Selenium compounds tetravalent

Selenium is more easily oxidized to selenium(rv), but the oxidation stage selenium(vi) is obtained with more difficulty than with the corresponding sulfur compounds. For example, selenuranes (tetravalent selenium compounds) are more easily obtained than the sulfur analogs. The synthesis as well as the chemistry of selected examples with selenium at higher oxidation stages is reviewed in Section 9.11.2.6. 

Tetravalent selenium compounds which are tetracoordinated have one lone pair and are also called selenuranes. Some of these compounds have already been mentioned earlier as these compounds can be used in rearrangement reactions. Their chemistry is still not much explored although some stable compounds have been synthesized. The dialkoxyselenurane 192 was found to give oxiranes 193 by thermolysis (Scheme 56).335... 

The structures of organic selenium compounds are closely related to those of the homologous sulfur compounds. However, in sharp contrast to organosulfur compounds, most of the organoselenium derivatives contain the divalent, dicoordinate selenium atom. Tetravalent, tricoordinate selenium compounds such as selenoxides, selenonium salts and seleninic acids are much less common. Only a few hcxavalent selenium compounds such as selenonic acids and selenones are known. Thus, asymmetric C —Se bond formation is limited to creation of a carbon — selenenyl bond. 

The generation of alkoxyl radicals from alcohols is an important process which has been used for remote functionalization via hydrogen atom abstraction and for rearrangement via -fragmentation. Hypervalent organoiodine compounds have been applied with success in this type of chemistry. More recently, tetravalent selenium compounds have been used for the same purpose [120]. An efficient way of making ether linkage from non-functionalized alcohols has been developed. The conversion of the alcohol 158 to the ether 159 is a key step in the synthesis of (-l-)-8-deoxyvernolepin [Eq. (51)] [121]. This transformation is best... 

The known nonclassical A,B-diheteropentalenes consist of compounds containing an annelated thiophene or selenophene ring, the only uncharged nonradical representations of which contain a tetravalent sulfur or selenium, while the charged structures represent carbonyl, azomethine, thiocarbonyl or selenocarbonyl ylides. The parent systems have not yet been synthesized, only substituted compounds being known. The properties of these substituted derivatives provide a good measure of understanding of the reactivities of the parent systems. 

The elements yield tetravalent derivatives m special circumstances, iu which they function as the basic elements. In the case of oxygen they are termed oxonium salts sulphur yields sulphonium salts whilst selenium and tellurium offer an even wider range of derivatives. Thus, the following types of compounds are now well known ... 

The first poly chalcogenide complex, K4USeg, was obtained by a solid-state reaction. It has a molecular sfructure with a distorted dodecahedral anion, [U(Se2)4] , which is isosfructural with the known peroxoanions [M(02)4]" , where M = V, Nb, Ta, Cr (n = 3) or Mo, W (n = 2). Recently, two additional uranium selenides have been synthesized, MU2Se6 (M = K+, Cs+), using a reactive flux method. The oxidation state of the uranium in these compounds was found to be tetravalent. The selenium has two distinct oxidation states, Se and one similar to a polyselenide network. 

The reactions with 2,3-diamino compounds are specific for tetravalent selenium, but since sample preparations are normally carried out under oxidizing conditions which could... 

Tetravalent selenium-oxo units (including Se02) interact with many organic compounds, particularly carbon-carbon double bonds and carbonyl groups, via peri-cyclic reactions. 

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