Nitrogen redox transformations

Table 1 summarizes several redox transformations that can be accomplished in artificial photosynthetic assemblies including the photolysis of water, carbon dioxide reduction, and nitrogen fixation processes. The endoergicities of these transformations, and the number of electrons involved in the reduction processes, are also indicated in the table. It is evident that the energy per electron to drive the various transformations are met by visible light quanta. 

We began studies with flavin coenzyme analogs in 1972 to probe what structural features in the flavin ring system were requisite for specific aspects of the enzymic catalyses noted above. In particular, evaluations of the 5-carba-5-deazaflavin and the 1-carba-l-deazaflavin system were selected, given the pivotal role of these nitrogens in the redox transformations. 

Chemical differences between imide and sulfide ligand types, however, are substantive and dictate synthetic tactics. In ionic form, N-anions are considerably more basic than sulfur anions [e.g. in DMSO PhNH2, = 30.6 PhSH, 10.3) and, when coordinated to weak-field iron, the former remain more reactive than the latter. Furthermore, redox transformations coupled to weak-field iron are much more accessible with sulfur than nitrogen. As a result, imide ligation is introduced in Scheme 5.9 by protolysis rather than the salt-metathesis or redox routes typical in Fe-S chemistry. Protolysis requires iron precursors with reactive ligands as latent bases the relative instability of these complexes forces the incorporation of imide (or equivalent N-anions) early in the synthetic sequence. 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

A simplified depiction of the marine nitrogen cycie iiiustrating redox and phase transitions mediated by microbes. The boxes contain the nitrogen species and its oxidation number. The arrows represent transformation reactions as foiiows (1) nitrogen fixation, (2) soiubiiization, (3) ammonification,... 

If ammonium concentrations in seawater are low, phytoplankton will assimilate nitrate and nitrite using chemical-specific permeases. Once inside the cell, these DIN species are transformed into ammonium via redox reactions in which nitrogen is reduced to the -III oxidation state ... 

Figure 4.10 Nitrogen transformations in submerged soils on a redox scale (McBride, 1994). Reproduced by permission of Oxford University Press...

Examples for the oxidation of nitrogen-containing compounds via halide ions as redox catalysts are listed in Table 4, No. 51-56. In this way, primary amines are transformed to nitriles using the system NaBr/MeOH (Table 4, No. 52) Thus, 1,2-diaminocyclohexane is cleaved to yield adiponitrile (Eq. (64))... 

When dealing with transformation reactions, it is important to know whether electrons have been transferred between the reactants. For evaluating the number of electrons transferred, it is convenient to examine the (formal) oxidation states of all atoms involved in the reaction. Of particular interest to us will be the oxidation state of carbon, nitrogen, and sulfur in a given organic molecule, since these are the elements most frequently involved in organic redox reactions. 

Triazenes can be prepared by the following three methods (Scheme 1) (1) redox reactions (reduction of azides, oxidation of triazanes), (2) building or splitting of nitrogen chains, and (3) exchange of substituents (mutual transformation of triazenes). 

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