Dendritic ruthenium porphyrins: A new class of highly selective catalysts for alkene epoxidation and cyclopropanation

Abstract

Attachment of Fréchet-type poly(benzyl ether) dendrons [G-n] to carbonylruthenium(II) meso-tetraphenylporphyrin (5) using covalent etheric bonds forms a series of dendritic ruthenium(II) porphyrins 5-[G-n]m (m=4, n=1, 2; m=8, n=0-2). The attachment was realized by treating the carbonylruthenium(II) complex of 5,10,15,20-tetrakis(4′-hydroxyphenyl)porphyrin or 5,10,15,20-tetrakis(3′,5′-dihydroxyphenyl)porphyrin with [G-n] OSO2Me in refluxing dry acetone in the presence of potassium carbonate and [18]crown-6. Complexes 5-[G-n]m were characterized by UV/Vis, IR, and NMR spectroscopy and mass spectrometry. All of the dendritic ruthenium porphyrins are highly selective catalysts for epoxidation of alkenes with 2,6-dichloropyridine N-oxide (Cl2pyNO). The chemo- or diastereoselectivity increases with the generation number of the dendron and the number of dendrons attached to 5, and complex 5-[G-2]8 exhibits remarkable selectivity or turnover number in catalyzing the Cl2pyNO epoxidation of a variety of alkene substrates including styrene, trans-/cis-stilbene, 2,2-dimethylchromene, cyclooctene, and unsaturated steroids such as cholesteryl esters and estratetraene derivative. The cyclopropanation of styrene and its para-substituted derivatives with ethyl diazoacetate catalyzed by 5-[G-2]8 is highly trans selective.