Selenium fundamental properties

Some of the fundamental properties of sulfur, selenium and tellurium differ significantly from those of oxygen, giving rise to disparities between the structures and properties of heavier chalcogen-nitrogen molecules and ions compared to those of their N-O counterparts. The major contributors to these differences include the following ... 

Chapter 5 considers the relationship between mechanism and regio- and stereoselectivity. The reactivity patterns of electrophiles such as protic acids, halogens, sulfur and selenium electrophiles, mercuric ion, and borane and its derivatives are explored and compared. These reactions differ in the extent to which they proceed through discrete carbocations or bridged intermediates and this distinction can explain variations in regio- and stereochemistry. This chapter also describes the El, E2, and Elcb mechanisms for elimination and the idea that these represent specific cases within a continuum of mechanisms. The concept of the variable mechanism can explain trends in reactivity and regiochemistry in elimination reactions. Chapter 6 focuses on the fundamental properties and reactivity of carbon nucleophiles, including... 

Fundamental properties are revealed in kinetic acidity studies (the kinetic acidity of m-CF3CgH4SMe is greater than that of the selenium analogue by a factor of 1.38), polarography (RSCH2CH2COCO2R is protonated at a carbonyl group rather than at sulphur), voltammetry (three successive anodic peaks for Ph2Se at Pt in MeCN), and complexation enthalpies of l,cu-bis-(n-butylthio)alkanes with... 

Chemically Modified Enzymes. Besides varying the physicochemical properties of enzymes (such as their solubility), the catalytic properties of an enzyme can be fundamentally altered by chemical modification of the chemical operator in the active site. This teclmique leads to semisynthetic enzymes, which often do not have much in conunon with their natural ancestors [492, 493]. Early efforts in this directiOTi focussed on the modification of nucleophilic OH- or SH-residues in Ser-or Cys-hydrolases, such as subtihsin or papain, respectively. For example, the Ser-hydroxy group within the active site of subtilisin was converted to its selenium... 

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