Acidic Ruthenium PNNP Complexes of Non-enolized 1,3-Dicarbonyl Compounds as Catalysts for Asymmetric Michael Addition

This is the authors' abstract. We don't add key points for this paper.

The chiral dicationic complexes [Ru( 4a )(PNNP)] 2+ ( 2a ) and [Ru( 4h )(PNNP)] 2+ ( 2h, PNNP is (1 S, 2 S )- N,N′ -bis[ o -(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine), containing non-enolized 2- tert -butoxycarbonylcyclopentanone ( 4a ) or α-acetyl- N -benzyl-δ-valerolactam ( 4h ), were prepared from [RuCl 2 (PNNP)] ( 1 ) by double chloride abstraction with (Et 3 O)PF 6, followed by reaction with the 1,3-dicarbonyl compound. The estimated aqueous p K a values for 2a (3.3 ± 0.3) and 2h (4.7 ± 0.1) were determined by deprotonation with Ph 2 NH and pyridine, respectively. The corresponding monocationic enolato complexes 3a and 3h were isolated and structurally characterized. Complex 2a catalyzes the 1,4-addition of 4a to methyl vinyl ketone with up to 93% ee when a CH 2 Cl 2 /Et 2 O (1:1) solvent mixture is used. Oxygen-containing cosolvents enhance both the rate and enantioselectivity of the Michael addition with a number of substrates. This observation is discussed in the context of our previous observation that the combined addition of diethyl ether and of β-keto ester 4a favors and accelerates the deprotonation of 2a in dichloromethane.