High-yield formation of substituted tetracyanobutadienes from reaction of ynamides with tetracyanoethylene

Article abstract of DOI:10.1002/chem.201402653

A high-yielding sequence of [2+2] cycloaddition-retroelectrocyclization of ynamides with tetracyanoethylene (TCNE) is described. The reaction provided tetracyanobutadiene (TCBD) species, which were characterized by various techniques. DFT and TD-DFT calculations were also performed to complement experimental findings. 2+2=Tetracyanobutadienes: The reaction between ynamides and tetracyanoethylene at room temperature in dichloromethane provides tetracyanobutadienes in good to quantitative yields, following a sequence of [2+2] cycloaddition-retroelectrocyclization (see scheme; EWG=electron- withdrawing group).

Full text of DOI:10.1002/chem.201402653

DOI: 10.1002/chem.201402653
Communication
&
Synthetic Methods
High-Yield Formation of Substituted Tetracyanobutadienes from
Reaction of Ynamides with Tetracyanoethylene
Marie Betou, Nicolas Kerisit, Esme Meledje, Yann R. Leroux, Claudine Katan, Jean-
FranÅois Halet, Jean-Claude Guillemin, and Yann Trolez*[a]
room temperature. However, the reactivity of TCNE with al-
Abstract: A high-yielding sequence of [2+2] cycloaddi-
tion–retroelectrocyclization of ynamides with tetracyano-
kynes directly substituted by an electron-donating heteroatom
has never been described so far.
ethylene (TCNE) is described. The reaction provided tetra-
As examples of such compounds, ynamides have received
cyanobutadiene (TCBD) species, which were characterized
considerable attention during the last decade.[9] This interest
by various techniques. DFT and TD-DFT calculations were
can be explained by the new efficient synthetic methodologies
also performed to complement experimental findings.
recently developed.[10] The ynamide CꢀC triple bond is activat-
ed by the donating ability of the nitrogen atom (Figure 2).
However, unlike ynamines, these compounds are stabilized by
The sequence of [2+2] cycloaddition–retroelectrocyclization
(CA–RE) between tetracyanoethylene (TCNE) and alkynyl-transi-
tion metal complexes has been known for several decades[1]
and has extensively been studied.[2] However, to the best of
our knowledge, this reactivity with purely organic alkynes has
been discovered only in 1999 with a-substituted thienyl-
alkynes.[3] Since then, other alkynes substituted by electron-do-
nating groups (EDG) have been shown to react the same way
(Figure 1).[4] This reaction has mostly been popularized by
Diederich and co-workers for the last decade.[5] Aniline-,[6] azu-
lene-[7] and heteroazulene-substituted[8] alkynes represent the
best examples of this reaction by providing yields over 90%,
by simply mixing the two reactants together in a solvent at
an electron-withdrawing group (EWG) on the nitrogen, which
Figure 2. Ynamide mesomeric forms highlighting the electron richness of
the CꢀC triple bond.
makes them air-stable and thus easy to handle. [2+2] Cycload-
ditions of ynamides are known and usually require a catalyst[11]
or a Lewis acid,[12] except for their reaction with the ketene.[13]
In this communication, we report on the reactivity of a varie-
ty of ynamides with TCNE to achieve tetracyanobutadiene
(TCBD) species in moderate to excellent yields (57% to quanti-
tative) at room temperature and without the need for any acti-
vating agent. In addition to their potential interesting opto-
electronic properties, these new TCBDs may be easily function-
alized in various positions thanks to the nature of their
ynamide precursors.
At first, three different ynamides, which differ from each
other by the nature of the electron-withdrawing group, were
synthesized. Ynamides 1 and 2 (Scheme 1) were prepared ac-
cording to literature procedures[14] whereas the synthesis of 3
has been inspired by a recent article from the Hsung group
(see the Supporting Information for details).[15] Compounds 1–
3 in dichloromethane were reacted with an equimolar amount
of TCNE at room temperature overnight. The same reactivity
was observed and TCBD adducts 4, 5 and 6 were obtained in
92, 98 and 93% yield, respectively.[16] This reaction is supposed
to proceed according to a sequence of [2+2] CA–RE as de-
scribed in Figure 1.
Figure 1. Previous work on [2+2] cycloaddition of TCNE with electron-rich
alkynes.
