Multi-redox indenofluorene chromophores incorporating dithiafulvene donor and ene/enediyne acceptor units

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  2. 1 ORCID Logo ,
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1Department of Chemistry, University of Copenhagen, Universitetsparken 5 DK-2100 Copenhagen Ø, Denmark
2Sino-Danish College (SDC), University of Chinese Academy of Sciences, Beijing, China
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Guest Editor: Y. Yamakoshi
Beilstein J. Org. Chem. 2024, 20, 59–73.
Received 13 Oct 2023, Accepted 28 Dec 2023, Published 15 Jan 2024
A non-peer-reviewed version of this article has been posted as a preprint
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Large donor–acceptor scaffolds derived from polycyclic aromatic hydrocarbons (PAHs) with tunable HOMO and LUMO energies are important for several applications, such as organic photovoltaics. Here, we present a large selection of PAHs based on central indenofluorene (IF) or fluorene cores and containing various dithiafulvene (DTF) donor units that gain aromaticity upon oxidation and a variety of acceptor units, such as vinylic diesters, enediynes, and cross-conjugated radiaannulenes (RAs) that gain aromaticity upon reduction. In some cases, the DTF units are expanded by pyrrolo annelation. The optical and redox properties of these compounds, in some cases carbon-rich, were studied by UV–vis absorption spectroscopy and cyclic voltammetry. Synthetically, the work explores IF diones or fluorenone as central building blocks by subjecting the carbonyl groups to a variety of reactions; that are, phosphite- or Lawesson’s reagent-mediated olefination reactions (to introduce DTF motifs), Ramirez/Corey–Fuchs dibromo-olefinations followed by Sonogashira couplings (to introduce enediynes motifs), and Knoevenagel condensations (to introduce the vinylic diester motif). By a subsequent Glaser–Hay coupling reaction, a RA acceptor unit was introduced to provide a DTF-IF-RA donor–acceptor scaffold with a low-energy charge-transfer absorption and multi-redox behavior.


Tetrathiafulvalene (TTF, Figure 1) is a redox-active molecule that has been widely explored in materials chemistry and supramolecular chemistry [1-8]. TTF reversibly undergoes two sequential one-electron oxidations, generating first a radical cation (TTF+•) and subsequently a dication (TTF2+) containing two 6π-aromatic 1,3-dithiolium rings. The redox properties and geometries of the redox states have been finely tuned by extending the conjugated system with various cores, such as polycyclic aromatic hydrocarbons (PAHs), resulting in so-called extended TTFs [9-12]. One example of this is the introduction of an indeno[1,2-b]fluorene (IF) core [13], providing indenofluorene-extended TTFs (IF-TTFs) of the general structure shown in Figure 1. The π-system can be further expanded as well at the dithiole rings. For example, we have recently developed a synthetic protocol for fusing a pyrrole unit to one of the dithiole rings of an IF-TTF, allowing for dimerization of extended TTFs via the nitrogen atom by different linkers [14].


Figure 1: Overview of structural motifs relevant for the work described herein.

Donor–acceptor chromophores can be obtained by replacing one of the dithiafulvene (DTF) rings of the IF-TTF by an electron acceptor. Cyclic and acyclic acetylenic scaffolds comprised of enediyne units are known to behave as good electron acceptors [15,16], and we became interested in combining the IF-DTF scaffold with such motifs to generate novel multi-redox systems. For example, the radiaannulene moiety RA shown in Figure 1 (or its truncated counterpart with one of the exocyclic enediyne units removed) [17,18] is a particularly good electron acceptor as it gains 14π-aromaticity upon reduction. In this work, we also want to further explore pyrrolo-annelated IF-DTFs with different substituents on the nitrogen atom, and the functionalization at the other end of the IF core with electron-accepting moieties. An overview of general motifs targeted in this work is shown in Figure 1.

Results and Discussion


The synthetic building blocks 18 used in this work are shown in Figure 2. The dione 1 and the ketones 4 and 6 were synthesized according to literature procedures [14,19,20], as were the 1,3-dithiole-2-thiones 2 and 3 [21]. Fluorenone 5 is commercially available. The new building blocks 7 and 8 were prepared according to related literature procedures [21], as described in Supporting Information File 1.


Figure 2: Dione/ketones 1, 46 and 1,3-dithiole-2-thione compounds 2, 3, 7, and 8 are building blocks used in this work.

Our first objective was to explore further annellation of dihydropyrrole and pyrrole units at the DTF moiety of an IF-DTF. A phosphite-mediated coupling of either 1,3-dithiole-2-thione 2, 7, or 8 with IF dione 1 afforded IF-DTFs 911, as shown in Scheme 1. Compound 11 was also obtained from building block 4 via the pyrrolo-annelated IF-DTF 12 by removal of the tosyl (Ts) group under alkaline conditions, followed by nucleophilic substitution to incorporate the hexyl chain on the pyrrole. Furthermore, treatment of the IF-DTF ketone 4 with Lawesson’s reagent (using a recently established protocol [20]) yielded the large dimer 13 as a mixture of E and Z isomers (ca. 4:1). Further functionalization of the IF-DTF ketone 11 was obtained by Ramirez/Corey–Fuchs dibromo-olefination and Knoevenagel condensation to yield vinylic dibromide 14 and diester 15, respectively, as illustrated in Scheme 2. We noted that the dibromo-olefination reaction was first discovered by Ramirez and co-workers [22] and used in the first step of the Corey–Fuchs reaction that ultimately provides an alkyne [23].


Scheme 1: Synthesis of IF-DTF ketones 9–12 and dimer 13.


Scheme 2: Further functionalization of the IF-DTF ketone 11 via Ramirez/Corey–Fuchs dibromo-olefination and Knoevenagel condensation.

To elucidate the properties of the donor part itself of the pyrrolo-annelated IF-DTF systems, we prepared compounds 16 and 17 containing a smaller fluorene PAH. These compounds were prepared by a Lawesson’s reagent-promoted coupling between fluorenone 5 and the Ts-protected 1,3-dithiole-2-thione building blocks 2 and 3, respectively, shown in Scheme 3 (albeit in modest yields). Fluorene-based DTF compounds have previously been explored in various elaborate systems [24-27].


Scheme 3: Coupling of 1,3-dithiole-2-thione building blocks 2 and 3 with fluorenone 5 to afford fluorene-extended DTFs 16 and 17.

