Isomerization of sulfoxides

The formation of stable disulfonium dications from certain bis-sulfides provided an impetus for re-evaluating the mechanisms of some old reactions. The isomerization of sulfoxide 124, using trifluoroacetic acid, involved an acid-catalyzed migration of an oxygen atom from a sulfoxide to sulfide (see Equation 36). This was one of the first examples in which an intermediate formation of an S-S dication was proposed. Recently, such a dication 125 was detected in a non-nucleophilic medium such as sulfuric acid <1995HAC145>. 

Acid-catalyzed migration of an oxygen atom from a sulfoxide to sulfide is one of the first examples where an intermediate formation of a S-S dication was proposed.26 Recently, such intermediates were observed by physical methods. Kinetic studies suggested that an intramolecularly formed S-S dication 54 was involved in isomerization of sulfoxide 53 to 55 under influence of trifluoroacetic acid (Scheme 18).82... 

Cleavage by nucleophiles 79 Acid-catalyzed reactions with nucleophiles 81 Reactions of dialkylalkylthiosulfonium ions 83 Cycloelimination reactions of alkyl thiolsulfinates 88 Homolytic reactions of aryl thiolsulfinates 92 Oxidation of thiolsulfinates by peracids 94 Isomerizations of sulfoxides and thiolsulfinates 95... 

Kinetic and Equilibrium Parameters for Linkage Isomerizations of Sulfoxide/Ruthenium Complexes [14]... 

Isomerization of sulfoxides. The naturaly securing sulfoxides to which the configuration of the oxygen has been assigned (cis to the C6-C7 bond) could be isomerized to the ones in the presence of hydrochloric acid. 

Note 2. Traces of unreacted triethylamine might cause partial isomerization of the allenyl sulfoxide into the propargyl sulfoxide. The methyl iodide is added to ensure that no triethylamine remains. 

Isomerization of 3-cephems (27) to 2-cephems (28) takes place in the presence of organic bases (e.g. pyridine) and is most facile when the carboxyl is esterified. Normally an equilibrium mixture of 3 7 (3-cephem/2-cephem) is reached. Since the 2-cephem isomers are not active as antibacterial agents, the rearrangement proved to be an undesirable side reaction that complicated acylation of the C-7 amine under certain conditions. A method for converting such mixtures to the desired 3-cephem isomer involves oxidation with concomitant rearrangement to the 3-cephem sulfoxide followed by reduction. Additions... 

Extension of the above method to 3-methoxyestra-3,5(10)-dien-17-one 17-ethylene ketal (46) prepared by base-catalyzed isomerization of 3-methoxy-estra-2,5(10)-dien-17-one 17-ketal (42) with potassium t-butoxide in dimethyl sulfoxide gives the isomeric tropone A-homo-estra-l,4,5(10)-triene-3,17-dione... 

As shown in Table 4, the same ratio of diastcreomeric sulfinylalkcnols was obtained from both (R)-(E)- and (/ )-(Z)-l-sulfinylalkenes (vinyl sulfoxides). This is explained by the fast cisjtrans isomerization of the organolithium intermediates in a strongly basic medium. Thus, EjZ mixtures of vinyl sulfoxides can be used without prior separation. 

A final, rather different example which fits in appropriately here, in that it involves hydrogen exchange, is the measurement of equilibrium and rate constants for the base-catalyzed isomerization of unsaturated sulfides, sulfoxides and sulfones193 ... 

Benzhydryl trichloromethanesulfenate rearranged to the corresponding sulfoxide after 10 min of reflux of a hexane solution, while the benzyl ester remained practically unchanged even after heating for 24 h at 120°C in benzene. Isomerization of optically active ( — )-a-phenylethyl trichloromethanesulfenate, [a]D20 = — 20.5° on heating in... 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

The reactions of the corresponding propargyl sulfoxides and sulfones now resemble the chemistry of the other acceptor-substituted derivatives such as ketones and aldehydes (see Section 1.2.4). Compared with the thioethers, here much milder bases are sufficient apart from aluminum oxide, often triethylamine or potassium carbonate are used. Sometimes even a spontaneous isomerization takes place. The selective isomerization of one triple bond in the presence of a second triple bond in 126 [313] (Scheme 1.56) or an allyl sulfone in 129 [314] (Scheme 1.57) are just two examples out of a whole series [178, 304, 313, 315-331]. When, on the other hand, the in situ oxidation of 126 was carried out in an aprotic solvent, no isomerization at all was observed. 

In the case of carboxylic acids [33], ketones [47] or sulfones [49], it was possible to prove a further prototropic rearrangement of allenes 19 with R2 = H yielding the alkynes 20. In other examples, the prototropic isomerization of the triple bond leads to conjugated dienes directly thus the allene of type 19 could only be postulated as an intermediate [50]. Braverman et al. showed that the allenes generated by prototropic isomerization of dipropargylic sulfones or sulfoxides, for example 24, are unstable. They are transferred rapidly via diradical intermediates to polycycles such as 27 [51-53],... 

Isomerization of benzyl p-toluenesulfenate to benzyl p-tolyl sulfoxide (95) can also be observed (Miller et al., 1968) but is much slower than the... 

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