Seleniranium ion

Internal nucleophilic capture of seleniranium ion is governed by general principles similar to those of other electrophilic cyclizations.96 The stereochemistry of cyclization can usually be predicted on the basis of a cyclic TS with favored pseudoequatorial orientation of the substituents. 

Areneselenenyl halides react with double bonds similarly to sulfenyl derivatives 1,2-additions are generally anti stereospecific, in agreement with the involvement of a bridged intermediate [episelenurane (a) and/or seleniranium ions (b)], prior to the product-forming step. 

The involvement of at least three different forms of the seleniranium ion intermediate, i.e. tight and solvent-separated ion pairs and free ions, has been invoked also to rationalize the different chemical behavior observed in the addition of benzeneselenenyl chloride to bicyclo[2.2.1]hepta-2,5-diene (49) in methanol and in methylene chloride140. As stressed by the authors, the addition of benzeneselenenyl chloride to 49 shows a number of interesting trends. Four products (151-154), all resulting from homoallylic attack, were isolated from the reaction carried out in methanol (equation 131). Furthermore, it... 

The mechanism of the asymmetric methoxyselenenylation of alkenes has been investigated using competition experiments and computational methods (Scheme 8). The experiments have demonstrated that the formation of the intermediate seleniranium ion (48) is reversible. Ions of type (49), generated in the addition of chiral selenium electrophiles to alkenes, are the key intermediates in the asymmetric methoxyselenenylation their stability is strongly dependent on the strength of the selenium-heteroatom interaction. Calculations have been carried out to determine the relative stabilities of the diastereoisomeric seleniranium ions (49). The results obtained from the calculations support the experimental flndings. ... 

The mechanism of selenocyclization of yS,y-unsaturated acids and their derivatives has been studied. The reactions of ( )-4-phenylbut-3-enoic acid and its silyl and alkyl esters (15 R = H, SiMe3, alkyl) with benzeneselenenyl halide PhSeX (X = Cl, Br) have been examined by VT-NMR and in situ IR spectroscopic methods. Whereas the reactions of the acid in the presence of a base were irreproducible and complicated, reactions of the silyl esters were clean and spontaneously and quantitatively afforded the corresponding chloroselenylation adduct at -70 °C as a single (Markovnikov) isomer. This adduct underwent three processes as the temperature was raised (1) reversal to the starting materials, (2) isomerization to the anti-Markovnikov product, and (3) cyclization to the selenolactone (16). All of these processes are believed to proceed via a seleniranium ion, the intermediacy of which was established by independent synthesis and spectroscopic identification. The reversible formation of chloroselenide adducts was unambiguously established by crossover experiments. The reaction of (15) with PhSeBr was found to be rapid but thermodynamically unfavourable at room temperature.29... 

Well before the wide use of organoselenium compounds in chemistry, it was discovered that electrophilic selenium compounds of the type RSeX add stereospecifically to alkenes.45 Since that time this reaction has been an important tool in the portfolio of organic chemists and has been used even for the construction of complex molecules. Comprehensive reviews on this chemistry have appeared46-49 and in recent times the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. As shown in Scheme 1, the addition reactions of selenium electrophiles to alkenes are stereospecific anti additions. They involve the initial formation of seleniranium ion intermediates 1 which are immediately opened in the presence of nucleophiles. External nucleophiles lead to the formation of addition products 2. The addition to unsymmetrically substituted alkenes follows the thermodynamically favored Markovnikov orientation. The seleniranium ion intermediates of alkenes with internal nucleophiles such as 3 will be attacked intramolecularly to yield cyclic products 4 and 5 via either an endo or an exo pathway. Depending on the reaction conditions, the formation of the seleniranium ions can be reversible. 

Experimentally, this was verified by the independent synthesis of the diastereomeric seleniranium ions 37 and 40. The corresponding /Fhydroxyselenides 36 and 39 were obtained by a reaction of the selenium anion with enantio-merically pure (R)- or (A)-styrene epoxide. These /Fhydroxyselenides were then treated with trifluoromethane... 

