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Synthetic Communications: AnInternational Journal for RapidCommunication of SyntheticOrganic ChemistryPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lsyc20

Improved, Highly Efficient,and Green Synthesis ofBromofluorenones andNitrofluorenones in WaterXin Zhang a , Jun-Bin Han a , Peng-Fei Li a , Xuan Ji a

& Zhao Zhang aa 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 andNitrofluorenones in Water, Synthetic Communications: An International Journal forRapid Communication of Synthetic Organic Chemistry, 39:21, 3804-3815

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Improved, Highly Efficient, and Green Synthesisof Bromofluorenones and Nitrofluorenones

in Water

Xin Zhang, Jun-Bin Han, Peng-Fei Li, Xuan Ji, and Zhao ZhangSchool of Chemistry and Chemical Engineering, Shanxi University,

Taiyuan, China

Abstract: A series of bromo-, nitro-, and bromonitrofluorenones were synthesizedchemo- and regioselectively in 90–98% yield via electrophilic aromatic bromina-tion and nitration under mild conditions using water as the sole solvent. Thesesynthetic methods involve simple workup procedures and use only minimalamounts of organic solvents during the purification of products. The newly devel-oped methods have the advantages of being cost-effective and environmentallyfriendly and could potentially be used for the large-scale synthesis of fluorenonederivatives.

Keywords: Aromatic bromination, aromatic nitration, fluorenones, greenchemistry, water

Fluorenones bearing versatile and synthetically useful groups, such as thebromo 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 plasticadditives to the preparation of the organic light-emitting materials.[1–4]

Because of the presence of the ketone functionality, electrophilic aro-matic substitution, such as halogenation and nitration, on fluorenoneand 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

Synthetic Communications1, 39: 3804–3815, 2009

Copyright # Taylor & Francis Group, LLC

ISSN: 0039-7911 print=1532-2432 online

DOI: 10.1080/00397910902838904

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necessarily harsh conditions would encounter further complicationsarising from multisubstitution and formation of various regioisomers.For example, bromination of 9-fluorenone entailed the use of methane-sulfonic acid (CH3SO3H) as the solvent and N-bromosuccinimide(NBS) as the brominating agent to give a mixture of 2-bromofluorenone(2a) and 2,7-dibromofluorenone (2b) (Scheme 1).[5] Similarly, nitrationof 9-fluorenone under standard conditions (HNO3–H2SO4 in aceticanhydride as solvent) gave rise to the formation of a 3:1 mixture of2-nitro- and 4-nitrofluorenones (3a and 3b).[6] Not surprisingly, variousindirect routes have been employed to obtain fluorenones with well-defined regiochemistry. The two most common indirect methods forthe synthesis of fluorenones are the oxidation of substituted fluorenes[2c,7]

and intramolecular Friedel–Crafts acylation of biphenyls.[8] However,these multistep synthetic methods suffer from various limitations becauseof the use of large amounts of hazardous organic solvents and expensivereagents and=or catalysts.

Increasing environmental awareness and economic concern have ledto the consideration of nontraditional solvents for organic synthesis. Inthis respect, water, being nonflammable, nontoxic, and environmentallybenign, is arguably the most attractive alternative to organic solventsand has been used in an increasing number of organic reactions as a greenreaction media.[9] We report here a simple, practical, and green synthesisof various bromo- and nitrofluorenones via bromination and nitration inwater 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 variousamounts (5–10%) of 2,7-dibromofluorenone (2b) when the reaction wasperformed with NBS as the brominating agent in the presence ofCH3SO3H as the solvent,[5] we have found that reaction of 1 with a slightexcess of Br2 in water under mild heating (bath temperature: 80�C) for 4 h

Scheme 1. Common methods for the synthesis of bromofluorenone andnitrofluorenone.

