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Aromatic Iodination with Lead Tetraacetate as the Oxidant

Synthesis, 1995, pp 926

An easy, quick, and fairly cheap laboratory method is presented for aromatic iodination of some highly activated arenes and heterocycles with molecular iodine in the presence of either pure lead tetraacetate dissolved in glacial acetic acid, or with the same oxidant but prepared in situ from Pb3O4 dissolved in hot AcOH/Ac2O prior to the iodination. 4-bromo- and 2,4-dibromoanisoles are obtained from anisole by the respective oxidative bromination.

Experimental:

Applicable for the steam-volatile iodo and bromo derivatives; higher boiling compounds may be distilled off with superheated steam to shorten the distillation times.

(a) Iodination in glacial AcOH in the presence of pure Pb(OAc)4; 4-iodoanisole from anisole:

Finely powdered iodine (5.1g, 20 mmol) was suspended in glacial AcOH (100mL) mixed with anisole (4.3g, 40 mmol). Pb(OAc)4 (9.25g, 22 mmol; 10% excess) was added with stirring, and the whole was stirred at room temperature for ~1 h, until the iodine colouration faded. Steam distillation gave the crystalline colourless product, recrystallized from hexane.

Yield: 85%.

The same procedure was applied to iodinate mesitylene, and 1,2-dimethoxy and 1,3-dimethoxybenzenes.

(b) Iodination in AcOH/Ac2O with Pb(OAc)4 prepared in situ; 4-iodoanisole from anisole:

Finely powdered Pb3O4 (15g, 22 mmol; 120% excess with respect to iodine consumed) was suspended, with stirring, in glacial AcOH (25mL) and Ac2O (10mL, 106 mmol). The temperature was raised to 55-60 C (it must not exceed 65 C) and stirring was continued until complete (or nearly so) dissolution of Pb3O4 (15-30 min). The nearly colourless (sometimes greyish) solution was cooled to room temperature, then finely powdered iodine (2.5g, 9.8 mmol) was added followed by anisole (2.1g, 19.6 mmol). The whole was stirred at 45 C for 15-30 min until the iodine colouration faded, leaving the solution yellowish or greyish. It was cooled to room temperature and saturated aq. Na2S2 3 (10-15mL) was slowly added dropwise with vigorous stirring (the reaction is exothermic, thus the temperature may rise to 45 C) until a white (or slightly coloured) slurry resembling cream was obtained. Steam distillation gave a fairly pure product, which was collected, washed with cold water, dried, and recrystallized from hexane.

Yield: 93%

The same procedure was applied, with slight modifications, to iodinate phenetole, 1,2- and 1,3-dimethoxybenzenes, mesitylene, and thiophene. The last compound gave a liquid product, hence the cooled steam distillate was extracted three times with CHCl3, the extract was dried over MgSO4, the solvent was distilled off, and the oily residue was fractionated under reduced pressure, bp 85-87 C/36mmHg, to yield the pure product. In the case of iodomesitylene, the oily crude product was fractionated under vacuum, bp 148-150 C/50mmHg, to yield the pure solid product.

(c) Bromination in AcOH with Pb(OAc)4; 4-bromoanisole and 2,4-dibromoanisole from anisole:

Pb(OAc)4 (9.75g, 22 mmol; 10% excess) was suspended in glacial AcOH(100mL) mixed with anisole (4.3g, 40 mmol). Then, at r.t., bromine (3.2g, 20 mmol) was added dropwise with stirring; the temperature rose to 40 C, and after ~10 min the bromine colouration faded. The contents of the flask were steam distilled and the cooled distillate was extracted with ether. The solvent was distilled off from the dried (MgSO4) extract, and the fraction boiling at 214-215C was collected.

Yield: 82%