Transition-metal-free C(sp3)–H oxidation of diarylmethanes

Article abstract of DOI:10.3390/molecules23081922

An efficient direct C(sp3)–H oxidation of diarylmethanes has been demonstrated by this study. This method employs environment-friendly O₂ as an oxidant and is promoted by commercially available MN(SiMe₃)₂ [M = K, Na or Li], which provides a facile method for the synthesis of various diaryl ketones in excellent yields. This protocol is metal-free, mild and compatible with a number of functional groups on substrates.

Full text of DOI:10.3390/molecules23081922

molecules
Communication
Transition-Metal-Free C(sp3)–H Oxidation
of Diarylmethanes
Fan Yang 1,2, Bihui Zhou 1,2, Pu Chen 1, Dong Zou 1,2, Qiannan Luo 1, Wenzhe Ren 1, Linlin Li 1,
Limei Fan 1 and Jie Li 1,
*
1
Department of Pharmacy, School of Medicine, Zhejiang University City College, No. 48, Huzhou Road,
Hangzhou 310015, China; yfvskd@zju.edu.cn (F.Y.); 18868816170@163.com (B.Z.);
cp15988801862@163.com (P.C.); lijie198455@126.com (D.Z.); luooooooo@163.com (Q.L.);
a71109005@163.com (W.R.); lill@zucc.edu.cn (L.L.); fanlm@zucc.edu.cn (L.F.)
College of Pharmaceutical Sciences, Zhejiang University, Hangzhou 310058, China
Correspondence: lijie@zucc.edu.cn; Tel.: +86-571-8801-6565
2
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 10 July 2018; Accepted: 27 July 2018; Published: 1 August 2018
Abstract: An efficient direct C(sp3)–H oxidation of diarylmethanes has been demonstrated by this
study. This method employs environment-friendly O2 as an oxidant and is promoted by commercially
available MN(SiMe3)2 [M = K, Na or Li], which provides a facile method for the synthesis of various
diaryl ketones in excellent yields. This protocol is metal-free, mild and compatible with a number of
functional groups on substrates.
Keywords: metal-free; diarylmethane; oxidation; ketone
1. Introduction
The oxidation reaction is one of the most important transformations in organic synthesis by
which oxygenated products of hydrocarbons were prepared and these compounds are valuable
structural core in chemical and pharmaceutical industries [1]. Of these transformations, direct oxidation
of methylene group of arylalkanes to ketones have attracted comprehensive attention, as diverse
aryl ketone motifs are important structural units in numerous pharmaceuticals, naturally occurring
molecules and organic functional materials [
as oxidant [ 10], advances in the synthetic method of aryl ketones were mainly based on three
approaches: (1) classical Friedel-Crafts acylation of arenes [11], (2) oxidation of secondary alcohols [12
and CO insertion reactions [13 14]. Recently, significant progress has been made for the formation of
aromatic ketones by transition-metal catalyzed oxidation of alkylarenes [15 17]. Notably, the latter
2–6]. Besides the traditional oxidation using KMnO4
7–
]
,
–
represent a powerful tool in organic synthesis, while inevitably they will suffer from the use of
toxic or expensive metals and ligands, harsh reaction conditions and generation of metal waste in
most cases. Obviously, it is desirable to develop more practical protocols to achieve the synthesis
of aryl ketones without the use of corrosive metal catalysts, hazardous stoichiometric oxidants and
reductants [18–21]. Therefore, transition-metal-free methods for oxidation of alkylarenes are desirable
in the pharmaceutical industry which can avoid the use of the heavy metal. For oxidation process,
Molecular oxygen (O2) represents one of the best choices because of its low cost and has attracted
substantial attention [22–33]. Herein, we wish to report the direct oxidation of diarylmethanes to diaryl
ketones using O2-mediation by MN(SiMe3)2 [M = K, Na or Li], which represent a green and efficient
synthetic method for this transformation.
