Sulphenes thermal

Sulphoxides and selenoxides undergo syn elimination under thermal conditions. A 1,4-elimination of sulphenic acid from an allyl sulphoxide leads to dienes (equation 20)50. Precursor sulphoxides are generated by oxidation of corresponding sulphides. This reaction, however, did not give good results when applied to more complicated systems51. 

Although the fragmentation reactions of the molecular ions of sul-phones and sulphoxides varied, the first step in rationalizing their behaviour was initial isomerization to sulphinate (64) and sulphenate esters (65). A variety of reactions of this type has been collated by Fields and Meyerson (1968), who pointed out that isomerization of sulphoxides to sulphenate esters was a known thermal reaction (see also, Kutz and Weininger, 1968, Kinstle et al., 1968b). In experiments designed to compare the mass-spectral and thermal behaviour of a series of sulphoxides, it was observed that the thermal isomerization appeared to proceed, but no comparable behaviour was observed in the mass spectra (Entwistle et al., 1967). 

Reactions.—Full details are now available" of the photochemical and thermal [1,3] phenylthio shifts of allyl phenyl sulphides [(73)- (74)]. Oxidation of such sulphides can lead to allyl alcohols through the known sulphoxide-sulphenate equilibrium with sulphenate trapping. 

Dichlorosulphine is formed in good yield by the thermal decomposition of p-anisyl trichloromethanesulphenate in acetonitrile or chloroform. The reaction is very sensitive to solvent polarity, and, in non-polar solvents, rearrangement of the sulphenate to the sulphoxide occurs by a polar mechanism. Since the trichloromethanesulphenate anion is known to give dichlorosulphine, an intermediate ion-pair is postulated, i.e. 

Bond Cleavage.—The thermal or acid-catalysed rearrangement of penicillin (S-oxides to cephalosporins has been shown to proceed through a reactive sulphenic acid (80 y), which may be isolated in crystalline form in low yield. This intermediate slowly reverts on standing to the starting sulphoxide, while under acid... 

The syntheses of five thiets (169) via the cycloadducts of sulphenes to enamines have been reported. They are thermally unstable and undergo... 

The first general method of transforming a penicillin into a cephalosporin, the acid-catalysed rearrangement of penicillin sulphoxides, continues to receive attention. That this rearrangement proceeds via a sulphenic acid derivative has been further confirmed by isolation of the crystalline sulphenic acid (74 f, y) from the thermal rearrangement of the penicillin sulphoxide (27 f, y). This intermediate slowly reverts to the penicillin at 38 0, and, on treatment with methanesulphonic acid in dimethylacetamide, cyclizes to cephalosporin (75 f, y). Trapping of (74) by oxidation to the sulphinyl chloride (76 f, y) has been reported cyclization to a cephalosporin sulphoxide was accomplished under base catalysis/ Treatment of penicillin sulphoxides with azo-compounds also effects rearrangement to... 

The sulphenic acids produced on thermal rearrangement of penicillin sulphoxides have been trapped by a variety of reagents, including thiols and acetylene and olefin derivatives such as norbomadiene, keten dimer, vinyl ethers, keten acetals, and acetylenedicarboxylates. Removal of the nitrogen substituent of the resulting 2-azetidinones, e.g. (98 c,v), was achieved by addition of diazomethane to the olefin followed by base treatment or reduction of the intermediate pyrazoline (99). ... 

In connection with some other work, the optically active lactone sulphoxides (83) and (84) have been observed to undergo a rapid, thermally induced racemization in boiling chloroform. Deuterium-labelling experiments indicate that the reaction probably involves the intermediacy of the sulphenic acids, e.g. (85), that are formed by a concerted hydrogen shift. Such a sigmatropic process finds analogy in the thermal isomerization of... 

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