Selenols diselenides

Stable hindered selenoketones222 are easily reduced to give selenolate ions with various reducing agents and oxidized by air to form diselenides (Scheme 39). 

The lO-E-4 chalcogen(IV) species diphenylselenium(IV) dibromide (1, Fig. 1) and diphenyltellurium(IV) dibromide (2, Fig. 1) oxidize thiophenol to diphenyl disulfide in nearly quantitative yield as shown in equations (13) and (14). Tellurium(IV) dihalides 6-11 also oxidize thiophenol to diphenyl disulfide and benzene selenol to diphenyl diselenide. Similarly, the 12-Te-5 molecule dioxatellurapentalene 45 (Fig. 19) is a mild oxidant for ethylmercaptan, thiophenol, and benzene selenol giving diethyl disulfide, diphenyl disulfide, and diphenyl diselenide in essentially quantitative yield. As shown in equation (15), 1,1,5,5,9,9-hexachloro-1,5,9-tritelluracyclododecane oxidizes six molecules of thiophenol to diphenyl disulfide and 1,5,9-tritelluracyclododecane in 90% yield. In contrast, 12-Te-5 pertellurane 44 and 12-Se-5 perselenane 46 do not oxidize thiophenol to diphenyl disulfide. Instead, these molecules undergo a nucleophilic addition of thiophenol followed by cleavage of the tellurium-carbon or selenium-carbon bond. ... 

Diethyl zinc undergoes enantioselective addition to 2-pyridinecarbaldeyde in 91% ee in the presence of 0.5 mol% optically pure diselenide 68 in toluene at room temperature <2003OL2635> (Equation 48). The reaction is postulated to proceed by in situ formation of a chiral zinc selenolate formed by nucleophilic attack of Et2Zn on diselenide 68. 

Various reducing agents have been used for the generation of selenolates from diselenides or selenocyanates. Alkali metals M (M = Li, Na, K)149,150 or alkali hydrides MH (M = Li, Na, K)151,152 can generate the corresponding selenolate anions such as 80 these are more reactive than the borane complexes of type 77 (Scheme 15). Diaryl diselenides are easier reduced than dialkyl diselenides, but the mechanism for the reduction of selenocyanates is complex and can lead to either diselenides or selenolates.153,154... 

Hydrazine155,156 or samarium diiodide157-162 are other useful reagents for the synthesis of selenolates, but electrochemical methods163,164 have also been employed for their selective synthesis from diselenides. Starting from elemental selenium, selenolates 80 can easily be prepared by insertion reactions of selenium into organolithium... 

Ring opening of larger cyclic ethers such as tetrahydrofuran does not normally proceed with selenolates of type 77 generated from diselenides and sodium borohydride in alcohols. If diselenides are reduced with lithium aluminum hydride in tetrahydrofuran,171 or with sodium borohydride in the presence of aluminum trichloride,172 ring-opened products 89 are obtained (Scheme 20). 

Selenolate 77, prepared from diselenides with sodium borohydride, is also not reactive enough for reactions with esters or lactones. Only under higher reaction temperatures, lactones 90 can be ring opened with selenolates to the corresponding carboxylic acids 91.173,174 The selenium moiety can then be used for further manipulations. Using similar reaction conditions, esters 92 can be converted to the corresponding carboxylic acids 93 and selenides 94 (Scheme 21).149... 

Also widely employed are organoselenium compounds containing group 14 elements. Phenyl trimethylsilyl selenide 102 can be easily prepared from diphenyl diselenide 8 and is a good source for selenium nucleophiles (Scheme 24). In the presence of methanol selenols are generated for use in Michael reactions or in ring-... 

The dihalides are susceptible to decomposition by hydrolysis. The dibromides oxidize thiols to disulfides, selenols to diselenides, and diphenyl tellurium to diphenyl tellurium oxide. Hydrazine reduces the dihalides to the parent compounds1. 

Oxidation cf Selenols to DIselenides and Further Oxidized Species by Chemical Reagents... 

Diselenides are generally prepared by the aerial oxidation of selenols or selenolates (see Section 2) or by the reaction of Li2Sc2, Na2Se2, or other 802 species with alkyl or aryl halides. In the latter case, elevated temperatures and DMF as the solvent are recommended. Aryl diselenides may also be obtained from the reaction of 802 with diazonium salts (Scheme 6). Relatively few unsymmetrical diselenides have been reported. Triselenides are also known but have not been as widely studied. 

As shown in Scheme 1, the reduction of diselenides generates selenols and selenolates, while their oxidation results in the formation of selenenic and seleninic acids and related compounds (see Section 7). Halogenation of diselenides with... 

Selenocyanates produce selenols or diselenides upon either reduction (e g. with sodium borohydride) or hydrolysis (see Scheme 1). They undergo displacement of the cyanide ion by various nucleophiles and add to alkenes in a maimer similar to selenenyl halides (see equation 14), except that catalysis with Lewis acids is required in the case of unactivated alkenes. The selenocyanates are also popular reagents for the preparation of selenides from alcohols, and (8) from carboxylic acids, as indicated in Scheme 3. 

Table 4. % Yield and % de (in parentheses) of asymmetric selenoetherification and selenol-actonization reactions using different diselenides...

Various diselenides can be reduced to the corresponding alkali selenolates by two equivalents of alkali hydrides (LiH, NaH, KH) in THF or DMF (Scheme 5)... 

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