Lennart Eberson

Radical ion reactivity—I : Application of the dewar-zimmerman rules to certain reactions of radical anions and cations

Abstract The application of the Dewar-Zimmerman rules to the interaction between radical cations, derived from 4n + 2 systems, shows that a nucleophile orbital interacting suprafacially with the ion should correspond to an antiaromatic transition state and hence to a less favored pathway relative to competing ones, e.g. electron transfer. An antarafacial interaction would on the other hand correspond to an aromatic transition state and be energetically favorable. Both types of interaction are generally possible for the same reagent, but since the suprafacial one for geometric reasons ensues and thus predominates in the early stage of the reaction, the net result should be an anomalously low reactivity of radical cations vs attack by nucleophiles. By calibration against known, qualitative reactivity data for perylene radical cation it is strongly indicated that halide ions belong to a class of reagents which do not react nucleophilically with radical cations but instead undergo electron transfer oxidation or do not react at all. This type of reaction is discussed in some detail. and several mechanisms involving radical cation/halide ion combination as a critical step can either be ruled out or considered open for reinvestigation. The same idea can be applied to the reaction between radical anions and electrophiles. Accordingly, the protonation of radical anions derived from 4n+2 aromatic systems is remarkably slow, as compared to that of analogous carbanions.

Structure and Mechanism in Organic Electrochemistry

Publisher Summary This chapter discusses the important aspects of structure–reactivity relationships and mechanisms in organic electrochemistry proper—the aspects relating to electrochemical reactions of organic compounds. The experimental prerequisites for running an electrochemical experiment with an organic system are simple: a solvent, capable of dissolving both the organic substrate and an electrolyte added to give a reasonably highly conducting medium (the electrolyte solution), two electrodes (the anode and the cathode), made from metallic materials but sometimes also from semiconducting ones, and a source of electric power to apply across the electrolyte solution via the two electrodes. Electrode processes are classified according to the nature of the final product and its formal mode of formation. The nature of the mechanistic problems originating from the reaction conditions peculiar to electrochemical experiments is discussed in the chapter.

‘Inverted spin trapping’. Reactions between the radical cation of α-phenyl-N-tert-butylnitrone and ionic and neutral nucleophiles

Tris(4-bromophenyl)aminium ion, TBPA˙+, is both an efficient electron transfer oxidant and an electrophile. Its reactivity toward nucleophiles reflects this dichotomy, in that bond formation occurs with nucleophiles that are not readily oxidized, such as carboxylate, cyanide and chloride ion, whereas less oxidation-resistant nucleophiles, such as bromide, iodide and trinitromethanide ion react with electron transfer.This property has been utilized to study spin-trapping reactions with α-phenyl-N-tert-butylnitrone (PBN) under oxidative conditions. The reaction between TBPA˙+ and a solution of PBN and nucleophile gave the corresponding spin adducts from both categories of nucleophiles, provided the nucleophilic/solvolytic reactivity of the spin adduct was not too high. Spin adducts from the oxidation-resistant nucleophiles [F–, CN–, CNO–, pyridine(s), succinimidate(s), triethyl phosphite, RCO2–] must then be formed in the reaction between the nucleophile and the radical cation of PBN, generated in an initial electron-transfer step (‘inverted spin trapping’). Only in the case of the more easily oxidizable nucleophiles [SCN–, (NO2)3C–, N3–] does proper spin trapping occur, i.e., the radical is formed by the TBPA˙+/nucleophile reaction and is then trapped by PBN.

The Marcus theory of electron transfer, a sorting device for toxic compounds

Abstract The Marcus theory for outer-sphere (non-bonded) electron transfer reactions is presented. In addition, it is applied to the problem of identifying compounds capable of generating radical ions and/or radicals via fast electron transfer to or from redox proteins. The use of the Marcus treatment is illustrated by several examples involving different types of xenobiotics.

