Thermoinduced Free-Radical C-H Acyloxylation of Tertiary Enaminones: Catalyst-Free Synthesis of Acyloxyl Chromones and Enaminones

Article abstract of DOI:10.1021/acs.orglett.8b01536

In this paper, the direct acyloxylation of the α-C(sp²)-H bond in tertiary β-enaminones is accomplished under catalyst-free conditions and ambient temperature by using aroyl peroxides as coupling partners. By means of a thermoinduced free-radical pathway, the present method enables facile and efficient synthesis of both acyloxylated chromones and enaminones.

Full text of DOI:10.1021/acs.orglett.8b01536

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Thermoinduced Free-Radical C−H Acyloxylation of Tertiary
Enaminones: Catalyst-Free Synthesis of Acyloxyl Chromones and
Enaminones
Yanhui Guo, Yunfeng Xiang, Li Wei, and Jie-Ping Wan*
College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, P. R. China
S
* Supporting Information
ABSTRACT: In this paper, the direct acyloxylation of the α-
C(sp2)−H bond in tertiary β-enaminones is accomplished
under catalyst-free conditions and ambient temperature by
using aroyl peroxides as coupling partners. By means of a
thermoinduced free-radical pathway, the present method
enables facile and efficient synthesis of both acyloxylated
chromones and enaminones.
hromones are the central backbone of flavonoid natural
products and many synthesized molecules with wide-
accessed via either the elaboration of prior prepared 3-hydroxyl
chromones resulting from the oxidation of unsubstituted
chromones via oxidative C−H/C−C bond cleavage4,8 or the
dehydrogenation/dehydrogenative hydroxylation of function-
alized dihydrochromones.9 Since these methods rely on
chromone or related derivatives as reactants, their broad
utilization is thus hampered by the tedious process required for
preparing those chromone reactants. In this context,
developing alternative methods allowing the synthesis of the
3-oxygen functionalized chromones with simple and easily
available acyclic substrates is currently an urgent issue.
Over the past decade, the direct cross-coupling of the vinyl
C−H bond in NH and NH2-functionalized enaminones has
exhibited widespread application in the synthesis of useful
organic products.10,11 Surprisingly, as a class of special
enaminones, N,N-disubstituted enaminones have received
much less attention as C−H bond donors, probably because
of the weaker HOMO heightening effect of the fully
substituted amino group to the vinyl C−H bond. Traditionally,
transition metal catalysis has been found to be applicable in
functionalizing the vinyl C−H bond of tertiary enaminones.12
It is in rather recent years that the transition-metal-free
operation has found application in the cross-coupling of such a
C−H bond, particularly in the construction of C−S and C−Se
bonds.13 However, as an important chemical fragment, the C−
O bond remains hard to access by a similar metal-free C−H
bond transformation.14 To address this challenge, the
identification of proper oxo reaction partners is obviously the
key point. In light of the free-radical pathway involved in the
previous chromone synthesis based on the enaminone C−H
coupling13c and the many known practical methods on radical-
based C−H functionalization reactions,15 we envisage that an
C
spread applications in clinical medicines, biological screenings,
and functional materials.1 Correspondingly, the synthetic
research toward densely functionalized chromones keeps
receiving extensive interest from the chemical community.2
In particular, C-3 oxygenated chromones such as 3-hydroxyl
chromones and related esters represent an important class of
chromone derivatives showing high merits in the discovery of
bioactive molecules and the designation of functional materials.
For example, the 3-hydroxy-2-(1-phenyl-3-aryl-4-pyrazolyl)
chromones A are promising lead compounds with potent
antifungal activity,3 aryl-linked 3-hydroxyl chromones B are
nucleobases possessing potential application in nucleic acid
labeling,4 and Karanjin (C) is a natural product with potent
mosquito larvicidal activity.5 In addition, Fisetin (D) and
Quercetin (E) are both typical flavonoid natural products with
enriched biological profiles (Figure 1).6 Moreover, 3-hydroxyl
chromone is also known as the central skeleton of many
functional fluorescent molecules, as well as the main precursor
in the synthesis of natural products.7
A survey of the literature indicates that functionalized
chromones such as chromone carboxylates can presently be
Received: May 15, 2018
Figure 1. Representative functional 3-oxygenated chromones.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01536
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
O-centered free-radical precursor would thus be applicable to
design the expected C−O bond forming reaction. With this
inspiration, the frequently utilized aroyl peroxides are thus
elected as the radical precursors.16 Herein, we report the first
example on the synthesis of 3-acyloxylated chromones and
tertiary enaminones via the free-radical-based C−H cross-
coupling of tertiary enaminones by using aroyl peroxide.
