Aromatic carbanionic chemistry

The polycyclic anions were first prepared by metal reduction in 1914 by Schlenk et al.5 a) and studied later by Schlenk and Bergmann 5 b). This class of conjugated anions opened a new era in carbanion chemistry by pointing out the electron transfer process as a source for charged species. The mechanism of the metal reduction of polycyclic hydrocarbons has been investigated and is well established 1,215 18-68>. The addition of two electrons to the fully conjugated (4n + 2)n-molecules yields 4mt paratropic systems 2°. 137 139>. The chemistry of this reaction is simple, with electrons initially on the alkali metal going to 7t-molecular orbitals associated with the aromatic hydrocarbon molecule (Eq. 13). 

Fe(III) salts are known to oxidise electron-rich centres to foster the formation of radical species. They are particularly efficient in the oxidation of aromatic systems or a carbanion to the corresponding carbon-centred radical which undergoes C-C bond formation to yield the coupled products. For a successful synthesis, it is important to work in the absence of reactive synthetic molecules other than those which form the combination of radicals. Barton et al. used a simple water-soluble diselenide derivative that shows radical scavenger properties towards alkyl and hydroxyl radicals in Fenton-type chemistry (Fe2+-H202)4 The reaction rate between the produced alkyl radical and the diselenide overwhelms self-termination and halogen transfer reactions. The ability of diselenide to scavenge alkyl and hydroxyl radicals [ 3(0 °C) = 6.1 x 108 M-1 s-1] could be exploited as a new tool in both synthetic and mechanistic work conducted in aqueous media (Scheme 8.5).4... 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

The major changes in the format and contents of this volume have been necessitated by economic considerations. Sulphur-containing heteroaromatic compounds which previously were included in these Reports and in the Reports on Aromatic and Heteroaromatic Chemistry are now covered in detail only in the new Reports, Heterocyclic Chemistry . The literature on these compounds has been reviewed up to March 1978 in Volume 5 of this series and thereafter in Volume 1 of the new series. Highlights of the literature of these compounds are given as Chapter 6 of this volume. The organization of the remainder of this volume is essentially the same as that of Volume 5 except that the section on the ylides and carbanionic compounds of selenium and tellurium has been considerably extended and the coverage of Chapter 3, Part III has been extended to include dithiocarbamates, xanthates, and trithiocarbonates. 

From a theoretical point of view, the key issue has been the basic nature of the metalation step, where the R groups moves from a R -H bond to a M-R bond. C-H activation is very common in organic chemistry as it allows the formation of functionalized hydrocarbons. Different mechanisms had been proposed for this metalation step, including electrophilic aromatic substitution, a-bond metathesis, oxidative addition/reductiveelimination and Heck-like insertion. Theoretical studies have facilitated narrowing the mechanistic possibilities to two main options oxidative addition/reductive elimination and proton abstraction by a base. In the oxidative addition/reductive elimination process the metal is inserted in the C-H bond with formal increase in the oxidation state of the metal, and the hydride leaves the metal coordination sphere of the metal afterwards. In the proton abstraction mechanism, the metal does not interact directly with the proton, which is captured by a base, with simultaneous formal creation of a carbanion that binds to the metal center. The mechanism of the reaction will depend on the presence of a base able to abstract the proton and of the existence of an energetically accessible oxidation state for the metal. 

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