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S. Naidu, S.R. Reddy / Journal of Molecular Liquids 222 (2016) 441–445
Table 1
Optimization of solvent for oxygenation of benzylic C\\H bonds.
Scheme 1. Oxygenation of benzylic C\\H bonds.
Entrya
1
2
3
4
5
6
7
8
9
10
11
12
Solvent
Ethyl acetate
Acetonitrile
Acetonitrile(2 mL)/[bmim]Br (50 mol%)
Toluene
Tetrahydrofuran
chloroform
–
[bmim]Br
[bmim]BF4
[bmim]PF6
[bmim]Cl
H2O
Oxidant
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
Yieldb
20
30
(multiplet). FTIR spectra were recorded on a Nicolet 6700 spectrometer
and are reported in frequency of absorption (cm−1). GC–MS are record-
ed on instrument Perkin Elmer mass spectrometer
90
Trace
Trace
21
Trace
95
95
62
55
42
3. Experimental
3.1. General procedure for oxygenation of benzylic C\\H bonds
Charged 0.5 mmol of 3-(9H-fluoren-2-yl)prop-2-yn-1-ol 13a
(110 mg, 0.5 mmol), 0.5 mL of [bmim]Br and TBHP (0.5 mL,
3.6 mmol) were taken in a round bottom flask and heated to 55 °C
until completion of the reaction. The reaction course was monitored
by TLC. After completion of the reaction, the reaction mixture was ex-
tracted using ethyl acetate (3 × 4 mL). The organic layer was concen-
trated under vacuum and purified by column chromatography on
silica gel to afford the desired product as 2-(3-hydroxyprop-1-yn-1-
yl)-9H-fluoren-9-one 13b Yellow Solid; yield: 75%, mp 148–150 °C; 1H
NMR (400 MHz, CDCl3): δ 7.69 (m, 2H), 7.56 (dd, 1H), 7.50 (m, 2H);
7.48 (d, 1H), 7.32 (m, 1H), 4.51 (s, 2H), 1.83 (br, 1H); 13C NMR
(100 MHz, CDCl3): δ 193.0, 144.0, 143.9, 137.9, 134.9, 134.3, 134.2,
129.5, 127.4, 124.5, 123.4, 120.7, 120.3, 88.9, 84.9, 51.6. ESI-MS Calculat-
ed for C16H10O2: Theoretical mass: 234.2, Observed mass: 234.1.
a
All experiments were carried out using 0.5 mmol substrate and TBHP 3.6 mmol (70%
t-BuOOH in H2O).
b
Isolated yields.
gave several undesired by-products. The reaction did not proceed at
all when THF and toluene were used as solvents. Next, the reaction
was carried out with similar ionic liquid by varying the counter anions
such as [bmim]BF4, [bmim]Br, [bmim]PF6 and [bmim]Cl. Interestingly,
[bmim]BF4 and [bmim]Br were found to be the best solvents which
gave the corresponding fluorenone with a maximum yield of 95% in
20 h (Table 1, Entries 8 and 9), whereas [bmim]PF6 and [bmim]Cl gave
moderate yield (Table 1, Entries 10 and 11). The reaction when carried
out using water as solvent gave the desired product with less yield
(42%) (Table 1, Entry 12). However, due to the easy availability and syn-
thesis of [bmim]Br, further experiments were carried out using the lat-
ter. Furthermore, the catalytic efficiency of the [bmim]Br was studied in
the presence of acetonitrile as solvent. Surprisingly, reaction was found
to be selective and gave quantitative yield of the corresponding ketone
(Table 1, Entry 3). Though [bmim]Br can be used in a catalytic amount,
further experiments were carried out using 0.5 mL of [bmim]Br attribut-
ed to its recyclable property.
Second set of experiments were carried out using various oxidants
such as hydrogen peroxide, urea hydrogen peroxide (UHP), and TBHP.
TBHP was found to be the best oxidant among the rest as it gave 95%
yield within 20 h of reaction time (Table 2, Entry 4). In addition, TBHP
was known to be the superior oxygen source for variety of organic oxy-
genation reactions and also possess good thermal stability [28,29]. Sub-
sequently, we have carried out the reactions at 55 °C using 0.5 mmol of
fluorene and varying concentrations of TBHP from 1.4 mmol to
3.2. General procedure for oxidation of secondary alcohol
Charged 0.5 mmol of 1,3-diphenylprop-2-yn-1-ol 15a (104 mg,
0.5 mmol), 0.5 mL of [bmim]Br and TBHP (0.5 mL, 3.6 mmol) were
taken in a round bottom flask and heated to 55 °C until completion of
the reaction. The reaction course was monitored by TLC. After comple-
tion of the reaction, the crude mixture was extracted with ethyl acetate
(3 × 4 mL). The organic layer was concentrated under reduced pressure
and purified by column chromatography on silica gel to afford the de-
sired product, 1,3-diphenylprop-2-yn-1-one 15b as yellow oil; 1H
NMR (400 MHz, CDCl3): δ 8.25 (d, 2H), 7.71–7.63 (m, 3H), 7.55–7.42
(m, 5H); 13C NMR (100 MHz, CDCl3): 178.0, 136.9, 134.2, 133.1, 130.9,
129.7, 128.8, 128.7, 120.2, 93.1, 86.9. GC–MS Calculated for C15H10O:
Theoretical mass: 206.24, Observed mass: 206.20.
3.3. General procedure for recyclability of the ionic liquid
Table 2
Optimization studies on the oxygenation of benzylic C\\H bonds.
Upon completion of the reaction, water and ethyl acetate were
added to the reaction mixture due to the their miscibility with IL
[bmim]Br and product respectively. The aqueous layer was extracted,
concentrated under high vacuum and the resulting IL was reused for
the subsequent experiments. The organic extracts upon evaporation
afforded the respective ketone in quantitative yields (Table 5).
Entrya
1
2
3
4
6
7
8
Oxidant
30% H2O2
50% H2O2
UHP
TBHP
TBHP
TBHP
–
TBHP
TBHP
mmol
4.5
4.5
3.6
3.6
2.1
1.4
–
Temp.
55 °C
55 °C
55 °C
55 °C
55 °C
55 °C
55 °C
45 °C
RT
Time
70 h
70 h
70 h
20 h
65 h
70 h
70 h
80 h
7 days
Yieldb
22
32
30
95
95
95
–
4. Results and discussion
First set of experiments were carried out using fluorene (0.5 mmol)
as a model substrate, oxidant, TBHP (3.6 mmol) at 55 °C up to 20 h using
[bmim]Br and various organic solvents such as toluene, tetrahydrofuran
(THF), chloroform, ethyl acetate and acetonitrile (Table 1). Among the
employed solvents, chloroform, acetonitrile and ethyl acetate afforded
the corresponding ketone (Table 1, Entries 6, 1 and 2). Nevertheless,
the reaction was incomplete even after 20 h and was not selective and
9
10
3.6
3.6
60
47
a
All experiments were carried out using 0.5 mmol (83 mg) fluorene, [bmim]Br (0. 5 mL),
Oxidant.
b
Isolated yields.