[a] Dr. M. Betou, N. Kerisit, E. Meledje, Dr. Y. R. Leroux, Dr. C. Katan,
Dr. J.-F. Halet, Dr. J.-C. Guillemin, Dr. Y. Trolez
Institut des Sciences Chimiques de Rennes, UMR 6226, CNRS,
Ecole Nationale Supꢀrieure de Chimie de Rennes,
Universitꢀ de Rennes 1, 35708 Rennes Cedex (France)
Fax: (+33)2-23238108
E-mail: yann.trolez@ensc-rennes.fr
Secondly, the scope of the reaction was investigated using
the tosylate group. This EWG was preferred over the carba-
mate and the phosphonate due to its ease of preparation and
higher degree of crystallinity.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402653 and contains synthetic proce-
dures, characterization of compounds 3–6, 12, 17–30, electrochemical
analysis, computational details and results.
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Communication
nylaniline, entries 4 and 6) did not change the yield (quantita-
tive and 92% respectively). However, electron-poor phenyl
groups (p-chlorophenyl and p-cyanophenyl, entries 3 and 5)
slightly to moderately decreased the reactivity (90% and 57%
respectively). Nevertheless, particularly noteworthy is the sig-
nificant increase of the yield with p-cyanophenyl group when
using 2 equivalents of TCNE instead of 1 (from 57% to 79%).
The n-hexyl group or a hydrogen in lieu of a phenyl one pro-
vided TCBD in quantitative yields (entries 7 and 8). Heteroaro-
matic substituents were also studied: Whereas a thiophen-2-yl
group did not really affect the yield of the reaction (87%,
entry 10), a pyridin-3-yl group significantly decreased the yield
to 58%, even using two equivalents of TCNE. Indeed, three
equivalents of TCNE and a longer reaction time of 64 h were
necessary to give 25 in 75% yield. In every instance, when the
yield was not over 90%, the conversion was not complete. It
was never due to decomposition during the purification pro-
cess. No other product was formed.
Scheme 1. Reactivity of ynamides 1–3 with TCNE giving the corresponding
TCBD adducts 4–6 in excellent yields.
From these figures, we can draw the conclusion that only
electron-poor groups might significantly decrease the yield of
the reaction in some particular cases but remain very well tol-
erated.
Table 1. Scope of the [2+2] CA–RE of ynamides and TCNE.
In addition, ynamide 29, bearing an azide functional group,
was synthesized in 19% yield over 3 steps starting from the
known azide 27 (Scheme 2).[18] A Sonogashira coupling using
(tri-isopropylsilyl)acetylene gave the protected alkyne 28 in
quantitative yield and the latter was then reacted with silver
fluoride and N-bromosuccinimide.[19] The bromide derivative
formed was unstable, and was further reacted under Hsung’s
reaction condition to afford ynamide 29. Reaction with one
equivalent of TCNE gave TCBD 30 in an excellent 93% yield.
This azide-functionalized TCBD 30 allows us to envisage further
incorporation of new functional groups by copper-catalyzed
azide–alkyne cycloaddition (CuAAC) reaction[20] for instance, as
it was performed in the past with other related TCBDs.[21]
This reactivity does not seem to be a general property of N-
substituted alkynes but a particularity of ynamides. Indeed,
when reacting the commercially available ynamine 31 with
TCNE using the same procedure as described above, a complex
Entry
Ynamide
TCBD
Yield [%]
1
2
3
4
5
6
7
8
9
10
7
8
9
10
11
12
13
14
15
16
R1 =CH3, R2 =Ph
17
18
19
20
21
22
23
24
25
26
100
95
90
100
57
92
100
100
58
R1 =CH2CH2Ph, R2 =Ph
R1 =CH2Ph, R2 =p-Cl-C6H4
R1 =CH2Ph, R2 =p-OMe-C6H4
R1 =CH2Ph, R2 =p-CN-C6H4
R1 =CH2Ph, R2 =p-NPh2-C6H4
R1 =CH2Ph, R2 =H
R1 =CH2Ph, R2 =n-hexyl
R1 =CH2Ph, R2 =pyridin-3-yl
R1 =CH2Ph, R2 =thiophen-2-yl
87
Ynamides 7–16 were synthe-
sized according to Evano’s[10c,17]
or Hsung’s[14a] procedures. The
influence of the second group
linked to the nitrogen (R1 in
Table 1) was first evaluated by
turning it into a methyl or a ho-
mobenzylic group (entries 1 and
2). No dramatic change was ob-
served, the yield of the reaction
remaining excellent (95% to
quantitative). The influence of
the group linked to the CꢀC
triple bond was also evaluated.
