A.
Diphenyliodonium-2-carboxylate. An
Erlenmeyer flask containing
80 ml. of concentrated sulfuric acid (Note
1) is placed in an
ice bath to cool. To a
2-l. three-necked flask are added (Note
2)
20 g. (0.081 mole) of lump-free o-iodobenzoic acid (Note
3) and
26 g. (0.096 mole) of potassium persulfate (Note
4). The flask is cooled in an ice bath, the chilled
sulfuric acid is added, and the flask is swirled in an ice bath for 4–5 minutes to produce an even suspension and to control the initial exothermal reaction. The flask is then removed from the ice bath and the time is noted. The reaction mixture foams somewhat and acquires a succession of colors. The flask is mounted in a pan of
acetone, a
mechanical stirrer with a curved
Teflon® blade is placed in the center neck of the flask and operated slowly, and a
250-ml. separatory funnel is mounted into a side neck. Three flasks are put in an ice bath to cool: one containing 190 ml. of distilled water, another
230 ml. of 29% ammonium hydroxide, and another
400 ml. of methylene chloride.
After the oxidation has proceeded for 20 minutes, the
acetone bath is brought to 10° by addition of crushed dry ice, and the solution is stirred for 2–3 minutes. There is added
20 ml. of thiophene-free benzene (17.6 g., 0.226 mole), and stirring at 10° is continued for 1 hour (Note
5). The temperature of the acetone bath is lowered to −15° by addition of crushed dry ice and the bath kept at this temperature while the chilled 190 ml. of distilled water is added with efficient stirring to precipitate the
potassium bisulfate salt of diphenyliodonium-2-carboxylic acid. The
400 ml. of chilled methylene chloride (Note
6) is added, and a 100°
thermometer is mounted in a side neck in such a way that the 15–25° section is visible. With efficient cooling (bath at −15°) and stirring, the
230 ml. of chilled 29% ammonium hydroxide (Note
7) is added at such a rate that the temperature of the reaction mixture remains between 15° and 25°. Approximately 10 minutes is required for the addition, and after the addition the aqueous layer should be alkaline to indicator paper (pH 9). The stirrer and the thermometer are removed and rinsed with water and
methylene chloride. The bulk of the aqueous layer is decanted into a
500-ml. Erlenmeyer flask for temporary storage, and the reaction flask is emptied and rinsed into a
500-ml. separatory funnel. The pale tan lower
methylene chloride layer is drained into a
1-l. Erlenmeyer flask, the decanted aqueous layer is added to the separatory funnel, and the combined aqueous layer is extracted with two
100-ml. portions of methylene chloride. The combined extract is dried over anhydrous
sodium sulfate, filtered into a 1-l. Erlenmeyer flask, and the filtrate is evaporated on the
steam bath until the product is left as a dry grayish cake. The cake is dislodged and broken up with a
stainless steel spatula, and the bulk of the product is transferred to a paper. Material adhering to the flask and spatula is dissolved in boiling
methylene chloride and the solution transferred to a tared
500-ml. flask and evaporated to dryness. The solid product is added to the flask and the combined solid is brought to constant weight at steam-bath temperature and water-aspirator pressure. The yield of crude product is
23.3–24.4 g. (Note
8).
Boiling water (275 ml.) is poured into the flask, the product brought into solution at the boiling point, and
0.4 g. of Norit® is added carefully to the slightly cooled solution. The solution is again heated to boiling, filtered, and allowed to stand for crystallization overnight, eventually at 0°. The colorless prisms of
diphenyliodonium-2-carboxylate monohydrate (Note
9) are collected and air-dried to constant weight at room temperature. The yield of product, m.p.
220–222° (dec.), is
20–22 g. (
72–79%, (Note
10).