Acidic ruthenium PNNP complexes of non-enolized 1,3-dicarbonyl compounds as catalysts for asymmetric Michael addition

The chiral dicationic complexes [Ru(4a)(PNNP)]2+ (2a) and [Ru(4h)(PNNP)]2+ (2h, PNNP is (IS,2S)N,N'-bis[o-(diphenylphosphino)benzylidenelcyclohexane-1,2-diamine), containing non-enolized 2-tertbutoxycarbonylcyclopentanone (4a) or (x-acetyl-N-benzyl-6-valerolactam (4h), were prepared from [RuC12(PNNP)] (1) by double chloride abstraction with (Et30)PF6, followed by reaction with the 1,3dicarbonyl compound. The estimated aqueous pKa values for 2a (3.3 0.3) and 2h (4.7 0.1) were determined by deprotonation with Ph2NH 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 CH2C12/Et2O 0:1) 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 P-keto ester 4a favors and accelerates the deprotonation of 2a in dichloromethane.