Electron-rich phenyl groups
(p-methoxyphenyl and p-diphe- Scheme 2. Synthesis of ynamide 29 and further reactivity to form TCBD 30.
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Communication
mixture of inseparable colored products was obtained, proba-
bly resulting from the over reaction of this very reactive spe-
cies, as it has already been described in the literature with
other related compounds.[22] By contrast, when ynehydrazide
32[23] was reacted with TCNE, no reaction occurred (Scheme 3).
In each case, no TCBD could be isolated. These observations
led us to conclude that the right balance was found with
ynamides for the reaction with TCNE to yield TCBDs in high
yields.
structures of compounds 4–5, 17, 19–22 and 25 reveal highly
distorted TCBD groups with significant twist between the two
dicyanovinyl planes (Figure 3). Indeed, the torsion angle be-
tween these groups ranges between 56 and 688, except for
compound 17 the torsion angle of which is 1108. The s-cis con-
formation in the solid state is consistent with that reported for
TCBD analogs.[4a,24] DFT optimized geometries both in vacuum
and in solvents, suggest that the s-cis conformation is pre-
ferred in solution even though the difference with the s-trans
conformation stays within a few hundreds of eV (Table S2 in
the Supporting Information). Both experiment and calculations
evidence clearly a single-bond character of the central CꢀC
bond of the TCBD group (Table S1 in the Supporting Informa-
tion).
UV/Vis absorption spectra are shown in Figure 4 and S11–
S16 (see the Supporting Information). Except for the donor-
substituted compounds, they reveal two broad absorption
bands in the UV range at approximately 340 and 260 nm with
little solvatochromism indicative of non-polar ground states.
TD-DFT calculations and tentative deconvolution of these opti-
cal spectra show that several electronic transitions contribute
to each band (Figure S15 and Table S5 in the Supporting Infor-
mation). Moreover, neither the HOMO nor the HOMO-
1 (Table S3) are involved in these transitions leading to the ab-
sence of absorption in the visible spectrum. Indeed, natural
transition orbital[25] plots reveal the electronic redistributions
upon excitation that mainly involve the dicyanovinyl moiety
connected to the phenyl ring for the first band and the other
dicyanovinyl group for the second one (Table S5). Significant
Scheme 3. Reactivity of an ynamine and an ynehydrazide with TCNE.
Products 4–6 and 17–26 were characterized by 1H and
13C NMR spectroscopy, mass spectrometry, UV/Vis spectroscopy
and electrochemistry. Additionally, compounds 4, 5, 17, 19–22
and 25 were also characterized by X-ray diffraction, confirming
unambiguously the structure of the adducts synthesized
(Figure 3 and S1–S10 in the Supporting Information). X-ray
Figure 3. X-ray structures of compounds 4, 5, 17, 19–22 and 25. Solvent and hydrogen atoms are omitted for clarity.
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Communication
of such adducts according to the literature,[4a,24] making them
potential super-acceptors. From these values were deduced
the two electronic affinities (Table 2) for each compound (see
the Supporting Information for details).[26] In addition, no oxi-
dation could be observed (except for compound 22), in major
contrast with other examples of adducts from reaction of TCNE
with electron-rich alkynes.[6a,7a,8]
From a theoretical point of view, calculated adiabatic elec-
tron affinities correlate nicely with the experimental values
given in Table 2. Electron affinities are only slightly affected by
the substitutions implemented in this work, the lowest value
being observed with the strongest donor group (compound
22). Optimized geometries of anions evidence sizeable diminu-
tion of both the dihedral angle and the CꢀC bond length be-
tween the two dicyanovinyl groups consistently with the orbi-
tal structure of the LUMO (Tables S3 and S4). This is even more
pronounced for di-anions and indicates electron removal from
orbitals delocalized over the whole TCBD unit. Moreover,
except for 22, calculated adiabatic ionization energies in aceto-
nitrile amount to approximately 7 eV (Tables S7 and S9 in the
Supporting Information), reaching almost that of benzene. For
the unsubstituted compounds, this is consistent with a HOMO
mainly localized on the benzyl ring (Table S3) leading to an in-
creased bond length alternation in their cationic forms
(Table S8). Such high oxidation potentials explain that no oxi-
dation could be observed by cyclic voltammetry in these con-
ditions.
