Heterocyclisation of ferrocenyl- and benzenetricarbonyl chromium-cyclopentadienes with o-quinone. Evidence for a SET route

A series of cyclopentadienyl ligands substituted by unsaturated systems including various transition metal groups ((C5H5)2Fe, PhCr(CO)3) were reacted with 3,5-di-tert-butyl orthoquinone producing 1,4-benzodioxines in high yields. These cycloaddition reactions were stereospecific. 1H and 13C NMR studies and X-ray structural analysis are in agreement with the exclusive formation of “endo” cycloadducts. Moreover, an ESR study shows the transient formation of both the radical anion of the o-quinone and the radical cation of the dienic system indicating an alternative single electron transfer pathway.