Reductive radical cyclization using low-valent iron pentacarbonyl [Fe(CO)5] to generate radical species from alkyl iodides was achieved. A range of pyrrolidines, tetrahydrofurans, and carbocycles were synthesized via 5-exo cyclization reactions of alkyl radical intermediates generated by electron transfer from a system involving Fe(CO)5, 1,10-phenanthroline, and diisopropylamine. In addition, we have successfully applied to the tandem radical reactions with Michael acceptor. We have found the presence of Fe2+(phen)3 species during the reaction through UV-vis absorption spectroscopy and electrospray ionization mass spectrometry (ESI-MS), which are the distinct evidence that Fe and phenanthroline system acts as electron donor agents. Moreover, we have proved that radical pathway is main route by the radical clock reaction, TEMPO experiment, and electron paramagnetic resonance (EPR) studies to investigate iron(III) complex and phenanthroline radical anion species.
Especially, iron(III) polypyridyl complexes can be used as single-electron oxidants with tertiary amines or anilines [Eox = 0.8-1.0 V] for formation amine radical cation as a hydrogen atom source. In this regard, we planned oxidative reaction of tertiary amines with iron polypyridyl complexes. After amine is oxidized by iron(III) complex, deprotonation of amine radical cation offers α-amino radical species, which is reactive to α,β-unsaturated carbonyl compounds as a radical acceptors. Herein, we present reductive radical cyclization of iodoalkene and oxidative cycloaddition of tertiary amines with maleimides under mild reaction condition.