Transforming Oxadiazolines through Nitrene Intermediates by Energy Transfer Catalysis: Access to Sulfoximines and Benzimidazoles

Article abstract of DOI:10.1021/acs.orglett.9b04646

Subtle differences in reaction conditions facilitated unprecedented photocatalytic reactions of oxadiazolines by energy transfer catalysis. A set of compounds, sulfoximines and benzimidazoles, were ingeniously prepared from oxadiazolines via nitrene intermediates by photocatalytic N-O/C-N bond cleavages. The synthesis of sulfoximines was realized through intermolecular N-S bond formation between nitrene intermediates and sulfoxides, whereas benzimidazoles were obtained via intramolecular aromatic substitution of the nitrene to the tethered aryl substituent.

Full text of DOI:10.1021/acs.orglett.9b04646

pubs.acs.org/OrgLett
Letter
Transforming Oxadiazolines through Nitrene Intermediates by
Energy Transfer Catalysis: Access to Sulfoximines and
Benzimidazoles
Do Dam Park, Kwan Hong Min, Jihee Kang, Ho Seong Hwang, Vineet Kumar Soni, Cheon-Gyu Cho,
and Eun Jin Cho*
Cite This: https://dx.doi.org/10.1021/acs.orglett.9b04646
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Subtle differences in reaction conditions facilitated
unprecedented photocatalytic reactions of oxadiazolines by energy
transfer catalysis. A set of compounds, sulfoximines and
benzimidazoles, were ingeniously prepared from oxadiazolines via
nitrene intermediates by photocatalytic N−O/C−N bond
cleavages. The synthesis of sulfoximines was realized through
intermolecular N−S bond formation between nitrene intermediates
and sulfoxides, whereas benzimidazoles were obtained via intra-
molecular aromatic substitution of the nitrene to the tethered aryl
substituent.
Scheme 1. Multidimensional Transformation of
Oxadiazolines
he selective formation of multiple valuable products from
Tthe same starting material via careful control of the
chemical reactivity is a highly attractive approach, though it
poses significant synthetic challenges.1 Heterocycles are
ubiquitous structural motifs that have found widespread
applications, including in synthetic chemistry, as synthons for
distinctive functional molecules.2 Oxadiazolines are one such
class of substrates, which provide quinazolinones by N−O bond
activation under thermal conditions and oxygen atmosphere
(Scheme 1a).3 The oxadiazoline moiety can also be converted
into spiro-azalactams in photocatalytic4 dual-energy transfer
pathways via imino radical intermediates.5 Our conducted
density functional theory (DFT) study shows that, in addition to
the known imino radical intermediate generated from oxadiazo-
lines,6 a nitrene intermediate7 can also be generated under
different photocatalytic conditions, particularly in the absence of
reactive oxygen species. Notably, the generation of nitrene from
the aromatic variant (i.e., oxadiazole) under UV irradiation was
reported previously.7a,b Building on the intermediacy of the
nitrene species, we envisioned the generation of other valuable
structural motifs in addition to the quinazolinone and spiro-
azalactam scaffolds by careful control of the reaction conditions
and designed intermolecular and intramolecular reactions to
access highly functional sulfoximines and benzimidazoles
(Scheme 1b). Particularly, the synthetic approach to sulfox-
imines with dimethyl sulfoxide (DMSO)8 as both the solvent
and the reagent is highly appealing due to its low cost, relative
stability, and low toxicity.
a
a
Spin density contours were visualized with an isovalue of 0.05 au.
