Catalysis continued coupling reactions

Pal et al. (1994) compared the catalysis of oxidative coupling reactions of various phenolic compounds by the enzymes, laccase and tyrosinase, and mineral catalyst, birnessite. Birnessite acts as a heterogeneous catalyst whereas laccase and tyrosinase function as homogeneous catalysts. Laccase and tyrosinase continue to oxidize catechol after repeated additions of the chemical, while birnessite lost its oxidizing activity after the first addition of catechol (Figure 2.20). In the case of birnessite,... 

Coupling reactions of aromatic, heteroaromatic, and vinylic substrates with alkynes via sp C—H bond cleavage under rhodium, iridium, ruthenium, palladium, or nickel catalysis to produce a wide range of cyclic compounds have been described in this chapter. These annulation reactions provide powerful methods for constructing a variety of -rr-conjugated molecules containing fused aromatic and heteroaromatic nuclei from simple, readily available substrates. Extensive efforts will continue to be made to extend the scope of starting materials for this catalysis. 

The unique versatility of ruthenium as an oxidation catalyst continues to provide a stimulus for research on a variety of oxidative transformations. Its juxtaposition in the periodic table and close similarity to the biological redox elements, iron and manganese, coupled with the accessibility of various high-valent oxo species by reaction of lower-valent complexes with dioxygen make ruthenium an ideal candidate for suprabiotic catalysis. 

Biocatalysed reactions assisted by US have mainly to do with enzymatic digestion [9], These processes, which can involve solid, liquid and heterogeneous samples, arefarfrom real digestions as only the target analytes are removed, the remaining sample components preserving their appearance, in most cases. This is a relatively novel use of the US-enzyme couple, particularly in hydrolysis reactions [33]. By contrast, US assistance to the joint use of enzymatic catalysis and supercritical fluids, which has aroused much analytical interest [34], continues to be unexplored as yet. 

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