Dendrimer-Metalloporphyrins:  Synthesis and Catalysis

A new class of sterically hindered dendrimer-metalloporphyrins was synthesized for use as shape-selective oxidation catalysts. A series of oxidatively robust poly(phenylesters) dendrimers were prepared through a convergent synthesis. Monodendrons were appended to the meta -positions of the 5,10,15,20-tetrakis(3‘,5‘-hydroxyphenyl)porphinatomanganese(III) chloride to obtain a sterically protected metal center. These complexes have been examined as regioselective oxidation catalysts for both intra- and intermolecular cases. Epoxidation of nonconjugated dienes and alkene mixtures of 1-alkene and cyclooctene catalyzed by dendrimer-metalloporphyrins has been carried out using iodosylbenzene as the oxygen donor. Significantly greater regioselectivity is observed with the dendrimer-metalloporphyrins, relative to the corresponding parent, 5,10,15,20-tetraphenylporphyrinatomanganese(III) cation. To examine the extent of steric crowding around the porphyrin from the dendrimers, molecular modeling studies were performed on these dendrimer-porphyrins. Although the top access is extremely limited in the dendrimer-metalloporphyrins, a significant side opening permits substrate entrance and limits the regioselectivity that can be achieved with meta -substitution of a tetraphenylporphyrin.