Functional carbo-butadienes: Nonaromatic conjugation effects through a 14-Carbon, 24-p-electron backbone

Article abstract of DOI:10.1002/chem.201303169

A systematic study of carbo-butadiene motifs not embedded in an aromatic carbo-benzene ring is described. Dibutatrienylacetylene (DBA) targets R ¹-C(R)=C=C=C(Ph)- C=C-C(Ph)=C=C=C(R)-R² are devised, in which R is C=CSiiPr₃ and R¹ and R² are R, H, or 4-X-C6H4, with the latter including three known representatives (X: H, NMe ₂, or NH₂). The synthesis method is based on the SnCl ₂-mediated reduction of pentaynediols prepared by early or late divergent strategies; the latter allows access to a OMe-NO₂ push-pull diaryl-DBA. If R¹ and R² are H, an over-reduced dialkynylbutatriene (DAB) with two allenyl caps was isolated instead of the unsubstituted DBA. If R¹=R²=R, the tetraalkynyl-DBA target was obtained, along with an over-reduced DBA product with a 12-membered 1,2-alkylidene-1H₂,2H₂-carbo-cyclobutadiene ring. X-ray crystallography shows that all of the acyclic DBAs adopt a planar trans-transoid-trans configuration. The maximum UV/Vis absorption wavelength is found to vary consistently with the overall p-conjugation extent and, more intriguingly, with the p-donor character of the aryl X substituents, which varies consistently with the first (reversible) reduction potential and first (irreversible) oxidation peak, as determined by voltammetry.

Full text of DOI:10.1002/chem.201303169

DOI: 10.1002/chem.201303169
Full Paper
&
Chromophores
Functional carbo-Butadienes: Nonaromatic Conjugation Effects
through a 14-Carbon, 24-p-Electron Backbone
Arnaud Rives,[a, b] Valꢀrie Maraval,*[a, b] Nathalie Saffon-Merceron,[c] and Remi Chauvin*[a, b]
Abstract: A systematic study of carbo-butadiene motifs not
embedded in an aromatic carbo-benzene ring is described.
the unsubstituted DBA. If R1 =R2 =R, the tetraalkynyl-DBA
target was obtained, along with an over-reduced DBA prod-
Dibutatrienylacetylene (DBA) targets R1ꢀC(R)=C=C=C(Ph)ꢀ uct with a 12-membered 1,2-alkylidene-1H2,2H2-carbo-cyclo-
CꢁCꢀC(Ph)=C=C=C(R)ꢀR2 are devised, in which
R
is
butadiene ring. X-ray crystallography shows that all of the
acyclic DBAs adopt a planar trans–transoid–trans configura-
tion. The maximum UV/Vis absorption wavelength is found
to vary consistently with the overall p-conjugation extent
and, more intriguingly, with the p-donor character of the
aryl X substituents, which varies consistently with the first
(reversible) reduction potential and first (irreversible) oxida-
tion peak, as determined by voltammetry.
CꢁCSiiPr3 and R1 and R2 are R, H, or 4-X-C6H4, with the latter
including three known representatives (X: H, NMe2, or NH2).
The synthesis method is based on the SnCl2-mediated reduc-
tion of pentaynediols prepared by early or late divergent
strategies; the latter allows access to a OMe–NO2 push–pull
diaryl-DBA. If R1 and R2 are H, an over-reduced dialkynylbuta-
triene (DAB) with two allenyl caps was isolated instead of
Introduction
tatriene (DaBEB) trimer, capped with tri(isopropyl)silyl (TIPS)
stabilizing groups (Figure 1, top).[4]
For 15 years, the chemistry of carbo-mers has mainly focused
on the synthesis of cyclic representatives, and more particularly
of carbo-benzenes,[1] in order to learn and benefit from their ar-
omatic character in the design of stable p-functional molecular
materials.[2] Recently however, carbo-mers of acyclic oligome-
thyne chains, containing more than one dialkynylbutatriene
(DAB) unit not embedded in a macroaromatic ring, were found
to be more stable than anticipated, while meeting a general
challenge in the chemistry of butatrienes (or [3]cumulenes).[1c]
Such carbo-oligoacetylenes, identified generically as “1,4-PTAs”,
that is, 1,4-isomers of the reference polytriacetylenes (1,2-
PTAs),[3] were exemplified in the perphenylated series by the
DAB monomer, the di(alkynylbutatrienyl)acetylene (DaBA or
DBA) dimer, and the less stable di(alkynylbutatrienylethynyl)bu-
Figure 1. Perphenylated carbo-oligoacetylenes exemplified for n=0–3 (1,4-
PTAs; E: SiiPr3) relative to the reference polytriacetylene (1,2-PTA) isomers
(top),[3,4] and functional n=2 derivatives exemplified for R1 and R2 being 4-X-
C6H4 (X: NH2, NMe2, indol-N-yl, or carbazol-N-yl; bottom).[5]
In the functional series, DBAs decorated with terminal p-ami-
nophenyl substituents (Figure 1, bottom) were also recently
described in comparison with their carbo-benzene counter-
parts.[5a] These preliminary results showed that regarding UV/Vis
absorption and redox properties, the DBA (or carbo-butadiene)
backbone is much more sensitive to substituent effects than
the p-carbo-benzene core. This trend is consistent with the p-
insulating character anticipated for an aromatic ring, with the
[a] Dr. A. Rives, Dr. V. Maraval, Prof. R. Chauvin
CNRS, LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, BP 44099, 31077 Toulouse Cedex 4 (France)
E-mail: vmaraval@lcc-toulouse.fr
chauvin@lcc-toulouse.fr
[b] Dr. A. Rives, Dr. V. Maraval, Prof. R. Chauvin
Universitꢀ de Toulouse, UPS, ICT-FR 2599
31062 Toulouse Cedex 9 (France)
[c] Dr. N. Saffon-Merceron
!
effect being enhanced in this case by the R1!C18 R2 p-frus-
Universitꢀ de Toulouse, UPS, Institut de Chimie de Toulouse
ICT-FR 2599, 118 route de Narbonne, 31062 Toulouse (France)
tration that results from the p donation of the two amino-
phenyl Ri substituents towards the p-electron-rich and soft C18
carbo-benzene core.[5b] With consideration of the photophysical
and liquid crystals properties of the parent 1,4-diaryl-substitut-
ed butadienes,[6] a systematic investigation of the substitution
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303169. It includes the synthesis proce-
1
dures and characterization data for new compounds (including H and
13C{1H} NMR spectra and cyclic and square-wave electrochemical voltam-
mograms), as well as a color version of Figure 7.
