1. Bringing Homogeneous Iron Catalysts on the Heterogeneous Side: Solutions for Immobilization
Fabio Moccia, Luca Rigamonti, Alessandro Messori, Valerio Zanotti, Rita Mazzoni Molecules. 2021 May 6;26(9):2728. doi: 10.3390/molecules26092728.
Noble metal catalysts currently dominate the landscape of chemical synthesis, but cheaper and less toxic derivatives are recently emerging as more sustainable solutions. Iron is among the possible alternative metals due to its biocompatibility and exceptional versatility. Nowadays, iron catalysts work essentially in homogeneous conditions, while heterogeneous catalysts would be better performing and more desirable systems for a broad industrial application. In this review, approaches for heterogenization of iron catalysts reported in the literature within the last two decades are summarized, and utility and critical points are discussed. The immobilization on silica of bis(arylimine)pyridyl iron complexes, good catalysts in the polymerization of olefins, is the first useful heterogeneous strategy described. Microporous molecular sieves also proved to be good iron catalyst carriers, able to provide confined geometries where olefin polymerization can occur. Same immobilizing supports (e.g., MCM-41 and MCM-48) are suitable for anchoring iron-based catalysts for styrene, cyclohexene and cyclohexane oxidation. Another excellent example is the anchoring to a Merrifield resin of an FeII-anthranilic acid complex, active in the catalytic reaction of urea with alcohols and amines for the synthesis of carbamates and N-substituted ureas, respectively. A SILP (Supported Ionic Liquid Phase) catalytic system has been successfully employed for the heterogenization of a chemoselective iron catalyst active in aldehyde hydrogenation. Finally, FeIII ions supported on polyvinylpyridine grafted chitosan made a useful heterogeneous catalytic system for C-H bond activation.
2. Organic Salts and Merrifield Resin Supported [PM12O40]3- (M = Mo or W) as Catalysts for Adipic Acid Synthesis
Jana Pisk, Dominique Agustin, Rinaldo Poli Molecules. 2019 Feb 21;24(4):783. doi: 10.3390/molecules24040783.
Adipic acid (AA) was obtained by catalyzed oxidation of cyclohexene, epoxycyclohexane, or cyclohexanediol under organic solvent-free conditions using aqueous hydrogen peroxide (30%) as an oxidizing agent and molybdenum- or tungsten-based Keggin polyoxometalates (POMs) surrounded by organic cations or ionically supported on functionalized Merrifield resins. Operating under these environmentally friendly, greener conditions and with low catalyst loading (0.025% for the molecular salts and 0.001⁻0.007% for the supported POMs), AA could be produced in interesting yields.
3. Compatibility study of Merrifield linker in Fmoc strategy peptide synthesis
Xiaoxiao Yang, Hao Lin, Wen Lu, Dexin Wang Protein Pept Lett. 2013 Feb;20(2):140-5. doi: 10.2174/092986613804725343.
The stability of Merrifield linker in Fmoc deprotection process was quantitatively investigated by establishing working curve of two major decomposition components from two resin bound dipeptide models. By sampling reaction solution and analyzing with RP-HPLC, decomposition rate was determined. The results indicated that either α-amino acid or β-amino acid anchored Merrifield linker was endurable for Fmoc strategy peptide synthesis in common de-Fmoc conditions such as 20% piperidine/DMF and 2% DBU/2% piperidine/DMF under room temperature treatments. However, Fmoc-deprotection with microwave assistance of α-amino acid anchored peptide resin with 20% piperidine/DMF more than 20 times or β-amino acid anchored peptide resin with 2% DBU/2% piperidine/DMF more than 30 times is not recommended. Feasibility of the proposed compatibility was verified by design and synthesis of a thymic humoral factor derived peptide via Fmoc strategy on Merrifield resin. Thus by choosing moderate de-Fmoc protocol, Merrifield resin is feasible for Fmoc strategy oligopeptide synthesis.
