New C2- and C1-symmetric phosphorus ligands based on carbohydrate scaffolds and their use in the iridium-catalysed hydrogenation of ketimines

The C-2-symmetric diphosphinite ligands 10a-d, which have different electron-donating or electron-withdrawing groups in the aryl group, and C-1-symmetric phosphinite-phosphite ligands 11a,b, were directly prepared from glucosamine. Various procedures for synthesising the phosphinite function were explored in order to improve the yield of the reaction. Results were best when Ph2PNEt2 was used in the presence of tetrazol as catalyst. These ligands were added to iridium complexes to give catalyst precursors that are active in the hydrogenation of imines 17 and 19. Cationic iridium complexes gave rise to catalytic systems that were more active than the neutral iridium complexes. The use of additives was, in general, detrimental to both the conversion and the enantioselectivity. In the hydrogenation of 17, results were best with ligand 11a (76% ee), but in the hydrogenation of 19 (70 % ee) they were best with ligand 10b. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)