Polymeric aryl halide

SCHEME 1.10 Polymeric aryl halide is more reactive than monomeric aryl halide. 

Early work performed by Lenz et al. in the 1960s demonstrated this approach by using electrophilic aromatic substitution to produce poly(phenylene sulfide) [91] (Scheme 1.10). In this case, an aryl halide is the electrophile, which is substituted by the metal thiophenoxide nucleophile. In the monomer, the metal sulfide is a strong electron-donating group, which deactivates the para position where electrophilic substitution must take place. Conversely, the polymer chain end is only weakly deactivated by the sulfide bond, rendering the polymeric aryl halide more reactive than the monomeric aryl halide. Unfortunately, Lenz was unable to characterize molecular weight distribution due to the insolubility of the resultant polymers. 

The direct preparation of arylboronic esters from aryl halides or triflates now allows a one-pot, two-step procedure for the synthesis of unsymmetrical biaryls (Scheme 1-41) [147]. The synthesis of biaryls is readily carried out in the same flask when the first coupling of the triflate with diboron 82 is followed by the next reaction with another triflate. The synthesis of naturally occurring biflavanoids and the couphng of N-(phenylfluorenyl)amino carbonyl compounds to polymeric supports are reported [154]. 

A superior and relatively versatile procedure for the synthesis of unsymmetrical dialkyl thioethers, which avoids the unattractive direct use of thiols, utilizes the stable l-alkylthioethaniminium halides, which are readily obtained from thioacet-amidc [32] (Scheme 4.4). The reaction has also been used for the synthesis of alkyl aryl thioethers from activated aryl halides [33], but it cannot be used for the synthesis of cyclic thioethers, as polymeric sulphides are formed from a,co-dihaloalkanes. A similar sequence to that which leads to the thioethers has been used for the synthesis of S-alkyl thioesters [34] (see 4.1.26). 

Meta-substituted AB-type monomer such as m-bromoaniline also underwent Pd-catalyzed polycondensation [65], but the para counterpart has not been reported, possibly because aryl halides bearing the NH group in the para position are generally poor substrates for the aryl amination process. Buchwald et al. prepared a dimeric para-substituted AB monomer bearing the BOC protecting group and polymerized it with Pd2(dba)3 and 2-(di-ferf-bulylphosphino)biphcnyl [73]. The polymerization carried out at room... 

Vinyl chloride [159], vinylidene chloride and chloroprene are polymerized in this way at ambient temperature [160]. Polymerization ceases before all the monomer has been exhaused. The chemistry of these processes is little known. Vinyl chloride [161] can be metallised by alkyllithium compounds and radicals are formed by the interaction of n-BuLi with alkyl and aryl halides [162]. Thus a contribution of the radical mechanism in polymerizations of chlorine-containing monomers cannot be excluded. The generation... 

The telluride-anions are sufficiently nucleophilic to react also with aryl halides to afford aryl tellurides, but generally the aryl group must be activated either by an additional substituent, or the reaction must be carried out at higher temperatures. For example,j9ara-iodobenzene reacts at 110-120 °C with sodium telluride to form polymeric poly-/ -phenylenetelluride. ... 

Silicon and germanium atoms also undergo oxidative addition reactions. However, in only a few cases do stable products result. Alkyl and aryl halides yield intermediate organosilicon halides that polymerize on warming Hydrogen halides yield stable products, but the method has no synthetic value ... 

The Pd-catalyzed coupling reaction of an aryl halide and olefin is a very efficient and practical method for making C—C bonds. The Heck alkenylation of aryl bromides with ethylene was used by Dow Chemical to make high-purity 2- and 4-vinyltoluenes, which are of interest as co-monomers in styrene polymers [159]. The monomer, o-vinyltoluene (99), has a low toxicity and an attractive co-monomer for styrene polymers. o-Vinyltoluene improved heat distortion properties of styrene and polymerization rate. It also minimized color formation or cross-linking and it was difficult to make by other routes [159]. Catalyst turnover, rate, and lifetime were significantly improved. 

In SPOC, either the aryl or vinyl halide or the boronate can be bound to the polymeric support. In most cases however, the aryl halide is bound to the polymer. A wide range of coupling conditions were developed within recent years, which are compatible with many polymeric supports (polystyrene, polyacrylamide, PEG, etc.) and are tolerated by many functional groups. 

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