| The ligands (R)- and (S)-1,2-bis(diphenylphosphino)-2-((diphenylphosphino)-methyl)-1-phenyl propane ((R)- and (S)-heliphos) were synthesized. The key step was an acid catalyzed diastereoselective Michael addition of diphenylphosphine to di-(1R,2S,5 R)-menthyl benzylidenemalonate (1) that yields diastereopure products on recrystallization. Tridentate coordination of heliphos was accomplished on reaction with [Rh(NBD)2](ClO4) yielding [Rh(heliphos)(NBD)](ClO 4). The crystal structure of [Rh((R)-heliphos)(NBD)](ClO 4) (2) was determined by X-ray diffraction. Solid state (X-ray) and solution (NMR) studies showed (R)-heliphos adopted the predicted locked Lambda-helical conformation. The sense of helicity was controlled by the absolute stereochemistry of the stereogenic carbon atom of the ligand. Reaction of heliphos with a number of ruthenium(II) complexes resulted mainly in bidentate coordination. Several synthetic schemes towards the derivatized heliphos ligand Me-heliphos (3) were analyzed, as sterics in 3 are proposed to favor tridentate coordination. The synthesis of a key alcohol intermediate CH3C(CH(OH)CH 3)(CH2CO2CH3)2 (4 ) has essentially finalized the synthesis.;[Ru((R)-BINAP)(1-3;5,6-eta-C8H11)(MeCN)](BF 4) (5) was examined as catalyst precursor in enantioselective hydrogenation reactions. The hydrogenations of alpha,beta-unsaturated acids and esters were performed in model protic (methanol) and aprotic (acetone) solutions to determine the effect of solvent. It was determined that the acids do not undergo reaction in aprotic solvent without added base (e.g. triethylamine). Deuterium labeling studies on the hydrogenations of tiglic acid (5), angelic acid (6), and dimethyl itaconate (7) further corroborate the mechanisms proposed by Halpern (alpha,beta-unsaturated acids) and Brown (alpha,beta-unsaturated esters).;Studies of the enantioselective hydrogenation of ketones led to the examination of dialkyl 3,3-dimethyloxaloacetate substrates (alkyl = methyl (10), iso-propyl (11), tert-butyl (12)). The ketones were hydrogenated, under 50 atm of H2 and at 50°C, in methanol solution yielding 59% ee (R), 69% ee ( R), 82% ee (R), respectively. Stoichiometric reaction of catalyst with ketones resulted in formation of ruthenium-alkoxide complexes, formed via ketone-hydride insertion, that were isolated and unambiguously characterized (solution NMR) as [Ru((R)-BINAP)(alkoxide)(MeCN)](BF 4) (13) where alkoxide is from 10, 11, or 12. Results obtained from the stoichiometric hydrogenolysis of 13 indicate that formation of 13 is rapid and reversible prior to the irreversible hydrogenolysis of the rutheniumoxygen bond. The stereoselectivities of the hydrogenolysis of 13 equals the overall stereoselectivity of the catalytic reaction. |
