Selenonium

Various sulfonium and carbosulfonium ions show remarkably enhanced reactivity upon superelectrophilic activation, similar to their oxygen analogs so do selenonium and telluronium ions. The alkylating ability of their trialkyl salts, for example, is greatly increased by protosolvation. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

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). 

Methylation of relatively acidic (pKa < 13) carbon nucleophiles occurs at neutral pH in aqueous media when substituted methylsul-fonium and -selenonium salt are used as electrophiles (e.g., 6.2).37... 

Table 9 Preparation of selenonium salts 69 from the corresponding chloroselenuranes 68 (Scheme 8)...

Although diselenonium-, ditelluronium- and mixed sulfonium-selenonium dications can exhibit either oxidative or electrophilic properties in reactions with nucleophiles, substitution at the onium chalcogen atom is more typical.96 Owing to the increased stability of heavier dichalcogenium-dications, they react only with highly activated substrates such as aniline and tV,A-dimethylaniline, while no reaction is observed with phenol and diphenylamine.113 Reactions of ditelluronium dications with activated aromatics are also not known (Scheme 44).114... 

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). 

Optically active sulfonium and selenonium salts are well known and the stereochemistry of the isomers has been studied.1 3 Optically active cyclic diaryl(alkoxy)-sulfonium salts 14, 15, and 16, stabilized by intramolecular sulfur-oxygen interaction, were synthesized in 2000 by reacting optically active spirosulfuranes with trimethyloxonium tetrafluoroborate.29 The absolute configurations were assigned on the basis of the reaction mechanism. The sulfonium salts were hydrolyzed in KHC03aq. to yield optically active sulfoxides in over 86% ee (Scheme 7). 

Cation-Anion Interactions Involving Selenonium and Telluronium Salts... 

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... 

Alkylation of Etiolates with Chiral Selenonium Salts, Sulfonium Salts and Mixed Sulfates... 

The enantioselective a-alkylation of /i-oxo esters, e.g., 2-methoxycarbonyl-l-oxoindane and 2-methoxycarbonylcyclohexanone, with optically active alkyl or aryl(cthyl)methyIselenonium and -sulfonium camphorsulfonates or perchlorates, which possess a stereogenic center at the selenonium or sulfonium group, led to a mixture of a-methylated and a-ethylated compounds (Table l)7-10. 

The selenonium and sulfonium salts act as chiral alkylating agents for carbon nucleophiles under weakly basic conditions which avoid any ylide formation from the salt. When 2-methoxycarbonyl-l-oxoindane is treated with an (.S )-etliyl(methyl)phenylselenoniurn perchlorate, the (S)-2-methyl and (i )-2-ethyl derivatives are obtained with a low enantiomeric excess (Table 1). Reac-... 

Table 1. Enantioselective 2-Alkylation of 1,3-Dicarbonyl Compounds with Chiral Selenonium or Sulfonium Salts...

Several cyclobutenes can be synthesized by elimination of selenoxides from the corresponding methyl- or phenylselanyl derivatives 58 or by elimination of dimethyl selenide from the corresponding selenonium salts 60. The results of these conversions are summarized in Table l.23 The cyclobutenes have been used as precursors for substituted butadienes.23... 

Trialkyl cations of oxygen, sulfur, and selenenium (and less commonly tellurium) are onium ions by virtue of their having one extra valence, although these three-coordinate ions invite comparison with carbenium ions. These ions (e.g., 28, 29, and 30) are not planar (as opposed to the analogous carbenium ions), but rather are pyramidal. These species have a well-characterized solution acid chemistry concentrated sulfuric acid is required for observation of the trimethyloxonium cation, but the sulfonium and selenonium ions require less strongly acidic solutions. 

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