Oxidative C-S Bond Cleavage of Benzyl Thiols Enabled by Visible-Light-Mediated Silver(II) Complexes

Article abstract of DOI:10.1021/acs.orglett.0c01399

The oxidative cleavage reaction of the C-S bond using singlet oxygen is challenging because of its uncontrollable nature. We have developed a novel method for the singlet-oxygen-mediated selective C-S bond cleavage reaction using silver(II)-ligand complexes. Visible-light-induced silver catalysis enables the controlled oxidative cleavage of benzyl thiols to afford carbonyl compounds, such as aldehydes or ketones, which are important synthetic components.

Full text of DOI:10.1021/acs.orglett.0c01399

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Letter
Oxidative C−S Bond Cleavage of Benzyl Thiols Enabled by Visible-
Light-Mediated Silver(II) Complexes
Boseok Hong,§ Kim Christopher C. Aganda,§ and Anna Lee*
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ABSTRACT: The oxidative cleavage reaction of the C−S bond using
singlet oxygen is challenging because of its uncontrollable nature. We
have developed a novel method for the singlet-oxygen-mediated
selective C−S bond cleavage reaction using silver(II)−ligand
complexes. Visible-light-induced silver catalysis enables the controlled
oxidative cleavage of benzyl thiols to afford carbonyl compounds, such
as aldehydes or ketones, which are important synthetic components.
inglet oxygen (1O2) is an important chemical species in
organic synthesis. Many natural products and bioactive
Scheme 1. Singlet-Oxygen-Mediated C−S Bond Cleavage
Reactions
S
compounds, as well as synthetic intermediates, have been
1
1
1
synthesized using O2. However, the use of O2 in organic
synthesis is mainly focused on specific transformations, such as
oxidation, annulation, and ene-type reactions (Scheme
1A).1b,e−h In contrast, other types of reactions using 1O2
1
have been rarely reported. In particular, O2-mediated C−S
bond cleavage has not been successful, despite the many
attempts to achieve this transformation.1c,2 The main reason
1
for this failure is the lack of control over the reactive O2
species during the reactions (Scheme 1B).3 The first C−S
1
bond cleavage reaction using O2 was reported by Corey and
Ouannes in 1976.2a Other research groups also attempted C−S
̀
bond cleavage reactions starting from sulfides.1c,2b−g,4 How-
ever, the reaction yields were low, and various side products
were observed. To determine the reaction pathway, insightful
mechanistic studies have been carried out. Major contributions
have been made by the Clennan group based on experimental
and computational studies.1c,2b−d In addition, the research
groups of Lapi, Baciocchi, and Albini have carried out
mechanistic studies and revealed the routes of the formation
of various byproducts as well as the carbonyl products.2e−g,4
Despite these efforts, however, the 1O2-mediated selective C−S
bond cleavage reaction remains out of reach. Nevertheless, the
development of this unsolved, challenging reaction would be
very valuable for organic synthesis.
In addition, the resulting products of the C−S cleavage
reaction are aldehydes and ketones, which are very important
synthetic key compounds in organic reactions.5 Herein, we
report a novel method for the oxidative C−S bond cleavage
reaction using visible-light-mediated Ag(II)−ligand complexes
under mild reaction conditions (Scheme 1C).6,7 Significantly,
organic reactions using Ag(II)−ligand complexes as catalysts
have been rarely reported even though the synthesis of silver
complexes has been well-established in inorganic chemis-
try.8−10 Our studies show that Ag(II) complexes catalyze the
Received: April 23, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01399
© XXXX American Chemical Society
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a
oxidative C−S bond cleavage of benzyl thiols to afford the
Table 1. Optimization of the Reaction Conditions
desired carbonyl compounds selectively.
