Iron(III)-catalyzed aerobic oxidation for the synthesis of 1-benzoxazolyl-o-carboranes

Article abstract of DOI:10.1016/j.jorganchem.2021.121881

A FeCl₃ catalyzed tandem condensation/cyclization/aerobic oxidation process for synthesis of 1-benzoxazolyl-o-carboranes has been developed. The degradation of o-carborane in the presence of aza-/oxa-nucleophiles was completely suppressed, and a series of 1-benzoxazolyl-o-carboranes, 1-benzothiazolyl-o-carborane and 1-benzimidazolyl-o-carborane have been synthesized with good to excellent yields. This work offers a general protocol for synthesis of 1-benzoxazolyl-o-carboranes, which has important reference for synthesis of aromatic heterocycle-carborane derivatives and promote its applications in miscellaneous areas.

Full text of DOI:10.1016/j.jorganchem.2021.121881

Journal of Organometallic Chemistry 945 (2021) 121881
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Iron(III)-catalyzed aerobic oxidation for the synthesis of
1-benzoxazolyl-o-carboranes
Ji Wua,b, Ke Caob,∗, Cai-Yan Zhangb, Xin-Yu Wen b, Bo Lia,∗∗, Junxiao Yang b
a Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, P. R. China
b State Key Laboratory of Environment-friendly Energy Materials & School of Material Science and Engineering, Southwest University of Science and
Technology, 59 Qinglong Road, Mianyang, Sichuan, 621010, P. R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
FeCl3 catalyzed tandem condensation/cyclization/aerobic oxidation process for synthesis of 1-
benzoxazolyl-o-carboranes has been developed. The degradation of o-carborane in the presence of
aza-/oxa-nucleophiles was completely suppressed, and series of 1-benzoxazolyl-o-carboranes, 1-
Received 5 March 2021
Revised 30 April 2021
Accepted 6 May 2021
Available online 15 May 2021
a
benzothiazolyl-o-carborane and 1-benzimidazolyl-o-carborane have been synthesized with good to ex-
cellent yields. This work offers a general protocol for synthesis of 1-benzoxazolyl-o-carboranes, which
has important reference for synthesis of aromatic heterocycle-carborane derivatives and promote its ap-
plications in miscellaneous areas.
Keywords:
o-carborane
benzoxazole
Iron
© 2021 Elsevier B.V. All rights reserved.
aerobic oxidation
1. Introduction
tramolecular oxidative cyclization of corresponding amides by Dut-
twyler and our group [35-36], respectively (Scheme 1, b and c).
Carboranes are a class of carbon-boron molecular clusters with
three dimensional aromaticity, which have proved to be the key
framework in designing pharmaceuticals for boron neutron cap-
ture therapy (BNCT) due to its high boron content as well as syn-
thetic flexibility [1-9] Additionally, the nitrogen or oxygen con-
taining carborane derivatives have also found extensive applica-
tions in functional materials [10-16], coordination chemistry and
organometallic chemistry [17-22].
However, the methods for synthesis of closo-dodecaborate amides
and 9-amide-o-carboranes are still a tedious subject as the par-
tial degradation of carborane would occur in the presence of aza-
nucleophiles [37-40].
In connection with our continued interest in selective func-
tionalization of o-carboranes [41-51], we envisioned to synthesis
the 1-benzoxazolyl-o-carborane from 1-formyl-o-carborane and 2-
aminophenol via tandem condensation/cyclization/oxidation pro-
cess (Scheme 1, d). As the o-carborane could be degraded
to nido-carborane in the presence of aza-/oxa-nucleophiles [37-
40], therefore, the condensation of 1-formyl-o-carborane with 2-
aminophenol to form the Schiff base and the following cycliza-
tion should be a fast reaction, and a mild oxidant is needed to
facilitate the oxidation dehydrogenation to form 1-benzoxazolyl-
o-carborane. With this proposal, we consider an aerobic oxidation
should be the optimal protocol to accomplish such a transforma-
tion [52-56]. Herein, we are pleased to present the FeCl3 catalyzed
tandem condensation/cyclization/aerobic oxidation process for syn-
thesis of 1-benzoxazolyl-o-carboranes.
Benzoxazoles are a kind of important building blocks in bioac-
tive natural products, pharmaceutical, agrochemical industries and
functional materials [23-27]. Based on the three dimensional aro-
maticity of carborane analogues to benzene, employing benzoxa-
zole unit into the carborane would offer a class of potential syn-
thons in designing drug candidates and molecular imaging regents
for targeted radionuclide therapy [28-33].
