Chemical Processes

Metalloporphyrins belong to a versatile family of transition metal catalysts. Nature uses a well-known family of heme-containing enzymes, cytochrome P-450, to catalyze a wide range of aerobic oxidation reactions in biological systems under mild conditions, including the highly selective oxidation of saturated C-H bonds of alkanes.

Due to the rigid square-planar coordination mode of porphyrin ligands, metalloporphyrin can only have two axial sites in a trans relationship for substrates coordination and/or activation, leading to an increase in the selectivity and catalyst robustness. The steric bulkiness of substituents on the porphyrin ring accounts for selectivity, while electron-withdrawing halogen substituents render the porphyrin ring difficult to oxidize. In addition, the properties of metalloporphyrins can be tuned by varying the metal ion and by modification of the porphyrin ligand. These complexes, which incorporate chiral peripheries into the metalloporphyrin framework, have provided researchers with scaffolds to directly manipulate the stereoselectivity of the reaction system.

Metalloporphyrins were applied primarily towards oxygen-transfer reactions. Further studies demonstrated that these catalysts were also active for nitrogen and carbon-group transfers, areas that have become of considerable interest in the last few decades. Nowadays, porphyrins have proved to be efficient catalysts for oxygen reduction reactions, cyclopropanation, C-H, N-H and S-H insertions, olefination, ylide-forming reactions, aziridination, amidation/amination, epoxide isomerization, carbonylation reaction, sulfoxidation, epoxidation, C-H hydroxylation, oxidation of alcohols, alkene cleavage, and oxime oxidation.

Metal phthalocyanine complexes are also considered as attractive catalysts because of their structural analogy with metalloporphyrins. Phthalocyanines are easily prepared on a large scale compared with some metalloporphyrins. These complexes are chemically and thermally stable. Among the most important industrial use of phthalocyanine is the Merox process. In petroleum refining, it consists in the catalytic oxidation of mercaptans performed by sulfonated cobalt phthalocyanines with the objective to remove a large part of the sulfur compounds from petroleum. A large range of chemical transformations including oxidation, reduction, preparation of nitrogen containing compounds and C-C bond formation reactions can be efficiently catalyzed by phthalocyanine metal complexes. The phthalocyanine complexes can be used not only in large processes for the preparation of bulk chemicals but also for the synthesis of elaborated fine chemicals up to applications in total synthesis.

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