Sulfur-covalent catalysis

In glucosides, hemiacetal hydroxyl activation/substitution can be achieved using a sulfonic anhydride and a nucleophile, plus a base as acid scavenger.12 The reaction is catalysed by dibutyl sulfoxide (Bu2S=0), and shows evidence of sulfur-covalent catalysis. Using benzenesulfonic anhydride [(PhS0)20], it is proposed to involve initial formation of a sulfonium sulfonate (6), the S(IV) centre of which then reacts with... 

Recently an expansion of the electrophile scope of the conjugate addition of sulfur nucleophiles has been reported by different groups. As depicted in Fig. 2.29 for selected examples, Cinchona-denwed catalysts 203 and 205 promote highly enantioselective additions to nitrooleflns [387] and a,p-unsaturated V-acylated oxazolidin-2-ones [388] through non-covalent catalysis. Especially interesting results the Michael reaction to P-substituted nitroacrylates catalyzed by chiral thio-... 

Highly enantioselective ylide-type covalent catalysis has been achieved with sulfides, phosphines, tertiary amines, selenides, and teUurides, and the reported reaction types include epoxidation, aziridination, cyclopropanation, and other cyclization reactions. So far, the sulfur ylide-mediated reactions are the best... 

Sulfides play an important role in hydrotreating catalysis. Whereas oxides are ionic structures, in which cations and anions preferably surround each other to minimize the repulsion between ions of the same charge, sulfides have largely covalent bonds as a consequence there is no repulsion which prevents sulfur atoms forming mutual bonds and hence the crystal structures of sulfides differ, in general, greatly from those of oxides. 

Another type of cysteine-containing metalloprotein which has M-S(cys) bonding at the active site is present in electron transfer proteins or metalloenzymes. Sulfur coordination is an important feature, and the covalency and soft environment are possible prerequisites for efficient electron transfer leading to redox catalysis. A distortion at the metal site is induced by the peptide ligands and is a significant feature of the active sites in metalloproteins containing transition metals Fe, Cu, Ni, Mo, etc. 

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