Selenium bonding

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... 

Fig. 6.3 Cluster model structure for Ru99Se54 showing the selenium bonding onto the ruthenium clusters (a) with a statistical distribution (b) with an ordered positioning. (Adapted with permission from [24]. Copyright 2009, American Chemical Society)...

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... 

This approach is used exclusively for the synthesis of the isomeric annelated isothiazoles and isoselenazoles to form the nitrogen-sulfur(selenium) bond and has been discussed in CHEC-II(1996) <1996CHEC-II(7)49>. A wide variety of oxidative cyclizations have been reported, some of which incorpotate an amination. Eor example, reaction of 47 with chloramine gives the corresponding isothiazolo[5,4-7]thiophene 40 (Equation 9) <2000P65>. 

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

Benzyllithiums. These reagents are generally prepared indirectly from ben-zyltin or mercury compounds or from benzyl ethers. They can also be prepared by reaction of benzylphenyl or benzylmethyl selenides with n-, sec- or f-BuLi, since cleavage occurs exclusively at the benzyl—selenium bond.2... 

Aluminum(III)—sulfur—selenium bonds, covalent and non-covalent, 9, 257... 

Element-element bonds, addition to G-G multiple bonds arsenic—selenium bonds, 10, 782 boron—boron bonds, 10, 727 boron—sulfur bonds, 10, 778 B-S and B-Ge bonds, 10, 758 chalcogen—chalcogen additions, 10, 752 germanium—germanium bonds, 10, 747 germanium-tin bonds, 10, 780 overview, 10, 725-787 phosphorus—phosphorus bonds, 10, 751 phosphorus—selenium bonds, 10, 782 phosphorus-sulfur bonds, 10, 781 Se-Si and Se-Ge bonds, 10, 779 silicon-germanium bonds, 10, 770 silicon-phosphorus bonds, 10, 780 silicon-silicon bonds, 10, 734 silicon-sulfur bonds, 10, 779 silicon-tin bonds, 10, 770 tin-boron bonds, 10, 767 tin-tin bonds, 10, 748... 

Phosphorus—selenium bonds, alkyne additions, 10, 782 Phosphorus—sulfur bonds, alkyne additions, 10, 781 Phosphorus ylides with gold(I), 2, 275 with trinuclear Os clusters, 6, 740 Phosphoylides, actinide complexes, 4, 203 Photoacoustic calorimetry, in thermochemistry, 1, 612-613 Photoactive molecular devices, and -[RefCO) ,] fragment, 5, 886... 

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],... 

I.r. and Raman data show C3v symmetry for the Se(HgMe)3 cation and C2v symmetry for the E(HgMe)2 compounds. A comparison of force constant values clearly indicates a weakening of the mercury-selenium bond on formation of the selenonium cation. 

R.A. Chittenden and L.C. Thomas, Characteristic infra-red absorption frequencies of organophosphorus compounds-III. Phosphorus-sulfur and phosphorus-selenium bonds, Spectrochim. Acta, 20, 1679-1696 (1964). 

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. 

Carbon-Oxygen, -Sulfur and -Selenium Bond Formation (Table 1)... 

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... 

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