Selenium—carbon bonds halogens

Unlike many other type of radical addition reactions, the product is most often an alkyl-cobalt(III) species capable of further manipulation. These product Co—C bonds have been converted in good yields to carbon-oxygen (alcohol, acetate), carbon-nitrogen (oxime, amine), carbon-halogen, carbon-sulfur (sulfide, sulfinic acid) and carbon-selenium bonds (equations 179 and 180)354. Exceptions to this rule are the intermolecular additions to electron-deficient olefins, in which the putative organocobalt(III) species eliminates to form an a,/ -unsaturated carbonyl compound or styrene353 or is reduced (under electrochemical conditions) to the alkane (equation 181)355. 

Addition Reactions with Formation of Carbon-Nitrogen Bonds Addition Reactions with Formation of Carbon-Sulfiir or Carbon-Selenium Bonds Addition Reactions with Formation of Carbon-Halogen Bonds Cleavage Reactions... 

While carbon and oxygen radicals add irreversibly to carbon-carbon double bonds, the fragmentation reaction is rapid (and often reversible) for elements like tin, sulfur, selenium and the halogens (Scheme 36). This elimination reaction can be very useful in synthesis if the eliminated radical Y- can either directly or indirectly react with a radical precursor to propagate a chain. Given this prerequisite, an addition chain can be devised with either an allylic or a vinylic precursor, as illustrated in Scheme 37. Carbon radicals are generated by the direct or indirect reaction with Y- and are removed by the -elimination of Y-. Selectivity is determined by the concentration of the alkene acceptor and the rate of -elimination... 

The condensation of two molecules of primary selenoamides proceeds with bromine under extrusion of selenium to give selenadiazoles [93]. The reaction of selenocarbonates, selenothiocarbonates [94], and selenourea [95] with bromine and iodine has been widely tested. The products depend on the amount of halogen used. For example, in the reaction of N-methylthiazoline-2(3H)-selone, the use of one equivalent of bromine gives hypervalent lO-Se-3 complexes 44, whereas two equivalents of bromine cleave the carbon selenium double bond to give product 45. A similar hypervalent compound is formed from 4-imidazolin-2-selone,but the iodination of bis(imidazolin-2-selone)methane gives iodinated product 46. The availability of some of halogen adducts has been tested as a conducting material. 

It should not be surprising that reaction at the allylic position is not restricted to halogenation. Thus, although the carbon-carbon double bond is easily oxidized (Part I of this chapter), an oxidizing agent such as selenium dioxide (Se02) preferentially directs oxidation to the allylic position and the process by which this is thought to occur (Scheme 6.52) is also indicative of the indirect path by which vinylic substitution can be accomplished. 

Identify the most common number of covalent bonds and lone pairs for the atoms of each of the following elements hydrogen, the halogens (group 17), oxygen, sulfur, selenium, nitrogen, phosphorus, and carbon. 

Molecular structure and thermochemistry are interrelated here for species chosen from contributions to the earlier Volume 3 of this book series. Discussion includes halogenated species gaseous nonmetal dioxides X-Y bond-containing species (X,Y = C, N, O) small carbon molecules arenols and substituted arenes steroids aromatic carbocycles difluoramines and nitro compounds selenium- and tellurium-nitrogen compounds. 

In bonds between carbon and nitrogen, phosphorus, oxygen, sulfur, selenium, or the halogens, the carbon is made more positive —E )... 

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