For synthesis of 1-benzoxazolyl-o-carborane, the insertion of
C-H bond of benzoxazole into in situ formed carboryne has
been developed in Xie’s group, and kinds of electron rich aro-
matic heterocycles were well compatible with this transformation
[34] (Scheme 1, a). Later, the closo-dodecaborate fused oxazoles
and nido-carborane fused oxazoles have been synthesized via in-
2. Material and methods
2.1. Generals
∗
Corresponding author.
∗∗
1a-1h were synthesized according to literature methods [57-
58]. Other materials were purchased from Acros, J&K and Al-
this author is a co-corresponding author
E-mail address: caoke@swust.edu.cn (K. Cao).
https://doi.org/10.1016/j.jorganchem.2021.121881
0022-328X/© 2021 Elsevier B.V. All rights reserved.

J. Wu, K. Cao, C.-Y. Zhang et al.
Journal of Organometallic Chemistry 945 (2021) 121881
Scheme 1. Strategies for synthesis of benzoxazole-carborane derivatives.
addin, and used as received unless otherwise specified. All re-
actions under standard conditions were monitored by thin-layer
chromatography (TLC) on gel F254 plates. The silica gel (200-300
meshes) was used for column chromatography, and the distilla-
tion range of petroleum ether was 60-90 °C. 1H NMR, 13C{1H}
and 11 B{1H} NMR spectra were recorded on the Bruker 600MHz
instruments. All 1H NMR and 13C{1H} NMR spectra data are re-
ported in ppm relative to tetramethylsilane (TMS) as internal stan-
dard, and 11 B{1H} NMR spectra data are referenced to external
BF3•Et2O.
2.2. General procedure for synthesis of 1-formyl-2-R-o-carboranes
(1a-1h)
To a 100 mL Schlenk flask were added 1-R-o-carborane (5
mmol), Et2O (30 mL) and cooled to -78°C. Then n-BuLi (5.5 mmol,
1.6 M in hexane, 3.4 mL) was added by syringe and the resulting
mixture was stirred for 1h at -78°C. Subsequently, methyl formate
(2 mL) was added into the mixture and stirred for 2h. The reaction
was quenched with diluted HCl (3M, 2 mL) at -78 °C and slowly
warmed to room temperature. The mixture was extracted with
2

J. Wu, K. Cao, C.-Y. Zhang et al.
Journal of Organometallic Chemistry 945 (2021) 121881
Table 1
a
Optimized conditions for synthesis of 1-benzoxazolyl-o-carborane
entry
catalyst
FeCl3
FeCl3
FeCl3
-
solvent
toluene
toluene
toluene
toluene
toluene
DCE
atmosphere
yield(%)b
75
1
Air
Ar
2
15
3
O2
65
4
Air
Air
Air
Air
Air
Air
Air
Air
0
19c
5
FeCl3
FeCl3
FeCl3
NiCl2
CuCl2
Cu(OTf)2
FeCl3
6
0
7
dioxane
toluene
toluene
toluene
toluene
31
8
57
9
54
10
11
65
80d
a
All reactions were carried out with 0.2 mmol 1a, 0.22 mmol 2a, 10 mol % catalyst
and 1 mL solvent at 110°C for 24 h.
b
Isolated yields.
c
Reacted at 80°C.
d
With 0.2 mmol 2a.
111.2, 77.4, 69.8, 23.8; 11 B{1H} NMR (192 MHz, CDCl3, ppm): δ -
1.3 (2B), -6.7 (2B), -12.3 (6B); HRMS: calculated for C10B10H17 NO−
(M-H)− 274.2241, found 274.2241.
EtOAc for three times (3 × 50 mL) and the combined organic pahse
was dried over anhydrous Na2SO4. After evaporation of the solvent,
the crude product was purified by column chromatography on
200-300 mesh silica gel with petroleum ether:EtOAc = 20:1~10:1.
3. Results and discussion
2.3. General procedure for synthesis of 3a-3k (Take 3a as an
example)
To evaluate the feasibility of this tandem transformation, 1-
formyl-o-carborane and 2-aminophenol were selected as model
substrates to screening conditions. After many efforts, we found
the tandem condensation/cyclization/oxidation process could be
accomplished in the presence of 10 mol % FeCl3 in toluene at
110°C for 24h under air atmosphere, and gave the desired 1-
benzoxazolyl-o-carborane with 75% yield (Table 1, entry 1). Con-
trol experiments demonstrated the FeCl3 was essential for this
reaction, and the oxygen in air was the terminal oxidant and
displayed more efficiency than pure oxygen (Table 1, entries 1-
4). The reaction temperature was a key factor for this trans-
formation, when the reaction was carried out at 80°C, the ex-
pected product was only obtained with 19% yield (Table 1, en-
try 5). Further studies indicated the toluene was more favor-
able for this reaction than 1,2-dichloroethane (DCE) and dioxane
(Table 1, entries 6-7). When the reaction was performed with
nickel or copper salts, the desired product was generated with
moderate yield (Table 1, entries 8-10), this result indicated that
the FeCl3 was the optimal catalyst for this transformation [59].