Next, we wanted to explore IF-DTFs as motifs for acetylenic scaffolding (Scheme 4). Starting from IF-DTF building block 6, dibromo-olefinated compound 18 was obtained by a Ramirez/Corey–Fuchs reaction. Two-fold Sonogashira couplings with trimethylsilylacetylene, ethynylbenzene, or 4-ethynylbenzonitrile yielded compounds 1921, while two-fold Sonogashira coupling with ((2-ethynylphenyl)ethynyl)triisopropylsilane resulted in compound 22. Desilylation of the alkynes of compound 22 with tetrabutylammonium fluoride (TBAF) and subsequent intramolecular Glaser–Hay coupling of the terminal alkynes afforded the macrocyclic DTF-IF-RA scaffold 23. Molecular sieves (4 Å) were added to the reaction mixture as this has previously been shown to significantly promote the Glaser–Hay coupling [28]. Compounds 20 and 21 were unfortunately very sensitive compounds that were found to easily degrade, which made their characterization somewhat difficult (vide infra).


Scheme 4: Synthesis of acetylenic scaffolds based on IF-DTF. Conditions: (a) Pd(PPh3)2Cl2, CuI, THF, Et3N, rt. (b) Pd2dba2, P(t-Bu)3, CuI, THF, Et3N, rt.

We also targeted other enediyne acetylenic scaffolds with IF as central core as shown in Scheme 5. Starting from IF dione 1, compounds 24 and 25 were synthesized via Ramirez/Corey–Fuchs dibromo-olefinations. Four-fold Sonogashira couplings of compound 25 with triisopropylsilylacetylene and ((2-ethynylphenyl)ethynyl)triisopropylsilane yielded compounds 26 and 27, respectively. A two-fold, intramolecular Glaser–Hay coupling of compound 27 (after desilylation) was attempted under the conditions that were successful in the synthesis of compound 23 (Scheme 4). A compound that may tentatively be assigned to 28 was observed by MALDI–MS analysis of the reaction mixture, but less than needed for an NMR sample was isolated. Furthermore, the isolated compound proved quite insoluble in all investigated deuterated solvents, and therefore it was not possible to determine the purity of the product by this method.


Scheme 5: Synthesis of acetylenic scaffolds with IF as central core. *Not fully characterized due to poor solubility. Conditions: (a) Pd(PPh3)2Cl2, CuI, THF, Et3N, rt.

In an initial attempt to investigate other synthetic pathways to extended IF compounds, the reduced IF 29 was synthesized from IF dione 1 by a Wolff–Kishner reduction of the two ketones as shown in Scheme 6. Compound 29 could potentially after deprotonation be reacted with electrophiles as previously established [29] for the parent structure [30] without tert-butyl substituents.


Scheme 6: Reduction of IF dione 1 to dihydro-IF 29.

UV–vis absorption spectroscopy

UV–vis absorption spectra of the known compound 4 [14] and new compounds 912 and 15 are depicted in Figure 3, and the data are presented in Table 1. A redshift of the longest-wavelength absorption maximum is observed for all new compounds compared to that of 4. For compounds 11 and 12, this indicates that the inductive electron-withdrawing or -donating influences of the substituent group (Ts group in 4 and Hex group in 11) on the nitrogen atom in the pyrrole ring have an effect on the absorption in the visible spectrum of pyrrolo-annelated IF-DTF ketones. Interestingly, the absorption of the dihydropyrrole IF-DTF 9 is redshifted relative to that of the pyrrole IF-DTF 4, while the absorption does not change significantly when comparing IF-DTFs 10 and 11, indicating that the extent to which the absorption changes upon oxidation from a dihydropyrrole to a pyrrole unit depends on the substituent on the N of the dihydropyrrole/pyrrole ring. Introducing the diester electron-acceptor in compound 15 does not change the absorption significantly, compared to compound 11. When changing the solvent from PhMe to CH2Cl2, we observed a redshift of the longest-wavelength absorption maximum for compounds 10 and 11, indicating some charge-transfer character of the absorption (see Figure S1 in Supporting Information File 1).


Figure 3: UV–vis absorption spectra of compounds 4, 912, and 15 in PhMe at 25 °C.

Table 1: UV–vis absorption data of compounds in PhMe or CH2Cl2 at 25 °C (absorption maxima λmax and molar absorptivities ε).

Compound λmax [nm] (ε [103 M−1 cm−1]) Compound λmax [nm] (ε [103 M−1 cm−1])
4a 297 (58), 445 (25) 16b 297 (3.6), 395 (21)*, 409 (22)
9a 297 (45), 462 (20) 17b 297 (7.1), 383 (27), 393 (25)*
10a 298 (68), 478 (24) 22b 262 (51), 300 (55), 402 (17) (broad), 489 (27)
11a 298 (74), 480 (31) 23b 297 (95), 401 (21)*, 426 (24), 444 (23)*, 529 (34)
12a 297 (79), 466 (30) 26b 296 (76), 413 (52), 440 (70)
13b,c 269 (69), 312 (84), 574 (43) 27b 306 (46), 444 (24)*, 461 (25), 534 (1.8) (broad)
15a 304 (60), 475 (34) 30b,d 251, 400*, 412

aPhMe; bCH2Cl2; cE/Z ratio of 4:1; dreference [14]; *shoulder peak.

UV–vis absorption spectra of the known compound 30 [20] and new compounds 13, 16, and 17 are shown in Figure 4, and the data are presented in Table 1. Compared to compound 30, the longest-wavelength absorption maximum of compound 16 is slightly blueshifted while the absorption maximum of compound 17 is significantly blueshifted. This indicates that annelation of the dihydropyrrole ring to the DTF moiety does not change the absorption maximum significantly compared to the two SHex substituents, while annelation of a pyrrole ring results in an absorption maximum at significantly shorter wavelength. These compounds have blueshifted longest-wavelength absorptions relative to the donor–acceptor scaffolds incorporating a pyrrolo-annelated DTF unit. Of these compounds, the large dimer 13 stands out with a significantly redshifted and intense longest-wavelength absorption maximum (λmax at 574 nm) expanding to ca. 680 nm.


Figure 4: UV–vis absorption spectra of compounds 13, 16, 17, and 30 in CH2Cl2 at 25 °C.

UV–vis absorption spectra of compounds 22, 23, 26, and 27 are depicted in Figure 5. By comparing donor–acceptor chromophores 22 and 23, it is observed that the RA moiety of DTF-IF-RA scaffold 23 induces a significant redshift, presumably due to the stronger electron-accepting character of the RA unit (and hence a lower-energy LUMO) compared to the acyclic acetylenic scaffold of compound 22 (in line with first reduction potentials, vide infra). For compound 27, a shorter longest-wavelength absorption maximum at 461 nm is observed; this is a symmetric compound for which no donor–acceptor “push–pull” system is present (albeit a broad tail to the absorption is observed), in contrast to 22 and 23. The absorption maxima of compound 26 are significantly blueshifted, presumably due to the smaller conjugated system. The same trend with a shorter longest-wavelength absorption maximum that was observed for compound 27 was also observed for this compound.


