Trialkylsulfonium salts

The catalysts must supply the system with lipophilic cations in order to form, with required anions, ion pairs able to enter nonpolar media. The most typical catalysts are tetraalkyl ammonium (TAA) salts R4N+X, particularly those having at least 16 carbon atoms in the four R groups. Similar lipophilic catalysts are tetraalkylphosphonium and -arsonium or trialkylsulfonium salts, which are less available and usually less stable. They are therefore of negligible practical use. There are a few reports on the use of trialkylamines as catalysts in some two-phase reactions. Usually these amines are qua-ternized by a reactant actually these reactions are catalyzed by TAA salts. More complicated is the generation of dihalocarbenes with trialkylamines. The amines form, with the carbene, an ammonium ylide, which acts as a base in the organic phase. 

Using one and the same base, one can reverse the normal Saytzeff preference of the E2 elimination into a preference for the Hofmann elimination through a variation of the leaving group. This is especially true for eliminations from tetraalkylam-monium salts R—NMe3+T and trialkylsulfonium salts R—SMe2+I. Their positively... 

Dialkyl sulfides can react with alkyl halides to form trialkylsulfonium salts, which are also good alkylating agents. 

Thiol sulfide mercapto group alkylthio group disulfide thiolate ion trialkylsulfonium salt sulfoxide sulfone... 

The third and final general protocol for the hydrolysis of 5 5-acetals exploits the very easy reaction of the sulfur atom of an S-acetal with alkylating agents such as iodomethane, trimethyl- or triethyl-oxonium tetrafluoroborate, and methyl triHuoromethanesuLfonate to form the corresponding trialkylsulfonium salts. Ley s approach to the potent insect antifeedant Azadirachtin [Scheme 2 81]135,171,172 benefited from an easy S-alkylation-hydrolysis sequence. In a synthesis of Epiantillatoxin, a more difficult liberation of an aldehyde from its dithiane derivative was accomplished without rearrangement of a p,y-alkene into conjugation [Scheme 2,82].173... 

Colichman and Love [104] assumed a one-step, two-electron cleavage to the sulfide and a carbanion, but later studies pointed to a cleavage after the uptake of one electron [105,106]. In trialkylsulfonium salts a dissociative electron uptake probably takes place, whereas in aryl dialkylsulfonium salts the reduction may be a sulfur-carbon bond breaking concomitantly with the electron uptake [106] or the sulfaranyl radical, formed by addition of an electron to the aryl r-system, may have a certain lifetime [107]. In favor of the former possibility, it is argued that the values in CV are very sensitive to the electronegativity of the fragmenting radical. 

Two-phase systems, usually consisting of a strongly alkaline aqueous rfiase and methylene chloride, are also effective and convenient. Trialkylsulfonium salts with methylsulfate counterions can be used alone under these conditions, whereas those with halide counterions generally benefit from added phase transfer catalysts (equation 2). Dodecyldimethylsulfonium chloride was found to be an effective phase transfer reagent. A sulfonium salt covalently attached to a polymeric resin was found most effective when a phase transfer agent was also employed. ... 

Because the valence electrons on sulfur are farther from the nucleus and are less tightly held than those on oxygen (3p electrons versus 2p electrons), sulfur compounds are more nucleophilic than their oxygen analogs. Unlike dialkyl ethers, dialkyl sulfides are good nucleophiles that react rapidly with primary alkyl halides by an SN2 mechanism to give trialkylsulfonium salts (R S ). 

Among these anions, two are often used to form ionic liquids, namely, hexafluorophosphate PFg and bis(trifluoromethanesulfonyl)amide NTf2. In Table 2.3, the melting points of tetraalkylammonium and trialkylsulfonium salts formed with the NTf2 anion are indicated, clearly showing that a large variety of ionic liquids with low melting points can be prepared. 

Diagram 3.11. Equilibrium between alkoxycarbenium ion (growing chain) and trialkylsulfonium salt... 

Many prospective photoinitiators fail to meet the basic requirements outlined above. Thus, for example, trialkylsulfonium salts are not useful photoinitiators because they do not absorb light in the ultraviolet and because as alkylating agents, they spontaneously initiate cationic polymerization even in the absence of light... 

Although somewhat less efficient than triarylsulfonium salts or diaryliodonium salts as photoinitiators, these compounds are nonetheless compensated for their reduced efficiency by their broader spectral response and by the facility of their synthesis. With respect to thermal stability, these compounds occupy a position intermediate between the very reactive trialkylsulfonium salts on one hand and the comparatively unreactive triarylsulfonium salts on the other. Clearly, this stability is derived from delocalization of the positive charge on sulfur by means of the conjugated enol form of the phenacyl group in III and by interaction with the electron rich phenolic ring in IV. 

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