To our delight, when the loading amount of 2-aminophenol was
reduced to equivalent with 1-formyl-o-carborane, the correspond-
ing product could be enhanced to 80% yield (Table 1, entry 11),
this result demonstrated the o-carborane would be degraded with
excess 2-aminophenol.
To a 10 mL dried flask were sequentially added 1-formyl-
o-carborane (1a, 34.4 mg, 0.2 mmol), toluene (1 mL), 2-
aminophenols (2a, 0.2 mmol) and FeCl3 (3.4 mg, 0.02 mmol) under
air atmosphere and stirred at 110 °C for 24h. Then the mixture was
cooled to room temperature and filtered through a short silica gel
column using ethyl acetate as eluent. After evaporation of the sol-
vent, the residue was purified by column chromatography on 200-
300 mesh silica gel with n-hexane/EtOAc=100:1 (v/v) as eluent
and gave 3a with 80% yield (41.5mg). 1H NMR (600 MHz, CDCl3,
ppm): δ 7.72-7.70 (d, 1H, J = 12Hz), 7.57-7.55 (d, 1H, J = 12Hz),
7.46-7.39 (m, 2H), 4.49 (s, 1H, Cage C-H); 13C{1H} NMR (150 MHz,
CDCl3, ppm): δ 156.0, 151.3, 140.2, 126.8, 125.7, 120.7, 111.1, 65.1,
59.0; 11B{1H} NMR (192 MHz, CDCl3, ppm): δ -2.1(2B), -8.5 (2B), -
10.7 (2B), -11.5 (2B), -12.9 (2B); HRMS: calculated for C9B10H14 NO−
(M-H)− 260.2084, found 260.2087.
2.4. General procedure for synthesis of 4a-4g (Take 4a as an
example)
To a 10 mL dried flask were sequentially added 1-formyl-2-
methyl-o-carborane (1b, 0.2 mmol), toluene (1 mL), 2-aminophenol
(2a, 0.2 mmol, 22 mg) and FeCl3 (3.4 mg, 0.02 mmol) under air at-
mosphere and stirred at 110 °C for 12h, then 20 mol % AgNO3 was
added and reacted for another 12h. Then the mixture was cooled
to room temperature and filtered through a short silica gel column
using ethyl acetate as eluent. After evaporation of the solvent, the
residue was purified by column chromatography on 200-300 mesh
silica gel with petroleum ether/EtOAc=100:1 (v/v) as eluent and
gave 4a with 98% yield (54mg). 1H NMR (600 MHz, CDCl3, ppm): δ
7.81-7.80 (d, 1H, J = 6Hz), 7.60-7.59 (d, 1H, J = 6Hz), 7.49-7.47 (dd,
1H, J = 6Hz), 7.44-7.42 (dd, 1H, J = 6Hz), 1.99 (s, 3H); 13C{1H} NMR
(150 MHz, CDCl3, ppm): δ 154.6, 151.3, 140.6, 127.2, 125.6, 121.2,
With the optimal reaction conditions in hand (Table 1, entry
11), the scope of 2-aminophenols were then examined. As can be
seen from Table 2, the 2-aminophenols substituted with electron
donating group or electron withdrawing group were all compati-
ble with this transformation well and gave the expected products
with good to excellent yields (3a-3h). When the reaction was con-
ducted with 2,2-bis(3-amino-4-hydroxylphenyl)propane, the de-
sired product was obtained with 50% yield (3i). Furthermore, the
o-aminothiophenol and o-phenylenediamine were also applicable
3

J. Wu, K. Cao, C.-Y. Zhang et al.
Journal of Organometallic Chemistry 945 (2021) 121881
Table 2
Synthesis of 1-benzoxazolyl-o-carboranes from 1-formyl-o-carborane with 2-aminophenolsa,b
a
All reactions were carried out with 0.2 mmol 1a, 0.2 mmol 2-aminophenols, 10 mol% FeCl3 and 1 mL toluene
at 110°C for 24 h under air atmosphere.
b
c
Isolated yields. 0.5 equivalent (0.1 mmol) of 2,2-bis(3-amino-4-hydroxylphenyl)propane was used.