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effect at the 1- and 10-positions
of the expanded carbo-buta-
diene backbone was undertak-
en. The synthesis and physico-
chemical properties (X-ray crys-
tal structures, electronic spectra,
and voltammograms) of homo-
and hetero-substituted repre-
sentatives bearing either donor
and/or acceptor R1 and R2 aryl
groups (Figure 1) is described
hereafter. Although it gave rise
to unexpected p-conjugated
structures, substitution by non-
aryl substituents (R1, R2: H,
CꢁCꢀTIPS) was also achieved.
Scheme 1. The two synthetic approaches envisaged for functional carbo-butadiene targets.
diastereoisomers) to p-anisaldehyde, p-nitrobenzaldehyde, p-
trifluoromethylbenzaldehyde, p-dimethylaminobenzaldehyde,
or paraformaldehyde gave the corresponding triynediols 4a–
e (Scheme 2). The dilithium salt of 1 was found to react selec-
tively with all of the aldehydes except p-nitrobenzaldehyde,
from which only undetermined polymeric products were ob-
tained. Instead, the use of the dibromomagnesium salt of 1 al-
lowed the bis(nitrophenyl)triynediol 4b to be isolated in 55%
yield. The double additions of 1 were found to proceed in
higher yields with aldehydes than with the propargylic phenyl-
ketone 5 that was previously reported to give perphenylated
pentaynediol 3 f in 51% yield.[4] From stereoisomeric mixtures
of triyne 1, the triynediols 4a–e were obtained as mixtures of
diastereoisomers.
Results and Discussion
Two strategies were envisaged for anchoring the R1 and R2
substituents to the carbo-butadiene backbone at earlier or
later stages of the synthesis from the same known triyne pre-
cursor 1.[7] The first one, inspired by the reported synthesis of
the reference tetraphenylated DBA,[4] relies on the addition of
disalts of 1 to selected aldehyde substrates, followed by oxida-
tion of the resulting carbinol centers and subsequent addition
of two equivalents of tri(isopropyl)silylacetylide (strategy A,
Scheme 1). The alternative method is based on the intermedia-
cy of the pentaynedione 2, the carbonyl groups of which serve
as electrophilic centers, for example, for the introduction of
various 4-aminophenyl R1 and R2 groups (strategy B,
Scheme 1).[5a] Pentaynedione 2 can be prepared in two steps
from triyne 1, so the latter is the convergence point of the two
strategies (Scheme 1), with the first one (A) being more versa-
tile (for allowing the introduc-
Oxidation of the secondary diols 4a–d to the corresponding
diketones 6a–d was performed with MnO2 in dichloromethane
at room temperature. Dialdehyde 6e was reported to decom-
pose upon purification by chromatography on silica gel,[7b] so
oxidation of the primary diol 4e into pure 6e required the use
tion of different R1 and R2 sub-
stituents) and the second one
(B) diverging later (from inter-
mediate 2). Both strategies end
up with an eliminative reduction
of the ultimate pentaynediol
precursors 3.
Synthesis of pentaynediol DBA
precursors directly from triyne
1 (strategy A)
Strategy A (Scheme 1) was first
applied to the synthesis of the
symmetrically substituted pen-
taynediols 3a–d and to the syn-
thesis of the unsubstituted pre-
cursor, 3e, of pentaynedione 2,
a key intermediate of strategy B
(see below). Addition of the di-
bromomagnesium or dilithium
Scheme 2. Synthesis of the pentaynediols 3a–f in three steps from triyne 1. DCM: dichloromethane; DCE: 1,2-di-
salt of 1 (as a mixture of two
chloroethane; IBX: 2-iodoxybenzoic acid.
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of 2-iodoxybenzoic acid in re-
fluxing 1,2-dichloroethane.[8] Fi-
nally, addition of two equiva-
lents of lithium tri(isopropyl)sily-
lacetylide to diketones 6a–c and
dialdehyde 6e afforded the cor-
responding pentaynediols 3a–
c and 3e in 57–90% yield. In
contrast, by using a similar pro-
cedure with dimethylaminophe-
nylketone 6d, pentaynediol 3d
could be neither isolated nor
observed (Scheme 2). The elec-
trophilicity of the carbonyl func-
tionalities of 6d is dramatically
reduced by phenylogous p-don-
ation from the dimethylamino
substituents,[5a] so the synthesis
of 3d was addressed through
Scheme 3. Synthesis of the dissymmetrically substituted pentaynediol 3g.
the
alternative
strategy B
(Scheme 1; see below).
Strategy A was also applied to the preparation of the dis-
symmetrically substituted pentaynediol 3g, the anticipated
precursor of a push–pull carbo-butadiene bearing a p-donating
anisyl group and a p-accepting nitrophenyl group (Scheme 3).
