Amphiphilic ligands for Cu-catalyzed aerobic oxidation to synthesize 9-fluorenones in water

Article abstract of DOI:10.1016/j.catcom.2019.04.022

A series of amphiphilic PEG-functionalized nitrogen ligands were developed for the highly efficient copper-catalyzed aerobic oxidation of 9-fluorenes, with molecular oxygen as the sole oxidant in neat water. A broad range of functional groups are well tolerated and thus offer the opportunity for further functionalization.

Full text of DOI:10.1016/j.catcom.2019.04.022

Accepted Manuscript
Amphiphilic ligands for Cu-catalyzed aerobic oxidation to
synthesize 9-fluorenones in water
Longjia Li, Zibo Liu, Shanyu Tang, Jiao Li, Xuanhe Ren, Guanyu
Yang, Heng Li, Bingxin Yuan
PII:
S1566-7367(19)30136-0
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2019.04.022
CATCOM 5690
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
23 January 2019
25 April 2019
27 April 2019
Please cite this article as: L. Li, Z. Liu, S. Tang, et al., Amphiphilic ligands for Cu-
catalyzed aerobic oxidation to synthesize 9-fluorenones in water, Catalysis
Communications, https://doi.org/10.1016/j.catcom.2019.04.022
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ACCEPTED MANUSCRIPT
Amphiphilic Ligands for Cu-Catalyzed Aerobic
Oxidation to Synthesize 9-Fluorenones in Water
Longjia Li, Zibo Liu, Shanyu Tang, Jiao Li, Xuanhe Ren, Guanyu Yang, Heng Li, and Bingxin
Yuan*
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
* Correspondence: bxyuan@zzu.edu.cn
Abstract: A series of amphiphilic PEG-functionalized nitrogen ligands were developed for the
highly efficient copper-catalyzed aerobic oxidation of 9-fluorenes, with molecular oxygen as the
sole oxidant in neat water. A broad range of functional groups are well tolerated and thus offer the
opportunity for further functionalization.
Keywords: Amphiphilic ligands; Aerobic oxidation; 9-Fluorenone; Water as solvent; Molecular
oxygen as the sole oxidant.
Highlights: 1) The covalent conjugation of PEG scaffolds to ligands confers surfactant-type
3
features to catalysts. 2) The surfactant-type ligand enables low catalyst loading. 3) Oxidation of sp
C-H to synthesize 9-fluorenones is accomplished with molecular oxygen as terminal oxidant in
water.
1. Introduction
9-Fluorenones have been found to be important building blocks in both synthetic chemistry and
material chemistry, widely ranging in organic light-emitting materials (OLEDs),[1-4] organic dyes,[5]
bioactive materials,[6-8] and organic optical materials.[9,10] The catalytic oxidation of 9H-fluorenes
represents one of the commonest synthetic methods of 9-fluorenones. Traditional oxidation has been

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well studied by adopting heavy metal,[11,12] hypervalent iodines,[13,14] and peroxides[15-19] as
oxidants. Recent years have increasingly seen chemistry research move toward an environmentally
friendly and sustainable process. Molecular oxygen as the sole oxidant and neat water as the solvent
represents one of the best strategies for green oxidation because of their low cost, natural abundance,
and environmental benignity.[20-23] Recently, Ma and coworkers reported nitrogen-doped
graphene-catalyzed C-H bond oxidation with TBHP as the initiator under 4 MPa O2 pressure in
water.[17] Shaabani et al introduced copper(II) tetrasulfophthalocyanine (CuTSPc) supported on
nitrogen-doped graphene for the aerobic oxidation of alkyl arenes at room temperature.[24] Yan
developed nanocrystalline cellulose-grafted CuTSPc for the oxidation of alkylarenes and alcohols
under atmospheric air.[25] Cobalt nanoparticles supported on γ-Al2O3 were investigated by
Mansournezhad.[26] Such heterogeneous catalysts exhibit considerable performance in the
oxidation of 9H-fluorenes using green oxidant in water; however, the preparation of such catalysts is
tedious.
2. Experimental Details
General Procedure: Synthesis of Ligands (L1-L10). Triethylene glycol monomethyl ether (0.96
mL, 6 mmol), NaH (0.173 g, 7.2 mmol), and DMF (2 mL) were added to a 25 mL Schlenk tube armed
with a magnetic stir bar. The reaction mixture was stirred at room temperature until no gas was
released from the reaction system, and then 4,7-dichloro-1,10-phenanthroline (0.15 g, 0.6 mmol) was
added. The tube was placed in an oil bath at 100 °C under stirring for 6 h. After reaction, the tube
was cooled to room temperature, and the mixture was diluted with DCM, washed with brine, dried
with Na2SO4, and concentrated on a rotovap. The residue was purified by column chromatography
to obtain the product (L1-L10).
Typical Procedure for the Products. A 25 mL Schlenk tube was charged with 9H-fluorene (33.2
.
mg, 0.2 mmol), CuCl2 2 H2O (0.17 mg, 0.001 mmol), L1 (0.5 mg, 0.001 mmol), NaOH (56 mg, 1.4
o
mmol), H2O (3 mL), and O2 balloon, and then, the resulting mixture was stirred at 50 C. After 16 h,
the reaction mixture was extracted with ethyl acetate (3 × 10 mL) and then dried over Na2SO4 and