Molecules 2018, 23, 1922; doi:10.3390/molecules23081922
www.mdpi.com/journal/molecules

Molecules 2018, 23, 1922
2 of 6
2. Results
Initially, we commenced the reaction studies using 4-benzylpyridine (1a) as the model substrate.
The control experiment was performed by stirring 4-benzylpyridine (1a) in THF under O2 at 60 ◦C
for 16 h and no reaction occurred (entry 1, Table 1). To optimize the reaction conditions, various
parameters such as temperature, bases, solvents were investigated. The strong bases play an important
role in this transformation. As indicated in Table 1, the silylamides [MN(SiMe3)2, M = Li, Na, K]
overwhelmed other bases (entries 2–8), giving the oxidative product in 76–85% yields. We anticipated
that the inert sp3 C–H bond was transferred to carbanion before the direct insertion of O2, which needs
the strong bases to achieve the deprotonation step. The solvent was next examined using LiN(SiMe3)2
as a base. Of the six solvents screened, THF showed the best performance (entries 4 and 9–13). Further
screen of the reaction temperature indicated 60 ◦C is most appropriate. Only 35% yield of oxidation
◦
product 2a was obtained at 40 C after 12 h (entry 14), while no elevated yield was observed when the
reaction was conducted in higher temperature 80 ◦C (entry 15). Furthermore, there is no oxidation
product if the O2 was replaced with N2.
Table 1. Optimization of the reaction conditions.
O
base (1.2 eq)
solvent, 60 oC,
N
N
O2 (1 atm)
1a
2a
◦
Entry a
Base
Solvent
Temp [ C]
Yield [%] b
◦
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16 c
–
THF
THF
THF
THF
THF
THF
THF
THF
dioxane
toluene
DME
CPME
CH2Cl2
THF
60 C
–
76
79
85
11
6
◦
KHMDS
NaHMDS
LiHMDS
KOtBu
60 C
◦
60 C
◦
60 C
◦
60 C
◦
NaOtBu
60 C
◦
LiOtBu
60 C
trace
–
◦
CS2CO3
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
LiHMDS
60 C
◦
60 C
23
15
79
56
11
84
35
–
◦
60 C
◦
60 C
◦
60 C
◦
60 C
◦
80 C
◦
THF
THF
40 C
◦
60 C
a
Reactions were carried out using 4-benzylpyridine (0.1 mmol) and base (0.15 mmol) in anhydrous solvent (1.0 mL)
b
c
under O2 for 12 h. Isolated yields. O2 was replaced with N2.
With the optimal condition in hand, we then turn our attention to investigate the generality
of this protocol. As presented in Figure 1, a variety of diarylmethanes were subjected to these
reaction conditions. These reactions were conducted at 60 ◦C, except where noted. Diarylmethanes
with different electronic and steric properties, such as 2-benzylpyridine (1b), diphenylmethane (1c),
xanthene (1d), 9,10-dihydroanthracene (1e) and fluorene (1f), were proved to be good substrates,
with corresponding products isolated in 79–91% yields. The family of fluorene analogues bearing
various functional groups was examined next. The carbon-halo (electron-withdrawing) groups were
compatible with this procedure and the oxidation products were obtained in excellent isolated yields
(2h, 2i, 2j, 2k). Additionally, remarkable chemoselectivity is observed with fluorene derivatives containing
acetal, nitro and amino moieties, which all underwent oxygenation delivering the corresponding
functionalized products (2l, 2m, 2n) in 81–87% yields.

Molecules 2018, 23, 1922
3 of 6
To illustrate the scalablitlity of this methodology, we commenced the oxidation reaction of
4-benzylpyridine (1a) on a 5 mmol scale. The oxidation product 2a was isolated in 80% yield (0.73 g).
Figure 1. Scope of diarylmethanes in metal-free oxidation a,b
.
a Reactions were conducted on a 0.1 mmol
c
scale using 1 equiv of 1, 1.5 equiv of LiN(SiMe3)2 at 0.1 M. b Isolated yield. Reaction conducted◦on
d
5 mmol scale. LiHMDS was replaced with NaHMDS. e LiHMDS was replaved with KHMDS. f 80 C.