Therefore, the anisyl group was first anchored by addition of
the monolithium salt of 1 to p-anisaldehyde, to give the inter-
mediate monoalcohol 7 in 65% yield. The nitrophenyl group
was then introduced by addition of the dilithium salt of 7 to p-
nitrobenzaldehyde, with the resulting triynediol 4g being iso-
lated in 75% yield. The last two steps in the synthesis of pen-
taynediol 3g were based on the same procedures as those
used for the preparation of the homosubstituted counterparts
3a–c and 3e (Scheme 2 and
philes in a complete study that has been detailed elsewhere
and will not be discussed herein.[5a] In summary, however,
whereas 1-(4-lithiophenyl)-1H-indole and 9-(4-lithiophenyl)-9H-
carbazole afforded the corresponding pentaynediols in 85 and
82% yields after isolation, respectively (Scheme 4, left),[5b] the
reaction of 2 with 4-R2N-phenylmetals (R: Me, SiMe3) did not
allow the isolation of the pentaynediol products in a pure
state. Nevertheless, by using the Grignard reactants of the p-
bromoaminophenyl derivatives, the method allowed the for-
mation and partial characterization of the silica-gel-sensitive
di(aminophenyl)pentaynediol derivatives 3d (R: Me; not de-
tected with strategy A: see Scheme 2) and 3h (R: SiMe3 or H).
Scheme 3): oxidation of the sec-
ondary carbinol centers of 4g
with MnO2 in 75% yield, fol-
lowed by double addition of
lithium tri(isopropyl)silylacetylide
to the resulting diketone 6g to
give 3g in 93% yield.
Synthesis of pentaynediol DBA
precursors via pentaynedione
2 (strategy B)
Pentaynedione 2, the key inter-
mediate
of
strategy B
(Scheme 1), can be quantitative-
ly prepared by MnO2-mediated
oxidation of pentaynediol 3e,
itself obtained in three steps
from triyne 1 (Scheme 2 and
Scheme 4).[5a] Double additions
of organolithium or -magnesium
reactants to 2 were exemplified
with p-aminophenyl nucleo-
Scheme 4. Preparation of the pentaynedione intermediate 2 for the late-divergent strategy B (Scheme 1), and re-
actions of 2 with C-nucleophiles.[5a]
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These compounds could, however, be used in the crude state
for the reduction step to the di(aminophenyl)-carbo-butadiene
targets 8d and 8h (see below).[5a]
The strategy was also used to anchor non-aryl substituents
on the DBA backbone by addition of lithium tri(isopropyl)silyla-
cetylide to pentaynedione 2. Heptaynediol 3i, with four tri(iso-
propyl)silylethynyl arms, was thus isolated in 80% yield
(Scheme 4, right).
Preparation of carbo-butadienes by eliminative reduction of
pentaynediols
Reduction of the pentaynediols to the carbo-butadiene targets
was performed with the classical acidic reductive system,
SnCl2/HCl (Scheme 5). As previously reported, the crude diols
Figure 2. 1H NMR spectroscopic evidence of the three isomers, in a 20:23:57
integrated ratio, of the carbo-butadiene 8b based on the deshielded meta
protons (H3) of the 4-nitrophenyl substituents (CDCl3, 400 MHz).
From solutions in suitable solvents, the seven carbo-buta-
dienes 8a–d and 8 f–h gave monocrystals suitable for X-ray
diffraction analysis (Figure 3). All of them crystallized in the all-
Scheme 5. Preparation of functional carbo-butadienes by SnCl2-mediated re-
duction of pentaynediol precursors. For 8d and 8h, see ref. [5a]; for 8 f, see
ref. [4].
3d and 3h[5a] and the tetraphenyl reference 3 f[4] were convert-
ed into the DBAs 8d, 8h, and 8 f in 29, 53, and 95% yield, re-
spectively. Likewise, after optimization of the procedure, similar
treatment of the pentaynediols 3a–c and 3g led to the carbo-
butadienes 8a–c and 8g as the main products of the crude re-
1
action mixtures, according to H NMR spectroscopic analyses.
The quite low isolated yields for 8b, 8d, and 8g (approximate-
ly (25ꢂ5)%) are due to the poor solubility of the compounds,
which makes them intractable by chromatography only and
thus requires recrystallizations of the most pure fractions. The
reductions were performed in dichloromethane, except for the
preparation of the bis(N,N-dimethylaminophenyl)-carbo-buta-
diene 8d, the formation of which required the use of diethyl
ether as the solvent.[5a]
All of the 1,10-diarylated carbo-butadienes were obtained as
mixtures of cis–cis, cis–trans, and trans–trans isomers, as evi-
Figure 3. Molecular views of the all-trans isomers of 8a–d, 8 f, 8 f–8h in the
crystal state (for clarity, the H atoms are omitted). All of the molecules
appear centrosymmetric (even 8g because of a NO2/OMe disorder, which is
artificially lifted here), and the 16 successive atoms of the common dialkyn-
yl-DBA chain are numbered as Si1ꢀC1ꢀC2ꢀC3ꢀC4ꢀC5ꢀC6ꢀC7ꢀC7A… and
the symmetric CiA (6ꢃiꢃ1) and Si1A (Table 1). For 8d and 8h, see ref. [5a];
for 8 f, see ref. [4].
1
denced for 8b by three deshielded H NMR doublet signals of
the meta protons of the 4-nitrophenyl substituents (in
a 20:23:57 ratio going upfield: see Figure 2). It can be assumed
that the most intense doublet corresponds to the less strained
trans–trans isomer of 8b.
