Intramolecular redox table

An indication that the intramolecular redox equilibrium between the first polypyridine reduction and the more negative vinyl ligand reduction is responsible for the radical initiation and subsequent film formation reaction of Ru(bpy)2(vinyl-ligand)22+ complexes is demonstrated in Figure 7. From the data for the various Ru(bpy)2(4 -X-stilb)2 + complexes in Table 2 it is seen that the first bipyridine reductions (Ej red.l)) all occur at approximately the same potential (-l.22jj3.03V). Thus, a disproportionation equilibrium will exist (eq. 13) which depends upon the reduction potential of the coordinated vinyl ligand. 

Table 4 Reductive defluorination by intramolecular redox reaction mediated by NHCs ...

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

Various aldehydes 184 and alcohols have been shown to be competent in the redox esterification of unsaturated aldehydes in the presence of the achiral mesityl triazo-lium pre-catalyst 186. Both aromatic and aliphatic enals participate in yields up to 99% (Table 13). Tri-substituted enals work well (entry 3), as do enals with additional olefins present in the substrate (entries 4 and 7). The nucleophile scope includes primary and secondary alcohols as well as phenols and allylic alcohols. Intramolecular esterification may also occur with the formation of a bicyclic lactone (entry 8). 

Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton.

In contrast, the closely related palladium acetate-promoted intramolecular alkylation of alkenes by tri-methylsilyl enol ethers (Scheme 4)6,7 has been used to synthesize a large number of bridged carbocyclic systems (Table 1). In principle, this process should be capable of being made catalytic in palladium(II), since silyl enol ethers are stable to a range of oxidants used to carry the Pd° -> Pd11 redox chemistry required for catalysis. In practice, catalytically efficient conditions have not yet been developed, and the reaction is usually carried out using a full equivalent of palladium(II) acetate. This chemistry has been used in the synthesis of quadrone (equation 2).8 With the more electrophilic palladium(II) trifluoroace-tate, methyl enol ethers underwent this cyclization process (equation 3).9... 

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. 

With the exception of the recently reported DMSO reductases from bacteria (71,72), all of the enzymes of Table I contain additional redox active prosthetic groups besides Mo-co. Substrate oxidation (reduction) occurs at the molybdenum center, and electrons are removed (added) via one of the other prosthetic groups. These two processes are coupled by intramolecular electron transfer between the molybdenum center and the other redox centers of the enzyme. Results for xanthine oxidase and sulfite oxidase and approaches to modeling the coupling in sulfite oxidase are summarized below. 

The jV-chloro-compounds were the first to be employed as radical cyclization precursors in the synthesis of pyrrolidines and piperidines, as well as fused and bridged heterocyclic skeletons [7], Aminyl and amidyl radicals were thus generated and used in intramolecular additions. Higher yields and selectivities are obtained with the metal-complexed species. Some selected examples are reported in Table 4. Generally, a typical radical chain mechanism is involved (with chlorine atom transfer from 7V-chloro-compound). In the particular case of copper-cornplexed aminyl radical cyclization, a redox chain process operates (with fast chlorine ligand transfer from cupric chloride)... 

The redox properties can be divided among those involved with its reduction by potential substrates of the enzyme, its reoxidation by molecular oxygen, and by electron transfer to and from other redox centers within the molecule. Type 1 Cu + is reduced rapidly by a variety of both one-electron and potential two-electron substrates via a one electron process 63, 64). A likely reason for this ease of reduction of Type 1 Cu2+ ion lies with its exceptionally high redox potential (0.77 V at pH 6.2, Table 2). Introduction of molecular oxygen to reduced laccase results in a rapid reappearance of the blue color characteristic of Type 1 Cu2+. The question of intramolecular electron transfer occurring during reoxidation will be considered below. 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

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