Fig. 1. Crystal structure of 4a
to this transformation and the corresponding 1-benzothiazolyl-o-
carborane and 1-benzimidazolyl-o-carborane were afforded with
56% and 73% yields, respectively (3j-3k).
der standard conditions for 12h, 20 mol % AgNO3 was added to
the reaction mixture for additional 12h could facilitate the cycliza-
tion/oxidation process and gave the expected product with 98%
yield (4a). Meanwhile, the exact structure of 4a was unambigu-
ously confirmed by X-ray crystallographic analysis [60] (Figure 1).
Under the improved conditions, the alkyl, aryl and benzyl substi-
tuted 1-formyl-2-R-o-carboranes were compatible with this trans-
formation well and generate the expected products with excellent
yields (Table 3, 4b-4g). For this result, we consider the AgNO3
would be a co-oxidant to accelerate the oxidative dehydrogenation
process.
Subsequently, the scope of 1-formyl-2-R-o-carboranes was then
explored. To our surprise, when the 1-formyl-2-methyl-o-carborane
was subjected to the standard conditions, the reaction was proceed
very sluggish, and only gave the desired product with 31% yield.
Meanwhile, a large amount of Schiff base intermediate (Scheme 1,
d) was isolated and characterized by 1H NMR. To address this
problem, some co-oxidants were examined (SI, Table S1). After
many efforts, we found when the reaction was carried out un-
4

J. Wu, K. Cao, C.-Y. Zhang et al.
Journal of Organometallic Chemistry 945 (2021) 121881
Table 3
Synthesis of 1-benzoxazolyl-o-carboranes from 1-formyl-2-R-o-carboranes with 2-aminophenola,b
a
All reactions were carried out with 0.2 mmol 1-formyl-2-R-o-carboranes, 0.2 mmol 2-aminophenol, 10 mol % FeCl3 and 1
mL solvent at 110°C for 12 h under air atmosphere, then 20 mol% AgNO3 was added and reacted for another 12 h.
b
Isolated yields.
Based on the experimental results and previous works for aer-
obic oxidation [61-64], the FeCl3 promoted condensation of 2-
aminophenol with 1-formyl-o-carborane should take place first.
Then the FeCl3 catalyzed addition of ortho-hydroxy to the Schiff
base would give the benzoxazoline intermediate, which would be
further oxidized into benzoxazole in situ under air atmosphere
with iron catalyst (Scheme 1, d). For 1-formyl-2-R-o-carboranes,
the electron donating effect and steric hindrance of R group at
C2 would restrain the forming of benzoxazoline intermediate, the
AgNO3 would be a co-oxidant to accelerate the oxidative dehydro-
genation of benzoxazoline to benzoxazole.
the Project of State Key Laboratory of Environment-friendly Energy
Materials of SWUST (20fksy03).
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2021.
121881.
References
[1] A.H. Soloway, W. Tjarks, B.A. Barnum, F.G. Rong, J.G. Wilson, The chemistry of
neutron capture therapy, Chem. Rev. 98 (1998) 1515–1562.
4. Conclusions
[2] M.F. Hawthorne, A. Maderna, Applications of Radiolabeled Boron Clusters to
the Diagnosis and Treatment of Cancer, Chem. Rev. 99 (1999) 3421–3434.
[3] F. Issa, M. Kassiou, L.M. Rendina, Boron in drug discovery: carboranes as
unique pharmacophores in biologically active compounds, Chem. Rev. 111
(2011) 5701–5722.
[4] M. Scholz, E. Hey-Hawkins, Carbaboranes as Pharmacophores: Properties, Syn-
thesis, and Application Strategies, Chem. Rev. 111 (2011) 7035–7062.
[5] P. Stockmann, M. Gozzi, R. Kuhnert, M.B. Sárosi, E. Hey-Hawkins, New keys
for old locks: carborane-containing drugs as platforms for mechanism-based
therapies, Chem. Soc. Rev. 48 (2019) 3497–3512.
[6] Z.J. Lesn´ ikowski, Challenges and Opportunities for the Application of Boron
Clusters in Drug Design, J. Med. Chem. 59 (2016) 7738–7758.
[7] D. Gabel, Boron clusters in medicinal chemistry: perspectives and problems,
Pure Appl. Chem. 87 (2015) 173–179.
[8] J.F. Valliant, K.J. Guenther, A.S. King, P. Morel, P. Schaffer, O.O. Sogbein,
K.A. Stephenson, The medicinal chemistry of carboranes, Coord. Chem. Rev.
232 (2002) 173–230.