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trans (that is, trans–transoid–trans) configuration, in a consistent
carbo-benzene Kꢀkulꢀ forms) to allenyl caps[9] and resembles
the core structure of the recently reported dialkynyl-carbo-hex-
atriene.[4] In spite of poor stability, even at low temperature,
which thus prevented selective crystallization for X-ray diffrac-
tion analysis, the diallene 9 could be unambigously identified
by MS, NMR spectroscopy, and UV/Vis spectroscopy (Figure 4
and Figure 7). In particular, the characteristic NMR signals of
manner with the proposed interpretation of the major signals
1
in the H NMR spectrum of 8b (Figure 2). Similar values were
obtained for the bond lengths and bond angles of the
common DBA backbone (Table 1). All of the molecules were
found to be centrosymmetric, or quasi-centrosymmetric for 8g
bearing the different but similar OMe and NO2 substituents,
which exchange in a crystallographic disorder.
1
the allenyl caps are the H resonances between d=5.36 and
5.47 ppm for the four sp2-CH units (Figure 4) and a 13C reso-
nance at d=216 ppm for the central sp-C atom.
Table 1. Selected crystallographic data for the carbo-butadienes 8a–d,
8 f–i, and 10.[a]
8a
8b
8c
8d
8h
8 f
8g
8i
10
¯
¯
P1
¯
P1
¯
P1
¯
P1
¯
P1
¯
P1
¯
P1
space group P1
R [%]
Bond lengths [ꢂ]
C2/c
5.49 4.99 7.04 4.48 4.76 5.57 5.93 4.22 9.02
Si1ꢀC1
C1ꢀC2
C2ꢀC3
C3ꢀC4
C4ꢀC5
C5ꢀC6
C6ꢀC7
C7ꢀC7A
Si1AꢀC9
C8ꢀC9
C8ꢀC3
C8ꢀC8A
1.833 1.837 1.829 1.829 1.837 1.840 1.833 1.842 1.830
1.208 1.207 1.209 1.203 1.207 1.207 1.208 1.201 1.204
1.433 1.435 1.433 1.433 1.430 1.432 1.430 1.426 1.421
1.358 1.351 1.352 1.354 1.358 1.351 1.359 1.352 1.351
1.232 1.237 1.242 1.230 1.235 1.237 1.233 1.234 1.245
1.359 1.356 1.361 1.357 1.358 1.354 1.358 1.356 1.372
1.412 1.413 1.418 1.413 1.414 1.418 1.410 1.418 1.421
1.209 1.201 1.204 1.206 1.210 1.203 1.207 1.200 1.139
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1.846 1.899
1.205 1.288
1.429 1.501
–
1.540
Bonds angles [8]
C2ꢀC3ꢀC4
C5ꢀC6ꢀC7
C4ꢀC3ꢀC8
C6ꢀC7ꢀC7A
C4ꢀC5ꢀC6
C3ꢀC4ꢀC5
117.4 117.2 117.4 116.1 117.0 116.7 117.6 121.2 123.8
117.8 119.6 117.5 116.9 117.7 117.9 118.2 117.5 112.2
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
122.0 116.8
–
–
–
162.6
169.6
168.9
Figure 4. 1H NMR spectrum of the diallenyl-capped DAB 9 (CDCl3, 400 MHz).
[a] Bond lengths [ꢂ] and bond angles [8] measured (within estimated
standard deviation errors) with Mercury 3.1.1 as the graphic interface. Mo-
lecular views and numbering of the pseudo-centrosymmetic DBA back-
bone are shown in Figure 3 and Figure 5. For 8d and 8h, see ref. [5a]; for
8 f, see ref. [4]; and for crystallographic parameters of the other com-
pounds, see Table 3.
With the view of preparing the 1,10-tetraalkynyl-DBA 8i,
pentaynediol 3i was subjected to treatment with SnCl2/HCl
(Scheme 7). However, as described above for 3e, this acidic re-
ductive system was found to be impracticable because sub-
strate 3i also bears non-aryl external substituents (unlike 3a–d
and 3 f–h). Standard conditions, which consisted of the addi-
tion of SnCl2 and ethereal HCl to a solution of 3i in dichloro-
methane at ꢀ788C, followed by a slow warming up of the re-
action mixture (in a cold bath) before quenching with aqueous
NaOH, produced only undetermined polymeric materials.
Target 8i could, however, be obtained by using a modified
procedure involving a fast warming up of the reaction mixture
and careful “visual monitoring”: when aqueous NaOH was
added at the dark-red stage (approximately 12 min after addi-
tion of HCl and stirring at room temperature), two main spots
were observed on TLC plates, and the corresponding products
could be separated by chromatography on silica gel. The less
polar red fraction afforded the target acyclic tetraalkynyl-carbo-
butadiene 8i in 15% yield. The more polar orange fraction
turned out to be the over-reduced cyclic DBA 10, which was
isolated in 16% yield. This strained structure was assigned on
the basis of MS and NMR spectroscopy and was confirmed by
X-ray crystallography (Figure 5; see below). However, the reac-
tion selectivity and yields for products 8i and 10 proved to be
highly sensitive to the exact conditions used. If the quenching
When applied to the bis-secondary pentaynediol 3e, the re-
duction procedure did not afford the unsubstituted carbo-bu-
tadiene target 8e (R1 =R2: H). Instead, the 1,4-di(allenylethy-
nyl)butatriene 9 was isolated in 42% yield (Scheme 6). The un-
precedented structure of 9 extends the possible caps of DABs
from trialkylsilyl, aryl, alkenyl, and butatrienyl caps (the latter in
Scheme 6. Synthesis of the diallenyl-capped DAB 9.
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with 50% unreacted diol 3i. If
the reaction was prolonged
over more than 15 min, a con-
siderable amount of polymeric
material was produced. These
observations suggest that the
cyclic DBA 10 arises from a sub-
sequent reductive cyclization of
the primary product 8i and
tends to release its strain by
polymerization under the acidic
conditions required for the re-
duction process.