Initially, the reaction of o-hydroxyphenyl enaminone 1a and
BPO was tentatively run in DMF, and we were delighted to
find that 3-benzoyloxyl chromone 3a was obtained in 44%
yield by simply heating at 60 °C (entry 1, Table 1). Therefore,
Scheme 1. Enaminone C−H Acyloxylation for Acyloxyl
a b
,
Chromone Synthesis
a
Table 1. Optimization of the Reaction Conditions
b
entry
solvent (mL)
t (°C)
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
DMF
DMF
DMF
DMF
60
50
40
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
44
52
56
60
68
45
10
62
70
65
10
75
83
79
84
88
82
DMSO
toluene
hexane
EL
EtOH
1,4-dioxane
H2O
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
a
General conditions: enaminone 1 (0.5 mmol), peroxide 2 (1 mmol)
b
in EtOH (2 mL). Stirred at room temperature for 6 h. Yield of
c
isolated product based on 1.
d
e
14
df
,
15
16
17
dg
,
enaminone substrate 1a was fixed to react with various aroyl
peroxides 2, corresponding products were given with similarly
good to excellent yields (3i−3o, Scheme 1). As expected, the
reactions employing both components containing additional
substitutions also proceeded well to yield the acyloxylated
products 3p−3s (Scheme 1). No evident effect of the
substituent on either substrate was observed with the present
data, regardless of their distinctive electronic property or
position. As an aliphatic acyl peroxide, the lauroyl peroxide was
also subjected to react with 1a, but only a trace amount of
expected product was observed, probably because of the lower
stability of the corresponding aliphatic carboxyl radical.
dh
,
a
General conditions: 1a (0.2 mmol), 2a (0.2 mmol) in solvent (2.0
b
c
mL) based on 1a, stirred for 12 h. Yield of isolated product. The
amount of 2a was 0.3 mmol. The amount of 2a was 0.4 mmol. The
amount of 2a was 0.5 mmol. The reaction time was 7 h. The
reaction time was 6 h, and benzoic acid was isolated in 81% yield from
d
e
f
g
h
this entry. The reaction time was 5 h.
the reaction was then executed under different temperatures,
which indicated that room temperature was the most favorable
(entries 2−4, Table 1). In addition, parallel reactions in
different organic solvents such as toluene, hexane, ethyl lactate
(EL), EtOH, dioxane, and water disclosed that EtOH was the
best medium (entries 5−11, Table 1). Increasing the loading of
2a led to evident improvement in the yield of 3a (entries 12−
14, Table 1). Further examination on the impact of reaction
time, on the other hand, proved that 6 h was enough to enable
the formation of 3a with satisfactory yield (entries 15−17,
Table 1). Notably, TLC analysis showed no occurrence of
transesterification on product 3a.
In the investigation of synthetic scope, a spectrum of
functionalized o-hydroxylphenyl enaminones 1 and aroyl
peroxides 2 were employed, respectively (product 3b, CCDC
1842991). The results obtained from this section are shown in
Scheme 1. First, when BPO 2a was utilized to react with
enaminones 1 containing different substituents in the phenyl
ring, all related chromone products were provided in excellent
yield (3a−3h, Scheme 1). On the other hand, when
Following the excellent results acquired in the synthesis of 3,
we then turned to further investigate the application of the C−
H functionalization to conventional tertiary enaminones 4.
Delightfully, with the identical operation, direct C−H
acyloxylation proceeded smoothly to provide various aroyloxyl
enaminones 5, demonstrating the good tolerance of this
synthetic protocol to functional substrates (Scheme 2). The
successful synthesis of products of both types 3 and 5
confirmed the broad application of this thermoinduced free-
radical coupling approach in affording divergent acyloxylated
products. However, the experiment employing E-chalcone to
react with BPO showed no transformation of the reactants,
indicating the importance of an amino group in enaminone in
activating the C−H bond for the target reaction.
