Preparation of Aryl and Heteroaryl Halides

Cross-linked polystyrene can be directly brominated in carbon tetrachloride using bromine in the presence of Lewis acids (Experimental Procedure 6.2 [55-58]). Thal-lium(III) acetate is a particularly suitable catalyst for this reaction [59]. Harsher bro-mination conditions should be avoided, because these can lead to decomposition of the polymer. Considering that isopropylbenzene is dealkylated when treated with bromine to yield hexabromobenzene [60], the expected products of the extensive bromi-nation of cross-linked polystyrene would be soluble poly(vinyl bromide) and hexabromobenzene. In fact, if the bromination of cross-linked polystyrene is attempted using bromine in acetic acid, the polymer dissolves and apparently depolymerizes [61]. 

Experimental Procedure 2 limminalion of cross-linked polystyrene [61] 

illium(TU) acetate (1.1 js g. 3.09 mmol) was added to a suspension of polystyrene (in I , cross-linked) in carbon tetrachloride (300 mL), and the mixture was slit red in the dark for 0.3 It. A solution of bromine (13.6 g, 85.1 mmol) in carbon tetiadiloride (20 mL) was then added, and the mixture was stirred at room temperature in the dark for 1 h and at 76 C for 1.5 h. Thereafter, the mixture was filtered and the resin was washed with carbon tetrachloride, acetone, acetone/water (2 1), acetone, benzene, and methanol. Drying of the resin under reduced pressure yielded 26.3 g of brominated polystyrene with a bromine content of 3.1 mmol/g, which corresponds to a bromination of 43% of all the available phenyl groups. 

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