Hypervalent selenium

For comparative purposes and in order to enhance our understanding of trends in chemical shifts, typical examples of hypervalent selenium and tellurium fluorides are provided in Scheme 7.20. 

Selenurane oxides are also one of the hypervalent selenium compounds. Recently, the enantiomers of chiral selenurane oxide 38 were isolated for the first time by enantioselective liquid chromatography of the racemate or by spontaneous resolution occurring during slow evaporation of its acetonitrile solution or slow crystallization from the same solvent.57 The absolute configurations of the enantiomers were determined by X-ray crystallographic analysis (Scheme 17). 

Very little is known about these hypervalent selenium and tellurium compounds. 

Three relatively stable compounds (210a-210c) of this class of hypervalent selenium heterocycles are known. The assignment of a bicyclic structure is based on physical and... 

Structure elucidation of this class of hypervalent selenium compounds (211) is based on spectroscopic methods and on a comparison of the results with those obtained for their established sulfur analogs. The compounds exist as stable, purple or violet crystals. UV, NMR and mass spectral data are reported in Table 7 <76JCS(P1)228,76ACS(B)600). 

The radical substitution at selenium is efficient when the leaving radical is stabilized (e.g. benzyl, tert-butyl). This reaction has been used for the preparation of selenium containing heterocycles [117]. In Eq. 50, the conversion of the Barton ester 156 to the tetrahydroselenophene derivative 157 is described [118, 119]. Mechanistic studies and ab initio calculations indicate that the substitution does not involve hypervalent selenium derivatives. 

Organic compounds possessing hypervalent selenium atoms are called selenuranes. Although selenuranes with four organyl residues had been originally considered to be unstable, tetraphenylselenurane 202 and some... 

The synthesis of several novel hypervalent selenium heterocycles has been reported. Thus the l,6-bis-(2,4-dinitrophenyl) l,6-dihydro-3,4-dimethyl-l,2,3-selenadiazolo[5,l-c]selenadiazole-7-Se (167) was obtained through... 

Selenanthrene 7 and related molecules react with halogens to form hypervalent selenium compounds (CHEC-11(1996), 6.24.4.2.1) and numerous unusual structures of this type have been reported recently (see Section 9.15.3.1). Theoretical investigations of these reactions have also been described in Section 9.15.2. Comparison of the electrochemistry of 7 with dibenzo(ce)-l,2-diselenine 89 has been discussed in Section 9.15.3.3. 

---