Reactions with electrophilic sulphur

The high stereoselectivity exhibited by the 2-carbanions of 1,3-dithians in reactions with electrophiles evoked further investigations on the structure of the lithiated species. Proton and n.m.r. spectra of the 2-lithio- and 2-potassio-derivatives of 2-phenyl-1,3-dithian suggested that neither lithium-sulphur interactions nor the concomitant intimate ion-pairing, but rather an intrinsically preferred equatorial orientation of the lone pair in carbanions derived from... 

It has been noted that the ortho para ratio for protodetritiation of p-[ H] toluene varies with acid medium, and although side reactions are present with both sulphuric acid and Lewis acids, they are usually absent in trifluoroacetic acid. See (a) Taylor, R. Electrophilic Aromatic Substitution. J. Wiley and Sons, New York, 1990, pp. 61-64 (b) Baker, R. Eabcm C. Taylor, R. J. Chem. Soc. 1961,4927. 

The 5-unsubstituted-l,2,3-triazol-4-ones (176, R = H) participate in electrophilic substitution reactions. Bromination in chloroform of anhydro-4-hydroxy-l,3-dimethyl-1,2,3-triazolium hydroxide (180) gave its 5-bromo derivative (182). The meso-ionic 3-aryl-1,2,3-triazol-4-ones (176, R = Me, R = Ar, R = H) gave 5-bromo derivatives (176, R = Me, R = Ar, R = Br) with bromine in acetic acid. Their reaction with sulphur monochloride gave the sulfide (189, X = S), and with thionyl chloride they gave the sulfoxide (189, X = SO). ... 

Umpolung The reversal of polarity of the carbonyl carbon atom is termed umpolung (German for polarity reversal). Normally the carbonyl carbon atom of an aldehyde (or a ketone) is partially positive i.e., electrophilic and therefore it reacts with nucleophiles. When the aldehyde is converted to a dithiane by reaction with 1,3-propanedithiol and reacted with butyl lithium the same carbon now becomes negatively charged to react with electrophiles. This reversed polarity of the carbonyl carbon is termed umpolung which increases the versatility of the carbonyl group in synthesis. The sulphur atoms stabilize... 

Phenol may be converted into a mixture of o- and p-nitrophenols (Expt 6.102) by reaction with dilute nitric acid the yield of p-nitrophenol is increased if a mixture of sodium nitrate and dilute sulphuric acid is employed. Upon steam distillation of the mixture of nitrophenols, the ortho isomer passes over in a substantially pure form the para isomer remains in the distillation flask, and can be readily isolated by extraction with hot 2 per cent hydrochloric acid. The mechanism of the substitution probably involves an electrophilic attack (cf. Section 6.2.1, p. 851) by a nitrosonium ion at a position either ortho or para to the activating hydroxyl group, to yield a mixture of o- and p-nitrosophenols, which are then oxidised by the nitric acid to the corresponding nitrophenols. The reaction depends upon the presence in the nitric acid of traces of nitrous acid which serve as the source of the nitrosonium ion. 

Further investigations on electrophilic additions to (65) (Poutsma and Ibarbia, 1971) found that the products of the reaction with tri-fluoroacetic acid in benzene at —15° to — 25° are mainly 66 and 67 (S = CFsCO), but minor amounts of 68 and 69 were also observed. A similar product distribution was obtained from 65 with acetic acid, with water in tetrahydrofuran-sulphuric acid, and with methanol in the presence of acids. The detection of compound 68 indicates that terminal protonation is not exclusive and some central attack also occurs. 

Reaction of electrophilic cyclopropanes with sulphur and selenium nucleophiles 526... 

Electrophilic Addition.—5a -Cholest-2-ene (67), on reaction with iodine, potassium iodate, and sulphuric acid in aqueous dioxan, gave the 2/3,3a-diol (69) as the main product. " The intermediate 2,3-epoxide (68) can be isolated by using a modified procedure, with iodine in aqueous dioxan in the presence of silver oxide to trap liberated hydrogen iodide." The 2-methyl and 3-methyl derivatives of 5a-cholest-... 

The chemical and metabolic reactions of sulphur mustard are dominated by reactions with nucleophiles at its two electrophilic carbon atoms, plus oxidation of the electron-rich sulphur atom. Nucleophilic reactions proceed by an internal SNi type mechanism, via the episulphonium ion shown in Figure 2. 

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