organic-semiconductor-based sys-
tem).
With a 5.0 Å longer N···N distance, the SMC of 1p is an-
ticipated to be lower than that of 1o and has not been ex-
perimentally determined yet.34 The di(anilinylethynyl)-
carbo-benzene 1p was, however, recently shown to act as
photosensitizer in a multicomponent nanocatalytic system
for the production of dihydrogen by formal water splitting
under solar irradiation (H2O + red –[cat][h ]–> H2 + redO).35
According to SEM and TEM imaging, the core of the [cat]
system, constituted by Ag plasmonic nanoparticles deposit-
ed on TiO2 particles, is wrapped by a layer of 1p. Specula-
tively, the latter might be a source of excitons expanding
throughout the TiO2 network to provide electrons for
remote reduction of water molecules. A lower solubili-
ty/processability prevents the use of the nonexpanded
counterpart 1o as an alternative photosensitizer. Neverthe-
Figure 3 Frontier orbitals of the bisanilinyl-carbo-benzene 1o and
bisethynylogue 1p. Optimized structures at the B3PW91/6-31G(d,p)
level of calculation (D2 symmetry for 1o, Ci symmetry for 1p); see Sup-
porting Information.
5 Carbo-oligo(phenyleneethynylene)s
Skeletal carbo-mers of OPPs or ring carbo-mers of OPEs,
containing several carbo-benzene rings connected by
ethyndiyl linkers, were recently considered under the des-
ignation (ring) ‘carbo-OPEs’ (Figure 4).36 For practical syn-
thesis reasons, aryl-substituted derivatives had to be target-
ed, and by virtue of the steric repulsion between facing
phenyl substituents in the parent ring carbo-PPPs (e.g., p,p′-
R–C18Ph4–C18Ph4–R), more efficient -conjugation between
C18 rings was indeed expected to occur in the carbo-OPE se-
ries (e.g., p,p′-R–C18Ph4–C≡C–C18Ph4–R). Concomitant ef-
fects of the -overlap and intrinsic conductance of the C18
rings (see section 4.5) could indeed provide such molecular
wires with remarkable electr(on)ical properties.37 The syn-
thesis of the three carbo-OPEs 1q, 1r, and 1s was thus tack-
led by methods previously implemented for the preparation
of dissymmetrically substituted carbo-benzenes (Scheme
8).11c The dissymmetrical precursors 29 and 30 were thus
obtained from the [6]pericyclynedione 16 (Scheme 8).36
Addition of lithiated 29 to the monoketone 30 and diketone
16 or 32 gave the bispericyclynediol 31 and trispericyclyne-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

K
K. Cocq et al.
Account
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diol 33 or 34, respectively. Ultimate reductive aromatiza-
tion furnished the carbo-biphenyl 1q from 31, and carbo-
terphenyls 1r and 1s from 33 and 34, respectively, with 9–
20% yield from 29 (Scheme 8).36
and/or acceptor end groups that could operate as wire junc-
tions, e.g., for SMC measurement. More generally any OPE-
based molecular device would deserve comparison with the
carbo-OPE counterpart.
C2
nOct
nOct
6 Conclusions
nOct
nOct
connectors
n
n
OPEn
OPPn
This Account illustrates how proper extension of the
structural variety for organic molecular materials relies on
endeavors in the design and implementation of specific
synthesis routes and methods. Indeed, while the highly ver-
satile Sonogashira's sp2C–spC coupling procedure employed
for the synthesis of alkynyl-benzenes is not applicable to
alkynyl-carbo-benzene targets, the latter are readily acces-
sible through the [6]pericyclynedione route, which proves
to be definitely more efficient for alkynyl nucleophiles than
for aryl and alkyl counterparts.11c
rings
skeleton
C2
C2
(+ Ph decoration)
(+ Ph decoration)
Ph
Ph
Ph
Ph
•
•
•
•
nOct
nOct
•
•
•
•
•
•
•
•
Ph
Ph
Ph
Ph
n
C2-tethering of electroactive substituents to the C18 car-
bo-benzene ring is thus shown to be a subtle way to modu-
late functional properties while preserving a ‘soft’ carbo-ar-
omatic character,1b featuring structural flexibility, visible
optical absorptivity, electrical conductivity, and magnetic
anisotropy. In the quadrupolar disubstituted series, a sa-
lient illustration is given for fluorenyl substituents: While
insertion of C2 connectors has a negligible effect on the OPA
spectrum, it has a strong effect on the TPA spectrum. For 4-
anilinyl substituents, the corresponding carbo-benzene 1o
exhibits an unprecedented single-molecule conductance,
naturally anticipated to be lower for the C2-expanded ver-
sion 1p. Nevertheless, the synthesis of 1p, described here
for the first time, allowed the finding of its remarkable sur-
face aggregation and photosensitization ability securing
the activity of a nanophotocatalytic system.35 In the carbo-
OPE series, the role of the C2 linkers, ensuring conjugation
between C18 rings while preserving their magnetic anisot-
ropy, is to be highlighted for the development of new mo-
lecular wire systems.
