Alkenes selenenylation

Terminal alkenes react with selenenyl halides with Markovnikov regioselectivity.64 However, the (J-selenyl halide addition products readily rearrange to the isomeric products.65... 

Terminal alkenes react with selenenyl halides with anti-Markownikoff regioselec-tivity.69 However, the /i-sclcncnyl halide addition product can readily rearrange to isomeric products70 ... 

Selenoxides are even more reactive than amine oxides toward [> elimination. In fact, many selenoxides react spontaneously when generated at room temperature. Synthetic procedures based on selenoxide eliminations usually involve synthesis of the corresponding selenide followed by oxidation and in situ elimination. We have already discussed examples of these procedures in Section 4.7, where the conversion of ketones and esters to their x,/J-unsatu rated derivatives was considered. Selenides can also be prepared by electrophilic addition of selenenyl halides and related compounds to alkenes (see Section... 

The use of mediators to improve reactivity or selectivity in nitrone cycloaddition chemistry begins with the nitrone generation step. As is well known, the N-alkyla-tion of oximes provides one of the most direct and convenient synthetic routes to N-alkylated nitrones from readily available aldehydes and ketones. Electrophilic mediators have been employed to activate alkenes for N-alkylation, both in intramolecular and intermolecular reactions. They include activation of the internal alkene function by the action of (a) strong nonmetallic electrophiles such as phenyl-selenenyl sulfate (159), and (b) metallic catalysts such as Ag(I) (160) and Pd(II) ions... 

Methoxyselenenylation of alkenes.1 This reaction can be effected by reaction of 1 with Br, in CH2C12 to form a red solid that is considered to be a selenenyl bromide. This intermediate reacts with an alkene in CH,OH to give a methoxy- denenylated alkene. The reaction with styrene gives only the Markovnikov product in a yield of 49% and 49% de. 

It is possible to perform selenenylation-deselenenylation sequences with only catalytic amounts of selenium species. This reaction sequence provides double bond transpositioned allylic ethers, allylic esters, or allylic alcohols 240 from the corresponding alkenes (Scheme 71). This sequence can be performed electrochemically, and the selenium electrophile is generated from catalytic amounts of diphenyl diselenide.467,468 It has been shown that the electrophilic selenium species can also be generated using diselenides and peroxosulfates together with copper (ii)... 

Benzeneselenenyl azide adds to alkenes readily. The addition of the selenenyl azide always occurs with trans stereochemistry. The yield of adducts is reliably high with several different alkenes. Unlike benzeneselenenyl chloride, mixtures of regioisomers are found with simple primary alkenes. No addition occurs between benzeneselenenyl azide and ethyl crotonate. The reagent adds to conjugated dienes in a trans 1,4-fashion which is thought to be due to an initial trans 1,2-addition, followed by a facile 1,3-allylic azide shift (equation 19). Unfortunately, this reagent must be prepared and used in situ ... 

Selenenyl chlorides add to alkenes, often via an AdE2 mechanism involving a bridged seleniranium ion intermediate (19) (equation 14). These reactions are therefore highly stereospecitic, resulting in anti addition. The regiochemistry of the process can be under either kinetic or thermodynamic control. In some cases, initial anti-Markovnikov products were observed at low temperature and Markovnikov adducts dominated after further equilibration. Analogous electrophilic additions to acetylenes and aUenes (Scheme 9) have also been reported. When selenenyl hahdes react with alkenes in the presence of other nucleophiles such... 

Other species of general stmcture RSeX, where X = a nonhalide leaving gronp, are also known and often show similar behavior to that of the selenenyl hahdes. " For example, benzeneselenenyl acetate (20), trifluoroacetate (21), and tosylate (22) can be generated in situ from the reactions of the selenenyl halides with silver acetate, trifluoroacetate, or tosylate, respectively (Scheme 10). The former two electrophiles react with enol acetates to produce a-seleno ketones and with alkenes and acetylenes to give 1,2-addition products, while the latter adds similarly to acetylenes. Examples are shown in equations (16) to (18). 

Selenocyanates produce selenols or diselenides upon either reduction (e g. with sodium borohydride) or hydrolysis (see Scheme 1). They undergo displacement of the cyanide ion by various nucleophiles and add to alkenes in a maimer similar to selenenyl halides (see equation 14), except that catalysis with Lewis acids is required in the case of unactivated alkenes. The selenocyanates are also popular reagents for the preparation of selenides from alcohols, and (8) from carboxylic acids, as indicated in Scheme 3. 

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. 

When selenenic acids are generated in the presence of alkenes, addition reactions similar to those of selenenyl halides occur. Oxidation and syn elimination of the adducts (42) provide a convenient synthesis of allylic alcohols (equation... 

Dehydrogenation reactions using sulfur reagents have been shown to tolerate a variety of other functional groups, including acetals, alkenes, epoxides and silyl ethers, - but the milder procedures available using selenenyl moieties may offer advantages in more sensitive molecules (see Section 2.2.4). 

Alkenes react with many selenenyl electrophiles to form 1,2-adducts thus, selenenyl chlorides, bromides and pseudohalides afford a-chloro-, a-bromo-, or other a-functionalized selenides, respectively. When the addition of a selenenyl halide to an alkene is carried out in the presence of a nucleophile, in some cases the nucleophile is incorporated at the a position instead of the halide atom. 

Table 3. Methoxyselenylation of Alkenes Using (/ )-1,1 -Bi-naphthyl-2-selenenyl Bromide and Methanol32...

Mcthoxyselenenylation of Alkenes using (/ )-1,l -Binaphthyl-2-selenenyl Bromide General Procedure32 ... 

The addition of selenenyl electrophiles can be employed for the conversion of alkenes to diastereomeric selenides containing various -nitrogen substituents. Such processes are generally and-stereoselective and produce mixtures of regioisomers in which the Markovnikov adduct dominates. 

Phenylseleno thiocyanate rapidly undergoes addition to alkenes without the presence of a catalyst, producing -thiocyanates and/or /1-isothiocyanates. The stereoselectivity of the addition is less predictable than with other selenenyl electrophiles. 

Tetravalent selenium chlorides, like selenenyl electrophiles, add readily to alkenes affording tetravalent /i-chloro alkylselenium dichlorides. 

Very recently, using polystyrene beads, the preparation of some polymer-sup-ported selenium reagents has been described by Nicolaou [28]. Polymer-bound selenium bromide and selenium phthahmide act as efficient selenenylating agents of alkenes. In view of their versatility and ease of handhng these reagents can find useful applications in sohd phase and combinatorial synthesis. 

The behavior of the sulfate counter anion is noteworthy. A considerably important advantage in using sulfates is that the reactions can be effected at room temperature. From the data collected in Tables 1 and 2 it can be seen that in the methoxyselenenylation of alkenes the diastereomeric excesses thus obtained are comparable with those observed with other selenenylating agents,... 

As previously observed for the diphenyl diselenide (Scheme 2) [24], Pandey has found that the phenyl alkyl selenides also can be transformed into the corresponding radical cations by photostimulated single electron transfer to 1,4-dicyanonaphthalene (DCN). These intermediates, as proposed above, suffer fragmentation to afford diphenyl diselenide and a carbocation, which can be trapped by a nucleophile. On the basis of these observations Pandey [121] described a sequential one-pot selenenylation-deselenenylation of alkenes in methanol using only catalytic amounts of diphenyl diselenide. An example is reported in Scheme 41 which illustrates the single steps of this process... 

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