Synthesis of triazene-substituted homoconjugated push-pull chromophores by formal [2?+?2] cycloadditions

Article abstract of DOI:10.1016/j.tetlet.2019.06.046

1-(4-Ethynylphenyl)-3,3-dialkyltriaz-1-enes, well-known building blocks for branched dendrimer syntheses, were utilized as a new type of electron-donor component in formal [2 + 2] cycloadditions. The click-type, atom-economic reactions proceed efficiently

Full text of DOI:10.1016/j.tetlet.2019.06.046

Accepted Manuscript
Synthesis of Triazene-Substituted Homoconjugated Push-Pull Chromophores
by Formal [2+2] Cycloadditions
Kübra Erden, İpek Savaş, Cagatay Dengiz
PII:
S0040-4039(19)30607-0
https://doi.org/10.1016/j.tetlet.2019.06.046
TETL 50887
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
7 June 2019
21 June 2019
23 June 2019
Please cite this article as: Erden, K., Savaş, I., Dengiz, C., Synthesis of Triazene-Substituted Homoconjugated Push-
Pull Chromophores by Formal [2+2] Cycloadditions, Tetrahedron Letters (2019), doi: https://doi.org/10.1016/
j.tetlet.2019.06.046
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of Triazene-Substituted Homoconjugated Push-Pull
Chromophores by Formal [2+2] Cycloadditions
Kübra Erden, İpek Savaş, Cagatay Dengiz*
Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey.
*Corresponding author: dengizc@metu.edu.tr
ABSTRACT.
1-(4-ethynylphenyl)-3,3-dialkyltriaz-1-enes, well-known building
blocks for branched dendrimer syntheses, were utilized as a new type of electron-donor
component in formal [2+2] cycloadditions. The click-type, atom-economic reactions
proceed efficiently under ambient conditions without any catalyst. The resulting
products are structurally interesting push-pull chromophores and were investigated in
the context of optoelectronic properties using UV/Vis spectroscopy, NMR studies, and
computational chemistry.
Keywords: homoconjugation; push–pull chromophores; [2+2] cycloaddition; triazenes

INTRODUCTION
The growing interest in conjugated materials in large industrial areas such as
textiles and electronics has been encouraging scientists for many years to develop
atomic-economic, efficient reactions.1 Most of the synthetic methods used to access
conjugated systems are insufficient since they involve demanding synthetic steps.
Accordingly, the rapid access to complex conjugated structures using short and
economic methods is the major necessity in the field.
The introduction of a fundamental research project arises at this point. One of the
most important organic transformations satisfying these conditions is the
cycloadditions.2 The most well-known reaction in this area, Diels-Alder [4+2]
cycloaddition, has entered the literature in the early 20th century with Otto Diels and
Kurt Alder’s pioneering study.2,3 In addition to Diels Alder cycloadditions, another
commonly used method to access carbocyclic materials is the [2+2] cycloadditions.4
[2+2] cycloadditions do not usually occur under thermal conditions, unlike [4+2]
cycloadditions. In order to initiate these valuable transformations, starting materials
should be stimulated by ultraviolet and/or visible light. Whether cycloaddition reactions
take place thermally or photochemically can be determined by the well-known
Woodward-Hoffmann rules.5 There are very few but quite important literatures that do
not comply with the Woodward-Hoffmann predictions. For example, Reinhoudt
reported the synthesis of thiepin derivatives by the thermal [2+2] cycloaddition reaction
of highly polarized alkynes and alkenes having opposite electron densities.6 The
resulting high-energy butene intermediates rearranged into thiepin derivatives without
being isolated at room temperature. The reactions of tetracyanoethene (TCNE) and
7,7,8,8-tetracyano-p-quinodimethane (TCNQ) with activated alkynes give non-planar
2

