Imidoyl radical intermediates

Trialkylsilyl radicals add to alkyl isocyanate to form imidoyl radicals 56 (Reaction 5.38). Detailed EPR studies established intermediates 56 to be strongly bent at the carbon bearing the unpaired electron [76], The absolute rate constant for the reaction of Et3Si radical with ieri-butyl isocyanate was found to be 5.5 x 10 s at 27 °C [13], whereas the relative rate of the addition of MesSi radicals to alkyl isocyanates was found to decrease in the... 

The addition of sulfur radicals to isocyanides produces imidoyl radicals as intermediates. The stereocontrolled 5-exo-trig cyclization of imidoyl radicals in the synthesis of substituted (alkylthio)pyrrolines, pyroglutamates, and thiopyroglutamates has been reported [95JOC6242], The intermediate 137 is not isolated but undergoes further thermal reaction to furnish a pyroglutamate derivative. 

Readily available thiocarbamates, thioamides, and thioureas provide direct routes to quinolines in moderate to good yields <03OL1765>. The reactive intermediate postulated is the synthetic equivalent of an imidoyl radical, but with greater utility. 

Imidoyl radicals are in principle very attractive intermediates for the synthesis of A-heterocycles. Actually, during the great upsurge of synthetic work carried out... 

It is worth noting that the same kind of reactions can be carried out by tin-radical-mediated ring closure of analogous isothiocyanates e.g., 33, 37, 39, NC = NCS) [22b,c], In this case, the intermediate a-thio-substituted imidoyl radical is generated by addition of a stannyl radical to the sulfur atom of the isothiocyanate. Although it had been known for a few decades, this way to imidoyl radicals had found very little application in organic synthesis. Bachi s work was the first example of synthesis of heterocyclic compounds by radical addition to isothiocyanates, showing the way to further possible applications [9]. 

Physical properties indicate that the dipolar contribution (1) is the major one and this structure actually accounts for the nucleophilic behavior of the terminal carbon of isonitriles. However, in terms of radical chemistry, the more interesting form is the divalent one (2). This clearly shows that the isonitrile group does not behave toward radical species like a vicinal radical acceptor/radical donor synthon, that is, like an usual unsaturated bond. It instead reacts like a geminal acceptor/donor synthon [2], where an incoming radical attacks the same carbon atom that will be the new radical center in the resulting imidoyl intermediate 3 (Scheme 1). Actually, isonitriles can serve as very efficient radical traps and this chapter reviews the structural, mechanistic, and synthetic studies carried out in this field, with outstanding results, in the last three decades. 

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