Sulfonium salts stabilized

The additional electronegative oxygen atom in the sulfoxonium salts stabilizes these ylides considerably, relative to the sulfonium ylides. ... 

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). 

Placing anion stabilizing groups on the cyclopropane greatly facilitates generation of cyclopropyl nucleophiles. The diphenylsulfonium ylide has proven to be an exceptionally versatile conjunctive reagent25). The sulfonium salt 9, available from either 3-chloro-l-iodopropane (Eq. 24a) or 3-chloro-l-propanol (Eq. 24b), is a nicely crystalline stable salt that can be stored indefinitely26,Z7). While substituted... 

The Pummerer reaction of sulfinyl compounds involves the formation of an a-functionalized sulfide [244, 245] from a sulfoxide. Acetic anhydride is commonly used as the electrophile, which adds to the sulfoxide to yield a sulfonium salt, and the rearrangement occurs through successive formations of an ylide (rate-determining step) and an alkylidene sulfonium, trapped by a nucleophile, or stabilized by a proton loss with formation of a vinyl sulfide. 

As expected on the basis of the higher stability of sulfonium salts as compared with oxonium salts, thiopyrylium cations are more stable and less reactive than pyrylium cations hydride-accepting ability decreases in the order pyrylium> selenopyrylium> thiopyrylium [95], 2,4,6-Triphenyl-thiopyrylium salts react with ammonia and primary alkylamines forming the corresponding pyridine and pyridinium salts, respectively, but they do not react with aniline or its derivatives [96-99], As described below, the Se- or Te-analogs are less stable than the thiopyrylium salts. 

Methylation of methylthiotropone 424a (Scheme 114), according to Seitz and The (87AP362), leads to unstable sulfonium salt 425. This salt can be demethylated and deprotonated (via tropone 426) to give the resonance-stabilized sulfonium ylide 428, accompanied by the product 427 of methyl transfer from 425. Ylide 428 at elevated temperatures undergoes an inter-molecular methyl transfer to yield sulfonium salt 424b. 

In the last decade more chemical attention has been focused on those Nuphar alkaloids which contain sulfur. The chemical behavior of this group is very much dependent on the presence of sulfur. Sulfur introduces additional steric hindrance to the molecule, creates a new nucleophilic center, and increases the stability of carbanions in sulfoxides the C—S bond in sulfonium salts is strongly polarized. 

The fourth type of photoirradiated cationic initiator is dialkyl-4-hydroxyphenyl-sulfonium salt 47> (Table 1). Photoexcitation of 4 gives rise to the formation of a resonance-stabilized ylid 6 and an acid HX. 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

Hurtley and Smiles4 had isolated fused-ring derivatives of 2 as early as 1926, but were understandably unable to find a satisfactory explanation for the valence relationships on the basis of the views held at that time. Wizinger and Soder,6 in 1958, were the first to suggest that these aryl-l,3-benzodithiol-l-sulfonium salts were derivatives of the resonance-stabilized benzo-l,3-dithiolium cation. 

The photolysis of dialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenyl-sulfonium salts is different from that of triphenylsulfonium salts. The latter compounds undergo irreversible photoinduced carbon-sulfur bond cleavage the former compounds, however, react by reversible photodissociation and form resonance-stabilized ylids as shown in Fig. 5. Because of the slow thermally induced reverse reaction, only small equilibrium concentrations of the ylid and acid arc present during irradiation and the concentration will rapidly decrease when photolysis has been terminated. Therefore, in contrast to triarylsulfonium salt initiation, no dark reaction will continue after the irradiation step. 

Sulfonium salts react in several ways. They may behave as a leaving group, undergoing substitution by a nucleophile or fragmenting with the formation of an alkene. However, the most important reaction of sulfonium salts involves the formation of an ylide in the presence of a base. The carbanion of this sulfur ylide is stabilized by the adjacent positively charged sulfonium ion. The reaction of the carbanion with a carbonyl group parallels that of a phosphonium ylide in the Wittig reaction. However, the decomposition of the intermediate dipolar species is different and leads to the formation of an epoxide (oxirane) rather than an alkene. 

NMR can be used for the characterization of axial hypervalent S-Cl bond and S- O close contact in chloro-A -sulfanes and sulfonium salts 24-26 with or7,4o-substitution in the aromatic ring. The solvent effect has also been investigated CDCI3 is the most favorable for stabilizing the hypervalent S-Cl bond (see Table 5) <2001J(P2)339>. 

A useful variant of the Darzens reaction has been reported for the preparation of fosfomycin from diethyl chloromethylphosphonate. Nucleophilic substitution of chlorine in diethyl chloromethylphosphonate with dimethyl sulfide gives the sulfonium salt, further converted into its stable ylide by the use of NaH in DMSO. The addition of the ylide to acetaldehyde produces diethyl 1,2-epoxypropylphosphonate, thus avoiding the problems associated with the stability of chloromethylphosphonate carbanions (Scheme 4.4). 

The sulfonium salt was shown to spontaneously oxidize highly stabilized free radicals such as the triphenylmethyl radical to form the corresponding triphenylmethyl carbonium ion.(20) It would also appear that the dimethoxybenzylic free radical (a Norrish Type I photocleavage product of 2,2-dimethoxy-2-phenyl-acetophenone) is similarily oxidized by the arylsulfonium salt ( ). 

The bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

A single sulfonium ylide is beUeved to be formed as alkylation of oxathiane 3a gave the equatorial sulfonium salt exclusively [29]. Ylide conformation has been studied by X-ray, NMR, and computation [30]. All of these studies indicate that the preferred conformation of sulfur yUdes is one in which the filled orbital on the ylide carbon is orthogonal to the lone pair on sulfur. The barrier to rotation around the C-S bond of the semi-stabilized ylide, dimethylsulfonium fluorenide, has been found to be 42 1.0 kJmol [30]. This impHes that the ylide will adopt conformations 6a and 6b and that these will be in rapid equilibrium at room temperature. Of these two, conformation 6b will be favored as 6a suffers from... 

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