Ligands chiral chelate

As Stated earlier, the combination of an organolithium reagent with sparteine has been widely used to generate chiral carbanions. We will focus here on recent examples where the efficiency of sparteine was compared with other chiral chelating ligands. 

Early work in the field of asymmetric hydroboration employed norbornene as a simple unsaturated substrate. A range of chiral-chelating phosphine ligands were probed (DIOP (5), 2,2 -bis(diphenyl-phosphino)-l,l -binaphthyl (BINAP) (6), 2,3-bis(diphenylphosphino)butane (CHIRAPHOS) (7), 2,4-bis(diphenylphosphino)pentane (BDPP) (8), and l,2-(bis(o-methoxyphenyl)(phenyl)phos-phino)ethane) (DIPAMP) (9)) in combination with [Rh(COD)Cl]2 and catecholborane at room temperature (Scheme 8).45 General observations were that enantioselectivities increased as the temperature was lowered below ambient, but that variations of solvent (THF, benzene, or toluene) had little impact. 

Figure 6-3. Mechanism for the hydrogenation of a prochiral substrate methyl acet-amidocinnamate (MAC) with a catalyst containing a chiral chelating diphosphine ligand...

This chapter reports principally on studies with ruthenium chiral phosphine and chiral sulfoxide complexes and their use for catalytic hydrogenation. We have used the familiar diop ligand, [2R,3R-(—)-2,3-Oisopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino) butane] (7) a related chiral chelating sulfoxide ligand dios, the bis(methyl sulfinyl)butane analog (21) (S,R S,S)-(+)-2-meth-ylbutyl methyl sulfoxide(MBMSO), chiral in the alkyl group and R-(+)-methyl para-tolyl sulfoxide(MPTSO), chiral at sulfur. Preliminary data on some corresponding Rh(I) complexes are presented also. 

Instead, complexes with chiral chelates of the 2,2 -bipyridine (bpy) ligand could lead to asymmetric induction in the synthesis of chiral p-lactams. 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

Scheme 2. Synthesis of chiral chelating cyclopentadienyl ligands...

Figure 9.2 Chiral chelating phosphines and their metal complexes. These ligands and complexes are used in asymmetric hydrogenation of C=C, C=0, and C=N double bonds.

Sequential allylic substitutions have also been used in an intermolecular-intramolecular fashion with 1,4-diacetoxy ds-2-butene 85 as a module, as shown by Hayashi and coworkers [81], to deliver morpholines 86 and piperazines with enantioselectivities up to 61% ee if a chiral chelating bisphos-phane such as BINAP is applied as a ligand (Scheme 26). 

Chiral chelating ligands that contain only one phosphine and another donor atom have been successful in recent years. A readily synthesized example is the phosphino-oxazoline ligand 87 shown in Scheme 31. These types of ligand can be prepared in two steps from an amino alcohol, a substituted benzonitrile and either a electrophillic or nucleophilic phosphorus reagent. They have been used in a number of asymmetric reactions. 

A series of chiral chelate phosphine ligands, mainly based on the ferrocene backbone (Figure 6.9), have been tested in [C4Ciim][PF6] as catalysts for enantioselective allylic alkylation reactions and results are listed in Table 6.9.[134]... 

The asymmetric hydroformylation of alkenes is catalyzed by a number of Rh and Pt complexes with chiral chelating ligands.69 A number of binaphthyl-based chiral phosphites10 and mixed phosphine-phosphite ligand like (22-XXIV) [ // ,S)-BINAPHOS ] have been found particularly effective, e.g., in the asymmetric hydroformylation of styrene in >90% optical yield.71... 

Figure 3.24 Some successful, chiral, chelating bisphosphane ligands.

Figure 4.84 Examples for chiral chelating sulfur donor ligands.

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