attractive for improving the physicochemical properties of
molecules, including metabolic stability, and, therefore, have
Received: December 26, 2019
Sulfoximines have gained increasing attention owing to their
remarkable biological activities and successful use as ligands and
auxiliaries in asymmetric synthesis.9 Their structural features are
https://dx.doi.org/10.1021/acs.orglett.9b04646
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
a
Table 1. Optimization Studies
b
yield (%)
entry
photocatalyst
[Ru(bpy)3]Cl2
[Ir(ppy)2(dtbbpy)]PF6
fac-Ir(ppy)3
[Ir{dF(CF3)ppy}2(dtbbpy)]PF6
Pt(ppy)(acac)
Eosin Y
solvent
variations
2a
3a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
DMA
MeCN
DCM
acetone
THF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMA
0
20
62
61
0
0
0
0
0
9
24
21
0
0
0
0
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
no light
79
91
7
37
20
58
20
22
19
35
12
31
71
89
c
CFL (23 W)
61
63
70
52
68
62
d
white LEDs
d
blue LEDs
d
blue LEDs, 60 °C
blue LEDs, 0.1 M
blue LEDs, 0.5 M
blue LEDs, 0.1 M
d
d
d
d
fac-Ir(ppy)3
DMA
blue LEDs, 0.5 M
a
b
c
d
Reaction scale: 0.1 mmol. Yields were determined by gas chromatography using n-dodecane as the internal standard. 48 h. 18 W LEDs were
used.
been of great interest in drug discovery programs.9c
Furthermore, sulfoximine moieties offer hydrogen bond accept-
or/donor properties, which aids the solubility of the molecule.10
In addition, they exhibit the potential for structural diversifica-
tion through nitrogen substitution at the stable stereogenic
sulfur center. The synthesis of sulfoximines is usually realized by
oxidative imidation of sulfoxides and nucleophilic substitutions
of sulfonimidoyl derivatives.11,12 There have also been efforts in
transition-metal-catalyzed N-transfer to sulfoxides via metal
nitrenoid species.10,13 The methodology reported in this study,
consisting of the generation of a nitrene intermediate from
oxadiazolines and its coupling with sulfoxides, is a comple-
mentary and straightforward approach toward sulfoximine
synthesis.
Whereas the reactions of nitrenes are typically carried out in
the presence of an external acceptor, in the absence of one,
intramolecular reactions can proceed depending on the pattern
of substitution. The insertion of a nitrene intermediate to the
tethered arene would proceed to give a benzimidazole, a widely
used structural motif with important biological and electronic
properties.14,15 Compared to the conventional benzimidazole
synthesis that generally involves condensation of a diamino
system, o-phenylenediamines with carbonyl derivatives,14 this
approach starting with a monoamino system is relatively
underdeveloped.16 In this transformation, the highly function-
alized benzimidazoles can be easily synthesized by utilizing a
substituted aryl moiety at the 4-position of 1,2,4-oxadiazoline.
We began the investigation using 1,2,4-oxadiazoline 1a as the
model substrate in DMSO in the presence of either complexes
containing transition metals (Ru, Ir, or Pt) or an organic dye,
Eosin Y, under photoirradiation using a 55 W compact
fluorescent lamp (CFL) (Table 1). To our delight, the desired
N−S coupled product 2a (S,S-dimethyl-N-[phenyl-
(phenylimino)methyl]sulfoximine) was formed along with the
intramolecular C−N coupled benzimidazole 3a, in the presence
of Ir complexes at 0.2 M concentration in DMSO at ambient
temperature under an argon atmosphere (entries 1−6).
Although fac-Ir(ppy)3 and [Ir{dF(CF3)ppy}2(dtbbpy)]PF6
showed similar reactivities, fac-Ir(ppy)3 was chosen for further
studies due to its lower cost and cleaner reaction profile. As
expected, control experiments showed that the reaction requires
the use of a photocatalyst and photon energy (entries 7 and 8).
Notably, removal of the intermolecular coupling partner by
changing DMSO to other organic solvents generated 3a
selectively (entries 9−14). Among the solvents examined,
dimethylacetamide (DMA) showed the best reactivity for the
benzimidazole synthesis and furnished 3a in 91% yield (entry
10). The choice of the photon source also affected the reaction
efficiency, and the reaction with blue LEDs (18 W) produced 2a
in slightly higher yield among the different photon sources
tested (entries 15−17). Interestingly, thermal energy rather
decreased the formation of 2a, generating a higher amount of 3a
even in DMSO (entry 18). Although further optimization of the
reaction concentration was attempted, no improvements in the
reaction efficiency were realized (entries 19−22).