[9] F. Ali, N.S Hosmane, Y. Zhu, Boron Chemistry for Medical Applications,
Molecules 25 (2020) 828–851.
In conclusion, we have developed
a FeCl3 catalyzed tan-
dem condensation/cyclization/aerobic oxidation process for syn-
thesis of 1-benzoxazolyl-o-carboranes, and the degradation of o-
carborane was completely suppressed. A series of 1-benzoxazolyl-
o-carboranes, 1-benzothiazolyl-o-carborane and 1-benzimidazolyl-
o-carborane have been synthesized with good to excellent yields.
This work offers a general and facile protocol for synthesis of 1-
benzoxazolyl-o-carboranes, which has important reference for syn-
thesis of aromatic heterocycle-carborane derivatives and promote
its applications in miscellaneous areas.
Declaration of Competing Interest
The authors declare no competing financial interests.
Acknowledgment
[10] C. Shi, H. Sun, X. Tang, W. Lv, H. Yan, Q. Zhao, J. Wang, W. Huang, Variable
Photophysical Properties of Phosphorescent Iridium(III) Complexes Triggered
by closo- and nido-Carborane Substitution, Angew. Chem., Int. Ed. 52 (2013)
13434–13438.
[11] H.J. Bae, J. Chung, H. Kim, J. Park, K.M. Lee, T.-W. Koh, Y.S. Lee, S. Yoo, Y. Do,
M.H. Lee, Deep Red Phosphorescence of Cyclometalated Iridium Complexes by
o-Carborane Substitution, Inorg. Chem. 53 (2014) 128–138.
[12] J.C. Axtell, K.O. Kirlikovali, P.I. Djurovich, D. Jung, V.T. Nguyen, B. Munekiyo,
A.T. Royappa, A.L. Rheingold, A.M. Spokoyny, Blue Phosphorescent Zwitterionic
Iridium(III) Complexes Featuring Weakly Coordinating nido-Carborane-Based
Ligands, J. Am. Chem. Soc. 138 (2016) 15758–15765.
This work is supported by National Natural Science Foundation
of China (22071201), Outstanding Youth Talents of Sichuan Science
and Technology Program (2020JDJQ0056), the Opening Project of
Key Laboratory of Drug-Targeting and Drug Delivery System of the
Education Ministry and Sichuan Province (Sichuan University), and
5

J. Wu, K. Cao, C.-Y. Zhang et al.
Journal of Organometallic Chemistry 945 (2021) 121881
[13] X. Li, X. Tong, Y. Yin, H. Yan, C. Lu, W. Huang, Q. Zhao, Using highly emissive
and environmentally sensitive o-carborane-functionalized metallophosphors to
monitor mitochondrial polarity, Chem. Sci. 8 (2017) 5930–5940.
[14] D. Jung, L.A.M. Saleh, Z.J. Berkson, M.F. El-Kady, J.Y. Hwang, N. Mohamed,
A.I. Wixtrom, E. Titarenko, Y. Shao, K. McCarthy, J. Guo, I.B. Martini, S. Kraemer,
E.C. Wegener, P. Saint-Cricq, B. Ruehle, R.R. Langeslay, M. Delferro, J.L. Bros-
mer, C.H. Hendon, M. Gallagher-Jones, J. Rodriguez, K.W. Chapman, J.T. Miller,
rane Derivatives: Entry to Enantiopure Carborane-Amine Combinations, Chem.
Eur. J. 15 (2009) 12030–12042.
[39] W. Neumann, R. Frankand, E. Hey-Hawkins, One-pot synthesis of an indole–
substituted 7,8-dicarba-nido-dodecahydroundecaborate, Dalton Trans 44 (2015)
1748–1753.
[40] Y. Nie, Y. Wang, J. Miao, Y. Li, Z. Zhang, Synthesis and characterization of carbo-
ranyl Schiff base compounds from 1-amino-o-carborane, J. Organomet. Chem.
798 (2015) 182–188.
X. Duan, R.B. Kaner, J.I. Zink, B.F. Chmelka, A.M. Spokoyny,
A molecular
cross-linking approach for hybrid metal oxides, Nat. Mater. 17 (2018) 341–348.
[15] S. Mandal, S. Natarajan, P. Mani, A. Pankajakshan, Post-Synthetic Modifica-
tion of Metal-Organic Frameworks Toward Applications, Adv. Funct. Mater. 31
(2021) 2006291.
