Dehydrogenative α-Oxygenation of Ethers with an Iron Catalyst

Abstract

Selective α-oxidation of ethers under aerobic conditions is a long-pursued transformation; however, a green and efficient catalytic version of this reaction remains challenging. Herein, we report a new family of iron catalysts capable of promoting chemoselective α-oxidation of a range of ethers with excellent mass balance and high turnover numbers under 1 atm of O2 with no need for any additives. Unlike metalloenzymes and related biomimetics, the catalyst produces H2 as the only byproduct. Mechanistic investigations provide evidence for an unexpected two-step reaction pathway, which involves dehydrogenative incorporation of O2 into the ether to give a peroxobisether intermediate followed by cleavage of the peroxy bond to form two ester molecules, releasing stoichiometric H2 gas in each step. The operational simplicity and environmental friendliness of this methodology affords a useful alternative for performing oxidation, while the unique ability of the catalyst in oxygenating a substrate via dehydrogenation points to a new direction for understanding metalloenzymes and designing new biomimetic catalysts.

Non‐Heme Imine‐Based Iron Complexes as Catalysts for Oxidative Processes

“…[147] Thea bovec omplex was found to catalyze the a-oxidation of ethers to lactones or esters with O 2 as the terminal oxidant (Scheme 15a). Thec atalyst loading was surprisingly low,a nd TONs up to 412 were obtained.…”

“…Mechanism of ether oxidation by complex L9Fe(II). [147] Scheme17. Ligandse mployedb yL ouloudia nd co-workers.…”

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

“…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.…”

“…When 0.2 equiv of CH 3 COONa was added in this catalysis, product yield and selectivity were improved to 76% and 77%, respectively (Entry 3), while other sodium salts, NaNO 3 and NaOTf, were less effective (Entries 4 and 5). 15 For other Fe salts, [Fe(acac) 3 ] catalyzed the oxidation to yield 2a in 55% yield with 56% selectivity (Entry 6), and Fe(OAc) 2 did not show catalytic activity due to low solubility (Entry 7). CF 3 COONa, a more electronegative carboxylate salt, increased product yield and selectivity (Entry 8), and thus trifluoroacetic acid and its salts were screened as additives.…”

“…19 Thereby, we attempted to isolate the FeNa complex by a reaction of Fe(NO 3 at m/z 1408 and 1544 in ESI-MS spectra, respectively ( Figure S2). Catalysis on oxidation of 1a using this compound gave 2a in 82% yield.…”