1
Previously, we reported the O2-mediated oxidation of aryl
thiols for the selective synthesis of diaryl sulfoxides and diaryl
sulfones (Scheme 2A).11,12 Unexpectedly, aldehydes, which are
Scheme 2. Preliminary Studies on the Oxidative C−S Bond
Cleavage of Benzyl Thiols
Ag(II)−ligand complex
yield
b
entry
(mol %)
solvent
DMSO
CH3CN green LEDs
light source
green LEDs
(%)
1
2
3
4
5
6
7
8
A (20)
A (20)
A (20)
A (20)
A (20)
A (20)
A (20)
B (20)
C (20)
D (20)
A (10)
A (10)
A (10)
A (5)
A (5)
A (10)
-
A (10)
A (10)
A (10)
A (10)
48
trace
16
trace
62
68
78
68
55
59
91
62
52
28
24
42
THF
DCM
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
green LEDs
green LEDs
green LEDs
blue LEDs
white LEDs
white LEDs
white LEDs
white LEDs
white LEDs
white LEDs
white LED
white LEDs
white LEDs
white LEDs
white LEDs
white LEDs
no light
9
10
11
c
12
d
13
14
c
15
e
16
17
f
the corresponding products of C−S bond cleavage reactions,
were obtained when we employed benzyl thiols as the starting
compounds (Scheme 2B). However, the reactions were not
clean, and the resulting aldehydes were obtained in low yields
(24−33%). In our EPR studies, we observed that a Ag(II)
species was generated in situ during the reaction.13
Interestingly, we found that the EPR spectrum of this species
is consistent with that of [Ag(II)(py)4]S2O8 in terms of the g-
value (Scheme 2C).9a,14
trace
trace
trace
trace
24
g
18
19
h
20
i
white LEDs
white LEDs
21
a
Conditions: 1a (0.65 mmol, 1.0 equiv), catalyst, K2S2O8 (2.0 equiv),
b
solvent (0.25 M) under O2 (1 atm) at 23 °C for 18 h. Isolated yield
after column chromatography. Used 1.0 equiv of K2S2O8. Used 3.0
equiv of K2S2O8. Used 2.0 equiv of (NH4)2S2O8. In the absence of
[Ag(II)(py)4]S2O8. In the absence of K2S2O8. Under Ar. Under air.
c
d
e
f
g
h
i
On the basis of these intriguing results, we envisioned that
Ag(II)−ligand complexes could lead to the formation of C−S
bond cleavage products in the presence of visible light. To test
our hypothesis, we synthesized Ag(II)−ligand complexes9b,15
and screened various experimental conditions. Selected
examples are summarized in Table 1. (4-Methoxyphenyl)-
methanethiol (1a) was selected as the starting thiol and
reacted with oxygen (1 atm) in the presence of the silver(II)−
ligand complex and K2S2O8 (2.0 equiv) at room temperature
and irradiated by visible light. Solvent screening showed that
the reaction does not take place in either acetonitrile or
dichloromethane (entries 2 and 4), while DMF allows the
formation of the corresponding aldehyde in 62% yield after 18
h (entry 5). Interestingly, by changing the light source, we
were able to improve the yield significantly, and the desired
product was obtained in 78% yield with white LEDs (entry 7).
Next, we screened various Ag(II)−ligand complexes (entries
7−10), and in terms of reactivity, the best result was obtained
with [Ag(II)(py)4]S2O8, where the desired product was
obtained in 78% yield (entry 7). Further improvement in the
yield was observed when we decreased the amount of
[Ag(II)(py)4]S2O8 (10 mol %, 91% yield, entry 11). Varying
the amount of K2S2O8, in contrast, did not improve the
reaction yield (entries 12 and 13). Control reactions were
performed in the absence of the silver catalyst, light, or the
oxidant (K2S2O8), which did not give any of the desired
products (entries 17−19). We also found that the reaction
atmosphere had a significant effect on the reaction progress.
Notably, synthesis under an Ar atmosphere did not give the
desired products (entry 20). The reaction yield decreased
when the transformation was performed under air (entry 21).
Based on these results, we performed further studies with the
silver catalyst ([Ag(II)(py)4]S2O8), oxidant (K2S2O8), oxygen
(O2), and visible light.
Next, the scope of the synthesis of the aldehydes and
ketones was explored (Schemes 3 and 4). The various benzyl
thiols provided the desired aldehydes in the presence of both
electron-donating (2a, 2c−2e, 2p, and 2q) and electron-
B
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a
a
Scheme 3. Substrate Scope: Aldehydes
Scheme 4. Substrate Scope: Ketones
a
Conditions: 1a (0.65 mmol, 1.0 equiv), [Ag(II)(py)4]S2O8 (20 mol
a
Conditions: 1a (0.65 mmol, 1.0 equiv), [Ag(II)(py)4]S2O8 (10 mol
%), K2S2O8 (2.0 equiv) in DMF (0.25 M), irradiation with 5 W white
LEDs at 23 °C for 18 h under O2 (1 atm). Isolated yield after column
chromatography.
