Sonochemical fluorination of heterocyclic nitro compounds with Selectfluor q Majid M. Sadeghi, a Hossein Loghmani-Khouzani, a, * Reza Ranjbar-Karimi, a Philip Butler b and Bernard T. Golding b, * a Department of Chemistry, Isfahan University, Isfahan 81746-73441, Iran b School of Natural Sciences—Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne NE1 7RU, UK Abstract—Methine and methylene groups attached to a nitro function and heterocycle (and Ph for CH) were rapidly mono- or di-fluorinated by reaction with 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2,2,2]octane bis-tetrafluoroborate (Selectfluor) in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), preferably with ultrasonic irradiation. Ó 2006 Elsevier Ltd. All rights reserved. Incorporation of fluorine selectively into organic mole- cules has provided a challenge to academic and indus- trial research. 1–3 Electrophilic sources of fluorine have been developed for introducing fluorine at centres of high electron density, and offer alternative strategies, where nucleophilic and free radical sources of fluorine have proved inefficient or have failed. Reagents used as a source of ‘F + ’ include acetyl hypofluorides, 4 N-flu- oroperfluoropiperidine, 5 dihydro-N-fluoro-2-pyridone, 6 N-fluoro-N-alkylsulfonamides, 7 N-fluoropyridinium salts, 8 N-fluoroquinuclidinium salts, 9 1-fluoro-substi- tuted 1,4-diazoniabicyclo[2.2.2]octane salts, 10 1,4-diflu- oro-1,4-diazoniabicyclo[2.2.2]octane salts, 11 1,1 0 -difluoro- bipyridinium salts, 12 trifluoroamine oxide 13 and 1-fluoro-2,4,6-trichloro-1,3,5-triazinium tetrafluoro- borate. 14 Reactions of carbanions with N-F reagents are generally easier than those with neutral nucleophilic substrates. Thus, fluorination of carbanions adjacent to CO, 15–18 CS, 19 COOR, 16,17 RSO 2 , 20,21 NO 2 , 22 CN, 8 PO(OR) 2 has been reported, 23 although the N-F reagent 1-chloro- methyl-4-fluoro-1,4-diazoniabicyclo-[2,2,2]-octane bis- tetrafluoroborate (Selectfluor) has also been shown to react with (E)-stilbene 24 and saturated hydrocarbons. 25 Electrophilic fluorination of carbanions within the side chains of imidazoles and indoles has been accom- plished. 26,27 Recently, Peng and Shreeve reported the fluorination of carbanions from several nitro and cyano compounds using Selectfluor. 28 Most of these reactions, which utilised, for example, potassium hydroxide as base for the nitro compounds, gave mainly products of mono-fluorination. In this letter we report fluorination of various nitro compounds under two sets of reaction conditions: one consisted of stirring the nitro compound for 4–6 h in the presence of DBU and 1-chloromethyl-4-fluoro- 1,4-diazoniabicyclo[2,2,2]octane bis-tetrafluoroborate (Selectfluor) (Scheme 1); the other conditions employed ultrasonic irradiation in the presence of ammonium ace- tate as a base and again using Selectfluor (Scheme 1). 29 The latter conditions gave a dramatic acceleration for mono- and di-fluorination and led to high yields of products (Table 1). Ultrasonic irradiation therefore provides the best conditions for the preparation of 0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2006.03.144 Keywords: Heterocycles; Nitro compounds; DBU; Selectfluor; Ultrasonic. DOI of original article: 10.1016/j.tetlet.2006.01.037 q Erratum to Tetrahedron Lett. 47 (2006) 2455. * Corresponding authors. Tel.: +98 311 7932722; fax: +98 311 6689732 (H.L.-K.); tel.: +44 191 2226647; fax: +44 191 2226929 (B.T.G.); e-mail addresses: h.log119@sci.ui.ac.ir; b.t.golding@ncl.ac.uk NO 2 H R 2 R 1 i or ii NO 2 F R 2 R 1 Scheme 1. Reagents and conditions: (i) DBU, Selectfluor, CH 2 Cl 2 ; 4– 6 h, rt; (ii) ))), CH 3 COONH 4 , Selectfluor, MeOH, 10–15 min, rt (n.b. all chiral compounds were racemic). Tetrahedron Letters 47 (2006) 4519–4522