Polymers dehalogenation reactions

Polymer-supported metallohydride carbonyl dehalogenation reactions... 

Conjugated polymers can also be prepared by condensation polymerisation, usually from a dihalide with the elimination of the related acid. The dehalogenation reaction for the preparation of polyfp-phenylenevinylene) is a typical example ... 

The dehalogenation reaction has also been used for preparing unusual polymers that contain the Si-Si o-bonds in conjugation with an organic n-system. Thus, reaction of dichlorosiloles with lithium metal results in the formation of l,r-polysiloles (see Eq. 1.7) [20, 21]. Every silicon in these polymers is also a part of a five-membered unsaturated heterocyclic ring. 

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

In a similar type of reaction, polymer-supported hydridoiron tetracarbonyl anion reacts with simple non-benzylic aliphatic bromides and iodides to produce aldehydes (Table 8.15), presumably through the intermediate formation of RCOFeH(CO)3, which undergoes reductive extrusion of the aldehydes [3], In contrast, benzylic halides and a-halocarbonyl compounds are reductively dehalogenated by the HFe(CO)4 anion (see Chapter 11). 

The reductive dehalogenation of haloalkanes has also been achieved in high yield using polymer supported hydridoiron tetracarbonyl anion (Table 11.15). In reactions where the structure of the alkyl group is such that anionic cleavage is not favoured, carbonylation of the intermediate alkyl(hydrido)iron complex produces an aldehyde (see Chapter 8) [3]. 

Ultrasonic irradiation of a mixture of zinc and a,a -dibromo-orthoxylene in dioxane results in dehalogenation and the generation of a xylylene intermediate (9) which readily adds to any dienophiles present in the reaction mixture e. g. with maleic anhydride or methyl propenoate to afford high yields of (10) and (11) respectively [95]. In the absence of dienophile the product is mainly polymer with a trace of (12) (Scheme 3.16). The work was performed in a cleaning bath at 25 °C on 10 mmol scale using 23 mmol zinc under N2. There was no reaction in the absence of ultrasound. 

A problem in these couplings is the identity of the end groups of the formed polymers. Pd-catalyzed dehalogenation and/or phosphonium salt formation are side reactions that are difficult to avoid. A concern for the structural integrity of the backbone is the formation of butadiyne defects. While there is no direct measure to determine the amount of butadiyne defects in PAEs, the numbers are estimated to range from 1 to 10% of all repeat units. 

The C-C coupling reaction between RMgX and R X is considered to proceed though an Ni(R)(R )Lm intermediate, and acceleration of the reductive elimination of R-R by coordination with olefinic or aromatic R X to Ni(R)(R )Lm is necessitated for a smooth catalytic reaction [15,16]. On these bases Ni-pro-moted dehalogenative polycondensation of dihalo organic compounds is suited to the preparation of 7i-conjugated aromatic and olefinic polymers. 

In many cases, high yields (>75% based on polymer loading) of the final product were obtained. Some drawbacks to this approach include partial dehalogenation of (67) back to (65) during the Stille coupling, the presence of Pd impurities in some products, and structural and chemical constraints imposed by the bromination reaction. 

Five catalyst types were used in these product distribution studies Pd/C, Pd/alumina, pure Pd metal powder, Pd/Fe, and Pd/Cu/support. The last two catalysts are not traditional materials. The Pd/Cu, which was used in the reduction of nitrate, was a bimetallic catalyst on an alumina, silica, or polymer support. Fe is an atypical support in that it has the intrinsic ability to reductively dehalogenate chlorinated compounds by oxidizing to Fe2+ however, the rapid reaction rates associated with Pd/Fe are indicative of Pd-catalyzed reactions, which are much faster than Fe reactions. 

In a subsequent study, Da Silva et al. investigated the photo degradation of 4-chlorophenol on the polysaccharide cellulose and on silica [41]. In both systems, transient spectra and products were consistent with the assumption of heterolytic dehalogenation to give 4-oxocyclohexa-2,5-dienylidene. In cellulose, unsubstituted phenoxyl radicals and phenol were the main products, indicating that the polymer serves as H-donor for the carbene. There was no effect of O2 on the reaction course, which was explained by the protective effect of the macromolecular structure. In silica, which contained... 

Optically active l,l -binaphthols are among the most important chiral ligands of a variety of metal species. Binaphthol-aluminum complexes have been used as chiral Lewis acid catalysts. The l,T-binaphthyl-based chiral ligands owe their success in a variety of asymmetric reactions to the chiral cavity they create around the metal center [107,108]. In contrast with the wide use of these binaphthyls, the polymer-supported variety has been less popular. The optically active and sterically regular poly(l,l -bi-naphthyls) 96 have been prepared by nickel-catalyzed dehalogenating polycondensation of dibromide monomer 95 (Sch. 7) [109] and used to prepare the polybinaphthyl aluminum(III) catalyst 97 this had much greater catalytic activity than the corresponding monomeric catalyst when used in the Mukaiyama aldol reaction (Eq. 29). Unfortunately no enantioselectivity was observed in the aldol reaction. 

Poly(pyrazine-2,5-diyl) has been prepared by Yamamoto et al. [74] by organome-tallic dehalogenative polycondensation of 2,5-dibromopyrazine (Scheme 14.37). The reaction was performed by irradiating a mixture of 2.62 mmol 2,5-dibromopyrazine, 5.23 mmol bis(l,5-cyclopentadiene)nickel(0), and 5.23 mmol 2,2 -bipyridyl in a single-mode microwave reactor for 10 min, either in toluene or DMF solution. Under microwave conditions, the polymer was afforded in 83-95%... 

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