push–pull systems through [2+2] cycloaddition/retroelectrocyclization cascade.7 The
highly reactive cyclobutene intermediates cannot be isolated similar to the ones
obserbed in the thiepin synthesis.6 Thermal [2+2] cycloaddition reactions have not been
able to find a wide range of applications for years due to limited substrate diversity,
low yields, and instability of products.8
However, serendipitous findings are important in chemistry.9 An example of
this is Trofimov’s unexpected discovery of a thermal [2+2] cycloaddition of DDQ with
pyrrole substituted electron-rich alkynes.10 An unsuccessful attempt to use DDQ as an
oxidizing agent provided pyrrole-substituted homoconjugated [2+2] cycloadducts. At
about the same time that Trofimov’s work was published, Diederich and his group also
reported a study on the reaction of DDQ with dialkylanilino (DAA)-substituted
electron-rich alkynes.11 Another important contribution of this study is the use of
synthesized [2+2] cycloadducts in nonlinear optical applications.11 Later, Trofimov,12
Diederich,13 Shoji,14 and Swager15 groups have published a number of studies on the
various alkyne derivatives which can be used in thermal [2+2] cycloaddition reactions.
Results in those reports showed that the adducts produced in the cycloadditions of
endocyclic double bonds were not suitable to undergo further retroelectrocyclization
step.
Despite all the efforts on [2+2] cycloadditions, the limitation of this strategy, the
poor substrate scope is still valid. The electron-rich alkyne derivatives used in these
studies are limited to ferrocene, indole, azulene, and dialkylanilino-substituted
substrates. Another issue is that electron-rich alkynes are generally very reactive and
therefore difficult to maintain under ambient conditions. Our aim in this study was to
3

prepare a new type of stable electron-donor material that can undergo click-type [2+2]
cycloaddition, thereby eliminating all the aforementioned synthetic limitations in the
literature. Historically electron-rich triazenes have been utilized in dendrimer chemistry
to mask aryl iodide substrates for branched dendrimer synthesis.16 Another important
field where triazenes found application is medicinal chemistry.17 However, little has
been reported on the electron-donor capabilities of triazene systems in the literature.
Recently, Severin and co-workers have reported that 1-alkynyl and 1-allenyl triazenes
can be utilized in cycloaddition and isomerization-type transformations.18 They
benefited from the reactions of lithium amides with nitrous oxide and alkynyl Grignard
reagents while synthesizing 1-alkynyl triazenes.19 Armed with the previous studies, we
aimed to investigate the reactivity of 1-(4-ethynylphenyl)-3,3-dialkyltriaz-1-enes in
[2+2] cycloadditions with electron-deficient DDQ.
RESULTS AND DISCUSSION
The key alkyne substrates are designed considering several crucial points such
as donor strength, reactivity, and solubility. The target triazene-functionalized
substrates 4a–f were obtained via simple-to-perform synthetic routes (Scheme 1).20 The
reaction sequence to access 4a–f began with the preparation of iodobenzenes 2a–f from
commercially available 4-iodoaniline (1). Compounds 2a–f were accessed by trapping
the in situ formed diazonium salt from (1) with corresponding dialkylamines.
Sonogashira cross-coupling of 2a–f with an excess of ethynyltrimethylsilane afforded
3a–f, which then underwent alkyne deprotection to give 4a–f in yields 30–77%. The
1H-NMR studies showed that alkyl signals are relatively broad compared to the rest of
the peaks due to restricted free rotation around the N−N bond as observed in the earlier
literature reports.21 Unfortunately, our attempts to prepare 1-(4-ethynylphenyl)-3,3-
4

diphenyl triaz-1-enes failed due to the insufficient nucleophilic character of the
diphenylamine.
Scheme 1. Three-step synthesis of electron-rich alkynes 4a–f.
Expanding the scope of [2+2] cycloadditions is crucial to access
homoconjugated push–pull dyes with enhanced electronic and optical properties. With
this motivation, triazene-capped alkyne substrates 4a–f were tested for their reactivities
towards formal [2+2] cycloadditions. In this part of the study, DDQ is selected as an
electron-poor olefin substrate although it is commonly known as an oxidizing agent for
dehydrogenation reactions.22 The reason behind this selection is the electron accepting
power of DDQ with its high reduction potential (Ered 1 = + 0.16 V, cyclic voltammetry
5

