Electrochemical and Photocatalytic Oxidative Coupling of Ketones via Silyl Bis-enol Ethers

Abstract

Diastereoselective oxidative coupling of ketones through a silyl bis-enol ether intermediate by anodic and photocatalytic oxidation is reported. These methods provide several 1,4-diketones in good yields without the need for stoichiometric metal oxidants. The strategic use of a silicon tether enables the coupling of both aromatic and aliphatic ketones as well as the synthesis of quaternary centers. Cyclic voltammetry is used to gain insight into the oxidation events of the reaction.

Electrochemical α-Arylation of Ketones via Anodic Oxidation of In Situ Generated Silyl Enol Ethers

“…Following the general procedure for the electrochemical α-arylation of ketones with acetophenone 5a as the ketone and naphthalene 6s as the arene, side product 9a could be purified and isolated by column chromatography using petroleum ether/ethyl acetate as an eluent as a pale yellow oil (15 mg, 12% yield): 1 H NMR (300 MHz, CDCl 3 ) δ 8.08–8.01 (m, 4H), 7.63–7.53 (m, 2H), 7.54–7.42 (m, 4H), 3.47 (s, 4H); 13 C­{ 1 H} NMR (75 MHz, CDCl 3 ) δ 198.8, 136.9, 133.3, 128.7, 128.3, 32.7; MS-EI m / z 238 (5%), 133 (7%), 105 (100%), 77 (73%), 51 (29%). These data are in agreement with those reported previously in the literature …”

“…Moreover, not surprisingly, all examples of electrochemical coupling of silyl enol ethers with arenes published to date comprised intramolecular reactions (Figure a) . Intermolecular couplings are likely tampered by the expected dimerization of the carbonyl species, which can readily take place by a reaction of the radical cation resulting from anodic oxidation of the silyl enol ether with another unreacted molecule of the enol intermediate …”

“…14 Intermolecular couplings are likely tampered by the expected dimerization of the carbonyl species, which can readily take place by a reaction of the radical cation resulting from anodic oxidation of the silyl enol ether with another unreacted molecule of the enol intermediate. 15 We envisioned that one-pot electrolysis of a ketone silylation reaction mixture, which includes a silylating agent and a base as reagents, would not require the addition of external supporting electrolytes to confer conductivity to the solution (Figure 1b). Such a strategy would avoid purification of the silyl enol ether and the use of additional salts for the electrochemical step, rendering a more convenient and sustainable methodology.…”

Electrochemical Synthesis of Fluorinated Ketones from Enol Acetates and Sodium Perfluoroalkyl Sulfinates

“…(See the SI.) Silyl enol ethers were found − to have oxidation potentials significantly lower than those of the enol acetates. The electron-accepting nature of the ester group in enol acetates is likely to be helpful in preventing the oxidation of the enol moiety.…”

Guanidinium Hypoiodite-Catalyzed Intramolecular Oxidative Coupling Reaction of Oxindoles with β-Dicarbonyls

“…The guanidinium hypoiodite derived from 1,5,7-triazabicyclo[4.4.0]dec-5-ene hydroiodide (TBD·HI) with cumene hydroperoxide (CHP) was found to be effective for the intermolecular oxidative coupling reaction of oxindoles 4 and β-dicarbonyls 5 , giving the coupling product 6 in an excellent yield, and the guanidinium cation was proposed to play significant role in activating the enolate through hydrogen-bonding interaction . In this context, we investigated the synthesis of spiro[indoline-4,3′-piperidine] derivatives via an intramolecular oxidative coupling reaction − of oxindoles with acyclic β-dicarbonyl compounds using the guanidinium hypoiodite catalyst.…”