Transfer hydrogenation active catalyst

Several other versions of these catalysts have been developed. Arene complexes of monotosyl-l,2-diphenylethylenediamine ruthenium chloride give good results with a,(3-ynones.55 The active catalysts are generated by KOH. These catalysts also function by hydrogen transfer, with isopropanol serving as the hydrogen source. Entries 6 to 8 in Scheme 5.3 are examples. 

The use of chiral ruthenium catalysts can hydrogenate ketones asymmetrically in water. The introduction of surfactants into a water-soluble Ru(II)-catalyzed asymmetric transfer hydrogenation of ketones led to an increase of the catalytic activity and reusability compared to the catalytic systems without surfactants.8 Water-soluble chiral ruthenium complexes with a (i-cyclodextrin unit can catalyze the reduction of aliphatic ketones with high enantiomeric excess and in good-to-excellent yields in the presence of sodium formate (Eq. 8.3).9 The high level of enantioselectivity observed was attributed to the preorganization of the substrates in the hydrophobic cavity of (t-cyclodextrin. 

Ir(cod)Cl]2 reacts with Q-diimines LL (derived from glyoxal and biacetyl) to yield cationic [Ir(cod)LL]+.523 If the reaction is carried out in the presence of SnCl2, then the pentacoordinate Ir(SnCl3)(cod)LL species results. The compounds are active catalysts in the homogeneous hydrogen transfer from isopropanol to cyclohexanone or to acetophenone followed by hydrogenation... 

Among the most active catalysts for the asymmetric transfer hydrogenation of prochiral ketones and imines to chiral alcohols and amines are arene-ruthenium(II) amino-alcohol (or primary/ secondary 1,2-diamine)-based systems, with an inorganic base as co-catalyst, developed by Noyori139-141 and further explored by others (Scheme 27).142-145... 

Gordon used a household microwave oven for the transfer hydrogenation of benz-aldehyde with (carbonyl)-chlorohydridotris-(triphenylphosphine)ruthenium(II) as catalyst and formic acid as hydrogen donor (Eq. 11.43) [61]. An improvement in the average catalytic activity from 280 to 6700 turnovers h-1 was achieved when the traditional reflux conditions were replaced by microwave heating. 

From all the above observations, it was concluded that, for diphosphine chelate complexes, the hydrogenation stage occurs after alkene association thus, the unsaturated pathway depicted in Scheme 1.21 was proposed [31 a, c, 74]. The monohydrido-alkyl complex is formed by addition of dihydrogen to the en-amide complex, followed by transfer of a single hydride. Reductive elimination of the product regenerates the active catalysts and restarts the cycle. The monohydrido-alkyl intermediate was also observed and characterized spectroscopically [31c, 75], but the catalyst-substrate-dihydrido complex was not detected. 

Scheme 3.8 Generation of the active dihydride catalyst by transfer hydrogenation by reductive elimination of the product to give a ruthenium(O) intermediate ([Ru] = Ru(PPh3)3).

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