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

Abstract

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(ClO 4), 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(ClO 4)-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 Br 2. When cyclohexanone is used as substrate, on the other hand, (OEP)Rh III(ClO 4) 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(ClO 4) 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(ClO 4)-assisted enolization of acetone at 30°C ( k 2 = 2.6 × 10 −4 M −1 sec −1) is 10 7 times as large as that of its spontaneous enolization in water, where water is both acid and bse.