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

Abstract

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_relvs. 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.

References

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