More generally, it is noteworthy that the topological
‘aromatic character’ the C2 tether (acetylene is the [2]annu-
lene)4f,10b has a weaker insulating effect than the 1,4-
phenylene tether: This can be interpreted by the weaker
conformational dependence of the C18 functionality -con-
jugation for the former.
The efficiency of the addition of alkynylmetals to
[6]pericyclynediones also encourages further exploration of
the C2-tethering modulation approach. In particular, novel
orientations beyond the quadrupolar series will be dis-
closed in due course.
carbo-OPEn
n = 0: carbo-benzene 1t
n = 1: carbo-biphenyl 1q
n = 2: carbo-terphenyl 1r
n = 3: carbo-quaterphenyl 1u
Figure 4 Design of carbo-OPEs, as ring carbo-mers of OPEs or skeletal
carbo-mers of OPPs
The carbo-OPEs were obtained as dark-red solids stable
at room temperature. As anticipated, the tetrapentylphenyl
carbo-terphenyl 1s proved to be significantly more soluble
than the all-phenyl counterpart 1r.
The UV-vis spectra of 1r and 1s exhibit three main ab-
sorption bands covering all the visible window, up to 750
nm. The -conjugation extent is also revealed by very low
absolute values of the first reversible reduction potentials,
at –0.39 and –0.45 V/SCE for 1r and 1s, respectively. In a
consistent manner, the carbo-biphenyl 1q and p-dioctyl-
carbo-benzene reference 1t are reduced at lower potentials,
–0.58 and –0.83 V/SCE, respectively.11c An accurate correla-
tion of the reduction potential with the LUMO energy
allows extrapolation to –0.30 V/SCE for the DFT-calculated
carbo-quaterphenyl 1u (Figure 4). This trend indicates the
efficiency of conjugation between the C18Ar4 units, enabled
by a low rotation barrier about the C2 tethers (3 kcal mol–1,
fictitious nonarylated carbo-OPEs constituted by C18H4
units being even planar in the equilibrium ground state).
This trend is to be related with the moderate energetic aro-
maticity of the C18 macrocycles,4f which, however, preserve
a strong magnetic aromaticity. A strong diatropic ring cur-
rent4d is indeed revealed by anisotropic NMR effects on the
ortho-1H nuclei of the aryl substituents ( = ca. 9.6 ± 0.2
ppm) and computed NICS values at the center of the rings
(NICS(0) = ca. –13 ± 1 ppm). This unique combination of
structural, electrochemical, chromophoric, and magnetic
features of the carbo-OPE backbone gives relevance to the
prospect of synthesizing functional derivatives with donor
Funding Information
The reported results have been obtained with fundings from the fol-
lowing sources: Agence Nationale de la Recherche (ANR-11-BS07-
016-01), the Centre National de la Recherche Scientifique (CNRS pre-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

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K. Cocq et al.