Because of the simple operation and reaction conditions, we
conducted a scaled-up experiment on the model reaction. As
outlined in eq 1, the gram-scale experiment gave 3a in 72%
B
DOI: 10.1021/acs.orglett.8b01536
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
proposed. As is shown in Scheme 4, the reaction starts from
the thermoinduced O−O bond cleavage providing O-centered
Scheme 2. Enaminone C−H Acyloxylation for the Synthesis
a b
,
of Acyloxylated Enaminones
Scheme 4. Proposed Reaction Mechanism
a
General conditions: enaminone 4 (0.5 mmol), peroxide 2 (1.0
b
mmol) in EtOH (2 mL). Stirred at room temperature for 6 h. Yield
radical species 6. The addition of 6 to enaminone 1 generates
another radical intermediate 7/7′, which undergoes the chain
propagation in the presence of 6 to give intermediate 8. The
elimination of carboxylic acid on 8 yields acyloxylated
enaminones 5. For 2-hydroxylphenyl functionalized enami-
nones, a classical chromone annulation further takes place to
yield products 3.
of isolated product based on 4.
yield, indicating the practicality of the present method in the
large-scale preparation of the chromone scaffold.
In conclusion, we have disclosed the first free-radical-based
C−H bond acyloxylation of tertiary enamionone, which
enables the synthesis of 3-acyloxyl chromones and α-acyloxyl
enaminones with high efficiency. By employing aroyl peroxides
as the coupling reagents, the reactions proceed well at room
temperature without using any catalyst, providing a reliable
route in the synthesis of these useful heterocyclic chromones
and densely functionalized alkenes.
In order to probe the possible reaction pathway, several
control experiments were conducted. First, when the model
reaction was performed under dark conditions, 3a was
provided with an equally excellent yield (eq 2, Scheme 3). In
Scheme 3. Control Experiments
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.8b01536.
General experimental information, procedures for the
1
synthesis of 3 and 5, full characterization data, H and
13C NMR spectra of all products, and the crystal
structure of 3b (PDF)
Accession Codes
CCDC 1842991 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
addition, another entry run at low temperature (−20 °C) gave
the target product with only a trace amount (eq 3, Scheme 3).
The above experiments supported that the reaction was a
thermoinduced transformation. In combination with the result
of the significantly lower product yield from the reaction
employing additional TEMPO (eq 4, Scheme 3), it can be
initially concluded that the reaction proceeds via a
thermoinduced free-radical pathway. Furthermore, the reaction
of chromone with BPO 2a was also conducted under the
standard conditions, wherein 3a was not observed (eq 5,
Scheme 3), suggesting that 5 is unlikely the reactive
intermediate during the product generation.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wanjieping@jxnu.edu.cn.
ORCID
Jie-Ping Wan: 0000-0002-9367-8384
Notes
According to the information provided by the control
experiments, the possible mechanism of these reactions is
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.8b01536
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(11) For metal-free coupling reactions, see: (a) Yuan, Y.; Hou, W.;
Zhang-Negrerie, D.; Zhao, K.; Du, Y. Org. Lett. 2014, 16, 5410.
(b) Zheng, C.; Wang, Y.; Fan, R. Org. Lett. 2015, 17, 916. (c) Sun, J.;
Zhang-Negrerie, D.; Du, Y. Adv. Synth. Catal. 2016, 358, 2035.
(d) Gu, Z.-Y.; Cao, J.-J.; Wang, S.-Y.; Ji, S.-J. Chem. Sci. 2016, 7, 4067.
(e) Yu, W.; Du, Y.; Zhao, K. Org. Lett. 2009, 11, 2417. (f) He, Z.; Liu,
W.; Li, Z. Chem. - Asian J. 2011, 6, 1340. (g) Gao, Y.; Wei, L.; Liu, Y.;
Wan, J.-P. Org. Biomol. Chem. 2017, 15, 4631. (h) Kang, L. S.; Luo,
M.-H.; Lam, C. M.; Hu, L.-M.; Little, R. D.; Zeng, C.-C. Green Chem.
2016, 18, 3767.
(12) For transition-metal-catalyzed functionalization on the C−H
bond of tertiary enaminones, see: (a) Ge, H.; Niphakis, M. J.; Georg,
G. I. J. Am. Chem. Soc. 2008, 130, 3708. (b) Yu, Y.-Y.; Georg, G. I. J.
Org. Chem. 2013, 78, 6163. (c) Chen, F.; Feng, Z.; He, C.-Y.; Wang,
H.-Y.; Guo, Y.-L.; Zhang, X. Org. Lett. 2012, 14, 1176. (d) Akram, M.