Green Synthesis of Fluorenone Derivatives 3805

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gave 2a, free from 2b, in 94% isolated yield after recrystallization fromethanol (Table 1, entry 1). (Although 1 is insoluble in water at roomtemperature, the reaction became pseudo-homogeneous when heated to80�C and facilitated by the formation of a fine liquid–liquid dispersionof the molten fluorenone into the reaction media.) Interestingly, when3.1 eq of Br2 was used, the reaction was performed at gentle reflux (bathtemperature: 100�C) for a longer period of time (12 h), and the pH ofreaction was maintained at �7, 2b was obtained in essentially quantita-tive yield (Table 1, entry 2). (An early attempt to directly brominate 1using Br2 in water led to the isolation of 2b in only 50% yield. SeeRef. 7a. It is important that Br2 be introduced in portions and that thepH of the reaction be adjusted to neutral in order to obtain a good yieldof 2b.) Thus, by substituting NBS, which is highly expensive (especiallywhen used for large-scale applications) and produces a large amount ofthe by-product succinimide, for the cheaply available Br2 as the bromi-nating agent and replacing the volatile and corrosive CH3SO3H by wateras solvent, we have developed a highly efficient, cost-effective, and envir-onmentally friendly synthesis of both the monobromofluorenone 2a andthe dibromofluorenone 2b. The new synthesis also eliminated the use of asurfactant as the additive and sulfuric acid as the catalyst for the bromi-nation of synthesis of 1 in water as described in a recently disclosedpatent procedure.[10] Our procedure is a green one and should be particu-larly attractive for large-scale production becuase the entire process doesnot involve the use of any catalyst or organic solvent, and no chroma-tography separation is needed during the purification stage. (It is welldocumented that bromination of deactivated arenes with Br2 normallyrequires the use of stoichiometric amounts of various Lewis acids, suchas FeBr3, HgO, Hg(O2CCH3)2, and Hg(O2CCF3)2. See Ref. 11.)

Table 1. Bromination of 9-fluorenone (1) in water

Entry Br2 Water (mL) Temp. (�C) Time (h) Product Yield (%)

1 1.2 equiv 30 80 4 2a 942a 3.1 equiv 50 100 12 2b 98

aBr2 was added in three portions with 4–5 h of interval for each addition. ThepH of the reaction was maintained at �7 by adding aqueous NaOH to neutralizethe HBr released during the reaction.

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Encouraged by the results of the previously mentioned brominationin 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 usefulmultifunctional fluorenone derivatives. In the event, nitration of 1

(50mmol) in the presence of HNO3 (65%, 2.8 eq) and H2SO4 (96%, 3.6eq) in water (10mL) 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 either2,7-dinitrofluorenone 4[12] or 2,4,7-trinitrofluorenone 5[13] (Table 2,entries 2 and 3, respectively). Thus, simply by increasing the amount ofboth HNO3 (65%, 10 eq) and H2SO4 (96%, 12.5 eq) and heating the reac-tion to reflux (bath temperature: 120�C) for 5 h, pure 4 was isolated in90%. For the formation of the 5, the concentration of HNO3 in the reac-tion system was crucial. In this case, the use of fuming HNO3 (95%, 11 eq)in combination with H2SO4 (96%, 9 eq) in water (10mL) as a morepowerful nitrating reagent delivered the trinitro derivative 5 in 94% yield.The synthesis of 4 and 5 described here has several advantages whencompared to other synthetic methods reported in the literature. Forexample, nitration of 1 was previously carried out in fuming nitric acidand gave a mixture of 4 and its regio-isomer, 2,5-dinitrofluorenones.[12]

2,4,7-Trinitrofluorenone (5) was previously synthesized by the nitrationof 1 in the presence of fuming nitric acid and concentrated sulfuric acidin 73% yield.[13a] The yield was improved only slightly when glacial aceticacid was added as a cosolvent.[13b] Recently, a further improvement forthe synthesis of 5 was reported,[13c] which gave 93% of 5 but utilized largeamounts of both the nitric acid and sulfuric acid. (The molar ratio of1=HNO3=H2SO4 in the synthesis of 5 in Ref. 13c is 1:80:120, whereas thisratio is only 1:11:9 in the synthesis of 5 in this work.) Finally, we havefound that the nitration of bromofluorenones 2a and 2b could also beachieved under similar conditions, and the nitration products 6 and 7

were isolated in good yield (Table 2, entries 4 and 5). [All the productsin Tables 1 and 2 are known compounds and their structures are charac-terized by spectroscopic methods (1H NMR and IR).]

In conclusion, we have shown that water can be used as a beneficialsolvent for the bromination of 9-fluorenone using elemental bromine asthe brominating agent and for the nitration of 9-fluorenone as well asbromofluorenone using HNO3=H2SO4 as the nitrating agent. Thesereactions 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 minimalamounts of organic solvents during the purification of products. Thenewly developed methods have the advantages of being cost-effective

Green Synthesis of Fluorenone Derivatives 3807

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Table

2.Nitrationoffluorenones

1,2a,and2b

Reactionconditions

Entry

Substrate

HNO

3a,b

H2SO

4a,c

Water

(mL)

Tem

p.

