Copper-Free Click Reaction Sequence: A Chemoselective Layer-by-Layer Approach

Article abstract of DOI:10.1021/acs.orglett.9b02891

An additive-free chemoselective ligation of dual clickable building blocks is demonstrated. The challenge of balancing reactivity and stability was achieved by employing a small, electron-deficient tetrazine bearing an azido group and an enol ether functionalized cyclooctyne. The chemoselective sequence of strain-promoted azide-alkyne cycloaddition (SPAAC) and inverse-electron-demand Diels-Alder (IEDDA) reaction is demonstrated with a cholic acid derived triazide as a molecular surface model for layer-by-layer synthesis.

Full text of DOI:10.1021/acs.orglett.9b02891

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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Free Click Reaction Sequence: A Chemoselective Layer-by-
Layer Approach
Jannick Meinecke and Ulrich Koert*
̈
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Straße 4, D-35032 Marburg, Germany
S
* Supporting Information
ABSTRACT: An additive-free chemoselective ligation of dual clickable building blocks is demonstrated. The challenge of
balancing reactivity and stability was achieved by employing a small, electron-deficient tetrazine bearing an azido group and an
enol ether functionalized cyclooctyne. The chemoselective sequence of strain-promoted azide−alkyne cycloaddition (SPAAC)
and inverse-electron-demand Diels−Alder (IEDDA) reaction is demonstrated with a cholic acid derived triazide as a molecular
surface model for layer-by-layer synthesis.
“Click” approaches combine rapid conversion and mild
reaction conditions with high selectivity and broad functional
group tolerance.1 Due to their modularity, they find wide
application in bio-orthogonal chemistry2 and material science.3
Bifunctional building blocks expand the scope of click reactions
toward more complex systems. Clicking two different
monofunctional units selectively to a bifunctional unit requires
orthogonal reactivity. Dual clickable organic molecules were
employed in material science (synthesis of polymers,
dendrimers, nanoparticles, hydrogels, surfactants)4 and life
science (drug development, bio-orthogonal labeling).5 Orthog-
onal clicking of two different bifunctional units is a formidable
challenge. Toward this end, copper-catalyzed azide−alkyne
cycloaddition (CuAAC) was combined with different other
click reactions (Figure 1a). Hawker combined the CuAAC
with thiol−ene coupling (TEC),6 Koert with strain-promoted
azide−alkyne cycloaddition (SPAAC),7 Lee with a thiol−
Figure 1. Orthogonal click reaction sequences of bifunctional building
blocks in previous work (a) and this work (b).
Michael reaction,8 Niu with sulfur(VI) fluoride exchange
(SuFEx),9 and Kakkar with a Diels−Alder reaction (DA).10
The necessity of copper catalysis in this earlier work displays a
major limitation for applications in living systems or surface
of a tetrazine with an alkene.12 In this work, we present an
functionalization via UHV vapor deposition. Similar drawbacks
orthogonal solution for a sequential combination of SPAAC
and IEDDA using a carefully balanced reactivity pattern of two
bifunctional units. While the intramolecular combination of a
tetrazine and a cyclooctyne was unfeasible,13 we intended to
investigate the promising additive-free, orthogonal click
exist for using SuFEx due to additional base and heating to 80
°C. TEC requires a photoinitiator, the DA reaction with furan
heating to 50 °C for 3 d, and the thiol−Michael reaction needs
reducing agent as additive to inhibit cystin formation. In
contrast, SPAAC, as developed by Bertozzi, proceeds metal-
and additive-free and can be accomplished at room temper-
ature with high efficiency.11,1b Another additive-free click
reaction is the inverse-electron-demand Diels−Alder (IEDDA)
Received: August 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02891
Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters
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reaction sequence alkene/cyclooctyne and azide/tetrazine
(Figure 1b).
Table 1. Reactivity Experiments with Enol Ether 11
The azido tetrazine 4 was chosen as starting point for the
bifunctional SPAAC/IEDDA units (Scheme 1). Its synthesis
Scheme 1. Synthesis of Tetrazine 4
commenced with boronic acid 1, which was converted via
Suzuki coupling with 4-bromobenzonitrile (2) and subsequent
reduction to the biaryl 3 (Scheme 1).14,15 After Lewis acid
mediated tetrazine formation adapted from Devaraj’s proce-
dure,16 the azide was installed with DPPA (diphenylphos-
phoryl azide) to deliver the azido tetrazine 4.
