Chalcogen substitution

TABLE 2.2. Impact of a-Chalcogen Substitution on Fluorine Chemical Shifts ... 

Fig. 10. Collected structural and spectroscopic data for 1,3-chalcogen-substituted stanno- ...

Figure 4.3 Chalcogen substitution in TTF-based donor molecules...

The chalcogen-substituted Ge, Sn and Pb derivatives 86-88 are stable towards both oxygen and moisture. Their stability appears to decrease in the following order E=S>Se Te M=OSn>Ge>Pb. The Te-substituted derivatives RCOTeMR (M=Ge, Sn, Pb) decompose in solutions (of CH2CI2 or CHCI3, for example) at -20 °C to give (R3M)2Te (M=Ge, Sn, Pb) with the hberation of black tellurium [306]. 

Carb-lS. Thio sugars and other chalcogen analogues 2-Carb-l6. Other substituted monosaccharides... 

Heteroatom-Substituted Germanium-Chalcogen Double Bond Compounds... 

Although the high reactivity of metal-chalcogen double bonds of isolated heavy ketones is somewhat suppressed by the steric protecting groups, Tbt-substituted heavy ketones allow the examination of their intermolecular reactions with relatively small substrates. The most important feature in the reactivity of a carbonyl functionality is reversibility in reactions across its carbon-oxygen double bond (addition-elimination mechanism via a tetracoordinate intermediate) as is observed, for example, in reactions with water and alcohols. The energetic basis... 

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

Heavy ketones have been synthesized by several synthetic methods shown in Scheme 19 (i) the reaction of divalent group 14 element species with chalcogen sources, (ii) the reaction of dihydrometallanes with chalcogen sources, (iii) the dechalcogenation of metal polychalcogenides, (iv) the reaction of dilithiomet-allanes with dihalochalcogenides, and (v) substitution of the chloro group of heavy acyl chlorides. The methods used for the synthesis of heavy ketones are summarized in Tables 3-5. 

Although diselenonium-, ditelluronium- and mixed sulfonium-selenonium dications can exhibit either oxidative or electrophilic properties in reactions with nucleophiles, substitution at the onium chalcogen atom is more typical.96 Owing to the increased stability of heavier dichalcogenium-dications, they react only with highly activated substrates such as aniline and tV,A-dimethylaniline, while no reaction is observed with phenol and diphenylamine.113 Reactions of ditelluronium dications with activated aromatics are also not known (Scheme 44).114... 

Thiophene and its derivatives, however, have become the most-studied chalcogen-containing monomers. Doped (oxidised) PT is highly conducting, and 3-alkyl-substituted PTs are soluble, easily characterised polymers,83 85 see Figure 7. Self-assembly is an important consideration in these substituted PTs, allowing control of the degree of planarity, n-orbital overlap, and hence optical... 

Finally, we discuss the effects of chalcogen (S to Se) substitution. Although in (EDS-TTF)2FeBr4 remarkably close Se---Br contacts exist between donor and anion layers, little magnetic exchange interaction exists between the magnetic anions. The molecular orbital calculation reveals that the contribution of Se 4p orbital to the HOMO of the EDS-TTF molecule is considerably smaller than the contribution of... 

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