Tunable chiral ligands for Ir and Rh hydrogenation processes. Synthesis of enantioamerically enriched amines.

by Guiu Rozas, Ester

Summary and Evaluation 173 Summary and Evaluation The development and study of new iridium and rhodium catalysts for the asymmetric hydrogenation of prochiral imines and enamides was the main objective in this present thesis. To achieve this important goal, several N-donor and P-donor ligands were synthesised. Further exploration of their coordination chemistry and catalytic behaviour was performed. As a result from this study, the following conclusions can be drawn: Chapter 2 deals with the synthesis of N-donor ligands based on oxazoline and imidazoline moieties, studies the coordination chemistry of these ligands towards Ir(I) complexes and explores the catalytic response of the Ir/NS and Ir/NP systems. The Ir/tioether-oxazoline catalysts reduced effectively the N-aryl imine N- (phenylethylidene)benzylamine, however, they did not provide any enantiocontrol of the process. This lack of steroselectivity could be due to the presence of diastereomeric species when the sulphur atom is coordinated to the metal center. The Ir/phosphine-imidazoline catalysts were active reducing N- (phenylethylidene)benzylamine, N-(phenylethylidene)aniline and 6,7-dimethoxyl-1- methyl-3,4-dihydroisoquinoline and provided enantioselectivities as high as 51%. It was not possible to establish a general rule between the activity and stereoselectivity of the process and the different electronic properties of the ligands. The addition of iodines or amines to these catalytic systems has a negative effect on the enantioselectivity and the configuration of the final product is reversed. To replace the sulphur atom by the phosphorus atoms is essential to obtain stereoselective catalytic systems based on N-donor ligands for reducing prochiral imines. In Chapter 3 is described the synthesis of C1- and C2-symmetric P-donor ligands based on carbohydrate backbones (D-xylose and D-mannitol) with different phosphorus functionalities. Diphosphites, diphosphinites and monophosphinites Summary and Evaluation 174 were prepared to be used in the Ir(I) and Rh(I)-catalysed hydrogenation of imines and enamides. Phosphorus ligands based on D-mannitol were more enantioselective than the phosphorus ligands based on D-xylose. The best result achieved in the hydrogenation of N- (phenylethylidene)benzylamine was provided by Ir(I)/D-mannitol-phosphinitephosphite catalysts. On the other hand, the catalysts Ir(I)/D-mannitol-diphosphinite gave the highest ees in the hydrogenation of N-(phenylethylidene)aniline. These results verify that the combination of the electronic and structural parameters of these ligands and their Ir(I) complexes must be optimal for each N-aryl imine. In general, the additives have a dramatic effect on the enantioselective and the addition of iodine derivatives to the catalytic systems promotes the formation of the opposite enantiomer. The Rh/monophosphinite ligands were more stereoselective catalysts in the reduction of prochiral enamides than the diphosphite and diphosphinite ligands. Resum 175