Radical anions dienes

The relevance of our PES and ETS studies to HT and ET processes is that the PES AIP it) splitting energies for 15(4)-15(6) are, to a good approximation, equal to twice the respective electronic coupling terms, Vel, for hole transfer in the diene radical cations (Scheme 6a). Likewise, the ETS AEA(if) values for these dienes are approximately equal to twice the respective electronic coupling terms, Vel, for ET in the diene radical anions (Scheme 6b). The derivation of these relationships is straightforward and has been explained in detail elsewhere.14,38,41 A simple MO... 

One important class of anionic-polymerization initiators are the organometallic compounds, especially those of lithium and sodium (see Chapter 5 to review the general properties of the metalloalkyls). Grignard reagents can induce polymerization of acrylonitrile and acrylate esters, but not of hydrocarbons. Initiation by organosodium compounds often involves in situ generation of the active species from sodium metal. Sodium metal catalyzes the polymerization of dienes, and the mechanism has been shown to involve a dianion formed by dimerization of the diene radical anion. An alternative initiation system for anionic polymerization involves... 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyclosiloxanes. Aromatic radical anions... 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

Interest in the synthesis of 19-norsteroids as orally active progestins prompted efforts to remove the C19 angular methyl substituent of readily available steroid precursors. Industrial applications include the direct conversion of androsta-l,4-diene-3,17-dione [897-06-3] (92) to estrone [53-16-7] (26) by thermolysis in mineral oil at about 500°C (136), and reductive elimination of the angular methyl group of the 17-ketal of the dione [2398-63-2] (93) with lithium biphenyl radical anion to form the 17-ketal of estrone [900-83-4] (94) (137). 

The Birch reduction has been used by several generations of synthetic organic chemists for the conversion of readily available aromatic compounds to alicyclic synthetic intermediates. Birch reductions are carried out with an alkali metal in liquid NH3 solution usually with a co-solvent such as THF and always with an alcohol or related acid to protonate intermediate radical anions or related species. One of the most important applications of the Birch reduction is the conversion of aryl alkyl ethers to l-alkoxycyclohexa-l,4-dienes. These extremely valuable dienol ethers provide cyclohex-3-en-l-ones by mild acid hydrolysis or cyclohex-2-en-l-ones when stronger acids are used (Scheme 1). 

Reduction of the aromatic nucleus in AjjV-dimethylbenza-mide occurs by an initial single electron transfer to give a radical anion. Protonation of the radical anion generates a radical and a second electron transfer gives the amide enolate 1. Protonation of the cross-conjugated trienolate moiety in 1 occurs carbonyl group to give the cyclohexa-1,4-diene 2. ... 

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]. 

These results provide strong evidence in favor of the involvement of radical-anions in electron-donor-sensitized l-aza-di-rr-methane rearrangements of aza-dienes 101, 117, 118, and 119 and also in the di-rr-methane reaction of diene 124. These observations open new lines of research in an area in which, due to the large number of smdies carried out for more than 30 years, apparendy there were very few things that remained to be uncovered. Further studies are in progress to determine the scope and synthetic applications of these reactions. 

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... 

A surprising new observation made by Rooney and coworkers shows that complex 12 generates radical anions when treated with jr-acceptors even dienes and simple alkenes.213... 

The reduction of the tetrasilacyclohepta-1,2-diene with a sodium mirror in Et20 gave dark red crystals that were shown by an X-ray structure determination to be the sodium salt of the allyl anion 61 produced by a series of intramolecular rearrangements of the initially formed radical anion (Fig. 37). The Na-C distances are 265.6(5)-288.2(5) pm.98... 

The use of alkali melals for anionic polymerization of diene monomers is primarily of historical interest. The electron-transfer mechanism of the anionic polymerization of styrenes and 1,3-diencs initiated by alkali metals has been described in detail the dimerization of radical anion intermediates is the important step. 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, arc efficient difunctionai initiators, However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization. 

The question of why the 1,3-diene is not formed, even though it would be more stable through conjugation, can be rationalized with a simple mnemonic. When viewed in valence bond terms, electron-electron repulsions in the radical anion will preferentially have the... 

Disilabicyclo[2.2.2]octa-2,5-dienes, prepared by the interaction of the radical anion of anthracene, naphthalene or biphenyl with 1,2-dichlorotetra-methyldisilane were found to undergo thermal decomposition472). 

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