Chiral compounds monodentate ligands

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... 

The Pfeiffer effect (the shift in a chiral equilibrium on the addition of an optical isomer of a different compound) of racemic [Cr(ox)3]3- has been examined using for the first time optically stable metal complexes cis-[MXY(AA)2]"+ (where M = Cr3+ or Co, AA = en or tmd and X and Y = anionic monodentate ligand). It was found that the chiral equilibrium of [Cr(ox)3]3-was always displaced in favour of its A enantiomer in the presence of A enantiomers of the cis complexes, and it is proposed that the absolute configurations of cis complexes could be inferred from the equilibrium shift induced in [Cr(ox)3], 438 Laser irradiation of an aqueous solution of racemic [Cr(ox)2(phen)] or [Cr(ox)(phen)2]+ in the presence of ( + )- or (- )-cinchonine hydrochloride rapidly shifts the chiral equilibrium in a direction opposite to that induced by the usual Pfeiffer effect in the dark.439... 

Whereas chirality in tetrahedral compounds of carbon requires four different groups to be bonded around the tetrahedral carbon atom, this is not necessarily the case for other central atoms with other stereochemistries. For example, octahedral complexes have more relaxed rules. Whereas a chiral tetrahedral organic compound is, as a consequence of the rule for chirality, asymmetric (or totally lacking in symmetry), chiral octahedral complexes need not be asymmetric, but may have axes of rotation (they are then dissymmetric). The common rule for chirality is simple - a compound must have non-superimposable mirror images. For an octahedral complex, this can occur even when three different pairs of monodentate ligands are coordinated, as discussed later. We shall look a little more closely at four- and six-coordinate complexes below. 

Each of the isomers will react at the same rate with simple monodentate ligands such as water. They differ only in their reactivity toward other chiral compounds and toward polarized light. 

A classical method for the preparation of enantiopure compounds is the resolution of racemate. However, it is much more effective to use the selective synthesis of the desired enantiopure substance via enantioselective approach. Stereoselective methods of synthesis have been widely developed in organic chemistry. The method of asymmetric synthesis has been known since the nineteenth century and asymmetric catalysis has witnessed an enormous amount of development in recent decades as shown in Chapter 3. In contrast, the asymmetric synthesis of coordination compounds has only recently become a subject of systematic investigation. This is no doubt related to the fact that the chirality of coordination compounds is a much more complex phenomenon than that of organic compounds, because of higher coordination and the multitude of possible central atoms. Furthermore, while in organic chemistry the chiral tetrahedral carbon centres can be prepared without racemization, in contrast T-4 metal centres are very often labile. In fact it is even difficult to prepare compounds with a metal centre coordinated to four different monodentate ligands, and thus the possibility of obtaining one enantiomer is excluded in most cases. 

In this way, some nickel and palladium compounds have been synthesized with the intention of obtaining chiral nematic (N ) and smectic (SmC ) phases. The compounds where the chiral element is situated in the chain of the monodentate ligand are not themselves mesogenic. The others are, and two of them have a modest twisting power in Merck MLC-6401 nematic solvent with a right-handed helix. 

To facilitate the identification of the absolute configuration of the phosphorus center, the nomenclature system for a coordination compound [MX(AB)2] (AB = hetero-bidentate ligand) that can have TBP or square-pyramidal (SP) geometry was used. In the TBP geometry, when a monodentate ligand X occupies one equatorial position, the chiral-at-metal configuration can be defined as /I or d (Figure 3). 

Transition metal-catalyzed hydrovinylation is one of a few practically useful carbon-carbon bond-forming reactions utilizing feedstock carbon sources for the synthesis of high-value fine chemicals. Asymmetric hydrovinylation has many potential applications in the synthesis of pharmacologically important compounds, such as ibuprofen and naproxen, and has attracted much attention [110]. Recently, chiral monodentate phosphines have proven to be highly efficient ligands for the asymmetric hydrovinylation of a-alkyl vinylarenes [111]. 

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