Single-electron process

Jacobsen et al. have made a convincing argument that these types of reaction proceed by way of discrete copper-nitrene complexes, rather than by some sort of single-electron process (Scheme 4.16) [13c]. 

Olefmation of carbonyls with low-valent titanium such as Ti(0) derived from TiCl3/LiAlH4. Single-electron process. 

To make the task more manageable this chapter will focus specifically on the interaction between the nucleophile and a double bond and not consider in any depth subsequent steps. We will also only briefly consider reactions in which there is a preassociation or complexation of the double bond with a Lewis acid prior to nucleophilic attack. Finally we shall concentrate on conventional nucleophilic attack and not discuss mechanisms involving single electron processes. In Section II we shall examine the types of double bonds that undergo nucleophilic attack, in particular examining relative reactivity, where available, and models for explaining this order. In Section III we shall review the orbital interactions that control the approach of a nucleophile to the double bond and the associated geometrical constraints. Then in Section IV we shall consider the implications of these constraints on selective reactions. 

An efficient modification of the manganese(III) mediated malonate radical addition to styrene has been examined. Use of cerium(IV) nitrate in methanol at room temperature results in the direct formation of the butyrolactone 17 in low yield along with other byproducts [95JCS(P1)1881]. Other intermolecular single electron processes for the formation of lactones have been reported [95JOC458] [95BCSF843],... 

Metal-14 anions react with alkyl halides (RX) mostly by nucleophilic substitution (Sn2), the stereochemistry of which is dependent on the structure of R and X, the solvent and the nature of the counterion. Other reactions were also observed nucleophilic substitution at halogen [also called halogen/metal exchange (HME)] and single electron processes. In some cases steric hindrance around the reactant results in elimination. 

Given that electrochemical rate constants are usually extremely sensitive to the electrode potential, there has been longstanding interest in examining the nature of the rate-potential dependence. Broadly speaking, these examinations are of two types. Firstly, for multistep (especially multielectron) processes, the slope of the log kob-E plots (so-called "Tafel slopes ) can yield information on the reaction mechanism. Such treatments, although beyond the scope of the present discussion, are detailed elsewhere [13, 72]. Secondly, for single-electron processes, the functional form of log k-E plots has come under detailed scrutiny in connection with the prediction of electron-transfer models that the activation free energy should depend non-linearly upon the overpotential (Sect. 3.2). 

Murray has demonstrated that soluble metallic clusters exhibit coulomb staircase-type behaviour [102]. The ionic space charge formed around the dissolved MFCs is reported to contribute to its capacitance, upon charging of the metal core. It is well known that small metal particles exhibit double layer charging (capacitive charging) properties in liquid electrolytes [104]. The sub-attofarad capacitance associated with the MFCs leads to charging of the tiny capacitor by single electron processes in potential intervals of A V that surpass ke T where is the Boltzmann constant and T is the temperature [102, 105]. 

An optimized design employing a tubular electrode in a cylindrical cavity has been described [638]. The mechanism and kinetics of the electrooxidation of several para-haloanilines and the follow-up reactions in acetonitrile have been investigated with this cell [639]. A similar design that is suitable for low temperature measurements (233 K) has been reported [640]. It has been employed in a study of the temperature dependence of the reduction of bromonitrobenzene in acetonitrile solution. The electroreduction of perinaphthenone in a single electron process has been investigated with this cell [641]. The lifetime of the neutral radical formed by deprotonation of the radical anion has been estimated to be around 1 min. A similar electrochemical behavior of benzanthrone was observed. 

A typical radical cyclization involves the attack of a radical center on an sp carbon of a double bond (or other unsaturated group) in the chain. If the chain includes one or more heteroatoms, then a heterocycle will be formed. In Scheme 4.44, we examine the cyclization of a radical to form a 5-membered ring (a pyrrolidine) by this process. Note the practice of showing single-electron processes with single-barbed arrows ( fish hooks ). 

Similar experiments with more tightly held adsorbates require higher bias voltages [54—59]. In this way, chlorobenzene adsorbed on Si(l 11) was subjected to the selective dissociation of C—Cl bonds, and it was concluded that a two-electron mechanism is operating that couples vibrational excitation and dissociative electron attachment processes. As can be seen from Fig. 4.9, the yield of desorption increases linearly with the electron current indicating a single-electron process, while for dissociation the yield increases with the second power. 

Improved descriptions of single-electron processes require treatments beyond the geometric approximation given by the Haxtree-Fock electron propagator in Eq. (4.25). More elaborate approximations of the equation of motion (9.1)... 

Based on the work of PC reduction on graphite, Aurbach et al. proposed the similar reaction mechanism for EC based on single-electron process, that is, LEDC. Considering that EC is an essential cosolvent of commercial electrolyte, therefore, LEDC is a key composition of SEI on anode surface in current Li-ion batteries. 

The use of secondary amine catalysis in combination with radical chemistry was first introduced by MacMillan in 2007 in a process he termed as organo-SOMO catalysis [32]. hi this system, the enamine that is generated in the condensation of a chiral secondary amine and a carbonyl, is oxidized via a single electron process. This generates a three-7i-electron radical cation with a singly occupied molecular orbital (SOMO) which can react asymmetrically in a variety of different processes (Scheme 1.25). 

The carbonyl group is polarised by the electronegativity difference between carbon and oxygen, making the carbon electrophilic. Magnesium is a reducing metal, and can donate two electrons, in two separate single electron processes. 

A classical and long-established electrochemical synthesis is the formation of alkanes (R-R) from the dimerisation of radicals generated via electro-decarboxylation of carboxylate salts known as the Kolbe reaction. The radical species are produced through a single electron process and... 

The complex [(pmcp) Pd( -PhCCPh) Pd(pmcp)] ( = M2) undergoes two one-electron reversible oxidation steps to give mono- and dipositive ions. It may also be reduced reversible in a single electron process. Therefore, in the case of this dimer, the formation of four quite stable oxidation states is indicated. 

Single-electron Process Corresponcting Auger Processes ... 

The processes in Fig. 6.1 have been arranged to show how Auger processes can compete with single-electron processes (for example with single-electron radiative processes). The trapping processes are analogous for donors and acceptors, which are shown separately. 

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