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filtered. After evaporation of the solvent under vacuum, the residue was subjected to column
chromatography on silica gel to afford products 2a-2l.
3. Results and Discussion
Inspired by the success of CuTSPc to synthesize 9-fluorenones, we designed a series of
amphiphilic nitrogen ligands for homogeneous oxidation in water, as shown in Figure 1.
Poly(ethylene glycol)-functionalized 1,10-phenanthrolines (phen) L1-5, 2,2’-bipyridines (bpy) L6-7,
and pyridines (py) L8-10 were prepared through one-step process in excellent yields (see Supporting
Information). The hydrophilic polyethylene glycol (PEG) chains grafted onto the commonly used
nitrogen ligands produces adequate aqueous solubility to the corresponding metal complex as
catalysts. More importantly, the covalent conjugation of PEG scaffolds to the ligand confers
surfactant-type properties to the metal–ligand complex.[27-31] These surfactant-type ligands might
accelerate the solubility of reaction substrates as well as the catalytic efficiency of copper in water
for the aerobic oxidation of 9H-fluorenes.
Figure 1. Designed amphiphilic nitrogen ligands.

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For the initial phase of our work, the transformation of 9H-fluorene into fluorenone was
performed by utilizing CuCl2∙2H2O as the catalyst, 4,7-diPEG-phen (L1) as the ligand, and NaOH as
the base under 1 atm O2 in neat water (Table 1). The catalyst amount was screened ranging from 1 to
0.1 mol% (Table 1, Entries 1-5). Fortunately, the catalyst loading could be as low as 0.5 mol%, while
the desired 9-fluorenone was produced in excellent yield (Table 1, Entry 2). The grafting sites of PEG
motifs on 1,10-phenanthroline significantly affected the catalytic efficiency. 3,8-DiPEG-phen (L2) and
2,9-diPEG-phen (L3) led to substantial decrease in product yields (Table 1, Entries 6 and 7).
Compared to L1, longer PEG chains consisting of four ethylene glycol units produced a similar
result (Table 1, Entry 8). 2-PEG-phen (L5) with one PEG chain grafted furnished the product at 84%
yield (Table 1, Entry 9), which is much higher than the result of L3. Amphiphilic ligands based on
2,2’-bipyridine and pyridine showed much lower catalytic activity (Table 1, Entries 10-14). Control
experiment confirmed that the desired product was formed in low yield in the absence of the ligand
(Table 1, Entry 15). Control experiments using 1,10-phenanthrolines or 2,2’-bipyridines as ligands in
water also led to low yields (Table 1, Entries 16 and 17). No product was formed when the reaction
was conducted in organic solvents, for instance, toluene (Table 1, Entry 18) and THF (Table 1, Entry
19). Various bases were screened, as shown in Table 2. Other copper salts such as Cu(OAc)2, CuSO4,
CuI, CuBr, and CuCl were also tested (Table S1). Finally, the optimal reaction condition was
obtained when 1a was treated with CuCl2∙2H2O (0.5 mol%), L1 (0.5 mol%), and NaOH (7 eq) in water
o
at 50 C for 16 h (Table 2, Entry 9).
Table 1. Ligand screening. a,b
a,b
Entry
Catalyst
mol%
1.0
Ligand
L1
Yield/%
94
1
CuCl2∙2H2O
CuCl2∙2H2O
2
0.5
L1
95

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3
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
CuCl2∙2H2O
0.3
0.1
0.05
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
L1
89
4
L1
59
5
L1
51
6
L2
54
7
L3
64
8
L4
94
9
L5
84
10
11
12
13
14
15
16
17
18
19
L6
11
L7
6
L8
32
L9
51
L10
/
43
31
phen
bpy
phen
phen
51
36
c
trace
trace
d
.
a Reaction conditions: 1a (0.2 mmol), CuCl2 2H2O (x mol%), Ligand (x mol%), NaOH (5.0 equiv.), H2O (3 mL), O2
balloon, 50 oC, 16 h. b Isolated yields. c Solvent: toluene. d Solvent: THF.
Table 2. Base screening. a,b
a,b
Entry
Base
Eq.
5
Yield/%
91.3
1
2
KOH
Na2CO3
5
trace