According to the redox potentials of photoexcited fac-
Ir(ppy)3 and 1a, either reductive electron transfer from fac-
Ir(ppy)3* (E*ox, −1.62 V vs SCE) to 1a (Ered, −2.16 V vs SCE)
B
https://dx.doi.org/10.1021/acs.orglett.9b04646
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Figure 1. Potential energy surface calculated at the M062X/6-311++G(d,p) level and proposed mechanism.
or oxidative electron transfer from 1a (Eox, +1.24 V vs SCE) to
fac-Ir(ppy)3* (E*red, +0.16 V vs SCE) is thermodynamically
forbidden.5 To gain better insight into the mechanism for the
transformation, we performed computational studies at the
M062X/6-311++G(d,p) level of DFT energy calculation
(Figure 1a). The time-dependent DFT calculations predicted
the occurrence of triplet−triplet energy transfer (TTET) from
the triplet state of fac-Ir(ppy)3 (ET, 61 kcal/mol) to 1a (ET, 55
kcal/mol).17,18 In addition, the spin density contours of
photoexcited 1a* generated by TTET from fac-Ir(ppy)3
revealed the localization of the spin densities on the N−O
bond of 1a*. The homolytic cleavage of the N−O bond of
photoactivated 1a* followed by C−N bond cleavage generates
the nitrene intermediate INT* and benzaldehyde. The inlet
images in Figure 1a show that the spin densities are located on
nitrene (INT* and INT*rot). In DMSO, INT* couples to
DMSO through TS1, producing sulfoximine 2a. On the other
hand, INT* can undergo rotation to generate the more stable
isomer INT*rot that undergoes intramolecular aromatic
substitution through TS2 to yield the benzimidazole product
3a. Based on these results, the plausible mechanistic pathways
for selective intermolecular N−S bond formation and intra-
molecular C−N bond formation are shown in Figure 1b.
smoothly with DMSO to form the corresponding sulfoximines
in moderate to good yields (2b−2g). The polycyclic naphthyl
substituent (2h) and heteroaryl substituents, including pyridyl
(2i) and furyl (2j), were tolerated under the mild reaction
conditions. To impart diversity to the functionalization of the
sulfoximine moieties, the N-substituents at the 4-position of
oxadiazoline were also varied, and the reactions proceeded to
give the corresponding functionalized sulfoximines (2k−2q).
Unfortunately, the reactions of 1,2,4-oxadiazolines with aliphatic
substituents at the 3- or 4-position did not proceed well, and the
substrates remained unreacted.
To further demonstrate the diversity and potential
applications of the transformation, we attempted the reaction
with other sulfoxide compounds such as di-n-butylsulfoxide and
tetrahydrothiophene-1-oxide (Scheme 3). Under the standard
conditions, the reactions proceeded to give the corresponding
sulfoximine derivatives 4 and 5.
Subsequently, the substrate scope for the intramolecular
process for generating the benzimidazole derivatives was
examined under optimized conditions (Scheme 4). The
oxadiazolines used for the sulfoximine synthesis also underwent
the intramolecular process efficiently in DMA to give the
corresponding benzimidazoles 3. Particularly, the reactions of
oxadiazolines with electron-rich aryl substituents at the 4-
position showed improved reactivity and afforded excellent
yields of the benzimidazole derivatives (3k, 3l, 3r, 3t). Notably,
in the case of 1r, 1s, and 1t, the reactions provided
benzimidazoles as major products even when the reaction was
conducted in DMSO. It is worth mentioning that the reaction of
naphthyl derivative 1t generated the corresponding tricyclic
Next, we turned attention to examine the generality of these
transformations by using oxadiazolines with a variety of
substitutions. The reactions were carried out under the
optimized conditions in DMSO for the sulfoximine synthesis
(Scheme 2). The scope of the substituent R1 at the 3-position in
1,2,4-oxadiazoline 1 was first examined. Oxadiazolines with both
electron-rich and electron-deficient aryl substituents reacted
C
https://dx.doi.org/10.1021/acs.orglett.9b04646
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
a
Scheme 2. Substrate Scope for Sulfoximine Synthesis in
DMSO
Scheme 4. Substrate Scope for Benzimidazole Synthesis
a
a
Reactions were performed on a 0.5 mmol scale under an argon
b
atmosphere, and the isolated yields are reported. On a 3 mmol scale.
