Radical-chain oxidative addition mechanism for the reaction of an [Re(CO)5]− anion with α-bromostilbene

Abstract

E-α-Bromostilbene spontaneously reacts with Na[Re(CO)₅] at 22 °C in THF to give Na[ReBr(CO)₄{Z-C(Ph)CHPh}] and Na[Re₂(CO)₉{Z-C(Ph)CHPh}] as the main products. Z-α-Bromostilbene is less reactive, but gives the same products. The reaction is stimulated by visible light or a source of solvated electrons (NaK_2.8) and can be inhibited by a quinomethide radical trap. With an excess of Na[Re(CO)₅] one can observe the initial formation of Na[ReBr(CO)₄{Z-C(Ph)CHPh}] and its complete transformation into Na[Re₂(CO)₉{Z-C(Ph)CHPh}]. Treatment of Na[ReBr(CO)₄{Z-C(Ph)CHPh}] with CO almost quantitatively converts it to [Re(CO)₅{Z-C(Ph)CHPh}], the structure of which is established by a single-crystal X-ray diffraction study. A radical-chain mechanism is proposed for the reaction comprising the following steps: (a) coupling of a Vin˙ radical with Na[Re(CO)₅], (b) CO-dissociation from the formed 19-electron radical-anion and (c) bromine atom abstraction by [Re(CO)₄{Z-C(Ph)CHPh}]˙⁻ from α-bromostilbene. The mechanism is confirmed by the formation of the same Na[ReBr(CO)₄{Z-C(Ph)CHPh}] product in the presence of NaI. When the radical-chain process is inhibited, a slow halogenophilic reaction is observed, mainly giving the Z and E-isomers of the acylrhenate Na[Re₂(CO)₉{C(O)C(Ph)CHPh}].