%), K2S2O8 (2.0 equiv) in DMF (0.25 M), irradiation with 5 W white
LEDs at 23 °C for 18 h under O2 (1 atm). Reaction conducted for
36 h. Isolated yield after column chromatography.
b
Scheme 5. Control Experiments
withdrawing (2f−2o) substituents (Scheme 3). In addition,
the products derived from the ortho-, meta-, and para-
substituted benzyl thiols were also obtained successfully
under the optimized reaction conditions. No noticeable
electronic effects of the substituents were observed in terms
of the reaction yield (2a or 2e vs 2f or 2h). Heteroaromatic
ring- or naphthyl-containing substrates (1r−1u) also provided
the desired aldehydes in high yields (80−94%).
The substrate scope for ketones was also explored (Scheme
4). The α-branched benzyl thiols 3 were varied, giving the
resulting ketones in moderate to excellent yields (60−95%).
For the synthesis of ketones, better reaction yields were
obtained when we employed 20 mol % of [Ag(II)(py)4]S2O8.
Similar to the results of the aldehydes, substituents at all
positions (ortho-, meta-, and para-) were tolerated, affording
the desired products (4b−4i) in moderate to high yields (60−
80%). On the other hand, decreased reaction yields were
observed with sterically demanding substituents at the α-
position of the thiols (4l: 70%, 4m: 81%, 4o: 70%). These
bulky substrates also required longer reaction times (36 h: 4m,
4o). No desired carbonyl product was obtained with
nonbenzylic substrates such as alkyl, allyl, and propargyl thiols.
In addition, 2-pyridinemethanethiol was not a suitable
substrate for this transformation (see the Supporting
Information).
this reaction, was detected by GC−MS during the reaction. We
were able to isolate 5a and confirmed its structure by NMR
(Scheme 5A). We also observed sulfide 6a by GC−MS, but 6a
was transformed to aldehyde 2a very quickly under the
reaction conditions. When we carried out the reaction at a
lower temperature (15 °C) to slow the reaction rate, sulfide 6a
was isolated (Scheme 5A). In addition, when disulfide 5a or
sulfide 6a was directly subjected to the reaction, the desired
To gain a mechanistic insight into this oxidative C−S bond
cleavage reaction, control experiments were carried out
(Scheme 5). First, disulfide 5a, a possible intermediate for
C
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aldehyde (2a) was obtained under the reaction conditions
(Scheme 5B,C). Furthermore, by monitoring the experiments
by GC−MS, we found that the ratio of disulfide 5a to sulfide
6a decreased over time (Scheme 5D). This indicates that after
the formation of disulfide 5a and the subsequent formation of
sulfide 6a as intermediates, further oxidative cleavage pathways
occur to give the desired carbonyl products. We also found
that the use of the silver catalyst was necessary to generate
sulfide 6a from disulfide 5a (Scheme 5D, see the Supporting
Information). Additionally, disulfide 5a can be formed from
thiol 1a by either an oxidation process or silver catalysis
(Scheme 5E,F). Next, when a radical inhibitor (TEMPO or
BHT) was employed, only trace amounts of the desired
products were observed (Scheme 5B). This indicates that the
oxidative cleavage pathway includes a radical process. In
addition, to check the oxygen species in this reaction, NaN3 or
6, paths A and B). In path A, a thiyl radical is generated from
thiol 1 by the silver catalyst.7,19 The subsequent formation of
disulfide 5 and sulfide 6 also occur. After the reaction with 1O2,
which would be generated by the silver(II)−pyridine complex,
α-hydroperoxy sulfide I is formed.1c,2b−f Then, the desired
carbonyl product would be formed by either an inter- (III-B)
or intramolecular (III-A) oxygen transfer process.1c,2b−f On
the basis of previous reports, persulfoxide II and the α-
hydroperoxy sulfide I subsequently formed by Pummerer-type
rearrangement are the key intermediates for the formation of
oxidative C−S bond cleavage compounds.1c,2b−f,20 We also
observed peaks that could correspond to α-hydroperoxy sulfide
1
I in the H NMR studies (see the Supporting Information).
