Radicals rearrangements, labeling experiments

A 1,2-shift has been observed in radicals bearing an OCOR group at the p-carbon where the oxygen group migrates as shown in the interconversion of 36 and 37. This has been proven by isotopic labeling experiments and other mechanistic explorations. A similar rearrangement was observed with phosphatoxy alkyl radicals such as 38. ... 

Isotope labeling experiments and EPR spectroscopic studies have shown that the cyclopropylmethyl radical is a discrete chemical species with a finite lifetime = 7 x 10 s at 25°C in methylcyclopropane solution). Unlike the corresponding cyclopropylmethyl cation, 1 has no nonclassical or fluxional characteristics, and it does not rearrange to cyclobutane derivatives. Its rate of formation from diazenes, peroxides, and other precursors is slightly greater than that of model primary acyclic radicals, which indicates that it has a small thermodynamic stabilization. EPR spectroscopic studies have shown that rotation of the methylene group carrying the unpaired electron is not free and that the preferred conformation is bisected rather than perpendicular. 

Labelling experiments demonstrate that a free radical is first generated on the (Si)-fadal methyl group, which rearranges to the cepham radical. This then either undergoes formal addition of a hydroxy-radical and dehydration, or transfer of one electron to the enzyme along a cationic mechanism, to produce desacetoxycaphalosporin C. 

FIGURE 20.50 A labeling experiment shows that the Cope rearrangement does not involve formation of a pair of free allyl radicals because the radical recombination products are not all formed. 

Heesing and coworkers have reported the rearrangement of 0-alkylsulfinyl-iV-benzoyl-iV-phenylhydroxylamine (143) at — 70°C to the corresponding sulfonamide together with the o- and p-alkylsulfonyl derivatives 144 and 145 (equation 89). The reaction has been suggested to proceed by an intramolecular radical pair mechanism, as evidenced by experiments with oxygen-18 labeling and C-CIDNP effects. 

There exist early examples of this transformation [507, 508], but due to the symmetric structure of the alkene part, only isotope labeling, etc., allowed the exclusion of a prototropic rearrangement. Furthermore, due to the high reaction temperatures of 340 °C and above, several different products are formed. A low-temperature version (77 K) of this reaction via the radical cation has been reported [509]. The chirality transfer has been studied and a detailed mechanistic investigation has been conducted [510] typical experiments in that context were the reactions of substrates such as 155 and 157 (Scheme 1.70). 

The mechanism has been the subject of much study.282 That the rearrangement is intramolecular was shown by crossover experiments, by 14C labeling,283 and by the fact that retention of configuration is found at R1.284 The first step is loss of the acidic proton to give the ylide 71, which has been isolated.285 The finding286 that CIDNP spectra287 could be obtained in many instances shows that in these cases the product is formed directly from a free-radical precursor. The following radical pair mechanism was proposed 288... 

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