Selenosulfonations

In the same manner as described before, arenesulfonyl thiocyanates are able to show self-addition to conjugated systems yielding sulfones243,244. More important, however, is that reactions of selenosulfonates with unsaturated systems as well as with nucleophilic carbon have been proved. 

TABLE 7. Sulfones from unsaturated C—C systems and selenosulfonates... 

One of the steps in the radical chain selenosulfonation of multiple bonds (vide infra) is equation 10, in which the R radical attacks via a SH2 reaction the Se-aryl areneselenosul-fonates to regenerate a sulfonyl radical, namely. 

Not only electrophilic 1,4-addition, as shown above, but also radical 1,4-addition to conjugated enynes such as selenosulfonation is known to yield acceptor-substituted allenes [118]. Finally, monotitanation of conjugated diynes followed by treatment with benzaldehyde and aqueous workup leads to an ester of penta-2,3,4-tri-enoic acid, which is formally also a product of 1,4-addition [147]. 

The preparation of polystyrene-supported selenosulfonates and their application for the synthesis of acetylenic sulfones have been reported by Huang et al. (Scheme 3.3) [126],... 

Disulfides, diselenides, and ditellurides can be oxidized by hypervalent iodine compounds quite easily. Depending on the reaction conditions disulfides can be oxidized to sulfinic esters [59] or thiosulfonic S-esters [60,61]. Diselenides can be transformed into selenosulfonates [62]. Arenetellurinic mixed anhydrides are mild oxidants and can be obtained by oxidation of the corresponding ditellurides as shown in Scheme 9 [63]. Recently it was shown that a thioacetal based linker for solid-phase synthesis can be cleaved oxidatively using [bis(trifluoro-acetoxy)iodo]benzene 4 [64]. 

In the presence of DABCO (30 mol%), the addition of various selenosulfonates, PhSChSeAr, to a,f3-unsaturated ketones proceeds smoothly at room temperature to give the corresponding Michael adducts in good yields under mild conditions.157... 

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. 

The [2,3]sigmatropic rearrangement of allylic selenides has proven to be a useful method for the preparation of allenic alcohols. Selenide 170 was obtained by a free-radical selenosulfonation of the corresponding enyne. Oxidation with mCPBA afforded the allenic alcohol 171 in 89% yield via an intermediate selenoxide (Scheme 49).295... 

R Dienophile Activation via Selenosulfonation. l-(Benzenesulfonyl)-cyclopentene... 

Selenosulfonation can also be used to achieve regiochemical reversal in Diels-Alder reactions. 

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). ... 

Selenosulfonates (see also Section 3.6.2.3) add to alkenes when catalyzed by Lewis acids. These adducts are transformed via oxidative elimination to vinyl sulfones in good overall yields. Similar to other methods only trans addition is observed. Mixtures of regioisomers are often produced with unsyminetri-cal unactivated alkenes. All other alkenes (i.e. styrene and acrylonitrile) provide Maikovnikov additions. Radical processes yield anti-Markovnikov adducts. Thus, either regioisomeric vinyl sulfone may be prepared from almost any activated alkene by simply varying the mode of addition. ... 

The selenosulfonates (26) comprise another class of selenenyl pseudohalides. They are stable, crystalline compounds available from the reaction of selenenyl halides with sulftnate salts (Scheme 10) or more conveniently from the oxidation of either sulfonohydrazides (ArS02NHNH2) or sulftnic acids (ArS02H) with benzeneseleninic acid (27) (equations 21 and 22). Selenosulfonates add to alkenes via an electrophilic mechanism catalyzed by boron trifluoride etherate, or via a radical mechanism initiated thermally or photolytically. The two reaction modes produce complementary regioselectivity, but only the electrophilic processes are stereospecific (anti). Similar radical additions to acetylenes and allenes have been reported, with the regio- and stereochemistry as shown in Scheme 11. When these selenosulfonation reactions are used in conjunction with subsequent selenoxide eliminations or [2,3] sigmatropic rearrangements, they provide access to a variety of unsaturated sulfone products. 

Several types of electrophilic selenium species have been immobilized on solid supports, for more convenient application to organic synthesis. These include selenenyl halides, " selenocyanates, and selenosulfonates. ... 

Selenosulfonate resin 44 has also been used in radical additions to alkynes in an approach to alkynyl sulfones (Scheme 11) [9]. Radical selenosulfonation... 

In a related study, Huang [34] has employed resin-bound selenosulfonate 202 in the radical cyclative-capture of 1,6-dienes such as 201 (Scheme 50). Oxidation and eliminative cleavage provided methylenetetrahydrofurans 204 in moderate yields. 

Keywoids Radicals, Group transfer reactions. Photoelectron transfers, Selenosulfonation, Acyl radicals, Azidoselenenylation, Selenides, Selenol... 

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