Photoredox-Mediated Direct Cross-Dehydrogenative Coupling of Heteroarenes and Amines

Article abstract of DOI:10.1021/acs.orglett.8b02389

A photoredox-mediated direct cross-dehydrogenative coupling reaction to accomplish α-aminoalkylation of N-heteroarenes is reported. This mild reaction has a broad substrate scope, offers the first general method for synthesis of aminoalkylated N-heteroarenes without the need for substrate prefunctionalization, and is scalable to the gram level. Furthermore, the reaction was found to be applicable to other hydrogen donors besides amines (i.e., ethers, an aldehyde, a formamide, p-xylene, and alkanes), thus enabling the preparation of N-heteroarenes bearing various types of substituents.

Full text of DOI:10.1021/acs.orglett.8b02389

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Photoredox-Mediated Direct Cross-Dehydrogenative Coupling of
Heteroarenes and Amines
Jianyang Dong,† Qing Xia,† Xueli Lv,† Changcun Yan,† Hongjian Song,† Yuxiu Liu,†
and Qingmin Wang*,†,‡
†State Key Laboratory of Elemento-Organic Chemistry, Research Institute of Elemento-Organic Chemistry, College of Chemistry,
Nankai University, Tianjin 300071, People’s Republic of China
‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: A photoredox-mediated direct cross-dehydrogenative
coupling reaction to accomplish α-aminoalkylation of N-hetero-
arenes is reported. This mild reaction has a broad substrate scope,
offers the first general method for synthesis of aminoalkylated N-
heteroarenes without the need for substrate prefunctionalization, and
is scalable to the gram level. Furthermore, the reaction was found to
be applicable to other hydrogen donors besides amines (i.e., ethers,
an aldehyde, a formamide, p-xylene, and alkanes), thus enabling the
preparation of N-heteroarenes bearing various types of substituents.
for coupling reactions between generated α-aminoalkyl radicals
and aromatic compounds under mild conditions.8 For example,
MacMillan used photoredox-catalyzed C−H arylation to
synthesize benzylic amines via the coupling of tertiary amines
with cyanoarenes and chloroheteroarenes.9 Subsequently,
MacMillan and Molander reported a protocol for amino-
methylation of aryl halides via Ni/photoredox dual catalysis.
However, this approach involves the use of prefunctionalized
heteroaromatic compounds or amino reagents (Scheme 1a).10
Cowden and Fu reported a decarboxylative α-aminoalkylation
of N-heteroarenes by means of a Minisci-type reaction, but the
substrates are limited to α-amino acids (Scheme 1a).11
he benzylic amine group is present in a wide range of
Tbioactive compounds, including pharmaceuticals, agro-
chemicals, and natural products.1 In addition to the classical
methods for benzylic amine synthesis,2 palladium-catalyzed
cross-coupling of α-metalated amines with aryl electrophiles, as
developed by Molander, Dieter, Campos, Nakamura, and
others, has emerged as an efficient alternative.3−7 However,
this method is limited to aryl halides and prefunctionalized
amino reagents (Scheme 1a). Recently, visible-light-driven
photoredox catalysis has been identified as a suitable method
for generation of α-aminoalkyl radicals, and this method allows
Considering the prevalence of amines and aromatic moieties
in biological systems, the development of a method for direct
cross-dehydrogenative coupling (CDC) of aromatic com-
pounds and amines without prefunctionalization would be
highly desirable.12 Cross-dehydrogenative Friedel−Crafts-type
aminomethylation of arenes in the presence of an oxidant was
first reported in 2005 by Li et al. In the following years,
proelectrophiles such as N,N-dialkylanilines and N-alkylamides
were successfully coupled with a variety of electron-rich arenes,
including indoles, indolizines, furans, and anisoles (Scheme
1b).13 However, the reactions of electron-poor arenes remain
unexplored due to the inherent problem of overoxidation of an
α-aminoalkyl radical to the iminium cation, which cannot react
with electron-deficient heteroaromatics.14 Herein, we describe
a photoredox CDC reaction for selective α-aminoalkylation of
a wide variety of N-heteroarenes with diverse amines under
mild conditions (Scheme 1c). This reaction, which is the first
Scheme 1. α-Arylation of Amines for the Synthesis of
Benzylic Amines
Received: July 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02389
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
method for the synthesis of α-aminoalkylated N-heteroarenes
without prefunctionalization, was also applicable to other
hydrogen donors besides amines, such as ethers, a formamide,
an aldehyde, p-xylene, and alkanes, and thus could be used to
introduce various types of substituents onto N-heteroarenes.
