Carbaporphyrins

It is noteworthy to mention that employment of silver(i) trifluoroacetate in place of silver(i) acetate, as in the case of A-confused porphyrin, did not give the desired products. This has been attributed to the better basicity of the acetate anion than the trifluoroacetate, which aided the deprotonation of the three interior GH/NH protons at the carbaporphyrin ligand. Besides, it has been noticed that an excessive amount of silver acetate was required for the synthesis. The mechanism of the silver insertion reaction for this type of ligands was proposed, according to what Bruckner had proposed for the synthesis of silver(m) w -triarylcorroles.218,236 The reaction was suggested to occur via a disproportionation reaction, with the supportive observation of silver deposit formation after the reaction.237... 

The crystal structure for 179b was determined which showed a planar geometry for the silver center and the carbaporphyrin ligand. A small tilt angle of 5.09° for the indene unit relative to the mean macrocyclic plane was determined, and the Ag-C bond length was 2.015(4) A. Extension of the work to the benzocarba-, benzi-, oxybenzi-,... 

Modification of the carbaporphyrin ligands could also be made by the interchange of an exocyclic methine group with a heteroatom such as oxygen. Ligands of this type are referred to as O-confused oxaporphyrins. Silver(m) complexes... 

Tetraphenyl-2-aza-21-carbaporphyrin 220 was reacted with various acyl chlorides and excess triethylamine in refluxing dry benzene, and unexpected pyrrolidin-2-one-fused N-confused calix[4]phyrins 221 were isolated in 60-70% yield (Scheme 54) <20060L1137>. 

Redox equilibrium of Ag(I I [-porphyrin /Ag(III) is characterized with = 0.59 V versus SCE [412]. Evidently, corroles and carbaporphyrins are able to stabilize the Ag(III) oxidation state, presumably due to the presence of 7r-electron donors, which reduce the formal oxidation state of the metal in such complex [396]. It is expected that such complexes have potential practical applications, for example, as the catalysts in the electron-transfer reactions. 

Inverted porphyrin or 2-aza-21-carbaporphyrin 88 also known as an NCP (Figure 3) was first isolated as a by-product of normal acid-catalyzed condensation of pyrrole and benzaldehyde (1994ACI779, 1994JA767). Latos-Grazynski and coworkers employed a Rothemund reaction between tolualdehyde and pyrrole in the presence of BF3-Et20 in DCM, followed by oxidation with 2,3,5,6-tetrachloro-p-benzoquinone (chloranil) to obtain NCP (1994ACI779). Simultaneously, Furuta and coworkers also reported... 

The first air-stable silver(lll) complex was reported by Furuta et al. (22) in 1999, who synthesized and characterized a silver(lll) complex with 5,10,15,20-tetraphenyl-2-aza-carbaporphyrin (nctpp) as the supporting ligand. The crystal structure shows an inequivalence in the Ag—N bonds and the Ag—C bond, which reflects the asymmetric nature of the porphyrin. Later, the same group reported another silver(lll) complex with ethoxy-5,10,15,20-tetrapentylflorophenyl-3,7-diaza-21,22-dicarbaporphyrin [(n2cp)2l as the supporting ligand (Fig. 3) (23). 

In 2002, Lash and co-workers (24) reported the first silver(III) complex supported by a non-nitrogen fused carbaporphyrin system by reacting silver(I) acetate with diphenylcarbaporphyrin (dpcp). Two years later they reported that reacting silver(I) acetate with semiquinone, cycloheptatriene, or indene under mild conditions yields stable silver(III) oganometallic complexes (Fig. 4) (25). 

Subsequent to the submission of this manuscript, a detailed communication describing the synthesis and properties of palladium(II) isoporphycenes appeared in the literature. Also appearing in the literature were two separate reports detailing the synthesis of derivatized forms of so-called carbaporphyrin 3.179. ... 

Bruckner C (2004) The silver complexes of porphyrins, corroles, and carbaporphyrins silver in the oxidation states II and III. J Chem Educ 81 1665-1669... 

Figure 1. Porphyrin (a) and several carbaporphyrins -confusedporphyrin (b) carbaporphyrin (c) m-benziporphyrin (d) tropiporphyrin (e) azuliporphyrin (f) benzocarbaporphyrin (g).

The sensitivity of the internal carbon to reaction would be a significant complicating factor for carbaporphyrins in protein active sites. Dioxygen binding and activation, for example, would readily result in oxidation at the core carbon position. In addition, the susceptibility of the internal carbon to nucleophiles could affect the coordination chemistry of the side chains of cysteine and histidine, frequently bound at the axial positions of heme sites. While novel inorganic structures do result from adducts formed upon substitution at the internal carbon, directly analogous chemistry to that seen in normal porphyrins is much less likely due to its susceptibility. 

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