3. Materials and Methods
3.1. General
1H- and 13C-NMR spectra were obtained on Bruker AVANCE III 500 MHz and 600 MHz
spectrometers (Bruker Co., Billerica, MA, USA) with TMS as the internal standard; MS spectra were
measured on a Finnigan LCQDECA XP instrument and an Agilent Q-TOF 1290 LC/6224 MS system
(Santa Clara, CA, USA); silica gel GF254 and H (10–40 mm, Qingdao Marine Chemical Factory, Qingdao,
China) were used for TLC and CC. Unless otherwise noted, all reactions were carried out under an
atmosphere of oxygen.
3.2. Representative Procedure for the Oxidation of Diarylmethane
To an 8 mL oven-dried vial, 4-benzylpyridine (0.1 mmol), dry THF (1 mL), LiHMDS (0.15 mmol)
were added subsequently. The reaction system was sealed by a rubber septum with a needle connected
with O2 balloon. After stirring at 60 ◦C for 12 h, the reaction mixture was passed through a short pad
of silica gel and eluted with ethyl acetate (1 mL
×
3). The combined organics were concentrated under
reduced pressure. The residue was purified by flash chromatography to give the diarylketone 2a as
1
white solid (15.6 mg, 85% yield). H NMR (500 MHz, CDCl3)
δ
7.65 (d, J = 7.3 Hz, 2H), 7.54–7.43 (m, 4H),
7.33–7.25 (m, 2H). The 1H NMR data of 2a was identical to those reported in the literature [34].
Analogous compounds 2b were prepared according to the similar procedure for 2a
1H NMR (500 MHz, CDCl3)
–
n
. 2b:
δ
8.73 (d, J = 4.1 Hz, 1H), 8.10–8.03 (m, 3H), 7.90 (td, J = 7.8, 1.7 Hz,
1
1H), 7.60 (t, J = 7.4 Hz, 1H), 7.52–7.44 (m, 3H). The H NMR data of 2b was identical to those

Molecules 2018, 23, 1922
4 of 6
reported in the literature [34]. 2c
:
1H NMR (500 MHz, CDCl3)
δ
7.72 (dd, J = 5.2, 3.2 Hz, 2H),
1
7.54–7.47 (m, 1H), 7.43–7.36 (m, 2H). The H NMR data of 2c was identical to those reported in
1
the literature [35]. 2d: H NMR (500 MHz, CDCl3)
δ 8.34 (d, J = 7.9 Hz, 2H), 7.73 (t, J = 7.7 Hz, 2H),
7.49 (d, J = 8.4 Hz, 2H), 7.38 (t, J = 7.5 Hz, 2H). The 1H NMR data of 2d was identical to those reported in
the literature [36]. 2e: 1H NMR (500 MHz, CDCl3):
8.26–8.37 (m, 2H), 7.56 (t, J = 7.4 Hz, 2H), 7.36–7.47
(m, 4H), 4.32 (s, 2H). The 1H NMR data of 2e was identical to those reported in the literature [37]. 2f
1H NMR (500 MHz, CDCl3) 7.65 (d, J = 7.3 Hz, 2H), 7.54–7.43 (m, 4H), 7.33–7.25 (m, 2H). The 1H
NMR data of 2f was identical to those reported in the literature [36]. 2g: H NMR (500 MHz, CDCl3):
8.14 (s, 1H), 7.88 (s, 1H), 7.85 (d, J = 7.5 Hz, 1H), 7.80 (d, J = 10.50 Hz, 1H), 7.73 (d, J = 9.5 Hz, 1H),
7.69 (d, J = 9.5 Hz, 1H), 7.75–7.55 (m, 2H), 7.45 (t, J = 9.2 Hz, 1H), 7.32 (t, J = 9.2 Hz, 1H). The 1H NMR
data of 2g was identical to those reported in the literature [38]. 2h
1H NMR (500 MHz, CDCl3):
δ
:
δ
1
δ
:
δ 7.78 (s, 1H), 7.68 (d, J = 7.3 Hz, 1H), 7.62 (d, J = 7.8 Hz, 1H), 7.53–7.52 (m, 2H), 7.41 (d, J = 7.8 Hz, 1H),
7.35–7.34 (m, 1H). The 1H NMR data of 2h was identical to those reported in the literature [39]. 2i: 1H NMR
(500 MHz, CDCl3):
δ
7.66 (d, J = 1.8 Hz, 2H), 7.52 (dd, J = 7.9 Hz, 1.8 Hz, 2H), 7.28 (d, J = 7.9 Hz, 2H).
