Improved, highly efficient, and green synthesis of bromofluorenones and nitrofluorenones in water

Article abstract of DOI:10.1080/00397910902838904

A series of bromo-, nitro-, and bromonitrofluorenones were synthesized chemo-and regioselectively in 90-98% yield via electrophilic aromatic bromination and nitration under mild conditions using water as the sole solvent. These synthetic methods involve simple workup procedures and use only minimal amounts of organic solvents during the purification of products. The newly developed methods have the advantages of being cost-effective and environmentally friendly and could potentially be used for the large-scale synthesis of fluorenone derivatives. Copyright Taylor & Francis Group, LLC.

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Improved, Highly Efficient,
and Green Synthesis of
Bromofluorenones and
Nitrofluorenones in Water
a
a
a
a
Xin Zhang , Jun-Bin Han , Peng-Fei Li , Xuan Ji
a
&
a
Zhao Zhang
School of Chemistry and Chemical Engineering,
Shanxi University, Taiyuan, China
Available online: 07 Oct 2009
To cite this article: Xin Zhang, Jun-Bin Han, Peng-Fei Li, Xuan Ji & Zhao Zhang
(2009): Improved, Highly Efficient, and Green Synthesis of Bromofluorenones and
Nitrofluorenones in Water, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 39:21, 3804-3815
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1
Synthetic Communications , 39: 3804–3815, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902838904
Improved, Highly Efficient, and Green Synthesis
of Bromofluorenones and Nitrofluorenones
in Water
Xin Zhang, Jun-Bin Han, Peng-Fei Li, Xuan Ji, and Zhao Zhang
School of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan, China
Abstract: A series of bromo-, nitro-, and bromonitrofluorenones were synthesized
chemo- and regioselectively in 90–98% yield via electrophilic aromatic bromina-
tion and nitration under mild conditions using water as the sole solvent. These
synthetic methods involve simple workup procedures and use only minimal
amounts of organic solvents during the purification of products. The newly devel-
oped methods have the advantages of being cost-effective and environmentally
friendly and could potentially be used for the large-scale synthesis of fluorenone
derivatives.
Keywords: Aromatic bromination, aromatic nitration, fluorenones, green
chemistry, water
Fluorenones bearing versatile and synthetically useful groups, such as the
bromo and the nitro group, are important building blocks and intermedi-
ates in organic synthesis and materials chemistry, ranging from the synth-
esis of medicinal and pharmaceutical agents, organic dyes, and plastic
[
1–4]
additives to the preparation of the organic light-emitting materials.
Because of the presence of the ketone functionality, electrophilic aro-
matic substitution, such as halogenation and nitration, on fluorenone
and its derivatives is intrinsically difficult, and reactions performed under
Received October 28, 2008.
Address correspondence to Zhao Zhang, School of Chemistry and Chemical
Engineering, Shanxi University, Taiyuan 030006, China. E-mail: z.zhang@sxu.
edu.cn
3
804

