Metal-acetylide addition to tetracyanoethylene

Article abstract of DOI:10.1016/j.tetlet.2017.05.009

Two push-pull chromophores that have shown utility in the field of molecular electronics and non-linear optics are DDMEBT (1, 2-(4-dimethylamino)phenyl)-3-((4-(dimethylamino)phenyl)ethynyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile) and TDMEE (2, 4-(4-dimet

Full text of DOI:10.1016/j.tetlet.2017.05.009

Accepted Manuscript
Metal-acetylide addition to tetracyanoethylene
Tristan A. Reekie, Jean-Paul Gisselbrecht, Corinne Boudon, Nils Trapp,
François Diederich
PII:
S0040-4039(17)30582-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.05.009
TETL 48902
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
6 March 2017
21 April 2017
4 May 2017
Please cite this article as: Reekie, T.A., Gisselbrecht, J-P., Boudon, C., Trapp, N., Diederich, F., Metal-acetylide
addition to tetracyanoethylene, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.05.009
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Metal-Acetylide Addition to Tetracyanoethylene
Tristan A. Reekie, Jean-Paul Gisselbrecht, Corinne Boudon,
Nils Trapp, and François Diederich
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1
Tetrahedron Letters
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Metal-acetylide addition to tetracyanoethylene
Tristan A. Reekie,a Jean-Paul Gisselbrecht,b Corinne Boudon,b Nils Trapp,a and François Diedericha, ∗
a Laboratorium für Organische Chemie, ETH Zürich, Vladimir-Prelog-Weg 3, 8093 Zürich, Switzerland.
bAffiliation 2, Address, City and Postal Code, Country
ARTICLE INFO
ABSTRACT
Article history:
Received
Two push-pull chromophores that have shown utility in the field of molecular electronics and
non-linear optics are DDMEBT (1, 2-(4-dimethylamino)phenyl)-3-((4-
Received in revised form
Accepted
Available online
(dimethylamino)phenyl)ethynyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile) and TDMEE (2, 4-(4-
dimethylamino)phenyl)but-1-en-3-yne-1,1,2-tricarbonitrile). Unfortunately, the methods
reported for their synthesis give variable yields, use toxic solvents, and only provide small
amounts of material. We report improved synthetic protocols, providing access to larger
quantities of material. By investigating multiple metal-acetylides of 4-ethynyl-N,N-
dimethylaniline and their subsequent addition to TCNE, we obtained various products
depending on the identity of the metal ion. This led to the simple synthesis of push-pull
chromophoric compounds.
Keywords:
push-pull chromophores
tetracyanoethene
metal-acetylide additions
optoelectronics
2009 Elsevier Ltd. All rights reserved.
Organic donor–acceptor chromophores are valuable systems
for nonlinear optical (NLO) materials applications, such as all-
optical computing and signal processing.1 Organic NLO
materials hold promise for the transmission of data at higher
speed while in turn reducing the size of devices required.2
and TCNE, affording 3 as the major product. The Cu(I)-acetylide
can undergo the CA–RE reaction followed by proto-
demetallation, or it may be that not all of the acetylene is
converted to the required metal species. In both cases, the
product expected to be formed would be 3. The Cu(I) source is
also reported as copper(I) acetate, which is highly reactive and
undergoes rapid air oxidation to Cu(II).
We have previously shown the high potential of 4-
(dimethylamino)phenyl-
(N,N-dimethylanilino,
DMA)
substituted cyanoethenes, such as 1–3 (Fig. 1), as efficient NLO
chromophores.3 Compounds 1 (DDMEBT) and 3 are both
obtained through the cycloaddition–retroelectrocyclization (CA–
RE) reaction with tetracyanoethene (TCNE),3b,4 whereas
compound 2 (TDMEE) can be prepared by cyano-substitution of
TCNE.3a DDMEBT and TDMEE have shown excellent two-
photon absorption, by an order of magnitude larger than regularly
used reference molecules.5 DDMEBT has been applied to the
formation of high-optical-quality amorphous thin films for use in
ultrafast, all-optical switching devices.6 Significant quantities of
these materials to further investigate their properties were needed
which required a reexamination of their preparation methods.
Figure 1. Previously synthesized organic donor–acceptor molecules.
The improved synthesis of compound 1 on multigram scale
could be easily achieved through a dimerization of compound 4
using Hay catalyst to afford compound 5 (Scheme 1). Subjecting
5 to the CA–RE reaction with tetracyanoethylene (TCNE) in
acetonitrile, rather than benzene, cleanly afforded high purity
compound following a short silica column. The combined yield
for these two steps affording more than 6 g of compound 1 was
95%.
The reported synthesis of 1 was conducted on a milligram
scale and used benzene as the solvent to effect the CA–RE
reaction.3b We aimed for conditions that would be successful on
the gram scale and avoid the use of benzene as solvent. The
reported synthesis of 2 involves the formation of the Cu(I)-
acetylide of N,N-dimethylanilino-acetylene (DMAA) (4) and its
subsequent addition to TCNE. The yields are variable and low,
less than 30%,3a and the protocol not amenable to scale up,
mostly due to a competing CA–RE reaction between the alkyne
———
∗ Corresponding author. Tel.: +41-44-632 2992; fax: + +41-44-632 1109; e-mail: diederich@org.chem.ethz.ch