The structures of 8i and 10
were confirmed by X-ray crystal-
lography (Figure 5, Table 1, and
Table 3 in the Experimental Sec-
tion), but whereas the data for
8i resemble those of the ho-
mologous diarylated DBAs 8a–
d and 8 f–h those for 10, al-
though less accurate (R=9%),
reveal unique features. The 12-
Scheme 7. Reactivity of the heptaynediol 3i with the SnCl2/HCl reducing system (top), and formal relationship of
10 with the antiaromatic (unknown) carbo-cyclobutadiene derivative (bottom).[10]
pz electron macrocycle of 10 can indeed be regarded as a dicar-
bene-stabilized form of the carbo-cyclobutadiene ring, which is
predicted to be strongly antiaromatic by the Hꢃckel rules and
by various energetic or magnetic indices calculated from the
DFT-calculated structure of the parent unsubstituted carbo-
cyclobutadiene (see explanative illustration at the bottom
of Scheme 7).[10] It is noteworthy that the two
C=CHTIPS bonds of the 1,2-alkylidene-1H2,2H2-carbo-cyclobuta-
diene 10 are coplanar and almost perpendicular to the endo-
cyclic butatriene units of the C12 macrocycle (dihedral angle
ꢄ998), which makes the latter just nonaromatic by cutting the
cyclic pz conjugation.
Absorption spectroscopy of functional carbo-butadienes
The UV/Vis absorption spectra of the functional DBAs 8a–d
and 8 f–i were recorded in chloroform solution at room tem-
perature (Figure 6). All of the chromophores display similar
spectral profiles, with an intense absorption band at high
wavelengths (with molar absorption coefficients (e) in the
range 40000–110000 Lmolꢀ1 cmꢀ1) and several secondary
bands at lower wavelengths. The range of variation of the lmax
values is 117 nm, with two extreme values for the bis-
(tri(isopropyl)silylethynyl) derivative 8i (lmax =552 nm) and the
bis(dimethylaminophenyl) derivative 8d (lmax =669 nm). The
maximum absorption wavelength is thus observed for the
compound with the most p-donating R1 and R2 substituents
on the DBA backbone, that is the N,N-dimethylaminophen-4-yl
groups of 8d.[5a] A bathochromic shift of 102 nm is induced by
substitution of the p-CH positions of the diphenyl DBA refer-
ence 8 f (lmax =567 nm) with NMe2 groups in 8d. In a related
monobutatrienic DAB series,[9] a similar bathochromic shift of
104 nm was reported by Diederich and co-workers upon re-
placement of the phenyl groups by dimethylaminophenyl
Figure 5. C2-symmetric molecular diagrams of DBAs 8i and 10 in the crystal
state (for clarity, the H atoms are omitted). The 16 atoms of the dialkynyl-
DBA chains are numbered as Si1ꢀC1ꢀC2ꢀC3ꢀC4ꢀC5ꢀC6ꢀC7ꢀC7A… and the
symmetric CiA (6ꢃiꢃ1) and Si1A (Table 1).
was performed at an earlier stage (after a reaction time shorter
than 10 min), 8i was obtained in only 8% yield, in a mixture
Chem. Eur. J. 2014, 20, 483 – 492
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absorption profiles (Figure 7).
The diallenyl-capped DAB 9 ap-
pears as a strongly absorbing
dye in the range 250–500 nm,
probably because of a broad
spectral superimposition of the
conjugated allene and buta-
triene chromophoric units in
the various stereoisomers or
conformers. The cyclic DBA 10
exhibits a maximum absorption
wavelength at lmax =402 nm,
a much smaller value than the
552 nm lower boundary at-
tained for 8i in the acyclic DBA
series. This hypsochromic shift
for 10 is actually consistent with
a further cut of the conjugation
extent with respect to the acy-
clic dehydro precursor 8i: the
C=CHꢀTIPS bonds, which corre-
Figure 6. UV/Vis absorption spectra of DBAs 8a–d and 8 f–i (in CHCl3). For 8d and 8h, see ref. [5a]; for 8 f, see
ref. [4].
groups.[9a] Apart from the dimethylaminophenyl derivative 8d
(lmax =669 nm) and the non-methylated diaminophenyl deriva-
tive 8h (lmax =622 nm),[5a] two families of DBAs with similar
spond to two of the four CꢁCꢀTIPS bonds in 8i, are indeed
no longer p-conjugated with the butatriene units of the origi-
nal DBA in the constrained C12 macrocycle of 10 (with a dihe-
dral angle of 998; Figure 5). The lmax value of 10 actually lies in
the range of lmax values of isolated DABs (lmax =(400ꢂ
25) nm).[4,9c] Nevertheless, the UV/Vis spectrum of 10 also ex-
hibits a weak band at 514 nm (Figure 7), which is reminiscent
of those bands observed in other macrocyclic p-conjugated
oligoacetylenics like carbo-benzenes and tentatively attibuted
to vibronic coupling.[2d,5b]
lmax values can be distinguished: the 8a,b,g family with lmax
=
(600ꢂ9) nm and the 8c,f,i family with lmax =(559.5ꢂ7.5) nm.
Just like 8d and 8h, the first family is constituted by aryl-sub-
stituted DBAs in which the 4-X-C6H4 aryl conjugation extends
beyond the C6H4 core to a p-conjugating X substituent, either
p donating (OMe) or p accepting (NO2). Shrinking of the p-con-
jugation extent to the C6H4 core only in 8 f (X: H) and 8c (X:
CF3) results in a 25 nm hypsochromic shift down to lmax
=
(566.5ꢂ0.5) nm. This second family is completed and bounded
by the alkynyl-substituted DBA 8i, which exhibits the shortest
conjugation extent and, thus, the lowest maximum absorption
wavelength at lmax =552 nm.
Electrochemistry of functional carbo-butadienes
The electrochemical properties of the DBAs 8a–d and 8 f–
i and the derivatives 9 and 10 were investigated by square-
wave (SWV) and cyclic (CV) voltammetry (Table 2). The DBAs
8a–d and 8 f–i exhibit similar reduction behaviors, with three
reduction waves for most of them, except for 8c, which under-
goes six reductions steps, and 8I, which undergoes only two.