Figure 4. Experimental (top) and calculated (bottom) normalized UV/Vis ab-
sorption spectra of compounds 5, 20, 22 and 26 in CH2Cl2. To ensure easy
comparison, calculated data have been shifted by an overall offset of
0.36 eV.
red shift occurs upon donor substitution with appearance of
a clear isolated band in the visible range for the strongest
donor (compound 22) (Figure 4). Comparison of molecular
(Table S4) and transition (Table S6) orbitals of compound 22
reveal the HOMO!LUMO nature of this band, while higher-
lying bright states show good correspondence with those dis-
cussed for compounds lacking donor substitution. Compounds
20 and 26 show an intermediary behavior due to smaller split-
tings between the first two excited states (Table S6 in the Sup-
porting Information).
To conclude, an original reactivity between fourteen differ-
ent ynamides and TCNE has been described. It allows for the
formation of new TCBD species in good to quantitative yields
by simply mixing equimolar quantities of ynamides and TCNE
in dichloromethane at room temperature. These new com-
pounds were characterized by various techniques and their
properties were explained by TD–DFT calculations. In this com-
munication, the nature of all the different functional groups of
the ynamides has been investigated, bringing us to the conclu-
sion that only a strong electron-withdrawing group linked to
the CꢀC triple bond can affect the yield of the reaction. More-
over, this reactivity paves the way to the construction of more
sophisticated systems that could exhibit interesting properties
for new materials in opto-electronic devices.
In order to investigate the electronic properties of these
new TCBD compounds, cyclic voltammetries were recorded in
acetonitrile (Figure S17). Two distinct reversible one-electron
reduction waves can be observed at approximately ꢀ0.5 and
ꢀ1.0 V vs Fc+/Fc (Table 2) that may be assigned to the subse-
quent reduction of the two dicyanovinyl groups. Moreover,
these TCBD species are among the easiest ones to reduce out
CCDC 905007(3),
CCDC 960613(17),
CCDC 881967(4),
CCDC 962305(19),
CCDC 884433(5),
CCDC 962971(20),
Table 2. Measured half-wave potentials (E1/2) and electronic affinities (EA)
of compounds 4–6, 20, 22 and 26 calculated on the basis of cyclic vol-
tammetry. Acetonitrile solution of 0.1 molLꢀ1 of nBu4NPF6 and 10ꢀ3
molLꢀ1 of TCBD at 100 mVsꢀ1. DFT calculated values (EAi,cal) obtained at
B3LYP/6-31+G* level in acetonitrile are also reported.
CCDC 962979(21), CCDC 966740(22) and CCDC 976260(25)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif
Compounds
4
5
6
20
22
26
E1/21(V vs Fc+/Fc)
E1/22(V vs Fc+/Fc)
EA1[ev]
ꢀ0.52 ꢀ0.54 ꢀ0.62 ꢀ0.58 ꢀ0.60
ꢀ1.06 ꢀ1.00 ꢀ1.04 ꢀ1.02 ꢀ1.01
ꢀ0.52
ꢀ0.93
4.31
4.46
3.90
Acknowledgements
4.32
4.54
3.77
3.69
4.29
4.50
3.83
3.71
4.21
4.37
3.78
3.85
4.26
4.44
3.81
3.70
4.24
EA1,cal[ev]
4.28[a]
3.81
3.71[a]
EA2 [ev]
EA2,cal[ev]
The authors gratefully acknowledge Dr. L. Toupet from Institut
de Physique de Rennes for solving X-ray structures, T. C.
Nguyen for help in performing DFT calculations and Pr. A. Bou-
cekkine for useful discussions. This work was granted access to
3.82
[a] Values derived from total electronic energies instead of Gibbs free en-
ergies
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Keywords: [2+2] cycloaddition
tetracyanoethylene · ynamides
·
tetracyanobutadienes
·
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Received: March 18, 2014
Published online on June 24, 2014
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