Thermal stability of N-heterocycle-stabilized iodanes-A systematic investigation

Article abstract of DOI:10.3762/bjoc.15.223

The thermal stability of pseudocyclic and cyclic N-heterocycle-stabilized (hydroxy)aryl-and mesityl(aryl)-λ³-iodanes (NHIs) through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) is investigated. Peak decomposition temperatures (T_peak) were observed within a wide range between 120 and 270 °C. Decomposition enthalpies (ΔH_dec) varied from-29.81 to 141.13 kJ/mol. A direct comparison between pseudocyclic and cyclic NHIs revealed high T_peak but also higher ΔH_dec values for the latter ones. NHIs bearing N-heterocycles with a high N/C-ratio such as triazoles show among the lowest T_peak and the highest ΔH_dec values. A comparison of NHIs with known (pseudo)cyclic benziodoxolones is made and we further correlated their thermal stability with reactivity in a model oxygenation.

Full text of DOI:10.3762/bjoc.15.223

Thermal stability of N-heterocycle-stabilized
iodanes – a systematic investigation
Andreas Boelke‡1, Yulia A. Vlasenko‡2, Mekhman S. Yusubov2, Boris J. Nachtsheim*1
and Pavel S. Postnikov*2,3
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 2311–2318.
1Institute for Organic and Analytical Chemistry, University of Bremen,
28359 Bremen, Germany, 2Research School of Chemistry and
Applied Biomedical Sciences, Tomsk Polytechnic University, 634050
Tomsk, Russian Federation and 3Department of Solid State
Engineering, University of Chemistry and Technology, 16628 Prague,
Czech Republic
doi:10.3762/bjoc.15.223
Received: 23 July 2019
Accepted: 20 September 2019
Published: 27 September 2019
Associate Editor: I. Baxendale
Email:
Boris J. Nachtsheim* - nachtsheim@uni-bremen.de;
Pavel S. Postnikov* - postnikov@tpu.ru
© 2019 Boelke et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author ‡ Equal contributors
Keywords:
differential scanning calorimetry; hypervalent iodine; N-heterocycle;
stability; thermogravimetry
Abstract
The thermal stability of pseudocyclic and cyclic N-heterocycle-stabilized (hydroxy)aryl- and mesityl(aryl)-λ3-iodanes (NHIs)
through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) is investigated. Peak decomposition tem-
peratures (Tpeak) were observed within a wide range between 120 and 270 °C. Decomposition enthalpies (ΔHdec) varied from
−29.81 to 141.13 kJ/mol. A direct comparison between pseudocyclic and cyclic NHIs revealed high Tpeak but also higher ΔHdec
values for the latter ones. NHIs bearing N-heterocycles with a high N/C-ratio such as triazoles show among the lowest Tpeak and the
highest ΔHdec values. A comparison of NHIs with known (pseudo)cyclic benziodoxolones is made and we further correlated their
thermal stability with reactivity in a model oxygenation.
Introduction
Hypervalent iodine compounds, in particular aryl-λ3-iodanes, One of the ligands gets either substituted during an iodane-
have found wide spread applications as oxidants and electro- mediated transformation or is directly transferred in an electro-
philic group transfer reagents in organic synthesis [1-11]. A philic group-transfer reaction onto a nucleophilic substrate. The
3-center 4-electron bond connects the central iodine atom, pro- other ligand stabilizes the electrophilic hypervalent iodine atom
viding two electrons, with two carbon- or heteroatom ligands L1 in its ground state or directly influences its reactivity by stabi-
and L2, providing one electron each (Figure 1). These ligands lizing reactive intermediates or transition states. In recent years,
can be arranged along the hypervalent iodine atom through an a plethora of cyclic and pseudocyclic iodanes have been de-
open-chained, a pseudocyclic or a cyclic structure.
veloped with covalently attached stabilizing ligands L2 and
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lized iodanes (NHIs). We demonstrated that the N-heterocycle
significantly influences the important I–L1 bond length and
subsequently has a profound impact on the reactivity of the
iodane in oxygen transfer reactions [29,30]. Since the combina-
tion of a highly oxidized hypervalent iodine species with
N-heterocycles with a high N/C-ratio might result in potential
hazardous high energy materials, we herein investigated their
thermal stability by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC).
