Captodative radical intermediate

Meso-dl isomerization of [47] was described by Koch (Koch et ah, 1975 Koch, 1986 Olson and Koch, 1986. The intermediate radical [48] is in equilibrium with the dimer and can be easily recognized by esr spectroscopy. The thermodynamic parameters for bond homolysis, as a function of medium, are reported in Table 18. A strong solvent effect is observed, in contrast to Riichardt s example ([24], [25]) reported above. This is interpreted as a manifestation of the polar character of the intermediate radical. The easy detection of [48] by esr spectroscopy is traced back, at least in part, to its captodative character. However, the strong solvent effect on homolysis of [47] need not necessarily be related to the captodative character of radicals [48]. 

The photo Diels-Alder reaction of a-acetylnaphthalene 82 with the chiral oc-enaminonitril 83 yielded the cycloadduct 84 with excellent diastereoselec-tivity (Sch. 16) [57]. The intermediary formed biradical Y is particular stable due to delocalization of the radical on the aromatic moiety and to a captodative effect on the enamine part. The chiral induction occurred in two steps [58]. In the first step, a stereogenic center is created in the a-position of the acyl group. In the second step of the diastereoselection, one of the two diasteromeric intermediates undergoes preferentially cyclization to yield the final product 84, while the other one is more readily decomposed to form the starting material. For a more detailed discussion of the mechanism see Ref. [59]. 

Scheme 10 is representative of the mechanism of these coupling reactions involving a captodatively stabhzed glycyl radical 15 from the initial reduction of the pyridyl sulfide group by the divalent lanthanide reagent. Further reduction of this carbon radical by a second equivalent of samarium diiodide leads to a Sm(lII) enolate intermediate 16 of unknown geometry, which ultimately reacts with the carbonyl compound to give 17. 

Radical mechanisms are also known to be involved in 1,2-rearrangement reactions, such as the R alkyl group migration leading from ammonium ylids 16 to tertiary amines 19 (Scheme 3). Whilst the possible ion pair intermediate 17 is destabilized by the electron-withdrawing carbonyl group, the radical pair 18 is favored by the captodative stabilization of one of the radicals. 

Many studies of substituent effects on radical stabilization involved C-C homolysis reactions leading to rather similar diradicals. Cis-trans isomerization of tetrasubstituted cyclopropanes 24 (Scheme 5) is found to occur fastest when the intermediate diradical 25 is stabilized by captodative substitution [25], with better donors yielding faster reactions. 

Ruggero Curd of the Universita di Bari has reported Tetrahedron Lett. 2007, 48, 3575) the stereospecific hydroxylation of 1,3-dimethyl cyclohexane 4 to the diol 6. Yasuyuki Kita of Osaka University has developed Organic Lett. 2007, 9, 3129) conditions for specific benzylic oxidation, converting 7 into 8 with high diastereocontrol. Larry E. Overman of the University of Cahfomia, Irvine has established Organic Lett 2007,9, 5267) that by using a slow H-atom donor, it was possible to effect intramolecular H abstraction, leading, by oxidation of the intermediate captodatively-stabilized radical, from 9 to the acetate 10. 

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