Diselenides, diphenyl reduction

The benzeneseleninic acid weighs 14.4-16 g. (73-82%) and melts at 123-124°. It may be reconverted to diphenyl diselenide by reduction with sodium thiosulfate or sodium bisulfite. ... 

Selenolate 1 [1,2], prepared from diphenyl diselenide by reduction with NaBH4 in ethanol, is not reactive enough towards esters and lactones. However, application of more rigorous conditions, i. e. at 110-120 °C in dry DMF for lactones 15, affords the corresponding ring-opened products, cu-phenylselenyl carboxylic acids 16, in good yields [44]. Compounds 16 can be easily converted to co-ole-finic methyl esters 17 by treatment with diazomethane followed by oxidative elimination of the phenylseleno group (Scheme 25). By contrast, the uncomplex-... 

Addition of the selenolate that is prepared from diphenyl diselenide by reduction with NaBH4 to conjugate enyne sulfones 20 occurs at the d-position of the sulfonyl group to give 4-phenylseleno-l-sulfonylbuta-1,3-dienes 21 regio- and stereoselectively (Scheme 28) [50]. On the other hand, addition of PhSeNa to the conjugated alkynyl sulfone 22 that has a -phenylseleno substituent affords the anti-Michael adduct 23 preferentially (Scheme 29) [51]. 

Selenolates prepared from diphenyl or dimethyl diselenide by reduction with NaBH4 smoothly transform various benzylic alcohols 24 into the corresponding selenides 25 in the presence of aluminum chloride (Scheme 30 a) [41]. AICI3 is considered to activate the alcohol substrate by coordinating to the oxygen. Similar transformations are possible by the reaction of alcohols with phenyl selenocyanate in the presence of tributyl phosphine [52]. When the selenolate is reacted with aromatic aldehydes or ketones 26 in the presence of AICI3, the corresponding benzylic selenides 27 are obtained in moderate yields (Scheme 30b) [41]. 

Recently, Kondo and coworkers reported on the polymerization of St with diphenyl diselenides (37) as the photoiniferters (Eq. 39) [ 162]. In the photopolymerization of St in the presence of 37a and 37b, the polymer yield and the molecular weight of the polymers increased with reaction time. The chain-end structure of the resulting polymer 38 was characterized. Polymer 38 underwent the reductive elimination of terminal seleno groups by reaction with tri-n-butyltin hydride in the presence of AIBN (Eq. 40). It also afforded the poly(St) with double bonds at both chain ends when it was treated with hydrogen peroxide (Eq. 41). They also reported the polymerization of St with diphenyl ditelluride to afford well-controlled molecular weight and its distribution [163]. 

Crich and Rumthao reported a new synthesis of carbazomycin B using a benzeneselenol-catalyzed, stannane-mediated addition of an aryl radical to the functionalized iodocarbamate 835, followed by cyclization and dehydrogenative aromatization (622). The iodocarbamate 835 required for the key radical reaction was obtained from the nitrophenol 784 (609) (see Scheme 5.85). lodination of 784, followed by acetylation, afforded 3,4-dimethyl-6-iodo-2-methoxy-5-nitrophenyl acetate 834. Reduction of 834 with iron and ferric chloride in acetic acid, followed by reaction with methyl chloroformate, led to the iodocarbamate 835. Reaction of 835 and diphenyl diselenide in refluxing benzene with tributyltin hydride and azobisisobutyronitrile (AIBN) gave the adduct 836 in 40% yield, along with 8% of the recovered substrate and 12% of the deiodinated carbamate 837. Treatment of 836 with phenylselenenyl bromide in dichloromethane afforded the phenylselenenyltetrahydrocarbazole 838. Oxidative... 

The third method makes use of the one-flask procedure, which is advantageous from the preparative point of view. However, opening of certain stereoisomeric epoxides (263) with selenophenol suffers from low regioselectivity, resulting in a low yield of the final product. The other disadvantage is the basic reaction-medium occasioned by the method used for the generation of selenophenol, namely reduction of diphenyl diselenide with sodium borohydride in solution in anhydrous alcohol (see Ref. 356) some epoxides are sensitive to basic media. However, David (see Ref. 356) did not observe side reactions in his syntheses of 256. 

Sodium phenylselenotriethoxyborate, Na+[C6H5SeB(OCH2CH3)3] (1) prepared by reduction of diphenyl diselenide with NaBH4 in ethanol. 

