Optically Pure 1,2‐Bis[(o‐alkylphenyl)phenylphosphino]ethanes and Their Use in Rhodium‐Catalyzed Asymmetric Hydrogenations of α‐(Acylamino)acrylic Derivatives

Abstract

Optically pure (S,S)-1,2-bis[(o-alkylphenyl)-phenylphosphino]ethanes 1a ± d were prepared in four steps from phenyldichlorophosphine via phosphine-boranes as the intermediates. The rhodium complexes 5a ± d of these diphosphines were used for the asymmetric hydrogenations of a-(acylamino)-acrylic derivatives including b-disubstituted derivatives. Markedly high enantioselectivity (78 ± > 99%) was observed for the reduction of b-monosubstituted derivatives. b-Disubstituted derivatives were also reduced in considerably high enantioselectivity (up to 90%). The single crystal X-ray analysis of the rhodium complex 5c of (S,S)-1,2-bis[phenyl(5',6',7',8'-tetrahydronaphthyl)phosphino]ethane (1c) revealed its dtype structure with face orientation of the two tetrahydronaphthyl groups and edge orientation of the two phenyl groups. This conformation corresponds to that of the rhodium complex of 1,2-bis[(o-methoxyphenyl)phenylphosphino]ethane (DIPAMP); the rhodium complex of (R,R)-DIPAMP, whose chirality at phosphorus is opposite that of 5c, exhibits a l-type structure with the face orientation of the two o-methoxyphenyl groups and the edge orientation of the two phenyl groups. The conformational similarity of these rhodium complexes as well as the stereochemical outcome in the asymmetric hydrogenations means that the coordinative interaction of the methoxy group of DIPAMP with rhodium metal is not the main factor that affects asymmetric induction.

Lithium Borohydride for Achiral and ­Stereospecific Reductive Boronation at Phosphorus: Lack of Electronic Effects on Stereoselective Formation of Alkoxyphosphonium Salts

“…The product boranes are mostly also known and fully characterised in the literature, including in many cases absolute configurations: 1b ,6b,17 2b ,6b,18 3b ,6b,19 4b ,6b,20 5b ,21 6b ,6b 7b ,6b except for the cases of 8b and 9b .…”

DiPAMP’s Big Brother “i‐Pr‐SMS‐Phos” Exhibits Exceptional Features Enhancing Rhodium(I)‐Catalyzed Hydrogenation of Olefins

“…Noteworthy, Imamoto et al. prepared a set of 1,2‐bis[( o ‐alkylphenyl)(phenyl)phosphino]ethanes, however the overall performance of the new ligands in the Rh(I)‐catalyzed hydrogenation of α‐acetamidoacrylic acids was not better than that of DiPAMP 5. Since the work of Knowles, the ( o ‐anisyl)(phenyl)phosphino moiety has been systematically appended to new ligand frameworks in order to assess the efficiency of the new P ‐stereogenic designs 6…”

Synthesis and applications of high-performance P-chiral phosphine ligands

“…Complexes 23c and 23d with larger ortho -alkyl substituents afforded almost perfect enantioselectivities (>99%), exceeding that of DIPAMP after 20 years. 56 , 57 ) Another significant fact is that the sense of the enantioselectivity of the newly synthesized ligands with ( S , S )-configuration is the same as that of ( S , S )-DIPAMP in the Rh-catalyzed asymmetric hydrogenation. These results clearly indicate that the coordinative interaction between methoxy oxygen atom and rhodium atom in the Rh-DIPAMP complex is not significant in the stereoregulation and the enantioselection is determined by the steric effects of the ligands.…”