Having an azide-functionalized tetrazine in hand, the
synthesis of the corresponding cyclooctyne enol ether 9 was
addressed next (Scheme 2). Starting with ethyl diazoacetate
Scheme 2. Synthesis of Cyclooctyne 9
a
b
3 equiv of enol ether 11, 1 equiv of tetrazine (0.35 M). Full
conversion of starting material; aromatization either in CHCl3/CDCl3
for 6 h or dioxane, 101 °C, 5 min.
classes were investigated for their reactivity toward tetrazines.19
Comparisons of tetrazine reactivities were mainly drawn within
one particular class (symmetric phenyl substituted,20a ether/
ester/amide substituted,20b and unsymmetric aryl and methyl/
proton substituted-d20c). An estimation attempt for a general
trend between different tetrazine classes was made by Slugovc
and Knall.21
A series of other known more electron-deficient tetrazines
12,20a 14,5b and 1512b were prepared, and their reactivity
toward enol ether 11 was examined. The dipyridyl tetrazine 12
furnished pyridazine 13 in excellent yield after heating to 70
°C for 20 h (Table 1, entry 2), whereas acid-functionalized
tetrazine 14 gave similar results as initial tetrazine 4 in means
of decomposition of the enol ether. The tetrazine diester 15
indicated full conversion after 40 s at room temperature. This
was monitored by TLC and vanishing of the strong red color
of the respective tetrazine was observed. In order to obtain
consistent and clear analytics, the unaromatized dihydropyr-
idazine byproduct was oxidized to the pyridazine 16 by either
stirring in CHCl3/CDCl3 in an open flask or short heating to
101 °C in dioxane.
Due to the promising reactivity of the diester−tetrazine
motif, we addressed the introduction of an azido group into
this structure. Initial attempts toward saponification, trans-
esterification, reduction, and amidation of tetrazine 15
altogether failed. Reactions on the dihydrotetrazine stage
were more promising (Scheme 3). The dihydrotetrazine 17
was prepared from ethyl diazoacetate (5).22 Breaking the
symmetry of dihydrotetrazine 17 succeeded via base-promoted
(5), a copper-mediated cyclopropanation with COD and
subsequent epimerization, accompanied by saponification,
furnished exo acid 6.17,18a Reduction with LiAlH4 followed
by bromination gave dibromo cyclooctane 7.18 Stepwise HBr
elimination with KOtBu to the corresponding vinyl bromide
and Swern oxidation furnished aldehyde 8. After Wittig
reaction to form the enol ether (E/Z = 1.9/1), a second
HBr elimination proceeded smoothly with LDA at low
temperatures to obtain cyclooctyne enol ether 9 as second
dual clickable unit.
In order to examine the IEDDA reactivity of tetrazine 4 and
an enol ether, a monofunctional enol ether 11 was prepared
first by Diels−Alder reaction of the cyclooctyne enol ether 9
and α-pyrone 10 (Table 1). No IEDDA reaction between 11
and 4 took place at rt. At higher temperatures (>85 °C), only
decomposition of enol ether 11 to the respective aldehyde was
observed (Table 1, entry 1). Hence, stability of the enol ether
was balanced for convenient accessibility and treatment at
room temperature, but the reactivity of azide-incorporated
tetrazine had to be enhanced. In the past, divergent olefin
B
DOI: 10.1021/acs.orglett.9b02891
Org. Lett. XXXX, XXX, XXX−XXX

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With the aim of developing chemoselective surface
functionalization, the orthogonal sequence of IEDDA and
SPAAC was next examined on a molecular surface model
system. Comparable to a former layer-by-layer approach,7
which entailed a combination of CuAAC and SPAAC and thus
the necessity of a copper additive, we employed literature
known24 cholic acid derived triazide 23. Due to its rigid
framework as well as its functionality, it is a suitable target for
the following concurrent layer-by-layer approach (Scheme 5).