O.; Bera, S.; Patil, N. T. Chem. Commun. 2016, 52, 12306. (e) Xiang,
H.; Zhao, Q.; Tang, Z.; Xiao, J.; Xia, P.; Wang, C.; Yang, C.; Chen, X.;
Yang, H. Org. Lett. 2017, 19, 146. (f) Jiang, Y.; Liang, G.; Zhang, C.;
Loh, T.-P. Eur. J. Org. Chem. 2016, 2016, 3326. (g) Joussot, J.;
Schoenfelder, A.; Larquetoux, L.; Nicolas, M.; Suffert, J.; Blond, G.
Synthesis 2016, 48, 3364.
(13) (a) Wan, J.-P.; Zhong, S.; Xie, L.; Cao, X.; Liu, Y.; Wei, L. Org.
Lett. 2016, 18, 584. (b) Zhong, S.; Liu, Y.; Cao, X.; Wan, J.-P.
ChemCatChem 2017, 9, 465. (c) Wan, J.-P.; Zhong, S.; Guo, Y.; Wei,
L. Eur. J. Org. Chem. 2017, 2017, 4401. (d) Siddaraju, Y.; Prabhu, R. J.
Org. Chem. 2017, 82, 3084. (e) Wan, J.-P.; Cao, S.; Hu, C.; Wen, C.
Asian J. Org. Chem. 2018, 7, 328.
(14) Recently, Loh and Jiang et al. reported the C−H acyloxyation
of enaminones using PhI(OAc)2 as the coupling reagent. This method
is thus applicable for the synthesis of MeCOO-functionalized
enaminones: Wang, F.; Sun, W.; Wang, Y.; Jiang, Y.; Loh, T.-P.
Org. Lett. 2018, 20, 1256. For other related examples, see: (b) Liu, X.;
Cheng, R.; Zhao, F.; Zhang-Negrerie, D.; Du, Y.; Zhao, K. Org. Lett.
2012, 14, 5480. (c) Yuan, J.; Zhang, Q.; Yu, M.; Huang, P.; Zhang, R.;
Dong, D. Org. Lett. 2015, 17, 5012.
ACKNOWLEDGMENTS
■
The work is supported by the National Natural Science
Foundation of China (21562025), the Natural Science
Foundation of Jiangxi Province (20161ACB21010), and the
Science Fund for Distinguished Young Scholars in Jiangxi
Province (20162BCB23023).
REFERENCES
■
(1) (a) Sharma, S. K.; Kumar, S.; Chand, K.; Kathuria, A.; Gupta, A.;
Jain, R. Curr. Med. Chem. 2011, 18, 3825. (b) Keri, R. S.; Budagumpi,
S.; Pai, R. K.; Balakrishna, R. G. Eur. J. Med. Chem. 2014, 78, 340.
(c) Grazul, M.; Budzisz, E. Coord. Chem. Rev. 2009, 253, 2588.
(d) Xu, J.; Kjer, J.; Sendker, J.; Wray, V.; Guan, H.; Edrada, R.; Lin,
W.; Wu, J.; Proksch, P. J. Nat. Prod. 2009, 72, 662. (e) Hu, Q.-F.;
Zhou, B.; Gao, X.-M.; Yang, L.-Y.; Shu, L.-D.; Shen, Y.; Li, G.-P.; Che,
C.-T.; Yang, G.-Y. J. Nat. Prod. 2012, 75, 1909.
(2) For selected examples, see: (a) Zhao, J.; Zhao, Y.; Fu, H. Angew.
Chem., Int. Ed. 2011, 50, 3769. (b) Friden-Saxin, M.; Pemberton, N.;
da Silva Andersson, K.; Dyrager, C.; Friberg, A.; Grøtli, M.; Luthman,
́
́
́
K. J. Org. Chem. 2009, 74, 2755. (c) Dahlen, K.; Wallen, E. A. A.;
Grøtli, M.; Luthman, K. J. Org. Chem. 2006, 71, 6863. (d) Anwar, H.
F.; Hansen, T. V. Org. Lett. 2009, 11, 587. (e) Shen, C.; Li, W.; Yin,
H.; Spannenberg, A.; Skrydstrup, T.; Wu, X.-F. Adv. Synth. Catal.