The α-aminoalkylation of benzothiazole (1, 1.0 equiv) with
N-Boc-pyrrolidine (2, 2.0 equiv) was used as a model reaction,
and several solvents, photocatalysts, and oxidants were
evaluated. We were delighted to find that in the presence of
1 mol % of [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 as a photocatalyst,
trifluoroacetic acid (TFA) as a proton source, and tert-butyl
peracetate (t-BPA) as an oxidant, reaction of 1 and 2 in
acetone under irradiation with a 36 W blue LED afforded an
excellent yield of desired product 3 (Table 1, entry 1; see the
Scheme 2. Substrate Scope of the CDC Reaction*
Table 1. Optimization of Conditions for α-Aminoalkylation
a
of Benzothiazole with N-Boc-pyrrolidine
b
entry
photocatalyst
oxidant
yield (%)
c
1
2
3
4
5
6
7
8
9
10
11
Ir[dF(CF3)ppy]2(dtbbpy)PF6
[Ir(dtbbpy)(ppy)2][PF6]
[Ru(bpy)3](PF6)2
Ir[dF(CF3)ppy]2(dtbbpy)PF6
Ir[dF(CF3)ppy]2(dtbbpy)PF6
Ir[dF(CF3)ppy]2(dtbbpy)PF6
Ir[dF(CF3)ppy]2(dtbbpy)PF6
Ir[dF(CF3)ppy]2(dtbbpy)PF6
t-BPA
t-BPA
t-BPA
t-BHP
t-BPB
Na2S2O8
K2S2O8
t-BPA
t-BPA
92 (90)
55
<5
47
45
24
20
<5
<5
<5
<5
d
Ir[dF(CF3)ppy]2(dtbbpy)PF6
Ir[dF(CF3)ppy]2(dtbbpy)PF6
e
t-BPA
a
General conditions: 1 (0.2 mmol), 2 (0.4 mmol), photocatalyst
(0.002 mmol), oxidant (0.4 mmol), TFA (0.4 mmol), and acetone (2
mL) under an argon atmosphere. Abbreviations: t-BHP, tert-
b
butylhydroperoxide; t-BPB, tert-butyl peroxybenzoate. Determined
by 1H NMR spectroscopy using 1,3-benzodioxole as an internal
c
d
standard. Isolated yield. Reaction performed in the absence of light.
e
No TFA.
Supporting Information). Subsequent variation of the photo-
catalyst revealed that [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 was the
most effective catalyst (entries 2 and 3). Other tert-alkyl
peroxides (i.e., tert-butylhydroperoxide and tert-butyl perox-
ybenzoate) gave lower yields (entries 4 and 5). Persulfate is an
inexpensive, environmentally benign oxidant that is often used
in CDC reactions between ethers and heteroaromatics,15 but
we found that sodium persulfate and potassium persulfate had
a deleterious effect on the reaction (entries 6 and 7). Control
experiments showed that the reaction failed to proceed in the
absence of light, photocatalyst, oxidant, or TFA (entries 8−
11).
With the optimized conditions in hand, we investigated the
scope of the reaction with respect to the heteroarene (Scheme
2, upper panel). Various electron-deficient heteroarenes were
readily α-aminoalkylated at the most electrophilic position
with N-Boc-pyrrolidine (2) in good to excellent yields.
Reactions of benzothiazole substrates with electron-with-
drawing or electron-donating substituents proceeded smoothly
with excellent selectivity for the C2 position to afford 3−6 in
50−90% isolated yields. The lower yield of 4 (50%) may have
been the result of lower reactivity of the heteroarene substrate
*
Reactions were performed on a 0.2 mmol scale under the conditions
listed in Table 1, unless otherwise noted; isolated yields are given.
a
b
Ten equivalents of N-Boc-pyrrolidine was used. DMSO as solvent.
c
Diastereomeric ratio (dr) was determined by 1H NMR spectroscopy.
d
N-Boc-fluoxetine (0.2 mmol) and benzothiazole (0.4 mmol) were
used.
toward the nucleophilic α-aminoalkyl radical. Quinoline
substrates with a chloro substituent at C4 were well tolerated,
undergoing reaction selectively at C2 to afford 7 (80% yield).