data in CH2Cl2 against the Fc+/Fc couple).13b Scheme 2 depicts the homoconjugated
dyes ()-6a–f that were obtained from alkynes 4a–f in yields changing from 19% to
73% using formal [2+2] cycloadditions. The reason for low yield as in the case of
compound ()-6c is probably due to competing oxidation reactions as reported earlier
for DDQ cycloadditions.13a,15 Additionally, we have not observed any correlation
between the electron-donor strength of the 1-(4-ethynylphenyl)-3,3-dialkyltriaz-1-enes
and the yields of the homoconjugated chromophores ()-6a–f.
Scheme 2. Formal [2+2] Cycloadditions of Triazene-substituted Electron-rich Alkynes
with DDQ.
6

To the best of our knowledge, there is no systematic study for the electron-donor
capabilities of triazene systems in the literature. However, Weringa and Janssen have
established electron-donation order for N-substituted aniline structures.23 Isolated
homoconjugated push–pull dyes ()-6a–f are all dark-orange colored solids that are
very stable under ambient conditions for weeks. Next, we sought to characterize the
resulting products in the context of optoelectronic properties.
A key question is about the strength of the donor group: are the triazene-based
donor group strong enough to initiate intramolecular charge transfer (ICT) to the
acceptor CN moieties? In order to answer this question, we examined the absorption
behavior of the homoconjugated dyes. It should also be noted that the efficiency of the
electron delocalization is hindered due to a quaternary carbon atom in between electron-
rich triazene groups and electron-poor CN groups. Almost all of the designed donor–
acceptor (D–A)-type chromophore structures are planar to keep efficient π-
conjugation.24 There are only a few studies on the chromophore structures in which
donor and acceptor groups are not parallel to each other.25 These results make the non-
linear homoconjugated chromophore architectures considerably more attractive.
UV/Vis spectroscopy provided solid evidence for the ICT behavior of the dark
orange-colored homoconjugated chromophores at room temperature in
dichloromethane. Compounds ()-6a–f feature intramolecular charge-transfer bands of
moderate intensity ( values between 3100 and 3800 M–1cm–1, see Figure 1 for details):
λmax = 440 nm (2.82 eV, 3700 M–1cm–1, ()-6a), 442 (2.81 eV, 3800 M–1cm–1, ()-6b),
7

438 (2.83 eV, 3100 M–1cm–1, ()-6c), 428 (2.90 eV, 3600 M–1cm–1, ()-6d), 430 (2.88
eV, 3800 M–1cm–1, ()-6e), and 448 (2.77 eV, 3500 M–1cm–1, ()-6f).
Figure 1. UV/Vis spectra of homoconjugate chromophores ()-6a (black line), ()-6b
(yellow line), ()-6c (purple line), ()-6d (blue line), ()-6e (red line), and ()-6f (green
line) in CH2Cl2 at 298 K.
Cycloadducts ()-6a–f shows positive solvatochromism which further supports
ICT behavior (Figure S1 in the SI).26 Less polar (e.g. toluene) solutions of
chromophores ()-6a–f are yellow and more polar solutions (e.g. dichloromethane) are
dark orange. The corresponding bathochromic shift for low energy bands is
approximately 25 nm. None of the chromophores are emissive in both solution and
solid-state. After analyzing the ICT behavior of the homoconjugated chromophores in
detail by experimental studies, the electronic properties of the cycloadducts were also
8