In path B, based on our results (Scheme 5B and Figure 1),
we suggest that the Ag(II) species was converted to a Ag(I)
species by a SET process, generating sulfur radical cation 7
(Scheme 6, path B). After the reaction with a superoxide
1
DABCO, which are well-known strong O2 quenchers, was
21
1
added to the reaction.16 As expected, the desired product was
not obtained. Moreover, only trace amounts of the product
were observed when the reaction was carried out in the
presence of FeSO4, which can react with the hydroperoxyl
radical (HOO·), indicating that both singlet oxygen and the
hydroperoxyl radical were involved in this reaction (Scheme
5B).17
radical anion, which would be generated from O2, IV is
formed. A subsequent reaction with IV and hydroperoxyl
radical would provide a key intermediate, α-hydroperoxy
sulfide I. Then, carbonyl compound 2 would be obtained
through III-A or III-B.1c,2b−f
In conclusion, we have developed a controlled oxidative C−
S bond cleavage reaction. This transformation features a dual
activation mode of silver catalysis and visible-light irradiation.
The reaction involves the visible-light-mediated silver(II)-
catalyzed C−S bond cleavage of benzyl thiols and focuses on
controlling the oxidation process to afford the resulting
carbonyl compounds. Various benzyl thiols can be converted
into the corresponding aldehydes or ketones in good to
excellent yields under mild reaction conditions. We anticipate
that this novel approach will allow the field of visible-light-
mediated organic reactions to be expanded.
1
Also, we could confirm the generation of O2 during the
reaction by EPR (see the Supporting Information). A
characteristic signal of TEMPO was detected when we
1
employed 2,2,6,6-tetramethylpiperidine (TEMP) to trap O2,
indicating that 1O2 is involved in this transformation.18
Furthermore, we found that the Ag(II) species was readily
quenched by the addition of sulfide 6a (Figure 1).13d
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01399.
Experimental details, characterization data for the
products, and NMR spectra (PDF)
Figure 1. Quenching studies by EPR.
AUTHOR INFORMATION
Corresponding Author
■
On the basis of previous reports and our results, a proposed
reaction pathway is shown in Scheme 6.1c,2a−f,4 Two different
reaction pathways are hypothesized for the formation of
oxidative C−S bond cleavage products from sulfide 6 (Scheme
Anna Lee − Department of Chemistry and Department of Energy
Science and Technology, Myongji University, Yongin 17058,
Republic of Korea; orcid.org/0000-0002-3450-7024;
Email: annalee@mju.ac.kr
Scheme 6. Proposed Reaction Pathway
Authors
Boseok Hong − Department of Chemistry, Myongji University,
Yongin 17058, Republic of Korea
Kim Christopher C. Aganda − Department of Energy Science
and Technology, Myongji University, Yongin 17058, Republic of
Korea
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01399
Author Contributions
§B.H. and K.C.C.A. contributed equally to this work.
Notes
The authors declare no competing financial interest.
D
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(9) For the synthesis of silver complexes, see: (a) Vosburgh, W. C.;
Cogswell, S. A. J. Am. Chem. Soc. 1943, 65, 2412−2413. (b) Po, H. N.
Coord. Chem. Rev. 1976, 20, 171−195.
ACKNOWLEDGMENTS
■
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIT) (2019R1A2C1008297) and the Basic
Science Research Program through the NRF funded by the
Ministry of Education (2016R1D1A1B03931335). This work
was also supported by the Korea Institute of Energy
Technology Evaluation and Planning (KETEP) and the
Ministry of Trade, Industry & Energy (MOTIE) of the
Republic of Korea (20194010201750).
(10) For selected examples of organic reactions using Ag(I)
complexes, see: (a) Dell’Acqua, M.; Castano, B.; Cecchini, C.;
Pedrazzini, T.; Pirovano, V.; Rossi, E.; Caselli, A.; Abbiati, G. J. Org.
Chem. 2014, 79, 3494−3505. (b) Trose, M.; Dell’Acqua, M.;
Pedrazzini, T.; Pirovano, V.; Gallo, E.; Rossi, E.; Caselli, A.;
Abbiati, G. J. Org. Chem. 2014, 79, 7311−7320. (c) Pedrazzini, T.;
Pirovano, P.; Dell’Acqua, M.; Ragaini, F.; Illiano, P.; Macchi, P.;
Abbiati, G.; Caselli, A. Eur. J. Inorg. Chem. 2015, 2015, 5089−5098.
(d) Alderson, J. M.; Corbin, J. R.; Schomaker, J. M. Acc. Chem. Res.
2017, 50, 2147−2158. (e) Ju, M.; Huang, M.; Vine, L. E.; Dehghany,
M.; Roberts, J. M.; Schomaker, J. M. Nat. Catal. 2019, 2, 899−908.
(11) Kim, D. H.; Lee, J.; Lee, A. Org. Lett. 2018, 20, 764−767.
(12) For examples of selective oxygenation of sulfides via
photoredox catalysis, see: (a) Li, Y.; Wang, M.; Jiang, X. ACS Catal.