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Pent
OMe
16
BrMg
H
1)
2) Oct-MgBr
3) nBuLi/MeI, DMSO
OMe Ph
Ph
Ph
OMe
OMe
Oct
OMe
Pent
OMe
Ph MeO
O
MeO
MeO
Oct
MeO
Ph
Ph
O
O
30
32
H
MeO
Pent
OMe Ph
OMe
OMe
Ph MeO
Pent
29
16 or 32 (0.5 equiv)
30
LiHMDS
OMe
LiHMDS
OMe Ar
OMe Ph
Ph MeO
Ph
OMe Ph
Ph MeO
Ar
Ar
OMe
OH
MeO
MeO
Ph
Ph
OMe
OMe
Oct
Ph
Ph
MeO
MeO
Ph
Ph
HO
HO
OMe
Oct
OMe
Oct
OMe
MeO
Oct
OMe
OMe
MeO
Ph
OMe
MeO
OMe Ar
Ph MeO
OMe Ph
OMe Ph
Ph MeO
31
Ar = Ph, 33
Ar = 4-Pent-C6H4–, 34
SnCl2/HCl
Ph
SnCl2/HCl
Ph
Ph
Ph
Ph
Ph
Ar
Ar
Ph
Ph
Oct
Ph
Oct
Oct Oct
Ph
Ph
Ph
Ph
Ph
Ph
Ar
Ar
Ph
dialkynyl-carbo-benzenic unit
1q, 18% from 16 and 29
Ar = Ph, 1r, 20% from 16
Ar = 4-Pent-C6H4, 1s, 9% from 32
Scheme 8 Synthesis of the carbo-OPEs 1q (carbo-biphenyl), 1r, and 1s (carbo-terphenyls)
maturation program, October 2016-November 2017, PHOTOH2 proj-
ect), and the Toulouse IDEX program 2015-2017 (CARBO-DEVICE
Supporting Information
DFT calculation data and comparative analysis of OPE[n] vs OPP[n]
(ref. 2), INPUT and OUTPUT file extracts of DFT calculation of the di-
anilinyl derivatives 1o and 1p. Supporting information for this article
project).
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is available online at https://doi.org/10.1055/s-0037-1610269.
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Acknowledgment
The Centre National de la Recherche Scientifique (CNRS) is acknowl-
edged for funding. K. C. and A. R. thank the Agence Nationale pour la
Recherche (ANR) for their doctoral and post-doctoral fellowship, re-
spectively. The authors are grateful to Alix Saquet for the electro-
chemical measurements and to all the co-authors of the cited
references.
References and Notes
(1) For reviews on carbo-mers see: (a) Maraval, V.; Chauvin, R.
Chem. Rev. 2006, 106, 5317. (b) Cocq, K.; Lepetit, C.; Maraval, V.;
Chauvin, R. Chem. Soc. Rev. 2015, 44, 6535. For the early defini-
tion, see: (c) Chauvin, R. Tetrahedron Lett. 1995, 36, 397. For ref-
erences invoking the ethynylogation process, see: (d) Hafner, K.;
Neuenschwander, M. Angew. Chem., Int. Ed. Engl. 1968, 7, 459.
(e) Arnold, D. P.; James, D. A. J. Org. Chem. 1997, 62, 3460.
(f) Young, B. S.; Herges, R.; Haley, M. M. Chem. Commun. 2012,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

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K. Cocq et al.
Account
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9441. (g) Haley, M. M.; Chase, D. T.; Rose, B.; Fix, A. G.
US9099660, 2015. (h) Listunov, D.; Saffon-Merceron, N.; Joly, E.;
Fabing, I.; Genisson, Y.; Maraval, V.; Chauvin, R. Tetrahedron
2016, 72, 6697.
(11) (a) Cocq, K.; Saffon-Merceron, N.; Poater, A.; Maraval, V.;
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Organometallics 2013, 32, 5015. For n = 4 and 8, see: (n) Yavari,
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(2) (a) A computational advance in this direction has been under-
taken by comparison of ethynylogous OPE[n] and OPP[n], for n =
0–10. For a given -conjugation extent nC (= 6n + 6 for OPP[n],
8n + 6 for OPE[n]), the HOMO–LUMO gap is found always
greater for OPE[(nC–6)/8] than for OPP[(nC–6)/6], becoming
greater than 3.2 eV for nC ≥ 54 (corresponding to OPP[8] and
OPE[6]). The OPE series is therefore definitely softer than the
OPP series vs nC. Calculations for ideal conjugation (under D2h
symmetry constraint) performed at the B3PW91/6-31G(d,p)
level with the program Firefly 8.1.1: Granovsky, A. A. Firefly
Version
8 http://classic.chem.msu.su/gran/firefly/index.html)
which is partially based on the GAMESS (US) source code.
(b) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K.
A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput.
Chem. 1993, 14, 1347; see Supporting Information.
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Ueda, I. Tetrahedron 1998, 54, 2477.
