Ligands chiral phosphines, influence

Hayashi et al.74 described a process of kinetic resolution in the coupling of Grignard reagents R Mgx (having a chiral center at the point of attachment to the metal) with various alkenyl halides under the influence of chiral phosphine-nickel complexes. Chiral amino acid derivatives (35) were used as ligands. 

The influence of the type of alcohol and of the carbon monoxide partial pressure on the asymmetric induction has been investigated in the asymmetric hydrocarbalkoxylation of 2-phenyl-1-propene using (-)DIOP as the chiral ligand (18). For the same substrate different chiral phosphines and diphosphines were used also (17, 24). 

Under the influence of chiral phosphine-transition meted complexes, a-amino ketones are hydrogenated to the corresponding optically active y9-amino alcohols (Scheme 16). Biologically active amino alcohols are obtainable in >90% ee by the Rh- and Ru-catalyzed homogeneous hydrogenation [43e, 54-56] using 55 or 56 as ligands. 

Our initial studies were dedicated to the design and synthesis of a new class of chiral phosphine ligands. Most of the successful chiral phosphine ligands available at the time possessed at least two aryl substituents on the phosphorus center. Aryl substituents render the phosphorus atoms, and thus the attached metal centers, relatively electron-poor. Because the electronic properties of phosphine ligands are well known to dramatically influence the reactivity and selectivity of transition metal centers, our first objective was to prepare new electron-rich (per-alkyl) chiral phosphines. At that time, the use of electron-rich chiral phosphines was largely unexplored. 

The asymmetric carbene transfer is governed by the chirality at iron, while the chiral phosphine ligand, required for resolution of the complexes, is of negligible influence. A broad mechanistic discussion has been presented83. 

Chiral phosphines are valuable as ligands in numerous important catalytic asymmetric processes. Intense research efforts have produced many classes of chiral phosphines with diverse steric and electronic properties, and researchers have examined how these properties influence activity and stereoselectivity in catalysis. Given the rapid development of new methods in asymmetric catalysis and the prominence of chiral phosphines in this area of ehemistry, it is surprising that few catalytic asymmetric syntheses of chiral phosphines have been described. Instead, enantiopure phosphines are most commonly prepared either via stereospecific reactions of resolved starting materials or through routes which require an additional resolution step, such as fractional crystallisation of diastereomers. On the other... 

There can be significant differences in the detailed structure and mechanism of these catalysts. For example, the geometry of the phosphine ligands may affect the reactivity at the metal ion, but the basic elements of the mechanism of enantioselection are similar. The phosphine ligands establish a chiral environment and provide an appropriate balance of reactivity and stability for the metal center. The reactants bind to the metal through the double bond and at least one other functional group, and mutual interaction with the chiral environment is the basis for enantioselectivity. The new stereocenters are established under the influence of the chiral environment. 

With the chiral diamine (S,S)-20 as a co-catalyst full conversion was obtained in all cases, indicating that the amine has a pronounced influence on reactivity and selectivity (entries 9-14). The combination (R Rc)-4ael/(S,S)-20 afforded 18a as an almost racemic mixture (entry 9). The value of 6% ee (R) obtained in this experiment reflects two opposite contributions. On one hand, the system chiral phosphorous ligand/achiral diamine (R Rc)-4ael/19 led to 18a with 65% ee (S) (entry 1). On the other hand, an ee value of 75% (R) in the hydrogenation of 1-acetonaphthone has been reported for the system achiral phosphine (PPh3)/(S,S)-20 [41] This indicates that two inductions are canceled in an almost additive way in the mixed system. 

The synthesis of acrylic acid or its ester (228) from ethylene has been investigated in AcOH from the standpoint of its practical production 12]. The carbonylation of styrene is a promising commercial process for cinnamate (229) production[207,213,214]. Asymmetric carbonylation of styrene with Pd(acac)2 and benzoquinone in the presence of TsOH using 2,2 -dimethoxy-6, 6 -bis(diphenylphosphino)biphenyl (231) as a chiral ligand gave dimethyl phenylsuccinate 230 in 93% ee, although the yield was not satisfactory, showing that phosphine coordination influences the stereochemical course of the oxidative carbonylation with Pd(II) salt[215]. 

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