Stoichiometric enantioselective alkene epoxidation with a chiral dioxoruthenium(VI) D4-porphyrinato complex

A dioxoruthenium(VI) complex containing a D₄-porphyrinato ligand por* {H₂por* = 5,10,15,20-tetrakis-[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8- dimethanoanthracen-9-yl]porphyrin} has been prepared by oxidation of its ruthenium(II) carbonyl precursor with m-chloroperoxybenzoic acid and characterised by spectroscopic methods. The [Ru^VI(por*)O₂] complex undergoes enantioselective epoxidation of alkenes and the highest enantiomeric excess (ee) attainable is 77%. In the presence of pyrazole the complex transforms to [Ru^IV(por*)(pz)₂] when reacting with alkenes. The kinetics of the epoxidation of para-substituted styrenes has been studied. The experimental rate law is - d[Ru^VI]/dt = k₂[Ru^VI][alkene]. The second order rate constants k₂ at 25°C fall in a narrow range, 2.1 × 10^-3-9.7 × 10^-3 dm³ mol^-1 s^-1. Comparison of the Hammett plot (log k_rel vs. σ*) with those for achiral analogues [Ru^VI(tpp)O₂] (H₂tpp = 5,10,15,20-tetraphenylporphyrin) and [Ru^VI(oep)O₂] (H₂oep = 2,3,7,8,12,13,17,18-octaethylporphyrin) suggests the formation of a radical intermediate for the alkene epoxidations. Both [Ru^II(por*)(CO)(EtOH)] and [Ru^VI(por*)O₂] were examined for enantioselective catalysis. Enantioselectivities of the stoichiometric and catalytic reactions showed good correlation. There is no solvent dependence on enantioselectivity when changing the solvent from dichloromethane to benzene.