Mechanism, radical pair

The Radical Pair Mechanism (RPM), first presented in 1969 [26-29] has undergone intense investigation to help explain experimental observations. According to the RPM, a radical pair is created in a non-stationary electronic spin state which in the 

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In certain cases, e.g. with Z = tert-butyl, the experimental findings may better be rationalized by an ion-pair mechanism rather than a radical-pair mechanism. A heterolytic cleavage of the N-R bond will lead to the ion-pair 4b, held together in a solvent cage ... 

The other mechanism which has been advocated58 is that known as the radical-pair mechanism , in which two cation radicals are thought of as intermediates held in a solvent cage so preserving the intramolecularity of the reaction, viz. 

SCHEME 6. Suggested radical/anion-radical pair mechanism for the reaction of a-sulfonyl carban-ions with perhaloalkanes (CC14 as the example)553, R" = R3S02CR1R2, k2> > k. ... 

The occurrence of characteristic by-products can be taken as indicative of a radical pair mechanism in rearrangements. Thus the observation... 

Scheme 5 Photo-Arbuzov rearrangement of arylethylphosphites via a short-lived singlet proximate radical pair mechanism. Reprinted with permission from [22]. Copyright 2001 American Chemical Society...

The MFEs on the CeoN cluster-MePH are explained by a radical pair mechanism between the C oN cluster, (C6oN + ) and MePH as shown in Figure 15.9a. MePH is mainly photoexcitedby the 355 nm laser light, because of the excess amount of MePH in comparison with C oN. The singlet excited state ofPH( PH )is generated by the... 

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. 

Further evidence consistent with the polar radical pair mechanism was provided by a crossover experiment (Scheme 6.26). A 1 1 mixture of labeled 8Z /8 and unlabeled 8Z/8E was heated in xylene at 125 °C for 2h and at 135 °C for 4h to afford hydroxypyrimidinones 3 and 3. Analysis of the products by high resolution mass spectrometry showed no crossover between the labeled and unlabeled fragments. This result reinforces the computational results discussed previously wherein PRP recombines to give product within the solvent cage (Scheme 6.24). 

One of the important possible mechanisms of MF action on biological systems is the influence of free radical production. Chemical studies predict that MFs may affect free radical reactions through the radical pair mechanism [201]. A reaction between two free radicals can generate a free radical pair in the triplet state with parallel electron spins. In this state free radicals cannot recombine. However, if one of the electrons overturns its spin, then free radicals can react with one another to form a diamagnetic product. Such electron spin transition may be induced by an alternative MF. 

Many more examples have been collected for the reaction of transition metal hydride complexes with 1,3-dienes, which appear to proceed via radical pair mechanisms, even without photochemical activation72-77. The following general mechanism has been assumed to be operative for the reaction of HMn(CO)572,73, HFe(CO)4SiCl374,75, HFe(CO)2Cp76 and HCo(CO)4 (H-[M]) (equation 18)77. 

Photolysis of diazomethane in carbon-tetrachloride in the presence of benzophenone yields 1,1,1,2-tetrachloroethane showing an enhanced absorption due to the triplet carbene. The direct photolysis of diazomethane proceeds via singlet methylene CIDNP-studies of the photolysis of methyl-diazoacetate, for which a radical pair mechanism was suggested, were recently challenged 2). 

If the photo-Fries reaction would occur via a concerted mechanism, the absence of solvent should be of minor importance for the formation of rearranged products. However, conclusive evidence supporting the radical pair mechanism arises from the experiments carried out with phenyl acetate (10) in the vapor phase. The major product in the irradiations of 10 is phenol (13), which accounts for 65% of the photoproducts. Under these conditions, less than 1% of ortho -hydroxyace-tophenone (11) appears to be formed [19,20]. Conversely, when a high cage effect is expected, as in rigid matrixes (i.e., polyethylene), the result is completely different, and phenol is practically absent from the reaction mixtures [29]. In the intermediate situation (liquid solution), both rearranged products and phenol are formed in variable amounts depending on solvent properties. These observations... 

Any CIDNP-based assignment of the sign and relative magnitude of hfcs is valid only if the radical pair mechanism (RPM) is operative they become invalid if an alternative process is the source of the observed effects. The triplet-Overhauser mechanism (TOM) is based on electron nuclear cross-relaxation. For effects induced via the TOM, the signal directions depend on the mechanism of cross-relaxation and the polarization intensities are proportional to the square of the hfc. Thus, they do not contain any information related to the signs of the hfcs. 

In 1956, Doering et al. reported that methylene (CH2) inserted into the C H bonds of pentane, 2,3-dimethylbutane, and cyclohexene with no discrimination (other than statistical) between chemically different sites CH2 was classed as the most indiscriminate reagent known in organic chemistry. Doering and Kirmse also demonstrated that the C—H insertion reactions of CH2 in solution were direct, single barrier concerted processes with transition states that could be represented as 27 (Fig. 7.12). In particular, they did not proceed via initial H abstraction to give radical pair intermediates that subsequently recombined. (Triplet carbene C H insertions, however, do follow abstraction-recombination, radical pair mechanisms, as demonstrated in classic experiments of Closs and Closs and Roth (see Chapter 9 in this volume). 

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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