Α-keto Radicals Research Articles

Nickel-catalyzed switchable asymmetric electrochemical functionalization of alkenes.

The development of general electrocatalytic methods for the diversity-oriented regio- and stereoselective functionalization of alkenes remains a challenge in organic synthesis. We present a switchable electrocatalytic method based on anodic oxidative activation for the controlled liberation of chiral α-keto radical species toward stereoselective organic transformations. Electrogenerated α-keto radical species capture alkene partners, allowing switchable intermolecular alkene difunctionalization and alkenylation in a highly stereoselective manner. In addition to acting as proton donors to facilitate H2 evolution at the cathode, the unique properties of alcohol additives play an important role in determining the distinct outcomes for alkene functionalization under electrocatalytic conditions.

Pd-Mediated Light-Controlled Living Radical Polymerization of Methyl Acrylate

Abstract The radical polymerization of methyl acrylate in the presence of a palladium dimer complex and ethyl iodoacetate as an initiator under photoirradiation conditions led to the formation of poly(methyl acrylate) having good Ð values. The polymerization could be controlled by switching the light on and off, exhibiting living characteristics. We propose that such α-Pd polyesters could function as dormant species, which would allow a pair of transient α-keto radicals and persistent Pd radicals to be generated upon photoirradiation.

Synthesis of α-Quaternary Bicyclo[1.1.1]pentanes through Synergistic Organophotoredox and Hydrogen Atom Transfer Catalysis.

Bicyclo[1.1.1]pentanes (BCPs) are important in drug design as sp3-rich bioisosteres of arenes and tert-butyl groups; however, the preparation of BCPs with adjacent quaternary carbons is barely known. We report a facile synthesis of α-quaternary BCPs using organophotoredox and hydrogen atom transfer catalysis in which α-keto radicals, generated through oxidation of β-ketocarbonyls, undergo efficient addition to [1.1.1]propellane. The BCP products can be transformed into a variety of useful derivatives, including enantioenriched BCPs featuring α-quaternary stereocenters.

Synthesis of β-Dicarbonylated Tetrahydropiperidines via Direct Oxidative Cross-Coupling between Different C(sp3)-H Bonds.

A novel and convenient synthesis of β-dicarbonylated tetrahydropiperidines from the cascade reactions of piperidine derivatives with methyl ketones is presented. Mechanistically, the formation of the title compounds involves a hitherto unreported oxidative cross-coupling between different C(sp3)-H bonds through the in situ generation of a cyclic enamine and an α-keto radical as the key intermediates followed by their radical addition and further oxidation of the addition adduct. To our knowledge, this is the first example in which β-dicarbonyl-substituted tetrahydropiperidines were prepared using readily available saturated cyclic amines as substrates and inexpensive and ubiquitous methyl ketones as dicarbonylation reagents. In addition, the product thus obtained could be easily transformed into structurally and pharmaceutically interesting polycyclic compounds in good efficiency.

Theoretical Calculations for the 1,4-Hydrogen Shift of 1-Hydroxyallyl Radicals Leading to α-Keto Radicals; Prediction of Facilitation by 1-Amino and 3-Tin Substituents

DFT calculations were carried out to elucidate the 1,4-hydrogen shift of 1-hydroxyallyl radicals to give α-keto radicals. The 1,4-H shift is predicted to be exothermic with large energy barriers. H...

Stabilizing influence of perfluoro-tert-butanol on α-keto radicals — the derivatives of perfluorinated α-diketones: quantum-chemical calculations of the formed complexes

Density functional theory calculations show that the heightened stability of α-keto radicals, the derivatives of perfluorinated α-diketones of general formula CF3C(O)C(O)RF (RF = i-C3F7; t-C4F9), in perfluoro-tert-butanol can be caused by the H-bond-imposed formation of cation radicals of vicinal dienols.

ChemInform Abstract: Free Radical Trapping of α-Keto Radicals Derived from α,. beta.-Epoxy Ketones via Photoinduced Single Electron Transfer Process.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Efficient access to functionalised medium-ring systems by radical fragmentation/radical addition to α-iodoketones

Functionalised medium-ring systems of various sizes can be efficiently prepared by a novel approach that embodies a radical-induced fragmentation of bicyclic β-hydroxy ketones, followed by a second radical-coupling reaction. The unexpected reactivity of α-keto radicals is also discussed.

Preparations of Furans from α-Bromo Ketones and Enol Ethers Catalyzed by a Rhenium(I) Nitrogen Complex

Abstract By the catalytic use of a rhenium(I) nitrogen complex, [ReCl(N2)(PMe2Ph)4], α-keto radicals are generated from α-bromo ketones and react with vinyl ethers and silyl enol ethers intermolecularly. Various substituted furans, including tetrasubstituted furans such as furoguaiacin, are prepared by this method.

Mn(III)-based oxidative free radical cyclizations of unsaturated 2-cyclohexenones and aldehydes

Oxidative free-radical cyclizations of unsaturated 2-cyclohexenones 9a, 9b, 17, and 24 with Mn(OAc) 3 afford unsaturated α′-keto radicals such as 10 that cyclize to afford bicyclic products. The major process with 2-cyclohexenone 27 is conjugate addition of acetate to form β-acetoxy α-keto radical 37. Unsaturated aldehydes 45, 57a, and 57b are oxidized to radicals that cyclize to give cyclopentane- and cyclohexanecarboxaldehdyes.