Figure 5: UV–vis absorption spectra of compounds 22, 23, 26, and 27 in CH2Cl2 at 25 °C.

The degradation of compound 20 in the presence of light and oxygen is visible as a color change upon leaving a sample of the compound in solution in an open vial, unshielded from light (Figure S2 in Supporting Information File 1). This degradation was investigated by UV–vis absorption spectroscopy; the absorption spectrum was measured over time for three different samples, and a notable change in the longest-wavelength absorption maximum was only observed for the sample that was exposed to both light and oxygen (see Figures S3 and S4 in Supporting Information File 1). We speculate that this degradation is due to the reaction with singlet oxygen generated by the compound as a photosensitizer; indeed, we have recently shown [31] that IF-TTF compounds are reactive towards singlet oxygen at the central fulvene bond but, in contrast, IF-TTFs (without an acetylenic moiety as in 20) are themselves poor photosensitizers for singlet oxygen.


Cyclic voltammograms of compounds 11, 13, 15, 16, and 17 (in MeCN for compounds 11 and 15 and in CH2Cl2 for compounds 13, 16, and 17, all with 0.1 M Bu4NPF6 as supporting electrolyte) are shown in Figure 6, and potentials against ferrocene (Fc/Fc+) (obtained from differential pulse voltammetry, see Supporting Information File 1) are summarized in Table 2. Compounds 11 and 15 showed two irreversible first oxidations at +0.34 V and +0.38 V vs Fc/Fc+, showing that replacing the ketone with the stronger electron withdrawing vinylic diester renders the first oxidation more difficult (by 40 mV). An anodic shift of 40 mV was also observed for the second oxidation. Oppositely, compound 15 underwent a significantly easier first reduction than 11 (−1.00 V vs −1.35 V), and it also underwent a second reduction. The pyrrolo-annelated dimer 13 showed a reversible oxidation at +0.42 V followed by an irreversible oxidation at +1.01 V, and two reversible reductions at −1.48 V and −1.81 V. Here, the acceptor properties are not promoted by incorporating an acceptor unit as in 15, but instead by the bifluorenylidene motif [32] obtained by dimerizing two pyrrolo-annelated IF-DTF units. Notably, the dimer 13 underwent a first oxidation more readily (by as much as 0.14 V) than the corresponding fluorene-DTF donor 17 (both containing the same N-tosylated pyrrolo-DTF unit). The low electrochemical HOMO–LUMO gap of 13 is paralleled by a low-energy longest-wavelength absorption maximum (vide supra, Figure 7).


Figure 6: Cyclic voltammograms of compounds 11 (in MeCN), 13 (in CH2Cl2), 15 (in MeCN), 16 (in CH2Cl2), and 17 (in CH2Cl2); supporting electrolyte: 0.1 M Bu4NPF6, scan rate: 0.1 V/s. All potentials are depicted against the Fc/Fc+ redox couple.

Table 2: Electrochemical data from differential pulse voltammetry of compounds in CH2Cl2 (with 0.1 M Bu4NPF6) if not otherwise stated; potentials in volts vs Fc/Fc+.

Compound E1ox E2ox E1red E2red
11a +0.34 +0.52 −1.35
13b +0.42 +1.01 −1.48 −1.81
15a +0.38 +0.56 −1.00 −1.21
16 +0.47 +0.99
17 +0.56 +1.07
22 +0.41 +0.76 −1.80
23 +0.41 +0.81 −1.50 −1.78
26 +0.84 −1.64 −1.98
27 +0.85 −1.63 −1.89

aIn MeCN. bE/Z ratio of 4:1.


Figure 7: Comparison of properties of compounds 13 and 17.

A quasi-reversible first oxidation was observed at +0.47 V for the fluorene compound 16 and an irreversible oxidation at +0.99 V. Compound 17 experienced a quasi-reversible first oxidation at +0.56 V and an irreversible oxidation at +1.07 V. Thus, the dihydropyrrolo-annelated DTF compound is more easily oxidized than the pyrrolo-annelated DTF compound. These fluorene compounds did not experience a reduction within the potential window.

Cyclic voltammograms of the acetylenic scaffolds 22, 23, 26, and 27 (in CH2Cl2 with 0.1 M Bu4NPF6 as supporting electrolyte) are shown in Figure 8. Quasi-reversible one-electron oxidations of the two DTF-functionalized compounds 22 and 23 are observed at +0.41 V followed by irreversible oxidations at +0.76 V (22) and +0.81 V (23), respectively. One reversible oxidation at +0.84 V and one reversible reduction at −1.64 V were observed for compound 26, along with one irreversible reduction at −1.98 V. These oxidation and reduction potentials are not significantly different from the potentials observed for compound 27, namely one quasi-reversible oxidation at +0.85 V and two one-electron reductions at −1.63 V and −1.89 V, indicating that the larger conjugated system of compound 27 does not significantly change the redox properties of the compound. Compounds 26 and 27 lack the DTF donor part and are hence oxidized at significantly higher potentials than the other compounds. On the other hand, they are stronger acceptors than the acetylenic scaffold 22 containing the DTF donor. We have previously [33] studied a related compound in which all four triisopropylsilylethynyl substituents of 26 are replaced by cyano groups; this compound showed superior acceptor properties, being reduced at −0.81 V and −1.09 V vs Fc/Fc+ (similar conditions), but no donor properties (thereby contrasting 26 and 27).


Figure 8: Cyclic voltammograms of compounds 22, 23, 26, and 27 in CH2Cl2; supporting electrolyte: 0.1 M Bu4NPF6, scan rate: 0.1 V/s. All potentials are depicted against the Fc/Fc+ redox couple.

Of the acetylenic scaffolds studied, DTF-IF-RA 23 containing an RA moiety is the strongest acceptor, which we ascribe to gain of 14πz-aromaticity of the cyclic moiety of the reduced species (in line with previously studied RA scaffolds [17,18,34]). Indeed, it is reduced more easily by as much as 0.3 V than its corresponding acyclic counterpart, compound 22, although it contains a π-system of the same size, and it is even reduced more easily by 0.13 V than the acetylenic scaffold 27 containing acetylenic acceptor motifs at both ends of the IF core and hence no DTF donor unit. Compound 23 also undergoes a reversible, second reduction to form the dianion. This compound should gain aromaticity upon either reduction or oxidation as illustrated in Figure 9.