(4) For theoretical studies of carbo-mers, see for example:
(a) Godard, C.; Lepetit, C.; Chauvin, R. Chem. Commun. 2000,
1833. (b) Lepetit, C.; Godard, C.; Chauvin, R. New J. Chem. 2001,
25, 572. (c) Ducere, J.-M.; Lepetit, C.; Lacroix, P. G.; Heully, J.-L.;
Chauvin, R. Chem. Mater. 2002, 14, 3332. (d) Lepetit, C.; Zou, C.;
Chauvin, R. J. Org. Chem. 2006, 71, 6317. (e) Soncini, A.; Fowler,
P. W.; Lepetit, C.; Chauvin, R. Phys. Chem. Chem. Phys. 2008, 10,
957. (f) Chauvin, R.; Lepetit, C.; Maraval, V.; Leroyer, L. Pure
Appl. Chem. 2010, 82, 769. (g) Wodrich, M. D.; Gonthier, J. F.;
Steinmann, S. N.; Corminboeuf, C. J. Phys. Chem. A 2010, 114,
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Ugalde, J. M.; Merino, G. J. Org. Chem. 2014, 79, 5463.
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F.; Alikhani, M. E.; Chauvin, R. Chem. Eur. J. 2016, 22, 5295.
(5) (a) Rives, A.; Baglai, I.; Malytskyi, V.; Maraval, V.; Saffon-Merce-
ron, N.; Voitenko, Z.; Chauvin, R. Chem. Commun. 2012, 48, 8763.
(b) Rives, A.; Maraval, V.; Saffon-Merceron, N.; Chauvin, R.
Chem. Eur. J. 2012, 18, 14702. (c) Rives, A.; Maraval, V.; Saffon-
Merceron, N.; Chauvin, R. Chem. Eur. J. 2014, 20, 483.
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Chauvin, R. Fr.-Ukr. J. Chem. 2015, 3, 60. (b) Rives, A.; Baglai, I.;
Barthes, C.; Maraval, V.; Saffon-Merceron, N.; Saquet, A.;
Voitenko, Z.; Volovenko, Y.; Chauvin, R. Chem. Sci. 2015, 6, 1139.
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C.; Lepetit, C.; Chauvin, R. Angew. Chem. Int. Ed. 2015, 54, 2703.
(8) For references on [N]pericyclynes, see: (a) Scott, L. T.; DeCicco,
G. J.; Hyun, J. L.; Reinhardt, G. J. Am. Chem. Soc. 1983, 105, 7760.
(b) Scott, L. T.; DeCicco, G. J.; Hyun, J. L.; Reinhardt, G. J. Am.
Chem. Soc. 1985, 107, 6546. For references on hexaoxy[6]pericy-
clynes, see: (c) Maurette, L.; Tedeschi, E.; Sermot, E.;
Soleilhavoup, M.; Hussain, F.; Donnadieu, B.; Chauvin, R. Tetra-
hedron 2004, 60, 10077. (d) Leroyer, L.; Zou, C.; Maraval, V.;
Chauvin, R. C. R. Chimie 2009, 12, 412.
(14) Basic calculations of the heterolytic bond dissociation energy
ΔE(R), corresponding to the equation (MeC≡C)2RC–OH
→
(MeC≡C)2RC+ + OH– in the gas phase, show that ∆E(MeC≡C) –
ΔE(Ph) = +3 kcal/mol: ΔE(MeC≡C) = 0.32935 Ha, ΔE(Ph) =
0.32458 Ha, ΔE(Me) = 0.34930 Ha, ΔE(H) = 0.36285 Ha. Calcula-
tions at the B3PW91/6-31G(d,p) level with the program Firefly
8.1.1: see ref. 2a,b.
(15) (a) Nicholas, K. M. Acc. Chem. Res. 1987, 20, 207.
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V.; Chauvin, R. Angew. Chem. Int. Ed. 2016, 55, 15133.
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4895.
(19) For the preparation of dibenzoylacetylene 12, see: Zhang, Z. Z.;
Schuster, G. B. J. Am. Chem. Soc. 1989, 111, 7149.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N

N
K. Cocq et al.
Account
Synlett
(23) For a recent review on discotic liquid crystals, see: Wöhrle, T.;
Wurzbach, I.; Kirres, J.; Kostidou, A.; Kapernaum, N.;
Litterscheidt, J.; Haenle, J. C.; Staffeld, P.; Baro, A.; Giesselmann,
F.; Laschat, S. Chem. Rev. 2016, 116, 1139.
(24) Rives, A.; Maraval, V.; Chauvin, R. unpublished results.
(25) Zhu, C.; Wang, T.-H.; Su, C.-J.; Lee, S.-L.; Rives, A.; Duhayon, C.;
Kauffmann, B.; Maraval, V.; Chen, C.-h.; Hsu, H.-F.; Chauvin, R.
