Full text of DOI:10.1002/anie.200701733

Angewandte
Chemie
DOI: 10.1002/anie.200701733
Charge-Transfer Chromophores
Charge-Transfer Chromophores by Cycloaddition–Retro-
electrocyclization: Multivalent Systems and Cascade Reactions**
Milan Kivala, Corinne Boudon, Jean-Paul Gisselbrecht, Paul Seiler, Maurice Gross, and
François Diederich*
In memoryof Yoshihiko Ito
Thermal [2+2] cycloadditions of tetracyanoethylene (TCNE)
with electron-rich alkynes yield cyclobutenes, which in
isolated cases have been shown to undergo retro-electro-
cyclization under formation of 1,1,4,4-tetracyanobuta-1,3-
dienes (TCBDs).[1,2] Recently, we found that a wide variety
and vice versa. This study culminates in a one-pot, eight-step
domino reaction, with the formation of a single product
resultingfrom four sequential cycloaddition/retro-electrocyc-
lizations of TCNE and TTF molecules to give a octa-1,3,5,7-
tetrayne.[6]
of
4-N,N-dialkylanilino-substituted
(DAA-substituted)
In our earlier work we showed that the fast addition of
TCNE to DAA-substituted alkynes at room temperature had
the character of a “click” reaction, thereby affordingproducts
in an atom-economic way with near quantitative yields.[3b]
Thus, the tris-TCBD derivative 1, which features an intra-
alkynes react smoothly with TCNE to furnish DAA-donor-
substituted TCBDs, often in quantitative yield. This new class
of chromophores features intense intramolecular charge-
transfer (CT) interactions with absorption maxima in the
visible spectral range as well as promising third-order optical
nonlinearities.[3]
A similar reaction occurs under the conditions of inverse
electron demand. Hopf et al. showed that the strongelectron
donor tetrathiafulvalene (TTF) undergoes cycloaddition with
electron-deficient acetylene moieties in cyanoethynylethenes,
and the subsequent electrocyclic ring-opening afforded buta-
diene derivatives, 1,2-di(1,3-dithiol-2-ylidene)ethanes.[4]
A
similar reaction was observed by Hirsch and co-workers at
the terminal acetylene moiety of a,w-dicyanopolyynes.[5]
Here, we report the application of the reaction of TCNE
with electron-rich alkynes to the high-yielding formation of
dendrimer-type, multivalent charge-transfer chromophores
that are capable of takingup an exceptional number of
electrons under electrochemical conditions, thereby actingas
a type of molecular batteries. Subsequently, we describe
cascade reactions of polyyne oligomers, in which the cyclo-
addition/retro-electrocyclization with TCNE activates the
ꢀ
adjacent C C bond electronically for the reaction with TTF
[*] M. Kivala, P. Seiler, Prof. Dr. F. Diederich
Laboratorium für Organische Chemie
ETH Zürich
Hönggerberg, HCI, 8093 Zürich(Switzerland)
Fax: (+41)44-632-1109
E-mail: diederich@org.chem.ethz.ch
Dr. C. Boudon, Dr. J.-P. Gisselbrecht, Prof. Dr. M. Gross
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Institut de Chimie—LC3—UMR 7177, C.N.R.S
UniversitØ Louis Pasteur
4, rue Blaise Pascal, 67000 Strasbourg (France)
[**] This work was supported by a grant from the ETH research council
and the NCCR “Nanoscale Science”, Basel. Dr. Carlo Thilgen
(ETHZ) is gratefully acknowledged for his help with the nomen-
clature and Prof. Dr. Bernhard Jaun (ETHZ) for the interpretation of
NMR data of the compounds described herein.
molecular CT band at 590 nm, was obtained from the
correspondingtriyne in 86% yield. The electrochemical
properties of 1 are remarkable in that it undergoes six
reversible one-electron reduction steps, each centered on a
dicyanovinyl moiety, in the unprecedently narrow potential
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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range between À0.69 and À1.69 V (versus the ferricinium/
Thus, the UV/Vis spectra of 2–4 in CH2Cl2 display CT bands
at lmax = 460 nm (2.70 eV, e = 114300mÀ1 cmÀ1), lmax = 476 nm
(2.60 eV, e = 110000mÀ1 cmÀ1), and lmax = 456 nm (2.72 eV,
e = 91400mÀ1 cmÀ1), respectively (see the SupportingInfor-
mation).