c
On a 0.3 mmol scale.
dazoles. Subtle differences in the reaction conditions yielded two
different products in addition to previously reported quinazo-
linone and spiro-azalactam compounds. The reaction proceeded
under mild photocatalytic conditions via Ir-catalyzed energy
transfer, where the triplet state of oxadiazoline underwent
homolytic N−O bond cleavage and concomitant C−N bond
cleavage to form the key nitrene intermediate. Sulfoximines were
selectively formed by intermolecular N−S bond formation
between the nitrene intermediate and sulfoxides, whereas the
intramolecular aromatic substitution of nitrene to the tethered
aryl substituent provided functionalized benzimidazoles. The
easy accessibility of substrates, mild reaction conditions, and
facile access to two different scaffolds will inspire further studies
in discovering the potential of such heterocycles to benefit
synthetic and medicinal chemistry.
a
Reactions were performed on a 0.5 mmol scale under an argon
b
atmosphere, and the isolated yields are reported. On a 3 mmol scale.
c
On a 0.3 mmol scale.
Scheme 3. Sulfoximine Synthesis with Other Sulfoxide
Derivatives
a
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04646.
Experimental details, DFT calculation details, analytical
data of the synthesized compounds, and NMR spectra
(PDF)
a
Reactions were performed on a 0.3 mmol scale under an argon
b
atmosphere, and the isolated yields are reported. Yield was
AUTHOR INFORMATION
Corresponding Author
■
determined by 1H NMR spectroscopy by using bromoform as an
internal standard.
Eun Jin Cho − Department of Chemistry, Chung-Ang University,
Seoul 06974, Republic of Korea; orcid.org/0000-0002-
6607-1851; Email: ejcho@cau.ac.kr
product, 3H-naphtho[1,2-d]imidazole 3t, showing high general-
ity of the process.
In conclusion, the careful selection of reaction conditions
revealed novel reaction pathways to facilitate unprecedented
transformations of oxadiazolines to sulfoximines and benzimi-
Authors
Do Dam Park − Department of Chemistry, Chung-Ang University,
Seoul 06974, Republic of Korea
D
https://dx.doi.org/10.1021/acs.orglett.9b04646
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
(9) (a) Okamura, H.; Bolm, C. Chem. Lett. 2004, 33, 482. (b) Lu
̈
cking,
Kwan Hong Min − Department of Chemistry, Chung-Ang
University, Seoul 06974, Republic of Korea
Jihee Kang − Department of Chemistry, Chung-Ang University,
Seoul 06974, Republic of Korea
Ho Seong Hwang − Department of Chemistry, Chung-Ang
University, Seoul 06974, Republic of Korea
Vineet Kumar Soni − Department of Chemistry, Chung-Ang
University, Seoul 06974, Republic of Korea; orcid.org/0000-
0001-8926-2557
U. Angew. Chem., Int. Ed. 2013, 52, 9399. (c) Sirvent, J. A.; Lu
ChemMedChem 2017, 12, 487.
̈
cking, U.
(10) Zenzola, M.; Doran, R.; Luisi, R.; Bull, J. A. J. Org. Chem. 2015,
80, 6391.
(11) (a) García Mancheno, O.; Bistri, O.; Bolm, C. Org. Lett. 2007, 9,
̃
3809. (b) Collet, F.; Dodd, R. H.; Dauban, P. Org. Lett. 2008, 10, 5473.
(c) Hendriks, C. M. M.; Lamers, P.; Engel, J.; Bolm, C. Adv. Synth.
Catal. 2013, 355, 3363. (d) Wang, J.; Frings, M.; Bolm, C. Angew.
́
Chem., Int. Ed. 2013, 52, 8661. (e) Lebel, H.; Piras, H.; Bartholomeus, J.
