Steric electron transfer kinetics

The quantitative effects of steric encumbrance on the electron-transfer kinetics reinforce the notion that the inner-sphere character of the contact ion pair D+, A- is critical to the electron-transfer paradigm in Scheme 1. Charge-transfer bonding as established in the encounter complex (see above) is doubtless an important consideration in the quantitative treatment of the energetics. None the less, the successful application of the electron-transfer paradigm to the... 

However, a more accurate comparison between the experimental reaction kinetics and the predictions of the dissociative electron transfer theory revealed that the agreement is good when steric hindrance is maximal (tertiary carbon acceptors) and that the reaction is increasingly faster than predicted as steric hindrance decreases.31 These results were interpreted as indicating an increase... 

Coming back to aromatic anion radicals, a more accurate comparison between the experimental reaction kinetics and the predictions of the dissociative electron transfer theory revealed that the agreement is good when steric hindrance is maximal (tertiary carbon acceptors) and that the reaction is faster and faster than predicted as steric hindrance decreases, as discussed in detail in Section 3.2.2 (see, particularly, Figure 3.1). These results were interpreted as indicating an increase in the ET character of the reaction as steric hindrance increases. Similar conclusions were drawn from the temperature dependence of the kinetics, showing that the entropy of activation increases with steric hindrance, paralleling the increase in the ET character of the reaction. 

Extensive efforts have been made to establish and rationalize the kinetics of dioxygen oxidation of iron(II) polyaminocarboxylate complexesThe kinetics of dioxygen oxidation of iron(II) complexes of 1,2- and of 1,3-propylenetetraacetate show the influence of steric factors on these electron transfer reactions7 " ... 

Rate constants (k X 10 9 M-1sec-1) were determined to be 7.0, 3.5, and 1.0 for the enumerated substrates, respectively. The change in kinetics for the three cation radicals with increasing steric hindrance at the (3-carbon is in accordance with the depicted addition reaction. In contrast with that, a reaction of the azide ion with these three cation radicals in acetonitrile proceeds with rate constants that are the same in all three cases ( 3 X 109 M-1sec-1). In acetonitrile, the reaction consists of one-electron transfer from the azide ion to a cation radical. As a result, a neutral styrene and the azidyl radical are formed. The azidyl radical reacts with the excess azide ion, and the addition reaction does not take place ... 

Both the thermodynamics and kinetics of electron transfer reactions (redox potentials and electron transfer rates) have steric contributions, and molecular mechanics calculations have been used to identity them. A large amount of data have been assembled on Co3+/Co2+ couples, and the majority of the molecular mechanics calculations reported so far have dealt with hexaaminecobalt (III/II) complexes. 

Cyclic voltammetry, kinetic studies, and DFT calculations using a BP functional and the TZVP basis set showed that the major pathway of the non-regiospeciflc zinc-reduced titanocene-mediated ring opening of epoxides was initiated by a titanium dimer-epoxide compound that reacted in a rate-determining electron transfer mechanism 25 The calculations showed that the transition state is early so the stereoselectivity is determined by steric effects rather than by the stability of intermediate radicals. This was confirmed by studies with more sterically crowded catalysts. 

The discovery that sterically crowded -carbon enols are kinetically stable constitutes the milestone towards the elucidation of the electron transfer ability of enolates . Based on this, Schmittel and coworkers started an accurate examination of their redox aptitude, and how it could be reflected on their chemical reactivity. In particular, they pointed out how the predominance of ketones over enols in the neutral keto/enol equilibrium could be inverted upon one-electron oxidation. These findings opened up interesting possibilities for new synthetic procedures. Starting from available ketones, the small amount of enol present in equilibrium can be oxidized by suitable oxidants and the resulting radical cation can be trapped by nucleophiles. For example, l-(p-methoxyphenyl)propan-2-one, which has an enol [l-(p-methoxyphenyl)propen-2-ol] content of only about 0.0001% , reacts with tris(p-methoxyphenyl)aminium hexachloroantimonate in methanol to give the a-methoxyketone 76 °. Comparable yields are obtained with [Fe(phen)3](PFe)3. The products isolated in the reactions are consistent with the mechanism reported in equation 52. 

For Cu 7Cu systems, the electron transfer takes place with a large rearrangement of the inner coordination sphere, as Cu generally prefers distorted octahedral or square pyramidal environments whereas Cu likes better a tetrahedral geometry. This consideration implies that, in unconstrained systems, one or two Cu-S bonds must be broken and bond angles must be drastically modified upon Cu reduction. Therefore, any steric constraint imposed by the coordinated ligand could strongly affect the kinetics of the Cu 7Cu electron transfer process. 

The relationship between ion pairing and steric congestion of the redox probes has been addressed in separate studies by Buttry s and Mirkin s groups. Ion pairing can be detected on the voltammetric time-scale if it kinetically limits electron transfer due to steric congestion. Using QCM, Buttry et al. measured chemically... 

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