Reactions of co-ordinated ligands. Part 10. Rhodium-catalysed cyclisation of 3-(2-fluorophenyl)propanols to chromans

The cyclisation of several 3-(2-fluorophenyl)-propanols to the corresponding chroman occurs in nitromethane–acetone solution at 80 °C when either the hexafluorophosphate (3) or tetrafluoroborate salt (4) of the (η5-ethyltetramethylcyclopentadienyl)(η6-benzene)rhodium(III) cation is used as a catalyst; the former salt is the more effective catalyst. The cyclisation is believed to involve the activation of the aryl fluoride (towards intramolecular nucleophilic substitution by the hydrpxy group) by the formation of a metal complex in which the aryl fluoride is π-bonded with the metal cation. It is suggested that the arene-exchange reaction which gives this π-bonded complex proceeds faster with the salt (3) than with the salt (4), and that this is the main factor for the greater efficiency of the former salt as a cyclisation catalyst.