Halogen selenium—oxygen bonds

Selenium-oxygen bonds. The Se chemical shifts of some inorganic selenium-oxygen compounds are listed in the Tables 2 and 3. It is apparent that Se (Vl)-species are much more shielded (5 = 944-1076 Table 2) than the Se (IV)-species (5 = 1263-1559 Table 3). This holds as well for SeO3, the anhydride of selenic acid (H2SeO4), the halogen derivatives and its alkyl esters and amides. 

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... 

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. 

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. 

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

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