Radical anion of biphenyl

The previously discussed characters will influence the electron transfer rates implying anions. One of the simplest examples was given by the rate constant difference observed in reactions in which pyrene (Py) reacts with an electron or an electron-cation pair [44,45]. The same type of difference was measured in the exchange between the radical anion of biphenyl (B) and pyrene (Py) [46]. The reduced reactivity is the consequence of the cation proximity in the ion pair. 

Figure 4-9. Schematic energy level diagrams, (a) The radical anion of biphenyl (PP2 ) (b) the dianion of biphenyl (PP2 ). , electron arrow, electronic transition. The molecular orbital levels are taken from [77],...

The diphenyl derivative is more stable than the radical anion of its carbon derivative, 9,9-diphenylfluorene, but, after extended periods of reduction, the spectrum of the biphenyl radical anion begins to grow in intensity. The 5,5-dimethyl derivative appears to be stable under these conditions. The enhanced stability of the silicon derivative might be due to stabilization of the carbon-silicon bonds by delocalization of charge into available d-orbitals 81). Methyl proton hyperfine splitting observed for the anion radicals of the 5,5-dimethyl- and 5,5-diethyldibenzosilole has been cited as evidence for d-7r interaction (56). 

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). 

The es reacts with PVB to give a polymer anion with a high efficiency [47]. The rate constant was evaluated as 4.7 x 109 mol -1 dm3 s-1 in hexamethylphos-phorictriamide. The absorption specra of the radical anions of PVB [47] and PVP [48] are similar to those of biphenyl anion and pyrene anion, respectively, to mean that the excess electrons trapped by the polymers are essentially localized on the side groups. 

A characteristic feature of some of these reactions is the dependence of their efficiency on the basicity of the radical anion [108], The differences are especially manifested in non-polar solvents, where the CIP are expected to dominate. Some of these cleavage processes are more efficient than expected, based on the thermodynamic evaluations of the unassisted fragmentation (Sect. 3.2). Also a stereochemical preference for cleavage is observed for erythro isomers as compared to the threo isomers (Scheme 6). In benzene erythro/threo selectivity is high, being highest for the relatively basic radical anion of dicyanonaphthalene and lowest for the relatively nonbasic radical anion of thioindigo. The stereochemical preference disappears in acetonitrile if biphenyl is used as a co-sensitizer [108]. 

The 1,1,4,4-tetraphenylbutane disodium salt was found to eject electrons scavenged by added 1,1-diphenylethylene [59, 60]. In the case of trans-stilbene dianion, the photoejection of an electron was accompanied by isomerization in ds-stilbene radical anion while biphenyl was used as electron scavenger [61]. ds-Stilbene itself was used as scavenger of electron photoejected from aromatic radical anions like sodium perylenide [46], These scavenging experiments are... 

The DCA-sensitized photooxygenation of biphenyl is reported by Mizuno and Otsuji [121-123]. In this photooxygenation, both DCA and biphenyl are oxidized by to give anthraquinone and benzoic acid (Scheme 37). Although the detailed mechanism of this reaction is still obscure, it is likely that the radical anion of DCA is oxidized with 2 after converted to either the anion or the radical. The radical cation of biphenyl is oxidized with 2 to give benzoic acid. 

Addition of electron acceptors to acetonitrile confirmed the presence of a transient reducing species through the formation of radical anions of solutes such as biphenyl [21a, 22], pyrene, trans-stilbene, and so forth [21b]. This reducing species has a broad absorption with Amax at 1450 nm, and was shown to exist in the monomeric and dimeric forms (see Eq. 36) from the effect of temperature on the absorbance at - max [21b]. The monomeric form is responsible for the 1450 nm peak, whilst the dimeric form exhibits a weak maximum at 550 nm superimposed on the tail of the monomeric band. The enthalpy change accompanying the reaction of Eq. 36 has been measured to be —34.9 kJ moE [21b] so that CH3CN is the dominant reducing species at room temperature. 

Reductive dimerization of benzene [246], cyano biphenyl ether [247], pyridine [248], and acridine [249] derivatives has also been investigated. Radical anions of diesters of pyridine and benzene undergo rapid reversible dimerization (A dim = 10 -10 M" s" ) [250]. 

Although biphenyl may improve formation of R3GeM from R Gej-M, the radical anion of napthalene, [CioH8] Na, cleaves the Ge—Ge bond of PhgGej in DME. This reagent does not cleave Ge—C bonds. 

Winkler et al. (1966) have obtained the spectra of the lithium salts of the radical-anions of aromatic hydrocarbons such as naphthalene and biphenyl by irradiating the corresponding hydrocarbon in the presence of phenyl-lithium this method has several advantages over others for generating hydrocarbon radical-anions, one being that studies may be made in a wide range of solvents. 

Photoinduced electron transfer from donors such as prenyl acetate, geranyl acetate, all-/ra/w-famesyl acetate, and all-/ra j-geranylgeranyl acetate to 1,4-dicyano-2,3,5,6-tetramethylbenzene, 1,4-dicyanonaphthalene, and 9,10-dicya-noanthracene in the presence of l,r-biphenyl as co-donor in acetonitrile produces the radical cation of biphenyl and the radical anion of the electron acceptors. Geranyl acetate is observed to photocyclise, and the mechanism of this process which involves reaction of its radical cation with water is discussed. Allyl glycosides (76) can be photodeprotected to give (77) via (78) by irradiating with di-t-butyl peroxide in the presence of bromotrichloro-methane. ... 

The radical anions of tram- and c/s-stilbene can be distinguished by ESR [503]. A radical ion pair has been observed by this method using trial-kylamines in acetonitrile [497], Electron back transfer (i.e., a reaction of the radical anion of frans-stilbene with the radical cation of the donor) opens a new pathway for intersystem crossing to the tram triplet state. Time-resolved resonance Raman spectroscopy of photoinduced electron transfer from amines to frans-stilbene has been reported [497,504], In the photooxidation of frans-stilbene the radical cation has been observed by flash photolysis using cyanoanthracenes [505-507], The radical cations of cis-and frans-stilbene were also produced by electron transfer from biphenyl to excited 9,10-dicyanoanthracene and subsequent electron transfer from stilbene to the radical cation of biphenyl [508]. External magnetic field effects... 

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). 

In the presence of methanol as solvent and 1,4-dicyanobenzene as acceptor, photoinduced electron transfer from 1,4-bis(methylene)cyclohexane gives 4-(methoxymethyl)-1 -methylenecyclohexane and 4-(4-cyanophenyl)-4-(methoxy-methyl)-l-methylenecyclohexane which arise by nucleophilic attack of the solvent on the radical cations, followed either by reduction and protonation, or by combination with the radical anion of the electron acceptor.These observations are in accordance with the proposed mechanism of the nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction. The same group has also investigated the use of cyanide ion as nucleophile and report that irradiation of a mixture of 1,4-dicyanobenzene in the presence of biphenyl as donor, KCN, and 18-crown-6 gives a mixture of (79) and (80). These workers have also extended the scope of NOCAS to fluoride ion. In particular, use of 2,3-dimethylbut-2-ene and 2-methylbut-2-ene gives 4-cyanophenyl substituted... 

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