Scheme 3. Synthesis of Azido Tetrazine 19
Scheme 5. Layer-by-Layer Assembly on Molecular Surface
transesterification using azidopropanol23 to furnish 18 in good
yield. The addition of molecular sieves turned out to be
essential in terms of scavenging developing methanol. This
way, we circumvented a late-stage nucleophilic substitution
with an azide (20 → 18), which was found to be difficult due
to nucleophilic NH groups of dihydrotetrazine derivatives.
Final oxidation of 18 to tetrazine 19 showed the best
performance (99% yield) by bubbling nitrous gases through
a solution of the substrate in CH2Cl2, which was superior to
aromatization with MnO2 (78% yield).
With both dual clickable units 19 and 9 in hand, a sequential
combination of IEDDA and SPAAC was examined in order to
confirm the orthogonality. The IEDDA between azide bearing
tetrazine 19 and enol ether 11 showed complete turnover of
the starting materials after 4 min (Scheme 4). This was
Scheme 4. Orthogonal Click Sequence Using Tetrazine 19
and Cyclooctyne 9
The initial 3-fold SPAAC to tris-triazole 24 proceeded
smoothly with nearly quantitative yield. Subsequent IEDDA
with azide bearing tetrazine 19 installed three pyridazine units
to generate the second layer 25. The layer-by-layer approach
was completed by showing final SPAAC to hexatriazole 26.
Each layer producing step was performed additive-free, at
room temperature, and in excellent yields.
In summary, we developed an additive-free click reaction
sequence of bifunctional cyclooctyne/alkene and azide/
tetrazine building blocks. On the way to a balanced reactivity
observable by TLC and vanishing of the strong red color of
tetrazine 19. The unaromatized dihydropyridazine byproduct
was subsequently oxidized via elimination to obtain the
pyridazine 21 as 1:1 regioisomeric mixture. SPAAC of the
azido group in 21 with cyclooctyne enol ether 9 led cleanly to
the triazole 22. The click sequence (19 → 21 → 22) proves
the postulated additive free combination of SPAAC and
IEDDA.
C
DOI: 10.1021/acs.orglett.9b02891
Org. Lett. XXXX, XXX, XXX−XXX

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C.; Lapthorn, C.; Roberts, L. R.; Selby, M. D.; Jones, L. H. Org.
Biomol. Chem. 2012, 10, 548.
(6) Antoni, P.; Robb, M. J.; Campos, L.; Montanez, M.; Hult, A.;
pattern, IEDDA experiments between known tetrazines and a
monofunctional dienophile concluded the diester tetrazine as
most promising diene. Subsequent repetition of an IEDDA
between this motif with an azide present resulted in the desired
pyridazine formation. Further conversion with dual clickable
cyclooctyne via SPAAC completed the alternating sequence. In
contrast to our previous example,7 the reaction sequence is
performed additive-free and at room temperature. Related to
the demonstrated layer-by-layer approach on a cholic acid
derived triazide in solution, application for solid surface-/
semiconductor functionalization via UHV vapor deposition
should be adaptable.25
̈
Malmstrom, E.; Malkoch, M.; Hawker, C. J. Macromolecules 2010, 43,
6625−6631.
(7) Munster, N.; Nikodemiak, P.; Koert, U. Org. Lett. 2016, 18,
̈
4296−4299.
(8) Shakiba, A.; Jamison, A. C.; Lee, T. R. Langmuir 2015, 31,
6154−6163.
(9) Yang, C.; Flynn, J. P.; Niu, J. Angew. Chem., Int. Ed. 2018, 57,
16194−16199.
(10) Vieyres, A.; Lam, T.; Gillet, R.; Franc, G.; Castonguay, A.;
Kakkar, A. Chem. Commun. 2010, 46, 1875−1877.
(11) (a) Turner, R. B.; Jarrett, A. D.; Goebel, P.; Mallon, B. J. J. J.
Am. Chem. Soc. 1973, 95, 790−792. (b) Yoshida, S.; Hatakeyama, Y.;
Johmoto, K.; Uekusa, H.; Hosoya, T. J. J. Am. Chem. Soc. 2014, 136,
13590−13593. (c) Gobbo, P.; Romagnoli, T.; Barbon, S. M.; Price, J.
