Selenium—carbon bonds

The wide variety of methods available for the synthesis of orga-noselenides,36 and the observation that the carbon-selenium bond can be easily cleaved homolytically to give a carbon-centered radical creates interesting possibilities in organic synthesis. For example, Burke and coworkers have shown that phenylselenolactone 86 (see Scheme 16), produced by phenylselenolactonization of y,S-unsaturated acid 85, can be converted to free radical intermediate 87 with triphenyltin hydride. In the presence of excess methyl acrylate, 87 is trapped stereoselectively, affording compound 88 in 70% yield 37 it is noteworthy that the intramolecular carbon-carbon bond forming event takes place on the less hindered convex face of bicyclic radical 87. 

Selenoaldehydes 104, like thioaldehydes, have also been generated in situ from acetals and then directly trapped with dienes, thus offering a useful one-pot procedure for preparing cyclic seleno-compounds [103,104], The construction of a carbon-selenium double bond was achieved by reacting acetal derivatives with dimethylaluminum selenide (Equation 2.30). Cycloadditions of seleno aldehydes occur even at 0 °C. In these reactions, however, the carbon-selenium bond formed by the nucleophilic attack of the electronegative selenium atom in 105 to the aluminum-coordinated acetal carbon, may require a high reaction temperature [103], The cycloaddition with cyclopentadiene preferentially gave the kinetically favorable endo isomer. 

A carbon-selenium bond can also be formed [106] by Diels-Alder reaction of the transient selenonitroso species 106 generated by phenylsulfinylselenylchlor-ide reacting with amines or trimethylsilylated amines. Selenonitroso compounds 106 were trapped with 2,3-dimethylbutadiene to afford 1,2-selenazine derivatives 107 (Scheme 2.44) in low yield. 1,2- Selenazines are interesting compounds which are quite unstable (2-3 h), except for the one having an... 

CARBON-SULFUR AND CARBON-SELENIUM BOND-FORMING CROSS-COUPLINGS 384... 

Allenes have also been used as substrates in free radical cyclizations. Dener and Hart demonstrated that such entries are valuable in constructing pyrrolizidine and indolizidine ring systems [71]. In a total synthesis of pyrrolizidine base (+)-heliotri-dine (340), compound 338 possessing an allene functionality was used as a key intermediate (Scheme 19.62). Tri-n-butyltin radical-mediated carbon-selenium bond homolysis of 338 followed by the addition of the free radical to the allene moiety... 

Reductive Cleavage of Carbon-Phosphorus, Carbon-Sulfur, and Carbon-Selenium Bonds... 

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],... 

Unlike many other type of radical addition reactions, the product is most often an alkyl-cobalt(III) species capable of further manipulation. These product Co—C bonds have been converted in good yields to carbon-oxygen (alcohol, acetate), carbon-nitrogen (oxime, amine), carbon-halogen, carbon-sulfur (sulfide, sulfinic acid) and carbon-selenium bonds (equations 179 and 180)354. Exceptions to this rule are the intermolecular additions to electron-deficient olefins, in which the putative organocobalt(III) species eliminates to form an a,/ -unsaturated carbonyl compound or styrene353 or is reduced (under electrochemical conditions) to the alkane (equation 181)355. 

Diselenolylium ions have been prepared by acid induced cyclization and dehydration of oxo esters of AT,AT-dialkyldiselenocarbamates (75JOC746, 77CC505, 80CC866, 80CC867). As shown in Scheme 14, protonation of the carbonyl oxygen results in carbon-selenium bond formation by electrophilic attack on selenium. Acid induced dehydration generates the... 

Besides the use of electrophilic or nucleophilic selenium reagents in synthesis, selenium has another advantage. It can stabilize adjacent carbanions or carbocations. While the deprotonation of selenides is difficult, the corresponding diselenoacetals can be used as efficient precursors through the cleavage of one of the carbon-selenium bonds. The chemistry of selenium-stabilized carbanions and carbocations is outlined in Section 9.11.2.3. 

The homolytic cleavage of the carbon-selenium bond provides an easy access to radicals, which can undergo various subsequent reactions. The plethora of radical chemistry is accessible using selenium-containing compounds and examples of free-radical reactions are described in Section 9.11.2.4. 

A photosensitized activation of carbon-selenium bonds was also used for performing phenylseleno group transfer reactions. This process involves a photosensitized electron transfer (PET) as the initial step in the reaction sequence. Fragmentation affords a radical and phenylselenolate, which is oxidized to diphenyl diselenide in the presence of oxygen. The cyclized radical is then trapped by diphenyl diselenide to afford the final product. This process is quite general for intramolecular radical reactions.70,266... 

Fig. 41. Molecular structure of [ (CO)5Fe2(jU.-Se2)2C(Ph)C(H)(jU.-dppe)] (129). [Reprinted with kind permission from Mathur, P. Hossain, M. M. Mahon, M. F. Carbon-selenium bond cleavage in the double butterfly complex [ (CO)6Fe2(jU.-Se)2)2(C(Ph)-C(H) ]. J Organomet. Chem. 1994, 471, 185. Copyright 1994 Elsevier Science S. A., P.O. Box 564, 1001 Lausanne, Switzerland.]...

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

FORMATION OF CARBON-SELENIUM BONDS FROM ADDITIONS TO -ir-BONDS... 

The weak carbon-selenium bonds tune the reactivity of electrophiHc selenium species as well. After the first publication in this area by Reich et al. in 1973 [17], intense activities can be recognized in this field of organoselenium chemistry. The versatility of electrophilic selenium species with respect to stereoselective synthesis is presented by M. Tiecco in Chap. 2. 

First, the homolytic cleavage of carbon-selenium bonds and selenium-selenium bonds is an easy process that can be initiated thermally or photochemi-cally. Sun lamp irradiation is usually sufficient to initiate a chain reaction involving organoselenium derivatives. 

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