The redox properties of 2–4 as well as their precursors (see
the SupportingInformation) were studied by cyclic voltam-
metry (CV) and rotating-disc voltammetry (RDV) in CH2Cl2
(+ 0.1m nBu4NPF6). Thus, the oxidation of all the DAA
moieties in 2–4 occurs in a single, reversible multielectron
transfer. As an example, dendrimer 4 is oxidized in a unique
12-electron transfer step at + 0.89 V (versus Fc+/Fc). Hence,
all the DAA moieties in a multivalent system behave as
independent redox centers (Table 1).[11a] Such a behavior has
been observed previously in the case of ferrocenyl dendrimers
by Astruc and co-workers[11b,c] and others.[11d]
ferrocene couple, Fc+/Fc) in CH2Cl2.[7] This findinginitiated
our search for larger multivalent CT systems that could act as
powerful electron reservoirs.[8–11]
We prepared a large library of dendrimer-like DAA-
substituted alkynes and report here on the new multivalent
CT chromophores 2–4.[12] In contrast to the tris-TCBD
derivative 1, buta-1,3-diyne-1,4-diyl rather than ethyne-1,2-
diyl fragments were used to attach the DAA moieties to the
central core to reduce the steric encumbrance and enhance
the distance between pairs of dicyanovinyl acceptor moieties,
thereby bringing the individual reduction potentials even
closer.
The new multivalent TCBD derivatives 2–4 were obtained
in excellent yields (77–96%) by the reaction of the corre-
spondingalkyne precursors with TCNE in CH Cl2 at 208C
2
(see the SupportingInformation). Thus, twelvefold addition
of TCBD to produce 4 with the hexaphenylbenzene core
Table 1: Electrochemical data of CT chromophores 2–5 observed by CV
and RDV in CH2Cl2 (+0.1m nBu4NPF6).[a]
CV
RDV
E8 [V][b] DEp [mV][c] Ep [V][d]
E1/2 [V][e]
slope [mV][f]
2
3
+0.88
90
+0.87 (3eÀ)
À0.73 (3eÀ)
À1.28 (3eÀ)
+0.90 (6eÀ)
60
120
150
85
À0.67 160
À1.13 180
+0.89 100
À0.46
À0.60
À0.70
À0.76
À0.95
À1.07
60
60
60
60
60
60
À0.80 (6eÀ)
400
À1.57 150
À1.55 (6eÀ)
200
50
4
5
+0.87
50
+0.89 (12eÀ)
À0.70 100
À1.10 220
+0.95 100
À0.73 (12eÀ) 130
[g]
+0.96 (3eÀ)
+0.81 (2eÀ)
+0.65 (2eÀ)
+0.41 (2eÀ)
70
70
60
70
+0.80
+0.61 115
+0.40 110
À1.12
À1.22
À1.40
300
À1.35
À1.40
À1.48
À1.55
[a] All potentials are given versus the Fc+/Fc couple used as internal
standard.The electrochemical data for all other compounds reported in
proceeded in a rather spectacular 86% yield, which means
nearly quantitative conversion in each of the twelve con-
current cycloaddition/retro-electrocyclization sequences (that
is, 98% yield for each TCNE addition). All the compounds
are stable at ambient temperature under air, and melt
undecomposed above 1008C. In agreement with our previous
observation,[3b] each buta-1,3-diynyl moiety of the corre-
spondingprecursor reacted exclusively with one equivalent of
this paper can be found in the Supporting Information. [b] E8=(Epc
+
Epa)/2, where Epc and Epa correspond to the cathodic and anodic peak
potentials, respectively. [c] DEp =EoxÀEred, where the subscripts ox and
red refer to the conjugated oxidation and reduction steps, respectively.
[d] Ep =Irreversible peak potential. [e] E1/2 =Half-wave potential.
[f] Slope of the linearized plot of E versus log[I/(IlimÀI)], where Ilim is
the limiting current and I the current. [g] Unresolved spread-out wave.