Figure 9: Radical anion (left), dianion (middle), and radical cation (right) of compound 23; the radical anion has a 14πz-aromatic ring (highlighted in blue; only counting 2π-electrons of each triple bond, here defined as those in πz orbitals), the dianion has an additional 6π-aromatic cyclopentadienyl anion (highlighted in green), while the cation has a 6π-aromatic 1,3-dithiolium ring (highlighted in red).

X-ray crystallographic analysis

Crystals suitable for single-crystal X-ray diffraction studies were obtained for compounds 25, 26, and 29. Their structures are shown in Figure 10, top, and their respective crystal packings below. All three compounds pack in a herringbone manner in the crystal structure, with the major difference that compound 29 is perpendicular with respect to the herringbone pattern and the related structures (see Figure 10, bottom). Compound 25 packs with an intramolecular distance of 3.41 Å between the planes of the π-systems. Neither compound 26 nor 29 shows π–π interactions in the crystal packing. The large bulkiness of the TIPS groups along with the tert-butyl groups in compound 26 prevent these interactions, while for compound 29, the lack of π–π interactions can be ascribed to the methylene bridges as the hydrogens along with the tert-butyl groups prevent good overlap of the π-systems.


Figure 10: ORTEP plots (50% probability) and crystal packing of compounds a) 25, b) 26, and c) 29. The respective crystal packing of each compound is shown below, in which the hydrogen atoms are omitted for clarity. Atoms are colored grey (carbon), white (hydrogen), brown (bromine), pale-yellow (silicon).

Table 3 lists the lengths of the bonds (bf) within the five-membered rings of the cores as well as the exocyclic C=C double bond (a) that is present in compounds 25 and 26 (for bond labels, see Figure 11). A small difference in the exocyclic C=C bond length is observed between 25 and 26, with the bond in 26 being slightly longer. Bonds b and f are affected by the moiety X, with the less π-delocalized structure 29 having the longest bonds of 1.51 Å, while only minor differences are observed for bonds c, d, and e.

Table 3: Bond lengths (Å) within five-membered rings and of exocyclic C=C double bond (for bond assignments, see Figure 11).

Bond Compound 25 Compound 26 Compound 29
a 1.343(3) 1.362(2)
b 1.495(3) 1.478(2) 1.5102(15)
c 1.408(3) 1.405(2) 1.4030(15)
d 1.463(3) 1.468(2) 1.4690(15)
e 1.412(3) 1.410(2) 1.4103(15)
f 1.495(2) 1.474(2) 1.5090(15)

Figure 11: Labels of bonds within five-membered ring.


In summary, various redox-active chromophores based on the indenofluorene scaffold were synthesized, incorporating different dithiafulvenes and acetylenic scaffolds, such as acetylenic radiaannulenes. The compounds have strong absorptions in the visible region and undergo reversible (or quasi-reversible) oxidations and reductions. We have also presented two new fluorene-extended dithiafulvenes, which also absorb strongly in the visible region and undergo one reversible oxidation, while no reductions were observed for these compounds. Systematic studies show that by small structural modifications, the optical and electrochemical HOMO–LUMO gaps can be finely tuned – with first oxidations and reductions that can be adjusted by several hundreds of millivolts for donor–acceptor IF scaffolds. Introduction of both the dithiafulvene and radiaannulene units along the indenofluorene scaffold provided a donor–acceptor compound covering a particularly broad absorption profile and with a redshifted longest-wavelength absorption maximum relative to most of the compounds (529 nm in dichloromethane), which can be related to the fact that it is both a good donor and a good acceptor as shown electrochemically. This compound stands out as gaining aromaticity in one of its appendages along the IF core upon either reduction (generation of 14πz-aromatic ring) or oxidation (generation of 1,3-dithiolium ring).

Synthetically, the work relies on using indenofluorene diones as key building blocks for performing olefination reactions, such as phosphite- or Lawesson’s reagent-mediated couplings, Ramirez/Corey–Fuchs dibromo-olefinations, and Knoevenagel condensations. In particular, the acetylenic scaffolds presented in this work may be useful precursors for even more elaborate, conjugated and carbon-rich structures in future work.


Anhydrous MeOH was obtained by distillation from activated Mg and stored over 3 Å molecular sieves, or by drying over 3 Å molecular sieves. All remaining anhydrous solvents were obtained from a solvent drying tower (IT model PS-MD-05). HPLC grade solvents were used unless otherwise specified. Purification by chromatography was performed using silica gel (flash: 40–63 μm, Sepacore® Flash Systems X10/X50: 40–63 μm). TLC was performed using aluminum sheets covered with silica gel coated with fluorescent indicator. NMR spectra were recorded on a Bruker instrument at 500 MHz and 126 MHz for 1H and 13C NMR, respectively. Deuterated chloroform (CDCl3, 1H = 7.26 ppm, 13C = 77.16 ppm), deuterated CH2Cl2 (CD2Cl2, 1H = 5.32 ppm, 13C = 54.00 ppm), deuterated DMSO ((CD3)2SO, 1H = 2.50 ppm, 13C = 39.53 ppm), deuterated acetone ((CD3)2CO, 1H = 2.05 ppm, 13C = 29.84 ppm), or deuterated benzene (C6D6, 1H = 7.16 ppm, 13C = 128.39 ppm) were used as solvents and internal references. Chemical shift values are referenced to the ppm scale and coupling constants are expressed in Hertz (Hz). HRMS analysis was performed on a Bruker SolariX XR MALDI-FT-ICR instrument with dithranol as matrix. Melting points are not corrected.

UV–vis absorption spectroscopy

UV–vis absorption spectra were recorded on a Varian Cary 50 UV–vis spectrophotometer scanning between 800 and 200 nm. All spectra were recorded with baseline correction in CH2Cl2 or toluene (HPLC grades) at 25 °C in a quartz cuvette with a 10 mm path length.


Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) were obtained using an Autolab PGSTAT12 instrument and Nova 1.11 software with a scan rate of 0.1 V/s for the CVs. A silver wire immersed in a 0.1 M Bu4NPF6 solution in CH2Cl2 separated from the analyte solution by a frit was used as the reference electrode, a Pt wire was used as the counter electrode, and a platinum disk (diameter = 1.6 mm) or a glassy carbon disk (3 mm) was used as the working electrode. The reference electrode was separated from the solution containing the substrate by a ceramic frit. Measured potentials were referenced to ferrocene/ferrocenium (Fc/Fc+) redox couple, measured before and after the experiment. A 0.1 M solution of NBu4PF6 was used as electrolyte. All solutions were purged with Ar prior to measurements.


