Catalytic enantioselective olefin hydrogenation

The quantitative and selective transformation of the precursor into the dinuclear complex in catalytic conditions, the different catalytic performance of the mononuclear derivative [(BDPBzP)Ru(CH3CN)j]OTf (lower activity and enantioselectivity), the identical catalytic performance of the precursor and of the T12-H2 complex, and the recognized capability of the structurally related Ru(II) complex [(ri2-H2)(dppb)Ru()a.-Cl)3RuCl(dppb)] to maintain the dimeric structure in olefin hydrogenation reactions [35-37] were taken as proofs for the direct involvement of [(BDPzP)(DMSO)Ru((1-C1)3Ru(ti2-H2)(BDPBzP)] in the enantioselective hydrogenation of acetylacetone [68]. 

Sawada, Y., Matsumoto, K., Kondo, S., et al. (2006). Titanium-Salan-Catalyzed Asymmetric Epoxidation with Aqueous Hydrogen Peroxide as the Oxidant, Angew. Chem. Int. Ed., 45, pp. 3478-3480 Matsumoto, K., Sawada, Y. and Katsuki, T. (2006). Catalytic Enantioselective Epoxidation of Unfunctionalized Olefins Utihty of a Ti(0i-Pr)4-salan-H202 System, Synlett, 20, pp. 3545-3547. 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

Phosphinodihydroxazole (PHOX) compounds, L2-4, act as P/N bidentate ligands showing excellent enantioselectivity in Ir-catalyzed hydrogenation of simple a,a-disubstituted and trisubstituted olefins (Figure 1.12). " The use of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArp) as a counter anion achieves high catalytic efficiency due to avoidance of an inert Ir trimer... 

Asymmetric catalytic hydrogenation is unquestionably one of the most significant transformations for academic and industrial-scale synthesis. The development of tunable chiral phosphorous ligands, and of their ability to control enantioselectivity and reactivity, has allowed asymmetric catalytic hydrogenation to become a reaction of unparalleled versatility and synthetic utility. This is exemplified in the ability to prepare en-antiomerically enriched intermediates from prochiral olefins, ketones, and imines through asymmetric hydrogenation, which has been exploited in industry for the synthesis of enantiomerically enriched drugs and fine chemicals. 

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