Alkyl azides radical cyclizations

N-centered radicals such as iminyl radicals can also be generated from a radical cyclization step onto nitrile groups. The fate of the iminyl radical depends on the nature of the a-substituent. When this latter is an alkyl group, further cychzation can take place [45]. A combination of both processes has been achieved addition of a staimyl radical onto an azide moiety generated a N-stannylaminyl radical that cyclized onto a nitrile and underwent further cyclization, thus opening access to pyrrolopyrroles and pyrrolopyridines derivatives [46]. 

Kim developed a new entry into A -heterocycles by radical cyclizations onto alkyl azides. Iodides, bromides and thionocarbonates (Scheme 28, Eq. 28.1) are suitable radical precursors. 5-Exo cyclizations afford 3,3-triazenyl radicals that lose N2 to furnish an aminyl radical [79]. Following this work, Kilburn has applied this strategy to the formation of spiro-heterocycles from methylenecyclopropanes [80]. Finally, this reaction was applied as a key step in a very elegant cascade synthesis of aspi-dospermidine developed by Murphy (Scheme 28, Eq. 28.2) [81]. 

Intramolecular attack of the radical center in 26 on the double bond then affords the alkyl radical 27. Lastly, addition of this alkyl radical to the azide function followed by loss of N2 gives the observed product 25 via the N-radical intermediate 28. The high stereoselectivity and the excellent yield of this tandem radical cyclization is noteworthy. 

Kim reported efficient radical cyclizations involving azides as radical traps (Scheme 8.33)7 This intramolecular amination process is efficient and proceeds via a 3,3-triazenyl radical that readily eliminates nitrogen to afford an aminyl radical. The use of (TMSlsSiH gives better results than BusSnH since azides are relatively inert toward tris(trimethylsilyl) silyl radicals. However, tin hydride can be used with alkyl iodides since the iodine atom abstraction by the stannyl radical is faster than its addition to the azide. 

Evidence for the formation of alkyl and aryl radicals in some cases following loss of SO2 (Scheme 1) has been obtained. Thus, a small amount of M-pentane was formed in the decomposition of M-pentanesulphonyl azide in mineral oil ). Thermolysis of diphenyl sulphone-2-sulphonyl azide (8) in dodecane at 150 °C gave diphenyl sulphone 9 (27%) and diphenyl sulphone-2-sulphonamide 10 (9%) which arise by hydrogen abstraction by the aryl radical and sulphonyl nitrene, respectively. When this thermolysis was carried out in Freon E-4 at 150 °C, the products were diphenylene sulphone 77 (1.3%) (Pshorr-type cyclization product of the aryl radical) and 10 (1.5%) together with tars 16h Ferro-... 

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