Photo-Mediated Intermolecular Coupling of Alkenes with Ketones via Acyloxy Nitroso Compounds

Article abstract of DOI:10.1002/anie.202016955

An atom-economic intermolecular radical addition reaction of acyloxy nitroso compounds to electron-deficient alkenes mediated by visible light is reported. The starting nitroso derivatives are readily prepared by oxidation of the corresponding oximes prepared from ketones and the overall transformation represents an oxidative coupling of a ketone with a Michael acceptor. The cascade proceeds smoothly under mild conditions, providing a series of valuable functionalized oximes in moderate to good yields. Mechanistic studies suggest that these cascades proceed via addition/coupling processes that are controlled by the persistent radical effect (PRE) with NO acting as the persistent species.

Full text of DOI:10.1002/anie.202016955

Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 8547–8551
International Edition: doi.org/10.1002/anie.202016955
German Edition: doi.org/10.1002/ange.202016955
Synthetic Methods
Photo-Mediated Intermolecular Coupling of Alkenes with Ketones via
Acyloxy Nitroso Compounds
Danqing Zheng, Stefanie Plçger, Constantin G. Daniliuc, and Armido Studer*
Abstract: An atom-economic intermolecular radical addition
reaction of acyloxy nitroso compounds to electron-deficient
alkenes mediated by visible light is reported. The starting
nitroso derivatives are readily prepared by oxidation of the
corresponding oximes prepared from ketones and the overall
transformation represents an oxidative coupling of a ketone
with a Michael acceptor. The cascade proceeds smoothly under
mild conditions, providing a series of valuable functionalized
oximes in moderate to good yields. Mechanistic studies suggest
that these cascades proceed via addition/coupling processes
that are controlled by the persistent radical effect (PRE) with
NO acting as the persistent species.
radical that has been successfully applied in the Barton nitrite
ester reaction as the oxidizing C radical trapping reagent
(Scheme 1B). This Barton reaction is highly valuable for the
remote radical d-functionalization of alcohols and has found
widespread application in organic synthesis. The cascade
comprises the homolytic OꢀNO bond cleavage of an in situ
[
5]
generated nitrite ester generating an alkoxyl radical and the
persistent nitric oxide radical (NO). 1,5-H atom transfer to
the alkoxyl radical gives a C radical that is oxidized in
a radical/radical cross-coupling reaction with the NO radical.
Tautomerization then provides an oxime as the final product.
The radical/radical cross-coupling is steered by the Persistent
[
6]
Radical Effect (PRE) and hence the persistent NO radical
[7]
C
arbon-centered radicals are key intermediates in organic
plays an important role in this process. Compared with the
homolytic cleavage of OꢀNO and NꢀNO s-bonds, the
homolytic cleavage of a CꢀNO bond to generate a C radical
transformations and have found many applications in organic
[
1]
synthesis. Considering the coupling of a keto carbonyl
moiety with a Michael acceptor, the ionic mode is not
applicable, since both C atoms to be connected are electro-
philic in nature. Along these lines, radical chemistry has
offered solutions. Fukuzawa and Inanaga independently
[8]
and the NO radical has been rarely studied, but we see great
potential along these lines. Herein, we present a visible-light-
mediated intermolecular radical addition of readily accessed
showed that SmI can be used to achieve reductive ketone–
2
[
2]
alkene coupling (Scheme 1A). These reactions proceed via
addition of an in situ generated nucleophilic Sm ketyl
III
radical to the Michael acceptor with subsequent reduction
and protonation. More recently, Baran elegantly showed that
formal reductive ketone–alkene coupling can be realized via
prior silyl enol ether formation and subsequent iron-catalyzed
[3a]
reductive alkene–alkene coupling. In both processes, the
intermolecular radical addition is followed by a reduction of
the electrophilic adduct radical. Due to the electrophilicity of
[4]
such a C radical, oxidation is difficult to achieve. Moreover,
the adduct radicals are stabilized intermediates and therefore
not well qualified as acceptors in atom or group transfer
processes.
Addressing this challenge, we planned a formal oxidative
coupling of ketones with various electrophilic alkenes follow-
ing a conceptually novel approach. Oxidation of the electro-
philic radical should be achieved via a radical/radical cross-
coupling reaction that is generally extremely fast and
independent of the electronics of a C radical intermediate.