T.; Keir, J.; Gilroy, J. B.; Workentin, M. S. Chem. Commun. 2015, 51,
6647−6650. (d) Dommerholt, J.; Rutjes, F. P. J. T.; van Delft, F. L.
Top Curr. Chem. (Cham) 2016, 374, 16.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02891.
■
S
(12) (a) Carboni, R. A.; Lindsey, R. V. J. Am. Chem. Soc. 1959, 81,
4342. (b) Sauer, J.; Mielert, A.; Lang, D.; Peter, D. Chem. Ber. 1965,
98, 1435−1445. (c) Boger, D. L.; Hong, J. Implementation into total
synthesis. J. Am. Chem. Soc. 2001, 123, 8515−8519.
(13) (a) Chen, W.; Wang, D.; Dai, C.; Hamelberg, D.; Wang, B.
Chem. Commun. 2012, 48, 1736−1738. (b) Sauer, J.; Heldmann, D.
K.; Hetzenegger, J.; Krauthan, J.; Sichert, H.; Schuster, J. Eur. J. Org.
Chem. 1998, 1998, 2885−2896.
Experimental details and spectroscopic and analytical
data of all new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: koert@chemie.uni-marburg.de.
ORCID
̈
(14) Urgel, J. I.; Ecija, D.; Auwarter, W.; Stassen, D.; Bonifazi, D.;
Barth, J. V. Angew. Chem., Int. Ed. 2015, 54, 6163−6167.
(15) Ballav, N.; Schupbach, B.; Dethloff, O.; Feulner, P.; Terfort, A.;
̈
Ulrich Koert: 0000-0002-4776-8549
Author Contributions
Zharnikov, M. J. Am. Chem. Soc. 2007, 129, 15416−15417.
(16) Yang, J.; Karver, M. R.; Li, W.; Sahu, S.; Devaraj, N. K. Angew.
Chem., Int. Ed. 2012, 51, 5222−5225.
J.M. accomplished the experimental work.
Notes
(17) Gential, G. P. P.; Ho, N. I.; Chiodo, F.; Meeuwenoord, N.;
Ossendorp, F.; Overkleeft, H. S.; van der Marel, G. A.; Filippov, D. V.
Bioorg. Med. Chem. Lett. 2016, 26, 3641−3645.
The authors declare no competing financial interest.
(18) (a) O’Brien, J. G. K.; Chintala, S. R.; Fox, J. M. J. Org. Chem.
2018, 83, 7500−7503. (b) Dommerholt, J.; Schmidt, S.; Temming,
R.; Hendriks, L. J.; Rutjes, F. P. J. T.; van Hest, J. C. M.; Lefeber, D.
J.; Friedl, P.; van Delft, F. L. Angew. Chem., Int. Ed. 2010, 49, 9422−
9425.
ACKNOWLEDGMENTS
Financial support by the Deutsche Forschungsgemeinschaft
(SFB 1083) is gratefully acknowledged.
■
(19) (a) Thalhammer, F.; Wallfahrer, U.; Sauer, J. Tetrahedron Lett.
1990, 31, 6851−6854. (b) Boger, D. L.; Sakya, S. M. J. Org. Chem.
1988, 53, 1415−1423. (c) Hamasaki, A.; Ducray, R.; Boger, D. L. J.
Org. Chem. 2006, 71, 185−193. (d) Xie, H.; Zu, L.; Oueis, H. R.; Li,
H.; Wang, J.; Wang, W. Org. Lett. 2008, 10, 1923−1926.
(20) (a) Wang, D.; Chen, W.; Zheng, Y.; Dai, C.; Wang, K.; Ke, B.;
Wang, B. Org. Biomol. Chem. 2014, 12, 3950−3955. (b) Boger, D. L.;
Schaum, R. P.; Garbaccio, R. M. J. Org. Chem. 1998, 63, 6329−6337.
(c) Karver, M. R.; Weissleder, R.; Hilderbrand, S. A. Bioconjugate
Chem. 2011, 22, 2263−2270. (d) Eising, S.; Engwerda, A. H. J.;
Riedijk, X.; Bickelhaupt, F. M.; Bonger, K. M. Bioconjugate Chem.