(�C)

Tim

e(h)

Product

Yield

d

(%)

12.8

equiv

ofA

3.6

equiv

10

80–100

2.5

92

2e

1(50mmol)

10equiv

ofA

12.5

equiv

10

Reflux(120� C

)f5

90

31(50mmol)

11equiv

ofC

9equiv

10

Reflux(120� C

)f2

94

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429equiv

ofA

36equiv

20

Reflux(120� C

)f3

93

540equiv

ofB

36equiv

20

Reflux(110� C

)f4

91

aHNO

3andH

2SO

4werepremixed

before

beingadded

tothereactionmedia.

bThefollowingconcentrationsofHNO

3wereused:A,65%;B,85%;C,95%.

c Conc.

H2SO

4(96%)wasused.

dRefersto

pure

product

after

recrystallization.

e Theacidmixture

wasadded

intw

oportions,withthesecondbatchbeingadded

after

1h.

f Thetemperature

inparentheses

refers

tothatoftheheatingoilbath.

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and environmentally friendly and could potentially be used for thelarge-scale synthesis of fluorenone derivatives.

EXPERIMENTAL

Synthesis of 2-Bromo-9-fluorenone (2a)

A mixture of 9-fluorenone (1) (50mmol) and water (30mL) was heated to80�C, and Br2 (60mmol) was added dropwise. After being stirred at 80�Cfor 4 h, the reaction mixture was quenched with water (100mL) and thentreated with aqueous NaHSO3 (10%, 100mL). After cooling to rt, thecrude product was filtered, washed with water (3� 100mL), and dried.The compound was purified by recrystallization from ethanol to give11.2 g of 2a as a yellow solid, yield 94%, mp 146–168�C. 1H NMR(300MHz, CDCl3): d 7.78 (s, 1 H), 7.68 (d, J¼ 7.3Hz, 1 H), 7.62 (d,J¼ 7.8Hz, 1 H), 7.52–7.53 (m, 2 H), 7.41 (d, J¼ 7.8Hz, 1H), 7.34–7.35(m, 1H). 13C NMR (75.5MHz, CDCl3): d 192.3, 143.6, 142.9, 137.0,135.7, 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) (50mmol) and water (50mL) was heated to80�C, and Br2(155mmol) was then added dropwise at reflux three times(the molar ratio of Br2 of each time was 4:3:2; the reaction time after eachaddition was 5, 4, and 3 h, respectively); between additions, the pH of thesystem was modified to 7. The reaction mixture was quenched with water(100mL) and then treated with aqueous NaHSO3 (10%, 100mL). Aftercooling to rt, the product was filtered, washed with water (3� 100mL),and dried. The reaction gave 16.56 g of yellow solid 2b without recrystal-lization, yield 98%, mp 203–205�C. 1H NMR (300MHz, CDCl3): d 7.66(d, J¼ 1.8Hz, 2 H), 7.52 (dd, J¼ 7.9Hz, 1.8Hz, 2 H), 7.28 (d, J¼ 7.9Hz,2 H).13C NMR (75.5MHz, CDCl3): d 191.1, 142.4, 137.7, 135.4, 128.0,123.5, 122.0.

Synthesis of 2-Nitro-9-fluorenone (3a)

A mixture of 9-fluorenone (1) (50mmol) and water (10mL) was heated to80�C. A mixture of HNO3 (65%, 144mmol) and H2SO4 (96%, 180mmol)was then added dropwise. After being stirred at 90�C for 2.5 h, the reac-tion mixture was quenched with water (200mL). The crude product was

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filtered, washed with water (3� 100mL), and dried. The compound waspurified by recrystallization from ethanol to give 10.35 g of 3a as a yellowsolid, yield 92%, mp 222–223�C. 1H NMR (300MHz, CDCl3): d 8.48 (d,J¼ 1.9Hz, 1H), 8.43 (dd, J¼ 1.9 and J¼ 8.2Hz, 1H), 7.77 (d, J¼ 7.3Hz,1H), 7.72� 7.67 (m, 2H), 7.64� 7.59 (t, 1H), 7.49� 7.44 (t, 1H). 13CNMR (75.5MHz, CDCl3): d 191.2, 150.0, 149.0, 135.7, 135.5, 135.3,131.3, 130.2, 125.4, 122.1, 121.0, 119.8.