analyzed with the help of computational chemistry (Figure 2). HOMO and LUMO
orbital depictions support ICT process involving transfer of electron density from
electron-rich triazene groups to the CN groups, which confirms the donor nature of the
proposed substrates with the HOMO is localized on the triazene unit while the LUMO
is on the CN groups and electron-deficient dichloro-dicyano-substituted
cyclohexenedione structure.
Figure 2. HOMO-LUMO orbital depictions of ()-6a.
Optimized molecular structure of the triazene-substituted dyes ()-6a–f were
obtained at the B3LYP/6-31G(d)27 level of theory with the CPCM solvation model in
CH2Cl2. The vertical optical transitions were calculated by time-dependent density
functional theory (TD-DFT) at the CAM-B3LYP/6-31G(d) level of theory, again with
the CPCM solvation model in CH2Cl2 using the software package Gaussian 09.28 The
lowest-energy transitions are mostly composed of HOMO to LUMO excitations and
observed in the region of 500 nm. The computed transition energies are in good
agreement with the experimental values (Tables S1–S6 in the SI). These values are
consistent with the values computed for the known substances in the literature.13,29–31
Figure 3 depicts the calculated and experimental UV/Vis spectra for the selected
homoconjugated dye ()-6a. Although the computational results are slightly off for the
9

high energy absorption bands, the results for the low energy ICT band is remarkable
with only 3 nm difference.
Figure 3. Calculated (not shifted, scaled by 0.6, red line) TD-DFT:CAM-B3LYP/6-
31G* level of theory in CH2Cl2 and experimental (blue line) UV/Vis absorption
spectrum of ()-6a.
CONCLUSION
In summary, we reported an elegant, one-step synthesis of triazene-substituted
structurally complex homoconjugated push–pull chromophores starting from easily
accessible and stable 1-(4-ethynylphenyl)-3,3-dialkyltriaz-1-enes. The electron-rich 1-
(4-ethynylphenyl)-3,3-dialkyltriaz-1-enes were used for the first time in the activation
of [2+2] cycloaddition transformations. The reported [2+2] cycloadditions occurred
under ambient conditions without any catalyst loading. The flexibility of the selected
method allowed for the easy variation of triazene substituents and rapid access to new
push–pull systems. Optoelectronic properties of the chromophores were investigated in
10

detail by UV/Vis studies. All homoconjugated dyes featured intramolecular charge-
transfer and positive solvatochromism which confirmed intramolecular charge transfer
behavior of the chromophores. Electronic/optical properties of the reported
cycloadducts were also investigated using computational chemistry. Theoretical
calculations indicated that target triazene-substituted alkynes are highly efficient donor
structures. This work expands the scope of formal [2+2] cycloadditions. Post-
transformations of the reported dyes to generate molecules with extended conjugation,
as well as optoelectronic device applications, are now being further investigated.
ACKNOWLEDGMENTS
This work was supported by the Scientific and Technological Research Council of
Turkey (TUBITAK, Grant No. 218Z126). We would like to express our sincere
gratitude to Prof. Dr. Metin Balcı for his constant scientific support. Dr. Sevil Özcan
(Roketsan Missiles Inc.) and Dr. Tristan Reekie (Australian National University) are
acknowledged for reviewing the manuscript.
DEDICATION
This paper is dedicated to Prof. François Diederich on the occasion of his retirement.
REFERENCES
(1) (a) Trost, B. M. Angew. Chem. Int. Ed. Engl. 1995, 34, 259–281. (b) Kolb, H. C.;
Finn, M. G.; Sharpless, K. B. Angew. Chem. Int. Ed. 2001, 40, 2004–2021.
(2) Nicolaou, K. C.; Snyder, S. A.; Montagnon, T.; Vassilikogiannakis, G. Angew.
Chem. Int. Ed. 2002, 41, 1668–1698.
(3) Diels, O.; Alder, K. Justus Liebigs Ann. Chem. 1928, 460, 98–122.
(4) Rabinovich, D.; Schmidt, G. M. J. J. Chem. Soc. B 1967, 144−149.
11