2018, 29, 3054−3059. For a review see: (e) Oliveira, B. L.; Guo, Z.;
Bernardes, G. J. L. Chem. Soc. Rev. 2017, 46, 4895−4950.
(21) Knall, A.-C.; Slugovc, C. Chem. Soc. Rev. 2013, 42, 5131−5142.
(22) Boger, D. L.; Panek, J. S.; Patel, M. Org. Synth. 1998, 9, 335.
(23) Tesch, M.; Kudruk, S.; Letzel, M.; Studer, A. Chem. - Eur. J.
2017, 23, 5915−5919.
REFERENCES
■
(1) (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem.
2001, 113, 2056−2075. (b) Sletten, E.; Bertozzi, C. Angew. Chem., Int.
Ed. 2009, 48, 6974−6998; Angew. Chem. 2009, 121, 7108−7133.
(2) For reviews see: (a) McKay, C. S.; Finn, M. G. Chem. Biol. 2014,
21, 1075−1101. (b) Jewett, J. C.; Bertozzi, C. R. Chem. Soc. Rev. 2010,
39, 1272−1279.
̈
(3) For reviews see: (a) Iha, R. K.; Wooley, K. L.; Nystrom, A. M.;
Burke, D. J.; Kade, M. J.; Hawker, C. J. Chem. Rev. 2009, 109, 5620−
5686. (b) Xi, W.; Scott, T. F.; Kloxin, C. J.; Bowman, C. N. Adv.
Funct. Mater. 2014, 24, 2572−2590.
(4) (a) Su, H.; Zheng, J.; Wang, Z.; Lin, F.; Feng, X.; Dong, X.-H.;
Becker, M. L.; Cheng, S. Z. D.; Zhang, W.-B.; Li, Y. ACS Macro Lett.
2013, 2, 645−650. (b) Campos, L. M.; Killops, K. L.; Sakai, R.;
Paulusse, J. M. J.; Damiron, D.; Drockenmuller, E.; Messmore, B. W.;
Hawker, C. J. Macromolecules 2008, 41, 7063−7070. (c) Luo, W.;
Luo, J.; Popik, V. V.; Workentin, M. S. Bioconjugate Chem. 2019, 30,
1140−1149. (d) Zong, H.; Goonewardena, S. N.; Chang, H.-N.; Otis,
J. B.; Baker, J. R. Bioorg. Med. Chem. 2014, 22, 6288−6296. (e) Han,
J.; Zhao, B.; Gao, Y.; Tang, A.; Gao, C. Polym. Chem. 2011, 2, 2175−
2178. (f) DeForest, C. A.; Polizzotti, B. D.; Anseth, K. S. Nat. Mater.
2009, 8, 659−664.
(24) Li, C.; Peters, A. S.; Meredith, E. L.; Allman, G. W.; Savage, P.
B. J. Am. Chem. Soc. 1998, 120, 2961−2962.
(25) Bifunctional cyclooctyne on Si(001): (a) Reutzel, M.; Munster,
̈
̈
̈
̈
N.; Lipponer, M. A.; Langer, C.; Hofer, U.; Koert, U.; Durr, M. J.
̈
Phys. Chem. C 2016, 120, 26284−26289. (b) Langer, C.; Heep, J.;
̈
̈
Nikodemiak, P.; Bohamud, T.; Kirsten, P.; Hofer, U.; Koert, U.; Durr,
M. J. Phys.: Condens. Matter 2019, 31, No. 034001.
̈
(5) (a) Kele, P.; Mezo, G.; Achatz, D.; Wolfbeis, O. S. Angew. Chem.,
Int. Ed. 2009, 48, 344−347. (b) Peng, B.; Thorsell, A.-G.; Karlberg,
T.; Schuler, H.; Yao, S. Q. Angew. Chem., Int. Ed. 2017, 56, 248.
̈
(c) Beal, D. M.; Albrow, V. E.; Burslem, G.; Hitchen, L.; Fernandes,
D
DOI: 10.1021/acs.orglett.9b02891
Org. Lett. XXXX, XXX, XXX−XXX