All single crystal X-ray diffraction data for compounds 25, 26, and 29 were collected on a Bruker D8 VENTURE diffractometer equipped with a Mo Kα X-ray (λ = 0.71073 Å). The data collections were done at 100 K. All data were integrated with SAINT and a multi-scan absorption correction using SADABS was applied [35,36]. The structure was solved by direct methods using SHELXT and refined by full-matrix least-squares methods against F2 by SHELXL-2019/2 [37,38]. The data for the compounds have been deposited with the Cambridge Crystallographic Data Centre [39]. The CIF files (Supporting Information Files 2–4) and reports were generated using FinalCIF [40].


Compounds 1 [19], 2 [21], 3 [21], 4 [14], and 6 [20] were synthesized according to literature procedures, and compounds 7 and 8 were synthesized according to modified literature procedures [21]. Representative synthetic protocols are provided below, while protocols for 7, 8, 1012, 14, 15, 17, 2022, 2427 are included in Supporting Information File 1.

Compound 9

A solution of 1 (139 mg, 352 μmol) and 2 (176 mg, 534 μmol) in anhydrous toluene (5 mL) and P(OEt)3 (10 mL) was heated to reflux for 5 h, resulting in a color change from orange to dark red. The reaction mixture was then allowed to cool to rt before it was concentrated under reduced pressure. The resulting dark red solid was purified by flash column chromatography (SiO2, 20% EtOAc/heptane), and recrystallization from CH2Cl2/MeOH followed by centrifugation yielded 9 (136 mg, 57%) as an orange solid. Rf = 0.18 (70% CH2Cl2/heptane); mp 178–181 °C; 1H NMR (500 MHz, CDCl3) δ 7.99 (s, 1H), 7.80 (d, J = 8.4 Hz, 2H), 7.76 (s, 1H), 7.72–7.71 (m, 2H), 7.68 (d, J = 8.0 Hz, 1H), 7.58–7.44 (m, 2H), 7.44–7.35 (m, 3H), 4.50 (s, 4H), 2.44 (s, 3H), 1.43 (s, 9H), 1.36 (s, 9H) ppm; 13C NMR (126 MHz, CDCl3) δ 194.1, 152.7, 151.1, 148.7, 148.0, 144.7, 143.5, 142.3, 142.3, 138.8, 137.1, 135.5, 135.1, 133.6, 132.3, 131.4, 130.4, 129.1, 128.8, 127.7, 121.0, 123.4, 121.6, 120.0, 119.7, 119.6, 115.7, 114.5, 52.7, 52.6, 35.4, 35.2, 31.9, 31.4, 21.8 ppm; five sp2 signals missing, presumably due to overlap. HRMS (MALDI+, FT-ICR, dithranol, m/z) [M + H+] calcd for C40H38NO3S3+, 676.2008; found, 676.2019.

Compound 13

A solution of 4 (62.0 mg, 92.0 μmol) and Lawesson’s reagent (23.1 mg, 57.0 μmol) in anhydrous, N2-degassed toluene (20 mL) was heated to reflux for 21 h. The reaction mixture was then allowed to cool to rt, diluted with toluene (50 mL), washed with 1 M NaOH (3 × 50 mL), and then with H2O (3 × 50 mL). The organic phase was dried over MgSO4 and concentrated under reduced pressure. The residue was purified by flash column chromatography (SiO2, 20% EtOAc/heptane), yielding 13 (15.5 mg, 26%) as a purple solid. Rf = 0.23 (20% EtOAc/heptane); 1H NMR (500 MHz, CDCl3) δ 8.64 (s, 2H), 8.50 (s, 2H), 8.12 (s, 2H), 7.96 (s, 2H), 7.83 (d, J = 8.4 Hz, 4H), 7.64 (d, J = 8.0 Hz, 2H), 7.50 (d, J = 8.0 Hz, 2H), 7.34 (m, 6H), 7.29 (d, J = 8.4 Hz, 2H), 7.23 (s, 2H), 7.20 (s, 2H), 2.42 (s, 6H), 1.44 (s, 18H), 1.27 (s, 18H) ppm (E:Z ratio 4:1; 1H NMR signals reported for the E isomer); 13C NMR (126 MHz, CDCl3) δ 150.2, 149.9, 145.8, 143.6, 140.7, 140.5, 139.1, 139.0, 138.6, 137.9, 137.4, 137.0, 136.4, 135.5, 130.4, 127.2, 126.7, 126.6, 126.1, 125.4, 124.1, 123.7, 120.8, 119.2, 119.1, 117.9, 115.0, 111.5, 111.4, 55.7, 35.3, 35.2, 35.2, 35.1, 32.0, 31.9, 31.8, 31.7, 31.6, 29.9, 29.5, 29.1, 22.8, 21.8, 14.3 ppm (E:Z ratio 4:1; sp2-C signals missing, presumably due to overlap): HRMS (MALDI+, FT-ICR, dithranol, m/z) [M•+] calcd for C80H70N2O4S6•+, 1314.3654; found, 1314.3631.

Compound 16

To a flame-dried vial equipped with a magnetic stir bar were added 2 (69 mg, 209 μmol), 5 (28 mg, 153 μmol), and Lawesson’s reagent (63 mg, 155 μmol). Dry toluene (5 mL) degassed with N2 for 15 min was added, and the solution was heated to 105 °C for 18.5 h. The reaction mixture was then allowed to cool to rt, diluted with toluene (20 mL), and washed with 1 M NaOH (3 × 20 mL), and then with H2O (20 mL). The yellow precipitate in the aqueous phase was isolated by filtration and washed with H2O before it was purified by flash column chromatography (SiO2, 50%–100% CH2Cl2/heptane) yielding 16 (18 mg, 39 μmol, 25%) as a yellow solid. Rf = 0.18 (50% CH2Cl2/heptane); mp > 260 °C; 1H NMR (500 MHz, CD2Cl2) δ 7.84 (d, J = 7.5 Hz, 2H), 7.77 (d, J = 8.1 Hz, 2H), 7.75 (d, J = 7.5 Hz, 2H), 7.41 (td, J = 7.5, 1.1 Hz, 2H), 7.39 (d, J = 8.1 Hz, 2H), 7.34 (td, J = 7.5, 1.1 Hz, 2H), 4.46 (s, 4H), 2.42 (s, 3H) ppm; 13C NMR (126 MHz, CD2Cl2) δ 145.0, 138.5, 137.3, 134.0, 130.6, 128.8, 128.0, 127.5, 126.4, 123.4, 120.2, 21.7 ppm; two sp2-C carbon signals missing, presumably due to overlap; HRMS (MALDI+, FT-ICR, dithranol, m/z) [M•+] calcd for C25H19NO2S3•+, 461.0572; found, 461.0577.