2016, 358, 466. (f) Shimizu, M.; Tsurugi, H.; Satoh, T.; Miura, M.
Chem. - Asian J. 2008, 3, 881. (g) Zha, J.; Zhao, Y.; Fu, H. Org. Lett.
2012, 14, 2710. (h) Zhu, F.; Wang, Z.; Li, Y.; Wu, X.-F. Chem. - Eur. J.
2017, 23, 3276.
(3) Prakash, O.; Kumar, R.; Parkash, V. Eur. J. Med. Chem. 2008, 43,
435.
(4) Spadafora, M.; Postupalenko, V. Y.; Shvadchak, V. V.;
́
Klymchenko, A. S.; Mely, Y.; Burger, A.; Benhida, R. Tetrahedron
2009, 65, 7809.
(5) Satyavani, S. R.; Kanjilal, S.; Rao, M. S.; Prasad, R. B. N.;
Murthy, U. S. N. Med. Chem. Res. 2015, 24, 842.
(15) (a) Yi, H.; Zhang, G.; Wang, H.; Huang, Z.; Wang, J.; Singh, A.
K.; Lei, A. Chem. Rev. 2017, 117, 9016. (b) Song, R.-J.; Liu, Y.; Xie,
Y.-X.; Li, J.-H. Synthesis 2015, 47, 1195. (c) Honraedt, A.; Le
Grognec, E.; Jacquemin, D.; Felpin, F.-X. Chem. Commun. 2014, 50,
5236. (d) Morofuji, T.; Shimizu, A.; Yoshida, J.-i. Angew. Chem., Int.
Ed. 2012, 51, 7259. (e) Xu, H.; Liu, P.-T.; Li, Y.-H.; Han, F.-S. Org.
Lett. 2013, 15, 3354. (f) Liu, X.; Yu, L.; Luo, M.; Zhu, J.; Wei, W.
Chem. - Eur. J. 2015, 21, 8745. (g) Zhang, L.; Liu, D.; Liu, Z. Org. Lett.
2015, 17, 2534. (h) Li, P.; Zhao, J.; Xia, C.; Li, F. Org. Chem. Front.
2015, 2, 1313. (i) Jiang, Q.; Xu, B.; Zhao, A.; Jia, J.; Liu, T.; Guo, C. J.
Org. Chem. 2014, 79, 8750. (j) Zhou, R.; Liu, H.; Tao, H.; Yu, X.; Wu,
J. Chem. Sci. 2017, 8, 4654. (k) Hu, M.; Fan, J.-H.; Liu, Y.; Ouyang,
X.-H.; Song, R.-J.; Li, J.-H. Angew. Chem., Int. Ed. 2015, 54, 9577.
(l) Achar, T. K.; Mal, P. J. Org. Chem. 2015, 80, 666. (m) Ahmed, J.;
Sreejyothi, P.; Vijaykumar, G.; Jose, A.; Raj, M.; Mandal, S. K. Chem.
Sci. 2017, 8, 7798.
(6) (a) Bayat, A.; Fattahi, A. J. Phys. Org. Chem. 2017, 30, e3692.
(b) Jo, W.-R.; Park, J.-J. J. Nutr. Biochem. 2017, 48, 103. (c) Maher, P.
Food Funct. 2017, 8, 3033. (d) Tang, Q.; Ji, F.; Wang, J.; Guo, L.; Li,
Y.; Bao, Y. Eur. J. Pharm. Sci. 2017, 109, 223. (e) Ren, J.; Fang, Z.;
Jiang, J.; Du, Q. J. Liposome Res. 2017, 27, 335.
(7) (a) Dziuba, D.; Karpenko, I. A.; Barthes, N. P. F.; Michel, B. Y.;
́
Klymchenko, A. S.; Benhida, R.; Demchenko, A. P.; Mely, Y.; Burger,
A. Chem. - Eur. J. 2014, 20, 1998. (b) Li, X.; Li, J.; Dong, X.; Gao, X.;
Zhang, D.; Liu, C. Sens. Actuators, B 2017, 245, 129. (c) Perveaux, A.;
Lorphelin, M.; Lasorne, B.; Lauvergnat, D. Phys. Chem. Chem. Phys.
2017, 19, 6579. (d) Nath, A.; Mal, J.; Venkateswaran, R. V. J. Org.
