Ylides, selenonium

The synthesis of a selenonium ylide with a tetronic acid anion moiety was reported (83TL75 84CPB2666). 3-Enamino-tetronie aeids have also been investigated (82SC431). 

Chiral sulfonium ylides have been known for some 30 years, and their stereochemistry and properties have been studied.15 Optically active selenonium ylides were obtained by reacting selenoxides with 1,3-cyclohexanedione under asymmetric conditions by Sakaki and Oae in 1976 for the first time,16 and also optically resolved by fractional recrystallization of the diastereomeric mixtures in the early 1990s.17 In 1995, optically active selenonium ylides 6 were obtained in over 99% de by nucleophilic substitution of optically active chloroselenurane or selenoxide with active methylene compounds with retention of configuration.18 The absolute configurations were determined by X-ray analysis of one... 

Optically active telluronium ylides were not obtained for a long time. Optically active diastereomeric telluronium ylides 7 were obtained for the first time in 1995 by fractional recrystallization of the diastereomeric mixture.19 The absolute configurations of the chiral telluronium ylides were determined by comparing their specific rotations and circular dichroism spectra with those of the corresponding selenonium ylide with known absolute configuration. The telluronium ylides were found to be much more stable toward racemization than the sulfonium and selenonium ylides (Scheme 4). 

In sharp contrast to the olefmation reaction employing stabilized telluronium ylides, semi-and non-stabilized ylides react with carbonyl compounds to give epoxides (like non-stabilized sulphonium and selenonium ylides). 

Several examples are of enantioselective rearrangement reactions of oxonium, sulfonium, or selenonium ylides, generated by the catalytic decomposition of diazo compounds (199-203), are known. The first reports of enantioselective dipolar... 

Methyl(phenyl)selenoniomethanide (1). The selenonium ylide is generated in situ from dimethyl(phenyl)selenonium methyl sulfate1 with NaH (equation I). 

The same authors examined the [2,3]-sigmatropic rearrangement of the allylic selenonium ylides 133 generated in situ from chloroselenuranes 132 and (phenylsulfonyl)acetonitrile (135) in the presence of triethylamine as base... 

Both the selenoxide and the selenonium ylide routes have been applied to cyclobutyl derivatives, themselves readily available 57) from selenoacetals of cyclo-butanones on one hand and primary alkyl halides, epoxides, or carbonyl compounds on the other. 

Syntheses of Alkylidene cyclopropanes Via the Selenonium route The selenonium route proved to be more valuable. It has been specifically designed by us to replace the deficient selenoxide route (Scheme 38). It was expected to produce alkylidene cyclopropanes by a mechanism which mimics the selenoxide elimination step but which involves a selenonium ylide in which a carbanion has replaced the oxide. Cyclopropyl selenides are readily transformed to the corresponding selenonium salts on reaction with methyl fluorosulfonate or methyl iodide in the presence of silver tetrafluoroborate in dichloromethane at 20 °C and, as expected, methylseleno derivatives are more reactive than phenyl-seleno analogs. Alkylidene cyclopropanes are, in turn, smoothly prepared on reaction of the selenium salts at 20 °C with potassium tert-butoxide in THF (Scheme 38). Mainly alkyl cyclopropenes form at the beginning of the reaction. They then slowly rearranges, in the basic medium, to the more stable alkylidene cyclopropanes( 6 kcal/mol). In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium fcrt-butoxide in THF. The reaction seems to occur via a selenonium ylide rather than via a P-elimina-tion reaction promoted by the direct attack of the /crt-butoxide anion on the P-hydrogen of the selenonium salt, since it has been shown in a separate experiment that the reaction does not occur when a diphenylselenonium salt (imable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs. It has also been found that the elimination reaction is the slow step in the process, since styrene oxide is formed if the reaction is performed in the presence of benzaldehyde which traps the ylide intermediately formed... 

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