Synthetic, spectral and catalytic activity studies of ruthenium bipyridine and terpyridine complexes: Implications in the mechanism of the ruthenium(pyridine-2,6-bisoxazoline)(pyridine-2,6-dicarboxylate)-catalyzed asymmetric epoxidation of olefins utilizi

Various Ru(L1)(L2) (1) complexes (L1 = 2,2′-bipyridines, 2,2′:6′,2″-terpyridines, 6-(4S)-4-phenyl-4,5-dihydro-oxazol-2-yl-2,2′-bipyridinyl or 2,2′-bipyridinyl-6-carboxylate; L2 = pyridine-2,6-dicarboxylate, pyridine-2-carboxylate or 2,2′-bipyridinyl-6-carboxylate) have been synthesized (or in situ generated) and tested on epoxidation of olefins utilizing 30% aqueous H2O2. The complexes containing pyridine-2,6-dicarboxylate show extraordinarily high catalytic activity. Based on the stereoselective performance of chiral ruthenium complexes containing non-racemic 2,2′-bipyridines including 6-[(4S)-4-phenyl-4,5-dihydro-oxazol-2-yl]-[2,2′]bipyridinyl new insights on the reaction intermediates and reaction pathway of the ruthenium-catalyzed enantioselective epoxidation are proposed. In addition, a simplified protocol for epoxidation of olefins using urea hydrogen peroxide complex as oxidizing agent has been developed.

Graphical abstractPossible active intermediates of the ruthenium-catalyzed epoxidation of olefins using H2O2 are proposed based on synthetic, spectral, catalytic activity and computational studies of ruthenium bipyridine and terpyridine complexes. This fragment based analysis of the catalytic system suggests also reasons for low catalytic activity and low enantiomeric induction of some catalysts. A simple catalytic system utilizing UHP has also been developed in which all six classes of aromatic olefins can be epoxidized in moderate to good yields with good chemoselectivity.Figure optionsDownload full-size imageDownload as PowerPoint slide