2
Tetrahedron
Considering we were already forming the lithium-acetylide,
we first examined how this may react with TCNE. The only
product isolated was 7, which presumably arises from a CA–RE
reaction to form 3, followed by conjugate addition of n-butylide.
Despite the multiple electrophilic sites present on 3, only one
product was observed. Compound 7 could be easily identified by
1
its H NMR spectrum which featured a doublet (J = 11.0 Hz, 1
H) at 4.16 ppm and a second doublet (J = 11.0 Hz, 1 H) at 7.64
ppm. The high-resolution mass spectrum also agreed with a mass
of 332.1870 ([M + H]+, calculated for C20H22N5: 332.1870).
We subsequently focused on the magnesium-acetylide
prepared from EtMgBr. In a one-pot reaction, the Mg-acetylide
was formed, then treated with TCNE. The only organic product
isolated was 8 in 24% yield. This is a known compound,
previously prepared in 3 steps and 30% overall yield.3a It is
interesting to note that despite the 1:1 ratio of Mg-acetylide to
TCNE, double, regioselective addition occurs to give the isolated
product.
The alkynylboronic ester was next prepared through lithiation
of 4 and quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane, following a reported procedure.7 The subsequent
reaction with TCNE only gave the CA–RE product 3 and in
lower yields than normally observed. It is likely that the
mechanism by which it forms is an initial CA–RE reaction
followed by proto-deborylation.
Scheme 1. Revised synthesis of compound 1. Reagents and conditions: a)
CuCl (10 mol%), tetramethylenediamine (TMEDA, 20 mol%), 4 Å molecular
sieves, CH2Cl2, 20 °C, 1.5 h; b) TCNE (1.0 equiv), MeCN, 20 °C, 1 h.
Greater issues existed around improving the synthesis of
TDMEE (2), and we aimed at improving the yields and operating
procedure. Instead of examining different variations of Cu(I), we
turned our attention to other metal- and metalloid-acetylides
(Scheme 2). When using CuOAc, it was difficult to determine if
all acetylene had been converted to the corresponding Cu(I)
species, so we first investigated the organostannane, which would
allow isolation. Stannylation was easily achieved through
deprotonation of DMAA (4) with n-BuLi, followed by reaction
with Me3SnCl. In this way, compound 6 could be obtained in
66% yield. Interestingly however, 6 undergoes a complete
dimerization process at ambient conditions overnight to give the
dialkyne 5. Confirmation of this process was obtained by reacting
the dimer with TCNE to give 1. Keeping compound 6 in CH2Cl2
solution at –20 °C prolonged the half-life to months.
Considering the softer metals tin and copper gave the desired
product and to avoid the toxic organotin reagents, we
