BINAP hgand, catalytic enantioselective

Asymmetric hydroboration of norbornene (27) is a synthetically useful transformation forming optically active norbornanol (28) which is an important chiral synthon. The catalytic enantioselective hydroboration with catecholborane was examined using rhodium complexes coordinated with several chiral phosphine ligands (Scheme 7 and Table 4) [14,15,17,23,24,27]. For this reaction, DIOP (10) and its derivatives 21 and 22, which are modified on the diphenyl-phosphino group, are more enantioselective Hgands than BINAP (7) or chira-phos (9). The highest enantioselectivity was observed in the reaction at -25 °C... 

Chiral bis(oxazoline) is an excellent chiral hgand as is BINAP for asymmetric reactions catalyzed by chiral metal complexes and has been applied to catalytic enantioselective allylation of aldehydes with allyltributyltin (33) by Cozzi, Um-ani-Ronchi and their colleagues [34]. Among various combinations of chiral bis(oxazohnes) and metal salts, they found that the zinc complex 13 is the most effective and the ee values are in the range of 40 to 46%. 

As a result of successful application of the BINAP hgand (Figure 29.7), several stmctural modifications on the biaryl skeleton have been developed to improve enantioselectivity in asymmetric hydrogenation as well as in other catalytic reactions. 

An iridium(I) complex with the l,2-bis(tcrt-butylmethylphosphino)ethane (4) and tetrakis(3,5-bis(trifluoromethyl)phenyl)borate as the counter anion catalyzes the hydrogenation of several acyclic aromatic Ai-arylimines under atmospheric hydrogen pressure at room temperature, giving the desired chiral amines with high-to-excellent enantioselectivities (up to 99%, Fig. 6) [19]. The authors also tested (S )-BINAP (Fig. 1) and (/ )-Ph-PHOX (PHOX = 2-[2-(diphenylphosphino) phenyl]-4,5-dihydrooxazole) hgands with lower enantioselectivities [19]. Both steric and electronic properties of the ligand and the combination with the BArF anion are in the base of the efficacy of this catalytic system. On the other hand, attempted hydrogenations of Ai-(2,2,2-trifluoro-l-phenylethylidene)aniline and M-(l,2,2-trimethyl-propylidene)aniline under the same conditions resulted in... 

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