Episelenonium ions

Oxyselenation of alkenes (8, 119 120). Details of this reaction have been published. The role of the metal catalyst is considered to be polarization of the Sc CN bond of the selcnocyanate, and in fact the pyridine complex of the metal salt is less effective. An episelenonium ion is postulated as the intermediate. CuBr2, CiCl, NiBr2, and NiCl2 are similar to CuCl2 in activity. CrCl2 and CoCI2 show limited activity, whereas halides of many other metals are inactive. 

Steric protection by the bulky 2,4,6-tri-f-butylphenyl (TTBP) group of the selenium atom in the episelenonium ion intermediate (17), generated in situ from (16), has been utilized in the selective formation of carbocyclic compounds (17)—>(19)—>(20) and (21)—>(22). The selenophilic attack (17)—>(18), leading to isomerization, can thus be avoided (Scheme 8).33... 

When a C —C double bond participates as a nucleophile in an intramolecular attack on an episelenonium ion the generation of a transient carbenium ion occurs which can be captured by another nucleophile or the solvent leading to a new carbocyclic ring. This type of cyclization is facile and synthetically useful. Thus, 1,6-cyclononadiene is converted to (1.5, 3a/ AV, 7a/ )-4-hydroxyoctahydro-l-phenylseleno-l//-diene (46) on treatment with phenylselenenyl chloride and subsequent hydrolysis57. 

An alternative approach is to use the readily available P-hydroxy phenyl selenides as Ritter substrates. Amide formation occurs with retention of configuration, indicating that fission of the carbon-oxygen bond is assisted by the neighboring phenylseleno group (Scheme 61). Diphenyl diselenide and iodine react with 1,5-dienes to give carbocyclic products. Initial formation of the episelenonium ion is followed by intramolecular attack and subsequent Ritter reaction (Scheme 62). ... 

Livinghouse has extended his studies on episulphonium ions (cf. Scheme 76) to include the corresponding episelenonium ions. The reagent of choice in this latter situation was... 

The Z-a, 3-unsaturated ester 2 is formed from the syn 3-hydroxy selenide by an anti elimination. 0-Mesylation and loss of the leaving group generates the cis episelenonium ion and hence the Z-alkene 2 (see Scheme 2.28). The syn 3-hydroxy selenide is formed by an aldol reaction of the Z-titanium enolate via a chair-shaped transition state (see Section 1.1.3). See S. Nakamura, T. Hayakawa, T. Nishi, Y. Watanabe andT. Torn, Tetrahedron, 57 (2001), 6703. 

Hairing SR, Edstrom ED, Livinghouse T (1992) Episulfonium ions and episelenonium ions. Versatile heterocyclic cations for the synthesis of natural products. Adv Heterocycl Nat Prod Synth 2 299-376... 

Sasaki M, Hatta M, Tanino K, Miyashita M (2004) Regio- and stereospecific alkyl and alkynyl substitution reactions of epoxy selenides with oiganoaluminums via episelenonium ions. Tetrahedron Lett 45 1911—1913... 

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