Porphyrinogen conformation

The complexation of transition metals " has been achieved in non-aqueous solvents using the lithiated form [EtgN4Li4(thf)4] of the m o-octaethylporphyrinogen. The metal helps fix one of the many possible porphyrinogen conformations, as observed both in solution and in the solid state. Reaction 1 exemplifies the complexation of several bivalent metals. ... 

Cyclization of 5, for entropic reasons via conformer 5, yields a porphyrinogen 6. Porphyrinogens 6 are extremely sensitive to oxidation so that even aereal oxidation gives the desired porphyrin 7 (contaminated sometimes by the cyclized dihydroporphyrin, i.e. chlorin 8). 

In contrast, porphyrinogens 16 (Fig. 5) possess only 16 Ti-electrons and as a consequence the delocalization over the whole macrocycle is absent. The conformations of porphyrinogens are not planar any more and can approximate to the conformations known for calix[4]arenes (vide supra). Compounds 16 (Fig. 5) may, therefore, be considered as heteroatomic calixarene derivatives, of which some have additional heteroatoms in the bridging positions [15, 28-30]. 

As in the case of the tetrameric macrocycles discussed above, compounds 17 and 18 can be considered as porphyrinogen analogues or as heteroatom-bridged heteroaromatic calix[4]arene derivatives. The two different conformations observed for 17 and 18 have analogues in compounds 16 (Fig, 5) [15, 28-30],... 

Molecular self-assembly will not be considered here (see [9.2]) in fact, it is a special type of synthetic procedure where several reactions between several reagents occur in one experimental operation to yield the final covalent structure it is subject to control by the intramolecular conformational features of intermediates and by the stereochemistry of the reaction(s) the efficient assembly of a covalent structure may require that the connecting reaction(s) be reversible so as to allow searching for the final structure. Examples are found in the generation of macropolycyclic structures by multiple (amine-aldehyde) condensations (see Section 4.1) or of porphyrinogens, porphyrins and phthalocyanins (see also in [9.13a]. 

Meso-porphyrinogens formally represent the structural transition between pyrroles and porphyrins. Stimulated by the work of Floriani et al. [251], porphyrinogens were considered as alternative ligand sets in lanthanide chemistry by Jubb and Teuben [68,252]. The tetraamide ligand offers a coordination environment which is obviously sterically and electronically more flexible than that provided by the standard Cp2Ln- fragment. The conformational flexibility... 

Zhang et al. [36] combine DFT-based conformation analysis with quantitative structure-activity relationship (QSAR) analysis. They looked at bioactive conformations for 25 cyclic imide derivatives as proto-porphyrinogen oxidase (PPO) inhibitors. PPO is the last common enzyme in the biosynthetic... 

The introduction of a cyclopropane moiety in the porphyrinogen skeleton causes a very little conformational change, as can be judged from the structure of 8 (Figure 6),... 

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