Green Synthesis of Fluorenone Derivatives
3805
necessarily harsh conditions would encounter further complications
arising from multisubstitution and formation of various regioisomers.
For example, bromination of 9-fluorenone entailed the use of methane-
sulfonic acid (CH SO H) as the solvent and N-bromosuccinimide
3
3
(
(
NBS) as the brominating agent to give a mixture of 2-bromofluorenone
[
2a) and 2,7-dibromofluorenone (2b) (Scheme 1). Similarly, nitration
5]
of 9-fluorenone under standard conditions (HNO –H SO in acetic
3
2
4
anhydride as solvent) gave rise to the formation of a 3:1 mixture of
2
[6]
-nitro- and 4-nitrofluorenones (3a and 3b). Not surprisingly, various
indirect routes have been employed to obtain fluorenones with well-
defined regiochemistry. The two most common indirect methods for
[
2c,7]
the synthesis of fluorenones are the oxidation of substituted fluorenes
[
8]
and intramolecular Friedel–Crafts acylation of biphenyls. However,
these multistep synthetic methods suffer from various limitations because
of the use of large amounts of hazardous organic solvents and expensive
reagents and=or catalysts.
Increasing environmental awareness and economic concern have led
to the consideration of nontraditional solvents for organic synthesis. In
this respect, water, being nonflammable, nontoxic, and environmentally
benign, is arguably the most attractive alternative to organic solvents
and has been used in an increasing number of organic reactions as a green
[
9]
reaction media. We report here a simple, practical, and green synthesis
of various bromo- and nitrofluorenones via bromination and nitration in
water without the use of any organic cosolvents.
In contrast to the previously reported bromination of 9-fluorenone
1), which gave 2-bromofluorenone (2a) in <70% yield along with various
(
amounts (5–10%) of 2,7-dibromofluorenone (2b) when the reaction was
performed with NBS as the brominating agent in the presence of
[
5]
CH SO H as the solvent, we have found that reaction of 1 with a slight
3
3
ꢀ
excess of Br in water under mild heating (bath temperature: 80 C) for 4 h
2
Scheme 1. Common methods for the synthesis of bromofluorenone and
nitrofluorenone.

3806
X. Zhang et al.
gave 2a, free from 2b, in 94% isolated yield after recrystallization from
ethanol (Table 1, entry 1). (Although 1 is insoluble in water at room
temperature, the reaction became pseudo-homogeneous when heated to
ꢀ
8
of the molten fluorenone into the reaction media.) Interestingly, when
0 C and facilitated by the formation of a fine liquid–liquid dispersion
3
.1 eq of Br was used, the reaction was performed at gentle reflux (bath
2
ꢀ
temperature: 100 C) for a longer period of time (12 h), and the pH of
reaction was maintained at ꢁ7, 2b was obtained in essentially quantita-
tive yield (Table 1, entry 2). (An early attempt to directly brominate 1
using Br in water led to the isolation of 2b in only 50% yield. See
2
Ref. 7a. It is important that Br be introduced in portions and that the
2
pH of the reaction be adjusted to neutral in order to obtain a good yield
of 2b.) Thus, by substituting NBS, which is highly expensive (especially
when used for large-scale applications) and produces a large amount of
the by-product succinimide, for the cheaply available Br as the bromi-
2
nating agent and replacing the volatile and corrosive CH SO H by water
3
3
as solvent, we have developed a highly efficient, cost-effective, and envir-
onmentally friendly synthesis of both the monobromofluorenone 2a and
the dibromofluorenone 2b. The new synthesis also eliminated the use of a
surfactant as the additive and sulfuric acid as the catalyst for the bromi-
nation of synthesis of 1 in water as described in a recently disclosed
[10]
patent procedure. Our procedure is a green one and should be particu-
larly attractive for large-scale production becuase the entire process does
not involve the use of any catalyst or organic solvent, and no chroma-
tography separation is needed during the purification stage. (It is well
documented that bromination of deactivated arenes with Br normally
2
requires the use of stoichiometric amounts of various Lewis acids, such
as FeBr , HgO, Hg(O CCH ) , and Hg(O CCF ) . See Ref. 11.)
3
2
3 2
2
3 2
Table 1. Bromination of 9-fluorenone (1) in water
ꢀ
Entry
1
Br
2
Water (mL) Temp. ( C) Time (h) Product Yield (%)
1.2 equiv
3.1 equiv
30
50
80
100
4
12
2a
2b
94
98
a
2
a
2
Br was added in three portions with 4–5 h of interval for each addition. The
pH of the reaction was maintained at ꢁ7 by adding aqueous NaOH to neutralize
the HBr released during the reaction.