Reactions of 2 with 4-bromo-, 5-bromo-, 6-bromo-, and 3-
methyl formate substituted isoquinolines afforded products of
selective α-aminoalkylation at C2, along with isoquinoline (8−
12, 40−83% yields). Quinazoline (13, 40%), 1H-benzo[d]-
imidazole (14, 54%), 6-chloroimidazo[1,2-b]pyridazine (15,
66%), 5-bromopyrimidine (16, 35%), and phenanthridine (17,
42%) were also acceptable substrates. In addition, the reaction
was applicable to the commercially available ligand 3,4,7,8-
tetramethyl-1,10-phenanthroline, affording biaminoalkylated
product 18 in good yield (51%). The reaction between 1
B
DOI: 10.1021/acs.orglett.8b02389
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and 2 is easy to upscale to gram scale, with 3 isolated in 84%
yield on a 4 mmol scale.
Scheme 3. Reactions of Other Hydrogen Donors*
Next, we explored the scope of the CDC reaction with
respect to the amine component by using 1 as the heteroarene
(Scheme 2, lower panel). The reaction was amenable to
amines with a wide range of substituents on the nitrogen atom,
including carbamate, amide, and urea groups (giving 3, 19−22,
and 30 in 52−92% yields). Furthermore, cyclic amines of
various ring sizes (azetidine to azepane) afforded desired
products 23−25 in 28−82% yields. An amine substituted with
an N-Boc-proline methyl ester underwent selective arylation at
the 5-methylene position to afford 26 in 58% yield. The CDC
reaction was not restricted to cyclic substrates; reactions of
acyclic amines afforded products 27−31 in 49−65% yields.
When the reaction was carried out with a substrate bearing an
exocyclic α-amino methyl group and an endocyclic methylene
group, completely regioselective α-aminoalkylation at the
former position was observed, affording 32 in 52% yield.
Indoline and 1,2,3,4-tetrahydroquinoline, moieties that are
present in various naturally occurring alkaloids, as well as in
bioactive synthetic compounds (including pharmaceuticals),16
reacted to give corresponding products 33 and 34 in good
yields (48% and 51%, respectively). Interestingly, the use of N-
Boc-morpholine resulted in selective alkylation at the α-
nitrogen atom (35, 46% yield). Finally, we successfully used
the CDC reaction for late-stage functionalization of some drug
molecules. For instance, when N-Boc-fluoxetine was used as a
representative drug molecule for diversification, α-amino-
alkylation derivative was readily accessed (36). Fasudil carrying
a free secondary NH group was selectively alkylated at C1 in
good yield (37). Pentoxifylline with an amide group can be
selectively alkylated at the C2 position (38). Considering the
prevalence of amine and N-heteroarene moieties in bioactive
molecules, this new coupling reaction may prove highly useful
in drug discovery research.17
*
Reactions were performed on a 0.2 mmol scale under the conditions
listed in Table 1, unless otherwise noted; isolated yields are given.
a
b
Thirty equivalents of R-H, 48 h. The ratio of regioisomers was
1
determined by H NMR spectroscopy; C2/C3 = 5:2.
Scheme 4. Use of the CDC Reaction to Prepared a Key
Intermediate in the Synthesis of an Antidiabetic Agent
Scheme 5. Mechanistic Experiments
Having accomplished the α-arylation of amines, we
suspected that other hydrogen donors might be suitable for
the CDC reaction,18 and in fact, we found that ethers were
acceptable substrates (Scheme 3). Reactions of 1 with
tetrahydrofuran, tetrahydropyran, 1,4-dioxane, and 1,2-dime-
thoxyethane afforded desired products 39−42, respectively, in
moderate yields (37−46%). In addition, butyraldehyde and
piperidine-1-carbaldehyde afforded satisfactory yields of
carbonylation products 43 and 44 (95% and 30%,
respectively). Finally, we demonstrated that the reaction was
applicable to alkanes, which afforded alkylation products in
good yields (45−49, 43−60%).
