Radical anions from dienes

In some instances, the preparation can be effected in the presence of the substrate to be deprotonated. From carboxylic acids, the dianions form in a few minutes at room temperature. The reaction is easily visualized in many cases the first 

Better yields are recorded from oo-chloroacids because of the electrophilic assistance by the generated lithium chloride, while bromides, iodides, or tosylate give poor results. 2 vVhen applied to a dipeptide precursor (Fig. 13), experiments using the sonochemically in situ generated LDA exhibit a higher stereoselectivity than with a conventionally prepared reagent. 

This is one of the rare examples of the influence of sonication on the stereoselectivity of a reaction, the origin of which is unclear. 

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The aza-di-TT-methane (ADPM) rearrangement of aza-1,4-dienes via radical-cat-ions suggests the possibility that other radical-ion intermediates (e.g., radical-anions) could also be responsible for this rearrangement reaction. In order to test this proposal, the azadiene 101 was irradiated for 20 min in acetonitrile using A,iV-dimethylaniline (DMA) as an electron-donor sensitizer. The reaction leads to formation of the cyclopropylimine 102. Separation of product mixture by column chromatography on silica gel affords the aldehyde 34 (21%) resulting from hydrolysis of the imine 102, (Scheme 18) [70]. 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

Fig. 17.82. Birch reduction of benzenes give 1,4-cyclohexa-dienes. The radical anion C is formed by capture of a solvated electron in an antibonding n -orbital of an aromatic compound. The alcohol protonates this radical anion to the radical D, which captures another electron from the solution to form the carbanion E. The carbanion is protonated by a second equivalent of the alcohol, and the 1,4-dihydroaromatic compound results.

A [2 + 2] photocycloaddition with two alkenes can also be induced by photochemical electron transfer [16,17]. In such cases, sensitizers are frequently used and the reactions therefore occur under photocatalysis [18]. Under photochemical electron transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12-membered cyclic polyether is built up. The reaction starts with excitation of the sensitizer 1,4-dicyanonaphthalene (DCN) only 0.1 equivalents of the sensitizer are added to the reaction mixture. Electron transfer occurs from the substrate 10 to the excited sensitizer, leading to the radical cation I. This intermediate then undergoes cycli-zation to the radical cation of the cyclobutane (II). Electron transfer from the radical anion of the sensitizer to the intermediate II leads to the final product 11, and regenerates the sensitizer. In some cases, for example the cydodimerization of N-vinylcarbazole, the effidency is particularly high because a chain mechanism is involved [20]. 

Within the area of SET-promoted di-7t-methane reactions, recent studies have shown that irradiation of 1-aza-1,4-dienes, such as 32, and the 1,4-diene 34, using AW-dimethylaniline (DMA) as electron-donor sensitizer, leads to production of the corresponding cyclopropane derivatives 33 and 35 resulting from 1-ADPM and DPM rearrangements, respectively, in reactions that take place via radical-anion intermediates (Sch. 11) [25]. 

In aromatic hydrocarbons, some substituted alkenes, dienes, substituted acetylenes and ketones, one half of the n orbitals are empty and an electron can easily be placed in these antibonding orbitals. The capture of an electron by the acceptor molecule is an exothermic process because the energy of the antibonding orbitals lies below the level of the ionization potential of the acceptor radical anion. Many radical anions formed from unsaturated molecules are themselves stable they do not decompose and may exist indefinitely under suitable experimental conditions [182a],On the other hand, they react easily with other molecules. 

A solution of sodium in ammonia may be considered as a source of solvated electrons. The alcohol functions as a proton source. The aromatic molecule accepts an electron from the solution to form a radical anion 1, protonation of which by the alcohol forms the radical 2 (Scheme 11.2). Acceptance of a second electron generates a new carbanion, which is also protonated and gives the 1,4-diene 3. The overall transformation is reduction of the aromatic compound to the 1,4-diene. 

