Sterically hindered metalloporphyrins

Sterically Hindered Metalloporphyrins Capable of Direct Aerobic Oxygenation. The catalytic aerobic olefin epoxidation system of Quinn and Groves, (tetramesitylporphyrinato)Ru/02/olefin substrate, effects equations 3-6, that is, the direct oxygenation of substrate using O2 as the oxidant without consumption of reducing agent 14), The (tetramesitylporphyrinato)Ru complex sterically... 

Scheme 29.25 Improved methods for the halogenation of sterically hindered metalloporphyrins.

The metalloporphyrin-initiated polymerizations are accelerated by the presence of steri-cally hindered Lewis acids [Inoue, 2000 Sugimoto and Inoue, 1999]. The Lewis acid coordinates with the oxygen of monomer to weaken the C— O bond and facilitate nucleophilic attack. The Lewis acid must be sterically hindered to prevent its reaction with the propagating center attached to the prophyrin structure. Thus, aluminm ortho-substituted phenolates such as methylaluminum bis(2,6-di-/-butyl-4-methylphenolate) accelerate the polymerization by factors of 102-103 or higher. Less sterically hindered Lewis acids, including the aluminum phenolates without ortho substituents, are much less effective. 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

Collman and his coworkers proposed a metallacycle intermediate in the olefin epoxidation by metalloporphyrin/hypochlorite systems due to saturation kinetics [206-209]. According to Nolte et al. these saturation kinetics were explained by a dimmer formation [210-212]. Bruice et al. also rather suggested involvement of comproportiona-tion process of higher valent 0x0 complex [213]. In addition, Bruice et al. prepared sterically hindered Fe (TDBPP)Cl, TDBPP tetrakis(2,6-dibromophenyl)porphyrin, and used as the catalyst for epoxidation of nine structurally different alkenes [214]. Fe (TDBPP) was found to be an extremely efficient catalyst for epoxidation and nearly quantitative yields of epoxide formation were obtained in all cases. A computer graphics docking study (Polygen programs Quanta and CHARMm) questioned possible intermediacy of metallacycle. A recent report by Collman indicated a direct reaction of OCl and olefins could also explain the saturation kinetics [215]. 

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