Cascade radical mediated macrocyclisation-transannulation reactions leading to ring-fused bicycles

The scope for tandem radical mediated macrocyclisation–transannulation processes in the elaboration of ring-fused carbocycles has been examined. Thus, a range of E-iodo dienones viz. 21, 30b, 40, 42 and 44 were first prepared using synthetic sequences based on sound literature precedent. Treatment of the iodo dienone 21 with Bu3SnH–AIBN led to a 3 : 2 mixture of trans-and cis-isomers of 1-decalone, 35 and 36, respectively, in a combined yield of 72%, whereas the positional isomer 30b of 21 underwent 10-endo macrocyclisation and transannulation to a 1 : 1 mixture of trans-1-decalone 35 and cis-octahydroazulen-1-one 39 (combined yield 68%), resulting from competitive 6-exo/5-exo transannular cyclisation from the intermediate cyclodecenone radical 38.In further investigations of the scope for sequential radical macrocyclisation–transannulations in the synthesis of bicyclic systems, the iodo dienone 40 was found to undergo regioselective cyclisation to the cis-tetralone 41(50%), whereas the iodo dienone 42 produced only (Z)-cyclooct-3-enone 54 and none of the expected bicyclo[3.3.0] octanone 43 on treatment with Bu3SnH–AIBN. Only the 4-cyclo-pentylcyclohexanone 61, and none of the hoped for 7,6-bicyclic ketone 45, was produced from radical cyclisation of the iodo dienone 44.The differing reaction pathways followed by the iodo dienones 21, 30b, 40 and 42 have been rationalised in terms of the conformational preferences of the macrocyclic α-keto radical intermediates, e.g. 32, 38 and 52 involved in the various cyclisations supported by some preliminary MM2 studies and calculations.

Manganese(III)-based oxidative fragmentation-cyclization reactions of unsaturated cyclobutanols

Allylic cyclobutanols 1, 10, 21, 27, 32, 39, 51, and 58 are oxidatively fragmented by Mn(OAc) 3 -2H 2 O in EtOH to give tertiary radicals as shown in eq 1. These tertiary radicals undergo both 6-endo-cyclization to the α,β-unsaturated ketone to afford α-keto radicals and 5-exo-cyclization to provide β-keto cyclopentylmethyl radicals. The α-keto radicals produced by 6-endo-cyclization are reduced to ketones or dimerize. The β-keto radicals formed by 5-exo-cyclization are oxidized by Cu(OAc) 2 to yield methylenecyclopentanones and rearrange to give 3-oxocyclohexyl radicals that are oxidized by Cu(OAc) 2 to afford cyclohexenones. Acetylenic cyclobutanols 47, 49, and 65 are oxidatively fragmented by Mn(OAc) 3 .2H 2 O in EtOH to provide tertiary radicals that cyclize to give α-keto vinyl radicals, which abstract a hydrogen atom to yield methylenecyclopentanones. Pentenylcyclobutanol 75 is oxidatively fragmented by Mn(OAc) 3 .2H 2 O to afford tertiary radical 74, which cyclizes to furnish cyclopentylmethyl radicals 77 and 78, which are oxidized by Cu(OAc) 2 to methylenecyclopentanes 76 and 79

The mechanism of gamma-radiolysis of polymethylene, polypropylene and poly-n-butylene oxides: An ESR investigation

The basic features of the mechanism of the solid state radiolysis of the title polyethers have been elucidated with an ESR study using the matrix isolation and spin-trapping techniques. The species identified at 77 K are α-alkoxy radicals arising from hydrogen abstractions probably involving the primary ether cation-radicals; chain scission radicals are formed on warming as a consequence of the thermal decomposition of the α-alkoxy radical precursors. The formation of acyl and α-keto radicals, which is observed on warming near room temperature, is identified with cage hydrogen abstraction reactions taking place between the products of the β-scission of the α-alkoxy radicals. Direct information on the stability and ESR properties of the polymeric ether cation-radicals, which are thought to be important intermediates in the radiolysis mechanism, have been obtained by irradiation in the CFCl 3 matrix.

Free radical trapping of α-keto radicals derived from α,β-epoxy ketones via photoinduced single electron transfer process

Irradiation of aromatic α,β-epoxy ketones in the presence of allyltributyltin afforded α-allyl-β-hydroxy ketones by a single electron transfer mechanism.

ChemInform Abstract: PHOTOREACTIONS OF α‐SULFONYLOXYKETONES

AbstractDie Tosyloxyketone (I), (IX), (XII) und (XV) (dargestellt in 5 l ‐66proz. Ausb. aus den entsprechenden Säurechloriden über die Diazoketone (Ausb. 76‐88%)) gehen bei der Photolyse in Abhängigkeit vom Lösungsmittel Umlagerungen, intramolekulare Cyclisierungen und Lösungsmitteleinfang‐ Reaktionen ein.

Photoreactions of α-sulfonyloxyketones

Earlier work on the photochemistry of α-sulfonyloxyketones uncovered their propensity to form the corresponding α-keto carbonium ions on photolysis. Typical reactions such as intramolecular cyclizations, carbonium ion rearrangements, and solvent trapping were found. This paper describes an attempt to extend this reaction to a general synthetic technique for polyene cyclizations. During the course of this work two other major reaction pathways were found. The first is an α-keto carbonium ion to acylium ion rearrangement and the second is a photoreduction–elimination process that leads to an α-keto radical rather than the corresponding cation.