A Study of Vinyl Radical Cyclization onto the Azido Group by Addition of Sulfanyl, Stannyl, and Silyl Radicals to Alkynyl Azides☆
Thermal radical reactions of azidoalkynes 2, 8, 14, and 21a–c with thiols 1a–c afford 2-sulfanylvinyl radicals by selective addition of sulfanyl radicals to the triple bond. 1-Phenylvinyl radicals 23 and 30a, as well as vinyl radical 30b, undergo fast 5-cyclization onto the aromatic azide function to give cyclized indoles. In contrast, both 1-phenyl (15, 17) and 1-alkyl (3a,b, 9) vinyl radicals fail to add to their aliphatic azido substituents and exclusively undergo cyclization onto the aromatic sulfanyl ring and H transfer from the thiol precursor. Azidoalkynes 14 and 21a react with Bu3SnH and TMSS under radical conditions to give instead the corresponding amines as a result of preferential attack of Bu3Sn · and (TMS)3Si · radicals on the azido group rather than on the triple bond. Evidence is provided that alkyl radical cyclizations onto azides are not feasible in the presence of thiol, in contrast with the reported utility of these cyclization reactions in the presence of Bu3SnH and TMSS.