The seleniranium ion intermediates in the cyclization reactions can be either generated from the corresponding /3-hydroxyselenides as shown in Scheme 6 or from suitably substituted alkenes. Depending on the alkene and on the selenium electrophile, cyclizations can be performed with high selectivities. The size of the electrophilic reagent has... 

Another method of hydroxyselenenation involves trapping the seleniranium ion by water. The use of N-phenyl eleno-succinimide (N-PSS) or -phthalimide (N-PSP) as the selenium electrophile facilitates the reaction, since the sucdnimide or phthalimide anion is not as nucleophilic as water. With dienes, trans-annular cyclizations can occur, forming bis(phenylseleno) ethers in good yields (equation 22). ... 

Another interesting sequence is the amidoselenenation of alkenes for the synthesis of allylic amides. The seleniranium ion is trapped by a nitrile group which is first converted to an iminium chloride and then hydrolyzed to the amide (similar to the Ritter amide synthesis). Several differing nitriles (e.g. methyl to phenyl) have been utilized and all provide good yields of amides. The stereochemistry of addition is always trans but mixtures of regioisomers occur with terminal and unsymmetrically substituted oleflns (equation 24). The -seleno amide is easily converted to the allylic amide by oxidation of the phenyl selenide using the standard conditions. ... 

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... 

In summary, several types of carbon ir-bonds react with benzeneselenenyl halides producing usually one or more regioisomers. Several of these regioisomers may be equilibrated via their seleniranium ions. 

Selenium reagents can be used to form carbocycles. A good example of the formation of carbocycles is the elegant synthesis of hirsutene (equation 31). The key step involves the attack of an enol on a seleniranium ion. This type of carbocyclization proceeds very nicely and in high yield. ... 

Okfin eycUzatUm. (Z,Z)-l,5-Cyclononadiene (1) reacts with benzeneselenenyl chloride in acetic acid buffered with NaOAc with transannular participation by the remote double bond to afiord 2 in 68% yield/ Under these conditions, geranyl acetate (3) does not afiord 6, but rather the usutd olefin addition products 4 and 5. Cyclization of 4 to 6 does occur on treatment with CF3CO2H in CH2CI2 the seleniranium ion a is probably an intermediate, since the compound analogous to 4 but lacking the CsHsSe— group fails to cyclize under these conditions. ... 

A.b initio calculations on the reaction of 39 and 40 with alkenes account for the diastereomeric product ratios and stabilities of the intermediary seleniranium ions <1998JA3376, 2000JA10914>. 

A novel 1,2-shift of silicon ascribed to seleniranium ion formation has been reported <1994JA2356>. The key rearrangement is shown in Equation (10). 

The rate of nucleophilic substitution at positions ft to selenium is greatly enhanced. This anchimeric assistance has been ascribed to neighboring group participation with formation of the corresponding seleniranium ion in the ratedetermining step as shown in Equation (11). 

As discussed in Section 1.07.6.4, addition of electrophilic selenium reagents to alkenes, which is suggested to lead to seleniranium ion intermediates, has been extensively studied. Some recent examples are also illustrated in that section. 

A recent study on the mechanism of selenocyclization of beta, gamma-unsaturated acids and their derivatives has revealed evidence for the intermediacy of seleniranium ions <2006JOC7293>. 

Elimination of water is particularly favored from p-chlorophenyF and (p-trifluoromethyl)phenyl selenides, whereas elimination of seleninic acid is mainly observed from methylseleno derivatives. Here, two different effects work in the same direction. The arylseleno group, more than the methylseleno group, makes the a-hydrogen acidic, and has at the same time a lower tendency to stabilize a p-caib-enium ion via the formation of a seleniranium ion. 

The reaction of 1 -hexene with phenylselenenyl hexafluorophosphate (PhSePF6) in toluene which resulted in the formation of the seleniranium ion (92) has been reported <75TL399l>. This seleni-ranium ion (92) was also shown to be an intermediate in the reaction of 2-bromohexyl phenyl selenide (93) with dimethylsulfoxide in the presence of silver hexafluorophosphate (AgPF6) and triethylamine which leads to the formation of a-phenylselenoketone (94) (Scheme 19) <78TL226i>. 

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