Olefins ketyl radical reactions

SET processes do not occur among moderately electrophilic olefinic acceptors, but are likely to be involved in highly electrophilic substrates. Some recent examples are the polyadditions of cuprate to fullerenes (Sect. 10.1.1). Fluorenone ketyl radical has been detected in a cuprate reaction of fluorenone [20]. Doubly activated olefins [67-69] and bromonaphthoquinone [70] also probably react through SET. 

Molander and co-workers have studied the stereoselective intramolecular addition of ketyl radicals to olefins [95JOC872]. The ketyl radicals are generated from ketone by treatment with samarium(II) diiodide. A similar reaction sequence using 61 gave only elimination products. 

One of the most frequently eneountered reactions is that with proton sources, as observed with arenes (Birch reduction) [189], aldehydes [ 190], alkynes [2d], fullerenes [191], ketones [192] (even enantioselective protonation of ketyl radical anions [193]), nitriles [194], nitro [195] and nitroso compounds [196], and olefins [197]. Protons are often replaced as electrophiles by trialkylsilyl chloride [198],... 

Ketyls generated by the reaction of SmE with aldehydes and ketones have been incorporated into numerous sequential processes in which a radical reaction is involved. Sequential radical processes, radical cyclization/carbonyl additions, radical cyclization/substitution reactions, nucleophilic acyl substitution/radical cyclizations, cyclization/elimination processes, and others have all been realized. Because these types of reactions have been extensively reviewed [2b, 25], further details will not be given here. Needless to say, new sequential processes based on SmE-promoted ketyl/olefin coupling reactions are still being developed (Eq. 75) [88]. 

Reactions herein significantly differ from most standard nBu SnH-mediated radical transformations that employ a variety of well-known precursors for carbon-centered radicals, including halides, thioacyl moieties, olefins, selenides and sulfides. Most of these potentially useful precursors are sacrificed and lost during the radical reaction and are not available for subsequent manipulations [7]. However, O-stannyl ketyl intermediates conserve the carbonyl oxygen for further functionalization. 

The carbon-centered radical stemming from the amine is able to initiate free-radical polymerizations of suitable monomers. a-Aminoradicals are especially suitable for the polymerization of acrylates [119] and are less efficient in styrene polymerization, which is explainable in terms of triplet quenching by styrene. In Table 9, rate constants for triplet quenching by various monomers are compiled. The ketyl radicals add due to resonance stabilization and, for steric reasons, only scarcely to olefinic double bonds but instead undergo recombination and disproportionation reactions, as shown in reaction (34). 

Samarium(II) iodide-promoted radical cydizations have also played a key role in the total synthesis of (-)-grayanotoxin III [43b]. Among the methods utilized for the synthesis of this molecule was an intramolecular ketyl olefin coupling reaction generating a bridged bicyclic ring system (Eq. 68). 

Annulation reactions are possible when a precursor monocyclic substrate contains an activated alkene in a tether [4a]. As demonstrated in Scheme 5, an ester was employed to activate the olefin appended to cycloalkanone 17. Upon generation of the 0-stannyl ketyl with tributyltin hydride, the carbon-centered radical attacks the electron-poor /(-position on the activated alkene. The corresponding cyclized adduct 18 is a bicyclic skeleton with a bridgehead hydroxyl group. An example of this reaction shows cyclopentanone 19 undergoing cyclization to diquinane 20 and tricycle 21 (76 24) in 69% yield. The presence of reasonable amounts of the minor, yet readily isolable, jn-diastereomers in the reaction indicated that the reaction may not be reversible. 

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