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3
K2CO3
Cs2CO3
CsF
5
5
5
5
5
10
7
3
1
/
trace
trace
trace
trace
trace
88
4
5
6
K3PO4
TEA
7
8
NaOH
NaOH
NaOH
NaOH
/
9
96
10
11
12
78
trace
0
.
a Reaction conditions: 1a (0.2 mmol), CuCl2 2H2O (0.5 mol%), L1 (0.5 mol%), NaOH (x equiv.), H2O (3 mL), O2
balloon, 50 oC, 16 h. b Isolated yields.
To further understand the catalytic performance of the water-soluble amiphiphilic ligands, we
performed UV–Vis studies. The absorption spectrum of L1 solution is characterized by bands
ranging from 200 to 370 nm (Figure 2a). On addition of CuCl2, the absorption bands showed red
shift, with an increasing shoulder at 220 nm. However, PEG-functionalized pyridines L8-10 showed
no coordination to the copper center, as the spectrum of the free ligands coincided with the
outcomes in the presence of CuCl2. (Figure S1, see Supporting Information).
Figure 2. Absorption spectra of L1 in water.

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With the optimal reaction conditions in hand, we continued to explore the substrate scope of
the aerobic oxidation of 9H-fluorene in water (Table 3). A wide range of 9H-fluorenes were
efficiently converted into the corresponding products in high yields. All reactions occurred with
absolute selectivity for ketones, and no other byproduct was detected in the reaction mixture.
Substrates bearing electron-withdrawing groups (1b-h) such as halogen (I, Br, Cl), nitro, and acetyl
groups and electron-donating groups (1i-l) such as amino, t-butyl, and phenyl groups were well
tolerated. It is to be noticed that the reaction could be conducted in the presence of halogen
substituents and thus offer the opportunity for further functionalization.
Table 3. Substrate scopes. a,b
.
a Reaction conditions: 1a (0.2 mmol), CuCl2 2H2O (0.5 mmol%), L1 (0.5 mol%), NaOH (7 equiv.), H2O (3 mL), O2
balloon, 50 oC, 16 h. b Isolated yields. c 24 h. d 0.3 MPa O2. e 120 oC, 0.3 MPa O2.

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The aerobic oxidation of alcohol was attempted by utilizing 9H-fluoren-9-ol as a model
substrate. The corresponding fluorenone was obtained in excellent yield.
Scheme 1. Aerobic oxidation of 9H-fluoren-9-ol.
According to our previous studies and literature,[32-35] a possible reaction mechanism for the
3
aerobic oxidation of sp C-H is proposed in Scheme 2. The formation of 9-fluorenones with
molecular oxygen presumably involves abstraction of proton from the benzylic site of the
-
9H-fluorene by OH , leading to the cleavage of C-H bond and formation of fluorenylperoxy-copper
intermediate through the single electron transfer process. The cleavage of peroxy bond results in the
formation of a carbonyl group.
Scheme 2. Proposed mechanisms for the formation of 9-fluorenones.
4. Conclusions
To conclude, we synthesized
a series of amphiphilic nitrogen ligands including
PEG-functionalized 1,10-phenanthrolines, 2,2’-bipyridines, and pyridines in quantitative yields by a
one-step reaction. Taking advantage of amphiphilic ligands, we successfully developed a novel
homogeneous catalytic system for the synthesis of 9-fluorenones in water, with molecular oxygen as
the sole oxidant. It is to be noticed that the catalyst loading could be as low as 0.5 mol%. A large

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variety of electron-withdrawing and electron-donating substituents are well tolerated. Notably, the
tolerance of reactive halogen groups (I, Br) allows further functionalization on the fluorenone unit.
Supplementary Materials: Electronic Supplementary Information (ESI) available: [Experimental procedures,
characterization data, and 1H and 13C NMR spectra].
Acknowledgments: The authors thank the National Natural Science Foundation of China (No. 21801229) for
the funding support.
Author Contributions: B.Y., L.H. and G.Y., conceived and designed the experiments; L.L., Z.L., S.T., J.L., and
X.R. performed the experiments; L.L. analyzed the data; and B.Y. wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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© 2017 by the authors. Submitted for possible open access publication under the
terms and conditions of the Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/).

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Graphical abstract:
PEG-functionalized nitrogen ligands for aerobic oxidation of sp3 C–H bonds to synthesize
9-Fluorenones in Water

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Highlights:
1) PEG-functionalized ligands confers surfactant-type features to catalysts.
2) The surfactant-type ligands enables low catalyst loading.
3) Oxidation of sp3 C-H to are done with molecular oxygen as sole oxidant in
water.