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Reduction of Aryl Halides into Arenes with 2-Propanol Promoted by a Substoichiometric Amount of a tert-Butoxy Radical Source
Received: 01 December 2015
Accepted after revision: 11 January 2016
27 January 2016 (online)
Aryl halides are reduced into the corresponding arenes in high yields, using 2-propanol, cesium carbonate, and di-tert-butyl peroxide (or di-tert-butyl hyponitrite) as a reductant/solvent, a base, and a radical initiator, respectively. This simple system reduces a wide variety of aryl bromides, chlorides, and iodides through single-electron-transfer mechanism with high functional-group tolerance.
References and Notes
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- 2 For overview on the methods to reduce aryl halides into arenes, see: Smith MB. March's Advanced Organic Chemistry . Wiley; Hoboken: 2013. 7th ed., 647-648
- 3 For a recent example, see: Bhattacharjya A, Klumphu P, Lipshutz BH. Org. Lett. 2015; 17: 1122 ; and references cited therein
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- 6 p-Methoxyphenyl radical possibly reacts also with anisole (2a). However, it would give p-(p-methoxyphenyl)anisole in addition to ortho and meta isomers. No generation of the para isomer shows that such a reaction hardly took place under the reaction conditions.
- 7 The half-life of t-BuOOt-Bu is reported to be more than 1000 h even at 80 °C. See: Walling C. Tetrahedron 1985; 41: 3887
- 8 t-BuON=NOt-Bu is reported to undergo decomposition into t-BuO• and N2 with t 1/2 = 29 min at 65 °C. See: Kiefer H, Traylor TG. Tetrahedron Lett. 1966; 7: 6163
- 9 General Procedure To a 3 mL vial equipped with a stir bar in a glove box were added successively an aryl halide 1 (0.25 mmol), 2-PrOH (1.5 mL, 0.020 mol), Cs2CO3 (97.7 mg, 0.30 mmol), and t-BuOOt-Bu (8.8 mg, 0.050 mmol). The vial was taken out of the glove box and stirred at 120 °C for 3 or 24 h. Decane was added to the reaction mixture as an internal standard for GC analysis. After dilution with EtOAc (1.0 mL), an aliquot was subjected to GC analysis. For products having a boiling point that is high enough not to be lost in evacuation process, addition of H2O (20 mL) was followed by extraction with EtOAc (3 × 20 mL). The combined organic layer was dried over MgSO4, filtered, and concentrated in vacuo. The residue was subjected to silica gel chromatography (PTLC) to give the corresponding arene 2.
A combination of LiAlH4 as a reductant with t-BuOOt-Bu as a radical initiator is used for reduction of aryl halides under photoirradiation, see: