A hydrogen atom transfer (HAT) step from acetone allowed the smooth generation of acetonyl radical that was then exploited as synthon in the mild formation of C–C bonds under flow conditions. The process was promoted by aryl radicals photocatalytically generated via single-electron transfer (SET) reduction of arenediazonium salts. The mechanism has been investigated by a combined experimental and computational approach and further supported by deuterium labeling experiments.

Photoredox-Catalyzed Generation of Acetonyl Radical in Flow: Theoretical Investigation and Synthetic Applications

Recoverable Phase‐Transfer Catalysts with Fluorinated Anions: Generation and Reactions of Dichlorocarbene and CCl3 Anion in the Heterogeneous System KOH(s)/CHCl3/nBu4NPF6

Copper(I)-catalyzed solvolysis of gem-chlorofluoro- and gem-bromofluorocyclopropanes. Preparation of 2-fluoroallylic ethers, esters and alcohols

A simple preparative method has been developed for the synthesis of aryl(furfuryl) ketones, amides, and furylacetic acid esters, based on radical alkylation of furan derivatives at the α-position with О-ethyl(phenacyl)xanthogenates and phenacyl iodides in the presence of Fenton's reagent (H2O2/FeSO4 · 7H2O) in DMSO. The range of applicability and mechanisms for the formation of major and side products have been considered.

A simple method for the synthesis of furfuryl ketones and furylacetic acid derivatives

In the past decade, major advances in radical chemistry have been made by the use of transition metals. This field has witnessed impressive accomplishments, and tremendous potential lies ahead. There are many transition metal-mediated methods for producing radicals, including (a) oxidation of CH bonds or unsaturated fragments by transition metals in higher oxidation states, (b) reduction of CX bonds or unsaturated moieties by transition metals in lower oxidation states, and (c) homolysis of metal–carbon σ bonds. Redox methods for generating radicals are well elaborated and utilize transition metals in different oxidation states. In the vast majority of these reactions, transient organometallic species have not been either isolated or identified. Their tentative structures have been proposed, in some cases based solely on chemical logic. Accordingly, the mechanisms of these multistep interactions have not been fully established. Particularly lacking is a clear recognition of those elementary steps that occur inside the ligand sphere of the transition metal.



In comparison with traditional methods of radical generation, redox initiators demonstrate remarkable regioselectivity and are especially efficient in polyfunctional organic compounds. Furthermore, new types of radicals, inaccessible by traditional approaches, can be successfully generated. The main difference lies in the multiple roles that metal oxidants play during the reaction, namely, one-electron transfer between proradical and transition metal to produce radical species, followed by redox interaction with intermediate adduct radicals. For this reason, metal-mediated reactions differ significantly from those of peroxide- or light-initiated processes.



Trivalent manganese occupies a rather unusual place among metal oxidants in higher oxidation states and is particularly useful in this field. Numerous novel regio-, chemo-, and stereoselective synthetic methods have been developed in both inter- and intramolecular reactions, and their applicability to the construction of complex natural and biologically active compounds has been demonstrated. Despite its growing significance for synthetic chemistry, manganese(III)-mediated reactions have not been comprehensively reviewed in recent years.



The subject of this chapter is the radical carbon–carbon bond-forming reaction induced by trivalent manganese derivatives such as Mn(OAc)3,Mn(acac)3, and Mn(pic)3. It includes the oxidative generation of α- or α,α-dioxoalkyl or alkyl radicals and their subsequent addition to unsaturated moieties. Both inter- and intramolecular processes are discussed, with special emphasis on the regio-, chemo-, and stereoselectivity issues, as well as natural products syntheses. A comprehensive representation of experimental data is accompanied by critical analyses to give a reader adequate ideas of the current status of this field, of what and can be achieved, of what can be anticipated in any new reaction or in any new application of a known process, and of predictions that can be made based on the collective accumulated knowledge. Discussions of oxidations of unsaturated compounds such as arenes and alkenes, of the α-acetoxylation and α-chlorination of ketones, and of chlorination of alkenes are beyond the scope of this review.


Keywords:

mechanism;
generation;
radicals;
formation;
reactivity patterns;
kinetics;
one multiple bond;
regioselectivity;
steroeselectivity;
conjugated systems;
aromatics;
scope;
limitations;
chemoselectivity;
addition-cyclization reactions;
intramolecular;
tandem reactions;
polycyclization reactions;
manganese chemistry;
new directions;
natural product synthesis;
pheromones;
terpenes;
terpenoids;
bioactive compounds;
formal synthesis;
experimental conditions;
experimental procedures;
tabular survey

Carbon–Carbon Bond‐Forming Reactions Promoted by Trivalent Manganese

1.

Anisole, toluene, chlorobenzene, ethyl benzoate, and benzonitrile have been substituted in the aromatic nucleus by the CH3COCH2 radical, generated in the redox system acetone-Mn(OAc)3-AcOH at 70°. The acetonyl radical displays electrophilic properties in this reaction.




2.

The radical reaction of acetone with thiophene proceeds selectively at the α-carbon atom of the thiophene ring to form 1-(2-thienyl)propan-2-one.

Radical reaction of acetone with aromatic compounds

1

The reaction of 1-fluoro-1-bromo-2-arylcyclopropanes with methanol occurs predominately with retention of the three -membered ring in the presence of bases and exclusively via ring cleavage in the presence of AgNO3




2

syn-1-Fluoro-1-bromo-2-arylcyclopropanes do not react in the presence of AgNO3 at 50°C, but are more reactive than the anti isomers in the presence of bases




3

Electron-donor substituents on the aromatic ring of 1-fluoro-1-bromo-2-arylcyclopropanes accelerate the reaction in the presence of electrophilic agents and inhibit the reaction on addition of a base




4

Logarithms of the relative rate constants for the reaction of 1-fluoro-1-bromo-2-arylcyclopropanes with methanol in the presence of AgNO3 correlate with Brown's σ* constants




5

The reaction of 1-fluoro-1-bromo-2-arylcyclopropanes with alcohol in the presence of bases occurs via sequential elimination and addition

Reactions of 1-fluoro-1-bromo-2-arylcyclopropanes in solvolysis conditions in the presence of electrophilic and nucleophilic agents

1.

Homolytic substitution in the heteroaromatic ring of furan,α-methylfuran, and thiophene by CH3COCH2 and CH3 COCHCOCH3 radicals generated from acetone and acetylacetone using Mn(III) acetate (a one-electron oxidizing agent) proceeds regiospecifically at theα-carbon atom of the ring and leads to 2-furyl- and 2-thienyl ketones.




2.

Acetoxylation and C-C or C-O dehydrodimerization occurs in the oxidation of 2-furyl- and 2-thienyl ketones by Mn(III) acetate. The reaction pathway depends on the concentration of Mn(OAc)3.

Homolytic oxoalkylation of furan, α-methylfuran, and thiophene

Kinetic data indicate that the principal path followed in the oxidative addition of acetone to aromatic compounds (ArH) under the action of Mn(III) acetate in AcOH solution to be the formation of a [CH2=C(CH3)-OMnIII·ArH] complex, which breaks down with rupture of the MnIII-O bond and subsequent addition of the acetonyl radical to the coordinated ArH (limiting step); this is, in turn, followed by rapid one-electron oxidation of the intermediate adduct radical by the Mn(III) ion, and formation of the final product.

Kinetics of the oxidative addition of acetone to aromatic compounds under the action of Mn(III) acetate

1.

We have determined the stereoisomeric composition of the products of the reaction of fluorobromocarbene with mono- and 1,2-disubstituted ethylenes. Addition of fluorobromocarbene to arylethylenes with monosubstituted rings forms a slight excess of the anti-isomer, while addition to arylethylenes with 2,6-di-, 2,4,6-tri-, and pentasubstituted rings gives an excess of the syn-isomer.




2.

Introduction of a methyl group at theβ-position of styrene increases the proportion of the isomer containing bromine and methyl on the same side of the cyclopropane ring by a factor of 2.5.

V. S. Aksenov

Papers

Radical reaction of acetone with aromatic compounds

Reactions of 1-fluoro-1-bromo-2-arylcyclopropanes in solvolysis conditions in the presence of electrophilic and nucleophilic agents

Homolytic oxoalkylation of furan, α-methylfuran, and thiophene

Kinetics of the oxidative addition of acetone to aromatic compounds under the action of Mn(III) acetate

The stereoisomeric composition of the products of addition of fluorobromocarbene to arylethylenes