Sulfenates from sulfoxides

The observations of the interconversion of allylic sulfenates and sulfoxides made by Braverman and Stabinsky34-38 are confirmed by the work of Mislow and coworkers44-47 who approached the problem from a different angle, namely, enhanced racemization of optically active allylic sulfoxides. 

Owing to the reversible nature of the allylic sulfenate/allylic sulfoxide interconversion, the stereochemical outcome of both processes is treated below in an integrated manner. However, before beginning the discussion of this subject it is important to point out that although the allylic sulfoxide-sulfenate rearrangement is reversible, and although the sulfenate ester is usually in low equilibrium concentration with the isomeric sulfoxide, desulfurization of the sulfenate by thiophilic interception using various nucleophiles, such as thiophenoxide or secondary amines, removes it from equilibrium, and provides a useful route to allylic alcohols (equation 11). 

Subsequently, these authors have also studied the effect of polar factors on the sulfenate-sulfoxide equilibrium and obtained similar results to those reported by Braverman and coworkers . For example, reaction of 2,4-dinitrobenzenesulfenyl chloride with lithium allyl-a-dj alcoholate gives only (or perhaps mainly ) allyl-a-d2 2,4-dinitrobenzenesulfenate, whereas the corresponding reaction with 4-nitrobenzenesul-fenyl chloride results in complete ( > 99%) rearrangement to the sulfoxide. However, when a single nitro group is located in the ortho position, the ratio (K) of sulfenate to sulfoxide approaches unity. This ratio is also affected by the polarity of the solvent and changes from 1.43 in CCI4 to 0.39 in chloroform, consistent with the results described above for the equilibrium shown in equation 9. 

We have thus established that sulfides, disulfides, and their initial oxidation products are not the actual preventive antioxidants. The active peroxide decomposers are the sulfenic acid from sulfoxide decomposition, the thiosulfoxylic acid from thiolsulfinate decomposition, and the acidic products formed when they react with hydroperoxides. The catalytic... 

Sulfenic esters and sultenes are relatively stable thermodynamically, lying only several kcal/mol over the sulfoxides, all other things being equal [50]. However, in the laboratory, sulfenic esters are very difficult to handle without significant decomposition, in the absence of stabilizing substitutions [51], Their absorption spectra often extend to the red of the isomeric sulfoxides, which contributes further to difficulty in their isolation by way of sulfoxide photochemistry. Thus it is exciting that two independent laboratories isolated stable sultene derivatives in the 1980s, derived from sulfoxide photolysis. 

The photochemistry of sulfenic esters has come up repeatedly in this review as a result of their formation from sulfoxide a-cleavage. With the exception of 50, the result is S-0 homolysis, just as one would expect intuitively. Indeed, photolysis of both simple and more complex sulfenic esters that will be discussed in this section appear to proceed along this pathway. 

The intermediate sulfenic acid derived from a penicillin sulfoxide has been trapped by a large assortment of reagents and, in one case, the sulfenic acid itself has been isolated (74JA1609). Only some of these products will be discussed here, and the reader is referred to the cited reviews (especially B-80MI51102) for additional examples. 

The main primary fragment ions of diaryl sulfoxides 13 and 14 have the structures 16a or 16b(C8H7S02 + m/z 167) and the ion m/z 152 (17) can be obtained from both by the loss of CH3 (equation 5)13. Ions 16a and 16b are formed from the sulfenate ester structure of the molecular ions of 13 and 14 through a cyclization process and a simultaneous loss of the other O Ar part. A similar ortho effect is not possible in 15 and hence its most intense ion is M+ (23% of the total ionization in comparison with 2.7 and 0.6% for 13 and 14, respectively) and its primary fragments are typical for a normal diaryl sulfoxide. 

Phenanthro[4,5-b, c, d]thiophene 4-oxide (19) and 4,4-dioxide (20)15 undergo first the well-known sulfenate (22) or sulfinate ester (23) rearrangements1 4,6, respectively (equation 6). The sulfoxide loses an oxygen atom and enters the fragmentation pathway of 18 or loses HCO (more likely in two steps) from 22. Both sulfenate and sulfinate ions can fragment further via 21 after losing a sulfur atom or eliminating SO5, respectively. 

One of the first uses of the allylic sulfoxide-sulfenate interconversion was made by Jones and coworkers64, who reported exclusive suprafacial rearrangement of the allyl group in the steroidal sulfoxide 17 shown in equation 13. Two other examples are shown in equations 1465 and 1566. Evans and coworkers have demonstrated the utility of the suprafacial allylic sulfoxide-sulfenate rearrangement in a new synthesis of the tetracyclic alcohol 24 (equation 16)67, as well as in a synthesis of prostaglandin intermediates as shown in equation 1768. The stereospecific rearrangement of the unstable sulfenate intermediate obtained from the cis diol 25 indicates the suprafacial nature of this process. 

A similar intramolecular nucleophilic capture of an allylic sulfenate generated thermally from the corresponding sulfoxide was also reported for the facile transformation of the azetidinone 61 into a new 3-acetylthio-2-thiacephem ring system 62 (equation 29)127. 

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