[16] L.A. Smyshliaeva, M.V. Varaksin, E.I. Fomina, M.N. Joy, V.A. Bakulev,
V.N. Charushin, O.N. Chupakhin, Cu(I)-Catalyzed Cycloaddition of Vinylacety-
lene ortho-Carborane and Arylazides in the Design of 1,2,3-Triazolyl-Modified
Vinylcarborane Fluorophores, Organometallics 39 (2020) 3679–3688.
[17] N.S. Hosmane, J.A. Maguire, Recent Advances in the Chemistry of Metallacarbo-
ranes, Eur. J. Inorg. Chem. 22 (2003) 3989–3999.
[41] K. Cao, Y. Huang, J. Yang, J. Wu, Palladium catalyzed selective mono-arylation
of o-carboranes via B-H activation, Chem. Commun. 51 (2015) 7257–7260.
[42] J. Wu, K. Cao, T.-T. Xu, X.-J. Zhang, L. Jiang, J. Yang, Y. Huang, Palladium cat-
alyzed regioselective mono-alkenylation of o-carboranes via Heck type cou-
pling reaction of a cage B-H bond, RSC Adv 5 (2015) 91683–91685.
[43] K. Cao, T.-T. Xu, J. Wu, L. Jiang, J. Yang, Palladium catalyzed/silver tuned selec-
tive mono-/tetra-acetoxylation of o-carboranes via B-H activation, Chem. Com-
mun. 52 (2016) 11446–11449.
[44] T.-T. Xu, C.-Y. Zhang, K. Cao, J. Wu, L. Jiang, J. Li, B. Li, J. Yang, Palladium–
Catalyzed Selective Mono-Chlorination of o-Carboranes: Changing the Concept
of FeCl3 from Lewis Acid to Chlorine Source in Carboranes, ChemistrySelect 2
(2017) 3396–3399.
[18] Z. Xie, Group 4 metallocenes incorporating constrained-geometry carboranyl
ligands, Coord. Chem. Rev. 250 (2006) 259–272.
[19] H. Shen, Z. Xie, Atom-Economical Synthesis of N-Heterocycles via Cascade In-
ter-/Intramolecular C-N Bond-Forming Reactions Catalyzed by Ti Amides, J.
Am. Chem. Soc. 132 (2010) 11473–11480.
[45] T.-T. Xu, K. Cao, J. Wu, C.-Y. Zhang, J. Yang, Palladium-Catalyzed Selective
Mono-/Tetraacetoxylation of o-Carboranes with Acetic Acid via Cross Dehydro-
genative Coupling of Cage B-H/O-H Bonds, Inorg. Chem. 57 (2018) 2925–2932.
[46] T.-T. Xu, K. Cao, C.-Y. Zhang, J. Wu, L. Jiang, J. Yang, Palladium catalyzed selec-
tive arylation of o-carboranes via B(4)-H activation: amide induced regioselec-
tivity reversal, Chem. Commun. 54 (2018) 13603–13606.
[20] D. Liu, Z. Qiu, Z. Xie, Synthesis, structure, and reactivity of mixed-sand-
wich zirconacarborane methyl complex (η5-C5Me5) [η1:η5-(Me2NCH2CH2)
C2B9H10 ]ZrMe, J. Organomet. Chem. 822 (2016) 144–153.
[21] R.N. Grimes, in: Carboranes in Catalysis, Carboranes, Elsevier, Oxford, UK, 2016,
pp. 929–944. Third ed..
[22] S.P. Fisher, A.W. Tomich, S.O. Lovera, J.F. Kleinsasser, J. Guo, M.J. Asay, H.M. Nel-
son, V. Lavallo, Nonclassical Applications of closo-Carborane Anions: From
Main Group Chemistry and Catalysis to Energy Storage, Chem. Rev. 119 (2019)
8262–8290.
[47] K. Cao, C.-Y. Zhang, T.-T. Xu, J. Wu, L.-F. Ding, L. Jiang, J. Yang, Palladium cat-
alyzed/counter ion tuned selective methylation of o-carboranes, J. Organomet.
Chem. 902 (2019) 120956.
[48] J. Wu, K. Cao, C.-Y. Zhang, T.-T. Xu, L.-F. Ding, B. Li, J. Yang, Catalytic Ox-
idative Dehydrogenative Coupling of Cage B-H/B-H Bonds for Synthesis of
Bis(o-carborane)s, Org. Lett. 21 (2019) 5986–5989.
[23] M.-J. Don, C.-C. Shen, Y.-L. Lin, W.-J. Syu, Y.-H. Ding, C.-M. Sun, Nitrogen-Con-
taining Compounds from Salvia miltiorrhiza, J. Nat. Prod. 68 (2005) 1066–1070.
