Bidentate chiral ligands

Contrary to the extended view that chirality in tetracoor-dinate complexes appears only when they have four different ligands—just as an asymmetric carbon atom—or when bis(chelate) complexes have asymmetric bidentate ligands, chirality is present even when the bidentate ligands are symmetric. The chirality of the well-known double-stranded helicates and trefoil knots based on tetrahedral metal centers can be nnderstood as due to the helicity of the bis(chelate) units linked by bridging ligands. 

Trisox as a Bidentate Ligand Chiral Trisoxazolines in Copper(ll) Lewis Acid Catalysis and Palladium-Catalyzed Asymmetric Allylic Substitutions... 

Complexes with chiral heterocycles possessing P-containing substituents as P-mono- andP,N-bidentate ligands and their use in homogeneous asymmetric catalysis 98KK883. 

Chiral P-heterocycles as P-mono- and P,N-bidentate ligands in the synthesis of coordination compounds and homogeneous asymmetric catalysis 98KK883. 

With the chiral methyltitanium reagents 39, modified by the bidentate ligands A-sulfonyl-norephedrine 38, a pronounced enantioselectivity is observed in reactions with aromatic aldehydes. Considerably lower enantiosclcction is obtained with aliphatic aldehydes36. 

The complexation of achiral metal enolates by chiral additives, e.g., solvents or complexing agents could, in principle, lead to reagent-induced stereoselectivity. In an early investigation, the Reformatsky reaction of ethyl bromoacetate was performed in the presence of the bidentate ligand (—)-sparteine20. The enantioselectivity of this reaction varies over a wide range and depends on the carbonyl Compound, as shown with bcnzaldehyde and acetophenone. 

Coordination to the central P atom of two different types of symmetrical bidentate ligands leads to structures of type P(aa)2(bb), which are this time C2-symmetric as detailed in Fig. 16. The same chiral descriptors A and A apply to these compounds. Derivatives like 3,4,14-17 and 19-22 fit this description and have only been reported in racemic form so far. If the ligand bb is itself chiral, then diastereomers are generated. This will be described in the next section. 

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

In 1997 the first asymmetric aza-Claisen rearrangement was reported by Overman et al. [55], which made use of diamines as bidentate ligands for Pd(II), allowing for moderate enantioselectivities. In the same year, Hollis and Overman described the application of the planar chiral ferrocenyl palladacycle 38 as a catalyst for the enantioselective aza-Claisen rearrangement of benzimidates 39 (Fig. 19) [56]. A related ferrocenyl imine palladacycle provided slightly inferior results, while a benzylamine palladacycle lacking the element of planar chirality was not able to provide any enantioselectivity [57]. 

Extending the same concept of a planar chiral nucleophilic or basic heterocyclic Fe-sandwich complex, aza-ferrocenes 65 were prepared. The latter have also been successfully applied as bidentate ligands in transition metal catalysis [85]. 

Kiindig EP, Dupre C, Bourdin B, Cunningham A Jr, Pons D (1994) New C2-chiral bidentate ligands bridging the gap between donor phosphine and acceptor carbonyl ligands. Helv Chim Acta 77 421 28... 

In 2001, Imamoto et al. reported the preparation of novel chiral S/P-bidentate ligands containing a chirogenic centre at the phosphorus atom and their stereoinduction capability in palladium-catalysed asymmetric allylic substitution reactions (Scheme 1.14)." ... 

Another class of chiral sulfur-containing ligands, diphosphine ligands based on a thiophene backbone, has been successfully developed by Sannicolo et For the first time, bis(diphenylphosphines) bidentate ligands incor-... 

Only a few reactions with bidentate ligands have been described. 32, reacting with the chiral carbohydrate derivative phenyl-4,6-o-(R)-benzylidene -2,3, 0-bis(diphenyl-phosphino-/ -D-glucopyranoside) (Ph-/ -glup), gave the compound [TcBr(CO)3 (Ph-/Tglup)] (56) in 53% yield. The complex was characterized by elemental analysis and i.r. spectroscopy [71]. Furthermore, reaction with dithio-... 

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