As a C radical oxidant we considered the nitric oxide (NO)
[*] Dr. D. Zheng, S. Plçger, Dr. C. G. Daniliuc, Prof. Dr. A. Studer
Westfꢀlische Wilhelms-Universitꢀt
Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: studer@uni-muenster.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016955.
Scheme 1. Reductive ketone–alkene couplings via radical intermediates
and the oxidative approach using NO as the oxidant.
Angew. Chem. Int. Ed. 2021, 60, 8547 –8551
ꢀ 2021 Wiley-VCH GmbH
8547

Angewandte
Communications
Chemie
[
12]
acyloxy nitroso compounds onto electron-deficient alkenes
steered by the PRE (Scheme 1C).
X-ray diffraction analysis (see Table 2). Solvent screening
revealed that acetone performs best in this cascade providing
3a in 49% yield (Table 1, entries 2–7). When the reaction was
conducted with a 15 W household CFL or 10 W yellow LED,
slightly lower yields were obtained (30–40%, Table 1,
entries 8 and 9). Irradiation with a 10 W red LED gave
a similar yield (47%, Table 1, entry 10). Target 3a was not
observed upon irradiation with a 10 W green LED (Table 1,
entry 11) or in the dark (Table 1, entry 12), indicating the
importance of a suitable light source for mediating this radical
cascade. These results are in accordance with the UV/Vis
absorption spectrum of 1-nitrosocyclohexyl acetate 1a that
C-Nitroso compounds have received continuous attention
from chemists and biologists due to their interesting reac-
[
9]
tivities and important bioactivities. Acyloxy nitroso com-
pounds can be easily prepared by the reaction of oximes with
[
10]
PhI(OAc) or Pb(OAc) . The oximes in turn are readily
2
4
accessed by condensing a ketone with hydroxylamine. Nota-
bly, C-nitroso compounds have been widely used as C radical
acceptors; however, their potential as C radical precursors is
[11]
not explored.
We decided to use acyloxy nitroso com-
pounds as C radical precursors to react with electron-defi-
cient alkenes. Visible-light-induced CꢀNO bond homolysis
[
13]
shows the maximum at 667 nm in methanol. A higher yield
(56%) was obtained when the amount of a,b-unsaturated
ketone 2a was increased to 3.0 equivalents (Table 1,
entry 13). We found that the yield could be further increased
to 77% by adding 1a in three portions. By using syringe pump
technique, the amount of 2a could be reduced to 2 equiv-
alents without decreasing the yield to a large extent (Table 1,
entries 14 and 15).
should lead to an electron-rich transient C radical and the
persistent NO radical in a reversible process. In the presence
of an electrophilic alkene, the nucleophilic a-oxy-C radical
should add to the alkene and the adduct radical will be
trapped by NO in a selective radical/radical cross-coupling
steered by the PRE to give the corresponding nitrosoalkene.
Tautomerization will eventually afford the final oxime
product. Importantly, this isomerization step irreversibly
terminates the radical cascade and prevents the adduct
nitroso compound from undergoing telomerization.
With the optimized reaction conditions, we examined the
scope with respect to the acceptor component keeping 1a as
the C radical precursor. A wide range of electron-deficient
alkenes were found to be eligible acceptors in this process, as
shown in Table 2. Aryl vinyl and alkyl vinyl ketones both
performed well and the desired a-oximinoketones 3a–3e
were obtained in good yields (75–83%). Acrylates can also be
used as coupling partners, as documented by the successful
preparation of the a-oximinoesters 3 f–3k that were isolated
in moderate to good yields. As expected, with the most
electrophilic trifluoroethyl acrylate, the highest yield was
achieved (3i, 71%). The corresponding hydroxyethyl ester
provided a significantly lower yield (3j, 35%). The free
primary alcohol moiety seems to interfere with the cascade.