[24] S. Singh, G. Veeraswamy, D. Bhattarai, J.-I. Goo, K. Lee, Y. Choi, Recent Advances
in the Development of Pharmacologically Active Compounds that Contain a
Benzoxazole Scaffold, Asian J. Org. Chem. 4 (2015) 1338–1361.
[49] T.-T. Xu, K. Cao, C.-Y. Zhang, J. Wu, L.-F. Ding, J. Yang, Old Key Opens the Lock
in Carborane: The in Situ NHC-Palladium Catalytic System for Selective Aryla-
tion of B(3,6)-H Bonds of o-Carboranes via B-H Activation, Org. Lett. 21 (2019)
9276–9279.
[50] J. Wu, K. Cao, C.-Y. Zhang, T.-T. Xu, X.-Y. Wen, B. Li, J. Yang, Palladium Cat-
alyzed Selective B(3)-H Activation/Oxidative Dehydrogenative Coupling for the
Synthesis of Bis(o-carborane)s, Inorg. Chem. 59 (2020) 17340–17346.
[51] K. Cao, T.-T. Xu, J. Wu, C.-Y. Zhang, X.-Y. Wen, J. Yang, The in situ NHC-Pal-
ladium catalyzed selective activation of B(3,6)- H bonds of o-carboranes for
hydroboration of alkynes: An efficient approach to alkenyl-o-carboranes, Inorg.
Chem. 60 (2021) 1080–1085.
[25] H. Song, C. Rao, Z. Deng, Y. Yu, J.H. Naismith, The Biosynthesis of the Benzox-
azole in Nataxazole Proceeds via an Unstable Ester and has Synthetic Utility,
Angew. Chem. Int. Ed. 59 (2020) 6054–6061.
[26] Y. Zhuang, J.G. Seong, Y.S. Do, H.J. Jo, M.J. Lee, G. Wang, M.D. Guiver, Y.M. Lee,
Effect of Isomerism on Molecular Packing and Gas Transport Properties of
Poly(benzoxazole-co-imide)s, Macromolecules 47 (2014) 7947–7957.
[27] P.-F. Wei, M.-Z. Qi, Z.-P. Wang, S.-Y. Ding, W. Yu, Q. Liu, L.-K. Wang, H.-Z. Wang,
W.-K. An, W. Wang, Benzoxazole-Linked Ultrastable Covalent Organic Frame-
works for Photocatalysis, J. Am. Chem. Soc. 140 (2018) 4623–4631.
[28] M. Couto, C. Alamón, S. Nievas, M. Perona, M.A. Dagrosa, F. Teixidor, P. Cabral,
C. Viñas, H. Cerecetto, Bimodal Therapeutic Agents Against Glioblastoma, One
of the Most Lethal Forms of Cancer, Chem. Eur. J. 26 (2020) 14335–14340.
[29] R. Li, J. Zhang, J. Guo, Y. Xu, K. Duan, J. Zheng, H. Wan, Z. Yuan, H. Chen, Appli-
cation of Nitroimidazole-Carborane-Modified Phenylalanine Derivatives as Du-
al-Target Boron Carriers in Boron Neutron Capture Therapy, Mol. Pharmaceu-
tics 17 (2020) 202–211.
[52] Y.-X. Chen, L.-F. Qian, W. Zhang, B. Han, Efficient aerobic oxidative synthesis of
2-substituted benzoxazoles, benzothiazoles, and benzimidazoles catalyzed by
4-methoxy-TEMPO, Angew. Chem. Int. Ed. 47 (2008) 9330–9333.
[53] A. Patra, A. James, T.K. Das, A.T. Biju, Oxidative NHC-Catalysis for the Genera-
tion of Imidoyl Azoliums: Synthesis of Benzoxazoles, J. Org. Chem. 83 (2018)
14820–14826.
[54] O. Koleda, T. Broese, J. Noetzel, M. Roemelt, E. Suna, R. Francke, Synthesis of
Benzoxazoles Using Electrochemically Generated Hypervalent Iodine, J. Org.
Chem. 82 (2017) 11669–11681.
[55] P.B. Gorepatil, Y.D. Mane, V.S. Ingle, Samarium(III) Triflate as an Efficient and
Reusable Catalyst for Facile Synthesis of Benzoxazoles and Benzothiazoles in
Aqueous Medium, Synlett 24 (2013) 2241–2244.
[30] J. Wang, L. Chen, J. Ye, Z. Li, H. Jiang, H. Yan, M.Y. Stogniy, I.B. Sivaev, V.I. Bre-
gadze, X. Wang, Carborane derivative conjugated with gold nanoclusters for
target cancer cell imaging, Biomacromolecules 18 (2017) 1466–1472.
