Preparation of asymmetrically substituted biaryldiphosphines and their use as cocatalysts for transition metal catalyzed enantioselective hydrogenation

Asym. substituted biaryldiphosphine ligands I (R1 = C1-6-alkyl or C3-10-cycloalkyl optionally substituted with one or more halogen atoms; R2 and R3 are equal and are C5-10-cycloalkyl or C1-6-alkyl, or R2 is C5-10-cycloalkyl or C1-6-alkyl, and R3 is aryl optionally substituted with one or more substituents selected from the group consisting of halogen atoms, nitro, amino, C1-6-alkyl, C1-6-alkoxy and di-C1-6-alkylamino groups, and each C1-6-alkyl, C1-6-alkoxy, di-C1-6-alkylamino and C5-10-cycloalkyl group in R2 and R3 optionally being substituted with one or more halogen atoms, from 2,2',6,6'-tetra-bromobiphenyl) were prepd. by a sequence of bromine-metal exchanges and subsequent reactions. Thus, lithiation of 2',6-dibromo-2-methoxy-1,1'-biphenyl (prepn. given) with BuLi followed by phosphination with chlorodicyclohexylphosphine in THF gave 74% title compd., 2',6-bis(dicyclohexylphosphino)-2-methoxy-1,1'-biphenyl (ligand 1). RuCl3/(-)ligand 1 catalyzed enantioselective hydrogenation of Et acetoacetate gave (R)-Et 3-hydroxybutyrate. [on SciFinder (R)]