Results and Discussion
We started our experiments with the preparation of appropriate
λ3-iodanes bearing N- and O-centered ligands according to our
reported procedures [29,30]. The synthesized compounds then
have been divided in the following discussion by their principle
structural features into pseudocyclic and cyclic hydroxy(phe-
Figure 1: General structure of aryl-λ3-iodanes.
applied in a variety of group-transfer reactions. Prominent ex- nyl)-λ3-iodanes (Figure 2) and pseudocyclic and cyclic
amples include ethynyl benziodoxolones (EBX, L1 = alkyne) mesityl(phenyl)-λ3-iodanes (Figure 6).
[12], the Zhdankin reagent azidobenziodoxole (ABX, L1 = N3)
[13], cyanobenziodoxole (CBX, L1 = CN) [14] or Togni’s We initially investigated the thermal decomposition of
reagent (L1 = CF3) [15]. Even though the transferable ligand N-heterocycle-stabilized pseudocyclic (hydroxy)aryl iodanes
(L1) has been varied extensively, the chemical design of the 2–15 (Figure 2 and Table S1, Supporting Information File 1).
stabilizing donor ligand L2 has not been investigated as inten- As the model oxygen-stabilized derivative, we evaluated the
sive. In general, carboxylic acid derivatives, ethers or free alco- thermal decomposition behavior of pseudocyclic iodosobenzio-
hols are utilized to stabilize the hypervalent iodine center by an doxolone 1. The decomposition of 1 has a two-step character
oxygen–iodine bond or through dative oxygen–iodine interac- and includes initial endothermic melting at 185.1 °C followed
tions in (pseudo)cyclic iodanes. Albeit aryl-λ3-iodanes are by exothermal decomposition at 206.8 °C with an ΔHdec of
viewed as safe and stable under ambient temperatures, system- 72.9 kJ/mol (Figure 3a). For the pseudocyclic NHI 2 with a tri-
atic thermal degradation studies of hypervalent iodine reagents azole ligand no initial melting process was detected. Instead the
are still rare. In 1992 Varvoglis and co-workers investigated the solid degraded with a pronounced and narrow (less than 1 °C)
thermal degradation of a variety of aryl iodine(III) dicarboxy- exothermal peak at 120.8 °C (Figure 3b). Decomposition was
lates into alkyl and aryl radicals through thermogravimetry associated with a higher ΔHdec of 116.3 kJ/mol.
[16,17]. In 2013 Kumar and co-workers compared the thermal
properties of open-chained aryl-λ3-iodanes with their polymer A similar decomposition behavior was observed for the other
bound derivatives and found an endothermic decomposition be- triazole-containing pseudocyclic NHIs 3–5. Introduction of a
havior [18]. In the same year Haller and co-workers investigat- methyl-substituent at C5 of the triazole 3 was accompanied by
ed the explosive properties of Togni’s reagent and very an increased decomposition enthalpy (ΔHdec = 125.1 kJ/mol).
recently, Williams and co-workers analyzed the sensitivity of However, thermal stability as indicated by a higher Tpeak, sig-
common oxidants including 2-iodoxybenzoic acid (IBX) and nificantly increased. Methyl substitution at N2 resulted in an
Dess–Martin periodinane (DMP) [19,20]. Waser and even higher Tpeak at 152.4 °C and a decreased decomposition
co-workers examined the thermal stability of the Zhdankin enthalpy. If the triazole is connected to the iodoarene via N1 as
reagent ABX and compared it with the amide-stabilized deriva- in 5, Tpeak decreases and ΔHdec increases. It should be con-
tive ABZ (azidobenziodazolone). They found a remarkable cluded that triazole 4 has the most advantageous decomposition
higher thermal stability of the latter compound by DSC analy- behavior: It is thermally the most stable among the pseudo-
sis indicated by a higher onset temperature and a lower heat cyclic triazoles with the lowest ΔHdec value. However, even the
release during decomposition [21]. ABZ is a rare example of a triazoles 2, 3, and 5 can be considered as safe compounds, but
nitrogen-stabilized iodane, showing promising properties in still deserve a common precaution due to the narrow decompo-
terms of reactivity and stability. However, iodanes stabilized by sition process.
