Iron-catalyzed Selective Oxidation of α,β-Unsaturated Aldehydes to α,β-Unsaturated Carboxylic Acids by Molecular Oxygen

Abstract

Selective oxidation of α,β-unsaturated aldehydes to α,β-unsaturated carboxylic acids was performed using O 2 as the oxidant in the presence of a simple iron catalyst. The addition of an alkali metal carboxylate as a cocatalyst enhanced the selectivity for the desired product. Redox tuning of the iron catalyst via association with the alkali metal led to a controlled radical generation during the catalytic O 2 oxidation.Molecular oxygen (O 2 ) is a readily available, inexpensive oxidant and is regarded as a promising natural resource for oxygen-containing chemical products.1,2 Conventional chemical processes including oxidations often use stoichiometric amounts of hazardous oxidants, leading to the formation of equimolar amounts of a by-product as waste. Although oxidation by O 2 would proceed with 100% atom efficiency under ideal conditions and thus generate no waste, such dioxygenase-type reactions have been reported less frequently than monooxygenaseand oxidase-type oxidation reactions, in which only one O atom of O 2 incorporates into substrate and O 2 is consumed by reoxidation of catalyst, respectively.3 In particular, the synthesis of value-added chemical products should be replaced by dioxygenase-type reactions to reduce waste generated during multistep reactions.α,β-Unsaturated carboxylic acids are among the most valuable intermediates and precursors for chemical production and pharmaceuticals. 4 Although the oxidation of aliphatic aldehyde into carboxylic acids proceeds by O 2 even without a catalyst, 5,6 several methods have been developed for the preparation of α,β-unsaturated carboxylic acids from their aldehyde derivatives, requiring severe and complex reaction conditions.7 Recently, N-heterocyclic carbene (NHC)-catalyzed O 2 oxidation reactions of α,β-unsaturated aldehyde were developed by several groups, yet more than equimolar amounts of base were essential for good reaction yields.8 From the standpoint of green-sustainable chemistry, catalytic oxidation using O 2 without large amounts of base is promising even if an organocatalyst could be employed.9 Nobile et al. reported a Fe catalyst bearing 2-(acetoacetoxy)ethyl methacrylate ligandcatalyzed O 2 oxidation of trans-2-hexenal; however, substrates were limited and a halogenated solvent was required.10 Mukaiyama et al. proposed the effectiveness of β-diketonate ligand to activate metal-catalyzed oxidation.11 More useful methods and systems to activate a base-metal complex must be developed to realize practical O 2 oxidation for the production of valuable fine chemicals.It is widely accepted that non-redox metal ions have considerable effect on the redox state of redox-active metal, promoting electron-transfer reactions 12 as well as catalytic reactions. 13 An appropriate combination of redox-active and non-redox-active base metals might act as simpler catalysts exhibiting enhanced activity in oxidation reactions. Herein, we report the preparation of α,β-unsaturated carboxylic acids from α,β-unsaturated aldehydes using a combination of O 2 and a...

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MOF-Zn-NHC as an efficient N-heterocyclic carbene catalyst for aerobic oxidation of aldehydes to their corresponding carboxylic acids via a cooperative geminal anomeric based oxidation

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