Harkesh B. Singh

Synthetic organoselenium compounds as antioxidants:glutathione peroxidase activity

Organoselenium compounds find applications in organic synthesis, materials synthesis, ligand chemistry and biologically relevant processes. This review deals with the use of various synthetic organoselenium compounds as mimics of glutathione peroxidase (GPx), a selenoenzyme which catalyses the reduction of a variety of hydroperoxides and protects the cell membranes from oxidative damage. The mechanism by which these compounds catalyse the reduction of peroxides is also reviewed. The cyclic selenenamides and diselenides with suitably positioned substituents exert their catalytic activity by a mechanism similar to that of the natural enzyme.

Organoselenium chemistry: role of intramolecular interactions.

In 1836 the first organoselenium compound, diethyl selenide, was prepared by Löwig,1 and it was isolated in the pure form in 1869.2 Early selenium chemistry involved the synthesis of simple aliphatic compounds such as selenols (RSeH), selenides (RSeR), and diselenides (RSeSeR); however, because of their malodorous nature, these compounds were difficult to handle. This, combined with the instability of certain derivatives and difficulties in purification, meant that selenium chemistry was slow to develop. By the 1950s, the number of known selenium compounds had increased significantly, but it was not until the 1970s, when several new reactions leading to novel compounds with unusual properties were discovered, that selenium chemistry began to attract more general interest.3-9 Aryl-substituted compounds were synthesized that were found to be less volatile and more pleasant to handle than the earlier aliphatic compounds. Compounds containing selenium in high oxidation states are relatively easy to manipulate using modern techniques.4c Organoselenium chemistry has now become a well-established field of research, and recent advances have been brought about by the potential technical applications of selenium compounds. Today selenium compounds find application in many areas including organic synthesis,4 biochemistry,5 xerography,6 the synthesis of conducting materials7 and semiconductors,8 and ligand chemistry.4c,9 Many of these aspects of selenium chemistry are wellcovered elsewhere in the literature; however, the subject of hypervalency has not attracted much attention and is the focus of this review.10

INTRAMOLECULAR SE ... N NONBONDING INTERACTIONS IN LOW-VALENT ORGANOSELENIUM DERIVATIVES : A DETAILED STUDY BY 1H AND 77SE NMR SPECTROSCOPY AND X-RAY CRYSTALLOGRAPHY

A series of novel low-valent organoselenium compounds stabilized by Se···N intramolecular interactions (such as the one in the figure) were synthesized, characterized, and examined for Se···N nonbonding interactions. A correlation between Se···N intramolecular distance and 77 Se chemical shift is attempted.

Intramolecular coordination in tellurium chemistry

Intramolecular coordination in organotellurium derivatives is reviewed. The various types of complexes are discussed according to the types of donor atoms involved. Structural and spectroscopic evidence in support of intramolecular coordination, where Te(II) and Te(IV) behave as Lewis acids, is emphasized.

Synthesis of some macrocycles/bicycles from bis(o-formylphenyl) selenide: X-ray crystal structure of bis(o-formylphenyl) selenide and the first 28-membered selenium containing macrocyclic ligand

Bis( o -formylphenyl) selenide ( 7 ) was synthesized using the ortholithiation methodology. The reaction of o -lithiobenzaldehyde acetal ( 5 ) with Se(dtc) 2 (dtc=diethyldithiacarbamate) afforded bis( o -formylphenyl) selenide acetal ( 6 ) in good yield. The key starting material 7 was isolated as pale yellow solid upon refluxing 6 with concentrated HCl. The structure of 7 was solved in the monoclinic space group P 2/ c with cell constants a =8.0170(6) A, b =8.4514(6) A and c =17.5289(12) A, Z =4. The condensation of 7 with diethylenetriamine yielded the novel macrocyclic ligand [C 36 H 38 N 6 Se 2 ] 8 via metal-free dimerization. Crystals of 8 are monoclinic, space group C 2/ c with a =18.732(3) A, b =8.6515(10) A, c =22.590(3) A and Z =4. Hydrogenation of macrocycle 8 provided the corresponding saturated tetraazamacrocycle [C 36 H 46 N 6 Se 2 ] ( 9 ), protonation of which with HBr afforded [C 36 H 52 N 6 Se 2 Br 6 ·H 2 O] ( 10 ). The two novel cryptands [C 54 H 54 N 8 Se 3 ] ( 12 ) and [C 54 H 54 N 8 Te 3 ] ( 13 ) were prepared from the reaction of tris(2-aminoethyl)amine (tren) and the chalcogenides ( 7 ) and bis( o -formylphenyl) telluride ( 11 ) respectively using cesium ion as the template.

