Conformationally Flexible Biphenyl-phosphane Ligands for Ru-Catalyzed Enantioselective Hydrogenation

Abstract

Stereomutation of a BIPHEP/RuCl /diamine complex (shown schematically) is possible because of the conformational flexibilty of BIPHEP ligands. The result is an asymmetric activation in the Ru-catalyzed hydrogenation of carbonyl compounds to optically active alcohols. Whereas a racemic BINAP/RuCl complex with a chiral diamine activator gives a 1:1 mixture of two diastereomers, unequal amounts of the diastereomers can be produced from a BIPHEP/RuCl complex and a chiral diamine. Ar=3,5-dimethylphenyl, BINAP=2,2'-bis(diphenylphosphanyl)-1,1'-binaphthyl, BIPHEP=2,2'-bis(diarylphosphanyl)biphenyl.

Asymmetric Activation

“…When the chirally rigid BINAPs is replaced by a flexible [71] and proatropisomeric BIPHEPs ligand (to form complex 4), [72] the diastereomeric complexes are formed, in principle, in unequal amounts (Scheme 17). [73] When the major diastereomer shows higher chiral efficiency than the minor isomer, this strategy becomes more effective than the use of similar, but chirally rigid, analogues. The initially formed mixture of ( (CD 3 ) 2 CDOD 1:2), when allowed to stand at room temperature (or at 80 8C), was found to give a 1:3 mixture of diastereomers, with (S)-4 b/(S,S)-DPEN as the major component (Scheme 17).…”

Resolution of BIPHEP on Rh with a chiral diene auxiliary

“…Mikami has performed dynamic resolutions of BIPHEP on Ru [28,29], Pd [30,31], and most recently Rh [32] with the use of enantiopure chiral diamine ligands. As the barriers to atropisomerization are generally higher when BIPHEP is complexed to a transition metal, these complexes can sometimes be used for asymmetric catalysis without significant erosion of the enantiopurity of the catalyst.…”

Asymmetric Hydrogenation and Transfer Hydrogenation of Ketones

“…However, it is expected that the enantioselectivities achieved in the hydrogenation of ketones with the catalysts C37 and C38 composed of chirally flexible biphenylphosphine ligands are lower than those of the C39 and C40 constructed with chiral BINAP-containing ligands. This might be explained using Mikami's mechanistic considerations obtained by an 1 H NMR study of the monomeric complex of DM-BIPHEP/RuCl2/(S,S)-DPEN (Mikami et al, 1999). Furthermore, this type of catalyst can be readily recovered and reused with the retention of enantioselectivity and reactivity.…”