Homolytic mechanism rearrangement

Reactions of (145) with potassium permanganate, performic acid, aqueous lithium carbonate, and silver acetate in acetic acid are described. Treatment of the cyclopentadiene-methyl a-bromovinyl sulphone adduct with sodium methoxide in dimethyl sulphoxide gives (141) by the Ramberg-Backlund rearrangement, but in refluxing aqueous 2N sodium hydroxide (146) is also formed. Good evidence in the latter case is presented for a homolytic mechanism. With added isopropyl alcohol, in addition to (146), products are obtained by hydrogen-atom abstraction. 

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

FIGURE 24.21 A mechanism for the methylmalonyl-CoA mntase reaction. In the first step, Co is rednced to Co dne to homolytic cleavage of the Co —C bond in cobalamin. Hydrogen atom transfer from methylmalonyl-CoA yields a methylmalonyl-CoA radical that can undergo rearrangement to form a snccinyl-CoA radical. Transfer of an H atom regenerates the coenzyme and yields snccinyl-CoA. 

More recently, a number of reports dealing with 1,3-sulfonyl shifts which proceed by other mechanisms have been published. For example, Baechler and coworkers suggested that the higher activation enthalpy observed for the isomerization of the deuterium labeled methallyl sulfone 72 in nitrobenzene at 150°C as compared to the corresponding sulfide, together with the positive entropy of activation may be taken as evidence for a homolytic dissociation mechanism (equation 44). A similar mechanism has also been suggested by Little and coworkers for the gas-phase thermal rearrangement of deuterium labelled allyl sec-butyl sulfone, which precedes its pyrolysis to alkene and sulfur dioxide. 

FIGURE 6 Speculative mechanism of Crl hydrocarbon biosynthesis from fatty acid hydroperoxides in algae. Homolytic cleavage of the hydroperoxide is assumed to give an allyl radical, which cyclizes to the thermolabile (1S,2R)-cyclopropane. The sequence is terminated by transfer of a hydrogen radical from C(16) to the -X-0 function. The cyclopropane rearranges to (6S)-ectocarpene as shown in Figure 4. 

Adenosylcobalamin (coenzyme 812) carries a covalently bound adenosyl residue at the metal atom. This is a coenzyme of various isomerases, which catalyze rearrangements following a radical mechanism. The radical arises here through homolytic cleavage of the bond between the metal and the adenosyl group. The most important reaction of this type in animal metabolism is the rearrangement of methylmalonyl-CoAto form succinyl-CoA, which completes the breakdown of odd-numbered fatty acids and of the branched amino acids valine and isoleucine (see pp. 166 and 414). 

Nitrogen ylides are also known to undergo [1,2]- (Stevens) and [2,3]-rearrangements. The Stevens rearrangement occurs through a mechanism that involves homolytic cleavage... 

In contrast, perfluorooxaziridines 30, containing a fluorine substituent on the ring carbon, are more stable than oxaziridines 28 and also rearrange thermally by a different pathway to give imidates 31 in quantitative yield.57 The mechanism presumably involves initial homolytic cleavage of the N-0 bond, as is the case for oxaziridines 28, followed by a fluoroalkyl shift from carbon to oxygen.56 57 Thus, the fluorine at C3 in oxaziridines 30 seems to have a dramatic effect on the mode of rearrangement. 

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