The new axially dissymmetric diphosphines (R)‐ and (S)‐(6,6′‐dimethoxybiphenyl‐2,2′‐diyl)bis(diphenyl phosphine) ((R)‐ and (S)‐5a; ‘MeO‐BIPHEP’) and the analogues (R)‐ and (S)‐5b and 5c have been synthesized in enantiomerically pure form. These ligands have become readily available by a synthetic scheme which employs, as key steps, an ortho‐lithiation/iodination reaction of the (m‐methoxyphenyl)diprienylphosphine oxides 8 and a subsequent Ullmann reaction of the resulting iodides 9 to provide the racemic bis(phosphine oxides) 10. The bis(phosphine oxides) 10 subsequently are resolved with (−)‐(2R,3R)‐ and (+)‐(2S,3S)‐O‐2,3‐dibenzoyltartaric acid and reduced to diphosphines 5. The Ullmann reaction constitutes a new and efficient route to 2,2′‐bis(phosphinoyl)‐substituted biphenyl systems. Absolute configurations were established for (R)‐5a by X‐ray analysis of the derived Pd complex (R,R)‐17a, and for 5b and 5c by means of 1H‐NMR comparisons of the derived Pd complexes 16 or 17, respectively, and by means of CD comparisons. The MeO‐BIPHEP diphosphine 5a proved to be as efficient as the previously described BIPHEMP diphosphine ((6,6′‐dimethylbiphenyl‐2,2′‐diyl)bis(diphenylphosphine)) in enantioselective isomerizations and hydrogenations.
The axially dissymmetric diphosphines (-)-(R)-and (+)-(S)-(6,6'-dimethylbiphenyl-2,2'-diyl)bis(diphenylphosphine) ((-)-(R)-10 and (+)-(S)-10; 'BIPHEMP') have been synthesized, starting from (R)-and (S)-6,6'-dimethylbiphenyl-2.2'-diamine ((R)-and (S)-16), respectively, via Sundmeyer reaction, lithiation, and phosphinylation. Moreover, racemic 4,4'-dimethyl-and 4,4'-bis(dimethylamino)-substituted analogues 11 and 12, respectively, and the 6,6'-bridged analogue 1,l l-bis(diphenylphosphino)-5,7-dihydrodibenz[c, eloxepin (13) were synthesized and resolved into optically pure (R)-and (S)-enantiomers via complexation with di-p-chlorobis{. The molecular structures of the diphosphines (S)-10 and (R)-13 and of two derived cationic Rh (1)
complexes, [Rh((S)-lO)(nbd)]BF, and [Rh((R)-13)(nbd)]BF4were determined by X-ray analyses. Absolute configurations were established for (+)-(S)-10 by X-ray analyses of both the free diphosphine and of the derived Rh(1) complex, and for (-)-(R)-l3 by X-ray analysis of the derived Rh(1) complex. Configurational assignments for the substituted BIPHEMP analogues 11 and 12 were achieved by means of 'H-NMR comparisons of the Pd(I1) complexes derived from the diphosphines and (R)-18, and by means of CD comparisons. The BIPHEMP ligand 10 and analogues 11, 12, and 13 are the first examples of optically active bis(triary1phosphines) containing the axially dissymmetric biphenyl moiety. All these new diphosphines proved to be excellent asymmetry-inducing ligands in Rh(1)-catalyzed isomerizations of N,N-diethylnerylamine affording citronella1 enamine of 98-99% ee.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.