Chem. Commun. 2017, 53, 5902.
1 M solution in THF, 2.8 mmol). The resulting mixture was
stirred for 1 h at –78 °C before dropwise addition of a solution
of 16 (706 mg, 1.04 mmol) in dry THF (60 mL) at the same tem-
perature. The resulting mixture was allowed to warm up to r.t.
under stirring over 18 h. After treatment with a saturated
aqueous solution of NH4Cl and extractions of the aqueous layer
with Et2O, the combined organic layers were washed with
brine, dried over anhydrous MgSO4, and concentrated under
reduced pressure to afford the diol 28b as an orange solid (1.48
g), which was used without further purification.
(26) Wu, J.; Warson, M. D.; Zhang, L.; Wang, Z.; Müllen, K. J. Am.
Chem. Soc. 2004, 126, 177.
(27) Rives, A.; Baglai, I.; Malytskyi, V.; Maraval, V.; Saffon-Merceron,
N.; Voitenko, Z.; Chauvin, R. Chem. Commun. 2012, 48, 8763.
(28) Baglai, I.; Maraval, V.; Bijani, C.; Saffon-Merceron, N.; Voitenko,
Z.; Volovenko, Y. M.; Chauvin, R. Chem. Commun. 2013, 49, 8374.
(29) Baglai, I.; Maraval, V.; Duhayon, C.; Chauvin, R. Acta Crystallogr.,
Sect. E: Struct. Rep. Online 2013, 69, o921.
(30) (a) Bhattacharya, A.; Vasques, T.; Ramirez, T.; Plata, R. E.; Wu, J.
Tetrahedron Lett. 2006, 47, 5581. (b) Nemoto, H.; Fujita, S.;
Nagai, M.; Fukumoto, K.; Kametani, T. J. Am. Chem. Soc. 1988,
110, 2931.
Second Step
A solution of the crude 28b (1.48 g) in dry DCM (330 mL) was
treated at –78 °C with SnCl2 (2.43 g, 12.8 mmol) and HCl·Et2O
(12.4 mL, 2 M solution in Et2O, 24.8 mmol). The resulting
mixture was allowed to warm up to –15 °C under stirring over
3.5 h, and then kept for 10 min. at r.t. before addition of NaOH
(25 mL, 2 M aqueous solution, 50 mmol). The resulting mixture
was stirred at r.t. for 16 h. After treatment with a saturated
aqueous solution of Na2CO3 and extractions of the aqueous layer
with DCM, the combined organic layers were dried over anhy-
drous MgSO4 before filtration through Celite® and concentration
under reduced pressure. Washings of the crude solid with
pentane and Et2O afforded the expected carbo-benzene 1p as a
dark violet solid (160 mg, 0.21 mmol, 20% yield over 2 steps).
Analytical Data
(31) Experimental Procedure and Characterization of 27b
First Step
Bis(benzonitrile)palladium(II)chloride (41.3 mg, 0.107 mmol)
and CuI (9.6 mg, 0.074 mmol) were placed into a Schlenk tube
under argon and dissolved in 4 mL of dry 1,4-dioxane. The
mixture was then treated with tri-tert-butylphosphine (0.22
mL, 1 M solution in toluene, 0.22 mmol) at r.t. After the solution
turned black, distilled diisopropylamine (0.65 mL, 4.99 mmol),
4-bromo-N,N-bis(trimethylsilyl)aniline (1.0 mL, 3.54 mmol)
and trimethylsilylacetylene (0.65 mL, 4.60 mmol) were added.
The resulting mixture was heated at 50 °C for 2 h, and then
cooled down to r.t. before addition of EtOAc, filtering through
Celite®, and concentration under reduced pressure to afford 29
as a black sticky liquid (1.42 g), which was used without further
purification.