Synthesis of 2,7-Dinitro-9-fluorenone (4)

A mixture of 9-fluorenone (1) (50mmol) and water (10mL) was heated to80�C, and HNO3 (65%, 500mmol)–H2SO4 (96%, 626mmol) was thenadded dropwise at reflux two times (the amount of each addition wasthe same, and the reaction times after each addition was 1 and 4 h, respec-tively). Subsequent workup and the purification of the product are thesame as described for the synthesis of 3a. The reaction gave 12.15 g ofyellow solid 4, yield 90%, mp 293–295�C. 1H NMR (300MHz, CDCl3):d 8.62 (d, J¼ 1.8Hz, 2 H), 8.58 (dd, J¼ 8.5Hz, 1.8Hz, 2 H), 7.91(d, J¼ 8.5Hz, 2 H).13C NMR (75.5MHz, CDCl3): d 190.8, 150.0,147.9, 136.0, 131.5, 124.7, 119.6.

Synthesis of 2,4,7-Trinitro-9-fluorenone (5)

A mixture of 9-fluorenone (1) (50mmol) and water (10mL) was heated to80�C, and HNO3 (95%, 560mmol)–H2SO4 (96%, 450mmol) was thenadded. After being stirred at reflux for 2 h, the reaction mixture wasquenched with water (200mL). The product was filtered, washed withaqueous NaOH (10%, 2� 100mL) and water (3� 100mL), and dried.The reaction gave 14.81 g of yellow solid 5 without recrystallization, yield94%, mp 173–175�C. 1H NMR (300MHz, CDCl3): d 9.00 (d, J¼ 2.0Hz,1 H), 8.89 (d, J¼ 2.0Hz, 1 H), 8.64 (d, J¼ 2.0Hz, 1 H), 8.53 (d, J¼ 2.0Hz,8.6Hz, 1 H), 8.34 (d, J¼ 8.6Hz, 1 H). 13C NMR (75.5MHz, CDCl3): d186.0, 150.4, 149.3, 145.2, 143.4, 139.6, 138.9, 136.4, 130.9, 128.3,125.9, 123.1, 120.3.

Synthesis of 2-Nitro-7-bromo-9-fluorenone (6)

A mixture of 2-bromo-9-fluorenone (2a) (10mmol) and water (10mL)was heated to 80�C; HNO3 (65%, 286mmol)–H2SO4 (96%, 358mmol)was then added dropwise. The mixture was stirred at reflux for 4 h.

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Subsequent workup and the purification of the product are the same asdescribed for the synthesis of 3a. Yellow solid 6 (2.82 g) was obtained,yield 93%, mp 228–230�C. 1H NMR (300MHz, CDCl3): d 8.52 (d,J¼ 1.8Hz, 1 H), 8.48 (dd, J¼ 1.8Hz, 8.2Hz, 1 H), 7.93 (d, J¼ 1.8Hz,1 H), 7.78 (dd, J¼ 1.8Hz, 8.0Hz, 1 H), 7.74 (d, J¼ 8.2Hz, 1 H), 7.69(d, J¼ 8.0Hz, 1 H). 13C NMR (75.5MHz, CDCl3): d 188.5, 144.9,138.5, 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) (10mmol) and water (10mL)was heated to 80�C, and HNO3 (85%, 396mmol)–H2SO4 (96%, 358mmol)was then added. The mixture was stirred at reflux for 4 h. Subsequentworkup and the purification of the product are the same as describedfor the synthesis of 3a. Yellow solid 7 (3.48 g) was obtained, yield 91%,mp 194–196�C. 1H NMR d 8.24 (d, J¼ 1.8Hz, 1 H), 8.06 (d, J¼ 1.8Hz,1 H), 7.95 (d, J¼ 8.3Hz, 1 H), 7.91 (d, J¼ 2.0Hz, 1 H), 7.74 (d, J¼ 8.3Hz,Hz, 2.0Hz, 1 H). 13C NMR (75.5MHz, CDCl3): d 190.3, 149.4, 141.9,139.2, 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 ShanxiProvince (2006011014).

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