Novel chiral organoselenenyl halides stabilized by Se···N nonbonded interactions: synthesis and structural characterization

The first examples of structurally characterized chiral organoselenenyl halides stabilized by Se···N intramolecular interactions are described. The existence of strong nonbonded Se···N interactions in the solid state as well as in solution was unambiguously determined by single-crystal X-ray analysis and 77 Se NMR spectroscopy.

Synthesis and characterization of monomeric tellurolato complexes of zinc and cadmium: crystal and molecular structure of bis[2-(4,4-dimethyl-2-oxazolinyl)phenyl]ditelluride

The synthesis and characterization of homoleptic zinc(II) and cadmium(II) tellurolates incorporating the intramolecularly chelating oxazoline ligand are described. The derivatives, M[Te(Ox)] 2 [M=Zn ( 4 ) or Cd ( 5 ) and Ox=2-(4,4-dimethyl-2-oxazolinyl)phenyl], are prepared in good yield via the metathesis reactions of MCl 2 with lithium arenetellurolate, OxTe − Li + ( 2 ). Attempts to synthesise the mercury complex led to isolation of the corresponding ditelluride ( 3 ). Crystal structure of the ditelluride was determined by X-ray diffraction method. Of particular interest in the structure is the intramolecular interaction of the sp 2 nitrogen with the tellurium. The strength of the Te…N nonbonded interactions in this compound [Te(1)…N(1): 2.864(5), Te(2)…N(2): 2.694(5) A] is found to be stronger than the similar interactions found in related compounds. The zinc and cadmium complexes are quite stable in the solid state and highly soluble in common non-polar organic solvents. The variable temperature NMR spectra of Zn[Te(Ox)] 2 ( 4 ) and Cd[Te(Ox)] 2 ( 5 ) show the complexes to be chiral at low temperatures.

Diferrocenyl diselenides: excellent thiol peroxidase-like antioxidants

Synthesis, structure and thiol peroxidase-like antioxidant activity of several diaryl diselenides having intramolecularly coordinating amino groups are described; the diselenides having both tertiary amino groups and redox-active ferrocenyl units show excellent peroxidase activity.

Synthesis of organochalcogens stabilized by intramolecular non-bonded interactions of sterically unhindered 2-phenyl-2-oxazoline

The synthesis and characterization of low-valent organoselenium and -sulfur compounds incorporating sterically unhindered 2-phenyl-2-oxazoline are described. Organylselenenyl halides, RSeX (X = Cl, Br, I) were prepared from diselenide and the benzyl selenide derivative was synthesized by the reaction of in situ generated lithium arylselenolate, OxSe−Li+ (Ox = 2-phenyl-2-oxazoline) with benzyl chloride. These compounds in general show strong Se⋯N intramolecular interactions as compared with the substituted oxazoline analogues. Bis[2-(2-oxazolinylphenyl)] disulfide and [2-(2-oxazolinyl)phenyl]benzyl sulfide were synthesized by the ortho-lithiation method and characterized by 1H and 13C NMR spectroscopy. The S⋯N intramolecular interactions were confirmed by single crystal X-ray crystallography.

Contrasting coordination behaviour of 22-membered chalcogenaaza (Se, Te) macrocylces towards Pd(II) and Pt(II): Isolation and structural characterization of the first metallamacrocyle with a C–Pt–Se linkage

Interaction of the 22-membered selenaaza macrocyle 3 with Pt(II) leads to the formation of the novel cationic Pt(IV) metallamacrocylic complex 5 via the oxidative addition of a C-Se bond to Pt(II), whereas corresponding reactions of 3 and 4 with Pd(II) afford cationic complexes 6 and 7 with differing ligating properties.