Selenium-carbon double bonds

Compounds with Selenium-Carbon Double Bonds 486... 

The equivalent to allylic oxidation of alkenes, but with allylic transposition of the carbon-carbon double bond, can be carried out by an indirect oxidative process involving addition of an electrophilic arylselenenyl reagent, followed by oxidative elimination of selenium. In one procedure, addition of an arylselenenyl halide is followed by solvolysis and oxidative elimination. 

A double oxidation at the 5,6-carbon-carbon double bond and N-9 by 2equiv of OT-chloroperbenzoic acid (MCPBA) was postulated as the mechanism for the oxidative rearrangement of 8-dialkylaminoxanthines to novel l-oxo-2,4,7,9-tetraazaspiro[4,5]dec-2-ene-6,8,10-triones <1999EJ02419>. The presence of an electron-donating 8-amino substituent was essential for this reaction to occur. Further oxidation of the products gave 1,3-dimethylpara-benic acid 45, which was also produced directly when selenium dioxide or hydrogen peroxide were used (Scheme 11). 

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... 

Trying to carry out 3 -exo carbolithiation reactions with the tertiary benzyllithium 147, generated by selenium-lithium exchange, Krief and Barbeaux have reported66 an isolated example of the reaction of this homoallylic lithium reagent with ethylene and further intramolecular carbolithiation reaction of intermediate 148 onto the suitably positioned carbon-carbon double bond. The resulting 1,3-dimethyl-l-phenylcyclopentane was isolated in modest yield and as a 1 1 mixture of diastereoisomers (Scheme 41). 

The aromatic 7r-electron system of selenophene is formed by the interaction of the n electrons of two carbon-carbon double bonds with the lone-pair of the selenium atom. Thus, it contains six electrons in the field of five nuclei the sixfold symmetry axis characteristic of benzene is removed there simply remains the plane and one twofold axis, leaving a molecule of C2 symmetry.15,23 Therefore, a uniform distribution of electron density is impossible in selenophene. This was confirmed, e.g., by MO calculations of the 7r-electron structure of the selenophene molecule. 

The most useful application of 3 is its use in photochemical [2+2] cycloadditions with alkenes and alkynes at the carbon-carbon double bond to afford bicyclo[4.2.0]octane-2,5-diones and bicyclo[4.2.0]oct-7-ene-2,5-diones in good to excellent yield.6 Since selenium dioxide oxidation of the resulting adducts furnishes the corresponding 3-ene-2,5-diones, diketone 3 can be regarded as a 1,4-benzoquinone equivalent leading to [2+2] cycloadducts at the carbon-carbon double bond. 

When the phosphorus-carbon double bond is reacted with selenium, the saturated product could be isolated. These systems seem to be more stable than their unsaturated counterparts. This behavior can be explained by the fact that more protecting groups are placed about the three-membered ring by saturation also, the antiaromaticity destabilizes the unsaturated systems. 

Due to the important role of nitrogen functional groups, the addition reactions of an electrophilic selenium reagent and a nitrogen nucleophile to a carbon-carbon double bond represent a synthetically relevant process with potential practical applications. Among the reactions of this type which have been described already, perhaps the most important contribution is represented by the Ritter-type amide synthesis described by Toshimitsu and Uemura [48a, 48b]. The addition of a phenylselenyl and of an acylamino group to a mono or a 1,2-disubstituted olefin was accomplished by treating the olefin with PhSeCl in acetonitrile and water in the presence of trifluoromethanesulfonic acid. 

Metallaheterocycles with the metal atom capable of supporting a carbon-metal double bond, and with oxygen, nitrogen, sulfur, or selenium as the second heteroatom are the only structures capable of full conjugation. These compounds show furan-like aromaticity with the heteroatom /i-electrons participating. No examples of this type of ring system were uncovered, undoubtedly due to the stability of the metalla-carbon double bonds required for their formation. 

As discussed previously, the ozonolysis of selenium in organic molecules leads to formation of the selenoxide, which is typically eliminated to form carbon-carbon double bonds. It is possible to use ozone in systems containing a selenium atom provided a more reactive functional group is present. Several examples of the successful ozonolysis of a carbon-carbon double bond in systems containing a phenylseleno moiety allow for functionalization of double bonds while preserving the synthetic handle resident in the starting olefin (eq 61). ... 

Selenium dioxide attacks allylic positions in preference to carbon-carbon double bonds and is a very useful reagent for allylic oxidation. A good deal of... 

Iv) Selenium dioxide oxidizes allylic positions of most nucleophilic double bond (trisubstituted) with -selectivity. As explained in the previous problem, trisubstituted alkenes are oxidized selectively at the more-substituted end of the carbon—carbon double bond. The position next to the unsaturated ester is not oxidized. 

Pericyclic reactions provide some of the most elegant examples of the importance of orbital symmetry in chemical reactions. Unlike in organic chemistry, however, pericyclic reactions are not of great importance in inorganic chemistry. That said, we will encounter a few significant examples in this book, including the reduction of carbon-carbon double bonds by diimide (Section 5.7a) and certain selenium dioxide oxidations (Section 6.16). 

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

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. 

Further reports have appeared on the oxyselenation "" and amidoselena-tion " of carbon-carbon double bonds. In some cases electrolytic methods have been used to generate the reactive form of the selenium reagent from diphenyldiselenide... 

C1/C2 chevrons, ferroelectric devices 644 cadmium selenium TFT address 233 Cano wedge, chiral nematics 347 f, 351 Canon technology, ferroelectric devices 648 capillaiy flow, shear viscosity 143 carbocyclic compounds, charge transfer systems 958 carbocyclic rings, smectogens 412 carbon atoms, intercalated smectics 808 carbon-carbon bonds, dimers 823 carbon-carbon double bonds, chiral smectics 498 carbonaceous phases 693 carbonyl connectors, antiferroelectrics 687 carbonyl groups... 

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