1H NMR (400 MHz, 298 K, (D8-THF)):  = 5.38 (br s, 4 H, –NH2),
6.83 (3JHH = 8.0 Hz, 4 H, m-C6H4–NH2), 7.70 (t, 3JHH = 8.0 Hz, 4 H,
p-C6H5), 7,80 (d, JHH = 8.0 Hz, 4 H, o-C6H4–NH2), 7.98 (t, JHH
8.0 Hz, 8 H, m-C6H5), 9.47 (t, JHH = 8.0 Hz, 8 H, o-C6H5) ppm.
13C{1H} NMR (100 MHz, 298 K, (D8-THF)):  = 87.00–113.90 (–
C≡C– and >C≡C≡C≡C<), 115,16 (m-C6H4–NH2), 119,85, 120,81
(>C(C6H4–NH2)– and >C(C6H5)–), 130.00–130.84 (o, m, p-C6H5),
134.85 (o-C6H4–NH2), 140.33, 152.07 (p-C6H4–NH2 and p-C6H5)
ppm. MS (MALDI-TOF/DCTB): m/z (%) = 756.2 (100) [M]+.HRMS
(MALDI-TOF/DCTB): m/z [M]+ calcd for C58H32N2: 756.2565;
found: 756.2602; [M + Na]+ calcd for C58H32N2Na: 779.2463;
3
3
=
3
Second Step
A solution of the crude product 29 (1.42 g) in methanol (20 mL)
was cooled down to 0 °C, before treatment with K2CO3 (884 mg,
6.40 mmol). The resulting mixture was stirred for 1 h at 0 °C
before addition of distilled water (20 mL). Then, the methanol
was removed under reduced pressure. The remaining aqueous
layer was extracted with Et2O, and the combined organic layers
were washed with brine, dried over anhydrous MgSO4, and con-
centrated under reduced pressure to afford 27b as a spectro-
scopically pure brown viscous liquid (920 mg, 3.50 mmol, 98%
yield over 2 steps).
found: 779.2437. UV-vis (THF): max() = 493 nm (131000
L/mol/cm). Voltammetry: reduction: E (V/SCE) = –0.67 (rev), –1.09
(rev), –1.56 (irrev), oxidation: 0.74 (irrev).
(33) (a) Li, Z.; Smeu, M.; Rives, A.; Maraval, V.; Chauvin, R.; Ratner,
M. A.; Borguet, E. Nature Commun. 2015, 6, 6321. For the STM-
break junction method, see: (b) Xu, B. Q.; Tao, N. J. Science 2003,
301, 1221.
(34) Choi, S. H.; Kim, B. S.; Frisbie, C. D. Science 2008, 320, 1482.
(35) (a) Cure, J.; Kahn, M.; Cocq, K.; Casterou, G.; Chauvin, R.;
Maraval, V. FR 1660040, 2016. (b) Cure, J.; Kahn, M.; Cocq, K.;
Casterou, G.; Chauvin, R.; Maraval, V.; Assi, H. PCT/FR
2017/052842, 2017.
(36) Zhu, C.; Poater, A.; Duhayon, C.; Kauffmann, B.; Saquet, A.;
Maraval, V.; Chauvin, R. Angew. Chem. Int. Ed. 2018, 57, 5640.
(37) See for example: (a) Lu, Q.; Yao, C.; Wang, X.; Wang, F. J. Phys.
Chem. C 2012, 116, 17853. (b) Parker, C. R.; Leary, E.; Frisenda,
R.; Wei, Z.; Jennum, K. S.; Glibstrup, E.; Abrahamsen, P. B.;
Santella, M.; Christensen, M. A.; Della Pia, E. A.; Li, T.; Gonzalez,
M. T.; Jiang, X.; Morsing, T. J.; Rubio-Bollinger, G.; Laursen, B.
W.; Nørgaard, K.; van der Zant, H.; Agrait, N.; Brønsted, Nielsen.
M. J. Am. Chem. Soc. 2014, 136, 16497.
Analytical Data
1H NMR (400 MHz, 298 K, (CD3)2CO):  = 0.10 (s, 18 H, -Si(CH3)3),
3.7 (s, 1 H, ≡CH) 6.96 (d, 3JHH = 8.0 Hz, 2 H, o-C6H4–), 7.39 (d, 3JHH
= 8.0 Hz, 2 H, m-C6H4–) ppm. 13C{1H} NMR (100 MHz, 298 K,
(CD3)2CO):  = 1 .31 (–Si(CH3)3), 77.66 (–C≡C–H), 83.37 (–C≡C–H),
117.54 (ipso-C6H4), 130.28 (o-C6H4), 132.77 (m-C6H4), 149.08
(p-C6H4) ppm.
(32) Experimental Preparation and Characterization of 1p
First Step
A solution of 27b (638 mg, 2 .44 mmol) in dry THF (15 mL) was
treated at –78 °C with lithium bis(trimethylsilyl)amide (2.8 mL,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–N