Disulfide catalysts, role

Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3. 

Metal sulfides play an important role in catalyzing a wide variety of hydrogenations (e.g., of fats, coal, or olefins) and also desulfurization reactions, which are used in pretreatment of fossil fuels to reduce the emission of sulfur oxides during combustion (Section 8.5). Molybdenum disulfide, an important defect catalyst, can be made to function as an n-type (Moi+xS2) or p-type (Mo1 xS2) semiconductor by exposure to an appropriate mixture of H2S and hydrogen at temperatures on the order of 600 °C. The equilibrium... 

The role of VO(acac) is presumably identical with that of the oxidative polymerization of diphenyl disulfide. The V(IV) catalysts disproportionate to give a V(ll) and V(III) species. The V(V) species oxidizes monomer 311 producing cation-radical 328 and V(1V). V(III) is readily oxidized in triflic acid by oxygen to regenerate the V(IV) catalyst [211]. 

The Transition Metal Sulfides are a group of solids which form the basis for an extremely useful class of industrial hydrotreating and hydroprocessing catalysts. Solid state chemistry plays an important role in understanding and controlling the catalytic properties of these sulfide catalysts. This report discusses the preparation of sulfide catalysts, the role of disorder and anisotropy in governing catalytic properties, and the role of structure in the promotion of molybdenum disulfide by cobalt. 

Disulfides play a prominent role in pharmaceuticals, pesticides, and rubber vulcanization reagents. In biological systems, disulfide bonding is essential for peptides and proteins to fold into their active conformation. Noel et al. reported a mild batch and continuous-flow method to access disulfides by the aerobic oxidation of thiols [48]. Initially, an open flask batch procedure with Eosin Y (1 mol%) as catalyst resulted in near quantitative yields for a variety of thiophenols and heteroaromatic and aliphatic thiols, within 16 h. Moreover, it was found that the... 

The selective aerobic (O2) oxidative coupling of thiols in several imidazolium-based ionic liquids is studied in the absence of any base/metal catalysts. Disulfides are obtained from the corresponding thiols in good to excellent yields in l-hexyl-3-methylimidazolium bromide ([hmim]Br). Furthermore, a H NMR-based mechanistic study of the S-S bond formation demonstrated the cooperative role of halide anion and imidazolium cation of [hmimJBr. ... 

---