Transformations with other electron-deficient alkenes such as
acrylamides, vinyl sulfones, and acrylonitrile proceeded
smoothly to afford the desired products 3l–3n in 38–83%
yields. Furthermore, styrenes bearing an electron-withdraw-
ing para-substituent such as trifluoromethyl (3o), cyano, (3p)
or methoxycarbonyl (3q) on the aromatic ring reacted with
We commenced the investigations by studying the reac-
tion of 1-nitrosocyclohexyl acetate 1a with the a,b-unsatu-
rated ketone 2a (2 equivalents). For the preparation of all
nitroso compounds used in this study, see the Supporting
Information (SI). Pleasingly, in dichloromethane (DCM)
under irradiation with a 10 W white LED at room temper-
ature for 1 h, the targeted a-oximinoketone 3a was obtained
in 43% yield of isolated product as single E-isomer (Table 1,
entry 1). The structure of 3a was confirmed by single-crystal
[
a]
Table 1: Reaction optimization.
[
b]
Entry
Solvent
Lamp
Yield [%]
1
a in moderate to good yields. Notably, for the less electro-
1
2
3
4
5
6
7
8
9
DCM
EtOAc
10 W white LED
10 W white LED
10 W white LED
10 W white LED
10 W white LED
10 W white LED
10 W white LED
15 W CFL
10 W yellow LED
10 W red LED
10 W green LED
–
43
46
43
40
49
32
44
31
40
47
n.d.
n.d.
56
77
72
69
philic parent styrene, only traces of the targeted product 3r
were observed, indicating the importance of electronic effects
in the acceptor component. If the reaction of the 1a-derived
nucleophilic C radical with the acceptor is too slow, direct
trapping with NO regenerating starting 1a will be the
exclusive reaction path. Along these lines, only traces of
product 3u were observed in the reaction of 1a with the
sterically more hindered (E)-1-phenylbut-2-en-1-one. 4-
Vinylpyridine and acrolein were found to be eligible accept-
ors (3s, 67%; 3t, 86%). A natural-product-derived acrylate
prepared from dehydroepiandrosterone was successfully
applied to this reaction, delivering 3v in 39% yield.
Various acyloxy nitroso compounds were studied next in
the cascade using 1-phenylprop-2-en-1-one 2a as the radical
acceptor (Table 3). 1-Nitrosocyclohexyl pivalate and 1-nitro-
socyclohexyl 2,2,2-trifluoroacetate reacted well with 2a to
provide the desired a-oximinoketones 3w and 3x in 83% and
85% yields, showing that the acetyl protecting group can be
CH CN
3
1,4-Dioxane
Acetone
Methanol
DMF
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
10
11
12
13
14
15
16
[
[
[
[
c]
10 W white LED
10 W white LED
10 W white LED
10 W white LED
c,d]
c,e]
a,e]
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol, 2.0 equiv),
solvent (2.0 mL), 10 W white LED, r.t., Ar, 1 h. [b] Yield of isolated
product based on 1a. [c] 3.0 equivalents of 2a were used. [d] 1a was
added in three portions. [e] 2a was added via a syringe pump over 2 h.
8548
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 8547 –8551

Angewandte
Communications
Chemie
[
a]
[a]
Table 2: Reaction of 1a with various electron-deficient alkenes.
Table 3: Varying the acyloxy nitroso compound.
[a] Reaction conditions: 1 (0.2 mmol), 2a (0.6 mmol, 3.0 equiv), acetone
(2.0 mL), 10 W white LED, r.t., Ar, 2 h, yields of isolated product.
this reaction setup would further improve the practicality
Table 4). PhI(OAc) oxidation was conducted in DCM for
(
2
0
.5 h. Acetone and 2a were then added to the reaction
mixture followed by white LED irradiation for 2 h at room
[
a]
Table 4: One-pot two-step process of various oximes with 2a.
[a] Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol, 3.0 equiv), acetone
(
2.0 mL), 10 W white LED, r.t., Ar, 2 h, yields of isolated product.
easily varied. Transformations of nitroso compounds contain-
ing a four-, five-, seven-, and twelve-membered ring all
proceeded in good yields (3y–3ab, 67–79%). Substrates
bearing a ketal functionality or a heterocyclic ring such as
tetrahydrothiopyran were also tolerated (3ac, 87%; 3ad,
7
8%). The 2-adamantone-derived nitroso compound partici-
pated in the reaction to give 3ae in 77% yield. Furthermore,
both symmetrical and unsymmetrical noncyclic tertiary
nitroso compounds could be efficiently converted to the
targeted oximes 3af–3ah in good yields (69–76%).