Compound 18

To an Ar-degassed solution of PPh3 (845 mg, 3.22 mmol) and CBr4 (560 mg, 1.69 mmol) in anhydrous toluene (20 mL) was added 6 (250 mg, 0.351 mmol). The reaction mixture was heated to reflux and stirred under a N2 atmosphere for 30 h before it was cooled to rt and filtered through a plug of SiO2 (CH2Cl2 as eluent) and concentrated in vacuum. Flash column chromatography (10% CH2Cl2/heptane) yielded 18 (246 mg, 81%) as an orange solid. Rf = 0.29 (10% CH2Cl2/heptane); 1H NMR (500 MHz, CDCl3) δ 8.99 (d, J = 0.7 Hz, 1H), 8.71 (d, J = 1.5 Hz, 1H), 7.94 (d, J = 0.7 Hz, 1H), 7.76–7.75 (m, 2H), 7.71 (d, J = 8.0 Hz, 1H), 7.48 (dd, J = 8.0, 1.5 Hz, 1H), 7.38 (dd, J = 8.0, 1.5 Hz, 1H), 3.01–2.96 (m, 4H), 1.85–1.68 (m, 4H), 1.51–1.47 (m, 4H), 1.45 (s, 9H), 1.40 (s, 9H), 1.37–1.30 (m, 8H), 0.92–0.88 (m, 6H) ppm; 13C NMR (126 MHz, CDCl3) δ 150.5, 150.4, 139.8, 139.6, 138.8, 138.5, 138.3, 138.1, 137.6, 137.3, 136.1, 135.8, 129.5, 128.4, 126.6, 123.3, 123.2, 121.4, 120.1, 119.2, 118.9, 117.3, 113.8, 89.1, 36.9, 36.8, 35.3, 35.3, 31.9, 31.7, 31.6, 31.5, 30.1, 30.0, 28.5, 22.7, 14.2, 14.2 ppm; two sp3-C signals missing, presumably due to overlap; HRMS (MALDI+, FT-ICR, dithranol, m/z) [M•+] calcd for C44H52Br2S4•+, 868.1293; found, 868.1287.

Compound 19

To a N2-degassed solution of 18 (90 mg, 0.10 mmol) in anhydrous THF (5 mL) and Et3N (5 mL) were added N2-degassed trimethylsilylacetylene (0.20 mL, 1.4 mmol), Pd(PPh3)2Cl2 (15 mg, 0.021 mmol), and CuI (5.0 mg, 0.026 mmol). The reaction mixture was stirred at rt under a N2 atmosphere for 4 h before it was filtered through a plug of SiO2 (CH2Cl2 as eluent) and concentrated under reduced pressure. Purification by flash column chromatography (SiO2, 10–15% CH2Cl2/heptane) yielded 19 (62 mg, 66%) as a purple solid (red in solution). Rf = 0.31 (15% CH2Cl2/heptane); 1H NMR (500 MHz, CDCl3) δ 9.07 (s, 1H), 8.81 (d, J = 1.7 Hz, 1H), 7.91 (s, 1H), 7.76 (d, J = 1.7 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H), 7.65 (d, J = 8.0 Hz, 1H), 7.41 (dd, J = 8.0, 1.7 Hz, 1H), 7.37 (dd, J = 8.0, 1.7 Hz, 1H), 3.01–2.96 (m, 4H), 1.79–1.72 (m, 4H), 1.54–1.48 (m, 4H), 1.45 (s, 9H), 1.40 (s, 9H), 1.35–1.32 (m, 8H), 0.92–0.89 (m, 6H), 0.44 (s, 9H), 0.35 (s, 9H) ppm; 13C NMR (126 MHz, CDCl3) δ 150.6, 150.2, 146.7, 139.6, 138.8, 138.3, 138.0, 137.6, 137.5, 136.0, 135.5, 129.5, 128.5, 126.9, 123.2, 123.2, 121.8, 120.1, 119.0, 118.8, 117.3, 113.9, 104.9, 104.5, 104.5, 104.3, 99.5, 36.9, 36.8, 35.3, 35.3, 31.9, 31.8, 31.6, 31.5, 30.1, 30.0, 28.5, 22.7, 22.7, 14.2, 14.2, 0.3, 0.1 ppm; one sp2-C signal and one sp3-C signal missing, presumably due to overlap; HRMS (MALDI+, FT-ICR, dithranol, m/z) [M•+] calcd for C54H70S4Si2•+, 903.3972; found, 903.3985.

Compound 23

In a manner similar to [41], TBAF (1 M in THF, 0.2 mL, 0.2 mmol) was added to a solution of 22 (93 mg, 0.073 mmol) in THF (10 mL), and the reaction mixture was stirred at rt for 45 min before it was filtered through a plug of SiO2 (CH2Cl2 as eluent) and concentrated under reduced pressure to a volume of approx. 2 mL. The resulting solution was diluted with CH2Cl2 (50 mL). A solution of CuCl (7.0 mg, 0.070 mmol) in CH2Cl2 (5 mL) and TMEDA (0.10 mL, 0.67 mmol) was added along with 4 Å molecular sieves, and the reaction mixture was stirred in an open flask at rt for 3 days before it was filtered through a plug of SiO2 (CH2Cl2 as eluent) and concentrated under reduced pressure. Flash column chromatography (30% CH2Cl2 (technical grade stabilized with 0.2% EtOH)/heptane) yielded 23 (33 mg, 47%) as a dark green solid. Rf = 0.20 (40% CH2Cl2/heptane); 1H NMR (500 MHz, CDCl3) δ 9.17 (s, 1H), 8.83 (d, J = 1.7 Hz, 1H), 7.89 (d, J = 7.8 Hz, 1H), 7.88 (s, 1H), 7.80 (d, J = 7.8 Hz, 1H), 7.76–7.71 (m, 2H), 7.60 (d, J = 8.0 Hz, 1H), 7.51–7.28 (m, 8H), 3.02–2.97 (m, 4H), 1.80–1.73 (m, 4H), 1.52–1.50 (m, 4H), 1.48 (s, 9H), 1.47 (s, 9H), 1.37–1.32 (m, 8H), 0.96–0.85 (m, 6H) ppm; 13C NMR (126 MHz, CDCl3) δ 150.5, 150.1, 148.3, 140.0, 139.0, 138.7, 138.7, 138.5, 137.8, 137.4, 136.0, 135.8, 131.4, 131.4, 130.6, 129.7, 129.5, 129.5, 129.0, 129.0, 128.8, 128.6, 128.6, 128.5, 127.0, 125.2, 125.0, 123.2, 123.1, 121.8, 120.2, 119.0, 117.9, 114.1, 99.6, 96.9, 95.9, 95.4, 94.5, 88.0, 87.8, 82.4, 81.2, 36.9, 36.8, 35.3, 35.2, 32.0, 31.9, 31.6, 31.5, 30.1, 30.0, 28.5, 22.7, 22.7, 14.2, 14 ppm; one signal missing in the aromatic region and one signal missing in the aliphatic region, presumably due to overlap; HRMS (MALDI+, FT-ICR, dithranol, m/z) [M•+] calcd for C82H102S4Si2•+, 956.3572; found, 956.3620.