investigated Zn-acetylides for the preparation of TDMEE
(Scheme 3). Lithiation, followed by treatment with ZnCl2 in THF
afforded the Zn-acetylide, which, in a one-pot reaction, was
treated with TCNE. Gratifyingly, the target compound 2 was
obtained cleanly, with no CA–RE side product, in reproducible
yields (60–65%). This transformation could be scaled up,
allowing access to significant, multi-gram quantities of TDMEE
in one run. Attempts to extend the methodology to
phenylacetylene derivatives were unsuccessful suggesting that a
donor-activated alkyne was required. Using ZnBr2 or ZnI2 were
also successful, but did not show a change in the yields, therefore
ZnCl2 was maintained.
When the Sn-acetylide 6 was freshly prepared and then
reacted with TCNE, a facile transformation occurred to give 2 in
63% yield (ca <30% reported previously from CuOAc).3a Despite
meeting our target, we hoped to eliminate the need for toxic tin
reagents and therefore investigated other metal-acetylides.
Encouraged by these simple and high-yielding conditions for
the addition of acetylides to TCNE, we applied it to enhance the
charge-transfer properties of this class of compounds, namely
donor-substituted cyanoethynylethenes (CEEs).3a We prepared
the dialkyne 9 by Glaser-Hay coupling of 4 with TMS-acetylene
and subsequent deprotection using previously reported
procedures.7 Forming the Zn-acetylide and treatment with TCNE
gave the expected product 10 in a similar yield to that observed
with the mono-alkyne (Scheme 3).
Scheme 2. The formation of metal-acetylides of 4 and their reaction with
TCNE. Reagents and conditions: a) n-BuLi (1.6 M in hexanes, 1.0 equiv), –
30 °C, 0.5 h, then Me3SnCl (1.0 equiv), –30 to 20 °C, 1 h; b) neat or in
CH2Cl2, ambient conditions; c) TCNE (1.0 equiv), MeCN, 20 °C, 1 h; d) n-
BuLi (1.6 M in hexanes, 1.0 equiv), –30 °C, 0.5 h, then TCNE (1.0 equiv), –
30 to 20 °C, 1 h; e) EtMgBr (1.0 M in THF, 1.1 equiv), 0 to 50 °C, 2.25 h,
then TCNE (1.0 equiv), 20 °C, 1 h; f)1) n-BuLi (1.6 M in hexanes, 1.0 equiv),
–30 °C, 0.5 h, then 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(1.0 equiv), –30 to 20 °C, 1 h; 2) TCNE (1.0 equiv), MeCN, 20 °C, 1 h.
Scheme 3. Zinc-acetylides and their reaction with TCNE. Reagents and
conditions: a) n-BuLi (1.6 M in hexanes, 1.0 equiv), –30 °C, 0.5 h, then ZnCl2
(1.0 M in THF, 1.2 equiv), –30 to 20 °C, 1.5 h, then TCNE (1.0 equiv), 20 °C,
0.5 h.
The structure of the new push-pull chromophore 10 was
determined by X-ray analysis (Fig. 2). The compound
crystallized as a solvate with CHCl3 in the triclinic space group
P . Of particular interest was the quinoid character9 of the (N,N-

