Mechanistic Studies of Ce(IV)-Mediated Oxidation of β-Dicarbonyls:  Solvent-Dependent Behavior of Radical Cation Intermediates

Jiao, Jingliang; Zhang, Yang; Devery, James J.; Xu, Luna; Deng, Jennifer; Flowers, Robert A.

doi:10.1021/jo0625406

Cited by 22 publications

(22 citation statements)

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“…Previous research from our group has shown that the oxidation of the enol tautomer of a diketone initially forms a radical cation, which is deprotonated readily by MeOH to form the radical under the reaction conditions [15–16]. …”

Section: Resultsmentioning

confidence: 99%

“…In a previous study by our research group, the rates of oxidation of several β-diketones and their related silyl enol ethers by CAN and the more lipophilic ceric tetra- n -butylammonium nitrate (CTAN) were measured in MeOH, MeCN and CH 2 Cl 2 by using stopped-flow spectrophotometry [16]. In these experiments, initial oxidation of substrates generated radical cation intermediates.…”

Section: Resultsmentioning

confidence: 99%

“…Using this method, a variety of 1,3-dicarbonyl compounds can be mildly converted to carboxylic acids in moderate to excellent yields. In follow up studies, we found that 2-tetralone 2a could be obtained as the major product when 1a was oxidized by CAN in MeOH [16]. In addition, when 2.2 equivalents of CAN were employed, no products of secondary oxidations were obtained.…”

Section: Introductionmentioning

confidence: 99%

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Mechanistic studies on the CAN-mediated intramolecular cyclization of δ-aryl-β-dicarbonyl compounds

Casey¹,

Sadasivam²,

Flowers³

2013

Beilstein J. Org. Chem.

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SummaryThe synthesis of 2-tetralones through the cyclization of δ-aryl-β-dicarbonyl substrates by using CAN is described. Appropriately functionalized aromatic substrates undergo intramolecular cyclizations generating 2-tetralone derivatives in moderate to good yields. DFT computational studies indicate that successful formation of 2-tetralones from δ-aryl-β-dicarbonyl radicals is dependent on the stability of the subsequent cyclohexadienyl radical intermediates. Furthermore, DFT computational studies were used to rationalize the observed site selectivity in the 2-tetralone products.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Mechanistic studies on the CAN-mediated intramolecular cyclization of δ-aryl-β-dicarbonyl compounds

Casey¹,

Sadasivam²,

Flowers³

2013

Beilstein J. Org. Chem.

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“…According to this mechanism, while Ce +4 is reduced to Ce +3 a radical cation B is formed. 48 Then, addition of the radical to the terminal double bond of the diene forms an allylic radical intermediate C. Radical C is oxidized to the carbocation D by CAN, followed by cyclization of D to give 5-(2-phenylvinyl)-4,5-dihydrofuran E. Intermediates F and H which is the another enol form of F were obtained from the radical addition of 1-phenyl-1,3-butanedione 1f to diene 2b. Intramolecular cyclization of these intermediates gave dihydrofurans G and I, respectively.…”

Section: Resultsmentioning

confidence: 99%

Synthesis of 2-(2-phenylethenyl) substituted 4,5-dihydrofurans by regioselective addition of 1,3-dicarbonyl compounds to dienes promoted by cerium(IV) ammonium nitrate

Yılmaz

Ustalar²

2016

Arkivoc

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Radical addition of 1,3-dicarbonyl compounds to conjugated dienes in the presence of cerium(IV) ammonium nitrate in THF produced 4,5-dihydrofurans in good to excellent yields. All radical additions occurred on the terminal double bond as regioselective. Two different dihydrofurans were obtained from the reaction of 1-phenyl-1,3-butanedione with 1-phenyl-1,3-butadiene and 3-methyl-1-phenyl-1,3-butadiene. All compounds were characterised by IR, 1 H, 13 C-NMR and HRMS spectra.

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“…Coupling of 2 and 6 produces intermediate 7. A second equivalent of Ce IV oxidizes 7 to 8, which undergoes b elimination of the silyl cation through reaction with a nucleophile present in solution according to a known nucleophilic displacement mechanism, 12 providing iminium intermediate 9. Finally, 3 is released by the addition of one equivalent of H 2 O to 9, generating product 4 and a proton which must be absorbed by a base to prevent protonation of 3.…”

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confidence: 99%

Mechanistic Complexity in Organo–SOMO Activation

et al. 2010

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Singly occupied molecular orbital (SOMO) activation provides a pathway for asymmetric a-addition to aldehydes. [1] The scope of SOMO activation includes the allylation, enolation, vinylation, styrenation, chlorination, polyene cyclization, and arylation [2] of a range of aldehydes. This union of organocatalysis with single-electron oxidative coupling is an intricate process involving the complex balance of catalyst, oxidant, radicophile, base, temperature, heterogeneous reaction conditions, and H 2 O. To examine the role of each component in this complex process, a series of spectroscopic and kinetic studies were carried out to study the asymmetric allylation of 1 shown in Equation (1).[1a] The data described herein show three important features: 1) Oxidation of the intermediate enamine is rapid and preferential to oxidation of the catalyst, 2) H 2 O concentration is critical for catalytic efficiency, and 3) the kinetic role of ceric ammonium nitrate (CAN) is masked by a physical phase-transfer process.The mechanism of the system is challenging to study since it is heterogeneous, but findings from initial work show that the presence of H 2 O is important for reaction success.[1a]Additionally, one of the key design features of the process involves reaction of the catalyst (3) with the substrate to form an intermediate enamine that is preferentially oxidized in the presence of catalyst or substrate. Previous studies show that enamines [3] are more readily oxidized than amines. [4] To further explore the selectivity of the oxidation, a series of amines and enamines derived from imidazolidinone catalysts were studied using stopped-flow spectrophotometry in an attempt to determine the impact of the structure on the rate of a single-electron oxidation. A range of single-electron oxidants based on Ce IV , Cu II , and Fe III were explored. In each case, enamine oxidation was faster than the mixing time of the stopped-flow spectrophotometer even at reduced temperatures (À10 8C). Conversely, oxidation of the catalyst was slower than the time-scale of the stopped-flow system (1000 s). Although these findings did not provide the structure-reactivity relationships initially desired, they indicated that oxidation of the enamine is significantly more rapid under homogeneous conditions than oxidation of the catalyst.Since the SOMO system is mechanistically complex, reaction progress kinetic analysis (RPKA) was employed to study the reaction in Equation (1). This approach, described by Blackmond, provides an elegant method for the study of mechanistic properties under synthetic conditions.[5] To utilize RPKA, two factors are necessary: 1) a detailed understanding of the stoichiometry of the reagents used, and 2) an understanding of the concept of excess. Excess (e) is defined as the difference in initial stoichiometric concentrations of a reagent (CAN) and the monitored substrate (1) [Eq 2].The "same excess" protocol [5] determines whether the active catalyst concentration remains constant throughout the reaction. Initially, t...

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Mechanistic Studies of Ce(IV)-Mediated Oxidation of β-Dicarbonyls: Solvent-Dependent Behavior of Radical Cation Intermediates

Cited by 22 publications

References 50 publications

Mechanistic studies on the CAN-mediated intramolecular cyclization of δ-aryl-β-dicarbonyl compounds

Mechanistic studies on the CAN-mediated intramolecular cyclization of δ-aryl-β-dicarbonyl compounds

Synthesis of 2-(2-phenylethenyl) substituted 4,5-dihydrofurans by regioselective addition of 1,3-dicarbonyl compounds to dienes promoted by cerium(IV) ammonium nitrate

Mechanistic Complexity in Organo–SOMO Activation

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