nitrogen-based ligands, in particular N-heterocycles are still
underexplored [22-28]. Recently, our groups investigated Pyrazoles 6 and 7 are thermally more stable (Tpeak = 168.9 and
systematically cyclic and pseudocyclic N-heterocycle-stabi- 196.5 °C) with a remarkably lower ΔHdec value. NH-pyrazole 6
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Figure 2: Tpeak and ΔHdec-values for a range of N- and O-substituted iodanes.
shows the lowest ΔHdec value among all investigated pseudo- values (193.9–210.1 °C). Following these results, it has been
cyclic iodanes (ΔHdec = 2.5 kJ/mol). Interestingly, the exother- intriguing to analyze the influence of the heteroatom in the
mal decomposition of 6 is superimposed by an endothermal heterocyclic moiety on the thermal decomposition process. The
melting process (Figure 3c). In direct comparison, indazole 8 is change of one nitrogen atom to sulfur as in thiazole 12 resulted
thermally less stable than 7 with a similar ΔHdec value. The in a drastic decrease of ΔHdec to 44.9 kJ/mol. In contrast,
throughout higher thermal stability of pyrazoles and indazoles oxazoles 13 and 14 both had a comparable ΔHdec to 9, however,
(6–8) in direct comparison to triazoles (2–5) is very likely these NHIs are thermally more labile (Tpeak = 159.2 and
connected with the lower C/N ratio.
144.0 °C). Compared to 9, diphenylimidazole-substituted NHI
15 exhibited a considerably lower ΔHdec value (39.7 kJ/mol).
Benzimidazoles 9–11 showed increased ΔHdec values Among the 1,3-azoles 15 is thermally the most labile one with a
(58.5–76.4 kJ/mol) in comparison with pyrazoles 6–8. Broad Tpeak of 118.9 °C. For an improved overview of the discussed
decomposition peaks (up to 14 °C peak width – see Supporting ΔHdec values a graphical comparison sorted by the respective
Information File 1) were observed at remarkable high Tpeak coordinating unit is given in Figure 4.
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Figure 3: TGA/DSC curves of (a) benziodoxolone 1, (b) triazole 2 and (c) pyrazole 6.
Figure 4: Decomposition enthalpy (ΔHdec) scale for pseudocyclic tosylates 1–15 and cyclic iodoso species 16 and 17.
After obtaining decomposition energies and temperatures for as pyrazole 7 and indazole 8. In our view thiazole 12 is the best
these pseudocyclic NHIs, we were intended to relate these find- compromise in this regard since it is thermally even more stable
ings with their reactivity. For this purpose, we chose the oxida- than 7 and 8 with a significant higher reactivity. However, pyra-
tion of thioanisole at room temperature as the initial model zole 6 still shows a good reactivity in this model reaction with a
reaction. This reaction shows quantitative conversion and there- concurrent outstanding thermal stability. If safety issues are a
fore the reactivity of the substrates can be described by the major concern, for example on a very large-scale synthesis,
ascertained reaction times [29]. However, because the reaction NHIs 6 or 12 should be the first choices. Except of 15 and 9, all
times differed widely a relative reactivity (based on the nega- diazoles are more stable and more reactive than the well-estab-
tive logarithm followed by normalization) was used for a better lished benziodoxolone 1. It is also worth mentioning, that even
comparison (Figure 5).
the least stable NHI 5 can be still regarded as “safe” to use [31].
Further investigations are needed to fully capture the synthetic
As expected, the thermally least stable triazoles 2–5 are also the potential of these pseudocyclic NHIs.
most reactive iodanes in this model reaction, especially
N-substituted triazoles 4 and 5. In contrast, the reactivity of tri- We also evaluated the decomposition of cyclic hypervalent
azole 3 is comparable to that of benzoxazoles 13 and 14 as well iodanes. Here, iodosobenzoic acid (IBA, 16) was chosen as an
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oxygen-bonded model substrate. In comparison to its pseudo-
cyclic congener 1, 16 demonstrated a higher Tpeak (267.3 °C
[19]) and a slightly lower ΔHdec (65.0 kJ/mol vs 72.9 kJ/mol).
Cyclic triazole 17 has a significantly higher Tpeak but also a
higher ΔHdec compared to the corresponding pseudocycle 2
(134.5 kJ/mol vs 116.3 kJ/mol).