3
dimethylamino)phenyl ring as a measure for ground state push-
pull conjugation. A value of 0.036 was calculated for compound
10, while compounds 1,3b (DMA ring on the acetylene), 2,3a and
33b have values of δr = 0.056, 0.037, and 0.036 Å, respectively
(see Supplementary Data for more details). The longer buta-1,3-
diyne-1,4-diyl spacer in 10 slightly reduces ground-state push-
pull conjugation as compared to 2 with an ethyne-1,2-diyl
spacer.10 In the crystal packing, chromophores 10 adopt an anti-
parallel alignment of dimers, with a π-stacking distance of 3.37
Å.
In conclusion, we investigated the reaction of different
metal-acetylides of N,N-dimethylanilino-acetylene (DMAA) (4)
with TCNE and found the zinc-acetylide readily undergoes
substitution of a single cyano moiety of TCNE to afford TDMEE
(2), a molecule of interest in molecular electronics. This
methodology was then extended to form the previously unknown
push-pull chromophore 10 with a larger buta-1,3-diyne-1,4-diyl
spacer. The substantial reduction in the optical (λmax 2: 2.10 eV;
10: 1.93 eV) and electrochemical (∆(Eox,1–Ered,1) 2: 1.65 eV; 10:
1.36 eV) HOMO-LUMO gap is in full agreement with other
study on different chromophore series,3a,10,11 showing that the
opto-electronic properties of push-pull chromophores are
enhanced with increasing length of the spacer between donor and
acceptor.
Acknowledgments
This work was supported by Swiss National Science
Foundation grant SNF 200020_159802. Dr Cagatay Dengiz
(ETH Zürich) is acknowledged for reviewing the manuscript.
References and notes
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P, Gross M, Biaggio I, Diederich F. Chem Commun. 2005: 737–739.
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Figure 2. a) ORTEP plot of 10.CHCl3. Atomic displacement parameters
at 100 K are drawn at 50% probability level. The solvent is omitted. For
selected bond lengths (Å) and angles (°), see Figs. S1 and S2 in the
Supplementary Data. Quinoid character: δr = (((a + a')/2– (b + b')/2) + ((c +
c')/2–(b + b')/2))/2) = 0.036. b) Antiparallel alignment of dimers of 10 in the
crystal lattice, at an average π-stacking distance of 3.37 Å. The solvent is
omitted.
The UV/Vis spectrum of 10 in CHCl3 (see Fig. S15 in the
Supplementary Data) features a strong intramolecular charge-
transfer (CT) band at λmax = 641 nm (1.93 eV, ε = 30000 M
2.
3.
4
–1
cm–1), which undergoes a hypsochromic shift as solvent polarity
decreases. When protonated with trifluoroacetic acid, the charge-
transfer absorption was lost, but the band was fully recovered
upon neutralization with Et3N. The CT band of 10 is substantially
bathochromically shifted with respect to the band in 2 (λmax = 591
nm (2.10 eV, ε = 43800 M–1 cm–1),3a but features a lower molar
extinction coefficient, in agreement with previous investigations
on the influence of spacer length on optical properties of push-
pull chromophores.3a,10
5.
6.
7.
8.
9.
10. Štefko M, Tzirakis MD, Breiten B, Ebert MO, Dumele O, Schweizer
WB, Gisselbrecht JP, Boudon C, Beels MTR, Biaggio I, Diederich F.
Chem Eur J. 2013; 19: 12693–12704.
11. Bures F, Schweizer WB, May JC, Boudon, C, Gisselbrecht JP, Gross
M, Biaggio I, Diederich F. Chem Eur J. 2007; 13: 5378-5387.
The redox properties of compound 10 were studied by cyclic
voltammetry (CV) and rotating-disk voltammetry (RDV). All
electrochemical measurements were carried out in CH2Cl2
containing nBu4NPF6 (0.1 M) in a classical three-electrode cell
with the potentials given versus the ferrocenium/ferrocene
(Fc+/Fc) couple and uncorrected for ohmic drop. The data for the
new chromophore 10 in comparison to those previously reported
for 1, 2, and 83 are given in Table S1 in the Supplementary Data.
Supplementary Material
The supplementary data contains experimental procedures,
1
full characterization of new compounds, copies of H and 13C
NMR spectra, and X-ray data (PDF).
For compound 10, reversible oxidation occurs at the
(dimethylamino)phenyl moiety at +0.68 V, well in the potential
range of the other chromophores. The first reduction potential of
10 is the most anodically shifted, at –0.68 V, showing around a
200 mV increase over the first reduction potentials linearly
conjugated compounds 1 (–0.89 V) and 2 (–0.86 V) and a very
large increase compared to cross-conjugated 8 (–1.31 V).

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Addition of metal-acetylides to TCNE
Different products depending on metal used
Synthesis of push-pull chromophoric compounds
Improved synthesis of useful materials for molecular electronics