To verify the utility of the CDC reaction for drug synthesis,
we prepared compound 50, a key intermediate in the synthesis
of an antidiabetic agent, by deprotection of product 27 under
acidic conditions (Scheme 4).15f
Having explored the substrate scope and utility of this
photoredox-mediated CDC reaction of heteroarenes and
amines, we turned our attention to the mechanism (Scheme
5). When the radical scavenger 2,2,6,6-tetramethyl-1-piper-
idinyloxy (TEMPO) was present in a reaction mixture
containing 1 and 2, compound 3 was obtained in less than
5% yield (see the SI). Byproduct 51 was observed by mass
spectrometry, suggesting that a tert-butoxy radical had been
generated. When vinyl sulfone 52 was used as a radical
scavenger, a small amount of compound 53, the product of α-
vinylation of the N-Boc-pyrrolidine, was observed by mass
spectrometry, which suggested that an α-aminoalkyl radical
had been generated.19 The reduction potential of t-BPA in
CH3CN was determined to be −1.56 V vs SCE (see the SI),
indicating that direct reduction of t-BPA (E0 = −1.56 V vs
SCE) by Ir*3+ (E1/2
*
= −0.89 V vs SCE) is
IV/ III
thermodynamically disfavored; however, we surmised that
proton-coupled electron transfer under acidic conditions
significantly lowers the barrier to reduction and may be
C
DOI: 10.1021/acs.orglett.8b02389
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Letter
kinetically feasible.20 Stern−Volmer fluorescence quenching
experiments clearly demonstrated this hypothesis, and the
excited state of the photocatalyst can be quenched by t-BPA in
the presence of TFA (no quenching was observed by t-BPA in
the absence of TFA). Moreover, we conducted a light/dark
experiment, which showed that coupling product 3 formed
only under continuous irradiation (Scheme 5). This result
suggests that radical-chain propagation was not involved in the
reaction.
Experimental procedures and spectroscopic data for all
new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wangqm@nankai.edu.cn.
ORCID
Qingmin Wang: 0000-0002-6062-3766
Notes
On the basis of these observations and the literature reports,
we propose the mechanism depicted in Scheme 6. Photo-
The authors declare no competing financial interest.
Scheme 6. Proposed Mechanism
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21732002, 21672117, 21602117) and the Tianjin
Natural Science Foundation (16JCZDJC32400) for generous
financial support for our programs.
REFERENCES
■
(1) (a) Lewis, J. R. Nat. Prod. Rep. 2001, 18, 95. (b) Lin, N.-H.;
Carrera, G. M.; Anderson, D. J. J. Med. Chem. 1994, 37, 3542.
(c) Hauel, N. H.; Nar, H.; Priepke, H.; Ries, U.; Stassen, J.-M.;
Wienen, W. J. Med. Chem. 2002, 45, 1757. (d) Reuveni, M. Eur. J.
Plant Pathol. 2003, 109, 243.
(2) For selected examples, see: (a) Nystrom, R. F.; Brown, W. G. J.
Am. Chem. Soc. 1948, 70, 3738. (b) Soffer, L. M.; Katz, M. J. Am.
Chem. Soc. 1956, 78, 1705. (c) Chandrasekharan, J.; Ramachandran,
P. V.; Brown, H. C. J. Org. Chem. 1985, 50, 5446. (d) Bair, K. W.;
Tuttle, R. L.; Knick, V. C.; Cory, M.; McKee, D. D. J. Med. Chem.
1990, 33, 2385.
(3) (a) Molander, G. A.; Sandrock, D. L. Org. Lett. 2007, 9, 1597.
(b) Molander, G. A.; Gormisky, P. E.; Sandrock, D. L. J. Org. Chem.
2008, 73, 2052. (c) Molander, G. A.; Hiebel, M.-A. Org. Lett. 2010,
12, 4876. (d) Molander, G. A.; Shin, I. Org. Lett. 2011, 13, 3956.
excitation of the [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 photocatalyst
with a blue LED produces a long-lived excited state Ir*3+. The
IV/ III
reduction of t-BPA (E0 = −1.56 V vs SCE) by Ir*3+ (E1/2
*
́
(e) Fleury-Bregeot, N.; Raushel, J.; Sandrock, D. L.; Dreher, S. D.;
= −0.89 V vs SCE) via proton-coupled electron transfer under
acidic conditions affords an oxidized iridium species (Ir4+),
acetic acid, and a tert-butoxy radical.20 α-Aminoalkyl radical A
is then generated by means of hydrogen-atom transfer (HAT)
between the N-protected amine and the tert-butoxy radical.