Compound 29

To a 250 mL round-bottomed flask equipped with a reflux condenser and containing a magnetic stir bar, diethylene glycol (125 mL) and KOH (2.67 g, 47.7 mmol) were added. The solution was degassed with Ar for 30 min after which 5 (461 mg, 1.17 mmol) was added. Then, N2H4·H2O (2.4 mL, 50.0 mmol) was added slowly, resulting in a color change to black within 30 min. The reaction was carried out under inert N2 atmosphere. The reaction mixture was then heated to 185–190 °C for 48 h after which it was cooled to 100 °C, poured onto ice (400 mL), and acidified with aq HCl (20 mL, 6 M), resulting in an orange precipitate. The ice was allowed to melt, and the precipitate was filtered, washed with H2O (100 mL), and dissolved in EtOAc (200 mL), after which the volatiles were removed under reduced pressure yielding compound 29 as a light orange crystalline solid (375 mg, 1.02 mmol, 88%). mp > 250 °C; 1H NMR (500 MHz, CDCl3) δ 7.89 (s, 2H), 7.71 (d, J = 8.0, 2H), 7.59 (s, 2H), 7.42 (d, J = 8.0, 2H), 3.95 (s, 4H), 1.39 (s, 18H) ppm; 13C NMR (126 MHz, CDCl3) δ 149.8, 143.8, 142.5, 140.6, 139.5, 124.1, 122.1, 119.2, 116.3, 37.0, 35.0, 31.8 ppm; HRMS (MALDI+, FT-ICR, dithranol, m/z) [M•+] calcd for C28H30•+, 366.2342; found, 366.2344.

Supporting Information

Supporting Information File 1: Synthetic protocols, UV–vis and NMR spectra, differential pulse voltammograms, and X-ray crystallographic data.
Format: PDF Size: 4.3 MB Download
Supporting Information File 2: Crystallographic information file of compound 25.
Format: CIF Size: 845.2 KB Download
Supporting Information File 3: Crystallographic information file of compound 26.
Format: CIF Size: 2.0 MB Download
Supporting Information File 4: Crystallographic information file of compound 29.
Format: CIF Size: 820.3 KB Download


The Novo Nordisk Foundation (NNF20OC0061574) and Sino-Danish College (SDC) are acknowledged for financial support.

Data Availability Statement

The data that supports the findings of this study is available from the corresponding author upon reasonable request.