ꢀ
TCNE at the more electron rich C C bond directly attached
to the DAA substituent (even in the presence of an excess of
TCNE at elevated temperature). The UV/Vis spectra of 2–4
are dominated by intense, broad CT bands accompanied by a
longtail or shoulder reachinginto the near infrared region.
The absorption maxima and their intensities are rather
indifferent to structural changes in the oligomeric TCBDs.
Compounds 2–4 undergo several reversible one-electron
reduction steps centered on the dicyanovinyl units: each
TCBD moiety can accommodate two electrons (Table 1).
Thus, 3 with six TCBD moieties shows six well-separated one-
electron reduction steps (from À0.46 V to À1.07 V) followed
by an unresolved six-electron transfer at À1.57 V. It can be
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assumed that the first six one-electron transfers are centered
on dicyanovinyl moieties of different TCBD moieties. Den-
drimer 4 accepts 24 electrons in two reversible 12-electron
reduction steps at À0.70 V and À1.10 V, respectively (for the
CV traces, see the SupportingInformation). The reversible
injection of 24 electrons into a molecule within a narrow
potential range between À0.70 V and À1.10 V is quite
remarkable, also in terms of the solubility of the highly
charged species formed. The differences in peak potentials for
the first and second reduction step are 100 mV and 220 mV,
respectively. The peak potentials are scan-rate independent
up to 1 VsÀ1. These characteristics are typical for unresolved
overlappingelectron transfers occurringat slightly different
potentials, which denotes very little interaction between the
different TCBD groups in 4.[11a]
While exploringthe reactivity of the tris-TCBD derivative
(2.57 eV, e = 132000mÀ1 cmÀ1; see the SupportingInforma-
tion).
Buildingon this result, we decided to construct rodlike
ꢀ
2, we found the C C bonds adjacent to the electron-accepting
TCBD units to be sufficiently electron-deficient and, hence,
activated for the [2+2] cycloaddition to TTF, as previously
observed by Hopf and co-workers and Hirsch and co-workers
for cyanoethynylethenes and a,w-dicyanopolyynes, respec-
tively.[4,5] Upon heatingin MeCN, 2 undergoes threefold
[2+2] cycloaddition to TTF followed by retro-electrocycliza-
tion to give adduct 5 in 47% yield (that is, 78% yield for each
TTF addition). The proposed constitution of this black-
metallic solid, which melts at 214–2178C, was unambiguously
proven by the spectral data (see the SupportingInformation),
although complex conformational equilibria complicate the
interpretation of the 1H and 13C NMR spectra. Highly
reproducible voltammograms were measured which showed
three successive reversible two-electron oxidation steps of the
three 1,2-di(1,3-dithiol-2-ylidene)ethane moieties at + 0.41,
+ 0.65, and + 0.81 V in addition to a three-electron oxidation
at + 0.96 V of the three DAA moieties and six individual
reduction waves between À1.12 V and À1.55 V for the six
dicyanovinyl units. The UV/Vis spectrum of 5 in CH2Cl2 is
dominated by an intense broad CT band at lmax = 482 nm
oligomeric donor–acceptor (D-A) systems where the donor
part consists of 1,2-di(1,3-dithiol-2-ylidene)ethane units,
while the 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) frame-
work serves as the acceptor. A cascade of successive TCNE/
TTF additions to end-capped polyynes, controlled by the
ꢀ
electronic properties of the reactingC C bonds, would
provide access to a new class of conjugated [AB]-type
oligomers and polymers[13] with dendralene-type back-
bones.[14]
Thus, bis-DAA-substituted tetrayne 6 (see the Supporting
Information) was treated with one equivalent of TCNE in
CH2Cl2 to yield TCBD 7 (72%; for the X-ray structure, see
the SupportingInformation) which reacted subsequently with
an excess of TTF in MeCN to afford the hybrid TCNE-TTF
adduct 8 in 80% yield (Scheme 1). In the next step, derivative
8 gave A-D-A system 9 (83%) upon reaction with TCNE
(Scheme 1). The regioselectivity of the TCNE addition is
determined by the stronger 1,2-di(1,3-dithiol-2-ylidene)-
ethane donor. However, the attempted cycloaddition of the
Scheme 1. Cascade of alternating [2+2] cycloadditions/retro-electrocyclizations of TCNE/TTF to octatetraynes 6 and 10. a) TCNE, CH2Cl2, 10–14 h,
208C, 72% (7), 95% (11). b) TTF, MeCN, 16–17 h, 608C, 80% (8), 78% (12). c) TCNE, CH2Cl2, 14–22 h, 208C, 83% (9), 92% (13). d) TTF,
CH2Cl2/MeCN 1:1, 3 h, 508C, 21% (14). e) TCNE, TTF, CH2Cl2/MeCN 1:1, 22 h, 508C, 21% (14).
