Catalytic reactions of metalloporphyrins.: II. Activation and catalytic aldol condensation of ketone with rhodium(III)-porphyrin perchlorate under neutral and mild conditions

Acetone and methyl ethyl kektone undergo facile and direct metalation at the methyl groups by a cationic (octaethylporphyrinato) rhodium (III) complex with a non-coordinating perchlorate counteranion, (OEP)Rh^III(ClO₄), under mild conditions. Acetylacetone and ethyl acetoacetate are similarly metalated at the internal methylene groups. The metalation of acetone is firs-order with respect to both rhodium complex and ketone, and involves the (OEP)Rh^III(ClO₄)-assisted, rate-determining enolization of the ketone. The resulting 2-oxopropyl-rhodium derivative undergoes facile cleavage of the CRh bond with electrophiles such as H⁺ and Br₂. When cyclohexanone is used as substrate, on the other hand, (OEP)Rh^III(ClO₄) catalyzes the aldol condensation of the ketone effectively, where the intermediate cyclohexanone enolate reacts with the ketone or other carbonyl compound present and regenerates the Rh^III complex. An essential aspect of the present reaction is the remarkable ability of (OEP)Rh^III(ClO₄) to promote enolization of simple ketones by activation with charge-separated (OEP)Rh^III+ (a Lewis acid) under mild and neutral conditions. The second-order rate constant of (OEP)Rh^III(ClO₄)-assisted enolization of acetone at 30°C (k₂ = 2.6 × 10⁻⁴ M⁻¹ sec⁻¹) is 10⁷ times as large as that of its spontaneous enolization in water, where water is both acid and bse.