· Bromine Monofluoride
· (MW 98.90)
(adds bromine and fluorine atoms across double and triple bonds;2 brominates aromatic rings;3 converts carbonyls, through their hydrazones4 or dithiolanes,5 to the CF2 group; converts benzylic dithiocarboxylates and orthothioesters to ArCF36)
Physical Data: mp -33 °C (dec to Br2 and BrF3); has not been isolated in pure form; orange-red crystals at low temperatures.7,9
Preparative Methods: unlike most organic reagents, the notation
[BrF] has been designated to indicate a two-step reaction taking place when a mixture of two different agents, one a source of electrophilic bromine and the other of nucleophilic fluorine, is reacted with a substrate. The true molecule BrF can be prepared in situ by reaction between the corresponding elements7 and by reacting bromine with BrF3.8 Many, but not all, reactions of [BrF] and BrF are quite similar.
Bromofluorination of Alkenes.
Adding the elements of bromine and fluorine across double bonds by using N-Bromosuccinimide (NBS), or similar N-Br derivatives, and anhydrous Hydrogen Fluoride is an established procedure employed extensively in steroid chemistry (eq 1).10 The addition proceeds with relatively good regio- and stereospecificity (anti addition). Olah suggested replacing anhydrous HF, which is inconvenient for laboratory use, with the easier to handle HF.py2a (eq 2). Many variations on this procedure have since appeared. These include the following combinations: NBS-Et3N/3HF,11 NBS-AgF/CaF2,12 NBS-Bu4NF,13 NBS-NH4HF2/ultrasound,14 and 1,3-dibromo-5,5-dimethylhydantoin (DBH)/SiF4.15
The combination of agents which results in [BrF] is also used for converting carbonyls, through their hydrazones, into the CF2 group (eq 3).4 The same eventual results can also be achieved through the action of [BrF], made from NBS or DBH with HF.py, on various dithiolanes (eq 4).5 A somewhat similar process is the reaction of [BrF] with methyl aryldithiocarboxylates, which can be directed to give either ArCF3 or the thioethers ArCF2SMe (eq 5).6a Trifluoromethylarenes are also obtained when [BrF] is reacted with aryl orthothioesters (eq 6).6b An interesting variation is the reaction with arylxanthates, which forms aryltrifluoromethyl ethers (eq 7).16
The molecular BrF, prepared in situ from the elements, adds across double bonds with similar results to [IF] (see Iodine Monofluoride), but it also adds to very deactivated enones and ynones (eq 8).2c The strong electrophilicity of the bromine in molecular BrF is the driving force behind the very efficient bromination of practically any deactivated aromatic compounds without the need for any Friedel-Crafts catalyst (eq 9).3
- 1. Boguslavskaya, L. S.; Chuvatkin, N. N. In New Fluorinating Agents in Organic Synthesis; German, L.; Zemkov, S., Eds.; Springer: Berlin, 1989.
- 2. (a) Olah, G. A.; Nojima, M.; Kerekes, I. S 1973, 780; (b) Rozen, S.; Brand, M. JOC 1985, 50, 3342; (c) Rozen, S.; Brand, M. JOC 1986, 51, 222.
- 3. Rozen, S.; Brand, M.; Lidor, R. JOC 1988, 53, 5545.
- 4. Prakash, G. K. S.; Reddy V. P.; Li, X.-Y.; Olah, G. A. SL 1990, 594.
- 5. (a) Sondej, S. C.; Katzenellenbogen, J. A. JOC 1986, 51, 3508; (b) Berglund, R. A.; Fuchs, P. L. SC 1989, 19, 1965.
- 6. (a) Kuroboshi, M.; Hiyama, T. CL 1992, 827; (b) Matthews, D. P.; Whitten, J. P.; McCarthy, J. R. TL 1986, 27, 4861.
- 7. Naumann, D.; Lehman, E. JFC 1975, 5, 307.
- 8. Steunenberg, R. K.; Vogel, R. C.; Fischer, J. JACS 1957, 79, 1320.
- 9. Meinert, H. ZC 1967, 7, 41.
- 10. Steroid Reactions; Djerassi, C., Ed.; Holden-Day: San Francisco, 1963.
- 11. Haufe, G.; Alvernhe, G.; Laurent, A. TL 1986, 27, 4449.
- 12. Ando, T.; Cork, D. G.; Fujita, M.; Kimura, T.; Tatsuno, T. CL 1988, 1877.
- 13. Maeda, M.; Abe, M.; Kojima, M. JFC 1987, 34, 337.
- 14. Ichihara, J.; Funabiki, K.; Hanafusa, T. TL 1990, 31, 3167.
- 15. Shimizu, M.; Nakahara, Y.; Yoshioka, H. CC 1989, 1881.
- 16. Kuroboshi, M.; Suzuki, K.; Hiyama, T. TL 1992, 33, 4173.
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