Platinum, asymmetric hydrogenations

Styrene, a-ethyl-asymmetric hydroformylation catalysts, platinum complexes, 6, 266 asymmetric hydrogenation catalysts, rhodium complexes, 6, 250 Styrene, a-methyl-asymmetric carbonylation catalysis by palladium complexes, 6, 293 carbonylation... 

In this chapter, we focus on the rhodium-catalyzed hydrogenation and the development of chiral phosphorous ligands for this process. Although there are other chiral phosphorous ligands, which are effective for ruthenium-, iridium-, platinum-, titanium-, zirconium-, and palladium-catalyzed hydrogenation, they are not discussed in this account. However, this does not preclude complexes of other transition metals as effective catalysts for asymmetric hydrogenation. Fortunately, there are numerous reviews and books that discuss this particular aspect of asymmetric hydrogenation [3]. 

Only one paper has reported on catalytic asymmetric hydrogenation. In this study by Corma et al., the neutral dimeric duphos-gold(I)complex 332 was used to catalyze the asymmetric hydrogenation of alkenes and imines. The use of the gold complex increased the enantioselectivity achieved with other platinum or iridium catalysts and activity was very high in the reaction tested [195] (Figure 8.5). 

More successful attempts at asymmetric hydroformylation have involved rhodium and platinum complexes. As in asymmetric hydrogenation, best results have been obtained with optically active chelating diphosphines as ligands, but some studies of monophosphines have been made. Using... 

Aryl crown ethers, in hexaruthenium carbido clusters, 6, 1008 Aryl cyanides, with Ni(0) complexes, 8, 47 Aryldiazoalkanes, with bis-Gp Ti, 4, 580-581 a-Arylenamides, asymmetric hydrogenation, 10, 28 Arylenes, with platinum(II), 8, 491 Aryl ethers, via cross-coupling with copper, 10, 650 with palladium, 10, 654 Aryl fluorides... 

Asymmetric hydrogenations have been reported with palladium on silk 123>, palladium on modified cellulose 124> and on modified ion exchange resins 125 >. Also with Raney Nickel modified with amino adds 126> and peptides, 27>. Platinum-carbon catalysts exhibiting shape selectivity have been made by coating them with a thermosetting resin, which is carbonized. In such a way an organic molecular sieve skin is formed over the original catalyst 128>. 

H.-Y. Peng, C.-K. Lam, T. C. W. Mak, Z. Cai, W.-T. Ma, Y.-X. Li, H. N. C. Wong, Chiral Rodlike Platinum Complexes, Double Helical Chains and Potential Asymmetric Hydrogenation Ligand Based on Linear Building Blocks ... 

Mainly the hydrogenations are mediated through a platinum-group-metal-catalysed reaction with chiral ligands bound to the metal. Koten and coworkers210 proposed the use of Ru- and Rh-containing catalysts immobilised on silica in asymmetric hydrogenation. As a chiral compound, they used BINAP (2,2 -bis(diphenylphosphino)-1,1 -binaphthyl) (Scheme 4). 

The present study aimed at revealing the mode of enantiodifferentiation in the asymmetric hydrogenation of ethyl pyruvate and isophorone over platinum and palladium catalysts. The effect of adding the modifier after an initial phase of racemic hydrogenation and the combined use of different vinca and cinchona type modifiers on enantioselectivity and activity were studied. A mechanistic rationale is proposed to account for the experimental observations. 

In contrast to Raney nickel catalysts ( 3.4.1), heterogeneous hydrogenation catalysts based on Pt, Rh or Pd do not induce asymmetry in the presence of tartaric acid [113, 578], Platinum catalysts modified by cinchona alkaloids 3.1 and 3.2 cause asymmetric hydrogenation of the carbonyl group of a-ketoesters with a high enantiomeric excess (> 90%). From other types of ketones, the enantioselectivities are lower. 

The next four procedures are based on some recent advances in catalysis. The first of these, an asymmetric hydrogenation of an allylic alcohol, illustrates yet another facet of the versatility of BINAP-ligated platinum group metals with a preparation of... 

New Diop-type ligands are derived from tartaric acid and were originally developed for asymmetric hydrogenation. Most important is the phosphole analog 2.2-dimethyl-4,5-bis(5//-dibenzophosphol-5-ylmethyl)-l,3-dioxolane (DTPHOL or Diop-DBP)57 l6°, which is successfully applied to asymmetric hydroformylation mainly if used with rhodium catalysts i r 7i. 119, ifio kut ajso platinum complexes47,88,124,138. 

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