Selenols halides

The most common reaction of selenolates is the nucleophilic substitution of halide compounds. For the synthesis of selenides 82 the selenolates 80 can be reacted with halides. Nucleophilic selenium species can also be reacted with a variety of other electrophilic compounds. The reaction with acid chlorides, chloroformates, or carbamoyl chlorides leads to the corresponding selenoesters 83 (R" = R ), selenocarbonates 83 (R" = OR ), or selenoamides 83 (R" = NR 2), respectively (Scheme 17).166... 

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. 

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]. 

Substitutions to halide compounds are the most typical reactions of nucleophilic selenium species (Scheme 18). Alkyl or aryl selenolates (R SeM), which are prepared in situ by the methods described in Sect. 2, react with halides to give... 

Two interesting applications of this type of reaction are presented in Schemes 19 and 20. Selenium-substituted cysteine (selenocysteine) derivatives are synthesized by condensation between selenocysteine and the corresponding alkyl and benzyl halides or between areneselenolate and j0-chloroalanine (Scheme 19) [31]. These compounds are considered as potential kidney selective prodrugs of biologically active selenol compounds. 

Bis(triphenylstannyl) selenide (56), which is prepared by the reaction of NaHSe with triphenylstannyl chloride [82,83], smoothly liberates a nucleophilic selenolate species with the aid of a fluoride ion [82]. The reaction conditions are neutral, producing the selenolate in a naked form. Therefore, in the reaction with halides, the corresponding symmetrical selenides are obtained in excellent yields without generating by-products (Scheme 42) [82]. When 56 is reacted... 

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. 

Thiols or selenols react with alkoxy compounds, halides and acetates to give R M(ER )3 (E = S, Se). Several examples are shown in equations 94" , 95 ° , 96 °... 

Synthesis of indium(III) thiolates containing halide donors have also been reported. The oxidative addition reactions of InX (X = C1, Br, I) with PhSSPh yields (PhS)2lnX." Related selenolates can also be synthesized, using PhSeSePh instead of PhSSPh. ... 

Areneselenyl magnesium bromide has been reported to react with /7-substituted benzoyl halides, giving aromatic selenol esters " as the first examples exhibiting liquid crystal properties (equation 2). ... 

Selenol esters have also been prepared from trimethylsilyl selenides and acyl halides, as shown in equation (5). ... 

As well as acylation of selenols, alkylation of selenocarboxylates with alkyl halides is one of the most straightforward methods for synthesis of selenol esters. However, known examples of selenol esters synthesized using this strategy have been limited to a few cases, due to the difficulty of the preparation of selenocarboxylates. 

Although alkylation of potassium selenobenzoate with alkyl halides led to the formation of selenol esters via 5e-alkylation, as described in Section 2,6.2.1.1 (equation 9), the reaction of (66 equation 26) with trimethylsilyl chloride proceeded through 0-silylation to give trimethylsilyl selenobenzoate (67) in 68% yield. The driving force of the reaction may be due to the strong affinity of silicon for oxygen. 

The parent compounds, methylselenomethyllithium and phenylselenomethyllithium, are less reactive towards alkylating agents than their higher homologs. They require higher temperatures (-20 C instead of -78 C) for a reasonable reaction rate irrespective of whether ether or THF is used. Unfortunately they produce, along with the expected alkylated selenides, appreciable amounts of the selenide resulting from substitution of the alkyl halide by the selenolate ion (Scheme 8, entry a). Apparently, under these conditions, the selenoalkyllithiums are in equilibrium with methylene and lithium selenolates. This side reaction can be avoided if the reactions are performed in THF-HMPA, under which conditions alkylation proceeds at temperatures as low as -78 C (Scheme 8, compare entry b to a). In several cases HMPA aids the alkylation of ot-selenoalkyllithiums. For example HMPA has been used for the alkylation of 1-phenylseleno-l-hexyllithium, and also for 1-selenocyclopropyl- and 1-selenocyclobutyl-lithiums (Scheme 8, entries c and e). 

Instead of alkylating reagents, the use of acyl or aroyl halides in fhis reaction conveniently leads to the synthesis of selenoesters as useful acylating reagents in organic synthesis. A mild and convenient method is reaction of trimethylsilyl selenides with acyl halides [27] or reaction of selenols with imidazolide (or triazolide) of carboxylic acids (Scheme 15.8) [28]. 

Selenocyanates (RSeCN), which are employed for the synthesis of selenides and selenoesters, are usually derived from selenenyl halides and cyanide ion (or silyl cyanate) [38 k, 40]. Alkylation of potassium selenolate (KSeCN) with alkyl halides is an alternative means of preparation of aliphatic selenocyanates (Scheme 15.13)... 

Selenides from halides via selenols RHal —v RSeH —> RSeR ... 

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