Episulfonium cation

This approach allows one to functionalize the j3-C atom of AN. For this purpose, AN are initially subjected to double silylation to prepare BENAs, which are then coupled with various stabilized carbocations (512, 513), as well as with sulfenyl and episulfonium cations (514). Molecules containing good leaving groups (e.g., arenesulfenyl chlorides (514)) are used as sources (or precursors) of... 

Table 1. Ab initio STO 3G miminized geometries of episulfonium cations 20 -24. Bond lengths in A ...

Addition of sulfur chlorides and sulfenyl halides to hydrocarbon olefins is a classic example of electrophilic reaction which usually proceeds under mild conditions and results in stereospecific trans-addition via intermediate formation of cyclic episulfonium cation [134]. Ring-opening reactions of episul-fonium cation with nucleophile is responsible for formation of regioisomers when nonsymmetrical olefins are used as substrates. 

Several disulfides (III) when electrolyzed at Pt electrodes in MeCN containing chlor-oacetic acid and Me4NBr, under controlled current conditions, yield bis(2-benzothiazolyl) disulfide and compound IV (X = Br), which is transformed into V during isolation [50]. An intermediate episulfonium cation is suggested for this reaction. The electrochemical conversion of III was thoroughly examined under different experimental conditions and was reviewed [51]. 

Oxymercuration and bromination, which occur by way of bridged transition states, show large kz/kE ratios for Z- (255a) and E-1,2-di-tert-butylethene (256). Steric hindrance of attack may account for the exceptionally high kz/kE value in the reaction of arylsulfenyl chloride with 255a and 256 (94). The formation of an S-phenyl-episulfonium cation (276) with a pyramidal sulfonium atom is less hindered in the Z- than in the -isomer (248). 

Neighboring group participation in Mitsunobu reactions occurs frequently if intermediate aziridinium or episulfonium cations can be formed. During such a process, subsequent attack by the nucleophile can lead to changes in ring size or products with retention instead of inversion at chiral alcohol centers. 

These contrasting results can be interpreted in terms of a relatively unreactive species, perhaps a chlorosulfurane, being the main intermediate in the absence of the salt. The presence of the lithium cation gives rise to a more reactive species such as the episulfonium ion, as the result of ion pairing with the chloride ion. 

Trichloroethylene is metabolized similarly and gives rise to dichlorovinyl cysteine. It has been found that S-(l,2-dichlorovinyl)-L-cysteine (DCVC) and S-(2-chloroethyl)-DL-cysteine (CEC) (Fig. 7.30) are both nephrotoxic when administered to animals causing renal proximal tubular necrosis. CEC does not require 3-lyase activation to be nephrotoxic, but can rearrange, possibly to a reactive episulfonium ion, by nucleophilic displacement of the chlorine atom. These compounds decrease the activity of the renal tubular anion and cation transport system. 

Episulfonium ions (ESI) are three-membered sulfur-containing cationic species that have been postulated as intermediates in many reactions. Mechanisms of the reaction of l-alkoxy-2-(arylsulfanyl)alkyl halides with different nucleophiles in the presence of a Lewis acid appear to be more complex because of the possibility of forming either ESIs or oxonium ions (IOS) as intermediates (Figure 27). Dudley212 reported equilibrium structures and harmonic vibrational frequencies of two model compounds related to the intermediates of nucleophilic attack on l-alkoxy-2-(arylsulfanyl)alkyl halides... 

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... 

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