Electron transfer ketones

As with other hydroperoxides, hydroxyaLkyl hydroperoxides are decomposed by transition-metal ions in an electron-transfer process. This is tme even for those hydroxyaLkyl hydroperoxides that only exist in equiUbrium. For example, those hydroperoxides from cycHc ketones (R, R = alkylene) form an oxygen-centered radical initially which then undergoes ring-opening -scission forming an intermediate carboxyalkyl radical (124) ... 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

The addition followed a radical chain mechanism initiated by photoinitiated electron transfer from the tertiary amine to the excited aromatic ketone and occurred with complete facial selectivity on the furanone ring (99TL3169). The yields increased and best results were obtained with sensitizers (4-methoxyacetophenone,... 

Cyclic and acyclic silyl enol ethers can be nitrated with tetranitromethane to give ct-nitro ketones in 64-96% yield fEqs. 2.42 and 2.43. " The mechanism involves the electron transfer from the silyl enol ether to tetranitromethane. A fast homolydc conphng of the resultant cadon radical of silyl enol ether with NO leads tn ct-nitro ketones. Tetranitromethane is a neutral reagent it is commercially available or readdy prepared. " ... 

In 1970, a new reacdon, the displacement of a nitro group from ct-nitro esters, ct-nitro nitnles, ct-nitro ketones, iind ct,ct-dinitro compounds by nitroalkiine salts, was described. These displacements, which are exemplified by the reacdon presented in Eq. 7.1, take place at room temperanire iind give excellent yields of pure products. The reacdon proceeds via a radiciil chain mechanism involving one electron-transfer processes as shovmin Scheme 7.1 the details of the mechanism are described in a review. ... 

Visible light systems comprising a photoreducible dye molecule e.g. 87)293 or an a-diketone e.g. 85)2% and an amine have also been described. The mechanism of radical production is probably similar to that described for the ketone amine systems described above (i.e. electron transfer from the amine to the photoexcited dye molecule and subsequent proton transfer). Ideally, the dye molecule is reduced to a colorless byproduct. 

The principles outlined above are, of course, important in electro-synthetic reactions. The pH of the electrolysis medium, however, also affects the occurrence and rate of proton transfers which follow the primary electron transfer and hence determine the stability of electrode intermediates to chemical reactions of further oxidation or reduction. These factors are well illustrated by the reduction at a mercury cathode of aryl alkyl ketones (Zuman et al., 1968). In acidic solution the ketone is protonated and reduces readily to a radical which may be reduced further only at more negative potentials. 

Thus a single two-electron wave is observed and only one product, the alcohol, can be isolated. Finally, at high pH neither the ketone nor the radical anion are protonated by this basic medium and it is not until the dianion, formed by successive electron transfers, that protonation occurs. 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Diols (pinacols) can be synthesized by reduction of aldehydes and ketones with active metals such as sodium, magnesium, or aluminum. Aromatic ketones give better yields than aliphatic ones. The use of a Mg—Mgl2 mixture has been called the Gomberg-Bachmann pinacol synthesis. As with a number of other reactions involving sodium, there is a direct electron transfer here, converting the ketone or aldehyde to a ketyl, which dimerizes. 

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

SRNl substitution include ketone enolates,183 ester enolates,184 amide enolates,185 2,4-pentanedione dianion,186 pentadienyl and indenyl carbanions,187 phenolates,188 diethyl phosphite anion,189 phosphides,190 and thiolates.191 The reactions are frequently initiated by light, which promotes the initiating electron transfer. As for other radical chain processes, the reaction is sensitive to substances that can intercept the propagation intermediates. 

This oxidative process has been successful with ketones,244 esters,245 and lactones.246 Hydrogen peroxide can also be used as the oxidant, in which case the alcohol is formed directly.247 The mechanisms for the oxidation of enolates by oxygen is a radical chain autoxidation in which the propagation step involves electron transfer from the carbanion to a hydroperoxy radical.248... 

These last two points are consistent with electron transfer from the tertiary amines to the triplet ketone in competition with hydrogen abstraction from benzhydrol ... 

In contrast, the use of SmI2/DMPU as an electron-transfer system led to the smooth production of spiroketone 3-171a in 79% yield, without any side products (Scheme 3.45). It was also possible to cydize the unprotected cydopropyl ketone 3-170b to give the spiroketone 3-171b in 57% yield. 

The use of samarium(II) iodide in synthesis permits the assembly of complex molecules as already shown in many examples. They profit from the electron-transfer ability of samarium(II) iodide thus, if ketones are employed as substrates the furnished ketyl-radical can react in a multitude of different ways. 

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

Various diorganozinc compounds (ZnR2 R = Me, Et, Pr, Pr1, Buc, Ph) reacted with o-quinones by two mechanisms, namely (i) a single-electron transfer from ZnR2 to the quinone to yield, after hydrolysis, alkyl(phenyl)oxyphenols, and (ii) a polar 1,2- and 1,4-addition of ZnR2 similar to those of conjugated ketones.201 Diorganozinc compounds with low ionization potentials favor a polar mechanism. 

Enol silyl ethers (ESE) as electron donors 199 Ketones as electron acceptors 212 Electron transfer as the unifying theme 218... 

Mattay et al.5i suggested from the photoreaction of biacetyl with highly electron-rich olefins that an initial electron transfer from an electron-rich olefin to photoexcited ketone is the key step in the oxetane formation via the ion-radical pair (equation 26). 

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

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