Benieneselenol, 6, 28-29. This reagent has been prepared by reaction of selenium with phenybnagnesium bromide (57-71% yield) and by reduction of diphenyl diselenide with hypophosphorous acid or with NaBHa (86% yield). , ... 

A complex similar to 1 could not be isolated from the reaction of chloramine-T with diphenyl diselenide, but its formation was demonstrated by the reaction of the diselenide and chloramine-T in the presence of an olefin. The initial product of addition to cyclohexene is.also unstable and not isolable, but reduction with NaBH4 gave a frans-l,2-amidoselenide (5). 

Cyclohexene is converted to tnmv-1-(4-mcthylphcnylsulfonylamino)-2-(phenylseleno)cyclohexane (59) by treatment with diphenyl diselenide and chloramine-T, followed by reductive workup with sodium borohydride66. The intermediate product A formed from diselenide and chlor-amine-T is believed to play the role of an electrophile attacking the alkene. 

The cyclization of O-allyl oximes 1, performed with diphenyl diselenide/ammonium thiosulfate/ trifluorosulfonic acid or phenylselenenyl bromide, gave cyclic iminium salts 2 via an intramolecular antt -addition mechanism. By addition of water to the reaction mixture, 3-substi-tuted isoxazolidines 3 are formed in good yield. When the salts 2 were prepared by using phenylselenenyl bromide, A-alkyl 3-substituted isoxazolidines 4 were afforded in good yield by reduction of 2 with sodium borohydride in methanol247-249. 

Reduction of diphenyl diselenide by sodium metal in refluxing THF produces copious precipitates of the salt of a benzeneselenolate anion 4 (Scheme 4 a)... 

Reduction of diphenyl diselenide with the Sm-Me3SiCl-H20 system also leads to a benzeneselenolate, which reacts with organic halides, epoxides, a, -un-saturated esters or cr, -unsaturated nitriles to afford unsymmetrical phenyl selenides in good yields under mild and neutral conditions as shown in Scheme 13 [19]. This method is easier to handle than the use of air-sensitive Sml2. 

Cathodic reduction of diphenyl diselenide produces a benzeneselenolate, which catalyzes the reduction of a, -epoxy carbonyl compounds into -hydroxy carbonyl compounds in the presence of a proton source (Scheme 16) [21]. This electrochemical method is applicable to the selenation of haloalkanes, epoxides, and enones [22]. Electrochemical reduction of elemental selenium to diselenide dianion (Se ) and selenide dianion (Se ) is also possible [23]. 

Treatment of a, -unsaturated carbonyl compounds 18 with nucleophilic selenium species affords -seleno carbonyl compounds 19 in good yields via Michael addition (Scheme 27) [46]. This reaction has been applied to protect a, -unsa-turated lactones [47], in natural product synthesis [48], and in asymmetric Michael additions in the presence of an alkaloid [49]. Michael addition also proceeds with selenolates that are prepared from diphenyl diselenide by cathodic reduction [22], reduction with the Sm-Me3SiCl-H20 system [19], and reduction with tributyl phosphine [25]. 

First, small amount of impurities such as diphenyl diselenide can drastically enhance the rate of reduction by building phenylselenol in situ [3]. This can be a severe drawback, since direct reduction becomes more rapid than the desired radical reaction (usually and inter- or intramolecular carbon-carbon bond forming reaction). This effect is discussed in detail in Sect. 4.2.2. 

An alternative interesting approach, which is based on the Mukaiyama reduction/oxidation procedure for peptide bond formationt uses tributylphosphine in the presence of diphenyl diselenide and the N-protected annino acid for in situ condensation with the a-azido compound. The phenyl carboselenolate is formed as an activated carboxylic add and the resulting benzeneselenol reduces the azide to the amine for reaction with the active ester. 

Benzeneselenol, a representative selenol, is a colorless liquid of greater acidity than benzenefhiol [pK = 5.9 (PhSeH) 6.5 (PhSH)]. Benzeneselenol is usually synfhesized by the reaction of phenylmagnesium bromide with metallic selenium then quenching wifh aqueous hydrochloric acid [10]. Similar hydrolysis of the sodium phenylseleno(triethoxy)borate complex prepared by fhe reduction of diphenyl diselenide wifh sodium borohydride is a convenient alternative [11]. 

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