Angew. Chem. Int. Ed. 2007, 46, 6357 –6360
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Communications
Chim. Fr. 1972, 6, 2385 – 2387; for an early reference to the
addition of TCNE, see: M. I. Bruce, J. R. Rodgers, M. R. Snow,
A. G. Swincer, J. Chem. Soc. Chem. Commun. 1981, 271 – 272.
[2] For some more recent work, see a) C. Cai, I. Liakatas, M.-S.
Wong, M. Bösch, C. Bosshard, P. Günter, S. Concilio, N. Tirelli,
U. W. Suter, Org. Lett. 1999, 1, 1847 – 1849; b) K. Onitsuka, N.
Ose, F. Ozawa, S. Takahashi, J. Organomet. Chem. 1999, 578,
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Nishida, Y. Yamashita, Chem. Lett. 2004, 33, 1190 – 1191.
[3] a) T. Michinobu, J. C. May, J. H. Lim, C. Boudon, J.-P. Gissel-
brecht, P. Seiler, M. Gross, I. Biaggio, F. Diederich, Chem.
Commun. 2005, 737 – 739; b) T. Michinobu, C. Boudon, J.-P.
Gisselbrecht, P. Seiler, B. Frank, N. N. P. Moonen, M. Gross, F.
Diederich, Chem. Eur. J. 2006, 12, 1889 – 1905.
[4] H. Hopf, M. Kreutzer, P. G. Jones, Angew. Chem. 1991, 103,
1148 – 1149; Angew. Chem. Int. Ed. Engl. 1991, 30, 1127 – 1128.
[5] G. Schermann, O. Vostrowsky, A. Hirsch, Eur. J. Org. Chem.
1999, 2491 – 2500.
[6] For cascade and domino reactions, see: L. F. Tietze, G. Brasche,
K. Gericke, Domino Reactions in Organic Synthesis, Wiley-
VCH, Weinheim, 2006.
[7] Note that the six reduction steps of fullerene C60 in MeCN/
toluene occur within a much wider potential range between
À0.98 and À3.26 V versus Fc+/Fc: Q. Xie, E. PØrez-Cordero, L.
Echegoyen, J. Am. Chem. Soc. 1992, 114, 3978 – 3980.
[8] a) D. Astruc, Acc. Chem. Res. 1986, 19, 377 – 383; b) D. Astruc,
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[10] R. Schenk, H. Gregorius, K. Meerholz, J. Heinze, K. Müllen, J.
Am. Chem. Soc. 1991, 113, 2634 – 2647.
remainingacetylenic bond in 9 to TTF failed. This triple bond
is not sufficiently electron deficient to undergo [2+2] cyclo-
addition to TTF.
To eliminate this “electronic confusion” of the last triple
bond, we started from mono-DAA-, monophenyl-substituted
tetrayne 10 (see the SupportingInformation). The TCBD
derivative 11 was formed in nearly quantitive yield (95%)
usingone equivalent of TCNE, and subsequent reaction with
TTF afforded adduct 12 (78%; Scheme 1). Treatment of A-D
chromophore 12 with TCNE gave A-D-A derivative 13 in
ꢀ
nearly quantitative yield (92%). Finally, the C C bond in 13,
which is now electron deficient and no longer “electronically
confused”, was subjected to the reaction with an excess of
TTF to yield the A-D-A-D chromophore 14 as a black-
metallic solid (m.p. 2608C) in 21% yield (Scheme 1). The
reduced yield of 14 is presumably caused by steric crowding
ꢀ
around the reactingC C bond.