(5) Woodward, R. B.; Hoffmann, R. Angew. Chem. Int. Ed. Engl. 1969, 8, 781–853.
(6) Reinhoudt, D. N.; Kouwenhoven, C. G. Tetrahedron 1974, 30, 2093–2098.
(7) (a) Kivala, M.; Diederich, F. Acc. Chem. Res. 2009, 42, 235–248. (b) Kato, S.-i.;
Diederich, F. Chem. Commun. 2010, 46, 1994–2006. (c) Misra, R.; Maragani, R.;
Gautam, P.; Mobin, S. M. Tetrahedron Letters 2014, 55, 7102–7105. (d) Misra, R.;
Gautam, P. Org. Biomol. Chem. 2014, 12, 5448–5457. (e) Misra, R.; Gautam, P.;
Maragani, R. Tetrahedron Letters 2015, 56, 1664–1666. (f) Michinobu, T.;
Diederich, F. Angew. Chem. Int. Ed. 2018, 57, 3552–3577.
(8) Reinhoudt, D. N. Adv. Heterocyc. Chem. 1977, 21, 253–321.
(9) Bosenman, M. F. J. Creat. Behav. 1988, 22, 132–138.
(10) Trofimov, B. A.; Sobenina, L. N.; Stepanova, Z. V.; Ushakov, I. A.;
Sinegovskaya, L. M.; Vakul’skaya, T. I.; Mikhaleva, A. I. Synthesis 2010, 3, 470–
476.
(11) Kato, S-i.; Beels, M. T. R.; La Porta, P.; Schweizer, W. B.; Boudon, C.;
Gisselbrecht, J.-P.; Biaggio, I.; Diederich, F. Angew. Chem. Int. Ed. 2010, 49, 6207–
6211.
(12) (a) Trofimov, B. A.; Sobenina, L. N.; Stepanova, Z. V.; Ushakov, I. A.;
Mikhaleva, A. I.; Tomilin, D. N.; Kazheva, O. N.; Alexandrov, G. G.; Chekhlov, A.
N.; Dyachenko, O. A. Tetrahedron Lett. 2010, 51, 5028–5031. (b) Sobenina, L. N.;
Stepanova, Z. V.; Ushakov, I. A.; Mikhaleva, A. I.; Tomilin, D. N.; Kazheva, O. N.;
Alexandrov, G. G.; Dyachenko, O. A.; Trofimov, B. A. Tetrahedron, 2011, 67,
4832–4837.
(13) (a) Dengiz, C.; Dumele, D.; Kato, S-i.; Zalibera, M.; Cias, P.; Schweizer, W. B.;
Boudon, C.; Gisselbrecht, J.-P.; Gescheidt, G.; Diederich, F. Chem. Eur. J. 2014,
12

