Selenium-containing Rings

Selenium-containing Rings.—Selenan 1-oxide (70) has been shown, by low-temperature n.m.r. spectra, to have a greater preference for axial O (84%) than has thian 1-oxide (62%).  

Anteunis group have reported the n.m.r. spectral parameters of some 4-alkyl-6-methyI-l,3-oxaselenans using INDOR techniques. Low-temperature studies show that the conformational equilibria in the trans-senes are markedly one-sided, with the methyl next to selenium strongly preferring to be axial (AG ca. 1.5 kcal mol ) (71). Ring-inversion barriers were measured (AGfoo = 8.3 kcal mol ). 

A molecular orbital approach has also been adopted by Sustmann and Trill. Assuming a concerted mechanism, they find a reasonable agreement between prediction and observation for substituent effects upon reaction rates. 

Eberhard and Huisgen have shown that the cycloreversion of the pyrazolin-4-yl anion (72) to the allyl anion (73) may well be a concerted process. This, they claim, supports their contention that 1,3-dipolar cycloadditions are concerted processes. 

Huisgen s group have also investigated the possibility that 1,3-dipolar cycloadditions could proceed via three-membered-ring intermediates, (74) - (75). They have investigated this mechanism by synthesis of the possible intermediates (74) in the reactions between styrene and benzo-nitrile-4-nitrobenzylide (76), phenyldiazomethane, and 4-nitrophenyl azide. 

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Telluroformates 144 were prepared by the treatment of alcohols 143 with a solution of phosgene in toluene, followed by sodium phenyltelluroate, and were isolated as yellow/orange viscous oil in 74% yield. It seems reasonable to say that the saturated selenium-containing rings 145 were formed through intramolecular nucleophilic substitution of the benzylse-leno moiety in telluroformates 144 with decarboxylative loss of phenyl telluride (Scheme 15) <1998JOC3032>. 

Syntheses Involving Transformation of a Selenium-containing Ring... 

The selenium-containing ring (428) can be prepared by the action of hydrogen selenide on the appropriate amine with formaldehyde in aqueous ethanol,and n.m.r. data reveal that the action of RP(NEt)2 (R = Me,... 

Selenazole is the selenium-containing compound in the series of heterocyclic 5-membered ring azoles with two different hetero atoms, of which the first two members are oxazole and thiazole. The numbering of the ring system is according to the scheme given (1). 

Electrochemical studies indicate that the redox behaviours of the different derivatives are dominated by the nature of the chalcogen at position 2, with the selenium containing cations more readily reduced (by more than 0.1 V) than the sulfur analogues.82 The difference has been ascribed to the extent of delocalisation of the positive charge away from the heterocycle, onto the fused aromatic ring. 

As mentioned in CHEC-II(1996) (Section 5.11.9), there remains significant interest in the technical potential of compounds containing six-membered rings with one selenium or tellurium atom. The electronic properties of selenium-containing organic semiconductors such as 180 and 182 are particularly interesting <1993J(P2)1815>. As... 

Selenium-containing six-membered ring heterocycles have proved to be useful catalysts in a variety of transformations. The Baylis-Hillman reaction involves the reaction of alkenes containing electron-withdrawing groups such as a,/3-unsaturated carbonyl compounds with aldehydes leading to carbon-carbon bond formation (Equation 79). The reaction is promoted by tertiary amines such as l,4-diazabicyclo[2.2.2]octane (DABCO), or tertiary phosphines and Lewis acids. Unfortunately, the Baylis-Hillman reaction is severely limited because it proceeds only very slowly <1998CC197>. Much research has been carried out in attempts to increase the rate of this reaction. 

However, 3-methyl-2//-l-benzotellurin did not form a benzotellurophene on treatment with selenium dioxide in pyridine. Instead, the Te-containing ring was cleaved with formation of ditellurium derivatives1. 

Electrochemically generated cation radical 252 derived from the cyclic alkylphenyl selenide 251 can react via two parallel paths deprotonization with the formation of compounds containing double bonds in the selenium-bearing rings and homolytic cleavage of the C(sp3)-selenium bond, followed by dimerization and formation of a diselenide 253 with an extended ring (Scheme 38) <1987ZOB609>. 

Five-membered heterocycles with two vicinal chalcogen atoms in the ring system can be used as stable precursors for sulfur as well as for selenium-containing hetero-1,3-dienes in cycloaddition reactions. Consequently, 3//-1,2,4-thiaselenazoles have been used for the in situ formation of 4,4-bis(trifluoromethyl)-l-thia-3-azabuta-1,3-dienes, which exist at room temperature only as 4,4-bis(trifluoromethyl)-2//-l,3-thiazetes. This strategy was applied to the synthesis of the first stable selenophosphorane from bis(trifluoromethyl)-substituted 3//-diselenazol and 2-methoxy-1,3,2-dioxaphospholan [78AG(E)774] (Scheme 83). 

An ab initio MO study of eight-membered sulfur-rich tellurium selenium sulfide rings has shown that the most stable species contain Se Te bonds. This is exemplified by... 

IV. Monosaccharides Having a Sulfur- or Selenium-containing Hemiacetal Ring... 

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