Asymmetric Reactions. X. Asymmetric Hydrogenation Catalyzed by Bis(dimethylglyoximato)cobalt(II)–Chiral Cocatalyst (Amino Alcohol) System

Abstract

The catalytic asymmetric hydrogenations of α-diketones, α-keto carboxylates, α-(acylamino)acrylates, α-phenylacrylophenone, and α-phenylacrylate were examined with bis(dimethylglyoximato)cobalt(II)–chiral cocatalyst (amino alcohol) and with simple achiral base coordinated bis(dimethylglyoximato)cobalt(II)–chiral cocatalyst (amino alcohol) systems. These gave corresponding optically active reduction products, and in some cases, optically active reductive dimerization products. High degrees of enantioselectivities (≈78%) are achieved in the hydrogenation of α-diketones. Evidence for non-binding of chiral base to cobalt complexes was presented in the case of latter system, i.e., it was shown that the catalytic site and the enantioselectivity-determining site are separated in this system, as in enzymes. The importance of protonated chiral bases for enantioselection was also shown. Based on these results and the stereochemical correlations between structures of the chiral bases and those of the products, a mechanism for this asymmetric hydrogenation was proposed.

Base‐Free Asymmetric Transfer Hydrogenation of 1,2‐Di‐ and Monoketones Catalyzed by a (NH)2P2‐Macrocyclic Iron(II) Hydride

“…[19] However,most of the above methods have drawbacks in terms of catalyst loading, substrate scope, enantioselectivity,o ra tom economy,w hich might be overcome by an approach based on asymmetric hydrogenation. After Ohgosp ioneering cobalt-catalyzed H 2 hydrogenation of benzil to benzoin with modest enantioselectivity (62 % ee), [20] advances have been slow.ATH has been used to reduce 1-phenyl-1,2-propanedione to the enantiopure a-hydroxy ketone B (Figure 1). [5a] To the best of our knowledge, however, no highly enantioselective hydrogenation catalyst has been reported for the hydrogenation of benzils to benzoins (A).…”

EPR Characterisation of Bis(dimethylglyoximato)-Cobalt(II) Complexes and Their Oxygen Adducts Synthesised in an X-Zeolite Matrix

“…Moreover, it is an interest ing candidate for other reasons. Cobaloximes(II) are known to participate in a variety of organic reactions, where species with coordinated molecu lar H 2 [18] and 0 2 [19,20] have been postulated as reactive intermediates. C on (dmgH)2 itself serves as a chemical model for vitamin B12r and con sequently has been well-studied in the last decade [21 -2 4 ] .…”

Photocatalytic Generation of Hydrogen from Water under Aerobic Conditions

“…of 296 for 1 after 18 h and 54 for [Ru(bpy) 3 ](PF 6 ) 2 after 8 h. At longer timescales a reduction in TON is observed. This apparent decrease in hydrogen concentration can be explained by leakage of the used vials or by irreversible hydrogenation reactions of dimethylglyoxime28 or other hydrogenation reactions 29–32. Upon more detailed analysis of the data, a somewhat different oxygen consumption is observed for the two chromophores.…”