Green Synthesis of Fluorenone Derivatives
3807
Encouraged by the results of the previously mentioned bromination
in water, we next turned our attention to the nitration of 1 as well as bro-
mofluorenones 2a and 2b; the latter two would afford synthetically useful
multifunctional fluorenone derivatives. In the event, nitration of 1
(
50 mmol) in the presence of HNO (65%, 2.8 eq) and H SO (96%, 3.6
3 2 4
ꢀ
eq) in water (10 mL) at 80 C for 1.5 h cleanly gave the mononitration pro-
duct 2-nitrofluorenone 3a in 92% yield after recrystallization (Table 2,
entry 1). It was also found that by slightly modifying the reaction condi-
tions, the nitration could be further directed to the formation of either
[12]
[13]
2
,7-dinitrofluorenone 4
or 2,4,7-trinitrofluorenone 5
(Table 2,
entries 2 and 3, respectively). Thus, simply by increasing the amount of
both HNO (65%, 10 eq) and H SO (96%, 12.5 eq) and heating the reac-
3
2
4
ꢀ
tion to reflux (bath temperature: 120 C) for 5 h, pure 4 was isolated in
0%. For the formation of the 5, the concentration of HNO in the reac-
tion system was crucial. In this case, the use of fuming HNO (95%, 11 eq)
9
3
3
in combination with H SO (96%, 9 eq) in water (10 mL) as a more
2
4
powerful nitrating reagent delivered the trinitro derivative 5 in 94% yield.
The synthesis of 4 and 5 described here has several advantages when
compared to other synthetic methods reported in the literature. For
example, nitration of 1 was previously carried out in fuming nitric acid
[
12]
and gave a mixture of 4 and its regio-isomer, 2,5-dinitrofluorenones.
,4,7-Trinitrofluorenone (5) was previously synthesized by the nitration
of 1 in the presence of fuming nitric acid and concentrated sulfuric acid
2
[
13a]
in 73% yield.
acid was added as a cosolvent.
The yield was improved only slightly when glacial acetic
Recently, a further improvement for
which gave 93% of 5 but utilized large
[
13b]
[13c]
the synthesis of 5 was reported,
amounts of both the nitric acid and sulfuric acid. (The molar ratio of
=HNO =H SO in the synthesis of 5 in Ref. 13c is 1:80:120, whereas this
1
3
2
4
ratio is only 1:11:9 in the synthesis of 5 in this work.) Finally, we have
found that the nitration of bromofluorenones 2a and 2b could also be
achieved under similar conditions, and the nitration products 6 and 7
were isolated in good yield (Table 2, entries 4 and 5). [All the products
in Tables 1 and 2 are known compounds and their structures are charac-
1
terized by spectroscopic methods ( H NMR and IR).]
In conclusion, we have shown that water can be used as a beneficial
solvent for the bromination of 9-fluorenone using elemental bromine as
the brominating agent and for the nitration of 9-fluorenone as well as
bromofluorenone using HNO =H SO as the nitrating agent. These
3
2
4
reactions proceeded smoothly under mild conditions and delivered sub-
stituted fluorenones chemo- and regioselectively in good yield. These syn-
thetic methods involve simple workup procedures and use only minimal
amounts of organic solvents during the purification of products. The
newly developed methods have the advantages of being cost-effective