[31] A.F. Armstrong, J.M. Lebert, J.D. Brennan, J.F. Valliant, Functionalized Carbo-
[56] C. Chen, T. Andreani, H. Li, A Divergent and Selective Synthesis of Isomeric
Benzoxazoles from a Single N-Cl Imine, Org. Lett. 13 (2011) 6300–6303.
[57] P. Dozzo, R.A. Kasar, S.B. Kahl, Simple, High-yield methods for the synthesis of
aldehydes directly from o-, m-, and p-Carborane and their further conversions,
Inorg. Chem. 44 (2005) 8053–8057.
rane Complexes of the [M(CO)2(NO)]2+ Core (M
Class of Organometallic Probes for Correlated in Vitro and in Vivo Imaging,
Organometallics 28 (2009) 2986–2992.
=
99mTc, Re):
A New
[32] G. Chen, J. Yang, G. Lu, P.C. Liu, Q. Chen, Z. Xie, C. Wu, One Stone Kills Three
Birds: Novel Boron-Containing Vesicles for Potential BNCT, Controlled Drug Re-
lease, and Diagnostic Imaging, Mol. Pharmaceutics 11 (2014) 3291–3299.
[33] B. C.Das, D.P. Ojha, S. Das, T. Evans, in: Boron Compounds in Molecular
Imaging, Boron-Based Compounds: Potential and Emerging Applications in
Medicine, John Wiley & Sons Ltd, 2018, pp. 205–231. First Ed.
[58] X. Zhang, H. Zheng, J. Li, F. Xu, J. Zhao, H. Yan, Selective Catalytic B-H Arylation
of o-Carboranyl Aldehydes by a Transient Directing Strategy, J. Am. Chem. Soc.
139 (2017) 14511–14517.
[59] S.L. Buchwald, C. Bolm, On the Role of Metal Contaminants in Catalyses with
FeCl3, Angew. Chem. Int. Ed. 48 (2009) 5586–5587.
[60] CCDC number for 4a is 2052604.
[34] D. Zhao, J. Zhang, Z. Xie, Dearomative [2+2] Cycloaddition and Formal C-H
Insertion Reaction of o-Carboryne with Indoles: Synthesis of Carborane-Func-
tionalized Heterocycles, J. Am. Chem. Soc. 137 (2015) 9423–9428.
[61] F. Xiao, Y. Chen, Y. Liu, J. Wang, Sequential catalytic process: synthesis of
quinoline derivatives by AuCl3/CuBr-catalyzed three-component reaction of
aldehydes, amines, and alkynes, Tetrahedron 64 (2008) 2755–2761.
[62] T.P. Forrest, G.A. Douphinee, S.A. Deraniyagala, On the mechanism of dis-
proportionation reactions of 1,2-dihydroquinolines, Can. J. Chem. 63 (1985)
412–417.
[35] Y. Sun, J. Zhang, Y. Zhang, J. Liu, S.V.D. Veen, S. Duttwyler, The closo-Dodecab-
orate Dianion Fused with Oxazoles Provides 3D Diboraheterocycles with Selec-
tive Antimicrobial Activity, Chem. Eur. J. 24 (2018) 10364–10371.
[36] C.-Y. Zhang, K. Cao, T.-T. Xu, J. Wu, L. Jiang, J. Yang,
A
facile approach
[63] J.A. Damavandi, M.A. Zolfigol, B. Karami, Oxidation of 1,2-dihydroquino-
lines under mild and heterogenous conditions, Synth. Commun. 31 (2001)
3183–3187.
[64] K. Cao, F.-M. Zhang, Y.-Q. Tu, X.-T. Zhuo, C.-A. Fan, Iron (III)-Catalyzed and Air–
Mediated Tandem Reaction of Aldehydes, Alkynes and Amines: An Efficient
Approach to Substituted Quinolines, Chem. Eur. J. 15 (2009) 6332–6334.
for the synthesis of nido-carborane fused oxazoles via one pot deborona-
tion/cyclization of 9-amide-o-carboranes, Chem. Commun. 55 (2019) 830–833.
[37] D.-H. Kim, J.H. Won, S.-J. Kim, J. Ko, S.H. Kim, S. Cho, S.O. Kang, Di-
carbollide Analogues of the Constrained-Geometry Polymerization Catalyst,
Organometallics 20 (2001) 4298–4300.
[38] V. Terrasson, J.G. Planas, D. Prim, F. Teixidor, C. Viñas, M.E. Light, M.B. Hurst-
house, General Access to Aminobenzyl-o-carboranes as a New Class of Carbo-
6