Green processing, coupling

Supported versions of such silver-catalyzed, three-component couplings have been recently reported. Silver oxide on multiwall carbon nanotubes or on alumina,102 as well as silver-doped zeolites,103 proved to be efficient and reusable catalysts for such coupling reactions. In the former, water was used as solvent, while in the latter, no solvent was required, making it a truly green process. 

In addition to cross-coupling chemistry, one of the most direct routes to the formation of vinyl ethers entails the addition of alcohols and phenols to alkynes. This atom-efficient transformation is referred to as hydroalkoxylation and can be tuned to generate vinyl ethers in excellent yields under very mild conditions. Both intermolecular and intramolecular versions of this reaction are well known, and a vast array of catalysts and conditions has been used for the successful synthesis of vinyl ethers [111, 112]. The following sections will highlight a number of advances in this area with special attention paid to modifications such as the development of fast reactions, green processes, as well as the ability of the reactions to tolerate moisture or air. 

The use of a PTC, tetramethylammonium hydroxide, to carry out the coupling under aeration of nitrobenzene and aniline to give nitroso diphenylamine (which on catalytic reduction with H2 gives 4-aminodiphenylamine, required in the rubber industry) may be cited as a green chemistry route compared to the established process based on p-nitrochlorobenzene and aniline where Cl is wasted (Bashkin et al. 1999). 

It is of primary interest to avoid corrosive mineral acids in synthetic processes. This can easily be achieved by use of acidic solid supports coupled with microwave irradiation. This has been applied to the preparation of quinolines [53] (Scheme 8.35). This procedure is a safe, green alternative to the use of H2S04 at more than 150 °C. In the same way, quinoxaline-2,3-diones were prepared [54] by use of single-mode irradiation. Previous attempts in solution led to explosions, but the authors successfully used solvent-free conditions with acidic supports or catalysts (the best being p-toluenesulfonic acid) and irradiation times of 3 min (Scheme 8.36). 

Dow used the Suzuki coupling process to synthesize a variety of fluorene-based homopolymers and copolymers that are able to emit the three primary LED colors red, green, and blue. Structural representations of these polymers are shown in Figure 11.20. The U.S. patents granted to Dow Chemical on OLED emitting materials are listed in Table 11.4. 

It is briefly reported that the European Commission will issue a White Paper on PVC within the next couple of months. The Paper will probably be a mix of both legislation and voluntary commitments. A Green Paper on PVC was adopted by the EC in July 2000. This raised six issues to be addressed substitution, production process, cadmium stabilisers, lead stabilisers, phthalates and waste management. 

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