̈
Cheon-Gyu Cho − Department of Chemistry, Hanyang
University, Seoul 04763, Republic of Korea; orcid.org/0000-
0003-4851-5671
Angew. Chem., Int. Ed. 2014, 53, 7300. (f) Lebel, H.; Piras, H. J. Org.
Chem. 2015, 80, 3572. (g) Zenzola, M.; Doran, R.; Degennaro, L.; Luisi,
R.; Bull, J. A. Angew. Chem., Int. Ed. 2016, 55, 7203.
(12) Matos, P. M.; Lewis, W.; Moore, J. C.; Stockman, R. A. Org. Lett.
2018, 20, 3674.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.9b04646
̈
(13) (a) Bach, T.; Korber, C. Eur. J. Org. Chem. 1999, 1999, 1033.
(b) Okamura, H.; Bolm, C. Org. Lett. 2004, 6, 1305. (c) Wang, Y.-F.;
Zhu, X.; Chiba, S. J. Am. Chem. Soc. 2012, 134, 3679. (d) Miao, J.;
Richards, N. G. J.; Ge, H. Chem. Commun. 2014, 50, 9687. (e) Bizet, V.;
Hendriks, C. M. M.; Bolm, C. Chem. Soc. Rev. 2015, 44, 3378.
(14) Reviews on synthetic methods for benzimidazoles: (a) Preston,
P. N. Chem. Rev. 1974, 74, 279. (b) Horton, D. A.; Bourne, G. T.;
Smythe, M. L. Chem. Rev. 2003, 103, 893. (c) Candeias, N. R.; Branco,
L. C.; Gois, P. M. P.; Afonso, C. A. M.; Trindade, A. F. Chem. Rev. 2009,
109, 2703.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Chung-Ang University
Graduate Research Scholarship in 2018 for D.D.P. We gratefully
acknowledge the support from National Research Foundation of
Korea (NRF-2014011165 and NRF-2017R1A2B2004082).
́
́
(15) (a) Valdez, J.; Cedillo, R.; Hernandez-Campos, A.; Yepez, L.;
́
́
́
Hernandez-Luis, F.; Navarrete-Vazquez, G.; Tapia, A.; Cortes, R.;
́
Hernandez, M.; Castillo, R. Bioorg. Med. Chem. Lett. 2002, 12, 2221.
(b) Bhattacharya, S.; Chaudhuri, P. Curr. Med. Chem. 2008, 15, 1762.
REFERENCES
■
(1) (a) Iqbal, N.; Jung, J.; Park, S.; Cho, E. J. Angew. Chem., Int. Ed.
2014, 53, 539. (b) Cheng, Q.-Q.; Yedoyan, J.; Arman, H.; Doyle, M. P.
J. Am. Chem. Soc. 2016, 138, 44. (c) Mose, R.; Preegel, G.; Larsen, J.;
Jakobsen, S.; Iversen, E. H.; Jørgensen, K. A. Nat. Chem. 2017, 9, 487.
(d) Liu, Z.; Xu, H.; Yao, T.; Zhang, J.; Liu, L. Org. Lett. 2019, 21, 7539.
(2) (a) Quin, L. D.; Tyrell, J. A. Fundamentals of Heterocyclic
Chemistry: Importance in Nature and in the Synthesis of Pharmaceuticals;
John Wiley & Sons, 2010. (b) Joule, J. A.; Mills, K. Heterocyclic
Chemistry at a Glance, 2nd ed.; John Wiley & Sons, 2012.
́
́
(c) Coban, G.; Zencir, S.; Zupko, I.; Rethy, B.; Gunes, H. S.; Topcu, Z.
Eur. J. Med. Chem. 2009, 44, 2280.
(16) (a) Brasche, G.; Buchwald, S. L. Angew. Chem., Int. Ed. 2008, 47,
1932. (b) Alla, S. K.; Kumar, R. K.; Sadhu, P.; Punniyamurthy, T. Org.
Lett. 2013, 15, 1334. (c) Xu, L.; Wang, L.; Feng, Y.; Li, Y.; Yang, L.; Cui,
X. Org. Lett. 2017, 19, 4343.
(17) The photophysical and electrochemical studies in our previous
work confirmed that 1a is photoactivated by triplet−triplet energy
transfer not by electron transfer; see ref 5.
