Nanoporous iron(iii) porphyrin frameworks: an efficient catalyst for [4+2] cycloaddition reactions of unactivated aldehydes with a diene

Abstract

The one pot conversion of several unactivated aldehydes with a diene to their corresponding [4+2] cycloaddition products has been studied using newly synthesized nanoporous iron(III) porphyrin frameworks as catalysts. The heterogeneous catalysts have been synthesized from 5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato iron(III) chloride (PFe·X) with the help of Pinnar type synthesis. The catalytic activity of the monomeric catalyst (PFe·X) vs. that of the new nanoporous catalysts (1·X) where X = Cl⁻, BF₄⁻etc. in the reaction of dienes and aldehydes were then compared. The structure of the catalysts was investigated using different characterization techniques including Fourier transform infrared spectroscopy (FT-IR), CHN, atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM) and the BET surface area which was measured via a physisorption study of N₂ at 77 K. The reaction parameters determining yield, including the quantity of the catalyst and the solvent, were optimized to afford the hetero Diels–Alder product efficiently. It was observed that the nanoporous catalysts (1·X) were much superior to the monomeric (PFe·X) catalysts at giving selective products in high yields. Secondly, the new catalysts remained active for at least 8 cycles without any observable decay in their catalytic ability. Attempts are made to rationalize the results by considering the nature of X and the BET surface area of the 1·X catalysts.