3
808

3
809

3810
X. Zhang et al.
and environmentally friendly and could potentially be used for the
large-scale synthesis of fluorenone derivatives.
EXPERIMENTAL
Synthesis of 2-Bromo-9-fluorenone (2a)
A mixture of 9-fluorenone (1) (50 mmol) and water (30 mL) was heated to
ꢀ ꢀ
0 C, and Br (60 mmol) was added dropwise. After being stirred at 80 C
2
8
for 4 h, the reaction mixture was quenched with water (100 mL) and then
treated with aqueous NaHSO (10%, 100 mL). After cooling to rt, the
3
crude product was filtered, washed with water (3 ꢂ 100 mL), and dried.
The compound was purified by recrystallization from ethanol to give
1
ꢀ
1
1.2 g of 2a as a yellow solid, yield 94%, mp 146–168 C. H NMR
(
300 MHz, CDCl ): d 7.78 (s, 1 H), 7.68 (d, J ¼ 7.3 Hz, 1 H), 7.62 (d,
3
J ¼ 7.8 Hz, 1 H), 7.52–7.53 (m, 2 H), 7.41 (d, J ¼ 7.8 Hz, 1H), 7.34–7.35
1
3
(m, 1H). C NMR (75.5 MHz, CDCl ): d 192.3, 143.6, 142.9, 137.0,
3
1
35.7, 135.0, 133.6, 129.3, 127.5, 124.5, 122.8, 121.6, 120.3.
Synthesis of 2,7-Dibromo-9-fluorenone (2b)
A mixture of 9-fluorenone (1) (50 mmol) and water (50 mL) was heated to
ꢀ
8
0 C, and Br (155 mmol) was then added dropwise at reflux three times
2
(the molar ratio of Br of each time was 4:3:2; the reaction time after each
2
addition was 5, 4, and 3 h, respectively); between additions, the pH of the
system was modified to 7. The reaction mixture was quenched with water
(100 mL) and then treated with aqueous NaHSO (10%, 100 mL). After
3
cooling to rt, the product was filtered, washed with water (3 ꢂ 100 mL),
and dried. The reaction gave 16.56 g of yellow solid 2b without recrystal-
ꢀ
1
lization, yield 98%, mp 203–205 C. H NMR (300 MHz, CDCl ): d 7.66
3
(
2
1
d, J ¼ 1.8 Hz, 2 H), 7.52 (dd, J ¼ 7.9 Hz, 1.8 Hz, 2 H), 7.28 (d, J ¼ 7.9 Hz,
1
3
H). C NMR (75.5 MHz, CDCl ): d 191.1, 142.4, 137.7, 135.4, 128.0,
3
23.5, 122.0.
Synthesis of 2-Nitro-9-fluorenone (3a)
A mixture of 9-fluorenone (1) (50 mmol) and water (10 mL) was heated to
ꢀ
8
0 C. A mixture of HNO (65%, 144 mmol) and H SO (96%, 180 mmol)
3
2
ꢀ
4
was then added dropwise. After being stirred at 90 C for 2.5 h, the reac-
tion mixture was quenched with water (200 mL). The crude product was

Green Synthesis of Fluorenone Derivatives
3811
filtered, washed with water (3 ꢂ 100 mL), and dried. The compound was
purified by recrystallization from ethanol to give 10.35 g of 3a as a yellow
ꢀ
1
solid, yield 92%, mp 222–223 C. H NMR (300 MHz, CDCl ): d 8.48 (d,
3
J ¼ 1.9 Hz, 1H), 8.43 (dd, J ¼ 1.9 and J ¼ 8.2 Hz, 1H), 7.77 (d, J ¼ 7.3 Hz,
1
3
1
H), 7.72 ꢁ 7.67 (m, 2H), 7.64 ꢁ 7.59 (t, 1H), 7.49 ꢁ 7.44 (t, 1H).
C
NMR (75.5 MHz, CDCl ): d 191.2, 150.0, 149.0, 135.7, 135.5, 135.3,
3
1
31.3, 130.2, 125.4, 122.1, 121.0, 119.8.
Synthesis of 2,7-Dinitro-9-fluorenone (4)
A mixture of 9-fluorenone (1) (50 mmol) and water (10 mL) was heated to
ꢀ
8
0 C, and HNO (65%, 500 mmol)–H SO (96%, 626 mmol) was then
3
2
4
added dropwise at reflux two times (the amount of each addition was
the same, and the reaction times after each addition was 1 and 4 h, respec-
tively). Subsequent workup and the purification of the product are the
same as described for the synthesis of 3a. The reaction gave 12.15 g of
ꢀ
1
yellow solid 4, yield 90%, mp 293–295 C. H NMR (300 MHz, CDCl ):
3
d 8.62 (d, J ¼ 1.8 Hz, 2 H), 8.58 (dd, J ¼ 8.5 Hz, 1.8 Hz, 2 H), 7.91
1
3
(
1
d, J ¼ 8.5 Hz, 2 H). C NMR (75.5 MHz, CDCl ): d 190.8, 150.0,
3
47.9, 136.0, 131.5, 124.7, 119.6.
Synthesis of 2,4,7-Trinitro-9-fluorenone (5)
A mixture of 9-fluorenone (1) (50 mmol) and water (10 mL) was heated to
ꢀ
8
0 C, and HNO (95%, 560 mmol)–H SO (96%, 450 mmol) was then
3
2
4
added. After being stirred at reflux for 2 h, the reaction mixture was
quenched with water (200 mL). The product was filtered, washed with
aqueous NaOH (10%, 2 ꢂ 100 mL) and water (3 ꢂ 100 mL), and dried.
The reaction gave 14.81 g of yellow solid 5 without recrystallization, yield
ꢀ
1
9
1
8
1
1
4%, mp 173–175 C. H NMR (300 MHz, CDCl ): d 9.00 (d, J ¼ 2.0 Hz,
3
H), 8.89 (d, J ¼ 2.0 Hz, 1 H), 8.64 (d, J ¼ 2.0 Hz, 1 H), 8.53 (d, J ¼ 2.0 Hz,
1
3
.6 Hz, 1 H), 8.34 (d, J ¼ 8.6 Hz, 1 H). C NMR (75.5 MHz, CDCl ): d
3
86.0, 150.4, 149.3, 145.2, 143.4, 139.6, 138.9, 136.4, 130.9, 128.3,
25.9, 123.1, 120.3.
Synthesis of 2-Nitro-7-bromo-9-fluorenone (6)
A mixture of 2-bromo-9-fluorenone (2a) (10 mmol) and water (10 mL)
ꢀ
was heated to 80 C; HNO (65%, 286 mmol)–H SO (96%, 358 mmol)
2
3
4
was then added dropwise. The mixture was stirred at reflux for 4 h.