Chem. 1996, 61, 4391.
(8) (a) Krow, G. R. The Baeyer-Villiger Oxidation of Ketones and
Aldehydes. Organic Reactions 1993, 43, 251. (b) Constantino, M. G.;
́
Junior, V. L.; da Silva, G. V. J. J. Heterocycl. Chem. 2003, 40, 369.
(c) Liu, G.-B.; Xu, J.-L.; Geng, M.; Xu, R.; Hui, R.-R.; Zha, J.-W.; Xu,
(16) For recent references employing aroyl peroxide as radical
precursors, see: (a) Li, Y.; Ge, L.; Muhammad, M. T.; Bao, H.
Synthesis 2017, 49, 5263. (b) Zhu, N.; Zhao, J.; Bao, H. Chem. Sci.
2017, 8, 2081. (c) Chen, M.; Li, Y.; Tang, H.; Ding, H.; Wang, K.;
Yang, L.; Li, C.; Gao, M.; Lei, A. Org. Lett. 2017, 19, 3147. (d) Pan,
C.; Fu, Y.; Ni, Q.; Yu, J.-T. J. Org. Chem. 2017, 82, 5005. (e) Yu, W.
Y.; Sit, W. N.; Zhou, Z.; Chan, A. S. C. Org. Lett. 2009, 11, 3174.
(f) Li, D.; Xu, N.; Zhang, Y.; Wang, L. Chem. Commun. 2014, 50,
14862. (g) Pan, C.; Zhang, H.; Han, J.; Cheng, Y.; Zhu, C. Chem.
Commun. 2015, 51, 3786. (h) Qian, C.; Lin, D.; Deng, Y.; Zhang, X.-
Q.; Jiang, H.; Miao, G.; Tang, X.; Zeng, W. Org. Biomol. Chem. 2014,
12, 5866. (i) Mo, J.; Wang, L.; Cui, X. Org. Lett. 2015, 17, 4960.
(j) Zhou, Z.; Cheng, J.; Yu, J.-T. Org. Biomol. Chem. 2015, 13, 9751.
(k) Vinayak, B.; Navyasree, P.; Chandrasekharam, M. Org. Biomol.
Chem. 2017, 15, 9200.
Q.; Xu, H.-X.; Li, J.-X. Bioorg. Med. Chem. 2010, 18, 2864.
̈
(d) Kurzwernhart, A.; Kandioller, W.; Bartel, C.; Bachler, S.;
Trondl, R.; Muhlgassner, G.; Jakupec, M. A.; Arion, V. B.; Marko,
̈
D.; Keppler, B. K.; Hartinger, C. G. Chem. Commun. 2012, 48, 4839.
(9) (a) Irgashev, R. A.; Sosnovskikh, V. Y.; Sokovnina, A. A.;
̈
Roschenthaler, G.-V. J. Heterocycl. Chem. 2010, 47, 944. (b) Moriarty,
R. M.; Prakash, O. Oxidation of carbonyl compounds with
organohypervalent iodine reagents. Organic Reactions 1999, 54, 273.
(10) For transition-metal-catalyzed reactions, see: (a) Wurtz, S.;
̈
̈
Rakshit, S.; Neumann, J. J.; Droge, T.; Glorius, F. Angew. Chem., Int.
Ed. 2008, 47, 7230. (b) Zoller, J.; Fabry, D. C.; Ronge, M. A.;
Rueping, M. Angew. Chem., Int. Ed. 2014, 53, 13264. (c) Bernini, R.;
Fabrizi, G.; Sferrazza, A.; Cacchi, S. Angew. Chem., Int. Ed. 2009, 48,
8078. (d) Gu, Z.-Y.; Zhu, T.-H.; Cao, J.-J.; Xu, X.-P.; Wang, S.-Y.; Ji,
S.-J. ACS Catal. 2014, 4, 49. (e) Tang, S.; Gao, X.; Lei, A. Chem.
Commun. 2017, 53, 3354. (f) Ma, H.; Li, D.; Yu, W. Org. Lett. 2016,
18, 868. (g) Jiang, H.; Huang, W.; Yu, Y.; Yi, S.; Wu, W. Chem.
Commun. 2017, 53, 7473.
D
DOI: 10.1021/acs.orglett.8b01536
Org. Lett. XXXX, XXX, XXX−XXX