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[摘要]：The substituent effect on the reactivity and regioselectivity of N-(4-pentenyl)amidyl radical cyclization was investigated. Exclusive 6-endo cyclization was obsd. for N-(4-pentenyl)amidyl radicals with internal vinylic heteroatom substitution (Cl, Br, I, OMe, SEt). The substituent on the carbonyl group also showed a significant influence on the reactivity of amidyl radicals, which increases in the order of Ph < Me < OEt. As a result, the photostimulated reactions of N-(4-halopent-4-enyl)amides and carbamates (X = Cl, Br, I) with DIB/I2 or Pb(OAc)4/I2 led to the efficient and exclusive formation of the corresponding piperidines while those of N-(5-halopent-4-enyl)amides afforded the pyrrolidine products only. The halogen-substitution effect also allowed the 6-exo and 7-endo amidyl radical cyclization to proceed in a highly regioselective manner. The above exptl. results, in combination with theor. analyses, revealed that the lone pair-lone pair repulsion between the nitrogen radical and the vinylic heteroatom played an important role in controlling the regioselectivity of cyclization. |
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