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

Article abstract of DOI:10.1016/S0022-328X(00)99798-5

Acetone and methyl ethyl ketone 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 first-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(1+) 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 deg C (k₂ = 2.6 * E-4 M^-1 sec^-1) is E7 times as large as that of its spontaneous enolization in water, where water is both acid and base.