(3) Wang, Y.-F.; Zhang, F.-L.; Chiba, S. Org. Lett. 2013, 15, 2842.
(4) For some recent reviews on visible-light photocatalysis, see:
(a) Stephenson, C. R. J.; Yoon, T. P.; MacMillan, D. W. C. Visible Light
Photocatalysis in Organic Chemistry; John Wiley & Sons, 2018. (b) Prier,
C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322.
(c) Schultz, D. M.; Yoon, T. P. Science 2014, 343, 1239176. (d) Romero,
(18) Soni, V. K.; Lee, S.; Kang, J.; Moon, Y. K.; Hwang, H. S.; You, Y.;
Cho, E. J. ACS Catal. 2019, 9, 10454.
́
N. A.; Nicewicz, D. A. Chem. Rev. 2016, 116, 10075. (e) Cambie, D.;
̈
Bottecchia, C.; Straathof, N. J. W.; Hessel, V.; Noel, T. Chem. Rev. 2016,
116, 10276. (f) Chen, J.-R.; Hu, X.-Q.; Lu, L.-Q.; Xiao, W.-J. Chem. Soc.
Rev. 2016, 45, 2044. (g) Chatterjee, T.; Iqbal, N.; You, Y.; Cho, E. J. Acc.
Chem. Res. 2016, 49, 2284. (h) Marzo, L.; Pagire, S. K.; Reiser, O.;
̈
Konig, B. Angew. Chem., Int. Ed. 2018, 57, 10034.
(5) Soni, V. K.; Hwang, H. S.; Moon, Y. K.; Park, S.-W.; You, Y.; Cho,
E. J. J. Am. Chem. Soc. 2019, 141, 10538.
(6) Gold-catalyzed reaction of oxadiazolines: Xu, W.; Wang, G.; Sun,
N.; Liu, Y. Org. Lett. 2017, 19, 3307.
(7) Some reactions through nitrene intermediates: (a) Newman, H.
Tetrahedron Lett. 1968, 9, 2417. (b) Vivona, N.; Palumbo Piccionello,
A.; Pibiri, I.; Pace, A.; Angela Raccuglia, R.; Buscemi, S.; Giorgi, G.
Heterocycles 2007, 71, 1529. (c) Peng, H.; Dornevil, K. H.; Draganov, A.
B.; Chen, W.; Dai, C.; Nelson, W. H.; Liu, A.; Wang, B. Tetrahedron
2013, 69, 5079. (d) Bizet, V.; Buglioni, L.; Bolm, C. Angew. Chem., Int.
Ed. 2014, 53, 5639. (e) Gu, Z.-Y.; Liu, Y.; Wang, F.; Bao, X.; Wang, S.-
Y.; Ji, S.-J. ACS Catal. 2017, 7, 3893. (f) Shimbayashi, T.; Sasakura, K.;
Eguchi, A.; Okamoto, K.; Ohe, K. Chem. - Eur. J. 2018, 25, 3156.
(8) (a) Martin, D.; Weise, A.; Niclas, H.-J. Angew. Chem., Int. Ed. Engl.
1967, 6, 318. (b) Soroko, I.; Bhole, Y.; Livingston, A. G. Green Chem.
2011, 13, 162. (c) Jiang, Y.; Loh, T.-P. Chem. Sci. 2014, 5, 4939.
(d) Goldberg, F. W.; Kettle, J. G.; Xiong, J.; Lin, D. Tetrahedron 2014,
70, 6613. (e) Gao, X.; Pan, X.; Gao, J.; Huang, H.; Yuan, G.; Li, Y. Chem.
Commun. 2015, 51, 210. (f) Xu, N.; Zhang, Y.; Chen, W.; Li, P.; Wang,
L. Adv. Synth. Catal. 2018, 360, 1199.
E
https://dx.doi.org/10.1021/acs.orglett.9b04646
Org. Lett. XXXX, XXX, XXX−XXX