Alkyl halides aromatic anion radical reduction

The addition of alkyl halides to aromatic anion radicals, generated by alkalimetal reduction in ethereal solvents, was already known in the 1950s [201] and was reviewed by Garst in 1971 [202]. The first electrochemical analogue was observed by Lund etal. [203]. These authors cathodically reduced hydrocarbons such as naphthalene, anthracene, stilbene [145, 146], and pery-lene [147-150] in the presence of alkyl halides and isolated hydrogenated and alkylated products. Similar reactions are observed when the halides are replaced by ammonium or sulfonium [204]. 

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

Several research groups ha ve been involved in the study of ET reactions from an electrochemically generated aromatic radical anion to alkyl halides in order to describe the dichotomy between ET and polar substitution (SN2). The mechanism for indirect reduction of alkyl halides by aromatic mediators has been described in several papers. For all aliphatic alkyl halides and most benzylic halides the cleavage of the carbon-halogen bond takes place concertedly with the... 

Tertiary alkyl halides are easier to reduce than secondary alkyl halides, which are, in turn, easier to reduce than primary alkyl halides. Ease of reduction of a carbon-halogen bond is governed by the identity of the halogen atom (a) iodides are easier to reduce than bromides, (b) chlorides are so difficult to reduce that they often appear to undergo no direct reduction, and (c) no report of the direct reduction of an alkyl monofluoride has been published. Finally, the existence of the radical anion [RX ], formed by addition of one electron to an alkyl monohalide, has never been demonstrated Andrieux and coworkers [8] have discussed why such a species is not expected for simple alkyl monohalides, but why radical anions of aromatic halides are distinct intermediates in the electrochemical reduction of aromatic halides. Canadell and coworkers [9] have described the implications of theoretical calculations pertaining to the lifetime of the water-solvated radical anion of methyl chloride. 

The process now known as reductive alkylation of rc-conjugated anions (quenching of anions) is as old as the preparation of the ions themselves5). The highly colored solutions obtained by the addition of alkali metals to solutions of aromatic hydrocarbons in ether were reacted with electrohpiles such as protons or alkyl halides (Scheme 2). The products of such a process are reduced hydrocarbons. The Birch reduction is one example oT such a process, reaction of an anion with an alkyl halide leading to an alkylated reduced hydrocarbon is another example 165). The complexity of the quenching experiments is demonstrated by the naphthalene radical anion 150-1581... 

When RX is easily reduced, as in the case of allyl iodides and benzyl bromides, the competing further reduction of the intermediate radical is suppressed and radical reactions such as dimerization, addition to double bonds and aromatic compounds or reaction with anions can be favored. The radical pathway can be also promoted by catalysis with reduced forms of vitamin Bn, cobaloximes or nickel complexes. These react with the alkyl halide by oxidative addition and release the alkyl radical by homolytic cleavage. 

The first use of 5-hexenyl radical cyclization as a mechanistic tool was proposed in 1966 by Garst and Lamb for the study of alkyl halides reduction by naphthalene radical anion. Since then, Garst has extensively used this method in the study of the one-electron transfer reactions from aromatic radical anions to alkyl halides critical discussion of the use of the method may be... 

Although the reduction potentials of aUcenes are too high to permit reduction, it is possible to reduce polycyclic aromatic hydrocarbons and alkenes bearing groups that can stabilize anions in aprotic media to afford the corresponding anion radicals and dianions. These species are good nucleophiles and can be captured by added electrophiles such as protons, alkyl halides, silyl halides, and carbon dioxide (Fig. 7) [11],... 

When steric hindrance in substrates is increased, and when the leaving anion group in substrates is iodide, SET reaction is much induced (Cl < Br < I). This reason comes from the fact that steric hindrance retards the direct nucleophilic reduction of substrates by a hydride species, and the a energy level of C-I bond in substrates is lower than that of C-Br or C-Cl bond. Therefore, metal hydride reduction of alkyl chlorides, bromides, and tosylates generally proceeds mainly via a polar pathway, i.e. SN2. Since LUMO energy level in aromatic halides is lower than that of aliphatic halides, SET reaction in aromatic halides is induced not only in aromatic iodides but also in aromatic bromides. Eq. 9.2 shows reductive cyclization of o-bromophenyl allyl ether (4) via an sp2 carbon-centered radical with LiAlH4. 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

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