A single reduction was found to be reversible for 8a and 8 f,
whereas two reversible reductions were observed for all of the
other DBAs. In the case of the push–pull DBA 8g, the two first
processes became reversible only at high scan rates (5 Vsꢀ1).
As compared to related monobutatrienic DAB homologues,[9c]
the increase of the p-conjugation extent in the dibutatrienic
DBAs induces a greater ease of reduction: this is illustrated
qualitatively by the appearance of a second reversible reduc-
tion wave in the DBA series and quantitatively by the relative
Relative to those of the above isostructural DBAs (Figure 6),
the peculiar products 9 and 10 display quite different UV/Vis
reduction potentials of the diphenylated DBA 8 f (E1/2
=
ꢀ0.80 V) relative to those of the corresponding diphenylated
DAB (E1/2 =ꢀ1.57 V). In the diaryl-DBA series, the first reduction
potential varies from ꢀ0.95 V (8d) to ꢀ0.52 V (8b), in
a manner consistent with the acceptor character of the 4-X-
C6H4 aryl substituent: 8d (X: NMe2)<8h (X: NH2)<8a (X:
OMe)<8 f (X: H)<8c (X: CF3)ꢄ8g (X: NO2, OMe)<8b (X:
NO2). It is noteworthy that the first reduction potential of the
Figure 7. UV/Vis spectra of the diallenyl-capped DAB 9 and the cyclic DBA
10 (in CHCl3). For a color version, see the Supporting Information.
Chem. Eur. J. 2014, 20, 483 – 492
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dation regime, a single wave was recorded for both 9 and 10
(Table 2). The reversibility of the reduction of 10 might be cor-
related with the non(anti)aromatic character of a 13-pz-electron
carbo-cyclobutadienide ring,[11] which exhibits an extended
conjugation (relative to the neutral substrate) after possible
planarization of the macrocyclic radical anion. The formation of
the latter would indeed be consistent with the voltammetric
observations at high scan rates (Table 2).
Table 2. CV and SWV data for DBAs 8a–d, 8 f–i, and 10 and DAB 9.[a]
Reduction
Ipox
Oxidation
Ipred
E1/2 [V]/DEp
[mV][b]
/
[c]
Ep
[V][d]
E1/2 [V]/DEp
[mV][b]
/
[c]
Ep
[V][d]
Ipred
Ipox
8a ꢀ0.88/99
1.03
ꢀ1.22
ꢀ1.45
ꢀ1.71
–
–
–
–
0.95
1.03
1.39
8b ꢀ0.52/96
ꢀ1.24/114
0.99
1.01
1.01
1.02
–
–
8c ꢀ0.65/93
ꢀ0.87/92
ꢀ1.15
ꢀ1.59
ꢀ1.71
ꢀ1.83
1.31
1.59
Conclusion
8d ꢀ0.95/73
ꢀ1.25/64
1.06
0.78
1.12
0.96
1.20
ꢀ1.55 0.43/63
ꢀ1.53 0.51/63
0.99
0.87
–
1.65
1.70
The disclosed results establish the structural variability of
stable dibutatrienic species outside a macroaromatic environ-
ment, like the Kꢀkulꢀ-resonant carbo-benzene ring.[1c] They pro-
vide an experimental basis for the design of a well-defined
class of p-extended organic molecules with promising optical
or (opto)electronic properties. Following the carbo-mer princi-
ple,[1] further targets are indeed naturally suggested by the rec-
ognized properties of parent butadiene derivatives. For exam-
ple, the E,E!Z,Z photoisomerization ability of 1,4-diphenylbu-
tadiene[12] would deserve comparison with that of the corre-
sponding carbo-butadiene, the individual 1,4-disubstituted bu-
tatriene units of which are supposed to exhibit a much lower
cis–trans isomerization barrier.[9b,c] Other natural carbo-meriza-
tion candidates (that is, molecules with specific properties that
might be refined further in the carbo-meric version and there-
fore justify synthesis efforts toward the latter) are 1,1,4,4-tetra-
phenylbutadiene (TPB), an electroluminescent dye commonly
used as a blue-emitting wavelength shifter from the UV/Vis
spectral region (Scheme 8),[13] and, in the functional series, the
1,4-di(p-aminophenyl)butadiene (DAPB) chromophores.[5a,14] In
the proposed carbo-meric targets, the external CꢁCꢀTIPS
arms of 8a–g and 8i would no longer be essential, and auxili-
ary substitution of the internal, but distant, Csp2 centers, a sta-
bilizing factor for the butatriene units,[1c] would not induce tor-
sion of the DBA motif: these targets could thus achieve opti-
mal n p-conjugation over ten carbon atoms in still allowed
quasiplanar conformations (Scheme 8, Figure 3).
8h ꢀ0.92/73
ꢀ1.26/64
8 f ꢀ0.80/106
ꢀ1.12
ꢀ1.37
–
0.95
1.15(a)
1.34
8g ꢀ0.62/91[e]
1.32
0.66
0.97
0.98
1.28
ꢀ1.66 1.06/93[e]
1.10
0.97
–
ꢀ0.78/88[e]
8i ꢀ0.65/94
ꢀ0.94/115
–
1.29/95
–
–
9
ꢀ1.16/100[f]
ꢀ1.42
ꢀ1.55
1.03
–
10 ꢀ1.15/108
0.87
ꢀ1.48 1.21/103[g]
ꢀ1.66
1.00
[a] Data for 8d and 8h in ref. [5a]; data for 8 f in ref. [4]. Measurements
were performed at room temperature in dichloromethane; supporting
electrolyte: [n-Bu4N][PF6] (0.1m); working electrode: Pt; reference elec-
trode: saturated calomel electrode (SCE; 0.242 V versus the hydrogen
electrode); scan rate: 0.2 Vsꢀ1 unless otherwise noted. [b] E = = [Epc +Epa]/
1
2
2, the arithmetic mean of anodic and cathodic peak potentials; DEp: CV
peak-to-peak separation for reversible processes. [c] Ipox/Ipred: absolute cur-
rent ratio for reversible processes measured by CV. [d] Ep: half-wave po-
tential determined by SWV for irreversible processes. [e] Becomes reversi-
ble at a scan rate of 5 Vsꢀ1. [f] Becomes reversible at a scan rate of
1 Vsꢀ1. [g] The formation of radicals is observed at high scan rates, but
the reversibility of the first oxidation remains unchanged.
