Sulfonium ion intermediates

Tetramethyl-l,2-oxathietane (138) was prepared by diazotization of 139, which was prepared from the aziridine (140) (86JA3811).Tlie reaction presumably involves the decomposition of the sulfonium ion intermediate (141).Tire dichloromethane solution of 138 at -20°C is sufficiently stable to permit exploration of the chemical reactions. Tire oxathietane 138 undergoes a formal [[Pg.248]

A novel ring-contraction reaction which proceeds via an epi-sulfonium ion intermediate has been reported for the simple and regiospeciflc synthesis of mono-and symmetrically di-functionalized tetrathiamacrocycles, starting with the mono- or dichloro-substituted macrocycles that have one or two more ring atoms. 

The transannular formation of sulfonium ion intermediates is not limited to six-membered rings. A similar contraction of a benzo[6]thiepin derivative occurred when (288) was refluxed in dry dioxane, giving a quantitative yield of (289) (74CC626). 

A Pummerer rearrangement features in two approaches to 1,3-oxathiins. Refluxing y,8-unsaturated sulfinyl compounds (26) in xylene containing TsOH effects conversion to the 3,1-benzoxathiin (27) via a sulfonium ion intermediate (95CC1197), whilst 2-(hydroxymethyl)phenyl sulfoxides (28), readily cyclise to the benzoxathiin (95T6819). 

The product of this reaction, the sulfenyl chloride, is also a good soft electrophile towards carbon atoms, particularly towards alkenes. The reaction is very like bromination with a three-membered cyclic sulfonium ion intermediate replacing the bromonium ion of Chapter 20. The reaction is stereo-specific and anti. 

In both cases, intramolecular displacement of the chloride leaving group by the sulfur atom—or, as we should call it, participation by sulfur (see Chapter 37)—gives a three-membered cyclic sulfonium ion intermediate (an episulfonium or thiiranium ion). Nucleophilic attack on this electrophilic sulfonium ion, either by water or by the structural proteins of the skin, is very fast. Of course, mustard gas can react twice in this way. You will see several more examples of reactions in which a sulfonium ion intermediate acts as an electrophile in the next section. 

The kinetics of the addition have been interpreted in terms of a two-step process involving a cyclic sulfonium ion intermediate. 

In the case of sulfur-mustard, the situation is somewhat more complex. It is marginally soluble in water tending to form droplets, and hydrolysis occurs at the droplet surface. This property has made measuring the hydrolysis rate constant difficult, and half-lives anywhere from 2 to 30 hours are reported. Chemically, the hydrolysis of HD involves the sequential replacement of the chlorine atoms by hydroxyl groups through cyclic sulfonium ion intermediates to form thiodiglycol (TDG), Reaction (2). If a median... 

This reaction demonstrates the ambident reactivity of the nitronium ion, in analogy with the ambident reactivity of NO, and N02 The nitro-sulfonium ion intermediate can be observed by H-, C-, and N-NMR spectroscopy. 

The reactions take place at all temperatures, but industrially they are carried out from 50-75 C and above. At lower temperatures, however, the process may take days to complete. At temperatures of 135-155 °C approximately 8% of sulfur (by weight of rubber) reacts. Also, sulfur dissolves in unvulcanized rubber even at room temperature. The overall mechanism of the reaction is still being studied. Most evidence points to an ionic mechanism and a sulfonium ion intermediate. It was shown that a straightforward reaction of sulfur with rubber is insufficient. Somehow, between 40-100 atoms of sulfur must be combined in order to obtain one crosslink. Out of 40-100 atoms only 6-10 are actually engaged in the formation of the crosslinks. The rest of the atoms form cyclic sulfide units that become spread along the main chain. ... 

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