1
1
The H NMR data of 2i was identical to those reported in the literature [39]. 2j: H NMR (500 MHz,
CDCl3)
δ
7.62 (d, J = 1.8 Hz, 2H), 7.47 (dd, J = 8.0, 2.0 Hz, 2H), 7.44 (d, J = 7.9 Hz, 2H). The 1H NMR data
1
of 2j was identical to those reported in the literature [40]. 2k: H NMR (500 MHz, CDCl3)
δ 7.96
(d, J = 1.6 Hz, 1H), 7.84 (dd, J = 7.8, 1.6 Hz, 1H), 7.76 (d, J = 1.8 Hz, 1H), 7.63 (dd, J = 7.9, 1.9 Hz, 1H),
7.39 (d, J = 7.9 Hz, 1H), 7.28 (s, 1H). The 1H NMR data of 2k was identical to those reported in the
1
literature [41]. 2l: H NMR (500 MHz, CDCl3)
δ
8.17 (d, J = 0.98 Hz, 1H), 8.13 (dd, J = 7.8, 2.0 Hz, 1H),
7.70 (d, J = 7.3 Hz, 1H), 7.63–7.51 (m, 3H), 7.40–7.35 (m, 1H), 2.63 (s, 3H). The 1H NMR data of 2l was
identical to those reported in the literature [42]. 2m: H NMR (500 MHz, CDCl3):
1
δ 8.48 (d, J = 1.9 Hz,
1H), 8.43 (dd, J = 1.9 Hz, 8.2 Hz, 1H), 7.77 (d, J = 7.3 Hz, 1H), 7.72–7.67 (m, 2H), 7.62 (t, 1H), 7.46 (t, 1H).
1
The 1H NMR data of 2m was identical to those reported in the literature [39]. 2n: H NMR (500 MHz,
CDCl3) δ 7.49 (d, J = 7.3 Hz, 1H), 7.43–7.23 (m, 2H), 7.20 (d, J = 7.0 Hz, 1H), 7.16–7.00 (td, J = 7.2 Hz,
1.2 Hz, 1H), 6.89 (d, J = 2.3 Hz, 1H), 6.65 (dd, J = 7.9, 2.3 Hz, 1H), 3.82 (s, 2H). The 1H NMR data of 2n
was identical to those reported in the literature [43].
4. Conclusions
In conclusion, we have developed a metal-free, environmentally benign method for C(sp3)–H
oxidation of various diarylmethanes using silylamides [MN(SiMe3)2, M = Li, Na, K] as base and O2
as an oxidant. This protocol provides a complementary method to prepare diaryl ketones in good to
excellent yields. The detailed mechanism study is still underway.
Author Contributions: J.L. conceived and designed the experiments and wrote the manuscript; F.Y., B.Z., P.C.,
D.Z., Q.L. and W.R. performed the experiments and analyzed the data; L.F. and L.L. interpreted the results and
helped write the paper.
Funding: We are grateful to the National Natural Science Foundation of China (81302668) and Hangzhou Science
and Technology Information Institute of China (20150633B45).
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
Chen, B.C.; Zhou, P.; Davis, F.A.; Ciganek, E. Organic Reactions; Overman, L.E., Ed.; Wiley: New York,
NY, USA, 2003.
Wu, S.B.; Long, C.; Kennelly, E.J. Structural Diversity and Bioactivities of Natural Benzophenones.
