Lead reduction, initiatives electronic products

In addition to simple electron transfers in which no chemical bond is either broken or formed, numerous organic reactions, previously formulated by movements of electron pairs, are now understood as processes in which an initial electron transfer from a nucleophile (reductant) to an electrophile (oxidant) produces a radical ion pair, which leads to the final products via the follow-up steps involving cleavage and formation of chemical bonds [11-23], The follow-up steps are usually sufficiendy rapid to render the initial electron transfer the rate-determining step in an overall irreversible transformation [24], In such a case, the overall reactivity is determined by the initial electron-transfer step, which can also be well designed based on the redox potentials and the reorganization energies of a nucleophile (reductant) and an electrophile (oxidant). 

In the electrochemical reduction of aryl diethyl phosphates, the initial one-electron addition is to the aryl ring and this is concerted with expulsion of the diethyl phosphate anion leaving an aryl radical. Further electron addition and protonation leads to the reaction product in 43-73 % yields. Examples of this electro-... 

Two major side reactions compete with the coupling reaction protonation of the radical anion followed by further reduction and protonation leading to the saturated dihydro product, and polymerization induced by the basic dianion formed by coupling of two radical anions. Other, less typical reaction pathways include reaction between a radical anion and a molecule of substrate. Scheme 2, dimerization of two radicals formed by protonation of the initial radical anion. Scheme 3, or, infrequently, cleavage of the radical anion followed by coupling. However, for radical anions derived from monoactivated alkenes, the pathway in Scheme 2 has only been unequivocally established as a major pathway in a few cases in which the final zero-electron product is a cyclobutane, that is, a cycloaddition product. 

Benzene was introduced in Chapter 5 (Section 5.10). Chapter 21 will discuss many benzene derivatives, along with the chemical reactions that are characteristic of these compounds. In the context of dissolving metal reductions of aldehydes, ketones, and alkynes, however, one reaction of benzene must be introduced. When benzene (65) is treated with sodium metal in a mixture of liquid ammonia and ethanol, the product is 1,4-cyclohexadiene 66. Note that the nonconjugated diene is formed. The reaction follows a similar mechanism to that presented for alkynes. Initial electron transfer from sodium metal to benzene leads to radical anion 67. Resonance delocalization as shown shordd favor the resonance contributor 67B due to charge separation. 

The reduction of the nitro group of 3-nitrophthalhydrazide (20) with dithio-nate is mechanistically complex, but a possible sequence of events is illustrated in Scheme 20.5, with ArN02 representing 20. The reaction presumably is initiated by transfer of an electron from dithionate to produce the anion radical 23. Subsequent steps involving protonation and addition of a second electron afford the N,N-dihydroxy intermediate 24, which can dehydrate to produce the nitroso compound 25. Further addition of electrons from dithionate, protonation, and loss of water from the hydroxylamine 26 leads to the reduced product 15. 

The species formed by electron transfer may not be stable in the electrolysis medium it may only be an intermediate which undergoes chemical change to form the observed product. In favourable conditions there may be a single reaction pathway leading to one product, but with organic intermediates it is common for there to be competitive reactions leading to a mixture of products, e.g. in the reduction of p-iodonitrobenzene in aqueous acid, the anion radical initially formed can either lose iodide ion or protonate, leading to different final products. 

Treatment of n-type polyacetylene with any of a variety of alkyl halides in THF typically leads to a dramatic swelling of the film and the formation of an orange-red, soluble fraction. The amount of alkylation of the insoluble but swellable films corresponds to ga- 0.9-1.3 alkyl groups per 10 CH units. Studies of the amount of alkylation of the polyacetylene and the amount of reduction products (R-H and R-R) formed as a function of alkyl halide type leads us to conclude that the mechanism involves initial electron transfer (perhaps more appropriately an electron shift) to R-X as shown in Scheme I. 

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

In addition to the dark oxidation of S(IV) on surfaces, there may be photochemically induced processes as well. For example, irradiation of aqueous suspensions of solid a-Fe203 (hematite) containing S(IV) with light of A > 295 nm resulted in the production of Fe(II) in solution (Faust and Hoffmann, 1986 Faust et al., 1989 Hoffmann et al., 1995). This reductive dissolution of the hematite has been attributed to the absorption of light by surface Fe(III)-S(IV) complexes, which leads to the generation of electron-hole pairs, followed by an electron transfer in which the adsorbed S(IV) is oxidized to the SO-p radical anion. This initiates the free radical chemistry described earlier. 

---