dialkynyl-DBA 8i (ꢀ0.65 V) is close to those of the acceptor-
substituted diaryl-DBAs 8c and 8g, probably because of the
a effect of the silicon atoms, which is supposed to stabilize the
reducing electron at the terminal C atoms of the four triple
bonds of 8i.
In the oxidation regime, the carbo-butadienes 8a–d and 8 f–
i undergo one or two oxidation steps, most of them being irre-
versible. Nevertheless, by symmetry with respect to the reversi-
ble half-wave potentials recorded in the reduction regime
(groups that stabilize cations generally destabilize anions, and
conversely), the first oxidation peak potential varies from
+0.43 V (8d) to +1.39 V (8b), in a manner consistent with the
donor characteristics of the 4-X-C6H4 aryl substituent: 8d (X:
NMe2, rev.)<8h (X: NH2, rev.)<8a (X: OMe)ꢄ8 f (X: H)<8g
(X: NO2, OMe, rev.)<8 f (X: H, rev.)<8c (X: CF3)<8b (X: NO2).
Rankings are for irreversible waves, unless otherwise noted as
“rev.”).
Scheme 8. Simple DBA target suggested by the carbo-mer principle and the
known electro-optical properties of the parent molecule (1,1,4,4-tetraphenyl-
1,3-butadiene in this case).[13]
Experimental Section
General
In spite of a moderate stability, the diallenyl-DAB 9 and
cyclic-DBA 10 could also be studied by CV and SWV. In the re-
duction regime, both of them exhibit the same behavior, with
one reversible reduction at almost identical potentials (approx-
imately ꢀ1.15 V) and two irreversible counterparts. In the oxi-
THF and diethyl ether were dried and distilled over sodium/benzo-
phenone; pentane and dichloromethane were dried and distilled
over P2O5. All other reagents were used as commercially available.
In particular, commercial solutions of nBuLi were 2.5m in hexane,
Chem. Eur. J. 2014, 20, 483 – 492
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solutions of ethylmagnesium bromide were 3m in THF, and solu-
tions of HCl were 2m in diethyl ether. Previously described proce-
dures were used for the preparation of 1,[7] 2,[5] 3e,[5] 3 f,[4] 4e,[7]
5,[9c] 6e,[7] 8d,[5] 8h,[5] and 8 f.[4] All of the reactions were carried out
under a nitrogen or argon atmosphere by using Schlenk and
vacuum-line techniques. Column chromatography was carried out
on silica gel (60P, 70–200 mm). Silica gel thin-layer chromatogra-
phy plates (60F254, 0.25 mm) were revealed by treatment with an
ethanolic solution of phosphomolybdic acid (20%). The following
routine analytical instruments were used: mass spectrometry:
Quadrupolar Nermag R10–10H spectrometer; UV/Vis spectroscopy:
Perkin–Elmer UV/Vis Win-Lab Lambda 35 spectrometer; 1H and
13C NMR spectroscopy: Bruker DPX 300, Avance 300, Avance 400,
Avance 400WB, or Avance 500 spectrometers. Most of the NMR
spectra were recorded in CDCl3 solutions. NMR chemical shifts d
are in ppm, with positive values to high frequency relative to the
tetramethylsilane reference; coupling constants J are in Hz. 13C{1H}
chemical shifts refer to 13C NMR spectra recorded under broad
band decoupling of the 1H nuclei. The synthesis procedures and
characterization data for new compounds can be found in the Sup-
porting Information.
ments were performed at room temperature in a home-made air-
tight three-electrode cell connected to a vacuum/argon line. The
reference electrode consisted of a saturated calomel electrode
(SCE) separated from the solution by a bridge compartment. The
counter electrode was a platinum wire of approximately 1 cm2 ap-
parent surface area. The working electrode was a Pt microdisk
(0.5 mm diameter) or a glassy carbon microdisk (1 mm diameter).
The supporting electrolyte [nBu4N][PF6] (Fluka, 99% electrochemi-
cal grade) was used as received and simply degassed under argon.
Dichloromethane was freshly distilled prior to use. The solution
concentrations were typically 10ꢀ3 m for the studied molecule and
0.1m for the supporting electrolyte. Before each measurement, the
solutions were degassed by bubbling argon, and the working elec-
trode was polished with a polishing machine (Presi P230). Typical
instrument parameters for square-wave voltammograms were: SW
frequency, f=20 Hz; SW amplitude, Esw=20 mV; scan increment,
dE=0.5 mV.
Crystallography
X-ray intensity data of crystals of 8a–c, 8g, 8i, and 10 were collect-
ed at 193(2) K on a Bruker-AXS APEX II diffractometer by using
a 30 W air-cooled microfocus source (ImS) with focusing multilayer
optics and graphite-monochromated MoKa radiation (l=0.71073 ꢂ)
by using F and W scans. The data were integrated with SAINT soft-
ware, and an empirical absorption correction was applied with the
Voltammetry
Voltammetric measurements were carried out with an Autolab
PGSTAT100 potentiostat controlled by GPES 4.09 software. Experi-
Table 3. Crystallographic data and structural refinement parameters for 8a–c, 8g, 8i and 10.
8a
8b
8c
8g
8i
10
initial code
formula
Mr
ar216m
C58H66O2Si2
851.29
vm655m
C56H60N2O4Si2
881.24
ar454m
C58H60F6Si2
927.24
vm671m
C57H63NO3Si2
866.26
ar334bism
C66H94Si4
999.77
ar328m
C66H97O0.5Si4
1010.80
crystal system
space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
triclinic
¯
P1
triclinic
¯
P1
triclinic
¯
P1
triclinic
¯
P1
triclinic
¯
P1
monoclinic
C2/c
14.5221(14)
19.685(2)
c23.699(2)
90
102.284(3)
90
6619.6(11)
4
1.014
0.125
2212
0.34ꢄ0.20ꢄ0.02
5.10–24.71
ꢀ17ꢅhꢅ17
ꢀ23ꢅkꢅ23
ꢀ27ꢅlꢅ27
58357
7.7665(6)
7.8099(5)
21.4670(16)
95.546(2)
96.993(3)
94.385(3)
1281.22(16)
1
1.103
0.109
458
0.50ꢄ0.20ꢄ0.02
2.70–26.40
ꢀ9ꢅhꢅ9
ꢀ9ꢅkꢅ9
ꢀ26ꢅlꢅ26
18998
7.5677(5)
7.8409(5)
21.2471(14)
96.902(3)
93.895(3)
94.542(3)
1243.86(14)
1
1.176
0.118
470
0.26ꢄ0.10ꢄ0.02
5.21–25.68
ꢀ9ꢅhꢅ8
ꢀ9ꢅkꢅ9
ꢀ25ꢅlꢅ25
22870
7.6358(10)
18.453(2)
19.817(2)
103.407(7)
99.615(6)
97.888(6)
2632.6(6)
2
1.170
0.124
980
0.26ꢄ0.04ꢄ0.01
5.12–27.10
ꢀ9ꢅhꢅ9
ꢀ23ꢅkꢅ23
ꢀ25ꢅlꢅ25
66461
a7.6332(8)
7.7812(9)
21.635(2)
97.045(5)
95.339(5)
94.087(5)
1265.4(2)
1
1.137
0.113
464
0.70ꢄ0.30ꢄ0.02
5.15–25.35
ꢀ9ꢅhꢅ7
ꢀ9ꢅkꢅ8
ꢀ26ꢅlꢅ26
16685
11.1387(9)
12.1237(10)
14.3424(11)
70.552(3)
72.040(3)
64.835(3)
1621.1(2)
1
1.024
0.127
546
0.32ꢄ0.20ꢄ0.04
5.13–27.10
ꢀ14ꢅhꢅ14
ꢀ15ꢅkꢅ15
ꢀ17ꢅlꢅ18
38261
a [8]
b [8]
g [8]
V [ꢂ3]
Z
dcalcd [gcmꢀ1
]
m [mmꢀ1
F(000)
]
crystal size [mm3]
q range [8]
index ranges
reflns collected
independent reflns
5225
4687
11521
4577
7095
5599
[R(int)=0.0478]
99.2
[R(int)=0.0665]
98.9
[R(int)=0.1613]
99.1
[R(int)=0.0495]
98.5
[R(int)=0.0325]
99.1
[R(int)=0.0782]
98.9
qmax [%]
max–min transmission 0.9978–0.9476
0.9976–0.9699
4687/0/295
0.9988–0.9685
11521/334/738
0.9977–0.9250
4577/117/324
0.9949–0.9605
7095/78/354
0.9975–0.9586
5599/913/648
data/restraints/
parameters
GOF on F2
5225/35/304
1.020
1.016
0.978
1.036
1.026
1.024
final R indices [I>2s(I)] R1=0.0549,
wR2=0.1223
R1=0.0499,
wR2=0.0960
R1=0.1017,
wR2=0.1134
0.246/ꢀ0.208
R1=0.0704,
wR2=0.1325
R1=0.2371,
wR2=0.1816
0.260/ꢀ0.202
R1=0.0593,
wR2=0.1365
R1=0.0998,
wR2=0.1559
0.459/ꢀ0.314
R1=0.0422,
wR2=0.1084
R1=0.547,
wR2=0.1167
0.348/ꢀ0.296
R1=0.0902,
wR2=0.2386
R1=0.1592,
wR2=0.2902
0.257/ꢀ0.206
R indices
R1=0.0937,
(all data)
wR2=0.1403
largest diff. peak/hole 0.205/ꢀ0.229
[eꢂꢀ3
]
Chem. Eur. J. 2014, 20, 483 – 492
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
SADABS program.[15] The structures were solved by direct methods,
with SHELXS-97 software, and refined by using the full-matrix
least-squares method on F2.[16] All non-H atoms were treated aniso-
tropically. The H atoms were located by difference Fourier maps
and refined with a riding model. Crystallographic data and struc-
tural refinement parameters are listed in Table 3, and molecular
views are shown in Figures 3 and 5. CCDC 951898 (8a), 951902
(8b), 951901 (8c), 951903 (8g), 951900 (8i) and 951899 (10) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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As well as the Centre National de la Recherche Scientifique,
the Ministꢅre de l’Enseignement Supꢀrieur de la Recherche et
de la Technologie, and the Universitꢀ Paul-Sabatier, the authors
thank the ANR program (11-BS07-016-01) for the postdoctoral
grant of A.R. and for the support toward the purchase of sup-
plies and costs of analyses. The authors also thank Dr. Alix
Saquet for electrochemistry experiments.
Keywords: alkynes · cumulenes · carbo-mers · chromophores ·
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Received: August 11, 2013
Published online on December 5, 2013
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