Nat. Prod. Rep. 2014, 31, 1158–1174. [CrossRef] [PubMed]
Belluti, F.; de Simone, A.; Tarozzi, A.; Bartolini, M.; Djemil, A.; Bisi, A.; Gobbi, S.; Montanari, S.; Cavalli, A.;
Andrisano, V.; et al. Fluorinated Benzophenone Derivatives: Balanced Multipotent Agents for Alzheimer’s
Disease. Eur. J. Med. Chem. 2014, 78, 157–166. [CrossRef] [PubMed]

Molecules 2018, 23, 1922
5 of 6
4.
5.
6.
7.
Vooturi, S.K.; Cheung, C.M.; Rybak, M.J.; Firestine, S.M. Design, Synthesis and Structure-Activity
Relationships of Benzophenone-Based Tetraamides as Novel Antibacterial Agents. J. Med. Chem. 2009, 52,
5020–5031. [CrossRef] [PubMed]
Lee, S.Y.; Yasuda, T.; Yang, Y.S.; Zhang, Q.; Adachi, C. Luminous Butterflies: Efficient Exciton Harvesting
by Benzophenone Derivatives for Full-Color Delayed Fluorescence OLEDs. Angew. Chem. Int. Ed. 2014
53, 6402–6406. [CrossRef] [PubMed]
,
Ryabchun, A.; Sakhno, O.; Wegener, M. Conventional Elastomers Doped with Benzophenone Derivatives as
Effective Media for All-optical Fabrication of Tunable Diffraction Elements. RSC Adv. 2016, 6, 51791–51800.
[CrossRef]
Al-hunaiti, A.; Raisanen, M.; Repo, T. From DNA to Catalysis: A Thymine-Acetate Ligated Non-Heme
Iron(III) Catalyst for Oxidative Activation of Aliphatic C-H Bonds. Chem. Commun. 2016, 52, 2043–2046.
[CrossRef] [PubMed]
8.
9.
Gao, Q.; Li, S.; Pan, Y.; Xu, Y.; Wang, H. The Indium-Catalysed Hydration of Alkynes Using Substoichiometric
Amounts of PTSA as Additive. Tetrahedron 2013, 69, 3775–3781. [CrossRef]
Iosub, A.V.; Stahl, S.S. Palladium-Catalyzed Aerobic Oxidative Dehydrogenation of Cyclohexenes to
Substituted Arene Derivatives. J. Am. Chem. Soc. 2015, 137, 3454–3457. [CrossRef] [PubMed]
10. Martínez-Ferrate, O.; Britovsek, G.J.; Claver, C.; van Leeuwen, P.W. C-H Benzylic Oxidation Promoted by
Dinuclear Iron DBDOC Iminopyridine Complexes. Inorg. Chim. Acta 2015, 431, 156–160. [CrossRef]
11. Sartori, G.; Maggi, R. Advances in Friedel–Crafts Acylation Reactions: Catalytic and Green Processes; CRC Press:
Boca Raton, FL, USA, 2009.
12. Tojo, G.; Fernández, M.I. Oxidation of Alcohols to Aldehydes and Ketones: A Guide to Current Common Practice;
Springer Science & Business Media: New York, NY, USA, 2006.
13. Colquhoun, H.; Thompson, D.; Twigg, M.V. Carbonylation: Direct Synthesis of Carbonyl Compounds; Springer
Science & Business Media: New York, NY, USA, 1991.
14. Beller, M.; Wu, X.-F. Transition-Metal-Catalyzed Carbonylation Reactions; Springer: Berlin/Heidelberg,
Germany, 2013.
