Ruthenium DPEN catalysts

Catalysts for ketone hydrogenation continue to be developed but one of the best systems is still the BINAP-DPEN catalyst first reported by Ohkuma et al. in 1995. " In this system ruthenium is combined with both a chiral diphosphine and a chiral diamine, forming an octahedral complex which gives a high degree of enantioselectivity. This stereoselectivity is considered to be a result of the synergistic effect of the chiral diphosphine and diamine ligands. 

BrXuPHOS (1) may be prepared (Figure 3.4) from bis(dimethylamino)phos-phine (2), the preparation of which has been described in a previous volume in this series,in a condensation with BINOL (3). The preparation procedure for the ruthenium(II) complex (S, S, 55)-BrXuPHOS-Ru-DPEN (4) is a modification of that reported by Noyori. All reactions described below must be carried out under an inert atmosphere of argon or nitrogen. Examples of ketone reductions using (4) as the catalyst are given in Figure 3.5 and Table 3.5. 

A ruthenium(II) complex (5,5,55)-BrXuPHOS-Ru-DPEN (4) containing BINOL-based monodonor phosphorus ligand BrXuPHOS (1) has been prepared and applied as a catalyst (S/C = up to 10000) for the asymmetric hydrogenation of ketones, providing the enantiomerically pure secondary alcohols with up to 99 % ee. 

The use of an activation/deactivation protocol with a chiral poison, (R)-DM-DABN (148), has been achieved with ruthenium catalysts that contained rac-xyl-BINAP and rac-tol-BINAP with chiral diamine (S,S)-DPEN. Asymmetric hydrogenation of 2-napthyl methyl ketone (128, Ar = 2-Naph, R = Me) without 148 gave the alcohol with 41% ee, whereas an enantioselectivity of 91% ee is obtained with deactivator 148 present (Scheme 12.58).197... 

The catalyst composed of the mismatched ligands, (f )-Xyl-BlNAP and (5,5)-DPEN, was much slower and furnished the (5)-alcohol with 56% ee (equation 10). Employing racemic Xyl-BINAP and (5, 5 )-DPEN gave a 50 50 mixture of diastereomeric ruthenium complexes that reduced I -acetylnaphthone to the (f )-alcohol with excellent enantioselectivity (90%). 

Figure 4.5 Chiral dendritic ruthenium catalysts containing BINAP- and DPEN-cored dendrimer ligands.

Itsimo [25] has also shown that polymer-supported OPEN monosulfonamides containing sulfonated pendent group (Scheme 16) are able to catalyze the HTR reduction of ketones in water with sodium formiate as hydrogen donor (S/C = 100). However, TsDPEN immobilized on polystyrene crosslinked or not, polymer 30 and 31 respectively, shrank in water. Sodium /j-styrene sulfonate was copolymerized with chiral A-(vinylbenzene-p-sulfonyl)-DPEN (20) imder radical polymerization conditions with or without DVB leading respectively to ligand 32 and 33. Control of the balance hydrophilicity/hydrophobieity of the polymer support is carried out by changing the salt from Na to quaternary ammonium. All of these polymers swelled in water, and their respective ruthenium, rhodium or iridium complexes were prepared. Compared to sodium salt polymer-supported catalyst from 32a and 33a, ammonium... 

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