2017, 7, 7587−7592. (b) Li, Y.; Rizvi, S. A.-e.-A.; Hu, D.; Sun, D.;
Gao, A.; Zhou, Y.; Li, J.; Jiang, X. Angew. Chem., Int. Ed. 2019, 58,
13499−13506.
REFERENCES
■
(1) (a) Stephenson, L. M.; Grdina, M. J.; Orfanopoulos, M. Acc.
Chem. Res. 1980, 13, 419−425. (b) Wasserman, H. H.; Ives, J. L.
Tetrahedron 1981, 37, 1825−1852. (c) Clennan, E. L. Acc. Chem. Res.
2001, 34, 875−884. (d) Greer, A. Acc. Chem. Res. 2006, 39, 797−804.
(e) Ogilby, P. R. Chem. Soc. Rev. 2010, 39, 3181−3209. (f) Ghogare,
A. A.; Greer, A. Chem. Rev. 2016, 116, 9994−10034. (g) Bian, C.;
Singh, A. K.; Niu, L.; Yi, H.; Lei, A. Asian J. Org. Chem. 2017, 6, 386−
396. (h) Sagadevan, A.; Hwang, K. C.; Su, M.-D. Nat. Commun. 2017,
8, 1812.
̈
(13) For selected reviews on EPR studies, see: (a) Mobius, K. Chem.
Soc. Rev. 2000, 29, 129−139. (b) Rieger, A. L.; Rieger, P. H.
Organometallics 2004, 23, 154−162. (c) Rhodes, C. J. Annu. Rep. Prog.
Chem., Sect. C: Phys. Chem. 2011, 107, 47−87. (d) Roessler, M. M.;
Salvadori, E. Chem. Soc. Rev. 2018, 47, 2534−2553.
́
(2) (a) Corey, E. J.; Ouannes, C. Tetrahedron Lett. 1976, 17, 4263−
(14) (a) McMillan, J. A.; Smaller, B. J. Chem. Phys. 1961, 35, 1698−
1701. (b) Po, H. N. Coord. Chem. Rev. 1976, 20, 171−195.
(15) (a) Breitinger, D., Herrmann, W. A.; Hiller, W. Copper, Silver,
Gold, Zinc, Cadmium and Mercury; Thieme, 1999; Vol. 5;
4266. (b) Jensen, F.; Greer, A.; Clennan, E. L. J. Am. Chem. Soc. 1998,
120, 4439−4449. (c) Toutchkine, A.; Clennan, E. L. J. Am. Chem. Soc.
2000, 122, 1834−1835. (d) Toutchkine, A.; Aebisher, D.; Clennan, E.
L. J. Am. Chem. Soc. 2001, 123, 4966−4973. (e) Bonesi, S. M.;
Fagnoni, M.; Monti, S.; Albini, A. Tetrahedron 2006, 62, 10716−
10723. (f) Baciocchi, E.; Del Giacco, T.; Lanzalunga, O.; Lapi, A. J.
Org. Chem. 2007, 72, 9582−9589. (g) Baciocchi, E.; Del Giacco, T.;
Lanzalunga, O.; Mencarelli, P.; Procacci, B. J. Org. Chem. 2008, 73,
5675−5682.
́
́
(b) Leszczynski, P. J.; Budzianowski, A.; Dobrzycki, Ł.; Cyranski,
M. K.; Derzsi, M.; Grochala, W. Dalton Trans 2012, 41, 396−402.
(16) (a) Ouannes, C.; Wilson, T. J. Am. Chem. Soc. 1968, 90, 6527−
6528. (b) Bancirova, M. Luminescence 2011, 26, 685−688. (c) Al-
Nu’airat, J.; Dlugogorski, B. Z.; Gao, X.; Zeinali, N.; Skut, J.;
Westmoreland, P. R.; Oluwoye, I.; Altarawneh, M. Phys. Chem. Chem.
Phys. 2019, 21, 171−183.
(3) (a) Ogilby, P. R.; Foote, C. S. J. Am. Chem. Soc. 1983, 105,
3423−3430. (b) DeRosa, M. C.; Crutchley, R. J. Coord. Chem. Rev.
2002, 233−234, 351−371. (c) Lemp, E.; Valencia, C.; Zanocco, A. L.