15. Hughes, M.D.; Xu, Y.J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D.I.; Carley, A.F.; Attard, G.A.;
Hutchings, G.J.; King, F.; et al. Tunable Gold Catalysts for Selective Hydrocarbon Oxidation Under Mild
Conditions. Nature 2005, 437, 1132–1135. [CrossRef] [PubMed]
16. Wang, X.L.; Liu, M.J.; Wang, Y.Q.; Fan, H.Y.; Wu, J.; Huang, C.; Hou, H.W. Cu(I) Coordination Polymers as
the Green Heterogeneous Catalysts for Direct C H Bonds Activation of Arylalkanes to Ketones in Water
−
with Spatial Confinement Effect. Inorg. Chem. 2017, 56, 13329–13336. [CrossRef] [PubMed]
17. Wu, J.L.; Liu, Y.; Ma, X.W.; Liu, P.; Gu, C.Z.; Dai, B. Cu(II)-Catalyzed Ligand-Free Oxidation of
Diarylmethanes and Second Alcohols in Water. Chin. J. Chem. 2017, 35, 1391–1395. [CrossRef]
18. Yanagisawa, S.; Itami, K. Tert-Butoxide-Mediated C-H Bond Arylation of Aromatic Compounds with
Haloarenes. ChemCatChem 2011, 3, 827–829. [CrossRef]
19. Shirakawa, E.; Hayashi, T. Transition-Metal-Free Coupling Reactions of Aryl Halides. Chem. Lett. 2012
41, 130–134. [CrossRef]
,
20. Mehta, V.P.; Punji, B. Recent Advances in Transition-Metal-Free Direct C-C and C-Heteroatom Bond Forming
Reactions. RSC Adv. 2013, 3, 11957–11986. [CrossRef]
21. Jin, F.; Han, W. Transition-Metal-Free, Ambient-Pressure Carbonylative Cross-Coupling Reactions of Aryl
Halides with Potassium Aryltrifluoroborates. Chem. Commun. 2015, 51, 9133–9136. [CrossRef] [PubMed]
22. Stahl, S.S. Palladium-Catalyzed Oxidation of Organic Chemicals with O2. Science 2005, 309, 1824–1826.
[CrossRef] [PubMed]
23. Que, L.; Tolman, W.B. Biologically Inspired Oxidation Catalysis. Nature 2008, 455, 333–340. [CrossRef]
[PubMed]
24. Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Recent Advances in Transition Metal Catalyzed Oxidation of
Organic Substrates with Molecular Oxygen. Chem. Rev. 2005, 105, 2329–2363. [CrossRef] [PubMed]
25. Zhang, C.; Tang, C.; Jiao, N. Recent Advances in Copper-Catalyzed Dehydrogenative Functionalization via
A Single Electron Transfer (SET) Process. Chem. Soc. Rev. 2012, 41, 3464–3484. [CrossRef] [PubMed]
26. Pattillo, C.C.; Strambeanu, L.I.; Calleja, P.; Vermeulen, N.A.; Mizuno, T.; White, M.C. Aerobic Linear Allylic
C-H Amination: Overcoming Benzoquinone Inhibition. J. Am. Chem. Soc. 2016, 138, 1265–1272. [CrossRef]
[PubMed]

Molecules 2018, 23, 1922
6 of 6
27. Das, P.; Saha, D.; Saha, D.; Guin, J. Aerobic Direct C(sp2)-H Hydroxylation of 2-Arylpyridines by Palladium
Catalysis Induced with Aldehyde Auto-Oxidation. ACS Catal. 2016, 6, 6050–6054. [CrossRef]
28. Anson, C.W.; Ghosh, S.; Hammes-Schiffer, S.; Stahl, S.S. Co(salophen)-Catalyzed Aerobic Oxidation of
p-Hydroquinone: Mechanism and Implications for Aerobic Oxidation Catalysis. J. Am. Chem. Soc. 2016, 138,
4186–4193. [CrossRef] [PubMed]
29. Brink, G.J.T.; Arends, I.W.C.E.; Sheldon, R.A. Green, Catalytic Oxidation of Alcohols in Water. Science 2000
287, 1636–1639. [CrossRef] [PubMed]
,
30. Brice, J.L.; Harang, J.E.; Timokhin, V.I.; Anastasi, N.R.; Stahl, S.S. Aerobic Oxidative Amination of Unactivated
Alkenes Catalyzed by Palladium. J. Am. Chem. Soc. 2005, 127, 2868–2869. [CrossRef] [PubMed]
