Reactions selenenylation

The first stereoselective selenenylation reactions were reported using the binaphthyl-based diselenide 16 as the reagent precursor.71-76 After that, several other diselenides have been prepared initial attempts used quite long syntheses to obtain these compounds, for example, the C2 symmetrical diselenides 1777-79 and 2380 and the mannose-derived diselenide 18.81-84 The ferrocenyl-based diselenide 1985-89 is using expensive starting materials and in 1995 more simple and easy accessible diselenides like 20 and 21 have been described in literature.90-93 Camphor-based diselenides such as 2294-98 have been prepared and, based on the success of diselenides 20 and 21, structural variants 2499,100 and diselenides with different coordinating heteroatoms such as sulfur and selenium 25101-108 and 26109 have been described recently. 

The selenenylation reaction can be viewed as an electrophilic attack on the enolate, or as an Sn2 attack by the enolate on selenium. Selenenylation is useful because the phenyl selenide readily undergoes intramolecular elimination to yield the conjugated enone. 

Selenenamides (23) are obtained by the substitution of selenenyl halides with amines or by the metathesis of the former compounds with Af-silylamines. N-(Phenylseleno)phthalimide (24) is similarly obtained using potassium phthalimide (Scheme 10). These compounds can be isolated but are prone to hydrolyze when exposed to moisture. Selenenamides react with aldehydes or jS-dicarbonyl compounds to afford a-seleno derivatives (as in the process shown in equation 11), and add to activated double and triple bonds, as in the example in equation (19). The imide (24) is a useful alternative to PhSeCl in various selenenylation reactions, and to ArSeCN in the conversion of alcohols and carboxylic acids to selenides and selenoesters (8), as shown in Scheme 3. 

Phenylselenenyl chloride remains the reagent of choice for direct selenenylation of ketones. The bromide is less suitable since a competing bromination of the substrate may occur diphenyl diselenide and areneselenenyl amides are not sufficiently reactive towards ketones. The selenenylation reaction of ketones often displays high chemo-, regio- and stereoselectivity. Thus, in the reaction of phenylselenenyl chloride with the trione 1 only one carbonyl group is selectively functionalized 2. 

The value of the selenenylation reaction is that the product can be converted into an a,/S-unsaturated carbonyl compound. On treatment with dilute HsOg at room temperature, the selenium is oxidized, elimination occurs, and an a,j8-unsaturated carbonyl compound is formed. The net result is introduction of a Od bond into the a,fi position of the carbonyl starting material. Yields are usually excellent, and the method is often superior to the alternative a-bromination/dehydrobromination route (Section 22.3). No added base is required (as in dehydrobromina-tion), and the reaction occurs quickly at room temperature. 

In some reactions intramolecular chalcogen nitrogen interactions may lead to stereochemical control. For example, selenenyl bromides react with C=C double bonds, providing a convenient method of introducing various functional groups. The reaction proceeds readily, but affords a racemic mixture. The modified reagent 15.22 contains a chiral amine in close interaction with the selenium atom. It reacts with olefins affording up to 97% ee of isomer A (Scheme 15.2). ... 

Chiral selenenylating reagents have been developed and shown to be capable of effecting enantioselective additions and cyclizations. The reagent 4, for example, achieves more than 90% enantioselectivity in typical reactions.104... 

II and 12 indicate, the selenenylation of ketones can also be effected by reactions of enol acetates or enol silyl ethers. 

Pentenyl amides such as 15A cyclize to lactams 15B on reaction with phenyl selenenyl bromide. The 3-butenyl compound 15C, on the other hand, cyclizes to an imino ether 15D. What is the basis for the differing reactions ... 

It is noteworthy that 108 reacts in AcOH with benzenesulfenyl chloride to give a 1 1 mixture of the sulfur analogues of 138 and 140, but when the reaction is carried out in the presence of LiClC>4 a complex mixture of at least five products was detected. From this comparison the authors suggest that areneselenenylation is much less affected by the solvent than arenesulfenylation, and if the reaction profiles for the two product-forming processes are assumed to be similar, the difference in product distributions can be interpreted in terms of a more efficient bridging ability of selenium than that of sulfur. In the addition of selenenyl derivatives, the solvent-dependent product distribution has also... 

The larger (Z,Z)-l,5-cyclononadiene (169) reacts141 stereoselectively with PhSeCl in AcOH to give the substituted hydrindan 170 (equation 138). In consideration of the anti addition mode of selenenyl reagents to double bonds, the transannular reactions of 169 have been rationalized on the basis of the two reaction intermediates, 171 or 172, which are liable to place the PhSe- and AcO- groups in a cis- 1,4-relationship and trans to the bridgehead hydrogen (equation 139). The preferential formation of 170 has thus been attributed to the fact that the pathway via 172 should involve a boat transition state. 

Selenenyl groups can be abstracted from acyl selenides to generate radicals on reaction with stannyl radicals.201 202 203 Normally, some type of stabilization of the potential reaction site is necessary. Among the types of selenides that are generated by selenenyl abstraction are x-sclcncnyl cyanides and a-selenenyl phosphates. 

Reaction of the 1,3,2-benzodiselenastannole (166) with selenium tetrachloride gives the triselenole (40) which is converted into the triselenolium hexafluorophosphate (20b) by reaction with nitrosyl hexafluorophosphate <94CC1593>. Benzo[l,2-rf][l,2,3]triselenolium chloride (20c) was obtained by reaction of 1,2-benzene-bis(selenenyl chloride) with selenium <92AG(E)779>. The structure of the trifluoromethanesulfonate (20a) has been established by x-ray crystallography (see Section 4.21.3.1). 

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... 

OrPhenybelenenylation of a,f -unsaturated esters.3 (F)-3r,/MJnsaturated esters are converted to a-phenylseleno-a,0-unsaturated esters in 20-65% yield by reaction with LDA followed by C6H5SeBr. The reaction is related to the reaction of a,/ -enones with pyridine and C H5SeCl (9, 28-29), and is also believed to involve conjugate addition of the base followed by selenenylation of the enolate. 

Trifluoromethaneseleninyl chloride (3) can be prepared by the reaction of trifluoromethane-sclcnenyl chloride with ozone whereas nitrogen dioxide transforms the initially formed acid choride to the anhydride (CF3Se0)20.316 Trifluoromethaneselenenic acid chloride and bromide are oxidized with hypofluorous acid in acetonitrile to the corresponding seleninyl halides 3 and 4, respectively.204 When an excess of hypofluorous acid is employed, the selenenyl halides and trifluoromethaneseleninic acid (5) are oxidized to the anhydride 6.304... 

The versatile selenenylating agent N-(phenylseleno)phthalimide (NPSP) is an effective carbocyclization mediator, which is capable of effecting acid catalyzed cyclization reactions from open-chain olefins, including the formation of cyclopropanes1. This is demonstrated by the quantitative cyclization of 3-butenyltrimethyltin to (cyclopropyl-methylseleno)benzene upon treatment with 1.1 equivalents of NPSP in CH2C12 at 25 °C under acid (e.g. p-TsOH) catalysis (equation 47). 

Addition of a selenenyl halide to an alkyne takes a course analogous to that of the sulfenyl halide, as seen in the reaction of l,4-dichlorobut-2-yne with PhSeCl or PhSeBr (Scheme 19).28-62 Selenenyl halides also undergo facile addition to allenes with exclusive attack by selenium occurring at the central allenic carbon.63... 

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