Electron transfer across mechanical bonds

The mechanism of electron transfer across a liquid-liquid interface is probably quite similar to that of a homogeneous electron transfer [106, 107]. In either case, the role of changes in the oxidation state of both reactants, in their molecular structure (including valence bond deformation, breaking or formation), and in the polarization state of the solvent have to be considered. Owing to electrostatic interactions of charged reactants with polar solvent molecules, the electron energy levels of the reac-... 

Majda and co-workers provided an elegant demonstration of through-bond and chain-chain hopping processes as parallel mechanisms for electron transfer across SAMs [46, 133]. In n-alkanethiol monolayers formed on hanging drop mercury electrodes, the adsorbate coverage T is measured by chronocoulometry of the mercuric thiolate formed upon adsorption. For the as-adsorbed films, geometric con-... 

Another way of arranging the intramolecular transmembrane electron transfer is to use the so called molecular wires, i.e. molecules with the electron conduction chain of conjugated bonds, redox active polar terminal groups and the length sufficient to span across the membrane. Such molecules can in principle provide for electron transfer from the externally added or photogenerated reductant across the membrane to the oxidant. This mechanism was suggested [41, 94] to explain the action of carotene-containing System 1 and 38 of Table 1. However, as it was shown later, the transmembrane PET in these systems proceeded also without carotene. 

The mechanisms for these types of reactions depend upon nucleophilicity/electrophilicity of both the alkene and adding group, and the presence of solvent or catalyst. In many cases, the first step of the reaction is the attack of a positively charged proton by the pi electrons, causing a proton transfer across the double bond. This is followed by addition of the nucleophile to the remaining cation. 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

The analogous primary amine had been earlier studied by Wubbels and gave only the photo-Smiles reaction, but this process was not observed for the present system and the reaction is suggested to proceed by a photo-electron transfer mechanism to yield the radical ion pair. Photoaddition of N-H bonds to aromatic compounds has been known for some years, and the intramolecular process which leads to cyclization products has been reported for the reaction of 9-(aj-anilinolalkyl)phenanthrenes (220). The formation of the spiro-compounds (221) by addition of the N-H bond across the phenan-threne 9,10-positions is suggested to arise from the intramolecular exciplex in... 

It was found while studying the mechanism of electron transfer (ET) across the interface between two immiscible liquids that specific anion adsorption occurs at the octane-water interface in the presence of metalloporphyrins. This adsorption increases in the order of Cl , Br , I , and is caused by coordinative bonding of the anions as ligands of the porphyrin metal atoms. 

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