31. Zhang, Y.H.; Yu, J.Q. Pd(II)-Catalyzed Hydroxylation of Arenes with 1 atm of O2 or Air. J. Am. Chem. Soc.
2009, 131, 14654–14655. [CrossRef] [PubMed]
32. Chiba, S.; Zhang, L.; Lee, J.Y. Copper-Catalyzed Synthesis of Azaspirocyclohexadienones from Alpha-Azido-
N-Arylamides under an Oxygen Atmosphere. J. Am. Chem. Soc. 2010, 132, 7266–7267. [CrossRef] [PubMed]
33. Lyons, T.W.; Reinhard, C.T.; Planavsky, N.J. The Rise of Oxygen in Earth’s Early Ocean and Atmosphere.
Nature 2014, 506, 307–315. [CrossRef] [PubMed]
34. Sterckx, H.; Sambiagio, C.; Maes, B.U.W. Copper-Catalyzed Aerobic Oxygenation of Benzylpyridine
N-Oxides and Subsequent Post-Functionalization. Adv. Synth. Catal. 2017, 359, 3226–3236. [CrossRef]
35. Nambo, M.; Keske, E.C.; Rygus, J.P.G.; Yim, J.C.H.; Crudden, C.M. Development of Versatile Sulfone
Electrophiles for Suzuki-Miyaura Cross-Coupling Reactions. ACS Catal. 2016, 7, 1108–1112. [CrossRef]
36. Li, S.; Zhu, B.; Lee, R.; Qiao, B.; Jiang, Z. Visible light-induced selective aerobic oxidative transposition
of vinyl halides using a tetrahalogenoferrate(III) complex catalyst. Org. Chem. Front. 2017, 5, 380–385.
[CrossRef]
37. Prebil, R.; Stavber, G.; Stavber, S. Aerobic Oxidation of Alcohols by Using a Completely Metal-Free Catalytic
System. Eur. J. Org. Chem. 2014, 2014, 395–402. [CrossRef]
38. Chinnagolla, R.K.; Jeganmohan, M. Regioselective Ortho-Arylation and Alkenylation of N-Alkyl Benzamides
with Boronic Acids via Ruthenium-Catalyzed C–H Bond Activation: An Easy Route to Fluorenones Synthesis.
Org. Lett. 2012, 14, 5246–5249. [CrossRef] [PubMed]
39. Zhang, X.; Ji, X.; Jiang, S.S.; Liu, L.L.; Weeks, B.L.; Zhang, Z. Highly Efficient Synthesis of 9-Fluorenones
from 9H-Fluorenes by Air Oxidation. Green Chem. 2011, 13, 1891–1896. [CrossRef]
40. Yip, W.T.; Levy, D.H.; Kobetic, R.; Piotrowiak, P. Energy Transfer in Bichromophoric Molecules: The Effect of
Symmetry and Donor/Acceptor Energy Gap. J. Phys. Chem. A 2009, 103, 10–20. [CrossRef]
41. Valášek, M.; Edelmann, K.; Gerhard, L.; Fuhr, O.; Lukas, M.; Mayor, M. Synthesis of Molecular Tripods Based
0
on A Rigid 9,9 -Spirobifluorene Scaffold. J. Org. Chem. 2014, 79, 7342–7357. [CrossRef] [PubMed]
42. Kojima, M.; Oisaki, K.; Kanai, M. Chemoselective Aerobic Photo-Oxidation of 9H-Fluorenes for the Synthesis
of 9-Fluorenones. Tetrahedron Lett. 2015, 46, 4736–4738. [CrossRef]
43. Dufresne, S.; Callaghan, L.; Skene, W.G. Conjugated Fluorenes Prepared From Azomethines Connections-II:
The Effect of Alternating Fluorenones and Fluorenes on the Spectroscopic and Electrochemical Properties.
J. Phys. Chem. B 2009, 113, 15541–15549. [CrossRef] [PubMed]
Sample Availability: Samples of the compounds 2a–2n are available from the authors.
©
2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).