Radical A then adds to the protonated electron-deficient
heteroarene (1) via a Minisci-type pathway to afford radical
Molander, G. A. Chem. - Eur. J. 2012, 18, 9564. (f) Molander, G. A.;
́
Vargas, F. Org. Lett. 2007, 9, 203. (g) Molander, G. A.; Jean-Gerard,
L. J. Org. Chem. 2007, 72, 8422.
(4) (a) Dieter, R. K.; Li, S. Tetrahedron Lett. 1995, 36, 3613.
(b) Dieter, R. K.; Li, S. J. Org. Chem. 1997, 62, 7726.
(5) (a) Campos, K. R.; Klapars, A.; Waldman, J. H.; Dormer, P. G.;
Chen, C. J. Am. Chem. Soc. 2006, 128, 3538. (b) Klapars, A.; Campos,
K. R.; Waldman, J. H.; Zewge, D.; Dormer, P. G.; Chen, C. J. Org.
Chem. 2008, 73, 4986. (c) Barker, G.; O’Brien, P.; Campos, K. R. Org.
Lett. 2010, 12, 4176. (d) Barker, G.; McGrath, J. L.; Klapars, A.;
Stead, D.; Zhou, G.; Campos, K. R.; O’Brien, P. J. Org. Chem. 2011,
76, 5936.
cation B. Single-electron oxidation of this intermediate by Ir4+
IV/III
(E1/2
= +1.70 V vs SCE in MeCN/H2O = 2:1) and
deprotonation gives the final α-aminoalkylated product 3 and
closes the photoredox cycle.
(6) Yoshikai, N.; Mieczkowski, A.; Matsumoto, A.; Ilies, L.;
Nakamura, E. J. Am. Chem. Soc. 2010, 132, 5568.
In conclusion, we have described the first method for
photoredox-mediated direct CDC to accomplish α-amino-
alkylation of N-heteroarenes by a wide variety of amines. This
mild and efficient reaction was applicable to N-heterocycles
such as benzothiazoles, (iso)quinolones, phenanthridine, and
other heterocyclic derivatives without the need for substrate
prefunctionalization and is scalable to the gram level.
Furthermore, the reaction was also suitable for other hydrogen
donors, such as ethers, a formamide, an aldehyde, p-xylene, and
alkanes. We expect that this reaction will be a useful
complement to existing CDC technologies.
(7) (a) Coldham, I.; Leonori, D. Org. Lett. 2008, 10, 3923. (b) Beng,
T. K.; Gawley, R. E. Org. Lett. 2011, 13, 394. (c) Millet, A.; Dailler,
D.; Larini, P.; Baudoin, O. Angew. Chem., Int. Ed. 2014, 53, 2678.
(8) Shi, L.; Xia, W.-J. Chem. Soc. Rev. 2012, 41, 7687.
(9) (a) McNally, A.; Prier, C. K.; MacMillan, D. W. C. Science 2011,
334, 1114. (b) Prier, C. K.; MacMillan, D. W. C. Chem. Sci. 2014, 5,
4173.
(10) (a) Remeur, C.; Kelly, C. B.; Patel, N. R.; Molander, G. A. ACS
Catal. 2017, 7, 6065. (b) Zuo, Z.-W.; Ahneman, D. T.; Chu, L.;
Terrett, J. A.; Doyle, A. G.; MacMillan, D. W. C. Science 2014, 345,
437. (c) Shaw, M. H.; Shurtleff, V. W.; Terrett, J. A.; Cuthbertson, J.
D.; MacMillan, D. W. C. Science 2016, 352, 1304.
(11) (a) Cowden, C. J. Org. Lett. 2003, 5, 4497. (b) Cheng, W.-M.;
Shang, R.; Fu, Y. ACS Catal. 2017, 7, 907. (c) Proctor, R. S. J.; Davis,
H. J.; Phipps, R. J. Science 2018, 360, 419.
(12) For reviews of cross-dehydrogenative coupling reactions, see:
(a) Girard, S. A.; Knauber, T.; Li, C.-J. Angew. Chem., Int. Ed. 2014,
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02389.
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DOI: 10.1021/acs.orglett.8b02389
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
53, 74. (b) Li, C.-J. Acc. Chem. Res. 2009, 42, 335. (c) Yeung, C. S.;
Dong, V. M. Chem. Rev. 2011, 111, 1215.
(13) For selected examples, see: (a) Li, Z.; Li, C.-J. J. Am. Chem. Soc.
2005, 127, 6968. (b) Dai, C.; Meschini, F.; Narayanam, J. M. R.;
Stephenson, C. R. J. J. Org. Chem. 2012, 77, 4425. (c) Ghobrial, M.;
Harhammer, K.; Mihovilovic, M. D.; Schnurch, M. Chem. Commun.
