Substitution, nucleophilic porphyrin ring

The mechanism explaining the formation of this polymer is similar to that proposed for the electrosynthesis presented earlier, except for the first oxidation step. Indeed, whereas the formation of radical cations was sufficient to perform mono-substitutions on the porphyrin ring, the electropolymerization process requires formation of dications, undoubtedly due to kinetic problems (dications react faster with nucleophiles than radical cations) [138]. Hence an Ei(E2j CNmesoE2x+iCB)x = i nE2(n+i) mechanism can be proposed to explain the electropolymerization process [135, 138]. 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

The aziridination of olefins, which forms a three-membered nitrogen heterocycle, is one important nitrene transfer reaction. Aziridination shows an advantage over the more classic olefin hydroamination reaction in some syntheses because the three-membered ring that is formed can be further modified. More recently, intramolecular amidation and intermolecular amination of C-H bonds into new C-N bonds has been developed with various metal catalysts. When compared with conventional substitution or nucleophilic addition routes, the direct formation of C-N bonds from C-H bonds reduces the number of synthetic steps and improves overall efficiency.2 After early work on iron, manganese, and copper,6 Muller, Dauban, Dodd, Du Bois, and others developed different dirhodium carboxylate catalyst systems that catalyze C-N bond formation starting from nitrene precursors,7 while Che studied a ruthenium porphyrin catalyst system extensively.8 The rhodium and ruthenium systems are... 

Fig. 8 Examples of meso and p-mono-substituted porphyrins obtained by chemical oxidation of the macrocycle ring in the presence of nucleophiles...

Shine and coworkers showed that similar reactions also occur at me o-tetra-phenylporphyrins (TPP) P-positions (Fig. 8). Indeed, radical cations obtained by ring oxidation with iodine, can react with nucleophiles like pyridine, triphenylphosphane, triphenylarsine, and thiocyanate, to give P-substituted porphyrins [107, 108]. Similar P-substitution reactions were subsequentiy described by other groups [109-111]. 

Braune and Okuda have shown the possibility to substitute porphyrin initiators by simpler systems based on the association of ammonium salts with a bulky aluminum bis(phenolate) electrophile. According to their study, the ring-opening polymerization of PO cannot occur at simple Lewis add centers, but that nucleophilic ate complexes must be present at the same time. However, so far, only the synthesis of PPO oligomers with MWs less than 5000 has been reported for this system. The important contribution of Braune and Okuda is that they confirmed the Vandenberg binudear mechanism earlier proposed for epoxide polymerization (see Figure 12). °- ... 

---