Diaza annulenes

The 7/NMR spectrum displays signals of shielded protons = - 2.35, integral level 1) and of deshielded ones = 10.45 and 9.39, integral levels 1 1). This reflects a ring current due to aromaticity as described for annulenes and porphyrins in section 2.5.2. To conclude, the reaction involves an oxidative cyclisatlon of 2,5-bis(2-pyrrolylmethyl)-17/-pyrrole 2 with 47/-trlazole-3,5-dlaldehyde 3 to the corresponding 2,3-diazaporphyrin 4, following the 3-t-l pathway of porphyrin synthesis. Two non-equivalent tautomers may exist these are the diaza[ 18]annulene 4a and the tetraaza[18]annulene 4b. 

The equivalence of the inner NH protons (Sm = - 2.35) as well as correlation signals with the pyrrolic carbons only (5c = 143.8, 141.6, 139.8 and 136.7) provide evidence for the diaza[18]-annulene tautomer 4a. Two separate NT/ proton signals and cross signals with the a-carbon atoms (5c = 159.3) of the triazole ring are expected, in contrast, for the tetraaza[18]annulene tautomer 4b. 

Dihydro- 1,2-diazocines such as (183) have been referred to as l,2-diaza[8]annulenes (78AHC(23)55), and are of interest in connection with the potential aromaticity of the 107r-electron system provided by these rings (see Section 5.19.4.4). On the basis of UV and NMR data the dibenzodiazocine (183) was suggested to have a partially planar structure there is no direct evidence of aromatic character. 

Aside from the l,4-dihydro-l,4-diazocines (l,4-diaza[8]annulenes) discussed in Section 5.19.4.4.1, completely unsaturated 1,4-diazocines are represented in the literature only by benzo and dibenzo fused systems. 

In addition to the complexes listed above, the following chelates belonging to the class containing an MN4 unit have been obtained on the basis of l,4-diaza-l,3-butadiene [1, vol.2 470], tetra-aza-1,3-butadiene [1, vol.2], penta-azadienes [471], azomethines of 2,5-diformylpyrrole [472,473], cyclic hydrazones and thiosemi-carbazones [262], dibenzotetra-aza[14]annulene [474], hetarylaminoazocompounds [8,404,475], azoazine radicals [476], Schiff bases of heterocyclic series [132,133,477], and azoleazines [132,133,478,479]. 

The primary products can be of theoretical or practical interest, such as the 3,4-diaza-l, 6-methano[10]annulenes 9,308 or they can be transformed into other compounds of great interest such as lO320 (cf Houben-Weyl, Vol E6b, pp 163, 451), 11,339 and PDE-ll methyl ester 12.340... 

Porphyrins form a planar macrocycle that contains a conjugated, cyclic delocalized system of 22 n-electrons [17]. 18 Electrons can be assigned to the perimeter of a l,16-diaza[18]annulene, an arrangement which is also found in the chlorin and bacteriochlorin systems. Porphyrins and chlorins are intensely coloured (1 red to violet, Amax 500-700 nm 2-4 green, 600-700 nm). 

Figure 10,1. Key types of vanadium complexes in petroleum (a) porphyrins, (b) mixed ligands such as N2O2 type, the -ketimines, (c) pseudoaromatic pheophorbides such as a bacterio-chlorophyll (the outer conjugation is interrupted, but still belong to the diaza-ld-annulene system), and (d) highly aromatic porphins such as the dehydrogenated product of m-a-naphylporphyrin, which was identified in Nonesuch shale. Type (a) is found in all petroleum. Types (b), (c) and (d) are commonly referred to as the nonporphyrin type of vanadium...

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