̈
2009, 46, 8836. (d) Shirakawa, E.; Yoneda, T.; Moriya, K.; Ota, K.;
Uchiyama, N.; Nishikawa, R.; Hayashi, T. Chem. Lett. 2011, 40, 1041.
(e) Liu, P.; Zhou, C.-Y.; Xiang, S.; Che, C.-M. Chem. Commun. 2010,
46, 2739.
(14) For selected examples, see: (a) Miyake, Y.; Nakajima, K.;
Nishibayashi, Y. Chem. - Eur. J. 2012, 18, 16473. (b) Miyake, Y.;
Nakajima, K.; Nishibayashi, Y. J. Am. Chem. Soc. 2012, 134, 3338.
(c) Nakajima, K.; Miyake, Y.; Nishibayashi, Y. Acc. Chem. Res. 2016,
49, 1946. (d) Okugawa, N.; Moriyama, K.; Togo, H. J. Org. Chem.
2017, 82, 170.
(15) For selected examples, see: (a) Jin, J.; MacMillan, D. W. C.
Angew. Chem., Int. Ed. 2015, 54, 1565. (b) Douglas, J. J.; Cole, K. P.;
Stephenson, C. R. J. J. Org. Chem. 2014, 79, 11631. (c) Blum, T. R.;
Zhu, Y.; Nordeen, S. A.; Yoon, T. P. Angew. Chem., Int. Ed. 2014, 53,
11056. (d) Cheng, P.; Qing, Z.; Liu, S.; Liu, W.; Xie, H.; Zeng, J.
Tetrahedron Lett. 2014, 55, 6647. (e) Dai, C.; Meschini, F.;
Narayanam, J. M. R.; Stephenson, C. R. J. J. Org. Chem. 2012, 77,
4425. (f) Zhang, Y. − Q.; Teuscher, K. B.; Ji, H.-T. Chem. Sci. 2016, 7,
2111.
(16) For selected examples, see: (a) Zi, W.; Zuo, Z.; Ma, D. Acc.
Chem. Res. 2015, 48, 702. (b) Kato, D.; Sasaki, Y.; Boger, D. L. J. Am.
Chem. Soc. 2010, 132, 3685. (c) Wang, N.; Du, S.; Li, D.; Jiang, X.
Org. Lett. 2017, 19, 3167. (d) Katritzky, A. R.; Rachwal, S.; Rachwal,
B. Tetrahedron 1996, 52, 15031. (e) Glushkov, V. A.; Tolstikov, A. G.
Russ. Chem. Rev. 2008, 77, 137. (f) Sridharan, V.; Suryavanshi, P. A.;
Menendez, J. C. Chem. Rev. 2011, 111, 7157.
(17) Cernak, T.; Dykstra, K. D.; Tyagarajan, S.; Vachal, P.; Krska, S.
W. Chem. Soc. Rev. 2016, 45, 546.
(18) (a) Devari, S.; Shah, B. A. Chem. Commun. 2016, 52, 1490.
(b) He, T.; Yu, L.; Zhang, L.; Wang, L.; Wang, M. Org. Lett. 2011, 13,
5016. (c) Liu, S.; Liu, A.; Zhang, Y.; Wang, W. Chem. Sci. 2017, 8,
4044. (d) Correa, A.; Fiser, B.; Bengoa, E. G. Chem. Commun. 2015,
51, 13365. (e) Xie, Z.; Cai, Y.; Hu, H.; Lin, C.; Jiang, J.; Chen, Z.;
Wang, L.; Pan, Y. Org. Lett. 2013, 15, 4600.
(19) (a) Noble, A.; MacMillan, D. W. C. J. Am. Chem. Soc. 2014,
136, 11602. (b) Qing, Z.-X.; Cao, H.-L.; Cheng, P.; Wang, W.; Zeng,
J. − G.; Xie, H.-Q. Org. Chem. Front. 2018, 5, 353.
(20) (a) Tarantino, K. T.; Liu, P.; Knowles, R. R. J. Am. Chem. Soc.
2013, 135, 10022. (b) DiRocco, D. A.; Dykstra, K.; Krska, S.; Vachal,
P.; Conway, D. V.; Tudge, M. Angew. Chem., Int. Ed. 2014, 53, 4802.
E
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