Chiral diene ligand

Recently considerable attention has been given to chiral 72-bonded olefin spectator ligands for the asymmetric conjugate addition.122 Hayashi and Carreira independently reported that new chiral diene ligands 83,123 84,124 and 85125 showed high enantioselectivities in Rh(i)-catalyzed conjugate addition of arylboronic acids to enones (Scheme 41). 

Catalysts lacking phosphorus ligands have also been used as catalysts for allylic substitutions. [lr(COD)Cl]2 itself, which contains a 7i-accepting diolefin ligand, catalyzes the alkylation of allylic acetates, but the formation of branched products was only favored when the substitution reaction was performed with branched allylic esters. Takemoto and coworkers later reported the etherification of branched allylic acetates and carbonates with oximes catalyzed by [lr(COD)Cl]2 without added ligand [47]. Finally, as discussed in Sect. 6, Carreira reported kinetic resolutions of branched allylic carbonates from reactions of phenol catalyzed by the combination of [lr(COE)2Cl]2 and a chiral diene ligand [48]. 

Fumaric and maleic compounds are also suitable substrates in the rhodium-catalyzed asymmetric 1,4-addition reactions. While phosphorus-based chiral ligands, such as (/f)-binap, provide low enantioselectivity for these substrates (<51% ee), chiral diene ligands 38 and 45 are particularly effective for achieving high ee (Figures 3.39 and 3.40). 

This chiral diene ligand displays a clear superiority over chiral phosphorus ligands in both enantioselectivity and catalytic activity. A second generation catalytic system tvas later reported for the highly enantioselective addition to N nosyl imines, for which the product is more easily converted into the free amine than that bearing a N tosyl group [120]. The more effective chiral ligand 60 had to be used (Scheme 1.36). 

Figu re 8.6 Chiral diene ligands for the conjugate arylation. 

A rhodium(l) chloride with chiral diene ligands catalyses addition of potassium organotrifluoroborates (K+ ArBFg") to A-sulfanyl ketimines, effecting C-arylation in good to excellent yields, and ee s up to 99.5%. 0... 

Chiral Diene Ligands. In the past few years, chiral bicyclo [2,2,2] octadienes and bicylo [3,3,0] octadienes, developed by and Hayashi (95) and Carrera (96), were used as ligands in a variety of enantioselective transformations (Fig. 10). These novel chiral ligands exert stereogenic centers adjacent to the metal, and... 

Fig. 10. Most widely used chiral diene ligands.

Novel chiral diene ligands were reported to form highly active complexes with rhodium for asymmetric conjugate addition (236). Feng s ligand of a wedge structure provided a chiral environment around rhodium, giving excellent enan-tiocontrol in the asymmetric arylation of N-tosylarylimines (237) and in asymmetric 1,4-addition of aryl boronic acids (238). 

Figure 5.9 The results obtained independently by Hayashi and Van der Eycken s groups [37, 38], on the asymmetric 1,4-addition of phenylboronic acid to cyclic enones, catalyzed by Rh(l)/chiral diene ligands.

Figure 5.10 Chiral diene ligand synthesized by Brown and Corey [39]. BOM benzyloxymethyl group.

Figure 5.11 Chiral diene ligands synthesized by Hayashi and Lin groups [41, 42], independently, and applied in the 1,4-addition of phenylboronic acid to cyclohexen-2-one using Rh catalysis.

The use of C -symmetric diene ligands proved their efficiency in the asymmetric synthesis of diarylamines (with enantioselectivities of the order of about 95-99% ee) [14]. Good tolerance to steric and electronic effects on the substituents of the imine substrates was observed. The active complex [RhCl(i -diene)]2 was isolated and fully characterized (including X-ray crystallography). Eleven diarylamine products were prepared in excellent yields (94-99%). Among the chiral diene ligands synthesized and tested, the one with the phenyl substituent proved to be the most efficient,... 

Carreira has reported a conceptually different class of chiral ligands for iridium-catalyzed allylic displacement reactions (Equation 6) [87]. The use of chiral diene 89 in combination with Ir(I) led to kinetic resolution of racemic allylic carbonates. The resulting allylic ethers (cf 90) and recovered starting allylic carbonates are obtained with high enantioselectivity. The modular chiral diene ligands, exemplified by 89, are readily available by a short synthetic sequence starting from carvone. 

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