  1. Canevet, D.; Sallé, M.; Zhang, G.; Zhang, D.; Zhu, D. Chem. Commun. 2009, 2245–2269. doi:10.1039/b818607n
    Return to citation in text: [1]
  2. Martín, N. Chem. Commun. 2013, 49, 7025–7027. doi:10.1039/c3cc00240c
    Return to citation in text: [1]
  3. Bergkamp, J. J.; Decurtins, S.; Liu, S.-X. Chem. Soc. Rev. 2015, 44, 863–874. doi:10.1039/c4cs00255e
    Return to citation in text: [1]
  4. Azov, V. A. Tetrahedron Lett. 2016, 57, 5416–5425. doi:10.1016/j.tetlet.2016.10.082
    Return to citation in text: [1]
  5. Jana, A.; Ishida, M.; Park, J. S.; Bähring, S.; Jeppesen, J. O.; Sessler, J. L. Chem. Rev. 2017, 117, 2641–2710. doi:10.1021/acs.chemrev.6b00375
    Return to citation in text: [1]
  6. Jana, A.; Bähring, S.; Ishida, M.; Goeb, S.; Canevet, D.; Sallé, M.; Jeppesen, J. O.; Sessler, J. L. Chem. Soc. Rev. 2018, 47, 5614–5645. doi:10.1039/c8cs00035b
    Return to citation in text: [1]
  7. Yamada, H.; Yamashita, M.; Hayashi, H.; Suzuki, M.; Aratani, N. Chem. – Eur. J. 2018, 24, 18601–18612. doi:10.1002/chem.201802744
    Return to citation in text: [1]
  8. Schröder, H. V.; Schalley, C. A. Beilstein J. Org. Chem. 2018, 14, 2163–2185. doi:10.3762/bjoc.14.190
    Return to citation in text: [1]
  9. Bryce, M. R. J. Mater. Chem. 1995, 5, 1481–1496. doi:10.1039/jm9950501481
    Return to citation in text: [1]
  10. Roncali, J. J. Mater. Chem. 1997, 7, 2307–2321. doi:10.1039/a703956e
    Return to citation in text: [1]
  11. Brunetti, F. G.; López, J. L.; Atienza, C.; Martín, N. J. Mater. Chem. 2012, 22, 4188–4205. doi:10.1039/c2jm15710a
    Return to citation in text: [1]
  12. Hammerich, O.; Nielsen, M. B. J. Mater. Chem. C 2019, 7, 2809–2822. doi:10.1039/c9tc00120d
    Return to citation in text: [1]
  13. Christensen, M. A.; Parker, C. R.; Sørensen, T. J.; de Graaf, S.; Morsing, T. J.; Brock-Nannestad, T.; Bendix, J.; Haley, M. M.; Rapta, P.; Danilov, A.; Kubatkin, S.; Hammerich, O.; Nielsen, M. B. J. Mater. Chem. C 2014, 2, 10428–10438. doi:10.1039/c4tc02178a
    Return to citation in text: [1]
  14. Broløs, L.; Nielsen, M. B. RSC Adv. 2020, 10, 15030–15033. doi:10.1039/d0ra02787a
    Return to citation in text: [1] [2] [3] [4] [5]
  15. Nielsen, M. B.; Diederich, F. Chem. Rev. 2005, 105, 1837–1868. doi:10.1021/cr9903353
    Return to citation in text: [1]
  16. Diederich, F.; Kivala, M. Adv. Mater. (Weinheim, Ger.) 2010, 22, 803–812. doi:10.1002/adma.200902623
    Return to citation in text: [1]
  17. Lincke, K.; Floor Frellsen, A.; Parker, C. R.; Bond, A. D.; Hammerich, O.; Brøndsted Nielsen, M. Angew. Chem., Int. Ed. 2012, 51, 6099–6102. doi:10.1002/anie.201202324
    Return to citation in text: [1] [2]
  18. Kilde, M. D.; Murray, A. H.; Andersen, C. L.; Storm, F. E.; Schmidt, K.; Kadziola, A.; Mikkelsen, K. V.; Hampel, F.; Hammerich, O.; Tykwinski, R. R.; Nielsen, M. B. Nat. Commun. 2019, 10, 3714. doi:10.1038/s41467-019-11700-0
    Return to citation in text: [1] [2]
  19. Broløs, L.; Kilde, M. D.; Hammerich, O.; Nielsen, M. B. J. Org. Chem. 2020, 85, 3277–3286. doi:10.1021/acs.joc.9b03118
    Return to citation in text: [1] [2]
  20. Bliksted Roug Pedersen, V.; Granhøj, J.; Erbs Hillers‐Bendtsen, A.; Kadziola, A.; Mikkelsen, K. V.; Brøndsted Nielsen, M. Chem. – Eur. J. 2021, 27, 8315–8324. doi:10.1002/chem.202100984
    Return to citation in text: [1] [2] [3] [4]
  21. O’Driscoll, L. J.; Andersen, S. S.; Solano, M. V.; Bendixen, D.; Jensen, M.; Duedal, T.; Lycoops, J.; van der Pol, C.; Sørensen, R. E.; Larsen, K. R.; Myntman, K.; Henriksen, C.; Hansen, S. W.; Jeppesen, J. O. Beilstein J. Org. Chem. 2015, 11, 1112–1122. doi:10.3762/bjoc.11.125
    Return to citation in text: [1] [2] [3] [4] [5]
  22. Desai, N. B.; McKelvie, N.; Ramirez, F. J. Am. Chem. Soc. 1962, 84, 1745–1747. doi:10.1021/ja00868a057
    Return to citation in text: [1]
  23. Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769–3772. doi:10.1016/s0040-4039(01)94157-7
    Return to citation in text: [1]
  24. Mysyk, D. D.; Perepichka, I. F.; Perepichka, D. F.; Bryce, M. R.; Popov, A. F.; Goldenberg, L. M.; Moore, A. J. J. Org. Chem. 1999, 64, 6937–6950. doi:10.1021/jo990100r
    Return to citation in text: [1]
  25. Amriou, S.; Wang, C.; Batsanov, A. S.; Bryce, M. R.; Perepichka, D. F.; Ortí, E.; Viruela, R.; Vidal‐Gancedo, J.; Rovira, C. Chem. – Eur. J. 2006, 12, 3389–3400. doi:10.1002/chem.200501326
    Return to citation in text: [1]
  26. Pérez, E. M.; Sierra, M.; Sánchez, L.; Torres, M. R.; Viruela, R.; Viruela, P. M.; Ortí, E.; Martín, N. Angew. Chem., Int. Ed. 2007, 46, 1847–1851. doi:10.1002/anie.200604327
    Return to citation in text: [1]
  27. Dekhtiarenko, M.; Krykun, S.; Carré, V.; Aubriet, F.; Canevet, D.; Allain, M.; Voitenko, Z.; Sallé, M.; Goeb, S. Org. Chem. Front. 2020, 7, 2040–2046. doi:10.1039/d0qo00641f
    Return to citation in text: [1]
  28. Vilhelmsen, M. H.; Jensen, J.; Tortzen, C. G.; Nielsen, M. B. Eur. J. Org. Chem. 2013, 701–711. doi:10.1002/ejoc.201201159
    Return to citation in text: [1]
  29. Tan, G.; Li, S.; Chen, S.; Sui, Y.; Zhao, Y.; Wang, X. J. Am. Chem. Soc. 2016, 138, 6735–6738. doi:10.1021/jacs.6b04081
    Return to citation in text: [1]
  30. Merlet, S.; Birau, M.; Wang, Z. Y. Org. Lett. 2002, 4, 2157–2159. doi:10.1021/ol025972l
    Return to citation in text: [1]
  31. Henke, P.; Rindom, C.; Kanta Aryal, U.; Frydenlund Jespersen, M.; Broløs, L.; Mansø, M.; Turkovic, V.; Madsen, M.; Mikkelsen, K. V.; Ogilby, P. R.; Brøndsted Nielsen, M. ChemSusChem 2023, 16, e202202320. doi:10.1002/cssc.202202320
    Return to citation in text: [1]
  32. Eakins, G. L.; Cooper, M. W.; Gerasimchuk, N. N.; Phillips, T. J.; Breyfogle, B. E.; Stearman, C. J. Can. J. Chem. 2013, 91, 1059–1071. doi:10.1139/cjc-2013-0074
    Return to citation in text: [1]
  33. Andersen, D.; Nygaard, D. B.; Kragh, R. R.; Broløs, L.; Nielsen, M. B. Tetrahedron Lett. 2020, 61, 151939. doi:10.1016/j.tetlet.2020.151939
    Return to citation in text: [1]
  34. Jiang, H.; Mazzanti, V.; Parker, C. R.; Broman, S. L.; Wallberg, J. H.; Lušpai, K.; Brincko, A.; Kjaergaard, H. G.; Kadziola, A.; Rapta, P.; Hammerich, O.; Nielsen, M. B. Beilstein J. Org. Chem. 2015, 11, 930–948. doi:10.3762/bjoc.11.104
    Return to citation in text: [1]
  35. SAINT, V8.40A; Bruker AXS Inc.: Madison, Wisconsin, USA.
    Return to citation in text: [1]
  36. Krause, L.; Herbst-Irmer, R.; Sheldrick, G. M.; Stalke, D. J. Appl. Crystallogr. 2015, 48, 3–10. doi:10.1107/s1600576714022985
    Return to citation in text: [1]
  37. Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3–8. doi:10.1107/s2053273314026370
    Return to citation in text: [1]
  38. Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3–8. doi:10.1107/s2053229614024218
    Return to citation in text: [1]
  39. Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, 72, 171–179. doi:10.1107/s2052520616003954
    Return to citation in text: [1]
  40. Kratzert, D. FinalCif, V113,
    Return to citation in text: [1]
  41. Broløs, L.; Kilde, M. D.; Brock‐Nannestad, T.; Nielsen, M. B. Eur. J. Org. Chem. 2021, 3537–3544. doi:10.1002/ejoc.202100514
    Return to citation in text: [1]
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