Olefin complexes dissociative ligand substitutions

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

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