Carbanions anodic oxidation

Electrolysis of Grignard reagents in ether produces the saturated and unsaturated hydrocarbons to be expected if the anode oxidizes a real or potential carbanion to the corresponding free radical. 

The success of these reactions was intriguing because, under normal reaction conditions, group 14 organometallic compounds serve as carbanion synthons. The anodic oxidation reaction successfully reversed this polarity thereby expanding the overall synthetic utihty of group 14 organometallic intermediates. 

Anodic oxidation of the carbamates 17 and 23 in methanol, followed by reaction with chlorodiphenylphosphine affords the a-diphenylphosphinylcarbamates 20 and 25, from which the readily generated carbanions react with aldehydes to give the 4-phosphinyl-2-oxazolidinones 21 and 26. The removal of the diphenylphosphinyl group by a mild thermal treatment provides a route to the 2(3//)-oxazolones 22 and 27 (Fig. 5.6). ... 

The tetraphenylallene system represents a particularly elegant confluence of chemistry, electrochemistry, and EPR. The carbanion of 1,1,3,3-tetraphenyl-propene is formed spontaneously in alkaline medium. Or it can be formed by the two-electron reduction of 2-ethoxy-1,1,3,3-tetrapheny 1-prop- 1-ene (at 2.2 V) or of tetraphenylallene (at -2.11 V). The tetraphenylallyl radical is then produced by anodic oxidation of the carbanion at -0.95 V. Its EPR spectrum was obtained using the two-stage flow cell described earlier [37]. 

A typical cyclic voltammetric trace for the anodic oxidation of the fluorenyl anion 2 at platinum is shown in Figure 1. The oxidation potential for this and several other resonance stabilized carbanions lies conveniently within the band gap of n-type Ti02 in the non-aqueous solvents, and hence in a range susceptible to photoinduced charge transfer. Furthermore, dimeric products (e. g., bifluorenyl) can be isolated in good yield (55-80%) after a one Faraday/mole controlled potential (+1.0 eV vs Ag quasireference) oxidation at platinum. 

As the oxidation potential clearly shows, carbanions may easily be oxidized by the anodic method. The most typical process of the anodic oxidation of carbanions is the formation of radical species (equation 52). 

The anodic oxidation of a-sulfonyl carbanions is shown to involve a coupling reaction between the electrogenerated radical and the parent anionic species. The relevance of this mechanism to transition metal oxidations of ot-sulfonyl carbanions is examined [224d]. 

Only few studies of the stereochemistry of anodic oxidative dimerization of active hydrogen compounds (carbon acids) and their salts (carbanions) have been reported. 

A sequence of anodic oxidation followed by metallation provides quinone carbanions which allow specific electrophilic substitution of quinones.5°0CH3... 

For these reasons, we preferred a technique using an undivided cell with a sacrificial anode made of a rearhly oxidized metal. Anodic oxidation of metals has been previously shown to provide organometallic compounds from either preformed carbanions [22], or carbanions formed in situ by cathodic reduction of organic halides... 

Radicals are generated at the anode by oxidation of carbanions (Scheme lb), for example, alkoxides and carboxylates (see Chapter 5, 6), and at the cathode by reduction of protonated carbonyl compounds or onium salts (Scheme Ic) (see Chapter 7). Thereby, a wide choice of different radical structures can be mildly and simply... 

Previous work in our laboratory (3) and in others (4) has established that the primary photoprocess in a variety of excited carbanions involves electron ejection. This photooxidation will generate a reactive free radical if recapture of the electron is inhibited. Parallel generation of these same carbon radicals by electrochemical oxidation reveals an irreversible anodic wave, consistent with rapid chemical reaction by the oxidized organic species (5). Little chemical characterization of the products has been attempted, however (6). 

The propanedioate (malonate) carbanion can be oxidized directly at an anode to give ethanetetracarboxylate esters, presumably via a radical intermediate.2-4 Competing oxidation of solvent leads to a mixture of products3,4 and for preparative purposes it is advantageous to carry out the reaction via indirect electrolysis as reported here. Indirect electrolysis refers to the continuous generation and regeneration of a reagent at an electrode, which inter-... 

Carbanions (those with the largest use are Grignard reagents, organolithians, or anions at the vicinal position to an unsaturated system) are easily oxidized. The anode material may be platinum, vitreous carbon, graphite, as well as titanium oxide. 

This carbanionic species is associated to a metal cation either initially present in the medium as a dissociated salt (a supporting electrolyte) usually a quaternary ammonium salt or simultaneously electrogenerated from an easily oxidable metal used as the anode, e.g. Mg or A1 (Eq. (2)). This method has been called the sacrificial anode process. 

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