Besides pseudocyclic and cyclic hydroxy(aryl)-λ3-iodanes,
mesityl(phenyl)- λ3-iodanes 18–33 were systematically investi-
gated by thermogravimetric analysis. Initially, the thermal de-
composition of pseudocyclic diaryliodonium salts 18 and 19
was measured. For both salts, initial endothermic melting was
followed by exothermal decomposition (Supporting Informa-
tion File 1 and Figure 6).
Compared to hydroxy(phenyl)-λ3-iodanes 1 and 2, the pseudo-
cyclic diaryliodonium salts 18 and 19 show a significantly de-
creased ΔHdec from 72.9 kJ/mol to 10.2 kJ/mol for benziodox-
olones 1 and 18 and from 116.3 kJ/mol to 23.4 kJ/mol for the
Figure 5: Correlation between the relative reactivity for pseudocyclic
NHIs based on the reaction time in the oxidation of thioanisole with the
corresponding decomposition enthalpy ΔHdec. Relative reactivity is
based on the negative normalized logarithm.
Figure 6: Tpeak and ΔHdec values for a range of N- and O-substituted iodanes.
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triazole derivatives 2 and 19. The same trend of a higher ther-
mal stability and lower ΔHdec for diaryliodonium salts com-
pared the their hydroxy-substituted congeners can be observed
for all other pseudocyclic N-heterocycle-substituted derivatives
20–29. Pyrazole 24 is the only exception. All investigated
diaryliodonium salts can be defined as safe due to Tpeak values
of usually above 180 °C and ΔHdec values of less than
50 kJ/mol. A graphical comparison of the discussed ΔHdec
values for mesityl(phenyl)- λ3-iodanes is given in Figure 7. In
comparison with Figure 4 it clearly shows that the relative
stability of these heterocycle-stabilized diaryliodonium salts do
not show the same trend as observed for the initially discussed
hydroxy(phenyl)-λ3-iodanes. In particular, the (pseudo)cyclic
pyrazole derivatives 24 and 33 show a comparable high ΔHdec
value.
Figure 8: TGA/DSC curves for the benzimidazole based diaryliodo-
nium salt 25.
These overall significant lower decomposition energies are in
good agreement with published data in the field of reactivity
and stability of hypervalent iodine compounds [32]. As another
common key characteristic, the exothermic decomposition of
diaryliodonium salts occurs during an endothermic melting
process as shown in Figure 8. Only the phenylbenzimidazoles
26 and 27 do not show this apparent melting-associated
endothermic effect. Interestingly, 22 shows a very unusual en-
dothermal decomposition of −29.81 kJ/mol.
Cyclic mesitylene derivatives 30–33 have significantly in-
creased ΔHdec values. Their TGA/DSC curves reveal a decom-
position behavior similar to the pseudocyclic hydroxy(phenyl)-
λ3-iodanes with an exothermal decomposition without initial
melting as exemplarily shown in Figure 9 for compound 32.
Figure 9: TGA/DSC curves for the cyclic triazole 32.
We can thus conclude that the formation of cyclic iodanes
results in an undesired increase of ΔHdec value and therefore, Finally, we investigated the decomposition products of one
their pseudocyclic precursors should be used whenever pseudocyclic (25) and one cyclic (33) diaryliodonium salt.
possible.
Pseudocyclic salt 25 was heated to 185 °C and the resulting oily
Figure 7: Decomposition enthalpy (ΔHdec) scale for (pseudo)cyclic mesitylen(phenyl)- λ3-iodanes 18–33.
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residue was analyzed by GC–MS (Scheme 1a). Besides dearyla-
tion to aryl iodide 25a we observed the formation of an
N-arylated product 25b in significant amounts. In a similar ex-
periment compound 33 was heated to 160 °C and 210 °C ac-
cording to the two exothermic peaks observed in the TGA/DSC
curve (see Supporting Information File 1). The resulting prod-
uct mixture was further analyzed by TLC–MS. While at 160 °C
no significant decomposition was visible, pronounced decom-
position has been observed at 210 °C. The MS analysis revealed
the formation of the N-arylated pyrazoles 33a and 33b as the
main products (Scheme 1b).
