Diselenides halides

Schneemeyer LF, Wrighton MS (1980) n-Type molybdenum diselenide-based photoelectrochemical cells Evidence for Fermi level pinning and comparison of the efficiency for conversion of light to electricity with various solvent/halogen/halide combinations. J Am Chem Soc 102 6964-6971... 

A simple, rapid and efficient method has been reported for the synthesis of dibenzyl diselenides under the action of MW irradiation. Benzyl halides are reacted with selenium powder in the presence of a base and phase-transfer agent (Eq. 57 and Tab. 5.29) [80]. The reactions were performed either in THF or in C6H6-H20. 

For the synthesis of selenocysteine derivatives that are suitable for peptide synthesis essentially two approaches have been used to date (1) conversion of p-chloroalanine 23 or serine-O-tosylate derivatives 24 into the desired selenocysteine derivatives by a nucleophilic displacement reaction with an areneselenol and (2) full reduction of selenocystine and in situ reaction with aryl halides to produce the aryl selenides. 7 25 In this context, reduction of selenocystine in 2 M NaOH with 2-methyl-2-propanethiol for concomitant formation of the mixed selenide/sulfide derivative 5e-(tert-butylsulfanyl)selenocysteine in analogy to the formation of 5-(fett-butylsulfanyl)cysteine 26 fails as a consequence of the difficult reduction of the diselenide with monothiols. 27 ... 

AUcylSe and alkylTe groups can be introduced by reaction with diselenides and ditellurides, but the desired products can be prepared in a more practical and economical way, by successive addition of the elements and the alkyl halide. The insertion of the elements... 

The diselenides, RSe.SeR, are prepared by the interaction of potassium diselenide and dialkyl sulphates or alkyl halides ... 

In a rare example of the use of iodonium salts for heteroatom-heteroatom bond formation, diaryliodonium halides were employed with sodium 0,0-diethyl phosphoroselenolate for a one-pot synthesis of diaryl diselenides (Scheme 9) [27]. These transformations probably occur via arylation of the phosphoroselenolate salt with the diaryliodonium ions, hydrolysis of the resulting aryl phosphoroselenolates with sodium hydroxide, and air oxidation of the arene-selenide ions thus produced. 

AlkylSe and alkylTe groups can be introduced by reaction with diselenides and ditellurides, but the desired products can be prepared in a more practical and economical way, by successive addition of the elements and the alkyl halide. The insertion of the elements proceeds most easily in liquid ammonia, provided that grey Te powder and red Se powder is used (black Te powder, obtained by precipitation is unreactive, probably because of the presence of an oxide coating, while the black modification of Se,"selenium nigrum" is also less reactive). In Et20 or THF, temperatures in the range 0-20 C are necessary for the dissolution of the elements. 

Diselenides. These compounds can be prepared in satisfactory yield by reaction of 1 with alkyl halides, epoxides, or lactones in THF at 20°. 

NiBr2(2,2 -bipyridine) complex44. Aryl halides, such as 4-bromobenzophenone, or 2-bromoquinoline could be transformed into diaryl diselenides 20 or ditellurides 21 elec-trochemically using sacrificial selenium or tellurium cathodes as can be seen in equation 20. 

An improvement in this technique of sacrificial electrodes (Se and Mg) for the formation of the Z2-2 ions was achieved by using an undivided cell and by addition of fluoride ions to avoid the precipitation of the Se2Mg304. Thus, the reaction of 2-chloroquinoline with Se2-2 gave 79% yield of 248a. Following this methodology the diselenides 250 were prepared by reaction with aryl halides 249 (equation 162)303. 

There are various ways to generate and synthesize selenium electrophiles and some of these compounds are commercially available. The addition reaction can also be dependent on the counterion X of these reagents and several protocols have been developed to exchange the counterions. The most commonly used electrophile is the phenylselenyl electrophile and compounds like phenylselenenyl chloride 6 and phenylselenenyl bromide 7 are commercially available. They can also be easily generated from diphenyl diselenide 8 by treatment with sulfuryl chloride or elementary chlorine or bromine, respectively. Diselenides in general are very versatile precursors for selenium electrophiles. For addition reactions using external nucleophiles the use of selenenyl halides can lead to complications, because chloride or bromide ions can also act as nucleophiles and lead to undesired side-products. An... 

The lithiation of 1,3-oxathiane (296) takes place with s-BuLi at —78 °C to give 2-lithio-1,3-oxathiane (315), an analogue of 2-lithio-l,3-dithiane (161), but with lower stability487. This intermediate reacts with different electrophiles, such as alkyl halides, carbonyl compounds, benzonitrile, dimethyl disulfide, dimethyl diselenide, trimethylplumbyl acetate and trimethylsilyl, germyl and stannyl chlorides488,489. However, further deprotection of 2-substituted 1,3-oxathianes has not been reported yet. 

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. 

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. 

Perfluoroalkylselenenylation of alkenes in the reaction of diphenyl diselenide with sodium borohydride followed by perfluoroalkyl halides is relatively stereoselective since a mixture of irans- and cw-adducts is obtained for both cyclohexene [X = CII2 d.r. (trans/cis) 75 25] and dihydropyran [X = O d.r. (transjeis) 73 27]... 

Reaction of diphenyl diselenide or dimethyl diselenide with hydrazine hydrate and sodium hydroxide generates the corresponding selenolates smoothly in solvents like DMF or diethyl ether and in the presence of tetrabutylammonium chloride as a phase-transfer catalyst [13]. The selenolates react with organic halides to give various selenides (Scheme 9). Similar conditions have been applied to the synthesis of aryl vinyl selenides from diaryl diselenides and acetylene [14]. 

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. 

Treatment of diphenyl diselenide with tributylphosphine in an alkaline medium generates a benzeneselenolate anion [24], which reacts with various electrophiles, such as halides, epoxides, a, -unsaturated ketones, to generate various selenides [24,25]. The mechanism for the generation of the benzeneselenolate is proposed as shown in Scheme 17 [25]. A selenophosphonium ion and/or a pentavalent phosphorus species are initially formed in the reaction of diphenyl diselenide and tributylphosphine, and then the addition of sodium hydroxide liberates the benzeneselenolate and phosphine oxide. 

Organoselenium compounds are very versatile radical precm-sors which are widely used. Due to their stability and ease of preparation, they offer imique advantages over organic halides as radical precm-sors. They can be utilized in tin mediated radical reactions as weU as in group transfer reactions for the formation of carbon-carbon bonds and carbon-heteroatom bonds. Selenols and diselenides have found applications as reducing agents and radical traps, respectively. A survey of these different reaction types wiU be given. Information about new reactions based on electron and photoelectron transfers wiU also be provided. 

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