3812
X. Zhang et al.
Subsequent workup and the purification of the product are the same as
described for the synthesis of 3a. Yellow solid 6 (2.82 g) was obtained,
ꢀ
1
yield 93%, mp 228–230 C. H NMR (300 MHz, CDCl ): d 8.52 (d,
3
J ¼ 1.8 Hz, 1 H), 8.48 (dd, J ¼ 1.8 Hz, 8.2 Hz, 1 H), 7.93 (d, J ¼ 1.8 Hz,
1
H), 7.78 (dd, J ¼ 1.8 Hz, 8.0 Hz, 1 H), 7.74 (d, J ¼ 8.2 Hz, 1 H), 7.69
3
1
3
(d, J ¼ 8.0 Hz, 1 H). C NMR (75.5 MHz, CDCl ): d 188.5, 144.9,
1
38.5, 137.3, 135.6, 135.2, 132.7, 131.9, 128.3, 127.7, 125.9, 123.5.
Synthesis of 2,7-Dibromo-4-nitro-9-fluorenone (7)
A mixture of 2,7-dibromofluorenone (2b) (10 mmol) and water (10 mL)
ꢀ
was heated to 80 C, and HNO (85%, 396 mmol)–H SO (96%, 358 mmol)
4
3
2
was then added. The mixture was stirred at reflux for 4 h. Subsequent
workup and the purification of the product are the same as described
for the synthesis of 3a. Yellow solid 7 (3.48 g) was obtained, yield 91%,
ꢀ
1
mp 194–196 C. H NMR d 8.24 (d, J ¼ 1.8 Hz, 1 H), 8.06 (d, J ¼ 1.8 Hz,
H), 7.95 (d, J ¼ 8.3 Hz, 1 H), 7.91 (d, J ¼ 2.0 Hz, 1 H), 7.74 (d, J ¼ 8.3 Hz,
1
1
3
Hz, 2.0 Hz, 1 H). C NMR (75.5 MHz, CDCl ): d 190.3, 149.4, 141.9,
3
1
39.2, 137.1, 134.8, 131.7, 128.2, 125.8, 125.4, 123.5, 119.7.
ACKNOWLEDGMENT
This work was supported by the Natural Science Foundation of Shanxi
Province (2006011014).
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