Polystyrene brominated cross-linked

Alternatively, boranes can be prepared in solution and then coupled with support-bound carbon electrophiles. The Suzuki coupling of alkylboranes, generated in situ from 9-BBN and alkenes, with brominated cross-linked polystyrene has been used to link substituted alkyl chains directly to the polymer (Entry 4, Table 5.3). Alkylboranes have also been used to alkylate polystyrene-bound aryl iodides (Entries 3 and 5, Table 5.3). 

Polystyrene and its divinylbenzene cross-linked copolymer have been most widely exploited as the polymer support for anchoring metal complexes. A large variety of ligands containing N, P or S have been anchored on the polystyrene-divinylbenzene matrix either by the bromination-lithiation pathway or by direct interaction of the ligand with C1-, Br- or CN-methylated polystyrene-divinyl-benzene network [14] (Fig. 7). 

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]. 

The iodination of cross-linked polystyrene has been achieved using iodine under strongly acidic reaction conditions [55] or in the presence of thallium(III) acetate [61], but this reaction does not proceed as smoothly as the bromination. More electron-rich arenes, such as thiophenes [45,62-64], furans [46], purines [65], indoles [66], or phenols [67,68] are readily halogenated, even in the presence of oxidant-labile linkers (Figure 6.2). Polystyrene-bound thiophenes have also been iodinated by lithiation with LDA followed by treatment with iodine [64],... 

Cross-linked polystyrene can be acylated with aliphatic and aromatic acyl halides in the presence of A1C13 (Friedel-Crafts acylation, Table 12.1). This reaction has mainly been used for the functionalization of polystyrene-based supports, and only rarely for the modification of support-bound substrates. Electron-rich arenes (Entry 3, Table 12.1) or heteroarenes, such as indoles (Entry 5, Table 15.7), undergo smooth Friedel-Crafts acylation without severe deterioration of the support. Suitable solvents for Friedel-Crafts acylations of cross-linked polystyrene are tetrachloroethene [1], DCE [2], CS2 [3,4], nitrobenzene [5,6], and CC14 [7]. As in the bromination of polystyrene, Friedel-Crafts acylations at high temperatures (e.g. DCE, 83 °C, 15 min [2]) can lead to partial dealkylation of phenyl groups and yield a soluble polymer. 

The first successful solid-phase synthesis of a peptide (H-Leu-Ala-Gly-Val-OH) was reported by Merrifield in 1963 [12]. The strategy used is outlined in Figure 16.1. The support was chloromethylated, partially nitrated (or brominated), 2% cross-linked polystyrene. Nitration was necessary to suppress acidolytic cleavage of the benzyl ester attachment during Z-group removal. Acylations of deprotected, support-bound amino acids were performed with DCC, and at the end of the synthesis the peptide was cleaved from the support by saponification with sodium hydroxide. 

The bromination of 1% cross-linked polystyrene was done in the presence of Tl(0Ac)3 or TICI3, which are preferable to the inefficient FeCl3 and the inconvenient BF3 (15). The resin (2 g) was swollen in CCI4 (30 ml) and contacted with TICI3 (0.2 g). The reactants were stirred in the dark for 30 min, then 1.36 g of Br2 in 2 ml of CCI4 were added slowly. After stirring fori hr at room temperature in the dark, the mixture was heated to reflux for 1.5 hr. The reaction mixture was filtered, and the beads were washed in sequence with CCI4, acetone, water, benzene, and methanol. The beads were then dried in vacuum. 

Polymer-supported Wittig reagents were first prepared more than 20 years ago [32]. It has been shown that the success of the reaction depends strongly upon (i) the preparation of the reagent by bromination and phosphination of cross-linked polystyrene rather than by co-polymerization using styryldi-phenyl phosphine, and (ii) the generation of the phosphorane with a base/ solvent system that swells the phosphonium sites in the polymer network (Scheme 6) [33]. Thus, bromination of polystyrene 1 yielded phenyl bromide 32, and this was followed by phosphination with n-butyUithium and chlor-odiphenylphosphine or with Hthium diphenylphosphide to give 33, a compound which is commercially available (Scheme 6). 

Polymeric reagent. Hodge and Richardson have prepared a polymer-supported triphenylphosphine by bromination of a polystyrene cross-linked with divinylbenzene followed by reaction with lithium diphenylphosphide (4, 303). A similar reagent has been described by Regen and Lee. Polymeric material is available from Strem. 

The polymer-bound phosphinate 10 has been prepared by successive treatments of 2% cross-linked brominated polystyrene with n-butyllithium, diethylchlorophosphite, and ethyl bromoacetate. It was used to form alkenes from aldehydes and ketones, but the yields were not consistently high, and the reagent was contaminated with an unidentified polymer-bound phosphorus species (52). 

Rieke calcium is exceptionally highly reactive. Thus, fluorinated, chlorinated, brominated, and chloromethylated cross-linked polystyrene resins are all successfully converted to the corresponding calcium reagents. It should be noted that few metals undei o oxidative addition to aryl fluorides. Thus, it is noteworthy that Ca reacts with / -fluoropolystyrene at room temperature to... 

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