Supporting Information
Supporting Information File 1
Synthetic procedures as well as TGA/DSC curves and
NMR spectra for all investigated iodanes.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-223-S1.pdf]
Acknowledgements
Funding by the Fonds der Chemischen Industrie (PhD scholar-
ship to AB), Russian Science Foundation (grant No. 17-73-
20066) and GACR (19-24603Y) is gratefully acknowledged.
TG/DTA for the compound 16 and GCMS measurements of
thermal decomposition for the compound 25 were carried out
using the core facilities of TPU's “Physical and chemical
methods of analysis”.
Both decomposition studies let us conclude, that intermolecular
N-arylation is the major decomposition pathway of
(pseudo)cyclic N-heterocycle-stabilized mesityl(phenyl)-λ3-
iodanes.
Conclusion
ORCID® iDs
Based on these comprehensive thermoanalytic studies we con-
clude that (pseudo)cyclic NHIs are throughout safe to use
reagents with a sufficient thermal stability. Only the triazole de-
rivatives (2, 3, and 5) deserve common precautions due to the
relatively narrow decomposition peak. In particular, benzothia-
zole- and pyrazole-substituted hydroxy(phenyl)-λ3-iodanes (12
and 6–8) show an excellent relation between thermal stability
and reactivity, in particular in direct comparison with well-
known benziodoxolones. We can also conclude that the pseudo-
cyclic forms of aryl(phenyl)-λ3-iodanes should be the reagents
of choice as electrophilic aryl group transfer reagents. Thermal
decomposition studies indicate that they should be potent elec-
trophilic arene donors.
Boris J. Nachtsheim - https://orcid.org/0000-0002-3759-2770
Pavel S. Postnikov - https://orcid.org/0000-0001-9713-1290
Preprint
A non-peer-reviewed version of this article has been previously published
as a preprint https://doi.org/10.26434/chemrxiv.8964848.v1
References
1. Hypervalent Iodine Chemistry. Wirth, T., Ed.; Topics in Current
Chemistry, Vol. 373; Springer International Publishing: Cham,
Switzerland, 2016. doi:10.1007/978-3-319-33733-3
Scheme 1: The thermal decomposition of (pseudo)cyclic N-heterocycle-stabilized mesityl(aryl)-λ3-iodanes 25 and 33.
2317

Beilstein J. Org. Chem. 2019, 15, 2311–2318.
2. Yoshimura, A.; Zhdankin, V. V. Chem. Rev. 2016, 116, 3328–3435.
doi:10.1021/acs.chemrev.5b00547
28.Balthazor, T. M.; Godar, D. E.; Stults, B. R. J. Org. Chem. 1979, 44,
1447–1449. doi:10.1021/jo01323a018
3. Boelke, A.; Finkbeiner, P.; Nachtsheim, B. J. Beilstein J. Org. Chem.
2018, 14, 1263–1280. doi:10.3762/bjoc.14.108
29.Boelke, A.; Lork, E.; Nachtsheim, B. J. Chem. – Eur. J. 2018, 24,
18653–18657. doi:10.1002/chem.201804957
4. Grelier, G.; Darses, B.; Dauban, P. Beilstein J. Org. Chem. 2018, 14,
1508–1528. doi:10.3762/bjoc.14.128
5. Yusubov, M. S.; Zhdankin, V. V. Curr. Org. Synth. 2012, 9, 247–272.
doi:10.2174/157017912799829021
30.Vlasenko, Y. A.; Postnikov, P. S.; Trusova, M. E.; Shafir, A.;
Zhdankin, V. V.; Yoshimura, A.; Yusubov, M. S. J. Org. Chem. 2018,
83, 12056–12070. doi:10.1021/acs.joc.8b01995
31.UN/SCETDG/53/INF.17.
6. Zhdankin, V. V. ARKIVOC 2009, No. i, 1–62.
doi:10.3998/ark.5550190.0010.101
https://www.unece.org/fileadmin/DAM/trans/doc/2018/dgac10c3/UN-SC
ETDG-53-INF17e.pdf (accessed Sept 24, 2019).
