Phenazine Modifications

In the case of phenazine, substitution in the hetero ring is clearly not possible without complete disruption of the aromatic character of the molecule. Like pyrazine and quinoxa-line, phenazine is very resistant towards the usual electrophilic reagents employed in aromatic substitution reactions and substituted phenazines are generally prepared by a modification of one of the synthetic routes employed in their construction from monocyclic precursors. However, a limited range of substitution reactions has been reported. Thus, phenazine has been chlorinated in acid solution with molecular chlorine to yield the 1-chloro, 1,4-dichloro, 1,4,6-trichloro and 1,4,6,9-tetrachloro derivatives, whose gross structures have been proven by independent synthesis (53G327). 

The new phenazine dyes are prepared according to established procedures with some modifications ... 

Phenazine (39) is found in two crystalline modifications, a- and jS-(Herbstein and Schmidt, 1955a). The a-form (space group P2ja, with two molecules in the unit cell) has been studied at room temperature (Herbstein and Schmidt, 1955a, b) and at about 90°K (Hirshfeld and Schmidt, 1957). The more accurate low-temperature analysis using partial three-dimensional data indicates that the deviations from the expected mmm molecular symmetry only exceed the estimated experimental accuracy ( + 0-003 A) in a direction normal to the mean molecular plane, the maximum deviation being 0-010 A. This slight non-planarity of the molecule is ascribed to the action of intermolecular forces. 

Gorton and coworkers have been particularly active in this field and produced an excellent review of the methods and approaches used for the successful chemical modification of electrodes for NADH oxidation [33]. They concentrated mainly on the adsorption onto electrode surfaces of mediators which are known to oxidise NADH in solution. The resulting systems were based on phenazines [34], phenoxazines [35, 36] and pheno-thiazines [32]. To date, this approach has produced some of the most successful electrodes for NADH oxidation. However, attempts to use similar mediators attached to poly(siloxane) films at electrode surfaces have proved less successful. Kinetic analysis of the results indicates that this is because of the slow charge transfer between the redox centres within the film so that the catalytic oxidation of NADH is restricted to a thin layer nearest the electrode surface [37, 38]. This illustrates the importance of a charge transfer between mediator groups in polymer modified electrodes. 

We have emphasized that minor chemical modifications stabilize different structural minima in Fig. 21. The MBP-TCNQ complexes in Section 3.4 may illustrate, if supported by structure determinations, different structures for the same complex, presumably due to different preparative conditions. That one modification is metastable hardly matters in the solid state where there is no interconversion between different minima in Fig. 21. The precise control of crystallization conditions is obviously crucial. The impurity effects leading to MP-TCNQ or HMP-TCNQ, or to different MjP-TCNE adducts in Section 4.4, represent different structures for different complexes. We are not aware of definitive evidence for different structures of the scone complex, but suspect such behavior to be possible among phenazine complexes. 

Besides conventional substitutions and derivati-zations of functional groups attached to the phenazine ring, the amount of reported modifications is limited. Oxidation with H2O2 or mCPBA gives the corresponding N-dioxides in good yields. 

---