The chiral complexes [Ru(2)(PNNP)]2+ (4a) and [Ru(3)(PNNP)]2+ (4b), containing the non-enolized 1,3-dicarbonyl compounds 2-((tert-butoxy)carbonyl)cyclopentanone (2) or α-acetyl-N-benzyl-δ-valerolactam (3), were deprotonated to the enolato complexes 5a,b. Complex 4a has a pseudo-aqueous pK a value of 4.6 ± 0.5 (with pK a(Ph3PH+) = 2.7 as reference) and catalyzes the 1,4-addition of 2 to methyl vinyl ketone with up to 79% ee.

The ruthenium-catalyzed homogeneous hydrogenation of carbonyl groups has established itself as a mature synthetic method on an academic and industrial scale [1] for the selective generation of stereogenic centers as an alternative to highenergy, pyrophoric hydride reagents. The development of such catalytic systems has gone hand in hand with that of chiral bidentate diphosphine ligands, [2] which, however, are often tedious and expensive to prepare because their multistep synthesis requires air-and moisture-free conditions. In contrast, phosphine-free chiral catalysts that operate under the industrially preferred and atom-economical hydrogenation with H 2 (HY) are still rare, [3][4][5] the most successful examples being h 5 -cyclopentadienyl [3] and h 6 -arene [4] diamine Ru II complexes. Most of these catalysts, as with those operating under transfer hydrogenation (TRHY) conditions, [6] fail to match the efficiency and selectivity requirements of industrial application. Following the work carried out at Firmenich [7] on the HY of ketones with [RuCl 2 (PNNP)] catalysts, [8] where PNNP is a chiral tetradentate ligand with a P 2 N 2 donor, [9] we developed a family of ligands in which the phosphines are replaced by thioethers (SNNS). [10] Such chiral ligands are cheap, air-and moisture-stable, and easy to prepare. This is a preliminary report of ruthenium/SNNS complexes that catalyze the asymmetric HY of ketones and aldehydes with good chemo-and enantioselectivity. [10][11][12] Ligands 1 a-f (Scheme 1) were conveniently obtained in two quantitative steps without exclusion of air by nucleophilic aromatic substitution of 2-nitro-or 2-bromobenzyl aldehydes with the appropriate thiol, followed by condensation with the chiral 1,2-cyclohexanediamine. Ligand 1 g was obtained by NaBH 4 reduction of 1 a in quantitative yield. The ruthenium complexes [RuCl 2 (SNNS)] (2 a-e,g) were prepared from the reaction of [RuCl 2 (PPh 3 ) 3 ] with the appropriate tetradentate ligand (SNNS = 1 a-e,g) and were fully characterized, whereas ligand 1 f was used in situ (see below), as its complexation failed with a number of precursors. The dichloro complexes 2 a-e and 2 g are stable for several hours in CHCl 3 , toluene, and alcohol solutions when exposed to air.The crystal structure of (R,R)-[RuCl 2 (1 a)] (R,R)-(2 a) shows a weakly distorted octahedral coordination sphere around ruthenium with a trans Cl-Ru-Cl unit, and the R configuration at both Ru-thioether moieties (Figure 1).The "stepped" conformation of the tetradentate SNNS ligand is reminiscent of the achiral diimino complex trans-[RuCl 2 -(SNNS)] (SNNS = N,N'-bis(2-tert-butylthiobenzylidene)-1,3propanediamine) [11a] and of other chiral PNNP [8,13] and salen [14] analogues.

The chiral dicationic complexes [Ru(4a)(PNNP)]2+ (2a) and [Ru(4h)(PNNP)]2+ (2h, PNNP is (1S,2S)-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 [RuCl2(PNNP)] (1) by double chloride abstraction with (Et3O)PF6, followed by reaction with the 1,3-dicarbonyl compound. The estimated aqueous pK a 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 CH2Cl2/Et2O (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.

The dichloro complexes [RuCl2(PNNP)] (2a−c) undergo double chloride abstraction when treated with AgSbF6 (2 equiv) (PNNP is one of (1S,2S)-N,N‘-bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine, 1a; (1S,2S)-N,N‘-bis[o-(bis(4-trifluoromethylphenyl)phosphino)benzylidene]cyclohexane-1,2-diamine, 1b; (1S,2S)-N,N‘-bis[o-(bis(3,5-bis(trifluoromethyl)phenyl)phosphino)benzylidene]cyclohexane-1,2-diamine, 1c). The resulting elusive species form the corresponding bis(aqua) complexes [Ru(OH2)2(PNNP)]2+ by reaction with water. The bis(aqua) complexes [Ru(OH2)2(PNNP)](SbF6)2 (6a−c) were fully characterized, including an X-ray structure of 6c. The X-ray structure of 2c with the new electron-poor chiral tetradentate PNNP ligand 1c is also reported. Complexes 6a and 6c catalyze the cis-selective asymmetric cyclopropanation of styrene, albeit with low yield. The best cis:trans ratio and enantioselectivity were obtained with 6c (87:13 and 92% ee for the cis isomer, respectively). The products of double chloride abstraction from [RuCl2(PNNP)] gave moderate to high conversion of styrene (37−70%), but the yield of the cyclopropane product was generally modest (13−64%). The best cis- and enantioselectivity were 86:14 and 81% ee, respectively.