Cooperativity in Diiron(III)porphyrin Dication Diradical-Catalyzed Oxa-Diels–Alder Reactions: Spectroscopic and Mechanistic Insights

Here, we delineate a maiden example of a diiron(III) dication diradical porphyrin dimer as a competent catalyst for the oxa-Diels–Alder type reaction of aldehydes with 1,3-dienes, which is a cardinal reaction in the syntheses of natural products. This catalyzed process does not demand the use of electron-deficient aldehydes such as glyoxylic acid derivatives or activated electron-rich 1,3-dienes such as Danishefsky's, Brassard's, or Rawal's diene. The robust catalyst exhibited high functional group tolerance. The computational studies corroborated the detailed spectroscopic investigation, which focused on the pivotal roles played by the metal ion as the Lewis acidic center in combination with counteranions and also enabled us to delve deeper into the reaction mechanism. Previously developed methodologies invariably require dissociation of the iron-axial bond of the catalyst; on the contrary, the axial ligand of the catalyst remains intact during the catalysis reported here. The use of a dication diradical iron(III)porphyrin as the Lewis acid catalyst facilitates activation of the aldehyde via coordination whose formation has also been confirmed experimentally. Moreover, the counteranion has a considerable effect on the reaction pathway; its coordination to the metal inhibits the coordination of the substrate to form the product. The efficacy of employing such a diheme catalyst over a monoheme analogue is manifested in the cooperative effect, which resulted in a lower catalyst loading with excellent yields.