As discussed above, the first study of the distance dependence of electronic coupling associated with ET and HT applied PES, ETS and Koopmans theorem calculations to the rigid, symmetrical polynorbomane dienes 12( ), from which values of 0.88 and 1.2 bond were found for HT in the radical cations and ET in the radical anions, respectively. Although the electronic coupling could be calculated for a broad range of bridge lengths (up to 16 bonds), only the first two members of the series could be studied experimentally. It was clearly imperative to obtain an experimental determination of the distance dependence of ET rates based on more than two distances ... 

The diazocompounds, such as azibenzil [107], 2,3,4,5-tetraphenyldiazocyclopenta-diene [108], diazodimedone [109], and diethyl diazomalonate [106], whose anion radicals decompose to relatively stable carbene anion radicals, are all derived from compounds with relatively low pKa values (from 13 to 18 in DMSO). It thus seem essential for the lifetime of the carbene radical anion that the negative charge can be accommodated in the structure [110]. 

Polysaccharides that are mainly found in the gel are also a group of compounds that exhibit antioxidant activities. It was demonstrated that APS-1 (mainly composed b mannose glucose in ratio 18 5) was effective in scavenging superoxide anion radical (dose-dependant fashion), hydroxyl radical, suppressed conjugated diene formation from LDL oxidation induced by Cu, and exhibited a protective effect on hydrogen peroxide-induced injury in PC12 cells 64). Also Kardosova et al. (65) showed that in vitro experiments with acidic and neutral polysaccharides were able to prevent lipid peroxidation by scavenging hydroxyl radicals. 

The mapping shown in Fig. 1 includes references to SBR and styrene block copolymers in the PB search. Removing these citations from the database reduced the number to 4297, which can be seen mapped in Fig. 3. the area of high activity is centered on hydroxy terminated PB (HTPB). Low-Mn HTPB can be prepared by a variety of polymerization processes such as radical, anionic, or even using acyclic diene metathesis (ADMET).f The HTPB has a variety of uses as a propellant. " Other uses include reaction with epoxy resins, nylon, urethane, or even in the formulation of adhesives.t" The use of HTPB as an oxygen scavenger in polyamide, polyvinyl alcohol, and multilayer... 

The diyne ene (84) undergoes photochemical cyclization on irradiation at 350 nm. The product obtained was identified as the phenanthrene derivative (85). " A variant of the photo-Bergman cyclization has been reported following the irradiation of the enediynes (86). The reactions are brought about using electron transfer from cyclohexa-1,4-diene in acetonitrile. This affords the products (87) in the yields reported under the structure. The products are formed by cyclization via the fulvene radical anion intermediate (88). " ... 

Subsequently readily available cholesterol has been degraded microbiologically to androst-1,4-diene-3,17-dione which is then ketalised with ethanediol to form regiospecifaically the 3-ketal, aromatisation of which occurs upon mild reaction with lithium in THF containing diphenyl/diphenylmethane Reaction takes place by way of the radical anion obtained from the lihium and diphenyl and diphenylmethane quenches the methyllithium arising from the leaving angular... 

Other studies have sought to establish the scope and limitations of the photo-NOCAS process. Thus Arnold and co-workers have examined the reactions of alkenes with 1,4-dicyanobenzene (DCB). A typical result from this reaction is shown in Scheme 1. All of the products arise from the attack of the radical cation of the alkene on the DCB sensitizer with loss of the cyano function. A further study of photo-NOCAS reactivity has demonstrated that the radical cation of 2,3-dimethylbut-2-ene, formed by irradiation in the presence of DCB/biphenyl, can be trapped by fluoride ion. The resultant radical (39) reacts with the radical anion of DCB to yield the adduct (40). The radical cation of methylenecyclopro-pane (41) can be formed by irradiation in the presence of DCB as the sensitizer. The products are illustrated in Scheme 2 and, as shown, in all cases the cyclopropane ring remains intact. The diene (42) undergoes SET to dicyanoben-zene as the sensitizer with biphenyl as the co-sensitizer. In the absence of nucleophiles many products are formed such as (43) and (44) by reaction with the solvent acetonitrile or the sensitizer, respectively. In the presence of alcohols low yields of (45) and (46) are formed by reaction of the alcohol with the radical cation of the diene (42). 

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