Investigations into Radical Reactions Facilitated by η6-Ruthenium Intermediate Complexes

Abstract

η6-Coordination of arenes to transition metals results in a significant alteration of arene properties. While reactions incorporating stoichiometric metals are common, those that proceed by transient π-coordination are less so. A recently developed hydrodeiodination protocol is believed to react via η6-arene intermediate ruthenium complexes, and this offers the opportunity to develop new reactions that are mediated by this type of coordination bonding.
Firstly, an intramolecular radical cyclisation reaction was investigated. Initially tested under optimised hydrodeiodination conditions, purification was difficult and so a better solvent alternative was found. Optimisation of base, catalyst, solvent, time, and temperature returned a maximum yield of only 8% with [RuCp]+ catalyst, mainly as a result of large amounts of alkene hydrogenation and isomerisation occurring in very short reaction times. Synthesis of sterically hindered alkenes showed that trisubstituted alkenes were significantly more resistant to these reactions.
Secondly, a ruthenium-catalysed iodide to bromide halogen exchange protocol is described. Optimisations found [RuCp*]+ as the best catalyst, with yields of 51% for unsubstituted iodobenzene. Electron deficient arenes were tolerated better, with a yield of 57% for the formation of 4-bromoacetophenone. The reaction mechanism remains unclear, with evidence suggesting that it might proceed via an oxidative addition or SNAr-type mechanism.