7. Dohi, T.; Kita, Y. Chem. Commun. 2009, 2073–2085.
doi:10.1039/b821747e
32.Zhdankin, V. V. Hypervalent Iodine Chemistry; John Wiley & Sons Ltd.:
Chichester, United Kingdom, 2013. doi:10.1002/9781118341155
8. Wirth, T. Angew. Chem., Int. Ed. 2005, 44, 3656–3665.
doi:10.1002/anie.200500115
9. Moriarty, R. M. J. Org. Chem. 2005, 70, 2893–2903.
doi:10.1021/jo050117b
10.Wirth, T. Synthesis 1999, 1271–1287. doi:10.1055/s-1999-3540
11.Stang, P. J.; Zhdankin, V. V. Chem. Rev. 1996, 96, 1123–1178.
doi:10.1021/cr940424+
12.Hari, D. P.; Caramenti, P.; Waser, J. Acc. Chem. Res. 2018, 51,
3212–3225. doi:10.1021/acs.accounts.8b00468
13.Zhdankin, V. V.; Krasutsky, A. P.; Kuehl, C. J.; Simonsen, A. J.;
Woodward, J. K.; Mismash, B.; Bolz, J. T. J. Am. Chem. Soc. 1996,
118, 5192–5197. doi:10.1021/ja954119x
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
14.Zhdankin, V. V.; Kuehl, C. J.; Krasutsky, A. P.; Bolz, J. T.; Mismash, B.;
Woodward, J. K.; Simonsen, A. J. Tetrahedron Lett. 1995, 36,
7975–7978. doi:10.1016/0040-4039(95)01720-3
15.Charpentier, J.; Früh, N.; Togni, A. Chem. Rev. 2015, 115, 650–682.
doi:10.1021/cr500223h
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(https://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.15.223
16.Katsoulos, G. A.; Lalia-Kantouri, M.; Varvoglis, A. Thermochim. Acta
1992, 197, 285–294. doi:10.1016/0040-6031(92)85027-s
17.Varvoglis, A. Chem. Soc. Rev. 1981, 10, 377.
doi:10.1039/cs9811000377
18.Verma, V.; Singh, K.; Kumar, A.; Kumar, D. J. Therm. Anal. Calorim.
2013, 114, 339–344. doi:10.1007/s10973-012-2894-1
19.Fiederling, N.; Haller, J.; Schramm, H. Org. Process Res. Dev. 2013,
17, 318–319. doi:10.1021/op400035b
20.Dallaston, M. A.; Bettencourt, C. J.; Chow, S.; Gebhardt, J.;
Spangler, J.; Johnston, M. R.; Wall, C.; Brusnahan, J. S.;
Williams, C. M. Chem. – Eur. J. 2019, 25, 9614–9618.
doi:10.1002/chem.201902036
21.Alazet, S.; Preindl, J.; Simonet-Davin, R.; Nicolai, S.; Nanchen, A.;
Meyer, T.; Waser, J. J. Org. Chem. 2018, 83, 12334–12356.
doi:10.1021/acs.joc.8b02068
22.Aertker, K.; Rama, R. J.; Opalach, J.; Muñiz, K. Adv. Synth. Catal.
2017, 359, 1290–1294. doi:10.1002/adsc.201601178
23.Xie, F.; Zhang, Z.; Yu, X.; Tang, G.; Li, X. Angew. Chem., Int. Ed.
2015, 54, 7405–7409. doi:10.1002/anie.201502278
Angew. Chem. 2015, 127, 7513.
24.Boppisetti, J. K.; Birman, V. B. Org. Lett. 2009, 11, 1221–1223.
doi:10.1021/ol8029092
25.Hari, D. P.; Schouwey, L.; Barber, V.; Scopelliti, R.; Fadaei‐Tirani, F.;
Waser, J. Chem. – Eur. J. 2019, 25, 9522–9528.
doi:10.1002/chem.201900950
26.Zhdankin, V. V.; Koposov, A. Y.; Su, L.; Boyarskikh, V. V.;
Netzel, B. C.; Young, V. G. Org. Lett. 2003, 5, 1583–1586.
doi:10.1021/ol0344523
27.Zhdankin, V. V.; Arbit, R. M.; McSherry, M.; Mismash, B.; Young, V. G.
J. Am. Chem. Soc. 1997, 119, 7408–7409. doi:10.1021/ja971606z
2318