Palladium complex catalysis asymmetric

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 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

The a-arylation of carbonyl compounds (sometimes in enantioselective version) such as ketones,107-115 amides,114 115 lactones,116 azlactones,117 malonates,118 piperidinones,119,120 cyanoesters,121,122 nitriles,125,124 sul-fones, trimethylsilyl enolates, nitroalkanes, esters, amino acids, or acids has been reported using palladium catalysis. The asymmetric vinylation of ketone enolates has been developed with palladium complexes bearing electron-rich chiral monodentate ligands.155... 

Carbon-carbon bond-forming reactions are one of the most basic, but important, transformations in organic chemistry. In addition to conventional organic reactions, the use of transition metal-catalyzed reactions to construct new carbon-carbon bonds has also been a topic of great interest. Such transformations to create chiral molecules enantioselectively is therefore very valuable. While various carbon-carbon bond-forming asymmetric catalyses have been described in the literature, this chapter focuses mainly on the asymmetric 1,4-addition reactions under copper or rhodium catalysis and on the asymmetric cross-coupling reactions catalyzed by nickel or palladium complexes. 

Very recently, Stahl et al. reported the first synthesis of a 7-membered NHC ligand [98]. Despite substantial effort, the isolation of the free carbene 21 was not successful. However, palladium complexes of 21 could be formed and structurally characterized. Ligand 21 is C2 symmetric as a result of a torsional twist which is thought to attenuate the antiaromatic character of the 87r-electron carbene heterocycle [101,102]. It will be interesting to see, if the synthesis of conformationally stable analogues and their application in asymmetric catalysis will be feasible. 

I 6 Heterogeneous Asymmetric Catalysis in Aqueous Media Table 6.1 Asymmetric allylic alkylation in water with a chiral polymeric palladium complex. 

Catalysis by palladium complexes was actively developed during this decade. Allylic substitution gave excellent results in some cases, thanks to a good fit between the structures of catalyst and substrate. There were significant improvements in the enantioselectivities of the reactions and understanding to some extent of various mechanistic details (for example see [64,65,66]. Most of the time the product was formed with one or several asymmetric centers. In rare cases axial chirality may be created, too [67]. 

They have presented a further library synthesis of phosphines which show very different properties in catalysis to those above. A library of phosphines 98 was prepared based on the well-known /3-turn forming motif -Pro-D-Yyy-(where Yyy is a D-amino acid) with the phosphine-containing amino acids flanking this element.The structural features that were varied in the initial library prepared were the amino acids at the N- and C-termini and substitutions of D-amino acids next to the proline of the -Pro-D-Yyy- fragment. Palladium complexes of the library members were formed on the pins from [PdCl(77 -C3Hs)]2 and these screened for catalytic activity in the asymmetric addition of dimethyl malonate to cyclopentenyl acetate (Scheme 49). The selectivities obtained ranged from 34% ee for the... 

Among the many transition metal complexes, palladium complexes have been known as the classic but still the most effective catalysts for ene-type cycliza-tions leading to five-membered rings [59,60]. However, the precedent examples for the enantioselective catalysis thereof are few [61-63], and hence there is a challenge to establish high levels of asymmetric induction as well as chemical yields [64]. 

Late transition-metal-catalyzed asymmetric Claisen rearrangement takes place in a different mode from that of Lewis-acid-catalyzed Claisen rearrangement Late transition metal catalysis is based on affinity for the Claisen diene system. Among late transition metals, palladium complexes are the most useful and effective for the Claisen rearrangement. 

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