20, 1279–1286. (b) Dengiz, C.; Prange, C.; Gawel, P.; Trapp, N.; Ruhlmann, L.;
Boudon, C.; Diederich, F. Tetrahedron 2016, 72, 1213–1224.
(14) (a) Shoji, T.; Maruyama, M.; Shimomura, E.; Akifumi, M.; Ito, S.; Yasunami,
M.; Higashi, J.; Toyota, K.; Morita, N. Heterocycles 2014, 88, 319–329. (b) Shoji,
T.; Higashi, J.; Ito, S.; Yasunami, M.; Morita, N. Heterocycles 2011, 83, 2271–2274.
(15) Dengiz, C.; Swager, T. M. Synlett 2017, 28, 1427–1431.
(16) (a) Moore, J. S.; Xu, Z. Macromolecules 1991, 24, 5893−5894. (b) Pesak, D. J.;
Moore, J. S. Macromolecules 1997, 30, 6467−6482.
(17) (a) Marchesi, F.; Turriziani, M.; Tortorelli, G.; Avvisati, G.; Torino, F.; De
Vecchis, L. Pharmacol. Res. 2007, 56, 275–287. (b) Newell, D. R.; Foster, B. J.;
Carmichael, J.; Harris, A. L.; Jenns, K.; Gumbrell, L. A.; Calvert, A. H.; Triazenes:
Chemical, Biological and Clinical Aspects, Springer, Berlin, Heidelberg, 1990,
pp.119–131.
(18) (a) Perrin, F. G.; Kiefer, G.; Jeanbourguin, L.; Racine, S.; Perrotta, D.; Waser,
J.; Scopelliti, R.; Severin, K. Angew. Chem. Int. Ed. 2015, 54, 13393–13396. (b)
Jeanbourguin, L.; Scopelliti, R.; Tirani, F. F.; Severin, K. Org. Lett. 2017, 19, 2070–
2073. (c) Suleymanov, A. A.; Scopelliti, R.; Tirani, F. F.; Severin, K. Org. Lett.
2018, 20, 3323–3326.
(19) Kiefer, G.; Riedel, T.; Dyson, P. J.; Scopelliti, R.; Severin, K. Angew. Chem. Int.
Ed. 2015, 54, 302–305.
(20) Block, M. A. B.; Hecht, S. Macromolecules 2008, 41, 3219−3227.
(21) Zarei, A.; Khazdooz, L.; Aghaei, H.; Azizi, G.; Chermahini, A. N.; Hajipour, A.
R. Dyes Pigm. 2014, 101, 295−302.
(22) Walker, D.; Hiebert, J. D. Chem. Rev. 1967, 67, 153 –195.
(23) Weringa, W. D.; Janssen, M. J. Rec. Trav. Chim. Pays-Bas 1968, 87, 1372–1380.
13

(24) (a) Yamashita, Y.; Tomura, M. J. Mater. Chem. 1998, 8, 1933–1944. (b) Meier,
H. Angew. Chem. Int. Ed. 2005, 44, 2482–2506.
(25) (a) Iwamura, H.; Makino, K. J. Chem. Soc. Chem. Commun.1978, 720– 721. (b)
Yamamura, K.; Nakazawa, T.; Murata, I. Angew. Chem. Int. Ed. Engl. 1980, 19, 543
–545. (c) Yamamura, K.; Nakasuji, K.; Yamochi, H.; Murata, I.; Yasuoka, N.;
Kusunoki, M. Chem. Lett. 1986, 969–972. d) Gleiter, R.; Doerner, T.; Irngartinger,
H. Liebigs Ann. 1996, 381– 391. (e) Kim, Y.; Zhu, Z.; Swager, T. M. J. Am. Chem.
Soc. 2004, 126, 452–453. (f) Cao, J.; Lu, H.-Y.; Chen, C.-F. Tetrahedron 2009, 65,
8104–8112.
(26) (a) Bures, F.; Pytela, O.; Kivala, M.; Diederich, F. J. Phys. Org. Chem. 2011,
24, 274–281. (b) Solvents and Solvent Effects in Organic Chemistry, 4th ed.;
Reichardt, C., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2010.
(27) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys.
1988, 37, 785–789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(28) Frisch, et al. Gaussian 09, D.01 ed.; Gaussian, Inc., Wallingford, CT, 2009. See
the Supporting Information for full reference.
(29) Jödicke, C. J.; Lüthi, H. P. J. Am. Chem. Soc. 2003, 125, 252–264.
(30) Jödicke, C. J.; Lüthi, H. P. J. Chem. Phys. 2002, 117, 4146–4156.
(31) Jacquemin, D.; Wathelet, V.; Perpéte, E. A.; Adamo, C. J. Chem. Theory
Comput. 2009, 5, 2420–2435.
14

Graphical Abstract
15

HIGHLIGHTS
 This work expands the scope of formal [2+2] cycloadditions.
 The reported cycloadditions occur under ambient conditions without any
catalyst.
 The flexibility of the method allowed for the easy variation of triazenes.
16