All cascades presented so far were conducted with
preformed isolated nitroso compounds as substrates. Consid-
ering the limited stability of some nitroso derivatives, we also
explored the possibility of a one-pot, two-step process
utilizing oximes as starting materials. Importantly, even for
stable nitroso compounds this would be advantageous since
[
(
(
a] Reaction conditions: 4 (0.2 mmol), PhI(OAc) (0.2 mmol), DCM
2
1.5 mL), 08C!r.t., 0.5 h, then 2a (0.6 mmol, 3.0 equiv), acetone
1.5 mL), 10 W white LED, r.t., Ar, 2 h, yield of isolated product based on
oxime 4. For 3aj, only the major isomer is drawn.
Angew. Chem. Int. Ed. 2021, 60, 8547 –8551
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org 8549

Angewandte
Communications
Chemie
temperature. To our delight, the one-pot transformation
worked smoothly with cyclohexanone oxime and the a-
oximinoketone 3a was isolated in 60% yield. By using this
protocol, oximes prepared from l-menthone, (+)-dihydro-
carvone, and stanolone were successfully reacted with 2a to
afford the desired products 3ai–3ak in moderate to good
yields. The l-menthone derivative reacted with excellent
diastereoselectivity, whereas the (+)-dihydrocarvone deriva-
tive and the steroid system coupled with moderate diastereo-
selectivities.
Finally, an investigation of the follow-up chemistry using
3a and 3 f as the model substrates was carried out to examine
the synthetic value of the obtained products (Scheme 3A).
The oxime functionality in 3a was converted to the ketone
moiety upon reaction with the Dess–Martin periodinane
[14]
(DMP) and 9 was isolated in 51% yield. Reduction with
sodium borohydride afforded the a-hydroxyoxime 10 in 95%
yield. The oxime group could also be reduced to an amine by
zinc in acetic acid. N-acetyl protection then gave the a-amino
ketone 11 in 44% overall yield. Saponification of the ester
group in 3a with 10% NaOH (aq) in methanol afforded the
lactol 12 in 84% yield. Treatment of 3a with catalytic amounts
of acid afforded the dihydroisoxazole 13 in 67% yield.
Reduction of the oxime functionality in 3 f followed by N-
acetyl protection gave the N-protected amino acid ester 14 in
80% overall yield. A gram-scale reaction with 6.0 mmol 1-
nitrosocyclohexyl acetate 1a provided the desired product 3a
in 70% yield, proving the practicality of this process
(Scheme 3B).
To gain insights into the mechanism, we ran the reaction
of 1a with 2a under standard conditions in the presence of
2
.0 equivalents
of
2,2,6,6-tetramethyl-1-piperidinyloxy
(
TEMPO, Scheme 2a). The cascade was suppressed and
TEMPO-trapping product 5 could be detected by high-
resolution mass spectrometry (HRMS), indicating that the
corresponding a-carbonyl C radical is an intermediate in this
reaction. When the reaction was carried out without light
using 10 mol% triethylborane as the radical initiator, neither
3
a nor (E)-2-(hydroxyimino)-1-phenylpentan-1-one 6 were
In summary, we have shown that readily accessible
acyloxy nitroso compounds are efficient C radical precursors
that engage in intermolecular radical addition reactions with
electron-deficient alkenes. The cascades occur under mild
conditions upon white light irradiation and provide a wide
range of synthetically useful functionalized oximes. Further-
more, a one-pot, two-step protocol utilizing oximes as the
starting materials that are oxidized in situ to the correspond-
ing acyloxy nitroso derivatives was developed, further
improving the practicality of the cascade. This process,
which uses commercially available starting materials
observed. Moreover, alkoxyamines 7 and 8 could be identi-
fied by HRMS (Scheme 2b). Alkoxyamine 7 is formed by
reaction of an ethyl radical with the nitroxide A, which itself is
formed by reaction of 1a with the corresponding a-keto
radical. This result shows that if a type-A nitroxide is formed,
its lifetime is, as expected, long enough to be trapped by
a second C radical. Hence, type-A radicals will not fragment
to a tertiary C radical along with the corresponding nitroso
compound and should therefore not appear as intermediates
in our cascades. Hence, a radical chain reaction via NO group
transfer radical addition is unlikely. These results indicate that
the reactions proceed via a radical/radical cross-coupling of
an a-carbonyl C radical with nitric oxide controlled by the
PRE, as suggested above in Scheme 1C. The findings further
show that the tertiary C radicals generated by photoinduced
homolysis of the nitroso compounds surprisingly react
selectively with the electron-poor radical acceptor rather
than with the tertiary nitroso derivative, likely for steric
reasons.
