Charge reduction stepwise

Fig. 14 Potential energy profile for stepwise and concerted mechanisms with (solid lines) and without (dotted lines) an attractive interaction between the caged fragments in the product cluster. The case of the reduction of a neutral substrate is represented. It can be transposed for reductions of a positively charged substrate or for oxidations of neutral or negatively charged substrates.

One of the key issues in understanding these stepwise dissociative reductions is to define the thermodynamics (or at least relative thermodynamics) of the fragmentation itself. It is equally important to understand on which side of the scissile bond the charge actually resides, as this has important mechanistic implications. This is most easily inferred by comparison of the measured standard (or peak) potential with standard potentials of similarly substituted aromatic molecules that make stable radical anions. 

A tetraanion salt (84 )/4M has been formed by stepwise four-electron transfer from 4,7,12,15-tetrastyryl[2.2]paracyclophane (8) to lithium, sodium, and potassium metals in [2Hx]THF at 220 K a strong effect of the cyclophane hub on the charge distribution has been demonstrated and the influence of o1, o2, o3, o4-tetramethyl and px, p2,pi,pA-tetramethoxy substituents on the ease of reduction has been tested.9... 

This activation process can be assumed to be the initial step in the formation of dinuclear copper(II) acetylide complexes, as first proposed by Bohlmann and coworkers 40 years ago (Scheme 6) [10f]. Deprotonated alkyne units 11 (or the corresponding JT-complexes 10) generated therein, stepwise displace the negatively charged counter ions of copper(II) salt dimers (12). The dinuclear copper(II) acet-ylide complex which finally results (14) collapses to the coupled product under reductive elimination of copper(I). The existence of higher-order copper acetylide... 

The WOC is oxidized stepwise by a nearby tyrosine residue (Tyrz), which is itself oxidized by the chlorophyll cation radical P680+ (formed by light-induced charge separation). The electrons are eventually used by PSII for the reduction of plastoqui-none. After the WOC has lost four electrons, the accumulated oxidizing power drives the formation of molecular oxygen from two substrate water molecules, and the catalytic system is reset. The sequence of the four electron-transfer steps is summarized in the Kok cycle [32] of Figure 4.5.3, where the most probable spectroscopically derived oxidation states of the Mn ions [33] are shown for each of the five redox state intermediates S (n - 0-4). 

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