Intramolecular Electron Transfer with Bond Formation

Intramolecular Electron Transfer with Bond Formation 517... 

The initial reaction brings about Z, -isomerism of the ethene bond and prolonged irradiation converts this into the major product identified as (48, ca. 78%). The route to the major product involves an intramolecular electron transfer with the formation of a 2witterionic biradical and it is within this that the transformation to product occurs. 

Photoinduced intramolecular interaction of t-S and tertiary amine moieties linked with a polymethylene chain has also been studied24. The photoexcitation of fraws-stilbene in which a tertiary amine is attached to the ortho position with a (CH2)i-3 linker leads to fluorescent exciplexes by intramolecular electron transfer, and results in no more than trans-cis isomerization. The failure to give adducts from the intramolecular exciplexes could arise from the unfavourable exciplex geometry to undergo the necessary bond formation. 

Ratera et al. (2003) discovered valence tautomerism in the ferrocene connected through the ethylenic bond with perchlorotriphenylmethyl radical. As ascertained by Moessbauer spectroscopy, this species in the solid state exhibited a thermally induced intramolecular electron transfer resulting in the formation of ferrocenium and perchlorotriphenylmethyl anion moieties. The authors used the initial species in its trans form. If the cis form would be available, the possibility of rotation around the ethylenic bond would be interesting to disclose. According to the authors, the interconversion of the cation-radical and anion centers proceeds gradually. At ambient temperature, equilibrium composition of the tautomers is achieved. This peculiarity is important with respect to potential technical applications. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

Since these data indicate that both double bonds might be involved in the potential-determining step, a concerted electron transfer step involving transfer of the electron simultaneously with the formation of the intramolecular C—C bond was proposed for cyclic EHD (Petrovich et al., 1966, 1966a) by analogy, the proposal was extended to the linear process too. We shall return to the problem of concerted mechanisms for electron transfer later, since it requires special attention, and continue with other, mostly electrochemical evidence bearing on the EHD mechanism. 

It can be conceived that intramolecular electron transfer from N2H2 to the Fe(III) centers, cleavage of NH bonds, and formation of SH bonds converts 44 into 45, which is a doubly protonated N2 complex. Deprotonation of 45 would yield the neutral N2 complex 46. The reversal of this sequence would convert 46 into 43 and realize the first 2 H+/2 e reduction step of N2 fixation. Twofold protonation of 46 gives 45 in which all atoms necessary to form the neutral diazene complex 43 have already taken their positions. The above mentioned anodic redox potential shift upon protonation (cf. Section IV.F) could enable us to reduce species C at relatively mild redox potentials, in contrast with the neutral species 46, which might be irreducible when it is an 18 VE complex. 

A spectroscopic investigation of the formation of THF-Cu"Cl2 complexes has been described, and irradiation of [Cu(Dto)2] (Dto = dithiooxalate) has been found to induce an intramolecular Dto Cu two-electron transfer with cleavage of the C-C bond in the Dto ligand and the formation of SCO. The kinetics of photo-oxidation of pyrene by Cu" in SDS micelles have been measured, but oxidants such as Eu" and Hg do not produce pyrene cations. A non-exponential decay of fluorescence is observed, and this is interpreted in terms of a model due to Tachiya which restricts the numbers of quenchers in a micelle. Transient Cu"-alkyl species are formed on flash photolysis of Cu -bis(amino-acid) complexes such as those of serine and valine, and pseudo first order rate constants for the decay of the transients have been obtained. 

An elegant photochemical formation of an aryl-carbon bond through a PET mechanism was recently reported in the total synthesis of the potent antimitotic polycycle (-)-diazonamide A. The reaction was initiated by intramolecular electron transfer between the indole chromophore and the adjacent bromoarene (Scheme 2.10). Thus, compound 21 was treated with an aqueous-acetonitrile solution of LiOH and the resulting lithium phenoxide solution was degassed and photolyzed (Rayonet, 300 nm) to yield biaryl 22 (as a single atropodiastereomer) in a good yield. A radical-radical anion pair (23) was formed upon excitation, and... 

The importance of Marcus theoretical work on electron transfer reactions was recognized with a Nobel Prize in Chemistry in 1992, and its historical development is outlined in his Nobel Lecture.3 The aspects of his theoretical work most widely used by experimentalists concern outer-sphere electron transfer reactions. These are characterized by weak electronic interactions between electron donors and acceptors along the reaction coordinate and are distinct from inner-sphere electron transfer processes that proceed through the formation of chemical bonds between reacting species. Marcus theoretical work includes intermolecular (often bimolecular) reactions, intramolecular electron transfer, and heterogeneous (electrode) reactions. The background and models presented here are intended to serve as an introduction to bimolecular processes. 

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