(ketones, hydroxylamine, PhI(OAc) , and electron poor
2
alkenes), will broaden the portfolio of PRE-mediated radical
transformations and further applications of this strategy are
anticipated.
Scheme 3. Follow-up chemistry and gram-scale reaction. Reaction con-
ditions: (i) 3a, DMP, wet DCM, 3 h; (ii) 3a, NaBH , methanol, 08C!
4
r.t., 3 h; (iii) 3a, Zn, Ac O, AcOH, 12 h; (iv) 3a, 10% NaOH (aq),
2
methanol, reflux, 2 h; (v) 3a, TsOH, toluene, reflux, 18 h; (vi) 3 f, Zn,
Scheme 2. Mechanistic studies.
Ac O, AcOH, 688C, 18 h.
2
8550
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 8547 –8551

Angewandte
Communications
Chemie
Maryasin, L. Gonzꢁlez, N. Maulide, J. Am. Chem. Soc. 2019, 141,
8437 – 18443; d) A. Vidyasagar, J. Shi, P. Kreitmeier, O. Reiser,
Acknowledgements
1
Org. Lett. 2018, 20, 6984 – 6989; e) R. Jahjah, A. Gassama, F.
Dumur, S. Marinkovi c´ , S. Richert, S. Landgraf, A. Lebrun, C.
Cadiou, P. Sellꢂs, N. Hoffmann, J. Org. Chem. 2011, 76, 7104 –
We thank the University of Mꢀnster and the Fonds der
Chemischen Industrie (fellowship to S.P.) for supporting this
work.
7118.
[
5] a) D. H. R. Barton, J. M. Beaton, L. E. Geller, M. M. Pechet, J.
Am. Chem. Soc. 1960, 82, 2640 – 2641; b) D. H. R. Barton, J. M.
Beaton, J. Am. Chem. Soc. 1960, 82, 2641; c) D. H. R. Barton,
J. M. Beaton, L. E. Geller, M. M. Pechet, J. Am. Chem. Soc.
Conflict of interest
1
961, 83, 4076 – 4083; for a review, see: d) G. Majetich, K.
The authors declare no conflict of interest.
Wheless, Tetrahedron 1995, 51, 7095 – 7129.
[
6] a) D. Leifert, A. Studer, Angew. Chem. Int. Ed. 2020, 59, 74 –
Keywords: Barton reaction · carbon radicals ·
nitroso compounds · persistent radical effect · radical cascade
1
08; Angew. Chem. 2020, 132, 74 – 110; b) A. Studer, Chem. Eur.
J. 2001, 7, 1159 – 1164; c) H. Fischer, Chem. Rev. 2001, 101, 3581 –
601.
3
[
[
7] J. Hartung, Chem. Rev. 2009, 109, 4500 – 4517.
8] a) H. Chakrapani, M. D. Bartberger, E. J. Toone, J. Org. Chem.
[
1] a) S. Crespi, M. Fagnoni, Chem. Rev. 2020, 120, 9790 – 9833;
b) M. Yan, J. C. Lo, J. T. Edwards, P. S. Baran, J. Am. Chem. Soc.
2009, 74, 1450 – 1453; b) B. G. Gowenlock, J. Chem. Educ. 2008,
2016, 138, 12692 – 12714; c) C. Chatgilialoglu, A. Studer, Ency-
85, 1243 – 1245; c) B. G. Gowenlock, G. B. Richter-Addo, Chem.
clopedia of Radicals in Chemistry, Biology and Materials, Wiley,
Chichester, 2012; d) A. Studer, D. P. Curran, Angew. Chem. Int.
Ed. 2016, 55, 58 – 102; Angew. Chem. 2016, 128, 58 – 106; e) S. Z.
Zard, Radical Reactions in Organic Synthesis, Oxford University
Press, Oxford, 2003; f) P. Renaud, M. P. Sibi, Radicals in Organic
Synthesis, Wiley-VCH, Weinheim, 2001.
Rev. 2004, 104, 3315 – 3340.
