Selenides, P-hydroxy

Among the functionalized selenides, P-hydroxy-alkyl-selenides 3-9,11,12) and allylselenides 3 24-411 are those which possess a typical reactivity. 

Among the functionalizal selenides, P-hydroxy-alkyl-selenides and... 

Oxidation of -hydroxy selenides.s p-Hydroxy selenides (1), obtained by reaction of a dialkyl ketone with phenylselenenylrnethyllithium, are oxidized by... 

Selenides and tellurides can be prepared similarly.774 When epoxides are substrates, p-hydroxy sulfides are obtained in a manner analogous to that mentioned in 0-35. Epoxides can also be directly converted to episulfides,775 by treatment with a phosphine sulfide such as PI13PS776 or with thiourea and titanium tetraisopropoxide.777... 

C—SeMe and the C—Cl bonds and often faster than that of the C— Br bond The reduction is highly chemoselective and leads to alcohols usually in almost quantitative yield (Scheme 161, a Scheme 164, a Scheme 168, a and b). In rare cases, however, such as when a ca n-carbon double or triple bond is present in a suitable position, the formation of a five- or six-membered ring takes place by trapping of the radical intermediate (Scheme 118). ° Tin hydride reduction has been advantageously extend (g P-hydroxy-y-alkenyl and -hydroxy-a-alkenyl selenides displayed in Scheme IM (a) and Scheme 168 (a and b) and derived from a-selenoalkyllithiums and enenones, and from 1-seleno-l-alkenyl metals and carbonyl compounds, respectively. 

The reaction involving the alkylation of p-hydroxyalkyl selenides to give p-hydroxyalkylselenonium salts which are then cyclized with a base is by far the most general. It allows Ae synthesis of a large variety of epoxides such as tenninal, a,a- and a,p-disubstituted, tri-33-> and tetra-substituted, 3,i88 as oxaspiro[2.0.n]-hexanes, -heptanes and -octanes (Scheme 161, g Scheme 162, d Scheme 164, d Scheme 165, b) - and vinyl oxiranes (Schemes 166 and 181)33 -239 from both p-hydroxy-alkyl methyl33- 3 3 22>.222j36,263 phenyl selenides. ... 

The case of p-hydroxy-y-alkenyl selenides merits further comments. The rearrangement efficiently takes place using the thallium(I) ethoxide method and the presence of an additional double bond in the reactant does not introduce a serious problem associated with unwanted reaction with the dichlorocarbene intermediate. This is not the case when silver tetrafluoroborate is used. 

Procedures which utilize selenides are similar, but a-lithio selenides are not generally preparable via simple deprotonation chemistry, due to facile selenium-lithium exchange. - Selenium-stabilized anions are available, however, by transmetalladon reactions of selenium acetals and add readily to carbonyl compounds. The use of branched selenium-stabilized anions has been shown to result exclusively in 1,2-addidon to unhindered cyclohexenones, in contrast to the analogous sulfur ylides. The resulting 3-hydroxy selenides undergo elimination by treatment with base after activation by alkylation or oxidation (Scheme 10). An alternative method of activating either p-hydroxy selenides or sulfides toward elimination involves treatment of a chloroform solution of the adduct with thallium ethoxide (Scheme 11). A mechanism involving the intermediacy of a selenium ylide is proposed. 

PINACOL-TYPE REACTIONS OF P-HYDROXY SULRDES AND SELENIDES... 

Epoxides can also be prepared from a variety of 3-functionalized alcohols such as p-hydroxy sulfoxides," p-hydroxysulfonium salts," P-hydroxy selenides" and vicinal diols. In the use of unsymmetri-... 

An alternative approach is to use the readily available P-hydroxy phenyl selenides as Ritter substrates. Amide formation occurs with retention of configuration, indicating that fission of the carbon-oxygen bond is assisted by the neighboring phenylseleno group (Scheme 61). Diphenyl diselenide and iodine react with 1,5-dienes to give carbocyclic products. Initial formation of the episelenonium ion is followed by intramolecular attack and subsequent Ritter reaction (Scheme 62). ... 

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

In some way P-hydroxy selenides resemble pinacols in their reactivity (Scheme 8). 

The presence of the soft selenium atom and the hard oxygen however, make, the reaction of p-hydroxy selenides site selective. These have in fact been transformed selectively to vinylselenides 7) or olefins4-9,11 12), by selective activation of the hydroxy group, inter alia, with thionyl chloride alone or with thionyl, mesyl and phosphoryl chloride, trifluoracetic anhydride, phosphorus triiodide or diphosphorus tetraiodide in the presence of triethyl amine (Schemes 8Ab 8Bb). The formation of olefins from p-hydroxyselenides is regio- and stereoselective and occurs by formal removal of the hydroxyl and selenenyl moiety in an anti fashion. 

Selective activation of the selenenyl moiety of P-hydroxy selenides has been achieved with methyl iodide, dimethyl sulfate or methyl fluorosulfonate. The... 

P-hydroxy-alkyl-selenides are also very powerful precursors of allyl alcohols 3 9, 11 12). The transformation requires the oxidation of the P-hydroxy-alkyl-selenides to P-hydroxy-alkyl-selenoxides which usually collapse to the allyl alcohol below 70 °C and often at room temperature. Hydrogen peroxide supported on alumina in THF are, among the conditions reported, the ones which can be recommended (Schemes 8Aa, BBa). 

Syntheses of Alkylidene cyclobutanes Via the Selenoxide Route The selenoxide route which was particularly inefficient with cyclopropyl derivatives in this case proved suitable. The reaction is completely regioselective in the case of P-hydroxy selenides, which produce exclusively the allyl alcohols possessing the endo-cyclic double bond (Scheme 39)s7), whereas a mixture of endo and exo olefins is... 

The presence of the soft selenium atom and the hard oxygen however, make, the reaction of p-hydroxy selenides site selective. These have in fact been transformed selectively to vinylselenidesor olefins by selective activation of the... 

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