J. Photochem. Photobiol., A 2004, 168, 91−96. (d) Varchola, J.;
(17) (a) Zhong, J.-J.; Meng, Q.-Y.; Wang, G.-X.; Liu, Q.; Chen, B.;
Feng, K.; Tung, C.-H.; Wu, L.-Z. Chem. - Eur. J. 2013, 19, 6443−
6450. (b) Aganda, K. C. C.; Hong, B.; Lee, A. Adv. Synth. Catal. 2019,
361, 1124−1129.
̌
́
́
́
Zelonkova, K.; Chorvat, D., Jr; Jancura, D.; Miskovsky, P.; Bano, G. J.
Lumin. 2016, 177, 17−21.
(18) (a) Zhu, X.; Li, P.; Shi, Q.; Wang, L. Green Chem. 2016, 18,
6373−6379. (b) Ji, W.; Li, P.; Yang, S.; Wang, L. Chem. Commun.
2017, 53, 8482−8485.
(4) Pasto, D. J.; Cottard, F.; Jumelle, L. J. Am. Chem. Soc. 1994, 116,
8978−8984.
(5) For selected reviews on organic reactions of carbonyl
compounds, see: (a) Lombardo, M.; Trombini, C. Chem. Rev. 2007,
107, 3843−3879. (b) Smith, A. M. R.; Hii, K. K. Chem. Rev. 2011,
111, 1637−1656. (c) Milnes, K. K.; Pavelka, L. C.; Baines, K. M.
Chem. Soc. Rev. 2016, 45, 1019−1035. (d) Holmes, M.; Schwartz, L.
A.; Krische, M. J. Chem. Rev. 2018, 118, 6026−6052. (e) Yamashita,
Y.; Yasukawa, T.; Yoo, W.-J.; Kitanosono, T.; Kobayashi, S. Chem. Soc.
Rev. 2018, 47, 4388−4480. (f) Murray, B. A. Organic Reaction
Mechanisms 2015 2019, 1−71.
́
̀
(19) (a) Denes, F.; Pichowicz, M.; Povie, G.; Renaud, P. Chem. Rev.
2014, 114, 2587−2693. (b) Fang, G.; Bi, X. Chem. Soc. Rev. 2015, 44,
8124−8173. (c) Zheng, Q.-Z.; Jiao, N. Chem. Soc. Rev. 2016, 45,
4590−4627.
(20) For selected reviews on Pummerer reactions, see: (a) Bur, S.
K.; Padwa, A. Chem. Rev. 2004, 104, 2401−2432. (b) Laleu, B.;
Machado, M. S.; Lacour, J. Chem. Commun. 2006, 2786−2788.
(c) Kaiser, D.; Klose, I.; Oost, R.; Neuhaus, J.; Maulide, N. Chem. Rev.
2019, 119, 8701−8780.
(21) (a) Saito, I.; Matsuura, T.; Inoue, K. J. Am. Chem. Soc. 1981,
103, 188−190. (b) Hayyan, M.; Hashim, M. A.; AlNashef, I. M. Chem.
Rev. 2016, 116, 3029−3085.
(6) For selected reviews on photoredox catalysis, see: (a) Nar-
ayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102−
113. (b) Shi, L.; Xia, W. Chem. Soc. Rev. 2012, 41, 7687−7697.
(c) Tucker, J. W.; Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617−
1622. (d) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev.
2013, 113, 5322−5363. (e) Schultz, D. M.; Yoon, T. P. Science 2014,
́
343, 1239176. (f) Afewerki, S.; Cordova, A. Chem. Rev. 2016, 116,
13512−13570. (g) Romero, N. A.; Nicewicz, D. A. Chem. Rev. 2016,
116, 10075−10166. (h) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Chem.
̈
Rev. 2016, 116, 10035−10074. (i) Konig, B. Eur. J. Org. Chem. 2017,
́
2017, 1979−1981. (j) Wang, C.-S.; Dixneuf, P. H.; Soule, J.-F. Chem.
Rev. 2018, 118, 7532−7585.
(7) For selected reviews on silver catalysis, see: (a) Pellissier, H.
Chem. Rev. 2016, 116, 14868−14917. (b) Fang, G.; Cong, X.; Zanoni,
G.; Liu, Q.; Bi, X. Adv. Synth. Catal. 2017, 359, 1422−1502.
(8) Nikolaev, A.; Legault, C. Y.; Zhang, M.; Orellana, A. Org. Lett.
2018, 20, 796−799.
E
https://dx.doi.org/10.1021/acs.orglett.0c01399
Org. Lett. XXXX, XXX, XXX−XXX