9] a) H. Van cˇ ik, Aromatic C-Nitroso Compound, Springer, New
[
York, 2013; b) S. Carosso, M. J. Miller, Org. Biomol. Chem. 2014,
12, 7445 – 7468; c) H. Yamamoto, M. Kawasaki, Bull. Chem. Soc.
Jpn. 2007, 80, 595 – 607; d) Y. Yamamoto, H. Yamamoto, Eur. J.
Org. Chem. 2006, 2031 – 2043; e) H. Yamamoto, N. Momiyama,
Chem. Commun. 2005, 3514 – 3525.
[
2] a) K. Otsubo, J. Inanaga, M. Yamaguchi, Tetrahedron Lett. 1986,
27, 5763 – 5764; b) S. Fukuzawa, A. Nakanishi, T. Fujinami, S.
[
10] a) M. B. Hadimani, R. Mukherjee, R. Banerjee, M. E. Shoman,
O. M. Aly, S. B. King, Tetrahedron Lett. 2015, 56, 5870 – 5873;
b) D. G. Calvet, M. Dussaussois, N. Blanchard, C. Kouklovsky,
Org. Lett. 2004, 6, 2449 – 2451; c) R. M. Moriarty, O. Prakash,
P. R. Vavilikolanu, Synth. Commun. 1986, 16, 1247 – 1254.
Sakai, J. Chem. Soc. Chem. Commun. 1986, 624 – 625; c) D. J.
Edmonds, D. Johnston, D. J. Procter, Chem. Rev. 2004, 104,
3
371 – 3404.
3] a) J. C. Loh, J. Gui, Y. Yabe, C.-M. Pan, P. S. Baran, Nature 2014,
16, 343 – 348; For reviews of photocatalysis involving ketyl
[
5
[
11] Y. Gao, S. Yang, W. Xiao, J. Nie, X.-Q. Hu, Chem. Commun.
radical coupling reactions: b) Q. Xia, J. Dong, H. Song, Q. Wang,
Chem. Eur. J. 2019, 25, 2949 – 2961; c) K. N. Lee, M.-Y. Ngai,
Chem. Commun. 2017, 53, 13093 – 13112; For other selected
examples of photoredox-catalysed coupling of a keto carbonyl
moiety with a Michael acceptor: d) L. Qi, Y. Chen, Angew.
Chem. Int. Ed. 2016, 55, 13312 – 13315; Angew. Chem. 2016, 128,
2
020, 56, 13719 – 13730.
[
12] Deposition number 2049049 (3a) contains the supplementary
crystallographic data for this paper. These data are provided free
of charge by the joint Cambridge Crystallographic Data Centre
and Fachinformationszentrum Karlsruhe Access Structures
service.
13] X. Sha, T. S. Isbell, R. P. Patel, C. S. Day, S. B. King, J. Am.
Chem. Soc. 2006, 128, 9687 – 9692.
14] S. S. Chaudhari, K. G. Akamanchi, Tetrahedron Lett. 1998, 39,
13506 – 13509; e) K. N. Lee, Z. Lei, M.-Y. Ngai, J. Am. Chem.
[
Soc. 2017, 139, 5003 – 5006; f) K. Cao, S. M. Tan, R. Lee, S. Yang,
H. Jia, X. Zhao, B. Qiao, Z. Jiang, J. Am. Chem. Soc. 2019, 141,
[
5437 – 5443; g) J. Cao, G. Wang, L. Gao, X. Cheng, S. Li, Chem.
3
209 – 3212.
Sci. 2018, 9, 3664 – 3671; h) J.-Y. Gu, W. Zhang, S. R. Jackson, Y.-
H. He, Z. Guan, Chem. Commun. 2020, 56, 13441 – 13444.
4] a) Z.-H. Luan, J.-P. Qu, Y.-B. Kang, J. Am. Chem. Soc. 2020, 142,
[
Manuscript received: December 21, 2020
Revised manuscript received: February 2, 2021
Accepted manuscript online: February 9, 2021
Version of record online: February 25, 2021
20942 – 20947; b) Y. Ando, T. Kamatsuka, H. Shinokubo, Y.
Miyake, Chem. Commun. 2017, 53, 9136 – 9138; c) C. R. Gon-
Åalves, M. Lemmerer, C. J. Teskey, P. Adler, D. Kaiser, B.
Angew. Chem. Int. Ed. 2021, 60, 8547 –8551
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
8551