Radicals selenyl

The phenylselenyl radical adds irreversibly to the central carbon atom of 2-methylbuta-l, 2-diene (Id) with a rate constant of 3 x 106 M-1 s-1 (23 1 °C) (Scheme 11.7) [45], On a synthetic scale, PhSe addition to cumulated Jt-bonds has been investigated by oxidizing phenylselenol with air in the presence of mono-, 1,1-di- or 1,3-di-substituted allenes to provide products of selective fi-addition. Trapping of 2-phenyl -selenyl-substituted allyl radicals with 02 did not interfere with the hydrogen atom delivery from PhSeH (Scheme 11.7) [31]. 

In the above-described displacement reactions (Sh2) of sulfur- and selenium-containing compounds, the mechanism could either be a synchronous or a stepwise process (Scheme 4.2). Thus, in the Sh2 stepwise mechanism an intermediate sulfuranyl or selenyl radical is formed, followed by a-cleavage [56]. Based on competitive studies, a stepwise process was suggested to occur in the reaction of (TMS)3Si radical with -decylphenylselenide [43]. On the other hand, ab initio calculations supported the synchronous process and the same observations were explained in terms of an overall reversible reaction [57]. 

Scheme 10.12 gives other examples of cyclization, including examples in which radicals are generated by sulfuryl and selenyl abstraction and by Mn(02CCH3)3 oxidation. 

Thiyl and selenyl radical-mediated domino reactions of dienes... 

DOC (dissolved organic carbon), 606, 607-8 Dodecyltrimethylammonium bromide, 744 Domino reactions, thiyl and selenyl radicals, 221-3, 224, 225, 226-7 DOPA (3,4-dihydroxyphenylalanine), 614, 615... 

Selenophenes, dioxirane oxidation, 1157 Selenyl radicals, triplet oxygen domino reactions, 223, 227... 

The synthesis of 32,3-peptides bearing allyl substituents has been achieved122 by the direct modification of a 33-peptide or the preparation of a ()23-amino acid and then peptide coupling. This novel approach makes use of radical-based chemistry to introduce the allyl substituent from a phenylselenyl-substituted peptide. The allyl substituent has been introduced both prior to the coupling of the amino acids and also after the coupling of several amino acids to form a selenylated oligopeptide. 

The modification of [T-amino acids by a-selenylation and then free radical allylation leads to the formation of (A-amino acids bearing an a-allyl group (Scheme 12). Hanessian and co-workers 22 initially used this approach to produce p2 3-amino acids which were then subjected to suitable coupling procedures. Interestingly, the coupling method chosen made use of PyBOP and hence demonstrates that other peptide coupling procedures can be used in the synthesis of (3-peptides. 

Treatment of diselenide with perfluoroalkyl iodide (66) in the presence of sodium hydroxymethanesulfinate gives perfluoroalkyl selenide (67) in good yield (eq. 2.32). The perfluoroalkyl radical generated by the reaction of perfluoroalkyl iodide with the S02 anion radical, reacts on the selenium atom of the diselenide to give perfluoroalkyl selenide and a selenyl radical which dimerizes to the starting diselenide [75, 76]. 

The reagent combination described for the azidoselenenylation can also be used for other reactions involving selenyl radicals. Aldehydes can be transformed into selenoesters by hydrogen atom abstraction and also methyl... 

Tsuchii et al. reported a very interesting four-component domino process where an alkyne, two olefins and diphenyl diselenide sequentially react to form a highly functionalized cyclopentane derivative, after a linear addition sequence and 5-exo-trig cyclization [136]. This reaction can be seen as an interrupted polymerization process initiated by the addition of selenyl radical to an electron-deficient alkyne in the presence of a large excess of a Michael acceptor. The identity of each reaction partner is important for the outcome of the reaction. For instance, use of (PhS)2 instead of (PhSe)2 leads to the polymerization product rather than to the cyclization one, while (PhTe)2 did... 

An interesting application of the asymmetric alkoxyselenenylation of alkenes to natural product synthesis was reported recently by Wirth, who described a short procedure to obtain some furofuran lignans 147]. The total synthesis of (+)-Samin 53 [47 a] is shown in Scheme 7. The protected allylic alcohol 50 was treated with the selenyl triflate derived from diselenide 29 in the presence of 2,3-butadien-l-ol, and afforded the addition product 51 in 55% yield and with a diastereomeric ratio of 15 1. The favored 5-exo-trig radical cyclization of the major isomer afforded the tetrahydrofuran derivative 52 from which the final product was obtained through few classical steps. 

Upon irradiation, thiyl radicals are generated and add to the alkyne 148 (the corresponding selenyl radicals are also formed but are less reactive than the thiyl radicals). The vinyl radical 149 cyclizes to 150. Finally, 150 reacts with diphenyl diselenide, the best radical trap present in the reaction mixture, to afford the product 151. 

If one of the reactions in a radical chain sequence is too slow to compete effectively with radical-radical reactions, the chain will collapse. Slow reactions of simple silanes such as Et3SiH with alkyl radicals precludes their use in the tin hydride method. Although quite reactive with alkyl radicals, thiols and selenols fail in the tin hydride method because the thiyl and selenyl radicals do not react rapidly with organic halide precursors. Nonetheless, it is possible to use thiols and selenols in tin hydride sequences when a Group 14 hydride is used as a sacrificial reducing agent. The thiyl or selenyl radical reacts with the silane or stannane rapidly, and the silicon- or tin-centered radical thus formed reacts rapidly with the organic halide [8], In practice, benzeneselenol in catalytic amounts has been used in radical clock studies where BusSnH served as the sacrificial reductant [9]. 

Gancarz and Kice280,286 have also studied the photochemical reactivity of the selenyl sulphones 321. Irradiation in carbon tetrachloride affords the products shown in Scheme 29. The formation of a selenyl and a sulphur radical can be demonstrated by irradiation in cyclohexene when the adduct 322 is formed. Addition also takes place to... 

There are many synthetic examples that use radical cyclization as a key step, and the radical precursor is not limited to iodides or bromides. In Pattenden s synthesis of pentalenene, conjugated selenyl ester 156 was treated with Bu3SnH and AIBN to give a 45% yield of tricyclic ketone 159. Loss of PhSe generated the acyl radical 157, which exists in equilibrium with the ketene radical 156. Radical cyclization via the latter intermediate leads to 159. Cyclization via aryl radicals is also possible. In Schultz s synthesis of hexahydro-phenanthren-2-one derivatives, " aryl bromide 160 was cyclized to 161 in 78% yield under standard conditions. Radical cascade reactions have become quite popular for the synthesis of polycyclic ring systems. In these reaction, polyenes are subjected to radical cyclization, generating tricyclic or even tetracyclic ring systems. 5 Chiral auxiliaries have been used effectively in radical cyclization reactions. ... 

Not aU radicals show the same reactivity, broadly speaking the character of radicals is affected by (i) the nature of the atom that is the radical center and (ii) the electronic properties of the groups attached to the radical. The importance of the atom bearing the unpaired electron is nicely illustrated by the different reactivities of group 6 radicals, and is rationalized by the hard/soft nature of the radical center. Thus, alkoxy radicals (RO ) are small and hard they typically undergo H-atom abstraction and p-scission reactions. However, they rarely add to C=C. Thiyl (RS ) and selenyl (RSe") radicals, however, are larger and softer. They do not usually abstract H, but they do readily add to C=C. This is useful for (Z/E) isomerization of alkenes. 

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