Seleno radicals

Radical cyclization is also an efficient method for the formation of tetrahydrofuran rings. Thus, treatment of the seleno radical precursor with n-Bu3SnH in hot toluene gave the multi-substituted tetrahydrofuran in high yield <02T2605>. 

In Tab. 15.2 the relative reactivity of seleno radicals toward unsaturated bonds is compared with that of other heteroatom-centered radicals [132, 136]. 

Seleno radicals are relatively less reactive toward olefins, and these kinetic data suggest that the reverse process from -seleno alkyl radical intermediates contributes to the inefficiency of the radical addition reactions of selenium compounds (Scheme 15.58). 

Radicals analogous to the above containing other chalcogens in place of oxygen are named by adding the prefixes thio-, seleno-, and so on for example, PS, thiophosphoryl CS, thiocarbonyl. 

Mixed aryl selenides have also proven to be excellent ree ents for group transfer reactions.Photolysis of selenides in an inert solvent such as benzene can initiate chain reactions. Substituted radicals can be generated in this manner, from a-selenoe-... 

Photolysis at room temperature of aryl telluroformates gives rise to oxoacyl radicals, which can be trapped with diphenyl diselenides giving the corresponding phenyl seleno-formates. ... 

Renaud and co-workers used 78 for the synthesis of (-)-phaseolinic acid (6) and (-)-pertusarinic acid (8) (Scheme 12) [32, 33]. Radical addition of dimethyl phenylselenomalonate to 78 proceeded with rearrangement of the bicyclics to yield the seleno-acetal 79 [34]. After reductive deselenylation and Baeyer-Villiger oxidation treatment of 80 with BU4NI and BBr3 led to a simultaneous cleavage of the ether, the lactone, and the methyl ester func-... 

C,H4S4)14 (NCSe), (2,2 -bi-l, 3-di-thiolylidene radical cation seleno-cyanate (14 8)), 19 31 (C4H4S4),4I43 2,2 -bi-l, 3-dithio-lylidene radical cation iodide (24 63)), 19 31... 

Selenium-containing molecules have also been used as precursors for radical seleno group transfer reactions. This is a very powerful method for radical additions to alkenes and alkynes it is especially interesting from an atom economy point of view since all atoms remain in the product molecule. The free-radical addition of selenosulfonates 146 can be initiated either photochemically or thermally using AIBN. The addition of 146 not only to alkynes 147,255-257 km also to alkenes258-261 or allenes,261 has been reported and the products such as 148 are versatile building blocks for subsequent reactions (Scheme 39). For example, vinyl selenides 148 can be easily transformed into allenes. 

A combination of a hetero Diels-Alder reaction of an aldehyde and a radical reaction in a sequential transformation to give the bridged pyrans 2-22 via 2-21 starting from 2-20 containing a seleno moiety and an electron-rich butadiene 2-10, was described by Clive (Fig. 2-6) [85]. 

Nitroxide mediated free radical polymerization is a living or controlled polymerization process. It can be used to initiated or terminate polymerization reactions as needed (1). The use of Phosphino, aryloxy, silyl, boryl and seleno mediating agents are described (2). 

Other compounds prepared by halolactamization of alkenes (and preceding the above woik ) include (64),(65), (66),2 (67), (bS), (69) and (70). Note that related sulfeno-, seleno- and tellu-ro-cyclizations have been discussed earlier (Section 3.5.5). Finally, related products may be obtained by radical reactions of N-haloamides, giving the following (71-73) representative products in reactions promoted thermally (71),photochemically (72 and 73),279.2so dibenzoyl peroxide (73) ° or chro-mous chloride (73). ... 

A large amount of woik has been accomplished using mixed sulfiir/selenium reagents, such as seleno-sulfides and selenosulfones. One example of selenosulfide addition via radicals is selenotbiolactoniza-tion. When selenosulfide (15) is treated with AIBN, a mixture of y-seleno-substituted thiolactones is produced (equation 9). Although these lactonizations result in mixtures of stereoisomers, they usually can be separated. In addition to these reports, selenosulfones have been used to form allenic sulfones, alkynic sulfones and (phenylsulfonyl)(Uenes (S< me4). ... 

Stabilized radicals are generated. A list of the main type of radical precursors used for selenium atom transfer is depicted in Fig. 1. Malonates [64, 66-73], -ketoesters [74, 75], malononitriles [76-79], haloalkanes [80], diphosphonate [81], glycolate derivatives [82, 83] and dithianes [84]. More substrates such as a-seleno-esters, -nitriles, -ketones and -sulfones can also be used, however, the efficiency of the process is reduced [68]. 

Tri-n-butyltin radicals can also be used to generate radicals from selenium compounds. An example is the formation of acyl radicals from seleno esters. 

The above-mentioned radical C-glycosylation has been applied for the synthesis of complex compounds. O Scheme 42 shows two such examples of the radical C-glycosylation of man-nosyl bromide with complex olefins such as unsaturated ketone and unsaturated lactone. This highlights the mildness of the reaction and these conditions have compatibility with many functional groups [68]. It is noteworthy that an acceptor bearing seleno and chloro groups can be employed without interference under the radical conditions. In both reactions, the a-C-gly-cosidic products were exclusively obtained, although in moderate yields. 

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. 

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