Catalysis by electron transfer in organic chemistry

Abstract Like a proton or a base, an electron or a positive hole can act as a very efficient catalyst in an organic reaction that in itself does not involve a change in oxidation level in going from substrate to product. If an electron is added to or removed from a neutral molecule, a radical anion (cation) is formed (eqn. i), and can undergo reactions at the new oxidation level (eqn. ii). At a later stage of the reaction sequence, a return to the original oxidation level can be achieved by a chain transfer step, i.e. , a chemically different radical anion (cation) can reduce (oxidize) a new molecule of the starting material (eqn. iii). A familiar example of this reaction type is the S RN 1 reaction (developed and studied by Bunnett, Kornblum, Russell and Saveant in 1965–1980) which enables otherwise extremely slow aromatic nucleophilic substitution reactions to take place under mild conditions. Other examples, to be discussed in this paper, include the S OE 1 and S ON 2 reactions, cyclodimerizations and cycloreversions, cycloadditions in general, rearrangements, oligomerizations and ligand exchange reactions. In general, catalytic effects of the order of 10–20 k cal mol can be attained by electron transfer catalysis.

Acetoxylation of aromatic compounds by potassium peroxydisulphate in acetic acid with palladium(II) complexes as catalysts

Aromatic compounds undergo a clean, predominantly meta-directing acetoxylation with potassium peroxydisulphate in glacial acetic acid, using PdII complexes with amines as catalysts.

Making radical cations live longer

By all measures, 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) appears as a solvent with properties at the extreme. Its combination of low nucleophilicity, high hydrogen bonding donor strength, low hydrogen bonding acceptor strength, high polarity and high ionizing power makes it an ideal solvent for radical cations. Applications of HFP as a solvent for EPR spectroscopy and mechanistic studies of radical cations as intermediates in electrophilic aromatic substitution, photochemistry and spin trapping are described.

Oxidation of aromatic compounds by metal ions. Part 8. Structure and selectivity in anodic and metal ion oxidations of polyalkylbenzenes

Positional selectivity and the partition deuterium isotope effect (k/sub H//k/sub D/) have been determined in the chemical (with cerium(IV) ammonium nitrate (CAN), cobalt(III) acetate, and N-bromosuccinimide (NBS)) and electro-chemical side-chain oxidation of alkyl aromatics by using 5-R-hemimellitenes (R = H, t-Bu) and 1,3-dimethyl-2-(trideuterimethyl)-5-tert-butylbenzene as the substrates. Considering also the already available data for isodurene, it has been found that the positional selectivity is strongly influenced by the substrate structure in the anodic and CAN-promoted oxidations, both reactions exhibiting a very similar pattern. In conrast, Co(OAc)/sub 3/ selectivities do not correlate with those of the anodic oxidation but with the selectivities of the side-chain bromination promoted by NBS. These results have been interpreted by suggesting that, as in the anodic oxidations, CAN-induced reactions involve first the formation of a radical cation intermediate which then loses a proton to give a benzylic free radical in the selectivity-determining step. The data for Co/sup III/ would instead suggest a mechanism involving a hydrogen atom transfer, but this conclusion cannot yet be considered definitive. No simple correlation exists between selectivity data and the k/sub H//k/sub D/ values.

Indirect Electrochemical Reduction of Some Benzyl Chlorides.

Etude par voltammetrie cyclique de la reduction des chlorures de benzyle, methoxy-4 benzyle α-methylbenzyle et (α-ethyl α-methyl) benzyle en presence de mediateurs chimiques (arenes, arenes condenses, cetones aromatiques)

The SON2 mechanism : A non-oxidative reaction that is initiated by electron transfer oxidation

Abstract Under oxidizing conditions, aromatic chloro and fluoro compounds undergo what formally are typical nucleophilic substitution reactions with surprising ease. As an example, 4-fluoroanisole is converted the 4-acetoxyanisole by anodic or metal ion oxidative initiation, and the reaction is shown to be a chain process. It is proposed that a mechanism analogous to that of the reductively initiated S RN 1 mechanism operates: The substrate is oxidized to a radical cation by the initiator system, and the radical cation then undergoes ipso attack by the nucleophile. In the third step, the leaving group leaves as a species at the same oxidation level as the nucleophile , giving the radical cation of the product to be formed. A chain transfer step involving this ion and a new substrate molecule then completes the propagation sequence. Previously reported cases of this phenomenon are discussed and the individual steps of the chain reaction are considered in terms of their thermochemistry. It is concluded that the S ON 2 mechanism should be more favoured with easily oxidizable substrates.