FACILE NUCLEOPHILIC-ADDITION OF METHYL KETONE ENOLATES TO (ETA-5-PENTAMETHYLCYCLOPENTADIENYL)RHODIUM ETA-6-PARA-XYLENE DICATION

The facile nucleophilic addition of methyl ketone enolates of acetone, 2-butanone, and 2-pentanone to (η5-pentamethylcyclopentadienyl)rhodium η6-p-xylene dication ([Cp*Rh(η6-1,4-(CH3)2C6H4)2+], 1) were studied by using 1,2,3,4-tetrahydroisoquinoline as the base at 25 °C to provide complexes 2-4, respectively. The regio-and stereochemistry of these ketone enolate addition reactions to 1 were unequivocably established by a single-crystal X-ray structural analysis of the acetone enolate addition product, complex 2, to reveal that the substituted arene ligand, a 6-β-keto-substituted 1,4-dimethylcyclo-hexadienyl group, was bonded η5 to Cp*Rh+. The acetone enolate added to the unsubstituted carbon position on the η6-p-xylene ligand of complex 1 by backside nucleophilic attack (exo to the Rh metal center). The scope of the reaction was briefly studied to show that only enolates of methyl ketones, i.e., 3-pentanone failed to react, were able to undergo this nucleophilic addition reaction to 1. Complex 2 was oxidized with Jones reagent to release the 2,5-dimethylbenzyl methyl ketone and provides a convenient synthetic method for this class of organic compounds. © 1990, American Chemical Society. All rights reserved.