By Electron Transfer

An Sh2 reaction apparently occurs at the tin centre in the reaction of tetramethyltin with oxygen, photoinitiated by electron transfer from a flavin analogue in the presence of Mg2+, where the chain process shown in equations 5-43 and 5-44 is thought to be involved.53 

The behaviour of the stannacyclopentanes is anomalous in that, even in the absence of an electronegative ligand on the tin, the Sh2 reaction of a variety of radicals occurs at the tin centre rather than at hydrogen.56 These systems are discussed in Section 10.1. 

Cleavage of an alkyltin bond via a radical cation occurs in the gas phase in mass spectroscopy (see Section 2.1.3), and can be induced by y-irradiation, or electrolytically, or by electron transfer from an oxidising agent. 

The study by ESR spectroscopy of organotin radical cations generated by y-irradiation of a dilute solution of the organotin parent in a frozen Freon matrix is discussed in Section 20.3. If the sample is allowed to warm, fragmentation occurs to give a tin cation and an alkyl radical. 

Similar reactions are involved with the organotin stablilisers in PVC wrapping film when food is sterilised by y-irradiation.57 

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Step 3 The cyclohexadienyl radical produced m step 2 is converted to an anion by electron transfer from sodium H H... 

Reactions involving the peroxodisulfate ion are usually slow at ca 20°C. The peroxodisulfate ion decomposes into free radicals, which are initiators for numerous chain reactions. These radicals act either thermally or by electron transfer with transition-metal ions or reducing agents (79). 

Electron Level Position. One essential condition of spectral sensitization by electron transfer is that the LUMO of the dye be positioned above the bottom of the conduction band, eg, > —3.23 eV in AgBr or > —4.25 eV in ZnO (108). To provide the desired frontier level position respectively to the valence and conduction bands of the semiconductor, it is necessary to use a polymethine with suitable electron-donor abiHty (Pq. Increasing the parameter (Pq leads to the frontier level shift up, and vice versa. Chain lengthening is known to be accompanied by a decrease of LUMO energy and hence by a decrease of sensitization properties. As a result, it is necessary to use dyes with high electron-donor abiHty for sensitization in the near-ir. The desired value of (Pq can be provided by end groups with the needed topological index Oq or suitable substituents (112). 

Hydrogen Abstra.ction. These important reactions have been carried out using a variety of substrates. In general, the reactions involve the removal of hydrogen either direcdy as a hydrogen atom or indirectly by electron transfer followed by proton transfer. The products are derived from ground-state reactions. For example, chlorarul probably reacts with cycloheptatrienyl radicals to produce ether (50) (39). This chemistry contrasts with the ground-state reaction in which DDQ produces tropyhum quinolate in 91% yield (40). 

Reductants such as zinc or sodium dithionite generate the semidione from diketones. Electrolytic reduction can also be used. Enolates can reduce diones to semidiones by electron transfer. 

The acetylide anion 3 is likely to form an alkynyl-copper complex by reaction with the cupric salt. By electron transfer the copper-II ion is reduced, while the acetylenic ligands dimerize to yield the -acetylene 2 ... 

Polymerization of ethylene oxide might be initiated by electron transfer process if metallic Na or Li is used as an initiator. On the other hand, initiation by sodium naphthalene involves not electron transfer but addition to naphthalene- ion. 

Quinones may react with carbon-centered radicals by addition at oxygen or carbon, or by electron transfer (Scheme 5.]6).l74, fi2 195 201 202 The preferred reaction pathway depends both on the attacking radical and the particular quinone (halogenated quinones react preferentially by electron transfer). The radical formed may then scavenge another radical. There is also evidence that certain quinones e.g. chloranil, benzoquinone (38)] may copolymerize under some conditions. ... 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

The symmetric series provides functional cyclohexadienes, whereas the non-symmetric one serves to build deuterated and/or functional arenes and tentacled compounds. In both series, several oxidation states can be used as precursors and provide different types of activation. The complexes bearing a number of valence, electrons over 18 react primarily by electron-transfer (ET). The ability of the sandwich structure to stabilize several oxidation states [21] also allows us to use them as ET reagents in stoichiometric and catalytic ET processes [18, 21, 22]. The last well-developed type of reactions is the nucleophilic substitution of one or two chlorine atoms in the FeCp+ complexes of mono- and o-dichlorobenzene. This chemistry is at least as rich as with the Cr(CO)3 activating group and more facile since FeCp+ activator is stronger than Cr(CO) 3. 

Two inorganic nucleophiles that react easily with arenediazonium ion, namely the nitrite ion and the hydroxide ion, provide good examples of the concept of the nucleophilic homolytic leaving group. By electron transfer to a diazonium ion the... 

The formation of aryl radicals from benzenediazonium ions, initiated by electron transfer from a nitrite ion, has already been discussed in Section 8.6. It is an excellent example of a dediazoniation assisted by a donor species that is capable of forming a relatively stable species on release of an electron, in this case a nitrogen dioxide radical NO2 (Opgenorth and Rtichardt, 1974). 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

Returning now to Beckwith s data we shall consider them on the basis of our discussion of aryl radical formation by electron transfer (Secs. 8.6, 10.5, and 10.6). It may appear surprising in Table 10-9 that Meijs and Beckwith (1986) carried out chloro- and bromo-de-diazoniations only with cupric salts. The authors point out, however, that they identified tiny amounts of cuprous impurities, and that relatively large concentrations of CunHlg2 are necessary for the second step in Scheme 10-80. 

Despite the differences in final products formed, these workers identify the initial step in the breakdown of all oxalates as C—C bond rupture in the anion (C204- 2 C02). This intermediate may be converted to the carbonate (through the carbonyl—carbonate intermediate) or to C02 (by electron transfer), viz. 

Nickel(IV) complexes react with dimethyl sulphoxide in acidic solution to give the sulphone and nickel(II) ions. The kinetics of this reaction have been studied and found to be very complex in nature. The reaction probably proceeds by initial complexation of the dimethyl sulphoxide to the nickel(IV) species followed by electron transfer and oxygen atom transfer producing the observed products149. 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

One potentially important example of CIDNP in products resulting from a radical pair formed by electron transfer involves a quinone, anthraquinone j5-sulphonic acid (23). When irradiated in the presence of the cis-syn dimer of 1,3-dimethylthymine (24), enhanced absorption due to vinylic protons and emission from the allylic methyls of the monomer (25) produced can be observed (Roth and Lamola, 1972). The phase of the polarizations fits Kaptein s rules for intermediate X... 

For electron transfer to and from a metal to species in solution the situation is complicated by the solvation energy of the species and in the general case changes in adsorption energy caused by electron transfer may also have to be taken into account. If we consider a simple reaction... 

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