Reactivity of ferrocenylboranes: rearrangements versus electron transfer reactions

The chemical behavior of ferrocenylboranes FcB(R)R‘ (Fc = C5H5FeC5H4) toward 2,5-bis(pyrazol-1-yl)benzoquinone (1) is highly dependent on the nature of the substituents R and R‘. Reaction of 1 with 2 equiv of the ferrocenylborane FcBMe2 (2a) results in an unprecedented sequence of methyl migration and elimination steps; the Fe(II) oxidation state of the Fc fragments remains unchanged. In contrast, electron-transfer processes involving iron oxidation and benzoquinone reduction dominate the reaction of 1 with FcB(Me)Br (2b) and FcBBr2 (2c). Despite these different reactivities, bis(ferrocenyl) complexes with a bridging hydroquinone ligand (3a, [3b]Br2, [3c]Br2) are formed in all three cases.