Selenium-derived reagents

Another example of the resin-capture-release technique which should see widespread applications in the future is the selenium-mediated functionalization of organic compounds. Polymer-supported selenium-derived reagents [34] are very versatile because a rich chemistry around the carbon-selenium bond has been established in solution and the difficulties arising from the odor and the toxicity of low-molecular weight selenium compounds can be avoided. Thus, reagent 26 (X = Cl) was first prepared by Michels, Kato and Heitz [35] and was employed in reactions with carbonyl compounds. This treatment yielded polymer-bound a-seleno intermediates, which were set free back into solution as enones from hydrogen peroxide induced elimination. Recently, new selenium-based functionalized polymers 26 (X = Br)-28 were developed, which have been utilized in syntheses according to Scheme 11 (refer also to Scheme 3) [36],... 

With the exception of a few examples of nucleophilic attack on the thiocarbonyl group, including a more recently published method using sodium hydroxide under phase transfer conditions,most conversions are carried out with oxidative reagents. Sodium peroxide, dimethyl sulfoxide with acid or iodine,selenium-derived reagents and bis(p-methoxyphenyl)telluroxide have been used. 

This section will focus primarily on a comparison of these ring systems with their heavier chalcogen analogues. The first selenium derivative benzo-l,2,5-selenadiazole was prepared more than 115 years ago by the condensation reaction of selenium dioxide with 1,2-diaminobenzene (Eq. 11.12) and other benzo derivatives may be prepared in a similar manner. The parent 1,2,5-selenadiazole has also been reported. This reagent has been employed to make the tellurium analogue via treatment with ethylmagnesium bromide followed by the addition of tellurium tetrachloride (Eq. 11.13). ... 

The aromatization of cyclohexenones is an important process that can be easily accomplished by the use of selenium-based reagents using similar techniques to those previously discussed for other carbonyl species. Thus, enolates derived from a,3-enones readily undergo selenenylation at the a -position and on oxidation and elimination afford the corresponding phenols. ... 

Within this class of selenium derivatives only selenium dioxide 41 alone, or in combination with other chemicals, and diimidoselenium reagents 42 are commonly used in organic synthesis. Because the use of these latter reagents is discussed in detail in Chap. 8 of this volume (devoted to rearrangements), our presentation will be limited to recent utilization of selenium dioxide as a reagent. 

Considering the examples of allylic oxidation with the selenium dioxide derived reagents presented above it is of interest to note that the selective oxidation of primary allylic alcohols 60 to the corresponding a, -unsaturat-ed aldehydes 61 was achieved [23] using an Se02/(TBHP)/Si02 system, while secondary allylic, benzylic and saturated alcohols remained unaffected (Eq. 10). 

In principle, any reagent capable of activating the N-oxide oxygen can promote the Polonovski reaction. However, acid anhydrides, and in particular trifluoroacetic anhydride (the modified Polonovski reaction), are usually employed. The discussion in this chapter will therefore be limited mainly to the use of acid anhydrides. Brief mention is made in Sections 4.7.2.5 and 4.7.2.6 of the use of iron salts, sulfur dioxide and organo-silicon and -selenium derivatives as activating agents. 

Ionic Inflate derivatives of nonmetallic elements such as selenium, sulfur, phosphorus, and iodine form an important class of reagents lor organic chemistry. Highly electrophilic phenylselenyl triflate can be used in the cyclization of 5- and 6-hydroxyalkenes, affording the corresponding tetrahydrofurans and pyrans [132] (equation 68). 

The preparation of Pans-1,2-cyclohexanediol by oxidation of cyclohexene with peroxyformic acid and subsequent hydrolysis of the diol monoformate has been described, and other methods for the preparation of both cis- and trans-l,2-cyclohexanediols were cited. Subsequently the trans diol has been prepared by oxidation of cyclohexene with various peroxy acids, with hydrogen peroxide and selenium dioxide, and with iodine and silver acetate by the Prevost reaction. Alternative methods for preparing the trans isomer are hydroboration of various enol derivatives of cyclohexanone and reduction of Pans-2-cyclohexen-l-ol epoxide with lithium aluminum hydride. cis-1,2-Cyclohexanediol has been prepared by cis hydroxylation of cyclohexene with various reagents or catalysts derived from osmium tetroxide, by solvolysis of Pans-2-halocyclohexanol esters in a manner similar to the Woodward-Prevost reaction, by reduction of cis-2-cyclohexen-l-ol epoxide with lithium aluminum hydride, and by oxymercuration of 2-cyclohexen-l-ol with mercury(II) trifluoro-acetate in the presence of ehloral and subsequent reduction. ... 

As a rule, the order of mixing reagents is of no importance. The addition of sulfur or selenium into the reaction mixture in a last stage makes it possible to obtain corresponding derivatives. 

As an alternative to hydrozirconation of acetylenic tellurides or selenides, Dabdoub and co-workers have more recently described the first additions of the Schwartz reagent (one equivalent) to acetylenic selenide salts 51 (Scheme 4.30) [52]. Subsequent alkylation at selenium produces 1,1-dimetallo intermediates 52, which are cleanly converted in a one-pot process to stereodefined products 53. It is noteworthy that ketene derivatives 52 are of ( )-geometry, the opposite regiochemistry to that which results from hydrozirconation of acetylenic tellurides (vide supra). This new route also avoids the mixtures of regio-isomers observed when seleno ethers are used as educts. The explanation for the stoichiometric use of Cp2Zr(H)Cl in these reactions, in contrast to the requirement for two equivalents with seleno ethers, may be based on cyclic intermediates 54, in which Li—Cl coordination provides an additional driving force. Curiously, attempted hydrozirconation of the corresponding telluride salt 55 under similar conditions was unsuccessful (Scheme 4.31) (Procedure 12, p. 143). 

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