Syntheses and applications of novel chiral ligands

Abstract

Homogeneous asymmetric catalysis is currently one of the most actively pursued research areas in chemistry. Asymmetric catalytic reactions promoted by chiral transition metal complexes have been significantly advanced in the past two decades. A great number of optically pure compounds have been synthesized and used as chiral ligands. From both academic and practical standpoints, it is highly desirable to design and synthesize new chiral ligands and to test their applications in the asymmetric syntheses of high-valued pharmaceuticals and chiral intermediates. The novel chiral phosphinite l,2,5,6-di-isopropylidene-3,4-bis(diphenylpho sphino)-D-mannitol (DIMOP) was designed and synthesized by reacting chlorodiphenylphosphine with a D-mannitol derivative. This ligand has been found to be highly effective in the Rh-catalyzed enantioselective hydrogenation of amidoacrylic acid and its derivatives. Excellent enantioselectivities ranging from 90% to 97% were obtained. A new C2-symmetric heterocyclic chiral ligand, 6,6'-dihydroxy-5,5'-biquinoline (BIQOL) was also synthesized and tested in asymmetric catalysis. BIQOL ligand was easily synthesized via the oxidative coupling of 6-hydroxyquinoline in the presence of a CuCl2-benzylamine complex. Resolution of the BIQOL racemate was carried out via the chromatographic separation of its triflate derivative on a chiral OD column and subsequent hydrolysis. As expected, Ti-BIQOL catalyst was highly effective for the enantioselective addition of diethylzinc to a variety of aldehydes. It has been found that the diethylzinc addition to aldehydes catalyzed by Ti-BIQOL complex provided chiral products with good enantioselectivities (81-91% ee). The trialkylaluminium addition to aldehydes catalyzed by Ti-BIQOL complex was also studied and good results were obtained. Besides the success in the synthesis of phosphinite ligand DIMOP and heterocyclic ligand BIQOL and their application in asymmetric catalysis, a chiral diamine, 6,6'-dimethoxy-2,2'-diaminobiphenyl was also prepared from m-nitrophenol via iodine mediated Ullmann coupling, reduction and resolution. Novel chiral aminophosphine ligand 6,6'-dimethoxy-2,2'-bis(diphenylphosphinoamino)-l,l'-biphenyl (DMBDPPABP) was synthesized through the reaction of the corresponding diamine with BuLi and chlorodiphenylphosphine. Good enantioselectivities were found for the asymmetric hydrogenation of dehydroamino acid and its derivatives catalyzed by the cationic Rh-DMBDPPABP complex. The asymmetric hydrogenation of amidoacrylic acid and its derivatives were carried out under mild conditions, the products were obtained in 73-81% ee. (Lower enantioselectivities were observed in the hydrogenation of enamides catalyzed by Rh-DMBDPPABP.) Another novel chiral diamine derivative Ts-DMBDPPA was synthesized. Application of the ligand in the Ru-catalyzed asymmetric transfer hydrogenation of aromatic ketones was studied, and chiral alcohols were obtained with moderate enantioselectivities (62-78 % ee).