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12th March 2014 @ 13:46


As per US2006199817 (page 53). p-toluenesulfonamide (4 g, 2.32 mmol) and iron (0.81 g, 250 mesh, 14.72 mmol, 6.35 equivalents) were stirred[1] and to the mixture was slowly added bromine (12 mL).[2] The reaction was stirred in air for 1 hour and then slowly poured into sodium thiosulfate (1M, 200 mL) with vigorous stirring.[3] The aqueous layer was extracted with dichloromethane (200 mL * 2) and the combined organic layers were dried over magnesium sulfate and concentrated in vacuo to give a yellow-white solid (7.41g). 1H NMR (B1) indicated 2 parts of the desired product, contaminated with 1 part starting material and no undesired regioisomers or multiply-brominated species (less than 0.2%). TLC indicated co-elution of starting material with product (dichloromethane, Rf ca. 0.3). Therefore, the reaction was repeated exactly as previously described and left stirring overnight.

After 16 hours, 1H NMR from a mini-workup (D1) indicated completion, and the reaction was diluted with dichloromethane (150 mL) and quenched by the addition of aqueous sodium thiosulfate (1M, 300 mL) with stirring. The organic layer was separated and the aqueous layer extracted with a further portion of dichloromethane (150 mL). The combined organic layers were dried over magnesium sulfate and filtered through a short plug of silica, which was then flushed with methanol/dichloromethane (1:9 v/v, 50 mL). The combined organic fractions were concentrated in vacuo to give a yellow solid containing only the desired product (by NMR) and significant insoluble impurities thought to be elemental sulfur.



Reactant: InChI=1S/C7H9NO2S/c1-6-2-4-7(5-3-6)11(8,9)10/h2-5H,1H3,(H2,8,9,10)

Product: InChI=1S/C7H8BrNO2S/c1-5-2-3-6(4-7(5)8)12(9,10)11/h2-4H,1H3,(H2,9,10,11)


Colloquial observations:

1) The iron sticks to the magnetic stirrer. This is fine.

2) There is a slight exotherm upon addition but temperature is effectively controlled by boil-off of bromine. 

3) The reaction flask was rinsed out with a little DCM until colourless; no trace of metallic iron remained.

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