We next attempted the cascade of successive
[2+2] TCNE/TTF additions to the end-capped tetrayne 10
in a one-pot setup. Mixing 10 with an excess of TCNE and
TTF in MeCN/CH2Cl2 at 508C indeed yielded the [ABAB]
system 14 in 21% yield, which corresponds to a yield of 68%
per cycloaddition/retro-electrocyclization step (Scheme 1).
As in the case of 5, the NMR characterization of the
hybrid TTF-TCNE chromophores 8, 9, and 12–14 was
seriously complicated by the presence of complex conforma-
tional equilibria in solution.[15] The UV/Vis spectra of 8, 9, and
12–14 are dominated by intense, broad CT bands with
absorption maxima lmax between 460 and 480 nm (see the
SupportingInformation).
[11] a) J. B. Flanagan, S. Margel, A. J. Bard, F. C. Anson, J. Am.
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Ruiz, J. Guittard, J.-C. Blais, D. Astruc, J. Am. Chem. Soc. 1997,
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Astruc, Chem. Eur. J. 2003, 9, 4371 – 4379; d) V. J. Chebny, D.
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A. Listorti, N. Armaroli, T. Nemoto, S. Isoda, J. Phys. Chem. C
2007, 111, 2777 – 2786.
While the redox properties of all the D-A chromophores
have already been studied by CV and RDV (for a listingand
discussion of the data for 6–14, see the SupportingInforma-
tion), the spin properties of the polyanions and polycations
with odd numbers of electrons by electron paramagnetic
resonance (EPR) are currently under investigation. Further-
more, the exploration of second- and third-order optical
nonlinearities and other advanced materials properties of the
fascinatingnew chareg-transfer chromophores reported in
this manuscript is now intensively beingpursued.
In summary, we have demonstrated the enormous syn-
thetic potential and complementarity of the [2+2] cycloaddi-
tions of TCNE and TTF to alkynes for the construction of
multivalent charge-transfer chromophores. A one-pot proto-
col for electronically controlled cascade TCNE/TTFadditions
to polyynes opens up an easy access to a new family of
conjugated [AB]-type oligomers and polymers, and applica-
tion to longer polyynes is now under investigation.
[12] All new compounds were fully characterized by m.p., IR, UV/
1
Vis, H and 13C NMR spectroscopy as well as HR-MALDI-MS
and/or elemental analysis; for full experimental details, see the
SupportingInformation. An account of the entire multivalent
CT compound library will be published as a full paper.
[13] a) Q. T. Zhang, J. M. Tour, J. Am. Chem. Soc. 1997, 119, 5065 –
5066; b) Q. T. Zhang, J. M. Tour, J. Am. Chem. Soc. 1998, 120,
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J. R. Reynolds, J. Am. Chem. Soc. 2004, 126, 16440 – 16450.
[14] a) H. Hopf, Angew. Chem. 1984, 96, 947 – 958; Angew. Chem. Int.
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113, 727 – 729; Angew. Chem. Int. Ed. 2001, 40, 705 – 707.
[15] The limited stability of chromophores 5, 8, 9, and 12–14 at high
temperatures together with their poor solubility at low temper-
atures reduced the available temperature range for VT-NMR
experiments (253–353 K). Thus, only the 1H NMR spectra of
derivatives 8 and 12 could be recorded beyond the coalescence
temperature of all signals. The temperature at which frozen
conformations could be observed by 1H NMR spectroscopy was
not reached because of the low solubility of the compounds.
Coalescence of the 13C NMR signals was not observed within the
available temperature range. Thus, complex 13C NMR spectra of
5, 8, 9 and 12–14 are reported in the SupportingInformation as
empiric enumeration of observed signals.
Received: April 19, 2007
Published online: July 20, 2007
Keywords: alkynes · cascade reactions · charge transfer ·
.
conjugation · cycloaddition · electrochemistry
[1] To the best of our knowledge, this cascade transformation was
first reported for the reaction of acrylonitriles with N,N-
diethylaminoprop-1-yne, see J. Ficini, A. M. Touzin, Bull. Soc.
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